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 2006, we conducted studies to improve our understanding of the biochemical properties of a recently identified Vif-sensitive host factor, APOBEC3G. We demonstrated earlier that inhibition of HIV-1 by APOBEC3G requires its packaging into HIV-1 virions through association with the viral RNA. Several studies found that APOBEC3G can be packaged into virus-like particles in an RNA-independent manner; however, our own data demonstrate that such RNA-independent packaging of APOBEC3G will not cause an antiviral effect. Thus, APOBEC3G not only has to be present in virions but, more specifically, must be associated with the viral core to exert antiviral activity. More recently, we determined that APOBEC3G is packaged into virions in monomeric form. This study is significant since intracellularly APOBEC3G efficiently multimerizes and forms large RNA-dependent multi-protein complexes. The functional significance of such APOBEC3G complexes remains to be determined; however, our data suggest that they most likely do not have functional significance for either encapsidation into HIV-1 virions or anti-viral activity. The HIV accessory protein Vif has the remarkable ability to inhibit the antiviral activity of APOBEC3G. We found that Vif can cause efficient depletion of APOBEC3G from virus-producing cells; however, we also found that such depletion could be functionally dissociated from the antiviral effect of APOBEC3G. We identified Vif variants that efficiently reduced cellular APOBEC3G without restoring viral infectivity and vice versa. We also identified APOBEC3G variants that were insensitive to Vif-induced degradation but whose antiviral activity remained Vif-sensitive. 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.