APOBEC3G (A3G) restricts the infectivity of human immunodeficiency virus 1 (HIV-1) by targeting newly synthesized minus strand DNAs, which are reverse-transcribed from HIV-1 RNA, and deaminating deoxycytidines within these single-stranded (ss) DNAs into deoxyuridines.
HIV-1 viral-infectivity-factor (Vif) neutralizes the anti-HIV-1 activities of A3G by forming a five-membered complex (VβBCC) that hijacks a transcription factor CBFβ and the components of human E3 ubiquitin ligase (Culin5, Elongin B, and Elongin C).
While Vif is known to bind to the C-terminal domain A3G, leading to its ubiquitination and degradation through ubiquitin-dependent proteolysis, other inactivation systems have also been observed. The hypothesis that Vif directly inhibits deaminase activity of A3G has been previously discussed, but it remains controversial.
In this study, we investigated the effects of VβBCC on the deaminase activities of A3G using real-time NMR and biochemical analyses. Our data demonstrate that VβBCC directly inhibits the deamination activity of A3G. Furthermore, VβBCC was shown to inhibit the deamination activities of A3F and A3B, which are the ubiquitination-sensitive and insensitive homologs of A3G, respectively. These results indicate that VβBCC inhibits the activities of A3 regardless of ubiquitination. To confirm the mechanism of VβBCC-derived deamination inhibition, we carefully examined the molecular binding properties among A3G, VβBCC, and DNA using analytical ultracentrifuge, fluorescence anisotropy, and NMR. Unexpectedly, we found that this inhibition is caused by a direct interaction between VβBCC and the C-terminal domain of A3G, which was previously not believed to interact with Vif. This implies that the interaction between VβBCC and both the C-terminal domain and N-terminal domains of A3G, the latter known to be targeted for ubiquitination, may need to be inhibited to prevent counteraction by Vif.