Specific proteolysis of a brain membrane phosphoprotein (B-50): effects of calcium and calmodulin

The membrane bound phosphoprotein B-50 (MW 48K) was isolated from rat brain tissue. The fraction containing the highest endogenous B-50 phosphorylating activity (ASP 57-82%) contains protease activity. In the absence of calcium a time-dependent decrease of the protein B-50 is observed. Under these conditions another phosphoprotein B-60 (MW 46K) appears in the incubation medium. Addition of calcium and/or calmodulin enhances the protease activity whereas the substrate specificity is lost. Results of both isoelectric focussing and peptide mapping indicate the B-50 and B-60 are related proteins. These data support our hypothesis that the recently isolated behaviorally active peptide PIP (MW approx. 1600 D) is the smaller cleavage product of the proteolytic degradation of B-50 to B-60.