Interactions between HIV-1 Vif and human ElonginB-ElonginC are important for CBF-beta binding to Vif

Background: The HIV-1 accessory factor Vif is necessary for efficient viral infection in non-permissive cells. Vif antagonizes the antiviral activity of human cytidine deaminase APOBEC3 proteins that confer the non-permissive phenotype by tethering them (APOBEC3DE/3F/3G) to the Vif-CBF-beta-ElonginB-ElonginC-Cullin5-Rbx (Vif-CBF-beta-EloB-EloC-Cul5-Rbx) E3 complex to induce their proteasomal degradation. EloB and EloC were initially reported as positive regulatory subunits of the Elongin (SIII) complex. Thereafter, EloB and EloC were found to be components of Cul-E3 complexes, contributing to proteasomal degradation of specific substrates. CBF-beta is a newly identified key regulator of Vif function, and more information is needed to further clarify its regulatory mechanism. Here, we comprehensively investigated the functions of EloB (together with EloC) in the Vif-CBF-beta-Cul5 E3 ligase complex. Results: The results revealed that: (1) EloB (and EloC) positively affected the recruitment of CBF-beta to Vif. Both knockdown of endogenous EloB and over-expression of its mutant with a 34-residue deletion in the COOH-terminal tail (EloB Delta C34/EB Delta C34) impaired the Vif-CBF-beta interaction. (2) Introduction of both the Vif SLQ -> AAA mutant (Vif Delta SLQ, which dramatically impairs Vif-EloB-EloC binding) and the Vif PPL -> AAA mutant (Vif Delta PPL, which is thought to reduce Vif-EloB binding) could reduce CBF-beta binding. (3) EloB-EloC but not CBF-beta could greatly enhance the folding of full-length Vif in Escherichia coli. (4) The over-expression of EloB or the N-terminal ubiquitin-like (UbL) domain of EloB could significantly improve the stability of Vif/Vif Delta SLQ/Vif Delta PPL through the region between residues 9 and 14. Conclusion: Our results indicate that the Vif interaction with EloB-EloC may contribute to recruitment of CBF-beta to Vif, demonstrating that the EloB C-teminus may play a role in improving Vif function and that the over-expression of EloB results in Vif stabilization.