HIV-1 encodes a number of genes that are crucial for replication in primate cells. Gag, Pol, and Env products represent the main virion components, while Tat and Rev products regulate intracellular transcriptional and post-transcriptional events for the controlled expression of viral genes. Of particular interest are the HIV accessory proteins Vif, Vpr, Vpu, Vpx, and Nef, which are unique to primate lentiviruses. There is increasing evidence that these proteins operate in conjunction with specific host factors. In fact, most if not all, of the accessory proteins appear to lack catalytic activities but instead seem to function as adaptors to link viral or cellular factors to pre-existing cellular pathways. In FY 2007, we conducted studies to improve our understanding of the functional interactions between HIV-1 Vif and a recently identified Vif-sensitive host factor, APOBEC3G. We and others recently demonstrated that Vif can cause proteasomal degradation of APOBEC3G. However, we also found that the effect of Vif on packaging of APOBEC3G into HIV virions was more pronounced than the effect on APOBEC3G degradation thus suggesting that Vif could target APOBEC3G through multiple pathways. In FY 2007 we identified an APOBEC3G variant that was completely resistant to degradation by Vif; however, Vif was still able to inhibit the antiviral activity of this APOBEC3G variant by preventing its encapsidation into viral particles. Similarly, we identified Vif variants that were able to cause degradation of APOBEC3G but did not inhibit APOBECs antiviral activity. The combined results of our studies demonstrate that virion exclusion of APOBEC3G and degradation by cellular proteasomes are functionally separable activities of the Vif protein. These studies are highly significant since they demonstrate that Vif does not merely act as an adaptor molecule to trigger the degradation of APOBEC3G, but has additional yet undefined functions required to control the antiviral effect of APOBEC3G. In addition to our studies on APOBEC3G, we identified in FY2007 a novel target of Vif, Cyclophilin A (CypA). It had been known for a number of years that CypA is packaged into HIV-1 virions where it has a positive effect on viral replication. CypA was, however, not seen in purified SIV virions. Interestingly, we found that the exclusion of CypA from SIV virions depended on the activity of SIV Vif. Accordingly, CypA was efficiently packaged SIV virions lacking Vif. Importantly, packaging of CypA while beneficial to HIV-1 caused a severe defect in SIV virions that led to the block of virus replication at an early post-entry step. Importantly, the inhibition of CypA encapsidation into SIV virions by Vif was not associated with intracellular degradation of CypA. Thus, we have identified a second cellular target whose encapsidation into virions was actively prevented by Vif protein. Characterizing the molecular mechanism that leads to the exclusion of CypA and APOBEC3G from viral particles will be subject of future research. Understanding the multiple activities of Vif is important for the design of assay systems to be used in high-throughput screens for specific pharmacological inhibitors of Vif.
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