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Murine Leukemia Viruses: Objects and Organisms

Murine Leukemia Viruses: Objects and Organisms Hindawi Publishing Corporation Advances in Virology Volume 2011, Article ID 403419, 14 pages doi:10.1155/2011/403419 Review Article Alan Rein HIV Drug Resistance Program, National Cancer Institute-Frederick, Frederick, MD 21702, USA Correspondence should be addressed to Alan Rein, reina@mail.nih.gov Received 3 June 2011; Accepted 25 July 2011 Academic Editor: Arifa S. Khan Copyright © 2011 Alan Rein. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Murine leukemia viruses (MLVs) are among the simplest retroviruses. Prototypical gammaretroviruses encode only the three polyproteins that will be used in the assembly of progeny virus particles. These are the Gag polyprotein, which is the structural protein of a retrovirus particle, the Pol protein, comprising the three retroviral enzymes—protease, which catalyzes the maturation of the particle, reverse transcriptase, which copies the viral RNA into DNA upon infection of a new host cell, and integrase, which inserts the DNA into the chromosomal DNA of the host cell, and the Env polyprotein, which induces the fusion of the viral membrane with that of the new host cell, initiating infection. In general, a productive MLV infection has no obvious effect upon host cells. Although gammaretroviral structure and replication follow the same broad outlines as those of other retroviruses, we point out a number of significant differences between different retroviral genera. 1. Introduction The best-studied retrovirus is, of course, human immun- odeficiency virus (HIV-1), which is a lentivirus. One striking A virus can be viewed as a rather regular, relatively simple contrast between MLVs and HIV-1 is the relative simplicity physical object. Alternatively, it can be seen as a living of MLVs. As discussed below, MLVs only encode the proteins organism, evolving in response to selective pressures. Both that will be assembled into the progeny virus particles, views are correct! This paper will outline very briefly some whereas HIV-1 encodes six additional so-called “accessory” of the characteristics of murine leukemia viruses (MLVs), proteins. Indeed, because of this distinction, HIV-1 has keeping both views in mind. We will try to point out the dis- frequently been called a “complex” retrovirus, in contrast to tinctive features of these retroviruses, which are often taken the “simple” retroviruses such as MLV, the proper objects of as prototypes of the gammaretrovirus genus. (Retroviruses study of simple retrovirologists. include Spumaretroviruses (also known as “foamy viruses”) The two viruses also differ in that HIV-1 can efficiently and Orthoretroviruses; the latter are divided into six genera, infect nondividing cells, while MLV generally does not [5, 6] that is, alpha-, beta-, gamma-, delta-, epsilon-, and lenti- (but see also [7, 8]). The ability of HIV-1 to infect nondivid- retroviruses [1].) ing cells is a critical element in its pathogenicity. MLVs have been studied for many years, beginning in Yet another cardinal difference between MLVs and HIV-1 the 1950s, when it was realized that leukemia could be is that HIV-1-infected cells usually die rapidly (within a few transmitted to newborn mice by a filterable agent [2–4]. They days at most) after infection. In contrast, at the cellular level have provided many insights into the general phenomenon MLV infection seems almost completely benign: in general, of leukemogenesis. The MLV genome has also been used as there are no detectable effects of productive MLV infection the starting material in the development of vectors for gene upon the growth, physiology, or morphology of the cells. therapy. Finally, MLVs have often been viewed as “model” HIV-1 viremia is maintained in infected people by continual retroviruses. In fact, while they have been very useful in answering questions about retroviruses and their hosts, there infection of new cells, replacing the cells killed by infection. We do not know how much infection is occurring in an are many ways in which gammaretroviruses differ from other retroviruses: it should never be assumed that a given property MLV-infected, viremic mouse, but since the virus does not of one genus will hold for another. generally kill its host cells, the rate of new infections may be 2 Advances in Virology far lower than with HIV-1. It should be noted that the drugs used in highly active antiretroviral therapy of HIV-infected people act by blocking new infections; thus, it is possible that analogous therapies would have only minimal effects on MLV viremia. 2. MLV: The Physical Object 2.1. MLV Virions. The overall structures of virus particles are probably very similar for all Orthoretroviridae. The virus is pleomorphic, but roughly spherical, with a diameter LB MA P12 CA NC of ∼100–120 nm [9]. It is released from the cell as an Figure 1: Structure of an immature MLV particle. A segment “immature particle”, in which several thousand rod-shaped from a rotationally averaged cryoelectron microscopic image of a Gag polyprotein molecules are arranged, in an incomplete single immature MLV particle is shown on the right. As indicated or imperfect hexameric lattice, as radii of the sphere (see on the left, the particle is bounded by a lipid bilayer (“LB”), Figure 1). The sphere is bounded by a lipid bilayer derived and the MA domain of Gag (pink) is associated with the inner from the plasma membrane of the virus-producing cell. The leaflet of the bilayer. Interior to the MA domain is a zone of N-terminal matrix (MA) domains of the Gag molecules are low density, presumably corresponding to the p12 domain. The most conspicuous feature of the image is the “railroad tracks”, in contact with the lipid bilayer and their C-terminal nucleo- representing the two domains within the CA domain (green), capsid (NC) domains project into the interior of the particle, followed by the NC domain (blue) with bound RNA. The Pol and presumably in contact with RNA. They are approximately Env proteins are not visible in this image. As the particle is ∼100 nm 20 nm long and only 2-3 nm in diameter. The particle also in diameter and the Gag molecules are ∼20 nm in length, there is a contains ∼1–300 Gag-Pol polyprotein molecules, in which region ∼60 nm in diameter largely occupied by solvent in the center Gag is extended at its C-terminus by protease (PR), reverse of the particle. (Reproduced from [9]. Copyright 1998, National transcriptase (RT), and integrase (IN). Finally, trimers of the Academy of Sciences, USA.) envelope (Env) polyprotein span the membrane, with the gp70 surface glycoprotein (SU) on the exterior of the particle, complexed with the p15E transmembrane (TM) protein. Roughly 2.5 × 10 nucleotides’ worth of RNA, representing only a few per cent of the mass of the particle, are also present in the virion. Some cellular proteins are also packaged: this has been documented in great detail in HIV-1 [10] but is also true in MLV [11]. After the particle is released from the cell, it undergoes maturation. PR cleaves Gag into four cleavage products, that is, MA, p12, capsid (CA), and NC. The Pol moiety of Gag-Pol 5 is also cleaved to release free PR, RT, and IN proteins, and the C-terminal 16 residues of TM (the “R peptide”) are removed, producing the mature TM protein p15E (in some papers, this shorter species is called p12E; the longer precursor has been 7 8 called either p15E or Pr15E). The cleavages in Gag cause a major change in the overall architecture of the virion, with Figure 2: The orthoretroviral replication cycle. Infection is initiated CA molecules reassembling in the interior of the particle when the mature, infectious virion binds to a receptor on the into a polygonal structure, the “mature core” of the particle. surface of the new host cell. The Env protein of the virus induces This new structure encloses a complex of the viral RNA with fusion between the viral membrane and the cell membrane (Step 1). NC protein; RT and IN are also presumed to be within this Within the cytoplasm, the mature core dissociates (Step 2) and structure. the dimeric viral RNA (shown in orange) is copied (Step 3) into double-stranded DNA (shown in green). The DNA copy enters the nucleus (probably when the nuclear membrane breaks down during 2.2. The MLV Replication Cycle. As with all orthoretro- mitosis) and is inserted into the chromosomal DNA of the cell viruses, infection is initiated by the binding of the SU (Step 4). The DNA is transcribed and the RNA product is exported glycoprotein on the exterior of the mature, infectious virion from the nucleus (Step 5); within the cytoplasm, some molecules to a receptor on the surface of the new host cell (see Figure 2). will be translated into viral proteins, and others are destined for This binding event triggers dramatic changes in Env, leading encapsidation into progeny virus particles. The viral components to the release of the SU component and conformational assemble into budding virions (Step 6), which are released from rearrangement of TM. The ultimate result is the fusion of the the cell as immature particles (Step 7). Finally, PR cleaves the viral viral membrane with the plasma membrane. proteins, converting immature particles into mature, infectious The fusion of the two membranes leads to the deposition virions (Step 8). It is possible that DNA synthesis actually occurs of the contents of the virion in the cytoplasm of the cell. Once within the mature core rather than after dissociation of the core as in the cytoplasm, the viral RNA is copied by the RT into a shown here. Advances in Virology 3 single molecule of dsDNA. This DNA is somehow conveyed into the nucleus, where the IN protein catalyzes its insertion ψ gag pol RU5 U3 R into chromosomal DNA. env Once the viral DNA is integrated into host DNA, it is termed the “provirus”. It is transcribed and translated Figure 3: The MLV genome. The viral RNA in Moloney MLV is by normal host-cell machinery. The encoded proteins are 8332 nt in length [18]. “R” sequence, 68 nt in length, is identical at trafficked to the plasma membrane, where they assemble both ends of the RNA. The 5 copy of RisfollowedbyU5sequences into progeny virus particles. The immature particles are and then by the PBS (nt 146–163). The long 5 untranslated region released from the cell with the help of the cellular “ESCRT” in the RNA also includes the ψ packaging signal; contained within this signal in some MLV isolates is the CUG codon at which glyco- machinery [23] and subsequently undergo maturation as the Gag translation is initiated (nt 357). The initiation codon for the PR in the virus cleaves the viral polyproteins. The particle is “normal” Gag protein is at nt 621. The gag and pol coding sequences not capable of initiating a new infection until maturation has are in the same frame; they are separated by a UAG termination taken place. codon, which in turn is followed immediately by a 57-base signal, including an RNA pseudoknot, inducing the inefficient translation 2.3. The MLV Genome. The RNA genome of MLV can be of the UAG as glutamine. The Env protein is translated from a divided into coding and noncoding regions and is shown spliced mRNA. The polypurine tract (PPT, nt 7803–7815) is the schematically in Figure 3. primer for +-strand DNA synthesis and is followed by the U3 and R regions. U3 (nt 7816–8264) is placed at the 5 end of the DNA copy of the genome synthesized during infection; it contains promoter 2.3.1. Coding Regions. The only proteins encoded by the and enhancer sequences governing the initiation of transcription at MLV genome are the three polyproteins that will make up the beginning of R. the progeny virus particles: Gag, the structural protein of the immature virus particle, Pol, comprising the PR, RT, and IN enzymes, and Env, the SU and TM proteins that jointly codon as a sense codon. In contrast, in all other genera, mediate the entry of an infectious virus particle into a new host cell to initiate infection [18]. In some MLV isolates, an the suppression occurs before the ribosomes encounter the alternative form of Gag, with an N-terminal extension, is also termination codon and is completely independent of this −1” frame synthesized; this “glyco-Gag” is discussed below. codon. In these viruses, Pol is encoded in the “ relative to Gag. A signal in these viral RNAs before the As in all orthoretroviruses, the three coding regions are end of the Gag coding region induces a fraction of the arranged, from 5 to 3 , Gag :Pol :Env. The Pol proteins are ribosomes to advance two, rather than three bases at a initially synthesized together with Gag, in a large Gag-Pol specific codon so that translation by this subset of ribosomes fusion polyprotein. Gag and Gag-Pol are both translated is shifted from the Gag frame to the Pol frame [29, 30]. (In from full-length viral RNA, identical in sequence to the some retroviruses, there are two frameshifting events, one genomic RNA present in the virion. It seems likely that extending Gag to produce Gag-PR and the second extending the Gag-Pol polyprotein is incorporated into assembling Gag-PR to yield Gag-PR-RT-IN.) A detailed discussion of virions due to “coassembly” of its Gag moiety with Gag translational suppression in retroviruses may be found in polyprotein molecules. Successful replication of the virus Hatfield et al. [31]. requires maintenance of an optimal ratio (on the order of The Env protein of MLV, like that of other orthoretro- 20 : 1) between the Gag and Gag-Pol proteins; indeed, no viruses, is translated from a singly spliced mRNA. There is an detectable virus particles are formed in cells expressing only overlap of 58 bases between the end of the Pol coding region Gag-Pol [24]. This may be because Gag-Pol is more than 3 and the beginning of the Env coding region. times the mass of Gag, and thus, there may not be space within the particles for very many Pol domains. This optimal ratio is achieved by finely tuned translational suppression of 2.3.2. Noncoding Regions. Like the RNAs of all orthoretro- the termination codon at the end of the Gag coding region. viruses, MLV RNA also contains a set of cis-acting signals Remarkably, different retroviruses use fundamentally that are essential for its function as a viral genome. These different mechanisms of translational suppression. In the include the “primer binding site” (PBS), the polypurine tract gammaretroviruses such as MLV (and epsilonretroviruses, a (PPT), the “packaging signal” or ψ, sequences required for genus about which very little is known), Gag and Pol are in insertion, by IN, of the DNA form of the viral genome into the same reading frame, separated by a single termination cellular DNA, and the promoter and enhancer sequences codon. MLV RNA contains a 57-base cis-acting signal within the LTR. immediately 3 of the termination codon [25]. This signal The PBS is an 18-base stretch that is complementary induces the insertion of glutamine (normally encoded by to the last 18 bases of a cellular tRNA molecule. In MLVs, Pro Gln CAG), rather than termination, in response to the UAG this is usually tRNA , but MLVs using tRNA have also termination codon in about 5% of the translation products; been found. Within the virion, the tRNA is hybridized to the resulting product is extended by translation of the entire the viral RNA; when the virus enters a new host cell, the Pol coding region [26]. Similar results are obtained when the tRNA serves as the primer for reverse transcription. The UAG is replaced by UGA or UAA [27, 28]. Thus, these viruses PBS is located ∼145 bases from the 5 end of the RNA and operate in essence by “mis-translation” of the termination ∼460 bases 5 of the beginning of the Gag coding region. PBS CUG AUG UAG PPT 4 Advances in Virology The first deoxynucleotide to be added to the tRNA during particles, and that they are crucial to selective packaging. reverse transcription is determined by pairing with the base These results explain why dimers, but not monomers, of immediately 5 of the PBS, and this base is the 5 terminus viral RNA are selectively packaged and also establish that the of the first (minus) strand in the final DNA product. In other specific, high-affinity binding of Gag to ψ is responsible for words, this site is the “right” end of the final double-stranded selective packaging. DNAproduct of reversetranscription. During reverse transcription, sequences from near the In general, during reverse transcription the RNA is 3 end of the viral RNA (“U3” sequences) are placed at copied by the polymerase activity of RT and is progressively the 5 end, as well as near the 3 end, of the viral DNA. degraded, shortly after being copied, by the RNase H (Conversely, U5 sequences, from near the 5 end of the activity of RT.However,anexceptional stretchof ∼15 RNA, are placed at the 3 endaswellasnearthe 5 purines near the 3 endofretroviralRNAs(the PPT)is end of the DNA.) Following integration of the viral DNA, specifically resistant to this degradation. Having survived the U3 sequences at the 5 end constitute the promoter reverse transcription, this fragment of the viral RNA is the and enhancers driving the transcription, by Pol II, of the primer for synthesis of the second (plus) strand of DNA. The integrated DNA. U3 sequences include a dense collection base immediately 3 of the PPT encodes the first base of the of transcription factor-binding sites; they were used in the plus strand of the DNA copy, that is, the 5 end of the plus experiments that originally demonstrated the existence of strand or “left” end of the double-stranded DNA. enhancers [40] and play a major role in determining the These sequences at the two ends of the final DNA product tissue tropism and pathogenicity of the virus (reviewed in are, of course, the sequences joined by IN to host-cell [41]). The placement of the U3 sequences, which are internal chromosomal DNA during the integration reaction. The two in viral RNA, upstream of the transcriptional start site in the ends form an inverted repeat (reviewed in [32]). In Moloney DNA is an elegant solution to the problem of how to ensure MLV, the sequence of the “plus” strand at the right edge that the viral sequences will lie 3 of a promoter, as required is 5 GGGGTCTTTCA 3 , while that at the left edge is 5 for Pol II transcription. TGAAAGACCCC 3 . The bases at the 3 ends of the plus strand on the right edge, and the 5 end of the left edge, are 2.4. MLV Proteins joined to cellular DNA, but it is the internal bases in these sequences that are essential for IN recognition [33, 34]. 2.4.1. Gag. In essence, the orthoretrovirus particle is con- All orthoretroviral genomic RNAs are, as noted above, structed by assembly of Gag protein molecules. All mRNAs. They resemble cellular mRNAs in having a 5 orthoretroviral Gag proteins contain at least three domains, cap and 3 poly (A) tail. In fact, under certain conditions, which will give rise to three distinct proteins in the mature retrovirus particles can encapsidate cellular mRNAs [35]. virus. The MA domain at the N-terminus of Gag is respon- Thus, the viral RNAs are evidently in competition with sible for targeting the protein to the plasma membrane of cellular mRNAs for incorporation into the virions. Intact the virus-producing cell. In MLV, as in most retroviruses, the retroviral RNAs are selectively incorporated because they N-terminus of Gag is modified by the 14-carbon saturated contain a “packaging signal”, giving them an advantage in fatty acid, myristic acid [42]; this modification is important this competition. for the plasma-membrane association of Gag [43]. The CA Recent structural studies have shed considerable light on domain is the locus of most, if not all, of the interactions the nature of the packaging signal in Moloney MLV RNA between Gag molecules leading to the assembly of the imma- (see Figure 4)[20, 36]. Briefly, in all orthoretroviruses, the ture virion. After the CA molecules are released from the Gag viral RNA is actually packaged in dimeric form, with two polyprotein by PR, they reassemble into the mature core. The molecules of the viral RNA linked by a limited number of NC domain plays a predominant role in the interactions of intermolecular base pairs. The primary location of these base Gag proteins with RNAs, and free NC protein is an essential pairs is in the “leader”, between the PBS and the beginning cofactor in reverse transcription during infection. In general, of the Gag coding sequence. MLV RNA, like that of all there is considerable structural conservation between the gammaretroviruses, contains a pair of stem loops in this Gag proteins in different orthoretroviral genera, despite the region with the sequence GACG in the loop [37]. Both almost complete lack of conservation of primary sequences. NMR and chemical-probing data show that when MLV RNA MLV Gag differsintwo importantrespectsfromthe dimerizes, the “CG” within each of these GACG’s pairs with canonical MA-CA-NC Gag structure (see Figure 5). First, the CG in the other monomer (note that “CG” is a 2-base it contains an additional domain, called p12, situated palindrome, the shortest possible palindromic sequence) [38, between MA and CA. p12 contains the Pro-Pro-Pro-Tyr “late 39]. Further, two other stem loops in the monomers open domain” of MLV [44]; this motif interacts with an Nedd4- out and pair intermolecularly. Most interestingly, this change like ubiquitin ligase to promote the release of the assembled entails a shift in register so that some of the bases which virion from the host cell [45]. p12 also participates in the are paired in intramolecular structures in the monomers infection process, but these additional functions are not well become unpaired in the dimers. These bases include two understood. It is part of the “preintegration complex”, a copies of the motif UCUG-UPu-UCUG. Several kinds of collection of proteins from the infecting virus particle that experiments [20] show that this motif is essential for high- accompany the newly synthesized viral DNA into the cell affinity binding by recombinant MLV Gag protein, that nucleus [46], and some mutations in p12 interfere with these bases are occupied by NC protein within mature MLV proper integration [47, 48]. Surprisingly, there are regions U A C G G A C G G A C G G A U G C G C G C U A C G C G C G G C G C G C A U U C G G A C C G C G U G C A G A A U G C G C U A C G C G C C G U C G G C U G A U U C G G SL1 U A C A G G 240 A U G C G C C U U A C A G C SL2 A U A U U A 5' C UG U A A G 3' Advances in Virology 5 UG C A A U G U U U A U U A Monomer Dimer C U U G SL0 G C A G U G G C U C U U A C 374 PAL2 U 280 U G C 3' A A G U C U U G U 300 A U 260 240 G C Flexible C G 226 235 domain C G UCG UUG UCU G U A 280 5' G 205 U C G U A A A 220 A U SL1 G C G C C G PAL1 U A Dimerization U A U A 320 A U G C G C G C PAL1 PAL2 C G C G U A C 340 A U A U A U G C 231 315 C G 205 227 316 348 355 374 C G U A A A A U G C A A U C U A U 5' G U C A U 360 SL2 UCG U UGC U U G 205 U C G U U C G G C A U U G G A A 3' G C 310 352 355 374 U G C U U C G C U G G C U G U U G U A U C A UG Figure 4: The Moloney MLV dimerization/packaging signal. The figure shows the secondary structure of the 170-base “minimal dimerization active sequence” (nt 205–374) [19] in both monomeric and dimeric forms. Two palindromic sequences, “PAL1” (green) and “PAL2” (red), are contained within stem loops in the monomer but open out and pair intermolecularly in the dimer. The two monomers are also connected in the dimer by base pairing between the “CG” moieties in the “GACG” loops of a pair of stem loops (“SL1” and “SL2”, orange). The RNA also contains two motifs with the sequence UCUG-UPu-UCUG (blue boxes); these are partially or fully base-paired in the monomer but become unpaired as a result of the RNA rearrangements accompanying the intermolecular base pairing of PAL1 and PAL2. These bases are a crucial element in ψ, as replacement of the four UCUG sequences with UCUA prevents selective packaging of the viral RNA; the exposure of these bases in dimers, but not monomers, presumably explains the selective packaging of dimeric RNA [20]. (Figure reproduced from Trends in Biochemical Sciences, Copyright 2011, with permission from Elsevier [21].) within p12 in which sequence changes seem to have no major MA p12 CA NC effect on viral function [49, 50], and the maturation cleavage between MA and p12, unlike the other cleavages, is not Figure 5: MLV Gag protein. The MLV Gag protein is modified at its absolutely essential for viral infectivity [51]. It is extremely N-terminus by the 14-carbon fatty acid myristic acid. It is cleaved proline-rich (18 of its 84 residues (21%) are prolines), and during virus maturation into MA, p12, CA, and NC; most of the it has been described as “unstructured” on the basis of NC molecules are also cleaved 4 residues before their C-terminus. NMR data [52]. However, recombinant MLV Gag protein is an extended rod in solution, and the prolines in the p12 domain contribute to its rigidity (Datta et al., manuscript in preparation). It seems likely that this domain in Gag can translation initiation, 264 bases 5 of the normal Gag AUG assume any of a number of rigid conformations containing initiation codon [53]. The N-terminal extension includes a short polyproline helices. signal sequence so that this protein (unlike standard Gag) Second, some, but not all, MLV isolates encode an is synthesized in the rough endoplasmic reticulum and alternative form of the Gag polyprotein, called “glyco-Gag” processed in the Golgi apparatus. Relatively little glyco-Gag Gag or gPr80 . This protein differs in sequence from “standard” is incorporated into virions [54]. Because of a sequence Gag in that it is extended N-terminally. Synthesis of glyco- polymorphism at the site of the CUG initiator, XMRV does Gag is initiated at a CUG codon in a favorable context for not encode glyco-Gag. C G A C G G U A A U A U G C U A G C G C G C C G C G C C G C C G G C G C C G C G C G U A C G A U A U G C G G G C G A U U A G C U A A U G C G C G C C G C G G C G C G C C 6 Advances in Virology The functional significance of glyco-Gag is still not clear. MLV Env is depicted schematically in Figure 6.Mature Early studies showed that it is not essential for replication SU of Moloney MLV is 435 residues in length, while TM is of MLV in cell culture, but is needed for efficient replication 180 residues. In turn, SU contains an N-terminal “receptor- and pathogenicity in mice [55, 56]. It was recently reported binding domain” (RBD) of ∼240 residues, a short, proline- that the correct assembly of standard MLV Gag into spherical rich “hinge” region, and a highly conserved C-terminal immature particles in cell cultures is impaired in the absence domain [74]. The RBD consists of an antiparallel β-sandwich of glyco-Gag [57]; new data indicates that the presence of projecting “up” from the surface of the virion, and a glyco-Gag directs virion assembly to lipid rafts and that this highly variable region resting atop this scaffold. Both ends function involves the cellular La protein [58]. Remarkably, of the RBD contribute to this β-sandwich [75]. Sequence glyco-Gag can also complement Nef deletions in HIV-1 alignments and analysis of chimeric SU proteins show that [59]. the variable sequences within the RBD make specific contacts with cell-surface receptors. Among the conserved features of MLV Gag is also unusual among orthoretroviral Gags in SU are a histidine residue near the extreme N-terminus and that its NC domain only contains a single zinc finger rather a CXXC motif in the C-terminal portion of SU. TM protein than two as in most genera. The zinc-coordinating residues begins with a very hydrophobic stretch, the “fusion peptide”. have the spacing C-X -C-X -H-X -C, as in all orthoretrovi- 2 4 4 Astretch betweenTMresidues43and 78 (inMoloney MLV) ral NC proteins. This 14-residue motif plays a critical role has a 4-3 repeating pattern of hydrophobic residues that in the selective packaging of genomic RNA, among other forms a coiled coil. TM also contains a CX CC motif; in the functions [60, 61]. The last 4 residues of NC are removed virus particle, there is a disulfide bond joining SU, via one of from the majority of Gag molecules, as they are from Gag- the cysteines in the CXXC, to TM, via the last cysteine in the Pol molecules, during virus maturation [26, 62]. CX CC [76–78]. The function of the Env complex is to induce fusion 2.4.2. Pol. As noted above, the products of cleavage of the between the membrane surrounding the virus particle and Gag-Pol polyprotein include PR, RT, and IN. PR catalyzes the the membrane of a new host cell. As in all orthoretroviruses, cleavages leading to virus maturation; like all retroviral PRs, the cleavage between SU and TM is absolutely required for it is an aspartic protease which is only active as a dimer [63, Env function [79]. Presumably, this is essential because it 64]. places the fusion peptide at the N-terminus of TM rather RT synthesizes the DNA copy of the viral genome during than in the interior of the Env polyprotein. The removal infection. This function involves three enzymatic activities: of the R peptide from the C-terminus of Prp15E during RNA-templated DNA synthesis, DNA-templated DNA syn- virus maturation is also necessary for the fusogenicity of thesis, and degradation of the RNA strand in an RNA:DNA Env [80, 81]. It seems likely that fusogenic activity would be hybrid, eliminating the RNA template immediately after harmful to the virus-producing cell and that the R peptide is synthesis of the complementary DNA strand. MLV RT is a “safety catch” suppressing this activity until the virus has apparently active as a monomeric protein [65, 66] unlike the left the cell. The mechanism by which the R peptide inhibits RT enzymes of alpharetroviruses and lentiretroviruses, which fusion is not known, but, remarkably, it has the same effect are both heterodimers [67]. when joined to the influenza HA protein [82]. Retroviral IN enzymes possess two catalytic activities: “3 The fusion between the two membranes by the mature end processing”, in which IN removes two nucleotides from Env complex is the end result of an amazing cascade the 3 end of each strand of the DNA to be integrated, and of events. Briefly, binding to the receptor on the plasma “strand transfer”, in which the new 3 ends are inserted into membrane induces a conformational change in the RBD. chromosomal DNA in the new host cell [32]. MLV IN has This change is propagated in SU, resulting in the ionization not been characterized in detail but is presumed to function of the one free thiol in its CXXC motif [83]. (The conserved as a tetramer [68, 69]. histidine near the N-terminus of SU, which is essential for Env function, may catalyze this ionization [84].) The 2.4.3. Env. As with all orthoretroviruses, the MLV Env gene ionized sulfur then attacks the neighboring cysteine, and product is synthesized in the rough endoplasmic reticulum the disulfide linkage between SU and TM is replaced by an and glycosylated in the Golgi apparatus. It is also cleaved in intra-SU bond between these two cysteines. Breaking the the Golgi by a cellular furin-like protease into two fragments, SU-TM bond releases SU from the Env complex, exposing SU the large, N-terminal surface glycoprotein (gp70 ) and the the fusion peptide at the N-terminus of TM. The fusion TM peptide inserts into the target membrane; this is followed C-terminal transmembrane protein p15E .Atrimer of these heterodimeric SU-TM complexes is then trafficked by a major conformational change in TM, in which a C- to the cell surface. As mentioned above, it undergoes an terminal heptad repeat-like sequence in the TM ectodomain additional cleavage during virus maturation: PR removes the folds against the N-terminal heptad repeat [76]. This shift to C-terminal 16 residues, also known as the “R peptide”, from a hairpin configuration brings the two membranes into very close apposition; this finally results in the fusion of the two the cytoplasmic tail of the TM protein [62, 70]. This matu- ration cleavage of TM is found in the gammaretroviruses, in membranes. Mason-Pfizer monkey virus, a betaretrovirus [71, 72], and in Further studies make it clear that RBD functions not only to bind a receptor on the target cell, but also to prevent the lentivirus equine infectious anemia virus [73], but not, as far as is known, in other retroviruses. the conformational change in TM, leading to membrane CX CC CXXC Advances in Virology 7 productively infected with an MLV, the viral Env protein sat- urates the receptors that it would use for infection, rendering the cell almost completely resistant to superinfection by virus FP particles that use the same receptor. This resistance makes it possible to group MLV isolates into families sharing common RBD receptors. “Interference” measurements of this kind showed that NIH/3T3 mouse cells have four distinct cell-surface Helix A molecules used as receptors by different MLVs, as indicated in Table 1 [91, 95]. This polymorphism is considered in detail in a comprehensive review [96], and is discussed in other articles in this series. It is notable that all receptors used by MLVs contain multiple membrane-spanning domains, unlike the known receptors for most other orthoretroviruses. Helix B 3. MLV: The Organism C-term -S-S- 3.1. Assays for Infectious MLV. Quantitative virology is virtually impossible without a reliable infectivity assay [97]. Since MLVs generally have no obvious effect on the cells they infect, the opportunities for developing a “plaque” or “focus” assay have been very limited. Two such assays have been devised, each exploiting a specific cell line with a unique response to MLV infection. One of these is the “UV-XC” test [98]. XC cells, derived Figure 6: MLV Env protein. MLV Env protein consists of a complex from a rat tumor induced by Rous sarcoma virus, undergo SU TM between gp70 and p15E . The cartoon shows that gp70 has two rapid syncytium formation when they come into contact domains, RBD at its N-terminus and “C-term” at its C-terminus, with cells producing ecotropic MLV. This property was used separated by a variable, proline-rich linker. P15E contains, from to develop an “indirect” plaque assay: a plate of permissive N- to C-terminus, the fusion peptide (FP), an N-terminal helical cells is first infected with the virus, and the virus is allowed domain (helix A), a short C-terminal helical domain (helix B), and to spread in these cells for 5–7 days. At the end of this period, a C-terminal domain (light pink) which spans the viral membrane (yellow). Gp70 is exclusively external to the virus and is connected the cells have grown into a confluent monolayer, and the to p15E by a disulfide linkage between one of the two cysteines in a plate contains invisible “foci” of MLV-producing cells. Each CXXC motif within its C-terminal domain and the last cysteine in a focus has arisen by the localized spread of virus from a single CX CC motif in p15E. cell, infected by a virus in the inoculum, to neighboring cells; several rounds of replication can occur during the assay. This monolayer is then killed by UV-irradiation and overlaid with XC cells. A day later, the XC cells have replaced the fusion, from occurring prematurely, that is, before contact original cells; they are fixed and stained, and “plaques”, that of the virus with the receptor [85, 86]. In fact, under special is, localized regions of syncytia, are counted. One particular circumstances infection can occur “in trans”, that is, when a advantage of this assay is that it can be used to measure soluble RBD binds a cell-surface receptor in proximity to the the infectivity of any ecotropic MLV on any cells; thus, for virion [87]. This activity of the MLV Env complex has special example, comparing the titer of a single virus preparation on consequences for the “MCF” class of MLVs. These “mink cell NIH/3T3 cells and Balb/3T3 cells tells one whether the virus focus-inducing” or “polytropic” MLVs arise in mice that are is N-tropic, B-tropic, or NB-tropic. On the other hand, the viremic for ecotropic MLVs, and are recombinants in which fact that it only detects ecotropic MLVs is a serious limitation the ecotropic RBD has been replaced by an RBD from an of the UV-XC test. endogenous MLV genome [88–90]. This substitution gives The other quantitative assay for replication-competent the MCF a different receptor specificity from that of its MLV is the S+L− assay [99]. S+L− cells are specific cell lines ecotropic parent [13–15, 91]. The complex of the ecotropic transformed by Moloney sarcoma virus. When these cells SU protein with the ecotropic receptor on target cells (as in are superinfected by an MLV, they become much rounder the viremic mice) has been shown to facilitate infection of and more refractile (this may reflect “hypertransformation”, the cells by MCF virions [92]. perhaps due to reinfection of the cells with additional copies Remarkably, TM protein performs yet another func- of Moloney sarcoma virus after it has been rescued by the tion for MLV. Immediately proximal to the CX CC motif MLV). In this assay, S+L− cells are infected and allowed discussed above is a 20-residue stretch which has potent to grow for ∼5 days; “foci” of rounded cells, which stand immunosuppressive activity; this activity is crucial in MLV out against the confluent monolayer of uninfected S+L− infections in mice [93, 94]. cells, are then scored under a low-power microscope. This As indicated above, MLVs are polymorphic with respect assay has the advantage that it will detect any replication- to their use of cell-surface receptors. In general, when a cell is competent MLV, not just members of a specific class. 8 Advances in Virology Table 1: MLV receptors on NIH/3T3 mouse cells. 3.3.1. Superinfection Interference. Two genes inducing strong resistance to specific envelope classes of MLV have been Virus class Example Receptor Reference described: Fv-4 and Rmcf [102, 103]. Both of these genes Moloney have been found to function by superinfection interference: [12] Ecotropic mCAT1 MLV in other words, the genes encode glycoproteins which Polytropic MCF247 XPR1 [13–15] bind MLV receptors, rendering the receptors unavailable Amphotropic 1504A SLC20A2 [16, 17] for incoming viruses. Fv-4 blocks the ecotropic receptor, mCAT1, whereas Rmcf blocks the MCF receptor XPR1. It SLC20A1 seems reasonable to imagine that these genes were originally 10A1 10A1 or [16, 17] introduced into the mouse genome as the Env genes of SLC20A2 endogenous MLVs. The table lists the receptors for MLVs found on NIH/3T3 mouse cells. The diversity of MLV receptors is discussed in more detail in other articles of this 3.3.2. Fv1 Restriction. Fv1 restriction was the first system series. for resistance to MLV to be described in mice [104]. Inbred mouse strains carry the “n” allele, the “b” allele, or the “nr” However, it is extremely time consuming. It can also be allele at the Fv1 locus. In turn, naturally occurring MLVs difficult to distinguish the foci from random irregularities in n nr may be N-tropic or B-tropic. Fv1 or Fv1 mice are partially the cell monolayer, so scoring the assay requires considerable resistant to B-tropic MLVs, while the Fv1 locus encodes skill and involves some judgment. nr partial resistance to N-tropic MLVs (Fv1 mice are resistant For many, but not all, kinds of experiments, replication- to some N-tropic MLVs as well as B-tropic MLVs). Passage defective “reporter” viruses rescued by MLV can be assayed of an MLV in the restrictive host may ultimately lead to the in lieu of assaying the MLV itself. The reporter viruses selection of a viral variant that has lost its sensitivity to Fv1 originally used in this way were acute transforming viruses; restriction; these laboratory isolates, such as Moloney MLV, for example, MLVs were grouped into interference families are termed NB-tropic. XMRV is unique in that it is restricted by measuring the ability of Harvey MSV pseudotypes to n b by both Fv1 and Fv1 [105]. transform MLV-infected cells [91, 95]. More recently, of Despite many years of investigation, the mechanism of course, MLV-derived vectors expressing a variety of genes, Fv1 restriction is still not well understood. The Fv1 gene such as luciferase, β-galactosidase, and green fluorescent productseems to be asomewhatdegenerateretroviralGag protein, have been constructed for use as reporter viruses protein [106]. Genetic data indicate that it binds to a specific (e.g., [100]). site in the N-terminal domain of CA in the mature core of Cell lines have also been developed in which a reporter the incoming virus particle. This interaction blocks infection gene is only expressed following replication in the cell of at a point between reverse transcription and integration an MLV. These cells contain an MLV-derived vector which of the viral DNA. The Fv1 protein is present in cells at carries a reporter gene in reverse orientation; the reporter extremely low levels [107]; in fact, restriction can be blocked gene is interrupted by an intron in the forward orientation. or “abrogated” by infection with a single particle of the Transcription and splicing yields an RNA in the cell with an restricted type [108]. Particles which have been inactivated uninterrupted, negative-sense copy of the reporter gene; if by heat or gamma irradiation can retain the ability to this RNA is rescued by an MLV, it can be copied into DNA, abrogate Fv1 restriction [109]. finally producing an intact reporter gene whose expression Biochemical analysis of the Fv1 restriction machinery has can be measured (Aloia et al., manuscript in preparation, proven extremely difficult, but it appears that the ability of but see [101]). This assay has the special advantage that it the Fv1 protein to multimerize [110] is an essential element can be performed by cocultivation of the assay cells with cells in restriction [111]. The specific binding of the protein to CA producing the virus to be assayed, as well as by infection of protein of the restricted type seems to occur only when the the assay cells with cell-free virus. mature CA is in a lattice, as in the viral core; this binding was recently demonstrated, for the first time, using CA protein 3.2. Endogenous MLVs. At least 100 times over the course of arrayed on lipid nanotubes [112]. evolution, MLVs have infected cells of the mouse germline. While the Fv1 restriction system is, as far as is known, Once the viral DNA has integrated into the germline DNA, it found only in mouse cells, human cells possess a somewhat is passed from parents to offspring just like any other mouse analogous restriction system effected by the TRIM5α protein. gene. The biology of these “endogenous” MLVs and their TRIM5α was discovered by virtue of its ability to restrict effects on their hosts are quite complex and are considered HIV-1, but it is also active against some MLVs; remarkably, in other articles in this series. like the Fv1 gene product, it distinguishes between N-tropic and B-tropic MLVs [113]. 3.3. Resistance to MLV. While MLVs are generally benign at the cellular level, they do induce both lymphomas and 3.3.3. APOBEC3 Restriction. All placental mammals have at neurological diseases in mice. Mice have evolved a number least one member of the APOBEC3 gene family; humans of resistance mechanisms that inhibit the growth of MLVs; and chimpanzees have seven APOBEC3 genes [114, 115]. MLVs have, in turn, developed strategies for evading these APOBEC3 proteins can be incorporated into retrovirus par- defense mechanisms. ticles, and they interfere with viral replication during reverse Advances in Virology 9 Virion transcription when the APOBEC3-bearing virus particle infects a new host cell. APOBEC3s are cytidine deaminases with one or two zinc-coordinating motifs that are instru- mental in the restriction of viral replication. It seems likely that the primary function of APOBEC3s is protection of the mammalian host against pathogens (or intracellular parasites GPI such as retrotransposons): mice lacking mouse APOBEC3 (mA3) survive and reproduce normally but are very sensitive to retrovirus infection [116, 117]. One way in which APOBEC3 proteins inactivate retro- viruses is by hypermutation. By deaminating deoxycytidine to deoxyuridine in minus-strand DNA during the synthesis of viral DNA, they bring about a G to A change in the plus- GPI strand. Many susceptible viruses have been shown to incur very high levels of G to A mutation as a result of APOBEC3 action. However, it is now clear that APOBEC3 proteins act on retroviruses in other ways as well. For example, the degree of inactivation of HIV-1 by human APOBEC3G (hA3G) does not necessarily correlate with the level of G to A mutation Host cell (reviewed in [118]), and hA3G has been shown to affect both the synthesis and integration of HIV-1 viral DNA [119]. Figure 7: Hypothetical mechanism of restriction by tetherin. The cellular restriction factor tetherin can act as a bridge between two There are two isoforms of mA3, containing or lacking membranes. Tetherin contains a transmembrane domain at its N- exon 5. Most studies on mA3 have used the form lacking the terminus and is anchored to a membrane by a glycophosphatidyl exon.MLVsshowdramaticdifferences in their sensitivity to linkage at its C-terminus. It also dimerizes due to a parallel coiled- this mA3: both XMRV and AKV (the endogenous ecotropic coil structure between the termini of the protein. Anchorage to MLV in AKR mice, a mouse line bred for high leukemia membranes at both ends apparently enables tetherin to “trap” virus incidence) are far more sensitive to inactivation by mA3 particles, preventing their escape from the virus-producing cell. It than Moloney MLV (which was selected for rapid growth is not known which end of the protein is embedded in the cellular and leukemogenicity by passage in mice over a period of membrane and which in the viral membrane. (Figure reproduced years) [105, 120–122]. Moreover, when DNA of XMRV or with permission from [22].) AKV is synthesized in the presence of mA3, it contains large numbers of G to A mutations [120, 121], but these host protein “tetherin” (also known as CD317, BST2, or mutations are not detectably induced in Moloney MLV HM1.24) [125]. Tetherin is a membrane protein with a very by mA3 [100, 123]. Presumably, the creation of the AKR unusual topology: it has a cytoplasmic N-terminus, followed mouse strain entailed the selection of mice that provide by a transmembrane helix, an extended ectodomain, and a maximally permissive environment for AKV, and thus, a C-terminus associated with the plasma membrane by a this virus has not faced selective pressure leading to mA3 glycophosphatidyl inositol linkage. Tetherin dimerizes via resistance. In contrast, selection during passage of Moloney the ectodomain, which forms a coiled coil ∼90Along. The MLV has led to partial resistance to inactivation by mA3, presence of membrane anchors at both ends of the molecule and apparently complete resistance to the hypermutational evidently gives it the ability to physically link released virus effects of mA3. The mechanisms underlying these resistance particles to the surface of the virus-producing cell, effectively phenomena are unknown. It should be noted that in HIV-1, preventing their escape into the surrounding medium (see one of the “accessory proteins”, that is, Vif, is responsible for Figure 7)[22]. viral resistance to hA3G. Vif functions by binding to hA3G Tetherins inhibit the release of all retroviruses tested, and inducing its proteasomal degradation. However, as and also of filoviruses such as Ebola, arenaviruses such as emphasized above, MLVs do not encode accessory proteins, Lassa, and herpesviruses such as Kaposi’s sarcoma-associated and the resistance of Moloney MLV to mA3 must reside in its herpesvirus. They are constitutively expressed on some cell Gag, Pol, and/or Env protein. As mA3 is packaged efficiently surfaces and are inducible by type I interferon in others. in Moloney MLV particles [100, 123], the resistance does not Mouse tetherin has been shown to inhibit the replication depend upon exclusion of mA3 from the virus. of MLV [126]. While lentiviruses have several alternative The biology of MLV restriction by the mA3 containing countermeasures against tetherins, including the HIV-1 exon5issomewhatdifferent from the foregoing: mA3 accessory protein Vpu (reviewed in [127]), no resistance protein containing this exon can be cleaved by MLV PR, mechanisms in MLVs have yet been described. leading to the inactivation of this mA3 within MLV particles [124]. 4. Concluding Remarks 3.3.4. Restriction by Tetherin. Recently, yet another antiviral It is clear that MLVs have provided an extraordinary wealth restriction system has been discovered, mediated by the of information about retroviruses, both as physical objects 10 Advances in Virology and as living organisms. They (and other gammaretro- [9] M. Yeager, E. M. Wilson-Kubalek, S. G. Weiner, P. O. Brown, and A. Rein, “Supramolecular organization of immature viruses, such as gibbon ape leukemia virus) are now being and mature murine leukemia virus revealed by electron developed as vectors for gene therapy. As has been indicated cryo-microscopy: implications for retroviral assembly mech- throughout this paper, the contrasts with other retroviruses anisms,” Proceedings of the National Academy of Sciences of the such as HIV-1 help to illustrate the range of possibilities United States of America, vol. 95, no. 13, pp. 7299–7304, 1998. by which viruses solve common problems. Finally, as with [10] D. E. Ott, “Cellular proteins detected in HIV-1,” Reviews in all viruses, MLVs provide a window into the “black box”, Medical Virology, vol. 18, no. 3, pp. 159–175, 2008. an unparalleled opportunity to learn about the cells and [11] J. E. Bubbers and F. Lilly, “Selective incorporation of H 2 organisms that they infect. Indeed, many cellular proteins antigenic determinants into Friend virus particles,” Nature, have been shown to participate in MLV replication; while this vol. 266, no. 5601, pp. 458–459, 1977. large topic is beyond the scope of this paper, it is the focus of [12] L. M. Albritton, L. Tseng, D. Scadden, and J. M. Cunning- a fascinating review by Goff [128]. ham, “A putative murine ecotropic retrovirus receptor gene encodes a multiple membrane-spanning protein and confers susceptibility to virus infection,” Cell, vol. 57, no. 4, pp. 659– Acknowledgments 666, 1989. [13] J. L. Battini, J. E. J. Rasko, and A. D. Miller, “A human The author thanks his colleagues John Coffinand Steve cell-surface receptor for xenotropic and polytropic murine Hughes for many helpful discussions and particularly Jim leukemia viruses: possible role in G protein-coupled signal Cunningham for insight into MLV Env gymnastics. He also transduction,” Proceedings of the National Academy of Sciences of the United States of America, vol. 96, no. 4, pp. 1385–1390, thanks Bob Bassin for his mentorship and generosity during his introduction to the study of MLVs. Research in his [14] C. S. Tailor,A.Nouri,C.G.Lee,C.Kozak,and D. Kabat, laboratory is supported by the Intramural Research Program “Cloning and characterization of a cell surface receptor of the NIH, National Cancer Institute, and Center for Cancer for xenotropic and polytropic marine leukemia viruses,” Research. Proceedings of the National Academy of Sciences of the United States of America, vol. 96, no. 3, pp. 927–932, 1999. [15] Y. L. Yang, L. Guo, S. Xu et al., “Receptors for polytropic and References xenotropic mouse leukaemia viruses encoded by a single gene [1] M. L. Linial et al., “Retroviridae. Virus taxonomy: clas- at Rmc1,” Nature Genetics, vol. 21, no. 2, pp. 216–219, 1999. [16] D. G. Miller and A. D. Miller, “A family of retroviruses that sification and nomenclature of viruses,” in Proceedings of the International Committee on Taxonomy of Viruses,C.M. utilize related phosphate transporters for cell entry,” Journal of Virology, vol. 68, no. 12, pp. 8270–8276, 1994. Fauquet, M. A. Mayo, J. Maniloff,U.Desselberger, andL.A. Ball, Eds., pp. 421–440, Elsevier/Academic Press, San Diego, [17] C. A. Wilson, K. B. Farrell, and M. V. Eiden, “Properties of a unique form of the murine amphotropic leukemia virus Calif, USA, 2004. [2] L. Gross, ““Spontaneous” leukemia developing in C3H mice receptor expressed on hamster cells,” Journal of Virology, vol. 68, no. 12, pp. 7697–7703, 1994. following inoculation in infancy, with AK-leukemic extracts, or AK-embrvos,” Proceedings of the Society for Experimental [18] T. M. Shinnick, R. A. Lerner, and J. G. Sutcliffe, “Nucleotide Biology and Medicine. Society for Experimental Biology and sequence of Moloney murine leukaemia virus,” Nature, vol. 293, no. 5833, pp. 543–548, 1981. Medicine, vol. 76, no. 1, pp. 27–32, 1951. [3] L. Gross, “Development and serial cellfree passage of a highly [19] C. S. Badorrek and K. M. Weeks, “RNA flexibility in the dimerization domain of a gamma retrovirus,” Nature potent strain of mouse leukemia virus,” Proceedings of the Society for Experimental Biology and Medicine, vol. 94, pp. Chemical Biology, vol. 1, no. 2, pp. 104–111, 2005. [20] C. Gherghe, T. Lombo, C. W. Leonard et al., “Definition of a 767–771, 1957. [4] J. B. Moloney, “Biological studies on a lymphoid-leukemia high-affinity Gag recognition structure mediating packaging of a retroviral RNA genome,” Proceedings of the National virus extracted from sarcoma 37. I. Origin and introductory investigations,” Journal of the National Cancer Institute, vol. Academy of Sciences of the United States of America, vol. 107, no. 45, pp. 19248–19253, 2010. 24, pp. 933–951, 1960. [5] P. F. Lewis and M. Emerman, “Passage through mitosis [21] A. Rein, S. A. Datta, C. P. Jones, and K. Musier-Forsyth, “Diverse interactions of retroviral Gag proteins with RNAs,” is required for oncoretroviruses but not for the human immunodeficiency virus,” Journal of Virology, vol. 68, no. 1, Trends in Biochemical Sciences, vol. 36, no. 7, pp. 373–380, pp. 510–516, 1994. 2011. [22] H. Yang, J. Wang, X. Jia et al., “Structural insight into the [6] T.Roe,T.C.Reynolds, G. Yu,and P. O. Brown, “Integration of murine leukemia virus DNA depends on mitosis,” EMBO mechanisms of enveloped virus tethering by tetherin,” Pro- ceedings of the National Academy of Sciences of the United Journal, vol. 12, no. 5, pp. 2099–2108, 1993. [7] L. Jarrosson-Wuilleme, C. Goujon, J. Bernaud, D. Rigal, States of America, vol. 107, no. 43, pp. 18428–18432, 2010. [23] O. Pornillos, J. E. Garrus, and W. I. Sundquist, “Mechanisms J. L. Darlix, and A. Cimarelli, “Transduction of nondi- viding human macrophages with gammaretrovirus-derived of enveloped RNA virus budding,” Trends in Cell Biology, vol. 12, no. 12, pp. 569–579, 2002. vectors,” Journal of Virology, vol. 80, no. 3, pp. 1152–1159, 2006. [24] K. M. Felsenstein and S. P. Goff, “Expression of the gag-pol fusion protein of Moloney murine leukemia virus without [8] X. H. Liu, W. Xu, J. Russ, L. E. Eiden, and M. V. Eiden, “The host range of gammaretroviruses and gammaretroviral vec- gag protein does not induce virion formation or proteolytic processing,” Journal of Virology, vol. 62, no. 6, pp. 2179–2182, tors includes post-mitotic neural cells,” PLoS ONE, vol. 6, no. 3, Article ID e18072, 2011. 1988. Advances in Virology 11 [25] Y. X. Feng, H. Yuan, A. Rein, and J. G. Levin, “Bipartite signal of Moloney sarcoma virus,” Nature, vol. 295, no. 5850, pp. for read-through suppression in murine leukemia virus 568–572, 1982. mRNA: an eight-nucleotide purine-rich sequence immedi- [41] A. B. Rabson and B. J. Graves, “Synthesis and processing ately downstream of the gag termination codon followed by of viral RNA,” in Retroviruses,J.M.Coffin, S. H. Hughes, an RNA pseudoknot,” Journal of Virology, vol. 66, no. 8, pp. and H. E. Varmus, Eds., pp. 205–262, Cold Spring Harbor 5127–5132, 1992. Laboratory Press, New York, NY, USA, 1997. [42] L. E. Henderson, H. C. Krutzsch, and S. Oroszlan, “Myristyl [26] Y. Yoshinaka, I. Katoh, T. D. Copeland, and S. Oroszlan, “Murine leukemia virus protease is encoded by the gag-pol amino-terminal acylation of murine retrovirus proteins: an unusual post-translational protein modification,” Proceed- gene and is synthesized through suppression of an amber termination codon,” Proceedings of the National Academy of ings of the National Academy of Sciences of the United States of America, vol. 80, no. 2, pp. 339–343, 1983. Sciences of the United States of America,vol. 82, no.6,pp. 1618–1622, 1985. [43] A. Rein, M. R. McClure, N. R. Rice, R. B. Luftig, and A. M. Schultz, “Myristylation site in Pr65(gag) is essential for [27] Y. X. Feng, T. D. Copeland, S. Oroszlan, A. Rein, and J. virus particle formation by Moloney murine leukemia virus,” G. Levin, “Identification of amino acids inserted during Proceedings of the National Academy of Sciences of the United suppression of UAA and UGA termination codons at the gag- States of America, vol. 83, no. 19, pp. 7246–7250, 1986. pol junction of Moloney murine leukemia virus,” Proceedings [44] B. Yuan, S. Campbell, E. Bacharach, A. Rein, and S. P. Goff, of the National Academy of Sciences of the United States of “Infectivity of Moloney murine leukemia virus defective in America, vol. 87, no. 22, pp. 8860–8863, 1990. late assembly events is restored by late assembly domains of [28] Y. X. Feng, J. G. Levin, D. L. Hatfield, T. S. Schaefer, R. other retroviruses,” Journal of Virology, vol. 74, no. 16, pp. J. Gorelick, and A. Rein, “Suppression of UAA and UGA 7250–7260, 2000. termination codons in mutant murine leukemia viruses,” [45] C. Segura-Morales, C. Pescia, C. Chatellard-Causse, R. Journal of Virology, vol. 63, no. 6, pp. 2870–2873, 1989. Sadoul, E. Bertrand, and E. Basyuk, “Tsg101 and Alix [29] T. Jacks, M. D. Power, F. R. Masiarz, P. A. Luciw, P. J. Barr, and interact with murine leukemia virus Gag and cooperate H. E. Varmus, “Characterization of ribosomal frameshifting with Nedd4 ubiquitin ligases during budding,” Journal of in HIV-1 gag-pol expression,” Nature, vol. 331, no. 6153, pp. Biological Chemistry, vol. 280, no. 29, pp. 27004–27012, 2005. 280–283, 1988. [46] A. Prizan-Ravid, E. Elis, N. Laham-Karam, S. Selig, M. [30] T. Jacks and H. E. Varmus, “Expression of the Rous sarcoma Ehrlich, and E. Bacharach, “The Gag cleavage product, p12, virus pol gene by ribosomal frameshifting,” Science, vol. 230, is a functional constituent of the murine leukemia virus pre- no. 4731, pp. 1237–1242, 1985. integration complex,” PLoS Pathogens, vol. 6, no. 11, Article [31] D. L. Hatfield, J. G. Levin, A. Rein, and S. Oroszlan, ID e1001183, 2010. “Translational suppression in retroviral GENE expression,” [47] B. Yuan,A.Fassati,A.Yueh, andS.P.Goff,“Characterization Advances in Virus Research, vol. 41, pp. 193–239, 1992. of Moloney murine leukemia virus p12 mutants blocked [32] P. O. Brown, “Integration,” in Retroviruses,J.M.Coffin, S. H. during early events of infection,” Journal of Virology, vol. 76, Hughes, and H. E. Varmus, Eds., pp. 161–203, Cold Spring no. 21, pp. 10801–10810, 2002. Harbor Laboratory Press, Plainview, NY, USA, 1997. [48] A. Yueh andS.P.Goff, “Phosphorylated serine residues and [33] J. Colicelli and S. P. Goff, “Mutants and pseudorevertants an arginine-rich domain of the Moloney murine leukemia of Moloney murine leukemia virus with alterations at the virus p12 protein are required for early events of viral integration site,” Cell, vol. 42, no. 2, pp. 573–580, 1985. infection,” Journal of Virology, vol. 77, no. 3, pp. 1820–1829, [34] J. Colicelli and S. P. Goff, “Sequence and spacing require- ments of a retrovirus integration site,” JournalofMolecular [49] M. R. Auerbach,C.Shu,A.Kaplan, andI.R.Singh, Biology, vol. 199, no. 1, pp. 47–59, 1988. “Functional characterization of a portion of the Moloney [35] S. J. RulliJr.,C.S.Hibbert,J.Mirro,T.Pederson, S. Biswal, murine leukemia virus gag gene by genetic footprinting,” and A. Rein, “Selective and nonselective packaging of cellular Proceedings of the National Academy of Sciences of the United RNAs in retrovirus particles,” Journal of Virology, vol. 81, no. States of America, vol. 100, no. 20, pp. 11678–11683, 2003. 12, pp. 6623–6631, 2007. [50] B. Yuan,X.Li, andS.P.Goff, “Mutations altering the [36] V. D’Souza and M. F. Summers, “Structural basis for pack- Moloney murine leukemia virus p12 Gag protein affect aging the dimeric genome of Moloney murine leukaemia virion production and early events of the virus life cycle,” virus,” Nature, vol. 431, no. 7008, pp. 586–590, 2004. EMBO Journal, vol. 18, no. 17, pp. 4700–4710, 1999. [37] D. A. M. Konings, M. A. Nash, J. V. Maizel, and R. B. [51] M. Oshima, D. Muriaux, J. Mirro et al., “Effects of blocking Arlinghaus, “Novel GACG-hairpin pair motif in the 5’ individual maturation cleavages in murine leukemia virus untranslated region of type C retroviruses related to murine Gag,” Journal of Virology, vol. 78, no. 3, pp. 1411–1420, 2004. leukemia virus,” Journal of Virology, vol. 66, no. 2, pp. 632– [52] S. K. Kyere, P. R. B. Joseph, and M. F. Summers, “The p12 640, 1992. domain is unstructured in a murine leukemia virus p12- [38] C. Gherghe, C. W. Leonard, R. J. Gorelick, and K. M. Weeks, CA(N) Gag construct,” PLoS ONE, vol. 3, no. 4, Article ID “Secondary structure of the mature ex virio Moloney murine e1902, 2008. leukemia virus genomic RNA dimerization domain,” Journal [53] A. C. Prats, G. De Billy, P. Wang, and J. L. Darlix, “CUG of Virology, vol. 84, no. 2, pp. 898–906, 2010. initiation codon used for the synthesis of a cell surface [39] C. H. Kim and I Tinoco Jr., “A retroviral RNA kissing antigen coded by the murine leukemia virus,” Journal of complex containing only two G·C base pairs,” Proceedings Molecular Biology, vol. 205, no. 2, pp. 363–372, 1989. of the National Academy of Sciences of the United States of [54] R. Fujisawa,F.J.McAtee, C. Favara,S.F.Hayes,and J. L. America, vol. 97, no. 17, pp. 9396–9401, 2000. Portis, “N-terminal cleavage fragment of glycosylated Gag is [40] B. Levinson, G. Khoury, G. Vande Woude, and P. Gruss, incorporated into murine oncornavirus particles,” Journal of “Activation of SV40 genome by 72-base pair tandem repeats Virology, vol. 75, no. 22, pp. 11239–11243, 2001. 12 Advances in Virology [55] A. Corbin, A. C. Prats, J. L. Darlix, and M. Sitbon, “A non- [70] A. Schultz and A. Rein, “Maturation of murine leukemia structural gag-encoded glycoprotein precursor is necessary virus env proteins in the absence of other viral proteins,” for efficient spreading and pathogenesis of murine leukemia Virology, vol. 145, no. 2, pp. 335–339, 1985. viruses,” Journal of Virology, vol. 68, no. 6, pp. 3857–3867, [71] B. A. Brody, S. S. Rhee, and E. Hunter, “Postassembly cleavage of a retroviral glycoprotein cytoplasmic domain removes a [56] P. Schwartzberg, J. Colicelli, and S. P. Goff, “Deletion mutants necessary incorporation signal and activates fusion activity,” of Moloney murine leukemia virus which lack glycosylated Journal of Virology, vol. 68, no. 7, pp. 4620–4627, 1994. gag protein are replication competent,” Journal of Virology, [72] M. A. Sommerfelt, S. R. Petteway Jr., G. B. Dreyer, and vol. 46, no. 2, pp. 538–546, 1983. E. Hunter, “Effect of retroviral proteinase inhibitors on [57] A. Low, S. Datta, Y. Kuznetsov et al., “Mutation in the Mason-Pfizer monkey virus maturation and transmembrane glycosylated gag protein of murine leukemia virus results in glycoprotein cleavage,” Journal of Virology,vol. 66, no.7,pp. reduced in vivo infectivity and a novel defect in viral budding 4220–4227, 1992. or release,” Journal of Virology, vol. 81, no. 8, pp. 3685–3692, [73] N. R. Rice,L.E.Henderson,R.C.Sowder, T. D. Copeland, S. Oroszlan, and J. F. Edwards, “Synthesis and processing [58] T. Nitta, R. Tam, J. W. Kim, and H. Fan, “The cellular protein of the transmembrane envelope protein of equine infectious La functions in enhancement of virus release through lipid anemia virus,” Journal of Virology, vol. 64, no. 8, pp. 3770– rafts facilitated by murine leukemia virus glycosylated Gag,” 3778, 1990. mBio, vol. 2, no. 1, 2011. [74] E. Hunter, “Viral entry and receptors,” in Retroviruses,J.M. [59] M. Pizzato, “MLV glycosylated-Gag is an infectivity factor Coffin, S. H. Hughes, and H. E. Varmus, Eds., pp. 71–119, that rescues Nef-deficient HIV-1,” Proceedings of the National Cold Spring Harbor Laboratory Press, Plainview, NY, USA, Academy of Sciences of the United States of America, vol. 107, no. 20, pp. 9364–9369, 2010. [75] D. Fass, R. A. Davey, C. A. Hamson, P. S. Kim, J. M. Cunning- [60] R. J. Gorelick, W. Fu, T. D. Gagliardi et al., “Characterization ham, and J. M. Berger, “Structure of a murine leukemia virus of the block in replication of nucleocapsid protein zinc finger receptor-binding glycoprotein at 2.0 angstrom resolution,” mutants from Moloney murine leukemia virus,” Journal of Science, vol. 277, no. 5332, pp. 1662–1666, 1997. Virology, vol. 73, no. 10, pp. 8185–8195, 1999. [76] D. Fass, S. C. Harrison, and P. S. Kim, “Retrovirus envelope [61] R. J. Gorelick,L.E.Henderson,J.P.Hanser, andA.Rein, domain at 1.7 A resolution,” Nature Structural Biology, vol. 3, “Point mutants of Moloney murine leukemia virus that fail no. 5, pp. 465–469, 1996. to package viral RNA: evidence for specific RNA recognition [77] B. Kobe, R. L. Center, B. E. Kemp, and P. Poumbourios, by a “zinc finger-like” protein sequence,” Proceedings of the “Crystal structure of human T cell leukemia virus type 1 National Academy of Sciences of the United States of America, gp21 ectodomain crystallized as a maltose-binding protein vol. 85, no. 22, pp. 8420–8424, 1988. chimera reveals structural evolution of retroviral transmem- [62] L. E. Henderson, R. Sowder, T. D. Copeland, G. Smythers, brane proteins,” Proceedings of the National Academy of and S. Oroszlan, “Quantitative separation of murine Sciences of the United States of America,vol. 96, no.8,pp. leukemia virus proteins by reversed-phase high-pressure 4319–4324, 1999. liquid chromatography reveals newly described gag and env [78] A. Pinter, R. Kopelman, Z. Li, S. C. Kayman, and D. A. cleavage products,” Journal of Virology, vol. 52, no. 2, pp. 492– Sanders, “Localization of the labile disulfide bond between 500, 1984. SU and TM of the murine leukemia virus envelope protein [63] L. Menendez-Arias, D. Gotte, and S. Oroszlan, “Moloney complex to a highly conserved CWLC motif in SU that murine leukemia virus protease: bacterial expression and resembles the active-site sequence of thiol-disulfide exchange characterization of the purified enzyme,” Virology, vol. 196, enzymes,” Journal of Virology, vol. 71, no. 10, pp. 8073–8077, no. 2, pp. 557–563, 1993. [64] R. Swanstrom and J. W. Wills, “Synthesis, assembly, and [79] E. O. Freed and R. Risser, “The role of envelope glycoprotein processing of viral proteins,” in Retroviruses,J.M.Coffin, S. processing in murine leukemia virus infection,” Journal of H. Hughes, and H. E. Varmus, Eds., pp. 263–334, Cold Spring Virology, vol. 61, no. 9, pp. 2852–2856, 1987. Harbor Laboratory Press, Plainview, NY, USA, 1997. [80] J. A. Ragheb and W. F. Anderson, “pH-independent murine [65] D. Das and M. M. Georgiadis, “The crystal structure of leukemia virus ecotropic envelope-mediated cell fusion: the monomeric reverse transcriptase from Moloney murine implications for the role of the R peptide and p12E TM in leukemia virus,” Structure, vol. 12, no. 5, pp. 819–829, 2004. viral entry,” Journal of Virology, vol. 68, no. 5, pp. 3220–3231, [66] K. Moelling, “Characterization of reverse transcriptase and RNase H from friend-murine leukemia virus,” Virology, vol. [81] A. Rein, J. Mirro, J. G. Haynes, S. M. Ernst, and K. 62, no. 1, pp. 46–59, 1974. Nagashima, “Function of the cytoplasmic domain of a retro- [67] A. Telesnitsky and S. P. Goff, “Reverse transcriptase and the viral transmembrane protein: p15E-p2E cleavage activates generation of retroviral DNA,” in Retroviruses,J.M.Coffin, S. the membrane fusion capability of the murine leukemia virus H. Hughes, and H. E. Varmus, Eds., pp. 121–160, Cold Spring Env protein,” Journal of Virology, vol. 68, no. 3, pp. 1773– Harbor Laboratory Press, Plainview, NY, USA, 1997. 1781, 1994. [68] S. Hare, F. Di Nunzio, A. Labeja, J. Wang, A. Engelman, and P. [82] M. Li, Z. N. Li, Q. Yao, C. Yang, D. A. Steinhauer, and R. Cherepanov, “Structural basis for functional tetramerization W. Compans, “Murine leukemia virus R peptide inhibits of lentiviral integrase,” PLoS Pathogens, vol. 5, no. 7, Article influenza virus hemagglutinin-induced membrane fusion,” ID e1000515, 2009. Journal of Virology, vol. 80, no. 12, pp. 6106–6114, 2006. [69] S. Hare,S.S.Gupta,E.Valkov, A. Engelman,and P. Cherepanov, “Retroviral intasome assembly and inhibition [83] M. Wallin, M. Ekstrom, ¨ and H. Garoff, “Isomerization of of DNA strand transfer,” Nature, vol. 464, no. 7286, pp. 232– the intersubunit disulphide-bond in Env controls retrovirus 236, 2010. fusion,” EMBO Journal, vol. 23, no. 1, pp. 54–65, 2004. Advances in Virology 13 [84] K. Li, S. Zhang, M. Kronqvist, M. Ekstrom, ¨ M. Wallin, and [99] R. H. Bassin, N. Tuttle, and P. J. Fischinger, “Rapid cell culture H. Garoff, “The conserved His8 of the Moloney murine assay technique for murine leukaemia viruses,” Nature, vol. leukemia virus Env SU subunit directs the activity of the SU- 229, no. 5286, pp. 564–566, 1971. TM disulphide bond isomerase,” Virology, vol. 361, no. 1, pp. [100] S. J. Rulli Jr., J. Mirro, S. A. Hill et al., “Interactions of murine 149–160, 2007. APOBEC3 and human APOBEC3G with murine leukemia [85] A. L. Barnett and J. M. Cunningham, “Receptor binding viruses,” Journal of Virology, vol. 82, no. 13, pp. 6566–6575, transforms the surface subunit of the mammalian C-type retrovirus envelope protein from an inhibitor to an activator [101] D. Mazurov, A. Ilinskaya, G. Heidecker, P. Lloyd, and D. of fusion,” Journal of Virology, vol. 75, no. 19, pp. 9096–9105, Derse, “Quantitative comparison of HTLV-1 and HIV-1 cell- 2001. to-cell infection with new replication dependent vectors,” PLoS Pathogens, vol. 6, no. 2, Article ID e1000788, 2010. [86] A. L. Barnett, R. A. Davey, and J. M. Cunningham, “Modular [102] R. H. Bassin, S. Ruscetti, I. Ali, D. K. Haapala, and A. Rein, organization of the Friend murine leukemia virus envelope “Normal DBA/2 mouse cells synthesize a glycoprotein which protein underlies the mechanism of infection,” Proceedings interferes with MCF virus infection,” Virology, vol. 123, no. of the National Academy of Sciences of the United States of 1, pp. 139–151, 1982. America, vol. 98, no. 7, pp. 4113–4118, 2001. [103] H. Ikeda and T. Odaka, “A cell membrane “gp70” associated [87] D. Lavillette, A. Ruggieri, S. J. Russell, and F. L. Cosset, with Fv-4 gene: immunological characterization and tissue “Activation of a cell entry pathway common to type C and strain distribution,” Virology, vol. 133, no. 1, pp. 65–76, mammalian retroviruses by soluble envelope fragments,” Journal of Virology, vol. 74, no. 1, pp. 295–304, 2000. [104] T. Pincus, W. P. Rowe, and F. Lilly, “A major genetic [88] S. K. Chattopadhyay, M. R. Lander, E. Rands, S. Gupta and D. locus affecting resistance to infection with murine leukemia R. Lowy, “Origin of mink cytopathic focus-forming (MCF) viruses. II. Apparent identity to a major locus described viruses: comparison with ecotropic and xenotropic murine for resistance to friend murine leukemia virus,” Journal of leukemia virus genomes,” Virology, vol. 113, no. 2, pp. 465– Experimental Medicine, vol. 133, no. 6, pp. 1234–1241, 1971. 483, 1981. [105] H. C. T. Groom, M. W. Yap, R. P. Gala˜o,S.J.D.Neil, and [89] J. W. Hartley, N. K. Wolford, L. J. Old, and W. P. Rowe, “A new K. N. Bishop, “Susceptibility of xenotropic murine leukemia class of murine leukemia virus associated with development virus-related virus (XMRV) to retroviral restriction factors,” of spontaneous lymphomas,” Proceedings of the National Proceedings of the National Academy of Sciences of the United Academy of Sciences of the United States of America, vol. 74, States of America, vol. 107, no. 11, pp. 5166–5171, 2010. no. 2, pp. 789–792, 1977. [106] S. Best, P. L. Tissier, G. Towers, and J. P. Stoye, “Positional [90] R. A. Bosselman, F. van Straaten, C. van Beveren, I. M. cloning of the mouse retrovirus restriction gene Fv1,” Nature, Verma, and M. Vogt, “Analysis of the env gene of a molecu- vol. 382, no. 6594, pp. 826–829, 1996. larly cloned and biologically active Moloney mink cell focus- [107] M. W. Yap and J. P. Stoye, “Intracellular localisation of Fv1,” forming proviral DNA,” Journal of Virology,vol. 44, no.1,pp. Virology, vol. 307, no. 1, pp. 76–89, 2003. 19–31, 1982. [108] G. Duran-Troise, R. H. Bassin, A. Rein, and B. I. Gerwin, [91] A. Rein, “Interference grouping of murine leukemia viruses: a “Loss of Fv 1 restriction in Balb/3T3 cells following infection distinct receptor for the MCF-recombinant viruses in mouse with a single N tropic murine leukemia virus particle,” Cell, cells,” Virology, vol. 120, no. 1, pp. 251–257, 1982. vol. 10, no. 3, pp. 479–488, 1977. [92] D. L. Wensel, W. Li, and J. M. Cunningham, “A virus-virus [109] R. H. Bassin, G. Duran-Troise, B. I. Gerwin, and A. Rein, interaction circumvents the virus receptor requirement for “Abrogation of Fv-1(b) restriction with murine leukemia infection by pathogenic retroviruses,” Journal of Virology, vol. viruses inactivated by heat or by gamma irradiation,” Journal 77, no. 6, pp. 3460–3469, 2003. of Virology, vol. 26, no. 2, pp. 306–315, 1978. [93] G. J. Cianciolo, T. D. Copeland, S. Oroszlan, and R. Snyder- [110] K. N. Bishop, G. B. Mortuza, S. Howell, M. W. Yap, J. P. Stoye, man, “Inhibition of lymphocyte proliferation by a synthetic and I. A. Taylor, “Characterization of an amino-terminal peptide homologous to retroviral envelope proteins,” Science, dimerization domain from retroviral restriction factor Fv1,” vol. 230, no. 4724, pp. 453–455, 1985. Journal of Virology, vol. 80, no. 16, pp. 8225–8235, 2006. [94] G. Schlecht-Louf, M. Renard, M. Mangeney et al., “Retroviral [111] M. W. Yap, G. B. Mortuza, I. A. Taylor, and J. P. Stoye, “The infection in vivo requires an immune escape virulence factor design of artificial retroviral restriction factors,” Virology, vol. encrypted in the envelope protein of oncoretroviruses,” 365, no. 2, pp. 302–314, 2007. Proceedings of the National Academy of Sciences of the United [112] L. Hilditch, R. Matadeen, D. C. Goldstone, P. B. Rosenthal, States of America, vol. 107, no. 8, pp. 3782–3787, 2010. I. A. Taylor, and J. P. Stoye, “Ordered assembly of murine [95] A. Rein and A. Schultz, “Different recombinant murine leukemia virus capsid protein on lipid nanotubes directs leukemia viruses use different cell surface receptors,” Virol- specific binding by the restriction factor, Fv1,” Proceedings ogy, vol. 136, no. 1, pp. 144–152, 1984. of the National Academy of Sciences of the United States of [96] C. S. Tailor, D. Lavillette, M. Marin, and D. Kabat, “Cell America, vol. 108, no. 14, pp. 5771–5776, 2011. surface receptors for gammaretroviruses,” Current Topics in [113] G. Towers, M. Bock, S. Martin, Y. Takeuchi, J. P. Stoye, and O. Microbiology and Immunology, vol. 281, pp. 29–106, 2003. Danos, “A conserved mechanism of retrovirus restriction in [97] H. M. Temin and H. Rubin, “Characteristics of an assay for mammals,” Proceedings of the National Academy of Sciences of Rous sarcoma virus and Rous sarcoma cells in tissue culture,” the United States of America, vol. 97, no. 22, pp. 12295–12299, Virology, vol. 6, no. 3, pp. 669–688, 1958. [98] W. P. Rowe,W.E.Pugh, andJ.W.Hartley,“Plaque assay [114] R. S. Harris and M. T. Liddament, “Retroviral restriction by techniques for murine leukemia viruses,” Virology, vol. 42, APOBEC proteins,” Nature Reviews Immunology, vol. 4, no. no. 4, pp. 1136–1139, 1970. 11, pp. 868–877, 2004. 14 Advances in Virology [115] R. S. LaRue, V. Andresd ´ ottir ´ , Y. Blanchard et al., “Guidelines for naming nonprimate APOBEC3 genes and proteins,” Journal of Virology, vol. 83, no. 2, pp. 494–497, 2009. [116] A. Low, C. M. Okeoma, N. Lovsin et al., “Enhanced replica- tion and pathogenesis of Moloney murine leukemia virus in mice defective in the murine APOBEC3 gene,” Virology, vol. 385, no. 2, pp. 455–463, 2009. [117] C. M. Okeoma, N. Lovsin, B. M. Peterlin, and S. R. Ross, “APOBEC3 inhibits mouse mammary tumour virus replica- tion in vivo,” Nature, vol. 445, no. 7130, pp. 927–930, 2007. [118] R. K. Holmes, M. H. Malim, and K. N. Bishop, “APOBEC- mediated viral restriction: not simply editing?” Trends in Biochemical Sciences, vol. 32, no. 3, pp. 118–128, 2007. [119] J. L. Mbisa, R. Barr, J. A. Thomas et al., “Human immun- odeficiency virus type 1 cDNAs produced in the presence of APOBEC3G exhibit defects in plus-strand DNA transfer and integration,” Journal of Virology, vol. 81, no. 13, pp. 7099– 7110, 2007. [120] M. A. Langlois, K. Kemmerich, C. Rada, and M. S. Neuberger, “The AKV murine leukemia virus is restricted and hypermu- tated by mouse APOBEC3,” Journal of Virology, vol. 83, no. 22, pp. 11550–11559, 2009. [121] T. Paprotka, N. J. Venkatachari, C. Chaipan et al., “Inhibi- tion of xenotropic murine leukemia virus-related virus by APOBEC3 proteins and antiviral drugs,” Journal of Virology, vol. 84, no. 11, pp. 5719–5729, 2010. [122] K. Stieler and N. Fischer, “Apobec 3G efficiently reduces infectivity of the human exogenous gammaretrovirus XMRV,” PLoS ONE, vol. 5, no. 7, Article ID e11738, 2010. [123] E. P. Browne and D. R. Littman, “Species-specific restriction of Apobec3-mediated hypermutation,” Journal of Virology, vol. 82, no. 3, pp. 1305–1313, 2008. [124] A. Abudu, A. Takaori-Kondo, T. Izumi et al., “Murine retrovirus escapes from murine APOBEC3 via two distinct novel mechanisms,” Current Biology, vol. 16, no. 15, pp. 1565–1570, 2006. [125] S. J. D. Neil, T. Zang, and P. D. Bieniasz, “Tetherin inhibits retrovirus release and is antagonized by HIV-1 Vpu,” Nature, vol. 451, no. 7177, pp. 425–430, 2008. [126] C. Goffinet, S. Schmidt, C. Kern, L. Oberbremer, and O. T. Keppler, “Endogenous CD317/tetherin limits replication of HIV-1 and murine leukemia virus in rodent cells and is resistant to antagonists from primate viruses,” Journal of Virology, vol. 84, no. 21, pp. 11374–11384, 2010. [127] D. T. Evans, R. Serra-Moreno, R. K. Singh, and J. C. Guatelli, “BST-2/tetherin: a new component of the innate immune response to enveloped viruses,” Trends in Microbiology, vol. 18, no. 9, pp. 388–396, 2010. [128] S. P. Goff, “Host factors exploited by retroviruses,” Nature Reviews Microbiology, vol. 5, no. 4, pp. 253–263, 2007. 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Murine Leukemia Viruses: Objects and Organisms

Advances in Virology , Volume 2011 – Nov 15, 2011

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Copyright © 2011 Alan Rein. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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10.1155/2011/403419
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Hindawi Publishing Corporation Advances in Virology Volume 2011, Article ID 403419, 14 pages doi:10.1155/2011/403419 Review Article Alan Rein HIV Drug Resistance Program, National Cancer Institute-Frederick, Frederick, MD 21702, USA Correspondence should be addressed to Alan Rein, reina@mail.nih.gov Received 3 June 2011; Accepted 25 July 2011 Academic Editor: Arifa S. Khan Copyright © 2011 Alan Rein. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Murine leukemia viruses (MLVs) are among the simplest retroviruses. Prototypical gammaretroviruses encode only the three polyproteins that will be used in the assembly of progeny virus particles. These are the Gag polyprotein, which is the structural protein of a retrovirus particle, the Pol protein, comprising the three retroviral enzymes—protease, which catalyzes the maturation of the particle, reverse transcriptase, which copies the viral RNA into DNA upon infection of a new host cell, and integrase, which inserts the DNA into the chromosomal DNA of the host cell, and the Env polyprotein, which induces the fusion of the viral membrane with that of the new host cell, initiating infection. In general, a productive MLV infection has no obvious effect upon host cells. Although gammaretroviral structure and replication follow the same broad outlines as those of other retroviruses, we point out a number of significant differences between different retroviral genera. 1. Introduction The best-studied retrovirus is, of course, human immun- odeficiency virus (HIV-1), which is a lentivirus. One striking A virus can be viewed as a rather regular, relatively simple contrast between MLVs and HIV-1 is the relative simplicity physical object. Alternatively, it can be seen as a living of MLVs. As discussed below, MLVs only encode the proteins organism, evolving in response to selective pressures. Both that will be assembled into the progeny virus particles, views are correct! This paper will outline very briefly some whereas HIV-1 encodes six additional so-called “accessory” of the characteristics of murine leukemia viruses (MLVs), proteins. Indeed, because of this distinction, HIV-1 has keeping both views in mind. We will try to point out the dis- frequently been called a “complex” retrovirus, in contrast to tinctive features of these retroviruses, which are often taken the “simple” retroviruses such as MLV, the proper objects of as prototypes of the gammaretrovirus genus. (Retroviruses study of simple retrovirologists. include Spumaretroviruses (also known as “foamy viruses”) The two viruses also differ in that HIV-1 can efficiently and Orthoretroviruses; the latter are divided into six genera, infect nondividing cells, while MLV generally does not [5, 6] that is, alpha-, beta-, gamma-, delta-, epsilon-, and lenti- (but see also [7, 8]). The ability of HIV-1 to infect nondivid- retroviruses [1].) ing cells is a critical element in its pathogenicity. MLVs have been studied for many years, beginning in Yet another cardinal difference between MLVs and HIV-1 the 1950s, when it was realized that leukemia could be is that HIV-1-infected cells usually die rapidly (within a few transmitted to newborn mice by a filterable agent [2–4]. They days at most) after infection. In contrast, at the cellular level have provided many insights into the general phenomenon MLV infection seems almost completely benign: in general, of leukemogenesis. The MLV genome has also been used as there are no detectable effects of productive MLV infection the starting material in the development of vectors for gene upon the growth, physiology, or morphology of the cells. therapy. Finally, MLVs have often been viewed as “model” HIV-1 viremia is maintained in infected people by continual retroviruses. In fact, while they have been very useful in answering questions about retroviruses and their hosts, there infection of new cells, replacing the cells killed by infection. We do not know how much infection is occurring in an are many ways in which gammaretroviruses differ from other retroviruses: it should never be assumed that a given property MLV-infected, viremic mouse, but since the virus does not of one genus will hold for another. generally kill its host cells, the rate of new infections may be 2 Advances in Virology far lower than with HIV-1. It should be noted that the drugs used in highly active antiretroviral therapy of HIV-infected people act by blocking new infections; thus, it is possible that analogous therapies would have only minimal effects on MLV viremia. 2. MLV: The Physical Object 2.1. MLV Virions. The overall structures of virus particles are probably very similar for all Orthoretroviridae. The virus is pleomorphic, but roughly spherical, with a diameter LB MA P12 CA NC of ∼100–120 nm [9]. It is released from the cell as an Figure 1: Structure of an immature MLV particle. A segment “immature particle”, in which several thousand rod-shaped from a rotationally averaged cryoelectron microscopic image of a Gag polyprotein molecules are arranged, in an incomplete single immature MLV particle is shown on the right. As indicated or imperfect hexameric lattice, as radii of the sphere (see on the left, the particle is bounded by a lipid bilayer (“LB”), Figure 1). The sphere is bounded by a lipid bilayer derived and the MA domain of Gag (pink) is associated with the inner from the plasma membrane of the virus-producing cell. The leaflet of the bilayer. Interior to the MA domain is a zone of N-terminal matrix (MA) domains of the Gag molecules are low density, presumably corresponding to the p12 domain. The most conspicuous feature of the image is the “railroad tracks”, in contact with the lipid bilayer and their C-terminal nucleo- representing the two domains within the CA domain (green), capsid (NC) domains project into the interior of the particle, followed by the NC domain (blue) with bound RNA. The Pol and presumably in contact with RNA. They are approximately Env proteins are not visible in this image. As the particle is ∼100 nm 20 nm long and only 2-3 nm in diameter. The particle also in diameter and the Gag molecules are ∼20 nm in length, there is a contains ∼1–300 Gag-Pol polyprotein molecules, in which region ∼60 nm in diameter largely occupied by solvent in the center Gag is extended at its C-terminus by protease (PR), reverse of the particle. (Reproduced from [9]. Copyright 1998, National transcriptase (RT), and integrase (IN). Finally, trimers of the Academy of Sciences, USA.) envelope (Env) polyprotein span the membrane, with the gp70 surface glycoprotein (SU) on the exterior of the particle, complexed with the p15E transmembrane (TM) protein. Roughly 2.5 × 10 nucleotides’ worth of RNA, representing only a few per cent of the mass of the particle, are also present in the virion. Some cellular proteins are also packaged: this has been documented in great detail in HIV-1 [10] but is also true in MLV [11]. After the particle is released from the cell, it undergoes maturation. PR cleaves Gag into four cleavage products, that is, MA, p12, capsid (CA), and NC. The Pol moiety of Gag-Pol 5 is also cleaved to release free PR, RT, and IN proteins, and the C-terminal 16 residues of TM (the “R peptide”) are removed, producing the mature TM protein p15E (in some papers, this shorter species is called p12E; the longer precursor has been 7 8 called either p15E or Pr15E). The cleavages in Gag cause a major change in the overall architecture of the virion, with Figure 2: The orthoretroviral replication cycle. Infection is initiated CA molecules reassembling in the interior of the particle when the mature, infectious virion binds to a receptor on the into a polygonal structure, the “mature core” of the particle. surface of the new host cell. The Env protein of the virus induces This new structure encloses a complex of the viral RNA with fusion between the viral membrane and the cell membrane (Step 1). NC protein; RT and IN are also presumed to be within this Within the cytoplasm, the mature core dissociates (Step 2) and structure. the dimeric viral RNA (shown in orange) is copied (Step 3) into double-stranded DNA (shown in green). The DNA copy enters the nucleus (probably when the nuclear membrane breaks down during 2.2. The MLV Replication Cycle. As with all orthoretro- mitosis) and is inserted into the chromosomal DNA of the cell viruses, infection is initiated by the binding of the SU (Step 4). The DNA is transcribed and the RNA product is exported glycoprotein on the exterior of the mature, infectious virion from the nucleus (Step 5); within the cytoplasm, some molecules to a receptor on the surface of the new host cell (see Figure 2). will be translated into viral proteins, and others are destined for This binding event triggers dramatic changes in Env, leading encapsidation into progeny virus particles. The viral components to the release of the SU component and conformational assemble into budding virions (Step 6), which are released from rearrangement of TM. The ultimate result is the fusion of the the cell as immature particles (Step 7). Finally, PR cleaves the viral viral membrane with the plasma membrane. proteins, converting immature particles into mature, infectious The fusion of the two membranes leads to the deposition virions (Step 8). It is possible that DNA synthesis actually occurs of the contents of the virion in the cytoplasm of the cell. Once within the mature core rather than after dissociation of the core as in the cytoplasm, the viral RNA is copied by the RT into a shown here. Advances in Virology 3 single molecule of dsDNA. This DNA is somehow conveyed into the nucleus, where the IN protein catalyzes its insertion ψ gag pol RU5 U3 R into chromosomal DNA. env Once the viral DNA is integrated into host DNA, it is termed the “provirus”. It is transcribed and translated Figure 3: The MLV genome. The viral RNA in Moloney MLV is by normal host-cell machinery. The encoded proteins are 8332 nt in length [18]. “R” sequence, 68 nt in length, is identical at trafficked to the plasma membrane, where they assemble both ends of the RNA. The 5 copy of RisfollowedbyU5sequences into progeny virus particles. The immature particles are and then by the PBS (nt 146–163). The long 5 untranslated region released from the cell with the help of the cellular “ESCRT” in the RNA also includes the ψ packaging signal; contained within this signal in some MLV isolates is the CUG codon at which glyco- machinery [23] and subsequently undergo maturation as the Gag translation is initiated (nt 357). The initiation codon for the PR in the virus cleaves the viral polyproteins. The particle is “normal” Gag protein is at nt 621. The gag and pol coding sequences not capable of initiating a new infection until maturation has are in the same frame; they are separated by a UAG termination taken place. codon, which in turn is followed immediately by a 57-base signal, including an RNA pseudoknot, inducing the inefficient translation 2.3. The MLV Genome. The RNA genome of MLV can be of the UAG as glutamine. The Env protein is translated from a divided into coding and noncoding regions and is shown spliced mRNA. The polypurine tract (PPT, nt 7803–7815) is the schematically in Figure 3. primer for +-strand DNA synthesis and is followed by the U3 and R regions. U3 (nt 7816–8264) is placed at the 5 end of the DNA copy of the genome synthesized during infection; it contains promoter 2.3.1. Coding Regions. The only proteins encoded by the and enhancer sequences governing the initiation of transcription at MLV genome are the three polyproteins that will make up the beginning of R. the progeny virus particles: Gag, the structural protein of the immature virus particle, Pol, comprising the PR, RT, and IN enzymes, and Env, the SU and TM proteins that jointly codon as a sense codon. In contrast, in all other genera, mediate the entry of an infectious virus particle into a new host cell to initiate infection [18]. In some MLV isolates, an the suppression occurs before the ribosomes encounter the alternative form of Gag, with an N-terminal extension, is also termination codon and is completely independent of this −1” frame synthesized; this “glyco-Gag” is discussed below. codon. In these viruses, Pol is encoded in the “ relative to Gag. A signal in these viral RNAs before the As in all orthoretroviruses, the three coding regions are end of the Gag coding region induces a fraction of the arranged, from 5 to 3 , Gag :Pol :Env. The Pol proteins are ribosomes to advance two, rather than three bases at a initially synthesized together with Gag, in a large Gag-Pol specific codon so that translation by this subset of ribosomes fusion polyprotein. Gag and Gag-Pol are both translated is shifted from the Gag frame to the Pol frame [29, 30]. (In from full-length viral RNA, identical in sequence to the some retroviruses, there are two frameshifting events, one genomic RNA present in the virion. It seems likely that extending Gag to produce Gag-PR and the second extending the Gag-Pol polyprotein is incorporated into assembling Gag-PR to yield Gag-PR-RT-IN.) A detailed discussion of virions due to “coassembly” of its Gag moiety with Gag translational suppression in retroviruses may be found in polyprotein molecules. Successful replication of the virus Hatfield et al. [31]. requires maintenance of an optimal ratio (on the order of The Env protein of MLV, like that of other orthoretro- 20 : 1) between the Gag and Gag-Pol proteins; indeed, no viruses, is translated from a singly spliced mRNA. There is an detectable virus particles are formed in cells expressing only overlap of 58 bases between the end of the Pol coding region Gag-Pol [24]. This may be because Gag-Pol is more than 3 and the beginning of the Env coding region. times the mass of Gag, and thus, there may not be space within the particles for very many Pol domains. This optimal ratio is achieved by finely tuned translational suppression of 2.3.2. Noncoding Regions. Like the RNAs of all orthoretro- the termination codon at the end of the Gag coding region. viruses, MLV RNA also contains a set of cis-acting signals Remarkably, different retroviruses use fundamentally that are essential for its function as a viral genome. These different mechanisms of translational suppression. In the include the “primer binding site” (PBS), the polypurine tract gammaretroviruses such as MLV (and epsilonretroviruses, a (PPT), the “packaging signal” or ψ, sequences required for genus about which very little is known), Gag and Pol are in insertion, by IN, of the DNA form of the viral genome into the same reading frame, separated by a single termination cellular DNA, and the promoter and enhancer sequences codon. MLV RNA contains a 57-base cis-acting signal within the LTR. immediately 3 of the termination codon [25]. This signal The PBS is an 18-base stretch that is complementary induces the insertion of glutamine (normally encoded by to the last 18 bases of a cellular tRNA molecule. In MLVs, Pro Gln CAG), rather than termination, in response to the UAG this is usually tRNA , but MLVs using tRNA have also termination codon in about 5% of the translation products; been found. Within the virion, the tRNA is hybridized to the resulting product is extended by translation of the entire the viral RNA; when the virus enters a new host cell, the Pol coding region [26]. Similar results are obtained when the tRNA serves as the primer for reverse transcription. The UAG is replaced by UGA or UAA [27, 28]. Thus, these viruses PBS is located ∼145 bases from the 5 end of the RNA and operate in essence by “mis-translation” of the termination ∼460 bases 5 of the beginning of the Gag coding region. PBS CUG AUG UAG PPT 4 Advances in Virology The first deoxynucleotide to be added to the tRNA during particles, and that they are crucial to selective packaging. reverse transcription is determined by pairing with the base These results explain why dimers, but not monomers, of immediately 5 of the PBS, and this base is the 5 terminus viral RNA are selectively packaged and also establish that the of the first (minus) strand in the final DNA product. In other specific, high-affinity binding of Gag to ψ is responsible for words, this site is the “right” end of the final double-stranded selective packaging. DNAproduct of reversetranscription. During reverse transcription, sequences from near the In general, during reverse transcription the RNA is 3 end of the viral RNA (“U3” sequences) are placed at copied by the polymerase activity of RT and is progressively the 5 end, as well as near the 3 end, of the viral DNA. degraded, shortly after being copied, by the RNase H (Conversely, U5 sequences, from near the 5 end of the activity of RT.However,anexceptional stretchof ∼15 RNA, are placed at the 3 endaswellasnearthe 5 purines near the 3 endofretroviralRNAs(the PPT)is end of the DNA.) Following integration of the viral DNA, specifically resistant to this degradation. Having survived the U3 sequences at the 5 end constitute the promoter reverse transcription, this fragment of the viral RNA is the and enhancers driving the transcription, by Pol II, of the primer for synthesis of the second (plus) strand of DNA. The integrated DNA. U3 sequences include a dense collection base immediately 3 of the PPT encodes the first base of the of transcription factor-binding sites; they were used in the plus strand of the DNA copy, that is, the 5 end of the plus experiments that originally demonstrated the existence of strand or “left” end of the double-stranded DNA. enhancers [40] and play a major role in determining the These sequences at the two ends of the final DNA product tissue tropism and pathogenicity of the virus (reviewed in are, of course, the sequences joined by IN to host-cell [41]). The placement of the U3 sequences, which are internal chromosomal DNA during the integration reaction. The two in viral RNA, upstream of the transcriptional start site in the ends form an inverted repeat (reviewed in [32]). In Moloney DNA is an elegant solution to the problem of how to ensure MLV, the sequence of the “plus” strand at the right edge that the viral sequences will lie 3 of a promoter, as required is 5 GGGGTCTTTCA 3 , while that at the left edge is 5 for Pol II transcription. TGAAAGACCCC 3 . The bases at the 3 ends of the plus strand on the right edge, and the 5 end of the left edge, are 2.4. MLV Proteins joined to cellular DNA, but it is the internal bases in these sequences that are essential for IN recognition [33, 34]. 2.4.1. Gag. In essence, the orthoretrovirus particle is con- All orthoretroviral genomic RNAs are, as noted above, structed by assembly of Gag protein molecules. All mRNAs. They resemble cellular mRNAs in having a 5 orthoretroviral Gag proteins contain at least three domains, cap and 3 poly (A) tail. In fact, under certain conditions, which will give rise to three distinct proteins in the mature retrovirus particles can encapsidate cellular mRNAs [35]. virus. The MA domain at the N-terminus of Gag is respon- Thus, the viral RNAs are evidently in competition with sible for targeting the protein to the plasma membrane of cellular mRNAs for incorporation into the virions. Intact the virus-producing cell. In MLV, as in most retroviruses, the retroviral RNAs are selectively incorporated because they N-terminus of Gag is modified by the 14-carbon saturated contain a “packaging signal”, giving them an advantage in fatty acid, myristic acid [42]; this modification is important this competition. for the plasma-membrane association of Gag [43]. The CA Recent structural studies have shed considerable light on domain is the locus of most, if not all, of the interactions the nature of the packaging signal in Moloney MLV RNA between Gag molecules leading to the assembly of the imma- (see Figure 4)[20, 36]. Briefly, in all orthoretroviruses, the ture virion. After the CA molecules are released from the Gag viral RNA is actually packaged in dimeric form, with two polyprotein by PR, they reassemble into the mature core. The molecules of the viral RNA linked by a limited number of NC domain plays a predominant role in the interactions of intermolecular base pairs. The primary location of these base Gag proteins with RNAs, and free NC protein is an essential pairs is in the “leader”, between the PBS and the beginning cofactor in reverse transcription during infection. In general, of the Gag coding sequence. MLV RNA, like that of all there is considerable structural conservation between the gammaretroviruses, contains a pair of stem loops in this Gag proteins in different orthoretroviral genera, despite the region with the sequence GACG in the loop [37]. Both almost complete lack of conservation of primary sequences. NMR and chemical-probing data show that when MLV RNA MLV Gag differsintwo importantrespectsfromthe dimerizes, the “CG” within each of these GACG’s pairs with canonical MA-CA-NC Gag structure (see Figure 5). First, the CG in the other monomer (note that “CG” is a 2-base it contains an additional domain, called p12, situated palindrome, the shortest possible palindromic sequence) [38, between MA and CA. p12 contains the Pro-Pro-Pro-Tyr “late 39]. Further, two other stem loops in the monomers open domain” of MLV [44]; this motif interacts with an Nedd4- out and pair intermolecularly. Most interestingly, this change like ubiquitin ligase to promote the release of the assembled entails a shift in register so that some of the bases which virion from the host cell [45]. p12 also participates in the are paired in intramolecular structures in the monomers infection process, but these additional functions are not well become unpaired in the dimers. These bases include two understood. It is part of the “preintegration complex”, a copies of the motif UCUG-UPu-UCUG. Several kinds of collection of proteins from the infecting virus particle that experiments [20] show that this motif is essential for high- accompany the newly synthesized viral DNA into the cell affinity binding by recombinant MLV Gag protein, that nucleus [46], and some mutations in p12 interfere with these bases are occupied by NC protein within mature MLV proper integration [47, 48]. Surprisingly, there are regions U A C G G A C G G A C G G A U G C G C G C U A C G C G C G G C G C G C A U U C G G A C C G C G U G C A G A A U G C G C U A C G C G C C G U C G G C U G A U U C G G SL1 U A C A G G 240 A U G C G C C U U A C A G C SL2 A U A U U A 5' C UG U A A G 3' Advances in Virology 5 UG C A A U G U U U A U U A Monomer Dimer C U U G SL0 G C A G U G G C U C U U A C 374 PAL2 U 280 U G C 3' A A G U C U U G U 300 A U 260 240 G C Flexible C G 226 235 domain C G UCG UUG UCU G U A 280 5' G 205 U C G U A A A 220 A U SL1 G C G C C G PAL1 U A Dimerization U A U A 320 A U G C G C G C PAL1 PAL2 C G C G U A C 340 A U A U A U G C 231 315 C G 205 227 316 348 355 374 C G U A A A A U G C A A U C U A U 5' G U C A U 360 SL2 UCG U UGC U U G 205 U C G U U C G G C A U U G G A A 3' G C 310 352 355 374 U G C U U C G C U G G C U G U U G U A U C A UG Figure 4: The Moloney MLV dimerization/packaging signal. The figure shows the secondary structure of the 170-base “minimal dimerization active sequence” (nt 205–374) [19] in both monomeric and dimeric forms. Two palindromic sequences, “PAL1” (green) and “PAL2” (red), are contained within stem loops in the monomer but open out and pair intermolecularly in the dimer. The two monomers are also connected in the dimer by base pairing between the “CG” moieties in the “GACG” loops of a pair of stem loops (“SL1” and “SL2”, orange). The RNA also contains two motifs with the sequence UCUG-UPu-UCUG (blue boxes); these are partially or fully base-paired in the monomer but become unpaired as a result of the RNA rearrangements accompanying the intermolecular base pairing of PAL1 and PAL2. These bases are a crucial element in ψ, as replacement of the four UCUG sequences with UCUA prevents selective packaging of the viral RNA; the exposure of these bases in dimers, but not monomers, presumably explains the selective packaging of dimeric RNA [20]. (Figure reproduced from Trends in Biochemical Sciences, Copyright 2011, with permission from Elsevier [21].) within p12 in which sequence changes seem to have no major MA p12 CA NC effect on viral function [49, 50], and the maturation cleavage between MA and p12, unlike the other cleavages, is not Figure 5: MLV Gag protein. The MLV Gag protein is modified at its absolutely essential for viral infectivity [51]. It is extremely N-terminus by the 14-carbon fatty acid myristic acid. It is cleaved proline-rich (18 of its 84 residues (21%) are prolines), and during virus maturation into MA, p12, CA, and NC; most of the it has been described as “unstructured” on the basis of NC molecules are also cleaved 4 residues before their C-terminus. NMR data [52]. However, recombinant MLV Gag protein is an extended rod in solution, and the prolines in the p12 domain contribute to its rigidity (Datta et al., manuscript in preparation). It seems likely that this domain in Gag can translation initiation, 264 bases 5 of the normal Gag AUG assume any of a number of rigid conformations containing initiation codon [53]. The N-terminal extension includes a short polyproline helices. signal sequence so that this protein (unlike standard Gag) Second, some, but not all, MLV isolates encode an is synthesized in the rough endoplasmic reticulum and alternative form of the Gag polyprotein, called “glyco-Gag” processed in the Golgi apparatus. Relatively little glyco-Gag Gag or gPr80 . This protein differs in sequence from “standard” is incorporated into virions [54]. Because of a sequence Gag in that it is extended N-terminally. Synthesis of glyco- polymorphism at the site of the CUG initiator, XMRV does Gag is initiated at a CUG codon in a favorable context for not encode glyco-Gag. C G A C G G U A A U A U G C U A G C G C G C C G C G C C G C C G G C G C C G C G C G U A C G A U A U G C G G G C G A U U A G C U A A U G C G C G C C G C G G C G C G C C 6 Advances in Virology The functional significance of glyco-Gag is still not clear. MLV Env is depicted schematically in Figure 6.Mature Early studies showed that it is not essential for replication SU of Moloney MLV is 435 residues in length, while TM is of MLV in cell culture, but is needed for efficient replication 180 residues. In turn, SU contains an N-terminal “receptor- and pathogenicity in mice [55, 56]. It was recently reported binding domain” (RBD) of ∼240 residues, a short, proline- that the correct assembly of standard MLV Gag into spherical rich “hinge” region, and a highly conserved C-terminal immature particles in cell cultures is impaired in the absence domain [74]. The RBD consists of an antiparallel β-sandwich of glyco-Gag [57]; new data indicates that the presence of projecting “up” from the surface of the virion, and a glyco-Gag directs virion assembly to lipid rafts and that this highly variable region resting atop this scaffold. Both ends function involves the cellular La protein [58]. Remarkably, of the RBD contribute to this β-sandwich [75]. Sequence glyco-Gag can also complement Nef deletions in HIV-1 alignments and analysis of chimeric SU proteins show that [59]. the variable sequences within the RBD make specific contacts with cell-surface receptors. Among the conserved features of MLV Gag is also unusual among orthoretroviral Gags in SU are a histidine residue near the extreme N-terminus and that its NC domain only contains a single zinc finger rather a CXXC motif in the C-terminal portion of SU. TM protein than two as in most genera. The zinc-coordinating residues begins with a very hydrophobic stretch, the “fusion peptide”. have the spacing C-X -C-X -H-X -C, as in all orthoretrovi- 2 4 4 Astretch betweenTMresidues43and 78 (inMoloney MLV) ral NC proteins. This 14-residue motif plays a critical role has a 4-3 repeating pattern of hydrophobic residues that in the selective packaging of genomic RNA, among other forms a coiled coil. TM also contains a CX CC motif; in the functions [60, 61]. The last 4 residues of NC are removed virus particle, there is a disulfide bond joining SU, via one of from the majority of Gag molecules, as they are from Gag- the cysteines in the CXXC, to TM, via the last cysteine in the Pol molecules, during virus maturation [26, 62]. CX CC [76–78]. The function of the Env complex is to induce fusion 2.4.2. Pol. As noted above, the products of cleavage of the between the membrane surrounding the virus particle and Gag-Pol polyprotein include PR, RT, and IN. PR catalyzes the the membrane of a new host cell. As in all orthoretroviruses, cleavages leading to virus maturation; like all retroviral PRs, the cleavage between SU and TM is absolutely required for it is an aspartic protease which is only active as a dimer [63, Env function [79]. Presumably, this is essential because it 64]. places the fusion peptide at the N-terminus of TM rather RT synthesizes the DNA copy of the viral genome during than in the interior of the Env polyprotein. The removal infection. This function involves three enzymatic activities: of the R peptide from the C-terminus of Prp15E during RNA-templated DNA synthesis, DNA-templated DNA syn- virus maturation is also necessary for the fusogenicity of thesis, and degradation of the RNA strand in an RNA:DNA Env [80, 81]. It seems likely that fusogenic activity would be hybrid, eliminating the RNA template immediately after harmful to the virus-producing cell and that the R peptide is synthesis of the complementary DNA strand. MLV RT is a “safety catch” suppressing this activity until the virus has apparently active as a monomeric protein [65, 66] unlike the left the cell. The mechanism by which the R peptide inhibits RT enzymes of alpharetroviruses and lentiretroviruses, which fusion is not known, but, remarkably, it has the same effect are both heterodimers [67]. when joined to the influenza HA protein [82]. Retroviral IN enzymes possess two catalytic activities: “3 The fusion between the two membranes by the mature end processing”, in which IN removes two nucleotides from Env complex is the end result of an amazing cascade the 3 end of each strand of the DNA to be integrated, and of events. Briefly, binding to the receptor on the plasma “strand transfer”, in which the new 3 ends are inserted into membrane induces a conformational change in the RBD. chromosomal DNA in the new host cell [32]. MLV IN has This change is propagated in SU, resulting in the ionization not been characterized in detail but is presumed to function of the one free thiol in its CXXC motif [83]. (The conserved as a tetramer [68, 69]. histidine near the N-terminus of SU, which is essential for Env function, may catalyze this ionization [84].) The 2.4.3. Env. As with all orthoretroviruses, the MLV Env gene ionized sulfur then attacks the neighboring cysteine, and product is synthesized in the rough endoplasmic reticulum the disulfide linkage between SU and TM is replaced by an and glycosylated in the Golgi apparatus. It is also cleaved in intra-SU bond between these two cysteines. Breaking the the Golgi by a cellular furin-like protease into two fragments, SU-TM bond releases SU from the Env complex, exposing SU the large, N-terminal surface glycoprotein (gp70 ) and the the fusion peptide at the N-terminus of TM. The fusion TM peptide inserts into the target membrane; this is followed C-terminal transmembrane protein p15E .Atrimer of these heterodimeric SU-TM complexes is then trafficked by a major conformational change in TM, in which a C- to the cell surface. As mentioned above, it undergoes an terminal heptad repeat-like sequence in the TM ectodomain additional cleavage during virus maturation: PR removes the folds against the N-terminal heptad repeat [76]. This shift to C-terminal 16 residues, also known as the “R peptide”, from a hairpin configuration brings the two membranes into very close apposition; this finally results in the fusion of the two the cytoplasmic tail of the TM protein [62, 70]. This matu- ration cleavage of TM is found in the gammaretroviruses, in membranes. Mason-Pfizer monkey virus, a betaretrovirus [71, 72], and in Further studies make it clear that RBD functions not only to bind a receptor on the target cell, but also to prevent the lentivirus equine infectious anemia virus [73], but not, as far as is known, in other retroviruses. the conformational change in TM, leading to membrane CX CC CXXC Advances in Virology 7 productively infected with an MLV, the viral Env protein sat- urates the receptors that it would use for infection, rendering the cell almost completely resistant to superinfection by virus FP particles that use the same receptor. This resistance makes it possible to group MLV isolates into families sharing common RBD receptors. “Interference” measurements of this kind showed that NIH/3T3 mouse cells have four distinct cell-surface Helix A molecules used as receptors by different MLVs, as indicated in Table 1 [91, 95]. This polymorphism is considered in detail in a comprehensive review [96], and is discussed in other articles in this series. It is notable that all receptors used by MLVs contain multiple membrane-spanning domains, unlike the known receptors for most other orthoretroviruses. Helix B 3. MLV: The Organism C-term -S-S- 3.1. Assays for Infectious MLV. Quantitative virology is virtually impossible without a reliable infectivity assay [97]. Since MLVs generally have no obvious effect on the cells they infect, the opportunities for developing a “plaque” or “focus” assay have been very limited. Two such assays have been devised, each exploiting a specific cell line with a unique response to MLV infection. One of these is the “UV-XC” test [98]. XC cells, derived Figure 6: MLV Env protein. MLV Env protein consists of a complex from a rat tumor induced by Rous sarcoma virus, undergo SU TM between gp70 and p15E . The cartoon shows that gp70 has two rapid syncytium formation when they come into contact domains, RBD at its N-terminus and “C-term” at its C-terminus, with cells producing ecotropic MLV. This property was used separated by a variable, proline-rich linker. P15E contains, from to develop an “indirect” plaque assay: a plate of permissive N- to C-terminus, the fusion peptide (FP), an N-terminal helical cells is first infected with the virus, and the virus is allowed domain (helix A), a short C-terminal helical domain (helix B), and to spread in these cells for 5–7 days. At the end of this period, a C-terminal domain (light pink) which spans the viral membrane (yellow). Gp70 is exclusively external to the virus and is connected the cells have grown into a confluent monolayer, and the to p15E by a disulfide linkage between one of the two cysteines in a plate contains invisible “foci” of MLV-producing cells. Each CXXC motif within its C-terminal domain and the last cysteine in a focus has arisen by the localized spread of virus from a single CX CC motif in p15E. cell, infected by a virus in the inoculum, to neighboring cells; several rounds of replication can occur during the assay. This monolayer is then killed by UV-irradiation and overlaid with XC cells. A day later, the XC cells have replaced the fusion, from occurring prematurely, that is, before contact original cells; they are fixed and stained, and “plaques”, that of the virus with the receptor [85, 86]. In fact, under special is, localized regions of syncytia, are counted. One particular circumstances infection can occur “in trans”, that is, when a advantage of this assay is that it can be used to measure soluble RBD binds a cell-surface receptor in proximity to the the infectivity of any ecotropic MLV on any cells; thus, for virion [87]. This activity of the MLV Env complex has special example, comparing the titer of a single virus preparation on consequences for the “MCF” class of MLVs. These “mink cell NIH/3T3 cells and Balb/3T3 cells tells one whether the virus focus-inducing” or “polytropic” MLVs arise in mice that are is N-tropic, B-tropic, or NB-tropic. On the other hand, the viremic for ecotropic MLVs, and are recombinants in which fact that it only detects ecotropic MLVs is a serious limitation the ecotropic RBD has been replaced by an RBD from an of the UV-XC test. endogenous MLV genome [88–90]. This substitution gives The other quantitative assay for replication-competent the MCF a different receptor specificity from that of its MLV is the S+L− assay [99]. S+L− cells are specific cell lines ecotropic parent [13–15, 91]. The complex of the ecotropic transformed by Moloney sarcoma virus. When these cells SU protein with the ecotropic receptor on target cells (as in are superinfected by an MLV, they become much rounder the viremic mice) has been shown to facilitate infection of and more refractile (this may reflect “hypertransformation”, the cells by MCF virions [92]. perhaps due to reinfection of the cells with additional copies Remarkably, TM protein performs yet another func- of Moloney sarcoma virus after it has been rescued by the tion for MLV. Immediately proximal to the CX CC motif MLV). In this assay, S+L− cells are infected and allowed discussed above is a 20-residue stretch which has potent to grow for ∼5 days; “foci” of rounded cells, which stand immunosuppressive activity; this activity is crucial in MLV out against the confluent monolayer of uninfected S+L− infections in mice [93, 94]. cells, are then scored under a low-power microscope. This As indicated above, MLVs are polymorphic with respect assay has the advantage that it will detect any replication- to their use of cell-surface receptors. In general, when a cell is competent MLV, not just members of a specific class. 8 Advances in Virology Table 1: MLV receptors on NIH/3T3 mouse cells. 3.3.1. Superinfection Interference. Two genes inducing strong resistance to specific envelope classes of MLV have been Virus class Example Receptor Reference described: Fv-4 and Rmcf [102, 103]. Both of these genes Moloney have been found to function by superinfection interference: [12] Ecotropic mCAT1 MLV in other words, the genes encode glycoproteins which Polytropic MCF247 XPR1 [13–15] bind MLV receptors, rendering the receptors unavailable Amphotropic 1504A SLC20A2 [16, 17] for incoming viruses. Fv-4 blocks the ecotropic receptor, mCAT1, whereas Rmcf blocks the MCF receptor XPR1. It SLC20A1 seems reasonable to imagine that these genes were originally 10A1 10A1 or [16, 17] introduced into the mouse genome as the Env genes of SLC20A2 endogenous MLVs. The table lists the receptors for MLVs found on NIH/3T3 mouse cells. The diversity of MLV receptors is discussed in more detail in other articles of this 3.3.2. Fv1 Restriction. Fv1 restriction was the first system series. for resistance to MLV to be described in mice [104]. Inbred mouse strains carry the “n” allele, the “b” allele, or the “nr” However, it is extremely time consuming. It can also be allele at the Fv1 locus. In turn, naturally occurring MLVs difficult to distinguish the foci from random irregularities in n nr may be N-tropic or B-tropic. Fv1 or Fv1 mice are partially the cell monolayer, so scoring the assay requires considerable resistant to B-tropic MLVs, while the Fv1 locus encodes skill and involves some judgment. nr partial resistance to N-tropic MLVs (Fv1 mice are resistant For many, but not all, kinds of experiments, replication- to some N-tropic MLVs as well as B-tropic MLVs). Passage defective “reporter” viruses rescued by MLV can be assayed of an MLV in the restrictive host may ultimately lead to the in lieu of assaying the MLV itself. The reporter viruses selection of a viral variant that has lost its sensitivity to Fv1 originally used in this way were acute transforming viruses; restriction; these laboratory isolates, such as Moloney MLV, for example, MLVs were grouped into interference families are termed NB-tropic. XMRV is unique in that it is restricted by measuring the ability of Harvey MSV pseudotypes to n b by both Fv1 and Fv1 [105]. transform MLV-infected cells [91, 95]. More recently, of Despite many years of investigation, the mechanism of course, MLV-derived vectors expressing a variety of genes, Fv1 restriction is still not well understood. The Fv1 gene such as luciferase, β-galactosidase, and green fluorescent productseems to be asomewhatdegenerateretroviralGag protein, have been constructed for use as reporter viruses protein [106]. Genetic data indicate that it binds to a specific (e.g., [100]). site in the N-terminal domain of CA in the mature core of Cell lines have also been developed in which a reporter the incoming virus particle. This interaction blocks infection gene is only expressed following replication in the cell of at a point between reverse transcription and integration an MLV. These cells contain an MLV-derived vector which of the viral DNA. The Fv1 protein is present in cells at carries a reporter gene in reverse orientation; the reporter extremely low levels [107]; in fact, restriction can be blocked gene is interrupted by an intron in the forward orientation. or “abrogated” by infection with a single particle of the Transcription and splicing yields an RNA in the cell with an restricted type [108]. Particles which have been inactivated uninterrupted, negative-sense copy of the reporter gene; if by heat or gamma irradiation can retain the ability to this RNA is rescued by an MLV, it can be copied into DNA, abrogate Fv1 restriction [109]. finally producing an intact reporter gene whose expression Biochemical analysis of the Fv1 restriction machinery has can be measured (Aloia et al., manuscript in preparation, proven extremely difficult, but it appears that the ability of but see [101]). This assay has the special advantage that it the Fv1 protein to multimerize [110] is an essential element can be performed by cocultivation of the assay cells with cells in restriction [111]. The specific binding of the protein to CA producing the virus to be assayed, as well as by infection of protein of the restricted type seems to occur only when the the assay cells with cell-free virus. mature CA is in a lattice, as in the viral core; this binding was recently demonstrated, for the first time, using CA protein 3.2. Endogenous MLVs. At least 100 times over the course of arrayed on lipid nanotubes [112]. evolution, MLVs have infected cells of the mouse germline. While the Fv1 restriction system is, as far as is known, Once the viral DNA has integrated into the germline DNA, it found only in mouse cells, human cells possess a somewhat is passed from parents to offspring just like any other mouse analogous restriction system effected by the TRIM5α protein. gene. The biology of these “endogenous” MLVs and their TRIM5α was discovered by virtue of its ability to restrict effects on their hosts are quite complex and are considered HIV-1, but it is also active against some MLVs; remarkably, in other articles in this series. like the Fv1 gene product, it distinguishes between N-tropic and B-tropic MLVs [113]. 3.3. Resistance to MLV. While MLVs are generally benign at the cellular level, they do induce both lymphomas and 3.3.3. APOBEC3 Restriction. All placental mammals have at neurological diseases in mice. Mice have evolved a number least one member of the APOBEC3 gene family; humans of resistance mechanisms that inhibit the growth of MLVs; and chimpanzees have seven APOBEC3 genes [114, 115]. MLVs have, in turn, developed strategies for evading these APOBEC3 proteins can be incorporated into retrovirus par- defense mechanisms. ticles, and they interfere with viral replication during reverse Advances in Virology 9 Virion transcription when the APOBEC3-bearing virus particle infects a new host cell. APOBEC3s are cytidine deaminases with one or two zinc-coordinating motifs that are instru- mental in the restriction of viral replication. It seems likely that the primary function of APOBEC3s is protection of the mammalian host against pathogens (or intracellular parasites GPI such as retrotransposons): mice lacking mouse APOBEC3 (mA3) survive and reproduce normally but are very sensitive to retrovirus infection [116, 117]. One way in which APOBEC3 proteins inactivate retro- viruses is by hypermutation. By deaminating deoxycytidine to deoxyuridine in minus-strand DNA during the synthesis of viral DNA, they bring about a G to A change in the plus- GPI strand. Many susceptible viruses have been shown to incur very high levels of G to A mutation as a result of APOBEC3 action. However, it is now clear that APOBEC3 proteins act on retroviruses in other ways as well. For example, the degree of inactivation of HIV-1 by human APOBEC3G (hA3G) does not necessarily correlate with the level of G to A mutation Host cell (reviewed in [118]), and hA3G has been shown to affect both the synthesis and integration of HIV-1 viral DNA [119]. Figure 7: Hypothetical mechanism of restriction by tetherin. The cellular restriction factor tetherin can act as a bridge between two There are two isoforms of mA3, containing or lacking membranes. Tetherin contains a transmembrane domain at its N- exon 5. Most studies on mA3 have used the form lacking the terminus and is anchored to a membrane by a glycophosphatidyl exon.MLVsshowdramaticdifferences in their sensitivity to linkage at its C-terminus. It also dimerizes due to a parallel coiled- this mA3: both XMRV and AKV (the endogenous ecotropic coil structure between the termini of the protein. Anchorage to MLV in AKR mice, a mouse line bred for high leukemia membranes at both ends apparently enables tetherin to “trap” virus incidence) are far more sensitive to inactivation by mA3 particles, preventing their escape from the virus-producing cell. It than Moloney MLV (which was selected for rapid growth is not known which end of the protein is embedded in the cellular and leukemogenicity by passage in mice over a period of membrane and which in the viral membrane. (Figure reproduced years) [105, 120–122]. Moreover, when DNA of XMRV or with permission from [22].) AKV is synthesized in the presence of mA3, it contains large numbers of G to A mutations [120, 121], but these host protein “tetherin” (also known as CD317, BST2, or mutations are not detectably induced in Moloney MLV HM1.24) [125]. Tetherin is a membrane protein with a very by mA3 [100, 123]. Presumably, the creation of the AKR unusual topology: it has a cytoplasmic N-terminus, followed mouse strain entailed the selection of mice that provide by a transmembrane helix, an extended ectodomain, and a maximally permissive environment for AKV, and thus, a C-terminus associated with the plasma membrane by a this virus has not faced selective pressure leading to mA3 glycophosphatidyl inositol linkage. Tetherin dimerizes via resistance. In contrast, selection during passage of Moloney the ectodomain, which forms a coiled coil ∼90Along. The MLV has led to partial resistance to inactivation by mA3, presence of membrane anchors at both ends of the molecule and apparently complete resistance to the hypermutational evidently gives it the ability to physically link released virus effects of mA3. The mechanisms underlying these resistance particles to the surface of the virus-producing cell, effectively phenomena are unknown. It should be noted that in HIV-1, preventing their escape into the surrounding medium (see one of the “accessory proteins”, that is, Vif, is responsible for Figure 7)[22]. viral resistance to hA3G. Vif functions by binding to hA3G Tetherins inhibit the release of all retroviruses tested, and inducing its proteasomal degradation. However, as and also of filoviruses such as Ebola, arenaviruses such as emphasized above, MLVs do not encode accessory proteins, Lassa, and herpesviruses such as Kaposi’s sarcoma-associated and the resistance of Moloney MLV to mA3 must reside in its herpesvirus. They are constitutively expressed on some cell Gag, Pol, and/or Env protein. As mA3 is packaged efficiently surfaces and are inducible by type I interferon in others. in Moloney MLV particles [100, 123], the resistance does not Mouse tetherin has been shown to inhibit the replication depend upon exclusion of mA3 from the virus. of MLV [126]. While lentiviruses have several alternative The biology of MLV restriction by the mA3 containing countermeasures against tetherins, including the HIV-1 exon5issomewhatdifferent from the foregoing: mA3 accessory protein Vpu (reviewed in [127]), no resistance protein containing this exon can be cleaved by MLV PR, mechanisms in MLVs have yet been described. leading to the inactivation of this mA3 within MLV particles [124]. 4. Concluding Remarks 3.3.4. Restriction by Tetherin. Recently, yet another antiviral It is clear that MLVs have provided an extraordinary wealth restriction system has been discovered, mediated by the of information about retroviruses, both as physical objects 10 Advances in Virology and as living organisms. They (and other gammaretro- [9] M. Yeager, E. M. Wilson-Kubalek, S. G. Weiner, P. O. Brown, and A. Rein, “Supramolecular organization of immature viruses, such as gibbon ape leukemia virus) are now being and mature murine leukemia virus revealed by electron developed as vectors for gene therapy. As has been indicated cryo-microscopy: implications for retroviral assembly mech- throughout this paper, the contrasts with other retroviruses anisms,” Proceedings of the National Academy of Sciences of the such as HIV-1 help to illustrate the range of possibilities United States of America, vol. 95, no. 13, pp. 7299–7304, 1998. by which viruses solve common problems. Finally, as with [10] D. E. Ott, “Cellular proteins detected in HIV-1,” Reviews in all viruses, MLVs provide a window into the “black box”, Medical Virology, vol. 18, no. 3, pp. 159–175, 2008. an unparalleled opportunity to learn about the cells and [11] J. E. Bubbers and F. Lilly, “Selective incorporation of H 2 organisms that they infect. Indeed, many cellular proteins antigenic determinants into Friend virus particles,” Nature, have been shown to participate in MLV replication; while this vol. 266, no. 5601, pp. 458–459, 1977. large topic is beyond the scope of this paper, it is the focus of [12] L. M. Albritton, L. Tseng, D. Scadden, and J. M. Cunning- a fascinating review by Goff [128]. ham, “A putative murine ecotropic retrovirus receptor gene encodes a multiple membrane-spanning protein and confers susceptibility to virus infection,” Cell, vol. 57, no. 4, pp. 659– Acknowledgments 666, 1989. [13] J. L. Battini, J. E. J. Rasko, and A. D. Miller, “A human The author thanks his colleagues John Coffinand Steve cell-surface receptor for xenotropic and polytropic murine Hughes for many helpful discussions and particularly Jim leukemia viruses: possible role in G protein-coupled signal Cunningham for insight into MLV Env gymnastics. He also transduction,” Proceedings of the National Academy of Sciences of the United States of America, vol. 96, no. 4, pp. 1385–1390, thanks Bob Bassin for his mentorship and generosity during his introduction to the study of MLVs. Research in his [14] C. S. Tailor,A.Nouri,C.G.Lee,C.Kozak,and D. Kabat, laboratory is supported by the Intramural Research Program “Cloning and characterization of a cell surface receptor of the NIH, National Cancer Institute, and Center for Cancer for xenotropic and polytropic marine leukemia viruses,” Research. Proceedings of the National Academy of Sciences of the United States of America, vol. 96, no. 3, pp. 927–932, 1999. [15] Y. L. Yang, L. Guo, S. Xu et al., “Receptors for polytropic and References xenotropic mouse leukaemia viruses encoded by a single gene [1] M. L. Linial et al., “Retroviridae. Virus taxonomy: clas- at Rmc1,” Nature Genetics, vol. 21, no. 2, pp. 216–219, 1999. [16] D. G. Miller and A. D. Miller, “A family of retroviruses that sification and nomenclature of viruses,” in Proceedings of the International Committee on Taxonomy of Viruses,C.M. utilize related phosphate transporters for cell entry,” Journal of Virology, vol. 68, no. 12, pp. 8270–8276, 1994. Fauquet, M. A. Mayo, J. Maniloff,U.Desselberger, andL.A. Ball, Eds., pp. 421–440, Elsevier/Academic Press, San Diego, [17] C. A. Wilson, K. B. Farrell, and M. V. Eiden, “Properties of a unique form of the murine amphotropic leukemia virus Calif, USA, 2004. [2] L. Gross, ““Spontaneous” leukemia developing in C3H mice receptor expressed on hamster cells,” Journal of Virology, vol. 68, no. 12, pp. 7697–7703, 1994. following inoculation in infancy, with AK-leukemic extracts, or AK-embrvos,” Proceedings of the Society for Experimental [18] T. M. Shinnick, R. A. Lerner, and J. G. Sutcliffe, “Nucleotide Biology and Medicine. Society for Experimental Biology and sequence of Moloney murine leukaemia virus,” Nature, vol. 293, no. 5833, pp. 543–548, 1981. Medicine, vol. 76, no. 1, pp. 27–32, 1951. [3] L. Gross, “Development and serial cellfree passage of a highly [19] C. S. Badorrek and K. M. Weeks, “RNA flexibility in the dimerization domain of a gamma retrovirus,” Nature potent strain of mouse leukemia virus,” Proceedings of the Society for Experimental Biology and Medicine, vol. 94, pp. Chemical Biology, vol. 1, no. 2, pp. 104–111, 2005. [20] C. Gherghe, T. Lombo, C. W. Leonard et al., “Definition of a 767–771, 1957. [4] J. B. Moloney, “Biological studies on a lymphoid-leukemia high-affinity Gag recognition structure mediating packaging of a retroviral RNA genome,” Proceedings of the National virus extracted from sarcoma 37. I. Origin and introductory investigations,” Journal of the National Cancer Institute, vol. Academy of Sciences of the United States of America, vol. 107, no. 45, pp. 19248–19253, 2010. 24, pp. 933–951, 1960. [5] P. F. Lewis and M. Emerman, “Passage through mitosis [21] A. Rein, S. A. Datta, C. P. Jones, and K. Musier-Forsyth, “Diverse interactions of retroviral Gag proteins with RNAs,” is required for oncoretroviruses but not for the human immunodeficiency virus,” Journal of Virology, vol. 68, no. 1, Trends in Biochemical Sciences, vol. 36, no. 7, pp. 373–380, pp. 510–516, 1994. 2011. [22] H. Yang, J. Wang, X. Jia et al., “Structural insight into the [6] T.Roe,T.C.Reynolds, G. Yu,and P. O. Brown, “Integration of murine leukemia virus DNA depends on mitosis,” EMBO mechanisms of enveloped virus tethering by tetherin,” Pro- ceedings of the National Academy of Sciences of the United Journal, vol. 12, no. 5, pp. 2099–2108, 1993. [7] L. Jarrosson-Wuilleme, C. Goujon, J. Bernaud, D. Rigal, States of America, vol. 107, no. 43, pp. 18428–18432, 2010. [23] O. Pornillos, J. E. Garrus, and W. I. Sundquist, “Mechanisms J. L. Darlix, and A. Cimarelli, “Transduction of nondi- viding human macrophages with gammaretrovirus-derived of enveloped RNA virus budding,” Trends in Cell Biology, vol. 12, no. 12, pp. 569–579, 2002. vectors,” Journal of Virology, vol. 80, no. 3, pp. 1152–1159, 2006. [24] K. M. Felsenstein and S. P. Goff, “Expression of the gag-pol fusion protein of Moloney murine leukemia virus without [8] X. H. Liu, W. Xu, J. Russ, L. E. Eiden, and M. V. Eiden, “The host range of gammaretroviruses and gammaretroviral vec- gag protein does not induce virion formation or proteolytic processing,” Journal of Virology, vol. 62, no. 6, pp. 2179–2182, tors includes post-mitotic neural cells,” PLoS ONE, vol. 6, no. 3, Article ID e18072, 2011. 1988. Advances in Virology 11 [25] Y. X. Feng, H. Yuan, A. Rein, and J. G. Levin, “Bipartite signal of Moloney sarcoma virus,” Nature, vol. 295, no. 5850, pp. for read-through suppression in murine leukemia virus 568–572, 1982. mRNA: an eight-nucleotide purine-rich sequence immedi- [41] A. B. Rabson and B. J. Graves, “Synthesis and processing ately downstream of the gag termination codon followed by of viral RNA,” in Retroviruses,J.M.Coffin, S. H. Hughes, an RNA pseudoknot,” Journal of Virology, vol. 66, no. 8, pp. and H. E. Varmus, Eds., pp. 205–262, Cold Spring Harbor 5127–5132, 1992. Laboratory Press, New York, NY, USA, 1997. [42] L. E. Henderson, H. C. Krutzsch, and S. Oroszlan, “Myristyl [26] Y. Yoshinaka, I. Katoh, T. D. Copeland, and S. Oroszlan, “Murine leukemia virus protease is encoded by the gag-pol amino-terminal acylation of murine retrovirus proteins: an unusual post-translational protein modification,” Proceed- gene and is synthesized through suppression of an amber termination codon,” Proceedings of the National Academy of ings of the National Academy of Sciences of the United States of America, vol. 80, no. 2, pp. 339–343, 1983. Sciences of the United States of America,vol. 82, no.6,pp. 1618–1622, 1985. [43] A. Rein, M. R. McClure, N. R. Rice, R. B. Luftig, and A. M. Schultz, “Myristylation site in Pr65(gag) is essential for [27] Y. X. Feng, T. D. Copeland, S. Oroszlan, A. Rein, and J. virus particle formation by Moloney murine leukemia virus,” G. Levin, “Identification of amino acids inserted during Proceedings of the National Academy of Sciences of the United suppression of UAA and UGA termination codons at the gag- States of America, vol. 83, no. 19, pp. 7246–7250, 1986. pol junction of Moloney murine leukemia virus,” Proceedings [44] B. Yuan, S. Campbell, E. Bacharach, A. Rein, and S. P. Goff, of the National Academy of Sciences of the United States of “Infectivity of Moloney murine leukemia virus defective in America, vol. 87, no. 22, pp. 8860–8863, 1990. late assembly events is restored by late assembly domains of [28] Y. X. Feng, J. G. Levin, D. L. Hatfield, T. S. Schaefer, R. other retroviruses,” Journal of Virology, vol. 74, no. 16, pp. J. Gorelick, and A. Rein, “Suppression of UAA and UGA 7250–7260, 2000. termination codons in mutant murine leukemia viruses,” [45] C. Segura-Morales, C. Pescia, C. Chatellard-Causse, R. Journal of Virology, vol. 63, no. 6, pp. 2870–2873, 1989. Sadoul, E. Bertrand, and E. Basyuk, “Tsg101 and Alix [29] T. Jacks, M. D. Power, F. R. Masiarz, P. A. Luciw, P. J. Barr, and interact with murine leukemia virus Gag and cooperate H. E. Varmus, “Characterization of ribosomal frameshifting with Nedd4 ubiquitin ligases during budding,” Journal of in HIV-1 gag-pol expression,” Nature, vol. 331, no. 6153, pp. Biological Chemistry, vol. 280, no. 29, pp. 27004–27012, 2005. 280–283, 1988. [46] A. Prizan-Ravid, E. Elis, N. Laham-Karam, S. Selig, M. [30] T. Jacks and H. E. Varmus, “Expression of the Rous sarcoma Ehrlich, and E. Bacharach, “The Gag cleavage product, p12, virus pol gene by ribosomal frameshifting,” Science, vol. 230, is a functional constituent of the murine leukemia virus pre- no. 4731, pp. 1237–1242, 1985. integration complex,” PLoS Pathogens, vol. 6, no. 11, Article [31] D. L. Hatfield, J. G. Levin, A. Rein, and S. Oroszlan, ID e1001183, 2010. “Translational suppression in retroviral GENE expression,” [47] B. Yuan,A.Fassati,A.Yueh, andS.P.Goff,“Characterization Advances in Virus Research, vol. 41, pp. 193–239, 1992. of Moloney murine leukemia virus p12 mutants blocked [32] P. O. Brown, “Integration,” in Retroviruses,J.M.Coffin, S. H. during early events of infection,” Journal of Virology, vol. 76, Hughes, and H. E. Varmus, Eds., pp. 161–203, Cold Spring no. 21, pp. 10801–10810, 2002. Harbor Laboratory Press, Plainview, NY, USA, 1997. [48] A. Yueh andS.P.Goff, “Phosphorylated serine residues and [33] J. Colicelli and S. P. Goff, “Mutants and pseudorevertants an arginine-rich domain of the Moloney murine leukemia of Moloney murine leukemia virus with alterations at the virus p12 protein are required for early events of viral integration site,” Cell, vol. 42, no. 2, pp. 573–580, 1985. infection,” Journal of Virology, vol. 77, no. 3, pp. 1820–1829, [34] J. Colicelli and S. P. Goff, “Sequence and spacing require- ments of a retrovirus integration site,” JournalofMolecular [49] M. R. Auerbach,C.Shu,A.Kaplan, andI.R.Singh, Biology, vol. 199, no. 1, pp. 47–59, 1988. “Functional characterization of a portion of the Moloney [35] S. J. RulliJr.,C.S.Hibbert,J.Mirro,T.Pederson, S. Biswal, murine leukemia virus gag gene by genetic footprinting,” and A. Rein, “Selective and nonselective packaging of cellular Proceedings of the National Academy of Sciences of the United RNAs in retrovirus particles,” Journal of Virology, vol. 81, no. States of America, vol. 100, no. 20, pp. 11678–11683, 2003. 12, pp. 6623–6631, 2007. [50] B. Yuan,X.Li, andS.P.Goff, “Mutations altering the [36] V. D’Souza and M. F. Summers, “Structural basis for pack- Moloney murine leukemia virus p12 Gag protein affect aging the dimeric genome of Moloney murine leukaemia virion production and early events of the virus life cycle,” virus,” Nature, vol. 431, no. 7008, pp. 586–590, 2004. EMBO Journal, vol. 18, no. 17, pp. 4700–4710, 1999. [37] D. A. M. Konings, M. A. Nash, J. V. Maizel, and R. B. [51] M. Oshima, D. Muriaux, J. Mirro et al., “Effects of blocking Arlinghaus, “Novel GACG-hairpin pair motif in the 5’ individual maturation cleavages in murine leukemia virus untranslated region of type C retroviruses related to murine Gag,” Journal of Virology, vol. 78, no. 3, pp. 1411–1420, 2004. leukemia virus,” Journal of Virology, vol. 66, no. 2, pp. 632– [52] S. K. Kyere, P. R. B. Joseph, and M. F. Summers, “The p12 640, 1992. domain is unstructured in a murine leukemia virus p12- [38] C. Gherghe, C. W. Leonard, R. J. Gorelick, and K. M. Weeks, CA(N) Gag construct,” PLoS ONE, vol. 3, no. 4, Article ID “Secondary structure of the mature ex virio Moloney murine e1902, 2008. leukemia virus genomic RNA dimerization domain,” Journal [53] A. C. Prats, G. De Billy, P. Wang, and J. L. Darlix, “CUG of Virology, vol. 84, no. 2, pp. 898–906, 2010. initiation codon used for the synthesis of a cell surface [39] C. H. Kim and I Tinoco Jr., “A retroviral RNA kissing antigen coded by the murine leukemia virus,” Journal of complex containing only two G·C base pairs,” Proceedings Molecular Biology, vol. 205, no. 2, pp. 363–372, 1989. of the National Academy of Sciences of the United States of [54] R. Fujisawa,F.J.McAtee, C. Favara,S.F.Hayes,and J. L. America, vol. 97, no. 17, pp. 9396–9401, 2000. Portis, “N-terminal cleavage fragment of glycosylated Gag is [40] B. Levinson, G. Khoury, G. Vande Woude, and P. Gruss, incorporated into murine oncornavirus particles,” Journal of “Activation of SV40 genome by 72-base pair tandem repeats Virology, vol. 75, no. 22, pp. 11239–11243, 2001. 12 Advances in Virology [55] A. Corbin, A. C. Prats, J. L. Darlix, and M. Sitbon, “A non- [70] A. Schultz and A. Rein, “Maturation of murine leukemia structural gag-encoded glycoprotein precursor is necessary virus env proteins in the absence of other viral proteins,” for efficient spreading and pathogenesis of murine leukemia Virology, vol. 145, no. 2, pp. 335–339, 1985. viruses,” Journal of Virology, vol. 68, no. 6, pp. 3857–3867, [71] B. A. Brody, S. S. Rhee, and E. Hunter, “Postassembly cleavage of a retroviral glycoprotein cytoplasmic domain removes a [56] P. Schwartzberg, J. Colicelli, and S. P. Goff, “Deletion mutants necessary incorporation signal and activates fusion activity,” of Moloney murine leukemia virus which lack glycosylated Journal of Virology, vol. 68, no. 7, pp. 4620–4627, 1994. gag protein are replication competent,” Journal of Virology, [72] M. A. Sommerfelt, S. R. Petteway Jr., G. B. Dreyer, and vol. 46, no. 2, pp. 538–546, 1983. E. Hunter, “Effect of retroviral proteinase inhibitors on [57] A. Low, S. Datta, Y. Kuznetsov et al., “Mutation in the Mason-Pfizer monkey virus maturation and transmembrane glycosylated gag protein of murine leukemia virus results in glycoprotein cleavage,” Journal of Virology,vol. 66, no.7,pp. reduced in vivo infectivity and a novel defect in viral budding 4220–4227, 1992. or release,” Journal of Virology, vol. 81, no. 8, pp. 3685–3692, [73] N. R. Rice,L.E.Henderson,R.C.Sowder, T. D. Copeland, S. Oroszlan, and J. F. Edwards, “Synthesis and processing [58] T. Nitta, R. Tam, J. W. Kim, and H. Fan, “The cellular protein of the transmembrane envelope protein of equine infectious La functions in enhancement of virus release through lipid anemia virus,” Journal of Virology, vol. 64, no. 8, pp. 3770– rafts facilitated by murine leukemia virus glycosylated Gag,” 3778, 1990. mBio, vol. 2, no. 1, 2011. [74] E. Hunter, “Viral entry and receptors,” in Retroviruses,J.M. [59] M. Pizzato, “MLV glycosylated-Gag is an infectivity factor Coffin, S. H. Hughes, and H. E. Varmus, Eds., pp. 71–119, that rescues Nef-deficient HIV-1,” Proceedings of the National Cold Spring Harbor Laboratory Press, Plainview, NY, USA, Academy of Sciences of the United States of America, vol. 107, no. 20, pp. 9364–9369, 2010. [75] D. Fass, R. A. Davey, C. A. Hamson, P. S. Kim, J. M. Cunning- [60] R. J. Gorelick, W. Fu, T. D. Gagliardi et al., “Characterization ham, and J. M. Berger, “Structure of a murine leukemia virus of the block in replication of nucleocapsid protein zinc finger receptor-binding glycoprotein at 2.0 angstrom resolution,” mutants from Moloney murine leukemia virus,” Journal of Science, vol. 277, no. 5332, pp. 1662–1666, 1997. Virology, vol. 73, no. 10, pp. 8185–8195, 1999. [76] D. Fass, S. C. Harrison, and P. S. Kim, “Retrovirus envelope [61] R. J. Gorelick,L.E.Henderson,J.P.Hanser, andA.Rein, domain at 1.7 A resolution,” Nature Structural Biology, vol. 3, “Point mutants of Moloney murine leukemia virus that fail no. 5, pp. 465–469, 1996. to package viral RNA: evidence for specific RNA recognition [77] B. Kobe, R. L. Center, B. E. Kemp, and P. Poumbourios, by a “zinc finger-like” protein sequence,” Proceedings of the “Crystal structure of human T cell leukemia virus type 1 National Academy of Sciences of the United States of America, gp21 ectodomain crystallized as a maltose-binding protein vol. 85, no. 22, pp. 8420–8424, 1988. chimera reveals structural evolution of retroviral transmem- [62] L. E. Henderson, R. Sowder, T. D. Copeland, G. Smythers, brane proteins,” Proceedings of the National Academy of and S. Oroszlan, “Quantitative separation of murine Sciences of the United States of America,vol. 96, no.8,pp. leukemia virus proteins by reversed-phase high-pressure 4319–4324, 1999. liquid chromatography reveals newly described gag and env [78] A. Pinter, R. Kopelman, Z. Li, S. C. Kayman, and D. A. cleavage products,” Journal of Virology, vol. 52, no. 2, pp. 492– Sanders, “Localization of the labile disulfide bond between 500, 1984. SU and TM of the murine leukemia virus envelope protein [63] L. Menendez-Arias, D. Gotte, and S. Oroszlan, “Moloney complex to a highly conserved CWLC motif in SU that murine leukemia virus protease: bacterial expression and resembles the active-site sequence of thiol-disulfide exchange characterization of the purified enzyme,” Virology, vol. 196, enzymes,” Journal of Virology, vol. 71, no. 10, pp. 8073–8077, no. 2, pp. 557–563, 1993. [64] R. Swanstrom and J. W. Wills, “Synthesis, assembly, and [79] E. O. Freed and R. Risser, “The role of envelope glycoprotein processing of viral proteins,” in Retroviruses,J.M.Coffin, S. processing in murine leukemia virus infection,” Journal of H. Hughes, and H. E. Varmus, Eds., pp. 263–334, Cold Spring Virology, vol. 61, no. 9, pp. 2852–2856, 1987. Harbor Laboratory Press, Plainview, NY, USA, 1997. [80] J. A. Ragheb and W. F. Anderson, “pH-independent murine [65] D. Das and M. M. Georgiadis, “The crystal structure of leukemia virus ecotropic envelope-mediated cell fusion: the monomeric reverse transcriptase from Moloney murine implications for the role of the R peptide and p12E TM in leukemia virus,” Structure, vol. 12, no. 5, pp. 819–829, 2004. viral entry,” Journal of Virology, vol. 68, no. 5, pp. 3220–3231, [66] K. Moelling, “Characterization of reverse transcriptase and RNase H from friend-murine leukemia virus,” Virology, vol. [81] A. Rein, J. Mirro, J. G. Haynes, S. M. Ernst, and K. 62, no. 1, pp. 46–59, 1974. Nagashima, “Function of the cytoplasmic domain of a retro- [67] A. Telesnitsky and S. P. Goff, “Reverse transcriptase and the viral transmembrane protein: p15E-p2E cleavage activates generation of retroviral DNA,” in Retroviruses,J.M.Coffin, S. the membrane fusion capability of the murine leukemia virus H. Hughes, and H. E. Varmus, Eds., pp. 121–160, Cold Spring Env protein,” Journal of Virology, vol. 68, no. 3, pp. 1773– Harbor Laboratory Press, Plainview, NY, USA, 1997. 1781, 1994. [68] S. Hare, F. Di Nunzio, A. Labeja, J. Wang, A. Engelman, and P. [82] M. Li, Z. N. Li, Q. Yao, C. Yang, D. A. Steinhauer, and R. Cherepanov, “Structural basis for functional tetramerization W. Compans, “Murine leukemia virus R peptide inhibits of lentiviral integrase,” PLoS Pathogens, vol. 5, no. 7, Article influenza virus hemagglutinin-induced membrane fusion,” ID e1000515, 2009. Journal of Virology, vol. 80, no. 12, pp. 6106–6114, 2006. [69] S. Hare,S.S.Gupta,E.Valkov, A. Engelman,and P. Cherepanov, “Retroviral intasome assembly and inhibition [83] M. Wallin, M. Ekstrom, ¨ and H. Garoff, “Isomerization of of DNA strand transfer,” Nature, vol. 464, no. 7286, pp. 232– the intersubunit disulphide-bond in Env controls retrovirus 236, 2010. fusion,” EMBO Journal, vol. 23, no. 1, pp. 54–65, 2004. Advances in Virology 13 [84] K. Li, S. Zhang, M. Kronqvist, M. Ekstrom, ¨ M. Wallin, and [99] R. H. Bassin, N. Tuttle, and P. J. Fischinger, “Rapid cell culture H. Garoff, “The conserved His8 of the Moloney murine assay technique for murine leukaemia viruses,” Nature, vol. leukemia virus Env SU subunit directs the activity of the SU- 229, no. 5286, pp. 564–566, 1971. TM disulphide bond isomerase,” Virology, vol. 361, no. 1, pp. [100] S. J. Rulli Jr., J. Mirro, S. A. Hill et al., “Interactions of murine 149–160, 2007. APOBEC3 and human APOBEC3G with murine leukemia [85] A. L. Barnett and J. M. Cunningham, “Receptor binding viruses,” Journal of Virology, vol. 82, no. 13, pp. 6566–6575, transforms the surface subunit of the mammalian C-type retrovirus envelope protein from an inhibitor to an activator [101] D. Mazurov, A. Ilinskaya, G. Heidecker, P. Lloyd, and D. of fusion,” Journal of Virology, vol. 75, no. 19, pp. 9096–9105, Derse, “Quantitative comparison of HTLV-1 and HIV-1 cell- 2001. to-cell infection with new replication dependent vectors,” PLoS Pathogens, vol. 6, no. 2, Article ID e1000788, 2010. [86] A. L. Barnett, R. A. Davey, and J. M. Cunningham, “Modular [102] R. H. Bassin, S. Ruscetti, I. Ali, D. K. Haapala, and A. Rein, organization of the Friend murine leukemia virus envelope “Normal DBA/2 mouse cells synthesize a glycoprotein which protein underlies the mechanism of infection,” Proceedings interferes with MCF virus infection,” Virology, vol. 123, no. of the National Academy of Sciences of the United States of 1, pp. 139–151, 1982. America, vol. 98, no. 7, pp. 4113–4118, 2001. [103] H. Ikeda and T. Odaka, “A cell membrane “gp70” associated [87] D. Lavillette, A. Ruggieri, S. J. Russell, and F. L. Cosset, with Fv-4 gene: immunological characterization and tissue “Activation of a cell entry pathway common to type C and strain distribution,” Virology, vol. 133, no. 1, pp. 65–76, mammalian retroviruses by soluble envelope fragments,” Journal of Virology, vol. 74, no. 1, pp. 295–304, 2000. [104] T. Pincus, W. P. Rowe, and F. Lilly, “A major genetic [88] S. K. Chattopadhyay, M. R. Lander, E. Rands, S. Gupta and D. locus affecting resistance to infection with murine leukemia R. Lowy, “Origin of mink cytopathic focus-forming (MCF) viruses. II. Apparent identity to a major locus described viruses: comparison with ecotropic and xenotropic murine for resistance to friend murine leukemia virus,” Journal of leukemia virus genomes,” Virology, vol. 113, no. 2, pp. 465– Experimental Medicine, vol. 133, no. 6, pp. 1234–1241, 1971. 483, 1981. [105] H. C. T. Groom, M. W. Yap, R. P. Gala˜o,S.J.D.Neil, and [89] J. W. Hartley, N. K. Wolford, L. J. Old, and W. P. Rowe, “A new K. N. Bishop, “Susceptibility of xenotropic murine leukemia class of murine leukemia virus associated with development virus-related virus (XMRV) to retroviral restriction factors,” of spontaneous lymphomas,” Proceedings of the National Proceedings of the National Academy of Sciences of the United Academy of Sciences of the United States of America, vol. 74, States of America, vol. 107, no. 11, pp. 5166–5171, 2010. no. 2, pp. 789–792, 1977. [106] S. Best, P. L. Tissier, G. Towers, and J. P. Stoye, “Positional [90] R. A. Bosselman, F. van Straaten, C. van Beveren, I. M. cloning of the mouse retrovirus restriction gene Fv1,” Nature, Verma, and M. Vogt, “Analysis of the env gene of a molecu- vol. 382, no. 6594, pp. 826–829, 1996. larly cloned and biologically active Moloney mink cell focus- [107] M. W. Yap and J. P. Stoye, “Intracellular localisation of Fv1,” forming proviral DNA,” Journal of Virology,vol. 44, no.1,pp. Virology, vol. 307, no. 1, pp. 76–89, 2003. 19–31, 1982. [108] G. Duran-Troise, R. H. Bassin, A. Rein, and B. I. Gerwin, [91] A. Rein, “Interference grouping of murine leukemia viruses: a “Loss of Fv 1 restriction in Balb/3T3 cells following infection distinct receptor for the MCF-recombinant viruses in mouse with a single N tropic murine leukemia virus particle,” Cell, cells,” Virology, vol. 120, no. 1, pp. 251–257, 1982. vol. 10, no. 3, pp. 479–488, 1977. [92] D. L. Wensel, W. Li, and J. M. Cunningham, “A virus-virus [109] R. H. Bassin, G. Duran-Troise, B. I. Gerwin, and A. Rein, interaction circumvents the virus receptor requirement for “Abrogation of Fv-1(b) restriction with murine leukemia infection by pathogenic retroviruses,” Journal of Virology, vol. viruses inactivated by heat or by gamma irradiation,” Journal 77, no. 6, pp. 3460–3469, 2003. of Virology, vol. 26, no. 2, pp. 306–315, 1978. [93] G. J. Cianciolo, T. D. Copeland, S. Oroszlan, and R. Snyder- [110] K. N. Bishop, G. B. Mortuza, S. Howell, M. W. Yap, J. P. Stoye, man, “Inhibition of lymphocyte proliferation by a synthetic and I. A. Taylor, “Characterization of an amino-terminal peptide homologous to retroviral envelope proteins,” Science, dimerization domain from retroviral restriction factor Fv1,” vol. 230, no. 4724, pp. 453–455, 1985. Journal of Virology, vol. 80, no. 16, pp. 8225–8235, 2006. [94] G. Schlecht-Louf, M. Renard, M. Mangeney et al., “Retroviral [111] M. W. Yap, G. B. Mortuza, I. A. Taylor, and J. P. Stoye, “The infection in vivo requires an immune escape virulence factor design of artificial retroviral restriction factors,” Virology, vol. encrypted in the envelope protein of oncoretroviruses,” 365, no. 2, pp. 302–314, 2007. Proceedings of the National Academy of Sciences of the United [112] L. Hilditch, R. Matadeen, D. C. Goldstone, P. B. Rosenthal, States of America, vol. 107, no. 8, pp. 3782–3787, 2010. I. A. Taylor, and J. P. Stoye, “Ordered assembly of murine [95] A. Rein and A. Schultz, “Different recombinant murine leukemia virus capsid protein on lipid nanotubes directs leukemia viruses use different cell surface receptors,” Virol- specific binding by the restriction factor, Fv1,” Proceedings ogy, vol. 136, no. 1, pp. 144–152, 1984. of the National Academy of Sciences of the United States of [96] C. S. Tailor, D. Lavillette, M. Marin, and D. Kabat, “Cell America, vol. 108, no. 14, pp. 5771–5776, 2011. surface receptors for gammaretroviruses,” Current Topics in [113] G. Towers, M. Bock, S. Martin, Y. Takeuchi, J. P. Stoye, and O. Microbiology and Immunology, vol. 281, pp. 29–106, 2003. Danos, “A conserved mechanism of retrovirus restriction in [97] H. M. Temin and H. Rubin, “Characteristics of an assay for mammals,” Proceedings of the National Academy of Sciences of Rous sarcoma virus and Rous sarcoma cells in tissue culture,” the United States of America, vol. 97, no. 22, pp. 12295–12299, Virology, vol. 6, no. 3, pp. 669–688, 1958. [98] W. P. Rowe,W.E.Pugh, andJ.W.Hartley,“Plaque assay [114] R. S. Harris and M. T. Liddament, “Retroviral restriction by techniques for murine leukemia viruses,” Virology, vol. 42, APOBEC proteins,” Nature Reviews Immunology, vol. 4, no. no. 4, pp. 1136–1139, 1970. 11, pp. 868–877, 2004. 14 Advances in Virology [115] R. S. LaRue, V. Andresd ´ ottir ´ , Y. Blanchard et al., “Guidelines for naming nonprimate APOBEC3 genes and proteins,” Journal of Virology, vol. 83, no. 2, pp. 494–497, 2009. [116] A. Low, C. M. Okeoma, N. Lovsin et al., “Enhanced replica- tion and pathogenesis of Moloney murine leukemia virus in mice defective in the murine APOBEC3 gene,” Virology, vol. 385, no. 2, pp. 455–463, 2009. [117] C. M. Okeoma, N. Lovsin, B. M. Peterlin, and S. R. Ross, “APOBEC3 inhibits mouse mammary tumour virus replica- tion in vivo,” Nature, vol. 445, no. 7130, pp. 927–930, 2007. [118] R. K. Holmes, M. H. Malim, and K. N. Bishop, “APOBEC- mediated viral restriction: not simply editing?” Trends in Biochemical Sciences, vol. 32, no. 3, pp. 118–128, 2007. [119] J. L. Mbisa, R. Barr, J. A. Thomas et al., “Human immun- odeficiency virus type 1 cDNAs produced in the presence of APOBEC3G exhibit defects in plus-strand DNA transfer and integration,” Journal of Virology, vol. 81, no. 13, pp. 7099– 7110, 2007. [120] M. A. Langlois, K. Kemmerich, C. Rada, and M. S. Neuberger, “The AKV murine leukemia virus is restricted and hypermu- tated by mouse APOBEC3,” Journal of Virology, vol. 83, no. 22, pp. 11550–11559, 2009. [121] T. Paprotka, N. J. Venkatachari, C. Chaipan et al., “Inhibi- tion of xenotropic murine leukemia virus-related virus by APOBEC3 proteins and antiviral drugs,” Journal of Virology, vol. 84, no. 11, pp. 5719–5729, 2010. [122] K. Stieler and N. Fischer, “Apobec 3G efficiently reduces infectivity of the human exogenous gammaretrovirus XMRV,” PLoS ONE, vol. 5, no. 7, Article ID e11738, 2010. [123] E. P. Browne and D. R. Littman, “Species-specific restriction of Apobec3-mediated hypermutation,” Journal of Virology, vol. 82, no. 3, pp. 1305–1313, 2008. [124] A. Abudu, A. Takaori-Kondo, T. Izumi et al., “Murine retrovirus escapes from murine APOBEC3 via two distinct novel mechanisms,” Current Biology, vol. 16, no. 15, pp. 1565–1570, 2006. [125] S. J. D. Neil, T. Zang, and P. D. Bieniasz, “Tetherin inhibits retrovirus release and is antagonized by HIV-1 Vpu,” Nature, vol. 451, no. 7177, pp. 425–430, 2008. [126] C. Goffinet, S. Schmidt, C. Kern, L. Oberbremer, and O. T. Keppler, “Endogenous CD317/tetherin limits replication of HIV-1 and murine leukemia virus in rodent cells and is resistant to antagonists from primate viruses,” Journal of Virology, vol. 84, no. 21, pp. 11374–11384, 2010. [127] D. T. Evans, R. Serra-Moreno, R. K. Singh, and J. C. Guatelli, “BST-2/tetherin: a new component of the innate immune response to enveloped viruses,” Trends in Microbiology, vol. 18, no. 9, pp. 388–396, 2010. [128] S. P. Goff, “Host factors exploited by retroviruses,” Nature Reviews Microbiology, vol. 5, no. 4, pp. 253–263, 2007. 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