Identification and characterization of loop7 motif and its role in regulating biological function of human APOBEC3G through molecular modeling and biological assay

Human APOBEC3G (hA3G) is a cytidine deaminase which inhibits HIV-1 replication. The HIV-1 accessory protein viral infectivity factor (Vif) counteracts with hA3G by targeting it for proteasomal degradation. In this work, we constructed and optimized molecular models of the hA3G dimer and the hA3G–Vif complex. The molecular modeling study revealed that the loop7 motif of hA3G appears on the interfaces of both the hA3G–Vif complex and the hA3G dimer. Biochemical analysis provided evidence suggesting that binding of Vif to hA3G results in steric blocking of hA3G dimerization, implying that monomeric hA3G serves as a substrate for Vif-mediated degradation. Furthermore, we presented evidence for the important roles of the loop7 motif, especially the central residues within the region, in hA3G dimerization, hA3G--Vif interaction, Vif-mediated hA3G degradation as well as subcellular localization of hA3G. This work highlights a multiple-task interface formed by loop7 motif, which regulates biological function of hA3G, thus providing the feasibility of the strategy of blocking Vif-mediated A3G degradation by targeting the putative site around loop7.