APOBEC3G is degraded by the proteasomal pathway in a vif-dependent manner without being polyubiquitylated

Although the covalent attachment of a polyubiquitin is the prevailing paradigm for entry into proteasomes, accumulating evidence suggests that poorly defined ubiquitin-free pathways also degrade proteins. The cytidine deaminase APOBEC3G (A3G) potently inhibits human immunodeficiency virus type 1 replication by disrupting viral reverse transcription. However, human immunodeficiency virus type 1 produces a viral infectivity factor (Vif) to destroy this antiretroviral protein. It was shown that Vif binds to both A3G and a Cullin 5 ubiquitin-protein isopeptide ligase. It is currently accepted that this enzyme polyubiquitylates A3G on lysine residues, resulting in its degradation by proteasomes. Here, we find that A3G without ubiquitylation is still degraded by proteasomes in a Vif-dependent manner. We further show that Vif is polyubiquitylated and that this event could be critical for A3G proteasomal degradation. Thus, A3G is degraded by a novel pathway that might involve ubiquitylation of one protein and then targets a second binding partner for proteasomal entry and degradation. We propose that instead of triggering A3G polyubiquitylation, polyubiquitylated Vif might serve as a vehicle to transport A3G into proteasomes for degradation.