Site-directed mutagenesis of the putative catalytic triad of poliovirus 3C proteinase

Based on predictions of the structure of proteinase 3C of poliovirus, mutations have been made at residues that are supposed to constitute the catalytic triad. Wild-type and mutant 3C were expressed in Escherichia coli, purified to homogeneity, and characterized by the ability to cleave a synthetic peptide substrate or an in vitro translated polypeptide consisting of part of the polyprotein of poliovirus. Additionally, the ability of autocatalytic processing of a precursor harboring wild-type or mutant 3C sequences was tested. Single substitutions of the residues His-40, Glu-71, and Cys147 by Tyr, Gln, and Ser, respectively, resulted in an inactive enzyme. Replacement of Asp-85 by Asn resulted in an enzyme that was as active as wild-type enzyme in trans cleavage assays but whose autoprocessing ability was impaired. Our results are consistent with the proposal that residues His-40, Glu-71, and Cys-147 constitute the catalytic triad of poliovirus 3C proteinase. Furthermore, residue Asp-85 is not required for proper proteolytic activity despite being highly conserved between different picornaviruses. This indicates that Asp-85 might be involved in a different function of 3C.