Core-Binding Factor Subunit Beta Is Not Required for Non-Primate Lentiviral Vif-Mediated APOBEC3 Degradation

Abstract

Viral infectivity factor (Vif) is required for lentivirus fitness and pathogenicity, except in equine infectious anemia virus (EIAV).Vif enhances viral infectivity by a Cullin5-Elongin B/C E3 complex to inactivate the host restriction factor APOBEC3. Corebinding factor subunit beta (CBF-␤) is a cell factor that was recently shown to be important for the primate lentiviral Vif function. Non-primate lentiviral Vif also degrades APOBEC3 through the proteasome pathway. However, it is unclear whether CBF-␤ is required for the non-primate lentiviral Vif function. In this study, we demonstrated that the Vifs of non-primate lentiviruses, including feline immunodeficiency virus (FIV), bovine immunodeficiency virus (BIV), caprine arthritis encephalitis virus (CAEV), and maedi-visna virus (MVV), do not interact with CBF-␤. In addition, CBF-␤ did not promote the stability of FIV, BIV, CAEV, and MVV Vifs. Furthermore, CBF-␤ silencing or overexpression did not affect non-primate lentiviral Vif-mediated APOBEC3 degradation. Our results suggest that non-primate lentiviral Vif induces APOBEC3 degradation through a different mechanism than primate lentiviral Vif. IMPORTANCEThe APOBEC3 protein family members are host restriction factors that block retrovirus replication. Vif, an accessory protein of lentivirus, degrades APOBEC3 to rescue viral infectivity by forming Cullin5-Elongin B/C-based E3 complex. CBF-␤ was proved to be a novel regulator of primate lentiviral Vif function. In this study, we found that CBF-␤ knockdown or overexpression did not affect FIV Vif's function, which induced polyubiquitination and degradation of APOBEC3 by recruiting the E3 complex in a manner similar to that of HIV-1 Vif. We also showed that other non-primate lentiviral Vifs did not require CBF-␤ to degrade APOBEC3. CBF-␤ did not interact with non-primate lentiviral Vifs or promote their stability. These results suggest that a different mechanism exists for the Vif-APOBEC interaction and that non-primates are not suitable animal models for exploring pharmacological interventions that disrupt Vif-CBF-␤ interaction.

Vif determines the requirement for CBF-β in APOBEC3 degradation

“…2). These data corroborate recent reports suggesting that not only FIV Vif but also BIV and SRLV Vif proteins do not require CBF-b to degrade APOBEC3 proteins of their hosts (Ai et al, 2014;Zhang et al, 2014). These observations indicate that only primate lentiviruses require CBF-b for APOBEC3 degradation and further suggest that Table 1.…”

“…These characteristics imply that FIV Vif is structurally dissimilar to HIV/SIV Vif. Additionally, Ai et al (2014) have shown that the conserved CBF-b interaction sequences in SIV/HIV Vif are not present in FIV Vif. Furthermore, Wang et al (2011) have reported that the FIV Vif has neither a CUL5 box nor HCCH zincbinding motif, despite a functional requirement for CUL5 in APOBEC3 degradation.…”

The Road Less Traveled: HIV's Use of Alternative Routes through Cellular Pathways

“…Either way, this example illustrates the existence of a potential alternative route which HIV-1 Vif may evolve to utilize as an escape mechanism should the availability of CBF-␤ become blocked. The SIVmac Vif (with a 22-amino-acid extension), while recruiting CBF-␤, has been reported to be less dependent on its presence (74). So far, it has been shown that chimeric HIV-1-containing Vif from FIV can successfully replicate in feline cells and escape the feline APOBEC3 protein, which is otherwise insensitive to HIV-1 Vif (76), and it will be illustrating to investigate the interactome of chimeric HIV-1 Vif variants containing mutations, truncations, and extensions inspired by the non-primate sequences.…”

“…As mentioned above, biochemical evidence and the recently revealed structure of human cullin5-ElonginB/C-CBF␤ complexed with HIV-1 Vif (PDB no. 4N9F [49]) suggest that CBF-␤ plays a chaperoning role, facilitating Vif stability, folding, and solubility (49,73,74).…”

“…Although non-primate lentiviruses use the same strategy for inhibiting APOBEC3 proteins via Vif manipulation of the E3 ligase pathway, it has recently been shown that CBF-␤ does not interact with or promote the stability of Vif from non-primate lentiviruses such as feline immunodeficiency virus (FIV; infects cats), caprine arthritis encephalitis virus (CAEV; infects goats), bovine immunodeficiency virus (BIV; infects cattle), and maedivisna virus (MVV; infects sheep) and is dispensable for Vif recruitment of the E3 complex and subsequent APOBEC3 degradation (54,74,75). Cross-species Vif sequence comparisons reveal that viruses shown not to require CBF-␤ for Vif degradation of APOBEC3 have ϳ40 to ϳ60-amino-acid extensions in their sequences compared to HIV-1 Vif (except for BIV Vif, with only an ϳ6-amino-acid extension) and that it is possible that these sequences substitute for the CBF-␤ role in chaperoning Vif folding and solubility (Fig.…”