Defining HIV-1 Vif residues that interact with CBFβ by site-directed mutagenesis

Design of Vif-Derived Peptide Inhibitors with Anti-HIV-1 Activity by Interrupting Vif-CBFβ Interaction

One of the major functions of the accessory protein Vif of human immunodeficiency virus type 1 (HIV-1) is to induce the degradation of APOBEC3 (A3) family proteins by recruiting a Cullin5-ElonginB/C-CBFβ E3 ubiquitin ligase complex to facilitate viral replication. Therefore, the interactions between Vif and the E3 complex proteins are promising targets for the development of novel anti-HIV-1 drugs. Here, peptides are designed for the Vif-CBFβ interaction based on the sequences of Vif mutants with higher affinity for CBFβ screened by a yeast surface display platform. We identified two peptides, VMP-63 and VMP-108, that could reduce the infectivity of HIV-1 produced from A3G-positive cells with IC50 values of 49.4 μM and 55.1 μM, respectively. They protected intracellular A3G from Vif-mediated degradation in HEK293T cells, consequently increasing A3G encapsulation into the progeny virions. The peptides could rapidly enter cells after addition to HEK293T cells and competitively inhibit the binding of Vif to CBFβ. Homology modeling analysis demonstrated the binding advantages of VMP-63 and VMP-108 with CBFβ over their corresponding wild-type peptides. However, only VMP-108 effectively restricted long-term HIV-1 replication and protected A3 functions in non-permissive T lymphocytes. Our findings suggest that competitive Vif-derived peptides targeting the Vif-CBFβ interaction are promising for the development of novel therapeutic strategies for acquired immune deficiency syndrome.

A computational exploration of global and temporal dynamics of selection pressure on HIV-1 Vif polymorphism

Virion infectivity factor (Vif), an accessory protein of HIV-1 (human immunodeficiency virus type 1), antagonizes host APOBEC3 protein (apolipoprotein B mRNA editing enzyme, catalytic polypeptide 3) or A3 via proteasomal degradation, facilitating viral replication. HLA (Human leukocyte antigens) alleles, host restriction factors, and error-prone reverse transcription contribute to the global polymorphic dynamics of HIV, impacting effective vaccine design. Our computational analysis of over 50,000 HIV-1 M vif sequences from the Los Alamos National Laboratory (LANL) database (1998-2021) revealed positive selection pressure on the vif gene (nonsynonymous to synonymous ratio, dn/ds=1.58) and an average entropy score of 0.372 in protein level. Interestingly, over the years (1998-2021), a decreasing trend of dn/ds (1.68 to 1.47) and an increasing trend of entropy (0.309 to 0.399) was observed. The predicted mutational frequency against Vif consensus sequence decreased over time (slope = -0.00024, p < 0.0001). Sequence conservation was observed in Vif functional motifs F1, F2, F3, G, BC box, and CBF β binding region, while variability was observed mainly in N- and C- terminal and Zinc finger region, which were dominantly under immune pressure by host HLA-I-restricted CD8+ T cell. Computational analysis of ∆∆Gstability through protein stability prediction tools suggested that missense mutation may affect Vif stability, especially in the Vif-A3 binding interface. Notably, mutations R17K and Y44F in F1 and G box were predicted to destabilize the Vif-A3 binding interface by altering bond formations with adjacent amino acids. Therefore, our analysis demonstrates Vif adaptation with host physiology by maintaining sequence conservation, especially in A3 interacting functional motifs, highlighting important therapeutic candidate regions of Vif against HIV-1 infections.

Dominant Negative Mutants of Human Immunodeficiency Virus Type 1 Viral Infectivity Factor (Vif) Disrupt Core-Binding Factor Beta-Vif Interaction

Apolipoprotein B mRNA-editing catalytic polypeptide-like 3 family members (APOBEC3s) are host restriction factors that inhibit viral replication. Viral infectivity factor (Vif), a human immunodeficiency virus type 1 (HIV-1) accessory protein, mediates the degradation of APOBEC3s by forming the Vif-E3 complex, in which core-binding factor beta (CBFβ) is an essential molecular chaperone. Here, we screened nonfunctional Vif mutants with high affinity for CBFβ to inhibit HIV-1 in a dominant negative manner. We applied the yeast surface display technology to express Vif random mutant libraries, and mutants showing high CBFβ affinity were screened using flow cytometry. Most of the screened Vif mutants containing random mutations of different frequencies were able to rescue APOBEC3G (A3G). In the subsequent screening, three of the mutants restricted HIV-1, recovered G-to-A hypermutation, and rescued APOBEC3s. Among them, Vif-6M showed a cross-protection effect toward APOBEC3C, APOBEC3F, and African green monkey A3G. Stable expression of Vif-6M in T lymphocytes inhibited the viral replication in newly HIV-1-infected cells and the chronically infected cell line H9/HXB2. Furthermore, the expression of Vif-6M provided a survival advantage to T lymphocytes infected with HIV-1. These results suggest that dominant negative Vif mutants acting on the Vif-CBFβ target potently restrict HIV-1. IMPORTANCE Antiviral therapy cannot eliminate HIV and exhibits disadvantages such as drug resistance and toxicity. Therefore, novel strategies for inhibiting viral replication in patients with HIV are urgently needed. APOBEC3s in host cells are able to inhibit viral replication but are antagonized by HIV-1 Vif-mediated degradation. Therefore, we screened nonfunctional Vif mutants with high affinity for CBFβ to compete with the wild-type Vif (wtVif) as a potential strategy to assist with HIV-1 treatment. Most screened mutants rescued the expression of A3G in the presence of wtVif, especially Vif-6M, which could protect various APOBEC3s and improve the incorporation of A3G into HIV-1 particles. Transduction of Vif-6M into T lymphocytes inhibited the replication of the newly infected virus and the chronically infected virus. These data suggest that Vif mutants targeting the Vif-CBFβ interaction may be promising in the development of a new AIDS therapeutic strategy.

Degradation-Independent Inhibition of APOBEC3G by the HIV-1 Vif Protein

The ubiquitin–proteasome system plays an important role in the cell under normal physiological conditions but also during viral infections. Indeed, many auxiliary proteins from the (HIV-1) divert this system to its own advantage, notably to induce the degradation of cellular restriction factors. For instance, the HIV-1 viral infectivity factor (Vif) has been shown to specifically counteract several cellular deaminases belonging to the apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like (APOBEC3 or A3) family (A3A to A3H) by recruiting an E3-ubiquitin ligase complex and inducing their polyubiquitination and degradation through the proteasome. Although this pathway has been extensively characterized so far, Vif has also been shown to impede A3s through degradation-independent processes, but research on this matter remains limited. In this review, we describe our current knowledge regarding the degradation-independent inhibition of A3s, and A3G in particular, by the HIV-1 Vif protein, the molecular mechanisms involved, and highlight important properties of this small viral protein.

A novel HIV-1 inhibitor that blocks viral replication and rescues APOBEC3s by interrupting vif/CBFβ interaction

HIV remains a health challenge worldwide, partly because of the continued development of resistance to drugs. Therefore, it is urgent to find new HIV inhibitors and targets. Apolipoprotein B mRNA-editing catalytic polypeptide-like 3 family members (APOBEC3) are important host restriction factors that inhibit HIV-1 replication by their cytidine deaminase activity. HIV-1 viral infectivity factor (Vif) promotes proteasomal degradation of APOBEC3 proteins by recruiting the E3 ubiquitin ligase complex, in which core-binding factor β (CBFβ) is a necessary molecular chaperone. Interrupting the interaction between Vif and CBFβ can release APOBEC3 proteins to inhibit HIV-1 replication and may be useful for developing new drug targets for HIV-1. In this study, we identified a potent small molecule inhibitor CBFβ/Vif-3 (CV-3) of HIV-1 replication by employing structure-based virtual screening using the crystal structure of Vif and CBFβ (PDB: 4N9F) and validated CV-3's antiviral activity. We found that CV-3 specifically inhibited HIV-1 replication (IC50 = 8.16 µm; 50% cytotoxic concentration >100 µm) in nonpermissive lymphocytes. Furthermore, CV-3 treatment rescued APOBEC3 family members (human APOBEC3G (hA3G), hA3C, and hA3F) in the presence of Vif and enabled hA3G packaging into HIV-1 virions, which resulted in Gly-to-Ala hypermutations in viral genomes. Finally, we used FRET to demonstrate that CV-3 inhibited the interaction between Vif and CBFβ by simultaneously forming hydrogen bonds with residues Gln-67, Ile-102, and Arg-131 of CBFβ. These findings demonstrate that CV-3 can effectively inhibit HIV-1 by blocking the interaction between Vif and CBFβ and that this interaction can serve as a new target for developing HIV-1 inhibitors.

Structural Insights into APOBEC3-Mediated Lentiviral Restriction

Mammals have developed clever adaptive and innate immune defense mechanisms to protect against invading bacterial and viral pathogens. Human innate immunity is continuously evolving to expand the repertoire of restriction factors and one such family of intrinsic restriction factors is the APOBEC3 (A3) family of cytidine deaminases. The coordinated expression of seven members of the A3 family of cytidine deaminases provides intrinsic immunity against numerous foreign infectious agents and protects the host from exogenous retroviruses and endogenous retroelements. Four members of the A3 proteins-A3G, A3F, A3H, and A3D-restrict HIV-1 in the absence of virion infectivity factor (Vif); their incorporation into progeny virions is a prerequisite for cytidine deaminase-dependent and -independent activities that inhibit viral replication in the host target cell. HIV-1 encodes Vif, an accessory protein that antagonizes A3 proteins by targeting them for polyubiquitination and subsequent proteasomal degradation in the virus producing cells. In this review, we summarize our current understanding of the role of human A3 proteins as barriers against HIV-1 infection, how Vif overcomes their antiviral activity, and highlight recent structural and functional insights into A3-mediated restriction of lentiviruses.

Inhibition of Vif-Mediated Degradation of APOBEC3G through Competitive Binding of Core-Binding Factor Beta

Vif counteracts the host restriction factor APOBEC3G (A3G) and other APOBEC3s by preventing the incorporation of A3G into progeny virions. We previously identified Vif mutants with a dominant-negative (D/N) phenotype that interfered with the function of wild-type Vif, inhibited the degradation of A3G, and reduced the infectivity of viral particles by increased packaging of A3G. However, the mechanism of interference remained unclear, in particular since all D/N Vif mutants were unable to bind Cul5 and some mutants additionally failed to bind A3G, ruling out competitive binding to A3G or the E3 ubiquitin ligase complex as the sole mechanism. The goal of the current study was to revisit the mechanism of D/N interference by Vif mutants and analyze the possible involvement of core binding factor beta (CBFβ) in this process. We found a clear correlation of D/N properties of Vif mutants with their ability to engage CBFβ. Only mutants that retained the ability to bind CBFβ exhibited the D/N phenotype. Competition studies revealed that D/N Vif mutants directly interfered with the association of CBFβ and wild-type Vif. Furthermore, overexpression of CBFβ counteracted the interference of D/N Vif mutants with A3G degradation by wild-type Vif. Finally, overexpression of Runx1 mimicked the effect of D/N Vif mutants and inhibited the degradation of A3G by wild-type Vif. Taken together, we identified CBFβ as the key player involved in D/N interference by Vif.IMPORTANCE Of all the accessory proteins encoded by HIV-1 and other primate lentiviruses, Vif has arguably the strongest potential as a target for antiviral therapy. This conclusion is based on the observation that replication of HIV-1 in vivo is critically dependent on Vif. Thus, inhibiting the function of Vif via small-molecule inhibitors or other approaches has significant therapeutic potential. We previously identified dominant-negative (D/N) Vif variants whose expression interferes with the function of virus-encoded wild-type Vif. We now show that D/N interference involves competitive binding of D/N Vif variants to the transcriptional cofactor core binding factor beta (CBFβ), which is expressed in cells in limiting quantities. Overexpression of CBFβ neutralized the D/N phenotype of Vif. In contrast, overexpression of Runx1, a cellular binding partner of CBFβ, phenocopied the D/N Vif phenotype by sequestering endogenous CBFβ. Thus, our results provide proof of principle that D/N Vif variants could have therapeutic potential.

Clues for two-step virion infectivity factor regulation by core binding factor beta

Lentiviruses threaten human and animal health. Virion infectivity factor (Vif) is essential for the infectivity of most lentiviruses, except for the equine infectious anaemia virus (EIAV). Vif promotes viral infectivity by recruiting a Cullin-based E3 ligase to induce the degradation of a class of host restriction factors, named APOBEC3. Core binding factor beta (CBF-β) is necessary for several primate lentiviral Vif functions, including HIV-1 Vif. Although much progress has been made in understanding the contribution of CBF-β to Vif function, the precise mechanism has not yet been fully elucidated. In this study, we found that an interaction with CBF-β altered the oligomerization and subcellular distribution pattern and increased the stability of two primate lentiviral Vifs, HIV-1 Vif and Macaca simian immunodeficiency virus (SIVmac) Vif. Moreover, using a CBF-β loss-of-function mutant, we demonstrated that the interaction between CBF-β and Vif was not sufficient for Vif assistance; a region including F68 in CBF-β was also required for the stability and function of Vif. For the first time, this study separates the binding and regulating processes of CBF-β when it is promoting Vif function, which further extends our understanding of the biochemical regulation of Vif by CBF-β.

Identification of a tripartite interaction between the N-terminus of HIV-1 Vif and CBFβ that is critical for Vif function

Background

HIV-1 Vif interacts with the cellular core-binding factor β (CBFβ) and counteracts the protective roles of certain human APOBEC3 (A3) proteins by targeting them for proteasomal degradation. Previous studies have identified some amino acids important for Vif–CBFβ interactions, and recently a co-crystal structure of a pentameric complex of HIV-1 Vif, CBFβ, Cul5, EloB, and EloC was resolved. However, a comprehensive analysis of Vif–CBFβ interactions that are important for Vif function has not been performed.

Results

Here, we carried out double-alanine scanning mutagenesis of the first 60 amino acids of Vif and determined their effects on interaction with CBFβ and their ability to induce A3G degradation as well as rescue HIV-1 replication in the presence of A3G. We found that multiple Vif residues are involved in the extensive N-terminal Vif–CBFβ interaction and that the ⁵WQVMIVW¹¹ region of Vif is the major determinant. A minimum of three alanine substitutions are required to completely abrogate the Vif–CBFβ interaction and Vif’s ability to rescue HIV-1 infectivity in the presence of A3G. Mutational analysis of CBFβ revealed that F68 and I55 residues are important and participate in a tripartite hydrophobic interaction with W5 of Vif to maintain a stable and functional Vif–CBFβ complex. We also determined that CBFβ amino acids ⁷³WQGEQR⁷⁸, which are not resolved in the structure of the pentameric complex, are not involved in interaction with HIV-1 Vif.

Conclusions

Our results provide detailed insight into the Vif–CBFβ interactions that are critical for Vif function and may contribute to the rational design of HIV-1 inhibitors that block Vif-mediated degradation of A3 proteins.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-017-0346-5) contains supplementary material, which is available to authorized users.

Core Binding Factor β Protects HIV, Type 1 Accessory Protein Viral Infectivity Factor from MDM2-mediated Degradation

HIV, type 1 overcomes host restriction factor apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like 3 (APOBEC3) proteins by organizing an E3 ubiquitin ligase complex together with viral infectivity factor (Vif) and a host transcription cofactor core binding factor β (CBFβ). CBFβ is essential for Vif to counteract APOBEC3 by enabling the recruitment of cullin 5 to the complex and increasing the steady-state level of Vif protein; however, the mechanisms by which CBFβ up-regulates Vif protein remains unclear. Because we have reported previously that mouse double minute 2 homolog (MDM2) is an E3 ligase for Vif, we hypothesized that CBFβ might protect Vif from MDM2-mediated degradation. Co-immunoprecipitation analyses showed that Vif mutants that do not bind to CBFβ preferentially interact with MDM2 and that overexpression of CBFβ disrupts the interaction between MDM2 and Vif. Knockdown of CBFβ reduced the steady-state level of Vif in MDM2-proficient cells but not in MDM2-null cells. Cycloheximide chase analyses revealed that Vif E88A/W89A, which does not interact with CBFβ, degraded faster than wild-type Vif in MDM2-proficient cells but not in MDM2-null cells, suggesting that Vif stabilization by CBFβ is mainly caused by impairing MDM2-mediated degradation. We identified Vif R93E as a Vif variant that does not bind to MDM2, and the virus with this substitution mutation was more resistant to APOBEC3G than the parental virus. Combinatory substitution of Vif residues required for CBFβ binding and MDM2 binding showed full recovery of Vif steady-state levels, supporting our hypothesis. Our data provide new insights into the mechanism of Vif augmentation by CBFβ.

Improving HIV proteome annotation: new features of BioAfrica HIV Proteomics Resource

The Human Immunodeficiency Virus (HIV) is one of the pathogens that cause the greatest global concern, with approximately 35 million people currently infected with HIV. Extensive HIV research has been performed, generating a large amount of HIV and host genomic data. However, no effective vaccine that protects the host from HIV infection is available and HIV is still spreading at an alarming rate, despite effective antiretroviral (ARV) treatment. In order to develop effective therapies, we need to expand our knowledge of the interaction between HIV and host proteins. In contrast to virus proteins, which often rapidly evolve drug resistance mutations, the host proteins are essentially invariant within all humans. Thus, if we can identify the host proteins needed for virus replication, such as those involved in transporting viral proteins to the cell surface, we have a chance of interrupting viral replication. There is no proteome resource that summarizes this interaction, making research on this subject a difficult enterprise. In order to fill this gap in knowledge, we curated a resource presents detailed annotation on the interaction between the HIV proteome and host proteins. Our resource was produced in collaboration with ViralZone and used manual curation techniques developed by UniProtKB/Swiss-Prot. Our new website also used previous annotations of the BioAfrica HIV-1 Proteome Resource, which has been accessed by approximately 10 000 unique users a year since its inception in 2005. The novel features include a dedicated new page for each HIV protein, a graphic display of its function and a section on its interaction with host proteins. Our new webpages also add information on the genomic location of each HIV protein and the position of ARV drug resistance mutations. Our improved BioAfrica HIV-1 Proteome Resource fills a gap in the current knowledge of biocuration.

Database URL: http://www.bioafrica.net/proteomics/HIVproteome.html

ASK1 restores the antiviral activity of APOBEC3G by disrupting HIV-1 Vif-mediated counteraction

APOBEC3G (A3G) is an innate antiviral restriction factor that strongly inhibits the replication of human immunodeficiency virus type 1 (HIV-1). An HIV-1 accessory protein, Vif, hijacks the host ubiquitin–proteasome system to execute A3G degradation. Identification of the host pathways that obstruct the action of Vif could provide a new strategy for blocking viral replication. We demonstrate here that the host protein ASK1 (apoptosis signal-regulating kinase 1) interferes with the counteraction by Vif and revitalizes A3G-mediated viral restriction. ASK1 binds the BC-box of Vif, thereby disrupting the assembly of the Vif–ubiquitin ligase complex. Consequently, ASK1 stabilizes A3G and promotes its incorporation into viral particles, ultimately reducing viral infectivity. Furthermore, treatment with the antiretroviral drug AZT (zidovudine) induces ASK1 expression and restores the antiviral activity of A3G in HIV-1-infected cells. This study thus demonstrates a distinct function of ASK1 in restoring the host antiviral system that can be enhanced by AZT treatment.

The human protein APOBEC3G (A3G) inhibits HIV-1 replication, but the viral protein Vif counteracts by inducing A3G degradation. Here Miyakawa et al. show that the antiretroviral drug AZT restores A3G function in vitro by stimulating expression of a host protein, ASK1, which interferes with the action of Vif.

Multiple lysines combined in HIV-1 Vif determines the responsiveness to CBF-β

The Vif (viral infectivity factor) protein of human immunodeficiency virus type-1 (HIV-1) is critical for HIV-1 infectivity. CBF-β is required for HIV-1 Vif function, as it increases the steady-state level of the HIV-1 Vif protein to promote host restriction factor APOBEC3 degradation. However, the precise mechanism by which CBF-β promotes HIV-1 Vif levels remains unclear. In the present study, we provided evidences that CBF-β promoted steady-state levels of HIV-1 Vif by inhibiting the degradation of HIV-1 Vif through the proteasome pathway. Our results reveal a new mechanism by which a cellular protein supports viral infectivity by inhibiting viral protein degradation.

Requirement of HIV-1 Vif C-terminus for Vif-CBF-β interaction and assembly of CUL5-containing E3 ligase

BACKGROUND

Human immunodeficiency virus type 1 (HIV-1) Vif hijacks an E3 ligase to suppress natural APOBEC3 restriction factors, and core binding factor β (CBF-β) is required for this process. Although an extensive region of Vif spanning most of its N-terminus is known to be critical for binding with CBF-β, involvement of the Vif C-terminus in the interaction with CBF-β has not been fully investigated.

RESULTS

Here, through immunoprecipitation analysis of Vif C-terminal truncated mutants of various lengths, we identified that CBF-β binding requires not only certain amino acids (G126A, E134A, Y135A and G138A) in the HCCH region but also the HCCH motif itself, which also affects the Vif-mediated suppression of APOBEC3G/APOBEC3F (A3G/A3F). These mutants still maintained interactions with substrate A3G or A3F as well as other cellular factors ElonginB/C (ELOB/C), indicating that their structures were not functionally affected. Moreover, by determining that the BC box also is necessary for CBF-β interaction in vivo, we speculate that binding to ELOB/C induces conformational changes in Vif, facilitating its interaction with CBF-β and consequent interaction with CUL5.

CONCLUSIONS

These results provide important information on the assembly of the Vif-CUL5-E3 ubiquitin ligase. Identification of the new binding interface with CBF-β at the C-terminus of HIV-1 Vif also provides novel targets for the development of HIV-1 inhibitors.

Core-binding factor subunit beta is not required for non-primate lentiviral Vif-mediated APOBEC3 degradation

Viral infectivity factor (Vif) is required for lentivirus fitness and pathogenicity, except in equine infectious anemia virus (EIAV). Vif enhances viral infectivity by a Cullin5-Elongin B/C E3 complex to inactivate the host restriction factor APOBEC3. Core-binding factor subunit beta (CBF-β) is a cell factor that was recently shown to be important for the primate lentiviral Vif function. Non-primate lentiviral Vif also degrades APOBEC3 through the proteasome pathway. However, it is unclear whether CBF-β is required for the non-primate lentiviral Vif function. In this study, we demonstrated that the Vifs of non-primate lentiviruses, including feline immunodeficiency virus (FIV), bovine immunodeficiency virus (BIV), caprine arthritis encephalitis virus (CAEV), and maedi-visna virus (MVV), do not interact with CBF-β. In addition, CBF-β did not promote the stability of FIV, BIV, CAEV, and MVV Vifs. Furthermore, CBF-β silencing or overexpression did not affect non-primate lentiviral Vif-mediated APOBEC3 degradation. Our results suggest that non-primate lentiviral Vif induces APOBEC3 degradation through a different mechanism than primate lentiviral Vif. Importance: The APOBEC3 protein family members are host restriction factors that block retrovirus replication. Vif, an accessory protein of lentivirus, degrades APOBEC3 to rescue viral infectivity by forming Cullin5-Elongin B/C-based E3 complex. CBF-β was proved to be a novel regulator of primate lentiviral Vif function. In this study, we found that CBF-β knockdown or overexpression did not affect FIV Vif's function, which induced polyubiquitination and degradation of APOBEC3 by recruiting the E3 complex in a manner similar to that of HIV-1 Vif. We also showed that other non-primate lentiviral Vifs did not require CBF-β to degrade APOBEC3. CBF-β did not interact with non-primate lentiviral Vifs or promote their stability. These results suggest that a different mechanism exists for the Vif-APOBEC interaction and that non-primates are not suitable animal models for exploring pharmacological interventions that disrupt Vif-CBF-β interaction.

Suppression of APOBEC3-mediated restriction of HIV-1 by Vif

The APOBEC3 restriction factors are a family of deoxycytidine deaminases that are able to suppress replication of viruses with a single-stranded DNA intermediate by inducing mutagenesis and functional inactivation of the virus. Of the seven human APOBEC3 enzymes, only APOBEC3-D, -F, -G, and -H appear relevant to restriction of HIV-1 in CD4+ T cells and will be the focus of this review. The restriction of HIV-1 occurs most potently in the absence of HIV-1 Vif that induces polyubiquitination and degradation of APOBEC3 enzymes through the proteasome pathway. To restrict HIV-1, APOBEC3 enzymes must be encapsidated into budding virions. Upon infection of the target cell during reverse transcription of the HIV-1 RNA into (-)DNA, APOBEC3 enzymes deaminate cytosines to form uracils in single-stranded (-)DNA regions. Upon replication of the (-)DNA to (+)DNA, the HIV-1 reverse transcriptase incorporates adenines opposite to the uracils thereby inducing C/G to T/A mutations that can functionally inactivate HIV-1. APOBEC3G is the most studied APOBEC3 enzyme and it is known that Vif attempts to thwart APOBEC3 function not only by inducing its proteasomal degradation but also by several degradation-independent mechanisms, such as inhibiting APOBEC3G virion encapsidation, mRNA translation, and for those APOBEC3G molecules that still become virion encapsidated, Vif can inhibit APOBEC3G mutagenic activity. Although most Vif variants can induce efficient degradation of APOBEC3-D, -F, and -G, there appears to be differential sensitivity to Vif-mediated degradation for APOBEC3H. This review examines APOBEC3-mediated HIV restriction mechanisms, how Vif acts as a substrate receptor for a Cullin5 ubiquitin ligase complex to induce degradation of APOBEC3s, and the determinants and functional consequences of the APOBEC3 and Vif interaction from a biological and biochemical perspective.

Cellular requirements for bovine immunodeficiency virus Vif-mediated inactivation of bovine APOBEC3 proteins

Human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus (SIV) viral infectivity factor (Vif) form a CRL5 E3 ubiquitin ligase complex to suppress virus restriction by host APOBEC3 (A3) proteins. The primate lentiviral Vif complex is composed of the unique cofactor core binding factor β (CBF-β) and canonical ligase components Cullin 5 (CUL5), Elongin B/C (ELOB/C), and RBX2. However, the mechanism by which the Vif protein of the related lentivirus bovine immunodeficiency virus (BIV) overcomes its host A3 proteins is less clear. In this study, we show that BIV Vif interacts with Cullin 2 (CUL2), ELOB/C, and RBX1, but not with CBF-β or CUL5, to form a CRL2 E3 ubiquitin ligase and degrade the restrictive bovine A3 proteins (A3Z2Z3 and A3Z3). RNA interference-mediated knockdown of ELOB or CUL2 inhibited BIV Vif-mediated degradation of these A3 proteins, whereas knockdown of CUL5 or CBF-β did not. BIV Vif with mutations in the BC box (Vif SLQ-AAA) or putative VHL box (Vif YI-AA), which cannot interact with ELOB/C or CUL2, respectively, lost the ability to counteract bovine A3 proteins. Moreover, CUL2 and UBE2M dominant negative mutants competitively inhibited the BIV Vif-mediated degradation mechanism. Thus, although the general strategy for inhibiting A3 proteins is conserved between HIV-1/SIV and BIV, the precise mechanisms can differ substantially, with only the HIV-1/SIV Vif proteins requiring CBF-β as a cofactor, HIV-1/SIV Vif using CUL5-RBX2, and BIV Vif using CUL2-RBX1.

IMPORTANCE

Primate lentivirus HIV-1 and SIV Vif proteins form a ubiquitin ligase complex to target host antiviral APOBEC3 proteins for degradation. However, the mechanism by which the nonprimate lentivirus BIV Vif inhibits bovine APOBEC3 proteins is unclear. In the present study, we determined the mechanism for BIV Vif-mediated degradation of bovine APOBEC3 proteins and found that it differs from the mechanism of HIV-1/SIV Vif by being CBF-β independent and requiring different ubiquitin ligase scaffolding proteins (CUL2-RBX1 instead of CUL5-RBX2). BIV Vif is the only known retroviral protein that can interact with CUL2. This information broadens our understanding of the distinct mechanisms by which the Vif proteins of different lentiviruses facilitate viral infection. This novel mechanism for assembly of the BIV Vif-APOBEC3 ubiquitin ligase complex advances our understanding of viral hijacking of host E3 ubiquitin ligases and illustrates the evolutionary flexibility of lentiviruses.

Identification of HIV-1 Vif regions required for CBF-β interaction and APOBEC3 suppression

Human immunodeficiency virus type 1 (HIV-1) Vif requires core binding factor β (CBF-β) to degrade the host APOBEC3 restriction factors. Although a minimum domain and certain amino acids of HIV-1 Vif, including hydrophobic residues at the N-terminal, have been identified as critical sites for binding with CBF-β, other regions that potentially mediate this interaction need to be further investigated. Here, we mapped two new regions of HIV-1 Vif that are required for interaction with CBF-β by generating a series of single-site or multiple-site Vif mutants and testing their effect on the suppression of APOBEC3G (A3G) and APOBEC3F (A3F). A number of the mutants, including G84A/SIEW86-89AAAA (84/86-89), E88A/W89A (88/89), G84A, W89A, L106S and I107S in the 84GxSIEW89 and L102ADQLI107 regions, affected Vif function by disrupting CBF-β binding. These Vif mutants also had altered interactions with CUL5, since CBF-β is known to facilitate the binding of Vif to CUL5. We further showed that this effect was not due to misfolding or conformational changes in Vif, as the mutants still maintained their interactions with other factors such as ElonginB, A3G and A3F. Notably, G84D and D104A had stronger effects on the Vif-CUL5 interaction than on the Vif-CBF-β interaction, indicating that they mainly influenced the CUL5 interaction and implying that the interaction of Vif with CUL5 contributes to the binding of Vif to CBF-β. These new binding interfaces with CBF-β in HIV-1 Vif provide novel targets for the development of HIV-1 inhibitors.