A BC-BOX DOMAIN-RELATED MECHANISM FOR VHL PROTEIN DEGRADATION

Viruses need to hijack cellular machineries both for their replication and propagation and to overcome cellular defenses. Since they often reconvert the functions of host proteins for their purposes, viruses are also powerful tool to better understand some molecular mechanisms underlying cellular processes. Gam1 is an early protein of the avian adenovirus CELO and it possesses a BC-box domain to interact with the host adaptor heterodimer ElonginB/C and, hence, to act as a substrate receptor. It reconstitutes active Cullin2- and Cullin5-based E3 ligase complexes to ubiquitylate the host SAE1 protein, a subunit of SUMO E1 activating enzyme, inducing its proteasomal degradation. Von Hippel-Lindau (VHL) protein is a cellular substrate receptor that associates to Cullin2 and ElonginB/C to specifically target the α subunits of HIF (Hypoxia-Inducible Factor) transcriptional factors for degradation. VHL is a tumor suppressor protein and its loss leads to the von Hippel-Lindau syndrome, characterized by the onset of renal cell carcinoma and other highly vascularized tumors. We analyzed the possible effects on VHL due to the hijacking of host Cullin2 E3 ligase complexes by Gam1. Interestingly, we observed that Gam1 leads to VHL proteasomal degradation and to the consequent stabilization and activation of HIF-1. Further experiments revealed that VHL protein degradation was not dependent on Gam1-related E3 ligase activity. Rather, the simple binding of Gam1 and other cellular and viral BC-box proteins to ElonginB/C was enough to induce VHL degradation, probably due to the reduced availability of free ElonginB/C complex that is essential for VHL stability. Indeed, since unbound VHL undergoes misfolding, we are currently investigating the possible involvement of heat shock proteins and chaperone-dependent CHIP E3 ligase in affecting VHL stability upon BC-box proteins overexpression, as suggested by our preliminary data. If verified, this will be an additional mechanism uncovered by the initial contribution of a viral protein.