CALPAIN DISSOCIATES INTO SUBUNITS IN THE PRESENCE IONS

Calpain is a calcium dependent cysteine protease consisting of a catal ytic 80K subunit and a regulatory 30K subunit. It has therefore been b elieved that calpain functions as a dimer. Here we have found that cal pain dissociates into subunits in the presence of the Ca2+ required fo r the expression of activity and that the dissociated 80K subunit is e nzymatically fully active. Moreover, the 80K subunit shows a calcium s ensitivity identical to the activated form of calpain but not to the o riginal control calpain. The results suggest that the activation of ca lpain corresponds to the dissociation into subunits in the presence of Ca2+ and that calpain functions as a monomer of the 80K subunit in vi vo. (C) 1995 Academic Press, Inc.