Purification and characterization of a calcium-calmodulin-dependent phospholamban kinase from canine myocardium

Abstract

A Ca2+-calmodulin-dependent protein kinase was purified to apparent homogeneity from the cytosolic fraction of canine myocardium, with phospholamban as substrate. Purification involved sequential chromatography on DEAE-cellulose, calmodulin-agarose, DEAE-Bio-Gel A, and phosphocellulose. This procedure resulted in a 987-fold purification with a 5.4% yield. THe purified enzyme migrated as a single band on native polyacrylamide gels, and it exhibited an apparent molecular weight of 550 000 upon gel filtration. Gel electrophoresis under denaturing conditions revealed a single protein band with Mr 55 000. The purified kinase could be autophosphorylated in a Ca2+-calmodulin-dependent manner, and under optimal conditions, 6 mol of PI was incorporated per mole of 55 000-dalton subunit. The activity of the enzyme was dependent on Ca2+, calmodulin, and ATP.cntdot.Mg2+. Other ions which could partially substitute for Ca2+ in the presence of Mg2+ and saturating calmodulin concentrations were Sr2+ > Mn2+ > Zn2+ > Fe2+. The substrate specificity of the purified Ca2+-acalmodulin-dependent protin kinase of cardiac proteins was determined by using phospholamban, troponin I, sarcoplasmic reticulum membranes, myofibrils, highly enriched sarcolemma, and mitochondria. The protein kinase could only phosphorylate phospholamban and troponin I either in their purified forms or in sarcoplasmic reticulum membranes and myofibrils, respectively. Exogenous proteins which could also be phosphorylated by the purified protein kinase were skeletal muscle glycogen synthease > gizzard myosin light chain > brain myelin basic protein > casein. However, phospholamban appeared to be phosphorylated with a higher rate as well as affinity than glycogen synthase. These findings suggest that a Ca2+-calmodulin-dependent protein kinase, composed of 10 identical subunits, may participate in regulation of sarcoplasmic reticulum function in cardiac muscle.