Proteasomal Turnover of Hepatitis C Virus Core Protein Is Regulated by Two Distinct Mechanisms: a Ubiquitin-Dependent Mechanism and a Ubiquitin-Independent but PA28γ-Dependent Mechanism

We have previously reported on the ubiquitylation and degradation of hepatitis C virus core protein. Here we demonstrate that proteasomal degradation of the core protein is mediated by two distinct mechanisms. One leads to polyubiquitylation, in which lysine residues in the N-terminal region are preferential ubiquitylation sites. The other is independent of the presence of ubiquitin. Gain- and loss-of-function analyses using lysineless mutants substantiate the hypothesis that the proteasome activator PA28 gamma, a binding partner of the core, is involved in the ubiquitin-independent degradation of the core protein. Our results suggest that turnover of this multifunctional viral protein can be tightly controlled via dual ubiquitin-dependent and -independent proteasomal pathways.