Effect of cAMP-dependent protein kinase A (PKA) on HCV nucleocapsid assembly and degradation.

The primary function of the hepatitis C virus (HCV) core protein is genome encapsidation. Core protein is also subject to post-translational modifications that can impact on the assembly process. In this report, we have studied the effect of cAMP-dependent protein kinase A (PKA) phosphorylation on its assembly and stability in a yeast Pichia pastoris expression system. We have recently shown that co-expression of the human signal peptide peptidase and core protein (amino acids 1-191) in yeast leads to the formation of nucleocapsid-like particles (NLPs) that are morphologically similar to the wild-type HCV capsid. In this system, we expressed mutants S53A and S116A and mutants S53D and S116D to abolish or mimic PKA phosphorylation, respectively. None of these mutations affected HCV assembly, but S116D led to the degradation of core protein. We also showed that nonenveloped NLPs were labelled in vitro by PKA, suggesting that the phosphorylation sites are available at the surface of the NLPs. The co-expression of human PKA with core and human signal peptide peptidase in yeast did not produce phosphorylated NLPs and led to a decreased accumulation of nonenveloped particles. Mutation S116A restored the core protein content. These results suggest that PKA phosphorylation can modulate HCV core levels in infected cells.