Generally, the maximum activities of the protein kinases A (PKA) and C (PKC) show an optimum value for their substrate concentrations rather than a saturation curve; at high substrate concentrations, the kinase activity is completely abolished. The C- and N-truncated form of the non-structural protein 3 (NS3) of hepatitis C virus (HCV) (HCV-polyprotein-(1,189-1,525)) abolishes the inhibiting effect of the substrate, yielding saturable Michaelis-Menten kinetics of PKA and its catalytical domain (C subunit). In contrast, HCV-polyprotein-(1,189-1,525) activates PKC with increasing Vmax, while it abolishes the substrate inhibition of its catalytical domain (M-kinase) through a mechanism analogous to that of PKA and C subunit. PKC isoforms alpha, beta and gamma investigated are similarly activated by HCV-polyprotein-(1,189-1,525). Our data suggest that NS3 attenuates the substrate inhibition through a generalized mechanism operating mainly on the substrate level that directly results from a specific protein-protein interaction. In the case of the PKC, an additional kinase activating mechanism operates on the enzyme level. Both actions of NS3, the attenuation of the substrate inhibition and the activation of PKC, could not be explained by classical means that predict autophosphorylation to enhance the rate of substrate phosphorylation. The results are discussed in view of similar activities displayed by matchmakers and some molecular chaperones.
