Proteolysis of protein kinase C: mM and μM calcium‐requiring calpains have different abilities to generate, and degrade the free catalytic subunit, protein kinase M

Abstract

Limited proteolysis of protein kinase C (PKC) by calpain under cell free conditions cleaves the regulatory and catalytic PKC subunits, generating a free, co-factor independent catalytic subunit, termed PKM. In the present study, we demonstrate distinct differences in the rate, nature, and lipid-sensitivity of PKC and PKM proteolysis by μM and mM calcium-requiring calpain isozymes (μ calpain or m calpain, respectively). PKC is a preferred substrate for m calpain; not even a 100-fold increase in μ calpain was capable of degrading PKC as fast as in calpain. PKM was generated by both m and μ calpains, but was itself rapidly degraded by m calpain and therefore was only transiently detectable. By contrast, PKM was formed but not degraded by μ calpain, and persisted in the presence of μ calpain long after all PKC had been degraded. Phosphatidyl serine (PS) inhibited PKC hydrolysis by m calpain yet enhanced PKC hydrolysis by μ calpain. The ability of either calpain isoenzyme to degrade [¹⁴C]azocasein was unaffected by PS, suggesting that the influence of PS was on PKC conformation. These findings point towards distinct roles for μ and m calpain in PKC regulation.