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Concept: Viral infectivity factor


The human apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like 3 (APOBEC3, referred to as A3) proteins are cellular cytidine deaminases that potently restrict retrovirus replication. However, HIV-1 viral infectivity factor (Vif) counteracts the antiviral activity of most A3 proteins by targeting them for proteasomal degradation. To date, the structure of an A3 protein containing a Vif-binding interface has not been solved. Here, we report a high-resolution crystal structure of APOBEC3C and identify the HIV-1 Vif-interaction interface. Extensive structure-guided mutagenesis revealed the role of a shallow cavity composed of hydrophobic or negatively charged residues between the α2 and α3 helices. This region is distant from the DPD motif (residues 128-130) of APOBEC3G that participates in HIV-1 Vif interaction. These findings provide insight into Vif-A3 interactions and could lead to the development of new pharmacologic anti-HIV-1 compounds.

Concepts: HIV, Protein, Cell, Metabolism, Enzyme, Interaction, Viral infectivity factor, APOBEC3G


The lentiviral protein Viral Infectivity Factor (Vif) counteracts the antiviral effects of host APOBEC3 (A3) proteins and contributes to persistent HIV infection. Vif targets A3 restriction factors for ubiquitination and proteasomal degradation by recruiting them to a multi-protein ubiquitin E3 ligase complex. Here, we describe a degradation-independent mechanism of Vif-mediated antagonism that was revealed through detailed structure-function studies of antibody antigen-binding fragments (Fabs) to the Vif complex. Two Fabs were found to inhibit Vif-mediated A3 neutralization through distinct mechanisms: shielding A3 from ubiquitin transfer and blocking Vif E3 assembly. Combined biochemical, cell biological and structural studies reveal that disruption of Vif E3 assembly inhibited A3 ubiquitination but was not sufficient to restore its packaging into viral particles and antiviral activity. These observations establish that Vif can neutralize A3 family members in a degradation-independent manner. Additionally, this work highlights the potential of Fabs as functional probes, and illuminate how Vif uses a multi-pronged approach involving both degradation dependent and independent mechanisms to suppress A3 innate immunity.

Concepts: HIV, Immune system, Protein, Proteasome, Viral infectivity factor, APOBEC3G


Human APOBEC3 cytidine deaminases are intrinsic resistance factors to HIV-1. However, HIV-1 encodes a viral infectivity factor (Vif) that degrades APOBEC3 proteins. In vitro APOBEC3F (A3F) anti-HIV-1 activity is weaker than A3G but is partially resistant to Vif degradation unlike A3G. It is unknown whether A3F protein affects HIV-1 disease in vivo. To assess the effect of A3F gene on host susceptibility to HIV- acquisition and disease progression, we performed a genetic association study in six well-characterized HIV-1 natural cohorts. A common six-Single Nucleotide Polymorphism (SNP) haplotype of A3F tagged by a codon-changing variant (p. I231V, with allele (V) frequency of 48% in European Americans) was associated with significantly lower set-point viral load and slower rate of progression to AIDS (Relative Hazards (RH) = 0.71, 95% CI: 0.56, 0.91) and delayed development of pneumocystis pneumonia (PCP) (RH = 0.53, 95% CI: 0.37-0.76). A validation study in the International Collaboration for the Genomics of HIV (ICGH) showed a consistent association with lower set-point viral load. An in vitro assay revealed that the A3F I231V variant may influence Vif mediated A3F degradation. Our results provide genetic epidemiological evidence that A3F modulates HIV-1/AIDS disease progression.

Concepts: HIV, AIDS, Gene, Genetics, Epidemiology, RNA, Viral infectivity factor, APOBEC3G


The apolipoprotein B mRNA-editing enzyme, catalytic polypeptide-like (APOBEC3, A3) family of DNA cytidine deaminases are intrinsic restriction factors against retroviruses. In felids such as the domestic cat (Felis catus), the APOBEC3 (A3) genes encode for the A3Z2s, A3Z3, and A3Z2Z3 antiviral cytidine deaminases. Only A3Z3 and A3Z2Z3 inhibit viral infectivity factor (Vif)-deficient feline immunodeficiency virus (FIV). FIV Vif protein interacts with Cullin (CUL), Elongin B (ELOB), and Elongin C (ELOC) to form an E3 ubiquitination complex to induce the degradation of feline A3s. However, the functional domains in FIV Vif for interaction with Cullin are poorly understood. Here, we found that the expression of dominant-negative CUL5 prevented the degradation of feline A3s by FIV Vif, while dominant-negative CUL2 had no influence on the degradation of A3. In co-immunoprecipitation assays, FIV Vif bound to CUL5 but not CUL2. To identify the CUL5 interaction site in FIV Vif, the conserved amino acids from position 47 to 160 of FIV Vif were mutated, but these mutations did not impair the binding of Vif to CUL5. By focusing on a potential zinc-binding motif (K175- C161-C184-C187) of FIV Vif, we found a conserved hydrophobic region (174IR175) that is important for CUL5 interaction. Mutating this region also impaired the FIV Vif-induced degradation of feline A3s. Based on a structural model of the FIV Vif/CUL5 interaction, residues 52LW53 in CUL5 were identified as mediating the binding to FIV Vif. By comparing our results to the HIV-1 Vif/CUL5 interaction surface (120IR121, a hydrophobic region that is localized in the zinc-binding motif), we suggest that the CUL5 interaction surface in the diverse HIV-1 and FIV Vif is evolutionarily conserved indicating a strong structural constraint. However, the FIV Vif/CUL5 interaction is zinc-independent, which contrasts with the zinc-dependence of HIV-1 Vif.IMPORTANCE Feline immunodeficiency virus (FIV), which is similar to human immunodeficiency virus (HIV)-1, replicates in its natural host in T-cells and macrophages that express antiviral restriction factors APOBEC3 (A3). To escape A3s, FIV and HIV induce degradation of these proteins by building ubiquitination ligase complex using the viral protein Vif to connect to cellular proteins, including Cullin 5. Here, we identified the protein residues that regulate this interaction in FIV Vif and Cullin 5. While our structural model suggests that the diverse FIV and HIV-1 Vifs use conserved residues for Cullin 5 binding, FIV Vif binds Cullin 5 independently of zinc in contrast to HIV-1 Vif.

Concepts: HIV, Immune system, DNA, Protein, Gene, Virus, Cat, Viral infectivity factor


APOBEC3G (A3G) is a restriction factor that provides innate immunity against HIV-1 in the absence of viral infectivity factor (Vif) protein. However, structural information about A3G, which can aid in unraveling the mechanisms that govern its interactions and define its antiviral activity, remains unknown. Here, we built a computer model of a full-length A3G using docking approaches and molecular dynamics simulations, based on the available X-ray and NMR structural data for the two protein domains. The model revealed a large-scale dynamics of the A3G monomer, as the two A3G domains can assume compact forms or extended dumbbell type forms with domains visibly separated from each other. To validate the A3G model, we performed time-lapse high-speed atomic force microscopy (HS-AFM) experiments enabling us to get images of a fully hydrated A3G and to directly visualize its dynamics. HS-AFM confirmed that A3G exists in two forms, a globular form (∼84% of the time) and a dumbbell form (∼16% of the time), and can dynamically switch from one form to the other. The obtained HS-AFM results are in line with the computer modeling, which demonstrates a similar distribution between two forms. Furthermore, our simulations capture the complete process of A3G switching from the DNA-bound state to the closed state. The revealed dynamic nature of monomeric A3G could aid in target recognition including scanning for cytosine locations along the DNA strand and in interactions with viral RNA during packaging into HIV-1 particles.

Concepts: DNA, Virus, Molecular dynamics, Polymer, Computer graphics, Monte Carlo method, Computer simulation, Viral infectivity factor


Human DNA cytosine-to-uracil deaminases catalyze mutations in both pathogen and cellular genomes. APOBEC3D, APOBEC3F, APOBEC3G, and APOBEC3H restrict human immunodeficiency virus 1 (HIV-1) infection in cells deficient in the viral infectivity factor (Vif), and have the potential to catalyze sublethal levels of mutation in viral genomes in Vif-proficient cells. At least two APOBEC3 enzymes, and in particular APOBEC3B, are sources of somatic mutagenesis in cancer cells that drive tumor evolution and may manifest clinically as recurrence, metastasis, and/or therapy resistance. Consequently, APOBEC3 enzymes are tantalizing targets for developing chemical probes and therapeutic molecules to harness mutational processes in human disease. This review highlights recent efforts to chemically manipulate APOBEC3 activities.

Concepts: HIV, Immune system, DNA, Cancer, Infectious disease, Mutation, Viral infectivity factor, APOBEC3G


The crystal structure of viral infectivity factor (Vif) was reported recently, which makes it possible to design new inhibitors against Vif by structure-based drug design. Through analysis of the protein surface of Vif, the C2 pocket located in the N-terminal was found, which is suit for developing small molecular inhibitors. Then, in our article, fragment-based virtual screening (FBVS) was conducted and a series of fragments was obtained, among which, Zif-1 bearing indole scaffold and pyridine ring can form H-bonds with Tyr148 and Ile155. Subsequently, 19 derivatives of Zif-1 were synthesized. Through the immune-fluorescence staining and Western blot assays, Zif-15 shows potent activity in inhibiting Vif-mediated A3G degradation. Further docking experiment shows that Zif-15 form H-bond interactions with residues His139, Tyr148 and Ile155. Therefore, Zif-15 is a promising lead compound against Vif that can be used to treat AIDS.

Concepts: Pharmacology, Bioinformatics, Molecular biology, Drug discovery, Drug design, Virtual screening, Medicinal chemistry, Viral infectivity factor


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.

Concepts: HIV, DNA, Protein, RNA interference, Primate, Lentivirus, Viral infectivity factor, Simian immunodeficiency virus


Apolipoprotein B mRNA editing enzyme catalytic polypeptide-like 3 (APOBEC3) proteins are mammalian-specific cellular deaminases and have a robust ability to restrain lentivirus replication. To antagonize APOBEC3-mediated antiviral action, lentiviruses have acquired viral infectivity factor (Vif) as an accessory gene. Mammalian APOBEC3 proteins inhibit lentiviral replication by enzymatically inserting G-to-A hypermutations in the viral genome, whereas lentiviral Vif proteins degrade host APOBEC3 via the ubiquitin/proteasome-dependent pathway. Recent investigations provide evidence that lentiviral vif genes evolved to combat mammalian APOBEC3 proteins. In corollary, mammalian APOBEC3 genes are under Darwinian selective pressure to escape from antagonism by Vif. Based on these observations, it is widely accepted that lentiviral Vif and mammalian APOBEC3 have co-evolved and this concept is called an “evolutionary arms race.” This review provides a comprehensive summary of current knowledge with respect to the evolutionary dynamics occurring at this pivotal host-virus interface.

Concepts: DNA, Protein, Gene, Genetics, Evolution, Virus, RNA, Viral infectivity factor


Viral infectivity factor (Vif) is protective against APOBEC3G (A3G)-mediated viral cDNA hypermutations, and development of molecules that inhibit Vif mediated A3G degradation is a novel strategy for blocking HIV-1 replication. Through optimizations of the central ring of N-(2-methoxyphenyl)-2-((4-nitrophenyl)thio)benzamide (RN-18), we found a potent compound 12c with EC50 value of 1.54 μM, enhancing the antiviral activity more than 150-fold compared with RN-18 in nonpermissive H9 cells. 12c protected A3G from degradation by inhibiting Vif function. Besides, 12c suppressed different HIV-1 clinical strains (HIV-1KM018, HIV-1TC-1 and HIV-1WAN) and drug-resistant strains (NRTI, NNRTI, PI, and FI) with relatively high activities. Amidation of 12c with glycine gave a prodrug 13a, improving the water solubility about 2600-fold compared with 12c. Moreover, 13a inhibited the virus replication efficiently with an EC50 value of 0.228 μM. These results suggested that the prodrug 13a is a promising candidate agent for the treatment of AIDS.

Concepts: Antiretroviral drug, DNA, Pharmacology, Virus, Molecule, Enzyme inhibitor, Viral infectivity factor, APOBEC3G