Identification of membrane-bound calcium, calmodulin-dependent protein kinase II in canine heart

Phospholamban, the putative regulatory proteolipid of the Ca²/Mg²⁺ ATPase in cardiac sarcoplasmic reticulum, was selectively phosphorylated by a Ca²⁺/calmodulin (CaM)-dependent protein kinase associated with a cardiac membrane preparation. This kinase also catalyzed the phosphorylation of two exogenous proteins known to be phosphorylated by the multifunctional Ca²⁺/CaM-dependent protein kinase II Ca²⁺/CaM-kinase II), i.e., smooth muscle myosin light chains and glycogen synthase a. The latter protein was phosphorylated at sites previously shown to be phosphorylated by the purified multifunctional Ca²⁺/CaM-kinase II from liver and brain. The membrane-bound kinase did not phosphorylate phosphorylase b or cardiac myosin light chains, although these proteins were phosphorylated by appropriate, specific calmodulin-dependent protein kinases added exogenously. In addition to phospholamban, several other membrane-associated proteins were phosphorylated in a calmodulin-dependent manner. The principal one exhibited a M_r of approximately 56,000, a value similar to that of the major protein (57,000) in a partially purified preparation of Ca²⁺/CaM-kinase II from the soluble fraction of canine heart that was autophosphorylated in a calmodulin-dependent manner. These data indicate that the membrane-bound, calmodulin-dependent protein kinase that phosphorylates phospholamban in cardiac membranes is not a specific calmodulin-dependent kinase, but resembles the multifunctional Ca²⁺/CaM-kinase II. Our data indicate that this kinase may be present in both the particulate and soluble fractions of canine heart.