Cyclin F/FBXO1 interacts with HIV-1 Vif and restricts progeny virion infectivity by ubiquitination and proteasomal degradation of Vif through SCF Cyclin F E3 ligase machinery [Protein Synthesis and Degradation]

Cyclin F, also known as FBXO1, is the largest among all cyclins which oscillates in the cell cycle like other cyclins. Apart from being a G2/M cyclin, Cyclin F functions as the substrate binding subunit of SCFCyclin F E3 ubiquitin ligase. In a gene expression analysis performed to identify novel gene modulations associated with cell cycle dysregulation during HIV-1 infection in CD4+ T cells, we observed down-regulation of Cyclin F (CCNF) gene. Later, using gene over expression and knockdown studies, we identified that Cyclin F negatively influences HIV-1 viral infectivity without any significant impact on virus production. Subsequently, we found that Cyclin F negatively regulates the expression of viral protein, Vif (Viral infectivity factor), at the protein level. We also identified a novel host-pathogen interaction between Cyclin F and Vif protein in T cells during HIV-1 infection. Mutational analysis of a Cyclin F-specific amino acid motif in the C-terminal region of Vif shows rescue of the protein from Cyclin F-mediated down-regulation. Subsequently, we have shown that Vif is a novel substrate of the SCFCyclin F E3 ligase, where Cyclin F mediates ubiquitination and proteasomal degradation of Vif through physical interaction. Finally, we have shown that Cyclin F augments APOBEC3G expression through degradation of Vif to regulate infectivity of progeny virions. Taken together, our results demonstrate Cyclin F as a novel F-box protein which functions as an intrinsic cellular regulator of HIV-1 Vif and imparts a negative regulatory effect on maintenance of viral infectivity by restoring APOBEC3G expression.

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