MECHANISM OF ACTION OF A NEW COMPONENT OF THE CA2-DEPENDENT PROTEOLYTIC SYSTEM IN RAT-BRAIN - THE CALPAIN ACTIVATOR()

Rat brain contains a calpain activator specific for the mu-form of the proteinase. We now report that this protein factor binds to the catal ytic 80 kDa calpain subunit, promoting the dissociation of the heterod imer structure of the proteinase. The successive steps of the activati on process, namely the two autoproteolytic steps producing the 78 kDa and the 75 kDa calpain forms, result in a 100 times faster rate. The a ctivator competes with calpastatin and associates with the inner surfa ce of plasma membranes. Based on its properties, the calpain activator can be visualised as the molecule indicating the sites for calpain ac tivation at which the proteinase can also elude the negative control e xerted by calpastatin. (C) 1998 Academic Press.