Effects of phospholamban phosphorylation catalyzed by adenosine 3':5'-monophosphate- and calmodulin-dependent protein kinases on calcium transport ATPase of cardiac sarcoplasmic reticulum.

To elucidate the role of 22000-dalton protein phospholamban, a putative regulator of Ca2+-dependent ATPase of cardiac sarcoplasmic reticulum, we examined the relationship between cyclic AMP- and calmodulin-dependent phosphorylation of phospholamban and their effects on ATPase activity and calcium transport of cardiac sarcoplasmic reticulum. Cardiac microsomes were incubated with [gamma-32P]ATP or unlabeled ATP, catalytic subunit of cyclic AMP-dependent protein kinase and/or exogenous calmodulin, and subsequently assayed for ATPase activity and calcium uptake by cardiac sarcoplasmic reticulum. Cyclic AMP-dependent phosphorylation of phospholamban was independent of Ca2+, whereas calmodulin-dependent phosphorylation of phospholamban was dependent on Ca2+ within a range between 0.2 and 50 microM. Cyclic AMP- and calmodulin-dependent phosphorylation of phospholamban occurred independently; when both kinases were operative, the amounts of phosphorylation were additive. Under these conditions, the phosphoproteins formed by cyclic AMP- and calmodulin-dependent protein kinases electrophoretically migrated as 11000-dalton components when sodium dodecyl sulfate-solubilized phosphoproteins were boiled prior to polyacrylamide gel electrophoresis. The ATPase activity was stimulated by either cyclic AMP- or calmodulin-dependent phosphorylation of phospholamban at Ca2+ concentrations up to 2 microM. The extents of stimulation of ATPase activity were additive when both types of phosphorylation were functional. Calcium uptake was similarly augmented by cyclic AMP- and/or calmodulin-dependent phosphorylation of phospholamban. These results indicate that Ca2+-dependent ATPase and calcium transport of cardiac sarcoplasmic reticulum are regulated by phospholamban phosphorylation catalyzed by cyclic AMP- and calmodulin-dependent protein kinases, thus suggesting a dual role of phospholamban in active calcium transport.