Active site residues in m-calpain: identification by site-directed mutagenesis

Site-directed mutagenesis was used to alter putative active site residues in the large subunit of calpain, and the activity of the mutants was measured following coexpression in E. coli of both calpain subunits and purification of the resultant dimers. Mutants Cys105Ser, His262Ala and Asn286Ala had no activity. Together with sequence comparisons among cysteine proteinases, the results suggest that these residues constitute the catalytic triad in calpain. Mutants Asn286Asp and Trp288Tyr had low activity, consistent with interaction of these residues with His262.