COMPLEX OF NS3 PROTEASE AND NS4A PEPTIDE OF BK STRAIN HEPATITIS-C VIRUS - A 2.2 ANGSTROM RESOLUTION STRUCTURE IN A HEXAGONAL CRYSTAL FORM

The crystal structure of the NS3 protease of the hepatitis C virus (BK strain) has been determined in the space group P6(3)22 to a resolutio n of 2.2 Angstrom. This protease is bound with a 14-mer peptide repres enting the central region of the NS4A protein. There are two molecules of the NS3(1-180)-NS4A(21'-34') complex per asymmetric unit. Each dis plays a familiar chymotrypsin-like fold that includes two beta-barrel domains and four short alpha-helices. The catalytic triad (Ser-139, Hi s-57, and Asp-81) is located in the crevice between the beta-barrel do mains. The NS4A peptide forms an almost completely enclosed peptide su rface association with the protease. In contrast to the reported H str ain complex of NS3 protease-NS4A peptide in a trigonal crystal form (K im JL et al., 1996, Cell 87:343-355), the N-terminus of the NS3 protea se is well-ordered in both molecules in the asymmetric unit of our hex agonal crystal form. The folding of the N-terminal region of the NS3 p rotease is due to the formation of a three-helix bundle as a result of crystal packing. When compared with the unbound structure (Love RA et al., 1996, Cell 87:331-342); the binding of the NS4A peptide leads to the ordering of the N-terminal 28 residues of the NS3 protease into a beta-strand and an alpha-helix and also causes local rearrangements i mportant for a catalytically favorable conformation at the active site . Our analysis provides experimental support for the proposal that bin ding of an NS4A-mimicking peptide, which increases catalytic rates, is necessary but not sufficient for formation of a well-ordered, compact and, hence, highly active protease molecule.