The Ca2+-pumping ATPase in skeletal muscle sarcolemma. Calmodulin dependence, regulation by cAMP-dependent phosphorylation, and purification.

A Ca2+-pumping ATPase has been shown to be an integral component of skeletal muscle sarcolemma. The enzyme is stimulated by calmodulin and inhibited by micromolar concentrations of vanadate. The presence of calmodulin results in the transition of the AT-Pase to a high Ca2+ affinity state; removal of calmodulin reverses this effect. The Ca2+-ATPase and the Ca2+ uptake associated with it are also regulated by a specific cAMP-dependent phosphorylation system. The phosphorylation of the membrane enhances the Ca2+-ATPase activity. The effect is reversible. Three sarcolemmal proteins (Mr 35,000, 28,000, and 26,000) were identified as specific substrates of the cAMP-dependent phosphorylation system. The sarcolemmal Ca2+-ATPase was isolated by means of a calmodulin affinity chromatography column as a protein band of Mr of about 140,000, which could be eluted from the column by EDTA. The band possesses Ca2+-ATPase activity and is phosphorylated with [gamma-32P]ATP in a Ca2+-dependent manner. The membrane of the transverse tubules does not contain the calmodulin-sensitive Ca2+-ATPase. It contains only one cAMP-dependent phosphoprotein (Mr 30,000).