Accessory Protein Vif Research Articles

Local Sequence Targeting in the AID/APOBEC Family Differentially Impacts Retroviral Restriction and Antibody Diversification

Nucleic acid cytidine deaminases of the activation-induced deaminase (AID)/APOBEC family are critical players in active and innate immune responses, playing roles as target-directed, purposeful mutators. AID specifically deaminates the host immunoglobulin (Ig) locus to evolve antibody specificity, whereas its close relative, APOBEC3G (A3G), lethally mutates the genomes of retroviral pathogens such as HIV. Understanding the basis for the target-specific action of these enzymes is essential, as mistargeting poses significant risks, potentially promoting oncogenesis (AID) or fostering drug resistance (A3G). AID prefers to deaminate cytosine in WRC (W = A/T, R = A/G) motifs, whereas A3G favors deamination of CCC motifs. This specificity is largely dictated by a single, divergent protein loop in the enzyme family that recognizes the DNA sequence. Through grafting of this substrate-recognition loop, we have created enzyme variants of A3G and AID with altered local targeting to directly evaluate the role of sequence specificity on immune function. We find that grafted loops placed in the A3G scaffold all produced efficient restriction of HIV but that foreign loops in the AID scaffold compromised hypermutation and class switch recombination. Local targeting, therefore, appears alterable for innate defense against retroviruses by A3G but important for adaptive antibody maturation catalyzed by AID. Notably, AID targeting within the Ig locus is proportionally correlated to its in vitro ability to target WRC sequences rather than non-WRC sequences. Although other mechanisms may also contribute, our results suggest that local sequence targeting by AID/APOBEC3 enzymes represents an elegant example of co-evolution of enzyme specificity with its target DNA sequence.

In-depth analysis of G-to-A hypermutation rate in HIV-1 env DNA induced by endogenous APOBEC3 proteins using massively parallel sequencing

Some APOBEC3 proteins cause G-to-A hypermutation in HIV-1 DNA when the accessory viral protein Vif is absent or non-functional. So far, cloning and sequencing has been performed to study G-to-A hypermutation. This is time-consuming and labour-intensive especially in the context of in vivo investigations where the number of hypermutated sequences can be very low. Thus, a massively parallel sequencing protocol has been developed for in-depth analysis of G-to-A hypermutation using the 454 pyrosequencing FLX system. Part of HIV-1 env was amplified and pyrosequenced after two rounds of infection in T cell lines and PBMCs using HIV-1 NL4-3Δvif. Specific criteria were applied to cope with major technical challenges: (1) the inclusion of hypermutated sequences, (2) the high genome diversity of HIV-1 env, and (3) the exclusion of sequences containing frameshift errors caused by pyrosequencing. In total, more than 140,000 sequences were obtained. 1.3–6.5% of guanines were mutated to adenine, most frequently in the GG dinucleotide context, the preferred deamination site of APOBEC3G. Non-G-to-A mutations occurred only in low frequencies (<0.6%). Single hypermutated sequences contained up to 24 G-to-A mutations. Overall, massively parallel sequencing is a very useful tool for in-depth analysis of G-to-A hypermutation in HIV-1 DNA induced by APOBEC3 proteins.

Lentiviral Vif Degrades the APOBEC3Z3/APOBEC3H Protein of Its Mammalian Host and Is Capable of Cross-Species Activity

All lentiviruses except equine infectious anemia virus (EIAV) use the small accessory protein Vif to counteract the restriction activity of the relevant APOBEC3 (A3) proteins of their host species. Prior studies have suggested that the Vif-A3 interaction is species specific. Here, using the APOBEC3H (Z3)-type proteins from five distinct mammals, we report that this is generally not the case: some lentiviral Vif proteins are capable of triggering the degradation of both the A3Z3-type protein of their normal host species and those of several other mammals. For instance, SIV(mac) Vif can mediate the degradation of the human, macaque, and cow A3Z3-type proteins but not of the sheep or cat A3Z3-type proteins. Maedi-visna virus (MVV) Vif is similarly promiscuous, degrading not only sheep A3Z3 but also the A3Z3-type proteins of humans, macaques, cows, and cats. In contrast to the neutralization capacity of these Vif proteins, human immunodeficiency virus (HIV), bovine immunodeficiency virus (BIV), and feline immunodeficiency virus (FIV) Vif appear specific to the A3Z3-type protein of their hosts. We conclude, first, that the Vif-A3Z3 interaction can be promiscuous and, second, despite this tendency, that each lentiviral Vif protein is optimized to degrade the A3Z3 protein of its mammalian host. Our results thereby suggest that the Vif-A3Z3 interaction is relevant to lentivirus biology.

Definition of the interacting interfaces of Apobec3G and HIV-1 Vif using MAPPIT mutagenesis analysis

The host restriction factor Apobec3G is a cytidine deaminase that incorporates into HIV-1 virions and interferes with viral replication. The HIV-1 accessory protein Vif subverts Apobec3G by targeting it for proteasomal degradation. We propose a model in which Apobec3G N-terminal domains symmetrically interact via a head-to-head interface containing residues 122 RLYYFW 127. To validate this model and to characterize the Apobec3G–Apobec3G and the Apobec3G–Vif interactions, the mammalian protein–protein interaction trap two-hybrid technique was used. Mutations in the head-to-head interface abrogate the Apobec3G–Apobec3G interaction. All mutations that inhibit Apobec3G–Apobec3G binding also inhibit the Apobec3G–Vif interaction, indicating that the head-to head interface plays an important role in the interaction with Vif. Only the D128K, P129A and T32Q mutations specifically affect the Apobec3G–Vif association. In our model, D128, P129 and T32 cluster at the edge of the head-to-head interface, possibly forming a Vif binding site composed of two Apobec3G molecules. We propose that Vif either binds at the Apobec3G head-to-head interface or associates with an RNA-stabilized Apobec3G oligomer.

Antiviral Activity of the Interferon-Induced Cellular Protein BST-2/Tetherin

Pathogenic microorganisms encode proteins that antagonize specific aspects of innate or adaptive immunity. Just as the study of the HIV-1 accessory protein Vif led to the identification of cellular cytidine deaminases as host defense proteins, the study of HIV-1 Vpu recently led to the discovery of the interferon-induced transmembrane protein BST-2 (CD317; tetherin) as a novel component of the innate defense against enveloped viruses. BST-2 is an unusually structured protein that restricts the release of fully formed progeny virions from infected cells, presumably by a direct retention mechanism that is independent of any viral protein target. Its spectrum of activity includes at least four virus families: retroviruses, filoviruses, arenaviruses, and herpesviruses. Viral antagonists of BST-2 include HIV-1 Vpu, HIV-2 and SIV Env, SIV Nef, the Ebola envelope glycoprotein, and the K5 protein of KSHV. The mechanisms of antagonism are diverse and currently include viral cooption of cellular endosomal trafficking and protein degradation pathways, including those mediated by ubiquitination. Orthologs of human BST-2 are present in mammals. Primate BST-2 proteins are differentially sensitive to antagonism by lentiviral Vpu and Nef proteins, suggesting that BST-2 has subjected lentiviruses to evolutionary pressure and presents barriers to cross-species transmission. BST-2 functions not only as an effector of the interferon-induced antiviral response but also as a negative feedback regulator of interferon production by plasmacytoid dendritic cells. Future work will focus on the role and regulation of BST-2 during the innate response to viral infection, on the mechanisms of restriction and of antagonism by viral gene products, and on the role of BST-2 in primate lentiviral evolution. The augmentation of BST-2 activity and the inhibition of virally encoded antagonists, in particular Vpu, represent new approaches to the prevention and treatment of HIV-1 infection.

Leveraging APOBEC3 Proteins to Alter the HIV Mutation Rate and Combat AIDS

At least two human APOBEC3 proteins - APOBEC3F and APOBEC3G - are capable of inhibiting HIV-1 replication by mutation of the viral cDNA. HIV-1 averts lethal restriction through its accessory protein Vif, which targets these APOBEC3 proteins for proteasomal degradation. The life-or-death interaction between human APOBEC3 proteins and HIV-1 Vif has stimulated much interest in developing novel therapeutics aimed at altering the deaminase activity of the APOBEC3s, thus changing the virus' mutation rate to either lethal or suboptimal levels. The current state of mechanistic information is reviewed and the possible risks and benefits of increasing (via hypermutation) or decreasing (via hypomutation) the HIV-1 mutation rate through APOBEC3 proteins are discussed.

Mutational analysis of the HIV-1 auxiliary protein Vif identifies independent domains important for the physical and functional interaction with HIV-1 reverse transcriptase

The HIV-1 accessory protein Vif plays a dual role: it counteracts the natural restriction factors APOBEC3G and 3F and ensures efficient retrotranscription of the HIV-1 RNA genome. We have previously shown that Vif can act as an auxiliary factor for HIV-1 reverse transcriptase (RT), increasing its rate of association to RNA or DNA templates. Here, by using seven different Vif mutants, we provide in vitro evidences that Vif stimulates HIV-1 RT through direct protein–protein interaction, which is mediated by its C-terminal domain. Physical interaction appears to require the proline-rich region comprised between amino acid (aa) 161 and 164 of Vif, whereas the RT stimulatory activity requires, in addition, the extreme C-terminal region (aa 169–192) of the Vif protein. Neither the RNA interaction domain, nor the Zn++-binding domain of Vif are required for its interaction with the viral RT. Pseudotyped HIV-1 lentiviral vectors bearing Vif mutants deleted in the RNA- or RT-binding domains show defects in retrotranscription/integration processes in both permissive and nonpermissive cells. Our results broaden our knowledge on how three important functions of Vif (RNA binding, RT binding and stimulation and Zn++ binding), are coordinated by different domains.

Regulation of vif mRNA Splicing by Human Immunodeficiency Virus Type 1 Requires 5′ Splice Site D2 and an Exonic Splicing Enhancer To Counteract Cellular Restriction Factor APOBEC3G

The human immunodeficiency virus type 1 (HIV-1) accessory protein Vif is encoded by an incompletely spliced mRNA resulting from splicing of the major splice donor in the HIV-1 genome, 5' splice site (5'ss) D1, to the first splice acceptor, 3'ss A1. We have shown previously that splicing of HIV-1 vif mRNA is tightly regulated by suboptimal 5'ss D2, which is 50 nucleotides downstream of 3'ss A1; a GGGG silencer motif proximal to 5'ss D2; and an SRp75-dependent exonic splicing enhancer (ESEVif). In agreement with the exon definition hypothesis, mutations within 5'ss D2 that are predicted to increase or decrease U1 snRNP binding affinity increase or decrease the usage of 3'ss A1 (D2-up and D2-down mutants, respectively). In this report, the importance of 5'ss D2 and ESEVif for avoiding restriction of HIV-1 by APOBEC3G (A3G) was determined by testing the infectivities of a panel of mutant viruses expressing different levels of Vif. The replication of D2-down and ESEVif mutants in permissive CEM-SS cells was not significantly different from that of wild-type HIV-1. Mutants that expressed Vif in 293T cells at levels greater than 10% of that of the wild type replicated similarly to the wild type in H9 cells, and Vif levels as low as 4% were affected only modestly in H9 cells. This is in contrast to Vif-deleted HIV-1, whose replication in H9 cells was completely inhibited. To test whether elevated levels of A3G inhibit replication of D2-down and ESEVif mutants relative to wild-type virus replication, a Tet-off Jurkat T-cell line that expressed approximately 15-fold-higher levels of A3G than control Tet-off cells was generated. Under these conditions, the fitness of all D2-down mutant viruses was reduced relative to that of wild-type HIV-1, and the extent of inhibition was correlated with the level of Vif expression. The replication of an ESEVif mutant was also inhibited only at higher levels of A3G. Thus, wild-type 5'ss D2 and ESEVif are required for production of sufficient Vif to allow efficient HIV-1 replication in cells expressing relatively high levels of A3G.

Accessory proteins of HIV and innate anti-retroviral factors

The HIV genome encodes several accessory proteins (Vif, Vpr, Vpx, Vpu, and Nef) unique to primate lentiviruses, in addition to the structural (Gag, Pol, and Env) and regulatory (Tat and Rev) proteins. Early studies showed that deletion of accessory proteins has a small or no effect on virus replication in cell cultures. However, recent studies have clearly demonstrated that these proteins are essential for efficient viral replication, dissemination, pathogenicity, and disease progression. Here, we summarize the current knowledge of HIV accessory proteins and their cellular targets, and discuss the functional roles of these biologically unique and important viral proteins for virus replication in vitro and in vivo.

Regulation of APOBEC3 Proteins by a Novel YXXL Motif in Human Immunodeficiency Virus Type 1 Vif and Simian Immunodeficiency Virus SIVagm Vif

The APOBEC3 cytidine deaminases are potent antiviral factors that restrict the replication of human immunodeficiency virus type 1 (HIV-1). In HIV-1-infected CD4+ T cells, the viral accessory protein Vif binds to APOBEC3G (A3G), APOBEC3F (A3F), and APOBEC3C (A3C) and targets these proteins for polyubiquitination by forming an E3 ubiquitin ligase with cullin 5. Previous studies identified regions of HIV-1 Vif, 40YRHHY44 and 12QVDRMR17, which are important for interaction with A3G and A3F, respectively, and showed that Vif residues 54 to 71 are sufficient for A3G binding. Here, we identify 69YXXL72 as a novel conserved motif in HIV-1 Vif that mediates binding to human A3G and its subsequent degradation. Studies on other APOBEC3 proteins revealed that Tyr69 and Leu72 are important for the degradation of A3F and A3C as well. Similar to A3F, A3C regulation is also mediated by Vif residues 12QVDRMR17. Simian immunodeficiency virus (SIV) Vif was shown to bind and degrade African green monkey A3G (agmA3G) and, unexpectedly, human A3C. The YXXL motif of SIVagm Vif was important for the inactivation of agmA3G and human A3C. Unlike HIV-1 Vif, however, SIVagm Vif does not require Tyr40 and His43 for agmA3G degradation. Tyr69 in the YXXL motif was critical for binding of recombinant glutathione S-transferase-Vif(1-94) to A3G in vitro. These results suggest that the YXXL motif in Vif is a potential target for small-molecule inhibitors to block Vif interaction with A3G, A3F, and A3C, and thereby protect cells against HIV-1 infection.

Should accessory proteins be structural components of lentiviral vaccines?: Lessons learned from the accessory ORF-A protein of FIV

The FIV regulatory protein Rev and accessory proteins Vif and ORF-A are essential for efficient viral replication and full-blown pathogenesis. Expressed at very low level during viral replication, they are nevertheless processed for recognition by cytotoxic T-lymphocytes (CTLs) and trigger cellular immune responses in FIV-infected cats. The observation that the accessory ORF-A protein of FIV is continuously expressed during viral replication and targeted by cellular immune responses in natural FIV infection, prompted us to investigate the protective potential of this protein. To this aim cats were immunized with three different strategies (protein alone in alum adjuvant, DNA alone, or DNA prime-protein boost) and generated clearly detectable immune responses. Upon challenge with ex vivo homologous FIV, ORF-A immunized cats showed distinct enhancement of acute-phase infection possibly due to an increased expression of the FIV receptor CD134. However, at subsequent sampling points plasma viremia was reduced and CD4+ T-lymphocytes in the circulation declined more slowly in ORF-A immunized than in control animals. These findings support the contention that a multicomponent vaccine, with the inclusion of both accessory and structural proteins, can not only improve the host's ability to control lentivirus replication and slow down disease progression but also draw attention to the fact that even simple immunogens that eventually contribute to protective activity can transiently exacerbate subsequent lentiviral infections.

HIV-1 accessory proteins VPR and Vif modulate antiviral response by targeting IRF-3 for degradation

The activation of IRF-3 during the early stages of viral infection is critical for the initiation of the antiviral response; however the activation of IRF-3 in HIV-1 infected cells has not yet been characterized. We demonstrate that the early steps of HIV-1 infection do not lead to the activation and nuclear translocation of IRF-3; instead, the relative levels of IRF-3 protein are decreased due to the ubiquitin-associated proteosome degradation. Addressing the molecular mechanism of this effect we show that the degradation is independent of HIV-1 replication and that virion-associated accessory proteins Vif and Vpr can independently degrade IRF-3. The null mutation of these two genes reduced the capacity of the HIV-1 virus to down modulate IRF-3 levels. The degradation was associated with Vif- and Vpr-mediated ubiquitination of IRF-3 and was independent of the activation of IRF-3. N-terminal lysine residues were shown to play a critical role in the Vif- and Vpr-mediated degradation of IRF-3. These data implicate Vif and Vpr in the disruption of the initial antiviral response and point to the need of HIV-1 to circumvent the antiviral response during the very early phase of replication.

Impact of viral accessory proteins of SIVsmmPBj on early steps of infection of quiescent cells

Although lentiviruses like HIV-1 are able to infect non-dividing cells, particular resting cells such as non-stimulated primary peripheral blood mononuclear cells (PBMC) are resistant to infection. In contrast to other lentiviruses, SIVsmmPBj can replicate in non-stimulated PBMC. Moreover, SIVsmmPBj-derived, but not HIV-1-derived, replication-incompetent vectors enable gene transfer into G 0-arrested human cell lines and primary human monocytes. Here, we demonstrate that transduction of G 0-arrested cell lines by SIVsmmPBj-derived vectors is independent of the viral accessory proteins Vif, Vpx, Vpr, or Nef. In contrast, for the transduction of primary human monocytes, the Vpx protein proved to be essential. However, trans-complementation of HIV-1 vectors with SIVsmmPBj Vpx did not provide the property of gene transfer into monocytes. Taken together, these data indicate that Vpx is essential for the infection of primary monocytes by SIVsmmPBj. Additionally, further genome functions besides the accessory proteins are required for the particular capacity of SIVsmmPBj in transduction or infection events.

Identification of Amino Acid Residues in APOBEC3G Required for Regulation by Human Immunodeficiency Virus Type 1 Vif and Virion Encapsidation

The human immunodeficiency virus type-1 (HIV-1) accessory protein Vif serves to neutralize the human antiviral proteins apolipoprotein B mRNA-editing enzyme, catalytic polypeptide-like 3G (APOBEC3G [A3G]) and A3F. As such, the therapeutic blockade of Vif function represents a logical objective for rational drug design. To facilitate such endeavors, we have employed molecular genetics to define features of A3G that are required for its interaction with Vif. Using alanine-scanning mutations and multiple different substitutions at key residues, we confirm the central role played by the aspartic acid at position 128 and identify proline 129 and aspartic acid 130 as important contributory residues. The overall negative charge of this 3-amino-acid motif appears critical for recognition by Vif, as single lysine substitutions are particularly deleterious and a double alanine substitution at positions 128 and 130 is far more inhibitory than single-residue mutations at either position. Our analyses also reveal that the immediately adjacent 4 amino acids, residues 124 to 127, are important for the packaging of A3G into HIV-1 particles. Most important are tyrosine 124 and tryptophan 127, and mutations at these positions can ablate virion incorporation, as well as the capacity to inhibit virus infection. Thus, while pharmacologic agents that target the acidic motif at residues 128 to 130 have the potential to rescue A3G expression by occluding recognition by Vif, care will have to be taken not to perturb the contributions of the neighboring 124-to-127 region to packaging if such agents are to have therapeutic benefit by promoting A3G incorporation into progeny virions.

Mutational Alteration of Human Immunodeficiency Virus Type 1 Vif Allows for Functional Interaction with Nonhuman Primate APOBEC3G

Human APOBEC3F (hA3F) and APOBEC3G (hA3G) are antiretroviral cytidine deaminases that can be encapsidated during virus assembly to catalyze C-->U deamination of the viral reverse transcripts in the next round of infection. Lentiviruses such as human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) have evolved the accessory protein Vif to induce their degradation before packaging. HIV type 1 (HIV-1) Vif counteracts hA3G but not rhesus macaque APOBEC3G (rhA3G) or African green monkey (AGM) APOBEC3G (agmA3G) because of a failure to bind the nonhuman primate proteins. The species specificity of the interaction is controlled by amino acid 128, which is aspartate in hA3G and lysine in rhA3G. With the objective of overcoming this species restriction, mutations were introduced into HIV-1 Vif at amino acid positions that differed in charge between HIV-1 Vif and SIV Vif. The mutant proteins were tested for the ability to counteract hA3G, rhA3G, and agmA3G. Alteration of the conserved sequence at positions 14 to 17 from DRMR to SERQ, which is the sequence in AGM Vif, caused HIV-1 Vif to functionally interact with rhA3G and agmA3G. Mutation of three residues to the sequence SEMQ allowed interaction with rhA3G. SEMQ Vif also counteracted D128K mutant hA3G and wild-type hA3G. Introduction of the sequence into an infectious molecular HIV-1 clone allowed the virus to replicate productively in human cells that expressed rhA3G or hA3G. These findings provide insight into the interaction of Vif with A3G and are a step toward the development of a novel primate model for AIDS.

APOBEC3F and APOBEC3G mRNA Levels Do Not Correlate with Human Immunodeficiency Virus Type 1 Plasma Viremia or CD4 + T-Cell Count

APOBEC3F and APOBEC3G (hA3F and hA3G) are part of an innate mechanism of antiretroviral defense. The human immunodeficiency virus type 1 (HIV-1) accessory protein Vif targets both proteins for proteasomal degradation. Using mRNA from peripheral blood mononuclear cells of 92 HIV-infected subjects not taking antiretroviral therapy and 19 HIV-uninfected controls, we found that hA3F (P < 0.001) and hA3G (P = 0.016) mRNA levels were lower in HIV-infected subjects and were positively correlated with one another (P = 0.003). However, we found no correlation in the abundance of either hA3F or hA3G mRNA with either viral load or CD4 counts in HIV-infected subjects.

Regulatory and Accessory HIV-1 Proteins: Potential Targets for HIV-1 Vaccines?

The HIV-1 regulatory proteins Tat and Rev and the accessory proteins Vpr, Vpu and Vif are essential for efficient viral replication, and their cytoplasmic production suggests that they should be processed for recognition by cytotoxic T lymphocytes. However, only limited data is available, evaluating the role of immune responses directed against these proteins in natural HIV-1 infection. Recent advances in the methods used for the characterization of HIV-1-specific cellular immune responses, including quantification of antigen-specific IFN-gamma production by ELISpot assay and flow-cytometry-based intracellular cytokine quantification, have allowed for a much more comprehensive assessment of virus-specific immune responses. Emerging data show that the regulatory and accessory proteins serve as important targets for HIV-1-specific T cell responses, and multiple CTL epitopes have been identified in functionally important regions of these proteins. Moreover, the use of autologous peptides have allowed for the detection of significantly stronger HIV-1-specific T cell responses in the more variable regulatory and accessory HIV-1 proteins Tat and Vpr. These data indicate that despite the small size of these proteins, regulatory and accessory proteins are targeted by cellular immune responses in natural HIV-1 infection and contribute importantly to the total HIV-1-specific CD8+ T cell response. A multi-component vaccine, with the inclusion of these proteins plus structural proteins remains the most promising choice for an effective AIDS vaccine.

Complementary function of the two catalytic domains of APOBEC3G

The HIV-1 viral accessory protein Vif prevents the encapsidation of the antiviral cellular cytidine deaminases APOBEC3F and APOBEC3G by inducing their proteasomal degradation. In the absence of Vif, APOBEC3G is encapsidated and blocks virus replication by deaminating cytosines of the viral cDNA. APOBEC3G encapsidation has been recently shown to depend on the viral nucleocapsid protein; however, the role of RNA remains unclear. Using APOBEC3G deletion and point mutants, we mapped the encapsidation determinant to the Zn 2+ coordination residues of the N-terminal catalytic domain (CD1). Notably, these residues were also required for RNA binding. Mutations in the two aromatic residues of CD1 but not CD2, which are conserved in cytidine deaminase core domains and are required for RNA binding, prevented encapsidation into HIV-1, HTLV-I and MLV. The Zn 2+ coordination residues of the C-terminal catalytic domain (CD2) were not required for encapsidation but were essential for cytidine deaminase activity and the antiviral effect. These findings suggest a model in which CD1 mediates encapsidation and RNA binding while CD2 mediates cytidine deaminase activity. Interestingly, HTLV-I was relatively resistant to the antiviral effects of encapsidated APOBEC3G.

Blocking HIV-1 Vif Restores a Natural Mechanism of Intracellular Antiviral Defense

AIDS has become the greatest pandemic in the human history counting approximately 40 millions people worldwide. To purge HIV-1 infection, new therapeutic approaches need to be searched in alternative and/or in addition to the current pharmacological ones. Recently, several independent laboratories have unveiled a non-immune intracellular anti-HIV-1 defense strategy based on the cytidine deaminase APOBEC3G, which restricts HIV-1 production by directly mutating the proviral DNA in infected cells. To counteract this defense pathway, HIV-1 has developed an evasion strategy by acquiring the accessory protein Vif, which blocks the action of APOBEC3G by inducing its proteasome-mediated degradation.

Vif is an auxiliary factor of the HIV-1 reverse transcriptase and facilitates abasic site bypass.

The HIV-1 accessory protein Vif was found to modulate the RNA- and DNA-dependent DNA synthesis activity of the viral RT (reverse transcriptase) in two ways: (i) it stimulated the binding of the viral RT to the primer by increasing the association rate kcat/K(m) and by decreasing the thermodynamic barrier DeltaH([ES]) for complex formation, and (ii) it increased the polymerization rate of HIV-1 RT. A Vif mutant lacking the final 56 amino acids at the C-terminus failed to stimulate the viral RT. On the other hand, another Vif mutant lacking the first 43 amino acids at the N-terminus, which are involved in RNA binding and interaction with the viral protease, was able to stimulate RT activity. In addition, Vif was found to promote the bypass of an abasic site by HIV-1 RT.