Vif Is Largely Absent from Human Immunodeficiency Virus Type 1 Mature Virions and Associates Mainly with Viral Particles Containing Unprocessed Gag

131

122

The Vif protein of human immunodeficiency virus type 1 (HIV-1) is essential for viral evasion of the host antiviral protein APOBEC3G, also known as CEM15. Vif mutant but not wild-type HIV-1 viruses produced in the presence of APOBEC3G have been shown to undergo hypermutations in newly synthesized viral DNA upon infection of target cells, presumably resulting from C-to-U modification during minus-strand viral DNA synthesis. We now report that HIV-1 Vif could induce rapid degradation of human APOBEC3G that was blocked by the proteasome inhibitor MG132. The efficiency of Vif-induced downregulation of APOBEC3G expression depended on the level of Vif expression. A single amino acid substitution in the conserved SLQXLA motif reduced Vif function. Vif proteins from distantly related primate lentiviruses such as SIVagm were unable to suppress the antiviral activity of human APOBEC3G or the packaging of APOBEC3G into HIV-1 Vif mutant virions, due to a lack of interaction with human APOBEC3G. In the presence of the proteasome inhibitor MG132, virion-associated Vif increased dramatically. However, increased virion packaging of Vif did not prevent virion packaging of APOBEC3G when proteasome function was impaired, and the infectivity of these virions was significantly reduced. These results suggest that Vif function is required during virus assembly to remove APOBEC3G from packaging into released virions. Once packaged, virion-associated Vif could not efficiently block the antiviral activity of APOBEC3G.

Section: Discussionsupporting

confidence: 73%

Influence of Primate Lentiviral Vif and Proteasome Inhibitors on Human Immunodeficiency Virus Type 1 Virion Packaging of APOBEC3G

Liu

Luo

et al. 2004

131

122

“…The packaging efficiency of Vif is therefore comparable to that of Env (58). In addition, the results from our study offer a possible explanation for the apparent paucity of virus-associated Vif reported in previous studies (14,54), which could be accounted for by the gradual processing of full-length virus-associated Vif into a short C-terminal fragment (p7‫,)ء‬ carrying the major antigenic epitope, and a longer N-terminal fragment that is generally not or only very inefficiently recognized by available polyclonal antibodies or MAbs (not shown). Thus, our results suggest that packaging of Vif occurs at levels that match or exceed those of reverse transcriptase or integrase, supporting the notion that packaging of Vif might be functionally relevant.…”

Section: Discussionsupporting

confidence: 68%

Intravirion Processing of the Human Immunodeficiency Virus Type 1 Vif Protein by the Viral Protease May Be Correlated with Vif Function

Khan

Akari

Kao

et al. 2002

The human immunodeficiency virus type 1 (HIV-1) Vif protein is specifically packaged into virus particles through an interaction with viral genomic RNA in which it associates with the viral nucleoprotein complex. We now demonstrate for the first time that virus-associated Vif is subject to proteolytic processing by the viral protease (Pr). Pr-dependent processing of Vif was observed both in vivo and in vitro. In vivo processing of Vif was cell type independent and evident by the appearance of a 7-kDa processing product, which was restricted to cell-free virus preparations. Processing of Vif required an active viral Pr and was sensitive to Pr inhibitors such as ritonavir. The processing site in Vif was characterized both in vivo and in vitro and mapped to Ala 150 . Interestingly, the Vif processing site is located in a domain that is highly conserved among HIV-1, HIV-2, and simian immunodeficiency virus Vif isolates. Mutations at or near the processing site did not affect protein stability or packaging efficiency but had dramatic effects on Vif processing. In general, mutations that markedly increased or decreased the sensitivity of Vif to proteolytic processing severely impaired or completely abolished Vif function. In contrast, mutations at the same site that had little or no effect on processing efficiency also did not influence Vif function. None of the mutants affected the ability of the virus to replicate in permissive cell lines. Our data suggest that mutations in Vif that cause a profound change in the sensitivity to Pr-dependent processing also severely impaired Vif function, suggesting that intravirion processing of Vif is important for the production of infectious viruses.

“…3) (6,11,22,38,39,42,61,64), even after removal of contaminating vesicles and extravirion proteins by subtilisin treatment. Although a consensus that Vif is associated with viral cores is generally emerging (11,39,42), the functional relevance of Vif packaging remains unresolved.…”

Section: Vol 77 2003mentioning

confidence: 99%

Comprehensive Investigation of the Molecular Defect in vif -Deficient Human Immunodeficiency Virus Type 1 Virions

Gaddis

Chertova

Sheehy

et al. 2003

Replication of human immunodeficiency virus type 1 (HIV-1) in primary blood lymphocytes, certain T-cell lines (nonpermissive cells), and most likely in vivo is highly dependent on the virally encoded Vif protein.Evidence suggests that Vif acts late in the viral life cycle during assembly, budding, and/or maturation to counteract the antiviral activity of the CEM15 protein and possibly other antiviral factors. Because HIV-1 virions produced in the absence of Vif are severely restricted at a postentry, preintegration step of infection, it is presumed that such virions differ from wild-type virions in some way. In the present study, we established a protocol for producing large quantities of vif-deficient HIV-1 (HIV-1/⌬vif) from an acute infection of nonpermissive T cells and performed a thorough examination of the defect in these virions. Aside from the expected lack of Vif, we observed no apparent abnormalities in the packaging, modification, processing, or function of proteins in ⌬vif virions. In addition, we found no consistent defect in the ability of ⌬vif virions to perform intravirion reverse transcription under a variety of assay conditions, suggesting that the reverse transcription complexes in these particles can behave normally under cell-free conditions. Consistent with this finding, neither the placement of the primer tRNA 3 Lys nor its ability to promote reverse transcription in an in vitro assay was affected by a lack of Vif. Based on the inability of this comprehensive analysis to uncover molecular defects in ⌬vif virions, we speculate that such defects are likely to be subtle and/or rare.