Origins of the difference in Ca2+ requirement for activation of mu- and m-calpain.

The mu- and m-calpains are closely related Ca(2+)-dependent cysteine proteases having different in vitro Ca(2+) requirements ( K (d)), of approx. 25 and 325 microM respectively. The two isoforms are heterodimers of slightly different large (80 kDa) subunits and an identical small (28 kDa) subunit, so that the difference in K (d) values must reside in the large subunits.