Enzymatic characterization of membrane-associated hepatitis C virus NS3-4A heterocomplex serine protease activity expressed in human cells.

The hepatitis C virus (HCV) nonstructural (NS)3-NS4A serine protease heterocomplex is a prime target for development of novel HCV therapies, due to its essential role in maturation of the viral polyprotein. While the mode of substrate/inhibitor recognition of the HCV NS3/NS4A serine protease has been extensively studied in vitro, important molecular aspects of the mechanism of action for this membrane-bound multifunctional enzyme remain unresolved in vivo. In particular, what influence does membrane association exert on the specificity and catalysis of NS3-4A protease? To carry out this study, we developed a specific and sensitive protease assay using a unique internally quenched fluorogenic substrate (IQFS). Our IQFS enables for the first time the direct, specific detection of NS3-4A protease activity within membrane fractions isolated from human cells expressing NS3-4A and the determination of its steady-state kinetic parameters, which were found to be K(m) = 51 +/- 3 microM and k(cat) = 0.39 min(-1). We also show that our fluorescence-based bioassay can be used to evaluate specifically the potency and mode of action of NS3-4A directed inhibitors, such as in the case of a known NS3-4A substrate-analogue inhibitor (K(i) = 22 nM). Our results indicate that the membrane anchoring of NS3 by NS4A does not affect the substrate/inhibitor recognition by the NS3-4A protease domain. Further investigation may reveal whether membrane association could be important for regulating other enzymatic activities associated with NS3 (e.g., helicase and/or ATPase) and/or regulating the recently proposed cross-talk between the protease and helicase activities.