Studies on phosphorylation of canine cardiac sarcoplasmic reticulum by calmodulin-dependent protein kinase

Abstract

Two endogenous protein kinase activities, cAMP-dependent and calmodulin-Ca2+-dependent, are associated with isolated cardiac sarcoplasmic reticulum (SR) vesicles. Both kinases posphorylate an endogenous substrate of ~22,000 daltons (phospholamban). The phosphorylation of phospholamban by either the intrinsic or by exogenous cAMP-dependent protein kinase is found to be Ca2+-independent between 0.05 and 100 μM free Ca2+. Calmodulin-dependent phosphorylation, on the other hand, does not require cAMP and is absolutely dependent on the presence of free Ca2+ over a concentration range that corresponds to physiological levels (10