The RNA-unwinding activity of hepatitis C virus non-structural protein 3 (NS3) is positively modulated by its protease domain

The nonstructural protein 3 (NS3) of hepatitis C virus contains a protease domain at its amino terminus and RNA helicase domain at its carboxyl terminus. To identify optimal NS3 protein for developing screening assays, we expressed full-length NS3 protease/helicase and helicase domains from both HCV type 1a (H77 strain) and 1b (Con1 strain), using either E. coli or baculovirus expression systems. Our studies showed that the full-length NS3 proteins, either with or without the presence of the NS4A domain, from either strains were at least 10-fold more efficient than the corresponding helicase domains in unwinding partial duplex RNA substrates. These findings provide a rationale for the use of full-length NS3 in high throughput screening assays to identify potent small molecule inhibitors of this important target of HCV.