Purified, reconstituted cardiac Ca21-ATPase is regulated by phospholamban but not by direct phosphorylation with Ca21/calmodulin-dependent protein kinase

Regulation of calcium transport by sarcoplasmic re-ticulum provides increased cardiac contractility in re-sponse to b-adrenergic stimulation. This is due to phos-phorylation of phospholamban by cAMP-dependent protein kinase or by calcium/calmodulin-dependent protein kinase, which activates the calcium pump (Ca21-ATPase). Recently, direct phosphorylation of Ca21-ATPase by calcium/calmodulin-dependent protein ki-nase has been proposed to provide additional regulation. To investigate these effects in detail, we have purified Ca21-ATPase from cardiac sarcoplasmic reticulum using affinity chromatography and reconsti-tuted it with purified, recombinant phospholamban. The resulting proteoliposomes had high rates of calcium transport, which was tightly coupled to ATP hydrolysi