Both Rbx1 and Rbx2 exhibit a functional role in the HIV-1 Vif-Cullin5 E3 ligase complex in vitro

• Both Rbx1 and Rbx2 can play a functional role in Vif E3 ligase in vitro.
• Rbx2 suppresses the interaction between Rbx1 and Cul5 in cells.
• Down-regulation of endogenous Rbx2 but not Rbx1 impairs Vif-induced A3G degradation.
• Two possible explanations for the selection of Cul5-Rbx module were proposed.

Rbx1 and Rbx2 are essential components of Cullin-RING E3 Ligases. Vif is generally believed to preferentially recruit the Cul5-Rbx2 module to induce proteasomal degradation of antiretroviral enzyme APOBEC3G, although some investigators have found that the Cul5-Rbx1 module is recruited. Here, to investigate the function of the two Rbx proteins in the Vif-Cul5 complex, we analyzed the performance of Cul5-Rbx1/Cul5-Rbx2 module in the activity of Vif E3 ligase and evaluated the interactions between Rbx1/Rbx2 and Cul5. We found that either Rbx1 or Rbx2 could promote ubiquitination of APOBE3G (A3G) in vitro. We also found that both Rbx1 and Rbx2 could bind Cul5 in cells and Rbx2 could dose-dependently inhibit the interaction of Rbx1 with Cul5. Furthermore, only the decrease of endogenous Rbx2 but not Rbx1 could impair the Vif-induced A3G degradation in cells. These findings indicate that Rbx1 and Rbx2 can both activate Cul5-Vif E3 ligase in vitro, but they may undergo a more delicate selection mechanism in vivo.