Mechanism Of HIV-1 And Other Species Vif Mediated The Suppression Of APOBEC3 Cytidine Deamineases

Vif(viral infectivity factor)is one of accessory proteins that was encoded by all lentiviruses except Equine infectious anemia virus. During HIV-1 infection, HIV-1 Vif suppresses native defensive factors, various APOBEC3 proteins through a common mechanism by recruiting Cullin5, ElonginB, ElonginC E3 ubiquitin ligase to induce targeted protein polyubiquitination and proteasome-mediated degradation. In the Cul5-containing E3 ligase complexes, HIV-1 Vif recruits ElonginB/C through the SLQXLA motif, which is highly conserved between HIV-1 and other lentiviral Vif molecules and a virus-specific BC-box that mediates the interaction with ElonginB/C. The HCCH motif that binds zinc and LPx4L motif in HIV1 Vif are required for Cul5 binding. SIVagmTan Vif has virus-specific BC-box and HCCH motif, but lacks LPx4L motif.Human cytidine deaminase apolipoprotein B mRNA-editing catalytic polypeptide-like 3G(APOBEC3G, A3G) and related cytidine deaminases of the APOBEC3 family of proteins which have cytidine deaminase activities that modify ssDNA and mRNA. They are potent inhibitors of many retroviruses, including HIV-1. In the absence of Vif, APOBEC3 proteins can be packaged into diverse retroviruses and they mediate potent antiviral functions in newly infected target cells. Encapsidation of human A3G and A3F into HIV-1 particles is mediated by the nucleocapsid domain (NC) of Gag with the help of viral genomic RNA and/or 7SL RNA. This virion-packaged A3G and A3F deaminate cytocines of the newly synthesized cDNA minus strand during reverse transcription in the next cycle of infection. As a result, numerous guanosine (G) to adenosine (A) changes in the plus strand of the provirus occur. These viral progency proved to be noninfectious. Except the cytidine deaminase-dependent activity, A3G and another APOBEC3 protein, A3F, also reduce the accumulation of viral reverse transcription products and inhibit proviral DNA formation.The interaction of Vif and APOBEC3 proteins is species specific. HIV-1 Vif interacts with human A3G but not rhesus macaque APOBEC3G (rhA3G) and African green monkey APOBEC3G (AGMA3G). In contrast, SIVagmTan Vif only interacts with AGMA3G but not human A3G.On the basis of HIV-1, we study the Vif proteins from other species such as SIVagmTan (African green monkey simian immunodeficiency virus) and BIV (bovine immunodeficiency virus).In the present study, we first showed like HIV-1 Vif, SIVagmTan Vif and BIV Vif all contain highly conserved sequence SLQXLA motif, which is a virus-specific BC-box that mediates the interaction with ElonginB and ElonginC. SIVagmTan Vif can form the Cul5-E3 ubiquitin ligase through HCCH motif and suppresses antiviral activity of AGMA3G and human A3C, despite it lacks LPx4L Cul5 box that is present in HIV-1 Vif. BIV Vif can form E3 ubiquitin ligase through downstream Cullin-box of BC-box and suppresses the activity of bA3F, despite it lacks HCCH motif. This conservation of function suggests that, despite a significant difference in protein structure HIV-1, BIV and SIVagmTan Vif molecules have evolved to use an evolutionarily conserved Cullin-containing E3 ubiquitin ligase to degrade APOBEC3 proteins.BIV Vif only inhibits its natural host cell antiviral factor, the bA3F, but not the other APOBEC3 proteins from other species such as mouse, sheep, pig and human. This is another evidence that the function of Vif is species specific. At the same time, we demonstrated for the first time that SIVagmTan Vif overcomes not only the antiviral activity of AGMA3G and A3F, but also that of human A3C across species. The APOBEC3 genes in AGM have not been fully characterized. However, the ability of SIVagm Vif to overcome human A3C suggests that primate lentiviral Vif proteins may also act on antiviral proteins that the virus may not even encounter in its natural host.Various amino-terminal domains of HIV-1 Vif are responsible for its specificity in recognizing the various human APOBEC3 proteins. Amino-terminal Y40RHHY44 (G-box) is important for A3G binding and degradation. On the other hand, D14RMR17 (F1-box) and W79 are required for A3F binding and degradation. He et al demonstrated that T74G/PERxW79 Motif (F2-box) is required for suppression of A3F and is dispensable for A3G binding and suppression. However, VxIPLx4LxIx2YWxL motif (FG-box) (which is rich in hydrophobic [Φ] residues:ΦxΦPΦx4-5ΦxΦx2YWxΦ) is important for binding to both A3G and A3F.Our data suggest that Vif employs distinct protein interfaces to recognize various human APOBEC3 proteins. The amino-terminal of HIV-1 Vif binds not only A3G and A3F, but also A3C and A3DE. The D14RMR17 and T74G/PERxW79 motifs of HIV-1 Vif are dispensable for the suppression of the potent anti-HIV-1 cytidine deaminase A3G yet are highly conserved among diverse HIV-1 strains. But they are important for suppression A3F, A3C and A3DE. K22和Y40RHHY44 motifs of HIV-1 Vif that are required for Vif-mediated degradation of A3G are dispensable for Vif-mediated A3F, A3C and A3DE degradation. HIV-1 Vif recognizes A3F, A3C, and A3DE through a similar mechanism that is distinct from that for A3G. One possible interpretation is that homolog of A3C and the carboxyl-terminal of A3F is very high, so that HIV-1 Vif recognizes the two molecules through the similar mechanism.We also demonstrated the distinct domains of various APOBEC3 proteins recognized by HIV-1 Vif. Human cytidine deaminases include the single cytidine deamination domain protein (A3C) and double cytidine deamination domain protein (A3G and A3F). The results of our comparative study of parental human A3A, A3C, and their chimeras have indicated that the amino-terminal region of human A3C upstream from the active site of A3C determines the molecule's sensitivity to Vif. This argument is further supported by the observation that amino-terminal deletion of amino acids 2 to 47 made mutant A3C resistant to Vif-induced degradation. Interestingly, amino acids 2 to 47 of human A3G were dispensable for Vif-induced degradation. Our results are consistent with the previous observation that amino acids 54-124 of human A3G is sufficient for interaction with HIV-1 Vif and residues 104-245 in human A3G are sufficient for HIV-1 Vif-induced degradation. Thus, both cytidine deamination domains of A3G are involved in Vif-mediated degradation. We have now found that, unlike the case for A3G, the carboxyl-terminal domain of another double-domain protein, human A3F, alone is sufficient for the interaction with HIV-1 Vif, and, more importantly, is all that is required for A3F to undergo interaction with Vif and Vif-mediated degradation. So, carboxyl-terminal domain of human A3F is required for Vif-mediated degradation.These information may be useful for the design and development of novel anti-HIV inhibitors.