Calpain dissociates into subunits in the presence of calcium ions.

Calpain is a calcium dependent cysteine protease consisting of a catalytic 80K subunit and a regulatory 30K subunit. It has therefore been believed that calpain functions as a dimer. Here we have found that calpain dissociates into subunits in the presence of the Ca2+ required for the expression of activity and that the dissociated 80K subunit is enzymatically fully active. Moreover, the 80K subunit shows a calcium sensitivity identical to the activated form of calpain but not to the original control calpain. The results suggest that the activation of calpain corresponds to the dissociation into subunits in the presence of Ca2+ and that calpain functions as a monomer of the 80K subunit in vivo.