Vif is largely absent from human immunodeficiency virus type 1 mature virions and associates mainly with viral particles containing unprocessed gag

HIV infection model of chronic obstructive pulmonary disease in mice

Cigarette smoke usage is prevalent in human immunodeficiency virus (HIV)-positive patients, and, despite highly active antiretroviral therapy, these individuals develop an accelerated form of chronic obstructive pulmonary disease (COPD). Studies investigating the mechanisms of COPD development in HIV have been limited by the lack of suitable mouse models. Here we describe a model of HIV-induced COPD in wild-type mice using EcoHIV, a chimeric HIV capable of establishing chronic infection in immunocompetent mice. A/J mice were infected with EcoHIV and subjected to whole body cigarette smoke exposure. EcoHIV was detected in alveolar macrophages of mice. Compared with uninfected mice, concomitant EcoHIV infection significantly reduced forced expiratory flow 50%/forced vital capacity and enhanced distal airspace enlargement following cigarette smoke exposure. Lung IL-6, granulocyte-macrophage colony-stimulating factor, neutrophil elastase, cathepsin G, and matrix metalloproteinase-9 expression was significantly enhanced in smoke-exposed EcoHIV-infected mice. These changes coincided with enhanced IκBα, ERK1/2, p38, and STAT3 phosphorylation and lung cell apoptosis. Thus, the EcoHIV smoke exposure mouse model reproduces several of the pathophysiological features of HIV-related COPD in humans, indicating that this murine model can be used to determine key parameters of HIV-related COPD and to test future therapies for this disorder.

HIV Genome-Wide Protein Associations: a Review of 30 Years of Research

The HIV genome encodes a small number of viral proteins (i.e., 16), invariably establishing cooperative associations among HIV proteins and between HIV and host proteins, to invade host cells and hijack their internal machineries. As a known example, the HIV envelope glycoprotein GP120 is closely associated with GP41 for viral entry. From a genome-wide perspective, a hypothesis can be worked out to determine whether 16 HIV proteins could develop 120 possible pairwise associations either by physical interactions or by functional associations mediated via HIV or host molecules. Here, we present the first systematic review of experimental evidence on HIV genome-wide protein associations using a large body of publications accumulated over the past 3 decades. Of 120 possible pairwise associations between 16 HIV proteins, at least 34 physical interactions and 17 functional associations have been identified. To achieve efficient viral replication and infection, HIV protein associations play essential roles (e.g., cleavage, inhibition, and activation) during the HIV life cycle. In either a dispensable or an indispensable manner, each HIV protein collaborates with another viral protein to accomplish specific activities that precisely take place at the proper stages of the HIV life cycle. In addition, HIV genome-wide protein associations have an impact on anti-HIV inhibitors due to the extensive cross talk between drug-inhibited proteins and other HIV proteins. Overall, this study presents for the first time a comprehensive overview of HIV genome-wide protein associations, highlighting meticulous collaborations between all viral proteins during the HIV life cycle.

HIV Genome-Wide Protein Associations: a Review of 30 Years of Research

The HIV genome encodes a small number of viral proteins (i.e., 16), invariably establishing cooperative associations among HIV proteins and between HIV and host proteins, to invade host cells and hijack their internal machineries. As a known example, the HIV envelope glycoprotein GP120 is closely associated with GP41 for viral entry. From a genome-wide perspective, a hypothesis can be worked out to determine whether 16 HIV proteins could develop 120 possible pairwise associations either by physical interactions or by functional associations mediated via HIV or host molecules. Here, we present the first systematic review of experimental evidence on HIV genome-wide protein associations using a large body of publications accumulated over the past 3 decades. Of 120 possible pairwise associations between 16 HIV proteins, at least 34 physical interactions and 17 functional associations have been identified. To achieve efficient viral replication and infection, HIV protein associations play essential roles (e.g., cleavage, inhibition, and activation) during the HIV life cycle. In either a dispensable or an indispensable manner, each HIV protein collaborates with another viral protein to accomplish specific activities that precisely take place at the proper stages of the HIV life cycle. In addition, HIV genome-wide protein associations have an impact on anti-HIV inhibitors due to the extensive cross talk between drug-inhibited proteins and other HIV proteins. Overall, this study presents for the first time a comprehensive overview of HIV genome-wide protein associations, highlighting meticulous collaborations between all viral proteins during the HIV life cycle.

HIV-1 infection of T cells and macrophages are differentially modulated by virion-associated Hck: a Nef-dependent phenomenon

The proline repeat motif (PxxP) of Nef is required for interaction with the SH3 domains of macrophage-specific Src kinase Hck. However, the implication of this interaction for viral replication and infectivity in macrophages and T lymphocytes remains unclear. Experiments in HIV-1 infected macrophages confirmed the presence of a Nef:Hck complex which was dependent on the Nef proline repeat motif. The proline repeat motif of Nef also enhanced both HIV-1 infection and replication in macrophages, and was required for incorporation of Hck into viral particles. Unexpectedly, wild-type Hck inhibited infection of macrophages, but Hck was shown to enhance infection of primary T lymphocytes. These results indicate that the interaction between Nef and Hck is important for Nef-dependent modulation of viral infectivity. Hck-dependent enhancement of HIV-1 infection of T cells suggests that Nef-Hck interaction may contribute to the spread of HIV-1 infection from macrophages to T cells by modulating events in the producer cell, virion and target cell.

Features, processing states, and heterologous protein interactions in the modulation of the retroviral nucleocapsid protein function

Retroviral nucleocapsid (NC) is central to viral replication. Nucleic acid chaperoning is a key function for NC through the action of its conserved basic amino acids and zinc-finger structures. NC manipulates genomic RNA from its packaging in the producer cell to reverse transcription into the infected host cell. This chaperone function, in conjunction with NC's aggregating properties, is up-modulated by successive NC processing events, from the Gag precursor to the fully mature protein, resulting in the condensation of the nucleocapsid within the capsid shell. Reverse transcription also depends on NC processing, whereas this process provokes NC dissociation from double-stranded DNA, leading to a preintegration complex (PIC), competent for host chromosomal integration. In addition NC interacts with cellular proteins, some of which are involved in viral budding, and also with several viral proteins. All of these properties are reviewed here, focusing on HIV-1 as a paradigmatic reference and highlighting the plasticity of the nucleocapsid architecture.

Lentivirus-associated MAPK/ERK2 phosphorylates EMD and regulates infectivity

Infection of a cell by lentiviruses, such as human immunodeficiency virus type 1 or feline immunodeficiency virus, results in the formation of a reverse transcription complex, the pre-integration complex (PIC), where viral DNA is synthesized. In non-dividing cells, efficient nuclear translocation of the PIC requires the presence of the inner nuclear lamina protein emerin (EMD). Here, we demonstrate that EMD phosphorylation is induced early after infection in primary non-dividing cells. Furthermore, we demonstrate that EMD phosphorylation is dependent on virion-associated mitogen-activated protein kinase (MAPK). Specific inhibition of MAPK activity with kinase inhibitors markedly reduced EMD phosphorylation and resulted in decreased integration of the proviral DNA into chromatin. Similarly, when a MEK1 kinase-inactive mutant was expressed in virus-producer cells, virus-induced phosphorylation of EMD was impaired and viral integration reduced during the subsequent infection. Expression of constitutively active MEK1 kinase in producer cells did not result in modulation of EMD phosphorylation or viral integration during subsequent infection. These studies demonstrate that, in addition to phosphorylating components of the PICs at an early step of infection, virion-associated MAPK plays a role in facilitating cDNA integration after nuclear translocation through phosphorylation of target-cell EMD.

Tumultuous relationship between the human immunodeficiency virus type 1 viral infectivity factor (Vif) and the human APOBEC-3G and APOBEC-3F restriction factors

The viral infectivity factor (Vif) is dispensable for human immunodeficiency virus type 1 (HIV-1) replication in so-called permissive cells but is required for replication in nonpermissive cell lines and for pathogenesis. Virions produced in the absence of Vif have an aberrant morphology and an unstable core and are unable to complete reverse transcription. Recent studies demonstrated that human APOBEC-3G (hA3G) and APOBEC-3F (hA3F), which are selectively expressed in nonpermissive cells, possess strong anti-HIV-1 activity and are sufficient to confer a nonpermissive phenotype. Vif induces the degradation of hA3G and hA3F, suggesting that its main function is to counteract these cellular factors. Most studies focused on the hypermutation induced by the cytidine deaminase activity of hA3G and hA3F and on their Vif-induced degradation by the proteasome. However, recent studies suggested that several mechanisms are involved both in the antiviral activity of hA3G and hA3F and in the way Vif counteracts these antiviral factors. Attempts to reconcile the studies involving Vif in virus assembly and stability with these recent findings suggest that hA3G and hA3F partially exert their antiviral activity independently of their catalytic activity by destabilizing the viral core and the reverse transcription complex, possibly by interfering with the assembly and/or maturation of the viral particles. Vif could then counteract hA3G and hA3F by excluding them from the viral assembly intermediates through competition for the viral genomic RNA, by regulating the proteolytic processing of Pr55(Gag), by enhancing the efficiency of the reverse transcription process, and by inhibiting the enzymatic activities of hA3G and hA3F.

The HIV-1 Vif protein mediates degradation of Vpr and reduces Vpr-induced cell cycle arrest

Prior work has implicated viral protein R (Vpr) in the arrest of human immunodeficiency virus type 1 (HIV-1)-infected cells in the G2 phase of the cell cycle, associated with increased viral replication and host cell apoptosis. We and others have recently shown that virion infectivity factor (Vif ) also plays a role in the G2 arrest of HIV-1-infected cells. Here, we demonstrate that, paradoxically, at early time points postinfection, Vif expression blocks Vpr-mediated G2 arrest, while deletion of Vif from the HIV-1 genome leads to a marked increase in G2 arrest of infected CD4 T-cells. Consistent with this increased G2 arrest, T-cells infected with Vif-deleted HIV-1 express higher levels of Vpr protein than cells infected with wild-type virus. Further, expression of exogenous Vif inhibits the expression of Vpr, associated with a decrease in G2 arrest of both infected and transfected cells. Treatment with the proteasome inhibitor MG132 increases Vpr protein expression and G2 arrest in wild-type, but not Vif-deleted, NL4-3-infected cells, and in cells cotransfected with Vif and Vpr. In addition, Vpr coimmunoprecipitates with Vif in cotransfected cells in the presence of MG132. This suggests that inhibition of Vpr by Vif is mediated at least in part by proteasomal degradation, similar to Vif-induced degradation of APOBEC3G. Together, these data show that Vif mediates the degradation of Vpr and modulates Vpr-induced G2 arrest in HIV-1-infected T-cells.

Human immunodeficiency virus 1 (HIV-1) virion infectivity factor (Vif) is part of reverse transcription complexes and acts as an accessory factor for reverse transcription

Virion infectivity factor (Vif) facilitates HIV infection by counteracting APOBEC3G late in replication in virus-producer cells. Here, we show that early after infection of new target cells Vif is part of the HIV reverse transcription machinery and acts as an accessory factor for reverse transcription. Vif protein was present in gradient fractions containing reverse transcription complexes (RTCs), and anti-Vif antibody immunoprecipitated HIV reverse transcription products from these gradient fractions. To investigate a role for Vif in RTCs independent of APOBEC3G, we created an intracellular environment that would restrict reverse transcription by pre-treating permissive target cells with 5-Fluoro 2-deoxyuridine, a thymidylate synthetase inhibitor, prior to infection with virus from permissive cells. Infectivity assays and quantitation of reverse transcription products demonstrated that replication of HIV lacking Vif was inhibited to a greater degree than wild type, without concurrent mutation of reverse transcription products, suggesting compromised reverse transcription in the absence of Vif.

Proteomic Analysis of Human Immunodeficiency Virus Using Liquid Chromatography/Tandem Mass Spectrometry Effectively Distinguishes Specific Incorporated Host Proteins

A major challenge to studying virus-incorporated host proteins is the fact that they are not encoded by the viral genome. We used Liquid Chromatography/Tandem Mass Spectrometry (LC-MS/MS) on whole virions to obtain a snapshot of the HIV-1 proteome. We identified known viral and host-cellular proteins and also identified novel components of HIV-1 and confirm these by traditional biochemical methods. Our comparison of wild-type and mutant viruses demonstrates that LC-MS/MS has the specificity to distinguish the presence/absence of a single host protein in intact virions.

Nuclear localization of HIV type 1 Vif isolated from a long-term asymptomatic individual and potential role in virus attenuation

Recent reports have determined that HIV-1 Vif counteracts an innate antiviral cellular factor, Apobec3G. However, the function of Vif during HIV-1 pathogenesis remains poorly understood. To gain a better understanding of Vif function, the viral isolate from an HIV-1-infected long-term nonprogressor (LTNP) that displayed a Vif-mutant replication phenotype was studied. This LTNP has been infected since before 1983 and has no HIV-related disease in the absence of antiretroviral therapy. From separate samples, obtained on more than one study visit, virus grew in cocultures of LTNP cells with Vif-complementing T cell lines, but not the parental T cell lines. An unusual amino acid motif (KKRK) was found in the Vif sequence at positions 90 to 93. Since this motif commonly functions as a nuclear localization sequence, experiments were performed to determine the ability of this KKRK motif to mediate nuclear localization of Vif. Wild-type Vif displayed a predominantly cytoplasmic distribution. In contrast, the KKRK Vif showed a predominantly nuclear localization. The effect of the KKRK mutation on virus production and infectivity was also studied. The KKRK motif that mislocalizes Vif to the nucleus also reduces viral replication and infectivity in nonpermissive cells. Our data highlight the importance of Vif in HIV-1 pathogenesis and also provide a unique tool to investigate the interaction of Vif and Apobec3G.

HIV-1 incorporates and proteolytically processes human NDR1 and NDR2 serine-threonine kinases

Mammalian genomes encode two related serine-threonine kinases, nuclear Dbf2 related (NDR)1 and NDR2, which are homologous to the Saccharomyces cerevisiae Dbf2 kinase. Recently, a yeast genetic screen implicated the Dbf2 kinase in Ty1 retrotransposition. Since several virion-incorporated kinases regulate the infectivity of human immunodeficiency virus type 1 (HIV-1), we speculated that the human NDR1 and NDR2 kinases might play a role in the HIV-1 life cycle. Here we show that the NDR1 and NDR2 kinases were incorporated into HIV-1 particles. Furthermore, NDR1 and NDR2 were cleaved by the HIV-1 protease (PR), both within virions and within producer cells. Truncation at the PR cleavage site altered NDR2 subcellular localization and inhibited NDR1 and NDR2 enzymatic activity. These studies identify two new virion-associated host cell enzymes and suggest a novel mechanism by which HIV-1 alters the intracellular environment of human cells.

The Interaction between HIV-1 Gag and APOBEC3G

APOBEC3G, a member of an RNA/DNA cytidine deaminase superfamily, has been identified as a cellular inhibitor of HIV-1 infectivity, possibly through the dC to dU deamination of the first minus strand cDNA synthesized during reverse transcription. Virions incorporate APOBEC3G during viral assembly in non-permissive cells, and this incorporation is inhibited by the viral protein Vif. The mechanism of APOBEC3G incorporation into HIV-1 is examined in this report. In the absence of Vif, cytoplasmic APOBEC3G becomes membrane-bound in cells expressing HIV-1 Gag, and its incorporation into Gag viral-like particles (VLPs) is proportional to the amount of APOBEC3G expressed in the cell. The expression of Vif, or mutant Gag unable to bind to membrane, prevents the APOBEC3G association with membrane. HIV-1 Gag alone among viral proteins is sufficient for packaging of APOBEC3G into Gag VLPs, and this incorporation requires the presence of Gag nucleocapsid. The presence of amino acids 104-156 in APOBEC3G, located in the linker region between two zinc coordination motifs, is also required for its incorporation into Gag VLPs. Evidence against an RNA bridge facilitating the Gag/APOBEC3G interaction includes data indicating that 1) the incorporation of APOBEC3G occurs independently of viral genomic RNA, 2) a Gag/APOBEC3G complex is immunoprecipitated from cell lysate after RNase treatment, and 3) the zinc coordination motif, rather than the regions flanking this motif, have been implicated in RNA binding in another family member, APOBEC1.

Influence of primate lentiviral Vif and proteasome inhibitors on human immunodeficiency virus type 1 virion packaging of APOBEC3G

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.

Comprehensive investigation of the molecular defect in vif-deficient human immunodeficiency virus type 1 virions

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/Delta 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 Delta vif virions. In addition, we found no consistent defect in the ability of Delta 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 tRNA3Lys 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 Delta vif virions, we speculate that such defects are likely to be subtle and/or rare.

The role of Vif during HIV-1 infection: interaction with novel host cellular factors

BACKGROUND

Current research suggests that human immunodeficiency virus type-1 (HIV-1) virion infectivity factor (Vif) acts during viral assembly in producer cells to ensure infectivity in target cells but the exact mechanism of action has not been defined. Vif interacts with Gag, viral protease and RNA and these interactions are proposed to be important for correct particle assembly and stability of the reverse transcription complex.

OBJECTIVES

The existence of cells that are either permissive or non-permissive for replication of Vif deficient viruses suggests the involvement of host cellular factors in its function. Current research suggests an association of Vif with the intermediate filament protein, vimentin, and the tyrosine kinase, Hck, but the significance of these associations remains to be defined. More recently HP68, a cellular ATP binding protein, has been shown to be important for capsid formation and an interaction between Vif and HP68 has been shown. Our aim was to further identify host cellular factors involved in Vif function.

STUDY DESIGN

We have employed the yeast 2-hybrid system to identify cellular proteins which interact with HIV-1 Vif. Sixteen clones were isolated from a high stringency yeast-2-hybrid screen of a human leucocyte cDNA library with Vif derived from the T-cell tropic HIV-1 strain NL4.3. Of these, 8 clones were confirmed as specifically binding Vif, fully sequenced and identified via GenBank homology searches.

RESULTS

Thus far 3 of these clones, spermine/spermidine N1-acetyltransferase, Triad 3 and a novel gene which we have termed 'novel Vif binding protein', have been characterised and represent attractive candidates for mediating Vif action during HIV replication.

CONCLUSIONS

Through identification and characterisation of cellular factors interacting with HIV-1 Vif we hope to unravel the mechanism of action of Vif which may ultimately aid therapeutic design.

Intravirion processing of the human immunodeficiency virus type 1 Vif protein by the viral protease may be correlated with Vif function

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.

Human immunodeficiency virus type 1 Vif binds the viral protease by interaction with its N-terminal region

The vif gene, one of the six auxiliary genes of human immunodeficiency virus (HIV), is essential for virus propagation in peripheral blood lymphocytes and macrophages and in certain T-cell lines. Previously, it was demonstrated that Vif inhibits the autoprocessing of truncated HIV type 1 (HIV-1) Gag-Pol polyproteins expressed in bacterial cells, as well as the protease-mediated cleavage of synthetic peptides in vitro. Peptides derived from the aa 78-98 region in the Vif molecule specifically inhibit and bind the HIV-1 protease in vitro and arrest the production of infectious viruses in HIV-1-infected cells. This study demonstrates that (i) purified recombinant Vif protein and HIV-1 but not avian sarcoma leukaemia virus protease specifically bind each other and (ii) the interaction between these two proteins takes place at the N terminus of the protease (aa 1-9) and the central part of Vif (aa 78-98). The data presented in this report suggest a model in which Vif interacts with the dimerization sites of the viral protease.

Viral and cellular specificities of caprine arthritis encephalitis virus Vif protein

The caprine arthritis encephalitis virus (CAEV) Vif protein is necessary for a productive infection of susceptible goat cells. The vif gene is conserved among all primate and most nonprimate lentiviruses. However, previous reports demonstrated that, in their respective host cells, primate Vif deleted lentiviruses could not be complemented by nonprimate Vif proteins, suggesting that species-specific restrictions between Vif and the virus-producing cells may modulate the Vif function on viral infectivity. Here we bring further support to this hypothesis since we show that CAEV Vif, when expressed in goat cells, is able to increase the infectivity of Vif deleted human immunodeficiency type-1 virus and of murine leukemia virus. Moreover, we demonstrate in vitro interactions between different Vif proteins and NC domains of heterologous Gag precursors, supporting the notion that species specificity of lentiviral infection is not due to molecular interactions between Vif and viral components.

Mechanistic aspects of HIV-1 reverse transcription initiation

During reverse transcription, the positive-strand HIV-1 RNA genome is converted into a double-stranded DNA copy which can be permanently integrated into the host cell genome. Recent analyses show that HIV-1 reverse transcription is a highly regulated process. The initiation reaction can be distinguished from a subsequent elongation reaction carried out by a reverse transcription complex composed of (at least) heterodimeric reverse transcriptase, cellular tRNA(lys3) and HIV-1 genomic RNA sequences. In addition, viral factors including Tat, Nef, Vif, Vpr, IN and NCp7, cellular proteins, and TAR RNA and other RNA stem-loop structures appear to influence this complex and contribute to the efficiency of the initiation reaction. As viral resistance to many antiretroviral compounds is a continuing problem, understanding the ways in which these factors influence the reverse transcription complex will likely lead to novel antiretroviral strategies.

Interaction of human immunodeficiency virus type 1 Vif with Gag and Gag-Pol precursors: co-encapsidation and interference with viral protease-mediated Gag processing

Interactions of human immunodeficiency virus type 1 (HIV-1) Vif protein with various forms of Gag and Gag-Pol precursors expressed in insect cells were investigated in vivo and in vitro by co-encapsidation, co-precipitation and viral protease (PR)-mediated Gag processing assays. Addressing of Gag to the plasma membrane, its budding as extracellular virus-like particles (VLP) and the presence of the p6 domain were apparently not required for Vif encapsidation, as non-N-myristoylated Deltap6-Gag and Vif proteins were co-encapsidated into intracellular VLP. Encapsidation of Vif occurred at significantly higher copy numbers in extracellular VLP formed from N-myristoylated, budding-competent Gag-Pol precursors harbouring an inactive PR domain or in chimaeric VLP composed of Gag and Gag-Pol precursors compared with the Vif content of Pr55Gag VLP. Vif encapsidation efficiency did not seem to correlate directly with VLP morphology, since these chimaeric VLP were comparable in size and shape to Pr55Gag VLP. Vif apparently inhibited PR-mediated Pr55Gag processing in vitro, with preferential protection of cleavage sites at the MA-CA and CA-NC junctions. Vif was resistant to PR action in vitro under conditions that allowed full Gag processing, and no direct interaction between Vif and PR was detected in vivo or in vitro. This suggested that inhibition by Vif of PR-mediated Gag processing resulted from interaction of Vif with the Gag substrate and not with the enzyme. Likewise, the higher efficiency of Vif encapsidation by Gag-Pol precursor compared with Pr55Gag was probably not mediated by direct binding of Vif to the Gag-Pol-embedded PR domain, but more likely resulted from a particular conformation of the Gag structural domains of the Gag-Pol precursor.