Complementary DNA portions coding for each domain (domain L internally repetitive domains, domains 1–4, each composed of approximately 140 amino acid residues) of pig calpastatin were subcloned into E. coli plasmids to express the respective portions of the proteinase inhibitor gene in bacteria. Cell extracts of E. coli harboring recombinant plasmids were assayed for calpain inhibition. All four internally repetitive domains showed inhibitory activities, essentially similar to one another, against calpains I and II. No inhibition was observed in the case of the N-terminal non-homologous domain (domain L). These results support our previous conclusion that the repetitive region is a functional unit of the proteinase inhibitor.
