UCSF

APOBEC3F (A3F) and APOBEC3G (A3G) are DNA cytosine deaminases that potently restrict human immunodeficiency virus type 1 replication when the virus is deprived of its accessory protein Vif (virion infectivity factor). Vif counteracts these restriction factors by recruiting A3F and A3G to an E3 ubiquitin (Ub) ligase complex that mediates their polyubiquitination (polyUb) and proteasomal degradation. While previous efforts have identified single amino acid residues in APOBEC3 proteins required for Vif recognition, less is known about the downstream Ub acceptor sites that are targeted. One prior report identified a cluster of polyubiquitinated residues in A3G and proposed an antiparallel model of A3G interaction with the Vif-E3 Ub ligase complex wherein Vif binding at one terminus of A3G orients the opposite terminus for polyUb [Iwatani et al. (2009). Proc. Natl. Acad. Sci. USA, 106, 19539-19544]. To test the generalizability of this model, we carried out a complete mutagenesis of the lysine residues in A3F and used a complementary, unbiased proteomic approach to identify Ub acceptor sites targeted by Vif. Our data indicate that internal lysines are the dominant Ub acceptor sites in both A3F and A3G. In contrast with the proposed antiparallel model, however, we find that the Vif-dependent polyUb of A3F and A3G can occur at multiple acceptor sites dispersed along predicted lysine-enriched surfaces of both the N- and C-terminal deaminase domains. These data suggest an alternative model for binding of APOBEC3 proteins to the Vif-E3 Ub ligase complex and diminish enthusiasm for the amenability of APOBEC3 Ub acceptor sites to therapeutic intervention.