A central hydrophobic domain of the hepatitis C virus NS4A protein is necessary and sufficient for the activation of the NS3 protease.

The processing at the NS3/4A, NS4A/4B, NS4B/5A and NS5A/5B junctions in the non-structural region of the hepatitis C virus (HCV) polyprotein is performed by a viral serine protease activity contained within the N-terminal 180 amino acids of the NS3 protein. Full protease activity is only achieved upon the interaction of a region at the N terminus of NS3 with the NS4A protein, this region is also involved in the modulation of the protease activity. Using the rabbit reticulocyte expression system, we have defined the minimal domain of NS4A that is necessary to increase the cleavage efficiency of NS3. A synthetic peptide containing the same region, NS4A amino acids 21 to 32, stimulates the proteolytic activity of NS3 at all the trans-cleavage sites.