title: Moloney murine leukemia virus glyco-gag facilitates xenotropic murine leukemia virus-related virus replication through human APOBEC3-independent mechanisms.
abstract: BACKGROUND
  One of the unique features of gammaretroviruses is that they contain an additional extended form of Gag, glyco-gag, which initiates in the leader sequence. MuLV glyco-gag, gPr80Gag, promotes retrovirus replication and disease progression. Although virtually all infectious MuLVs encode glyco-gag, XMRV (xenotropic murine leukemia virus-related virus) lacks the classical gPr80Gag sequence. We examined XMRV to determine if its leader sequence contains glyco-gag activity, whether the presence of conventional gPr80Gag affects replication of XMRV, and we describe the evolution of glyco-gag-deficient MuLVs in Mus.
  
  
  RESULTS
  We introduced several mutations disrupting two putative but noncanonical glyco-gag proteins in the leader sequence region in XMRV and found that those mutations did not affect virus release nor susceptibility to the antiviral activity of hA3G (human APOBEC3G). A chimeric XMRV encoding the Moloney MuLV (M-MuLV) leader sequence (MXMRV) demonstrated that M-MuLV glyco-gag facilitated MXMRV release and increased infectivity. Infectivity assays with several cell lines showed that glyco-gag increases XMRV infectivity in all cell lines tested, but the level of this increase varies in different cell lines. Because MuLV glyco-gag counteracts mouse APOBEC3, we investigated whether M-MuLV glyco-gag enhances XMRV infection by counteracting human APOBEC3. Comparison of hAPOBEC3 isoforms expressed in different cell lines indicated that hA3B was the most likely candidate for a restrictive hA3. However over-expression of hA3B showed no enhanced restriction of infection by XMRV compared to MXMRV. Endogenous MuLVs in the sequenced mouse genome were screened for canonical glyco-gag, which was identified in two clades of xenotropic MuLVs (X-MuLVs) and ecotropic MuLVs, but not in other X-MuLVs or in any polytropic MuLVs.
  
  
  CONCLUSIONS
  M-MuLV glyco-gag facilitates XMRV replication, and the leader sequence region in XMRV does not encode proteins equivalent to M-MuLV glyco-gag. The fact that the ability of glyco-gag to enhance XMRV infection varies in different cell lines suggests a glyco-gag sensitive restrictive factor that further reduces XMRV infectivity. The M-MuLV glyco-gag enhancement for XMRV replication is through a hAPOBEC3 independent mechanism. The absence of glyco-gag in MuLVs carried by western European mice suggests that loss of this sequence is a relatively recent event with limited subspecies distribution.
authors: Department of Molecular Biology and Biochemistry and Cancer Research Institute, University of California, Irvine, CA 92697-3905, USA.
authors: Takayuki
authors: T
authors: Nitta
authors: Sangouk
authors: S
authors: Lee
authors: Dat
authors: D
authors: Ha
authors: Maribel
authors: M
authors: Arias
authors: Christine A
authors: CA
authors: Kozak
authors: Hung
authors: H
authors: Fan
mesh_terms: D000071478:APOBEC Deaminases; D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D003564:Cytidine Deaminase; D043525:Cytosine Deaminase / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D019143:Evolution, Molecular; D015683:Gene Products, gag / Q000145:classification / Q000235:genetics / Q000378:metabolism*; D016679:Genome, Viral; D006023:Glycoproteins / Q000235:genetics / Q000378:metabolism*; D006031:Glycosylation; D057809:HEK293 Cells; D006367:HeLa Cells; D056945:Hep G2 Cells; D006801:Humans; D007527:Isoenzymes / Q000235:genetics / Q000378:metabolism; D051379:Mice; D008969:Molecular Sequence Data; D008979:Moloney murine leukemia virus / Q000235:genetics / Q000378:metabolism*; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D010802:Phylogeny; D051381:Rats; D057074:Virus Release; D014779:Virus Replication; D058008:Xenotropic murine leukemia virus-related virus / Q000235:genetics / Q000378:metabolism / Q000502:physiology*
chemical_list: D015683:Gene Products, gag; D006023:Glycoproteins; D007527:Isoenzymes; D043525:Cytosine Deaminase; D000071478:APOBEC Deaminases; C502889:APOBEC3 proteins, human; D003564:Cytidine Deaminase