Calpain autolyses in the presence of Ca2+. In the case of m-calpain (80 + 30 kDa) the first product is an 80 + 18 kDa species which has an intact large subunit and the C-terminal Ca(2+)-binding domain of the small subunit. It was possible to bind E64 into the active site of calpain in the presence of Ca2+ before cleavage of either calpain subunit. This suggests that the active site is functional before any autolysis has occurred and that calpain is not a proenzyme. Prolonged autolysis generates several fragments including a 42 kDa active-site domain fragment that showed no proteolytic activity and Ca(2+)-binding domain fragments. Some of the Ca(2+)-binding domain fragments were found to exist as heterodimers (23 + 18 kDa and 22 + 18 kDa), with the Ca(2+)-binding domain of the large subunit interacting with the Ca(2+)-binding domain of the small subunit. These species were true heterodimers, as they showed co-elution of the two Ca(2+)-binding domains on ion-exchange and gel-filtration chromatography, and immunoprecipitation of both polypeptides with an antiserum specific for the small-subunit Ca(2+)-binding domain. The generation of the dimer species after only 15 min autolysis suggests that the interaction between the Ca(2+)-binding domains is present in the native calpain structure. The interaction of calpain with calpastatin was investigated using an assay based on binding to calpastatin-Sepharose and a competitive binding assay. Calpain, active-site-blocked calpain and calpain fragments generated by autolysis were studied. Calpain bound to calpastatin in the presence of Ca2+; however, the isolated active-site-containing 80 kDa large subunit (proteolytically inactive), a 42 kDa active-site-containing fragment (proteolytically inactive) and Ca(2+)-binding domain fragments of calpain did not. Active-site-blocked calpain bound to calpastatin, but with an affinity reduced by approximately two orders of magnitude when compared with native calpain.

Crawford, C., Brown, N. R., & Willis, A. C. (1993). Studies of the active site of m-calpain and the interaction with calpastatin. Biochemical Journal, 296(1), 135–142.