THE HEPATITIS-C VIRUS NS3 SERINE PROTEINASE AND NS4A COFACTOR - ESTABLISHMENT OF A CELL-FREE TRANS-PROCESSING ASSAY

The hepatitis C virus RNA genome encodes a long polyprotein that is proteolytically processed into at least 10 products, The order of these cleavage products in the polyprotein is NH2-C-E1-E2-p7-NS2-NS3-NS4A-NS4B-NS5A-NS5B-COOH. A serine proteinase domain located in the N-terminal one-third of nonstructural protein NS3 mediates cleavage at four downstream sites (the 3/4A, 4A/4B, 4B/5A, and 5A/5B sites), In addition to the proteinase catalytic domain, the NS4A protein is required for processing at the 4B/5A site but not at the 5A/5B site. These cleavage events are likely to be essential for virus replication, making the serine proteinase an attractive antiviral target, Here we describe an in vitro assay where the NS3-4A polyprotein, NS3, the serine proteinase domain (the N-terminal 181 residues of NS3), and the NS4A cofactor were produced by cell-free translation and tested for trans-processing of radiolabeled substrates. Polyprotein substrates, NS4A-4B or truncated NS5A-5B, were cleaved in trans by all forms of the proteinase, whereas NS4A was also required for NS4B-5A processing. Proteolysis was abolished by substitution mutations previously shown to inactivate the proteinase or block cleavage at specific sites in vivo. Furthermore, N-terminal sequence analysis established that cleavage in vitro occurred at the authentic 4A/4B site. Translation in the presence of microsomal membranes enhanced processing for some, but not all, proteinase-substrate combinations, Trans-processing was both time and temperature dependent and was eliminated by treatment with a variety of detergents above their critical micelle concentrations, Among many common proteinase inhibitors tested, only high (millimolar) concentrations of serine proteinase inhibitors tosyllysyl chloromethyl ketone and 4-(2-aminoethyl)benzenesulfonyl fluoride inactivated the NS3 proteinase, This in vitro assay should facilitate purification and further characterization of the viral serine proteinase and identification of molecules which selectively inhibit its activity.