title: Role of phospholamban in regulating cardiac sarcoplasmic reticulum calcium pump.
abstract: Cardiac sarcoplasmic reticulum plays a critical role in the excitation-contraction cycle and hormonal regulation of heart cells. Catecholamines exert their ionotropic action through the regulation of calcium transport into the sarcoplasmic reticulum. Cyclic 3'-5'-adenosine monophosphate (cAMP) causes the cAMP-dependent protein kinase to phosphorylate the regulatory protein phospholamban, which results in the stimulation of calcium transport. Calmodulin also phosphorylates phospholamban by a calcium-dependent mechanism. We have reported the isolation and purification of phospholamban with low deoxycholate (DOC) concentrations (5 X 10(-6) M). We have also reported the isolation and purification of Ca2+ + Mg2+-ATPase with a similar procedure. Both phospholamban and Ca2+ + Mg2+-ATPase retained their native properties associated with sarcoplasmic reticulum vesicles. Further, we have shown that the removal of phospholamban from membranes of sarcoplasmic reticulum vesicles uncouples Ca2+-uptake from ATPase without any effect on Ca2+ + Mg2+-ATPase activity or Ca2+ efflux. Phospholamban appears to be the substrate for both the Ca2+-calmodulin system and the cAMP-dependent protein kinase system. It is found that the phosphorylation of phospholamban by the Ca2+-calmodulin system is required for the normal basal level of Ca2+ transport, and that the phosphorylation of phospholamban at another site by the cAMP-dependent protein kinase system causes the stimulation of Ca2+-transport above the basal level. The functional effects of the phosphorylation of phospholamban by cAMP-dependent protein kinase system are expressed only after the phosphorylation of phospholamban with Ca2+-calmodulin system. We propose a model for the cardiac Ca2+ + Mg2+-ATPase, whereby the enzyme is normally uncoupled from Ca2+ uptake. The enzyme becomes coupled to Ca2+ transport after the first site of phospholamban is phosphorylated with the Ca2+-calmodulin system. When the second site of phospholamban is phosphorylated with cAMP-dependent protein kinase both Ca2+ transport and ATPase are stimulated and phospholamban becomes inaccessible to DOC solubilization and trypsin.
mesh_terms: D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects; D017301:Ca(2+) Mg(2+)-ATPase; D002118:Calcium / Q000378:metabolism*; D002135:Calcium-Binding Proteins / Q000502:physiology*; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D002147:Calmodulin / Q000494:pharmacology; D002463:Cell Membrane Permeability / Q000187:drug effects; D003840:Deoxycholic Acid / Q000494:pharmacology; D007473:Ion Channels / Q000378:metabolism; D008954:Models, Biological; D009203:Myocardial Infarction / Q000378:metabolism; D009206:Myocardium / Q000378:metabolism*; D010766:Phosphorylation; D012519:Sarcoplasmic Reticulum / Q000378:metabolism; D000099211:Phospholamban
chemical_list: D017301:Ca(2+) Mg(2+)-ATPase; D002118:Calcium; D002135:Calcium-Binding Proteins; D000252:Calcium-Transporting ATPases; D002147:Calmodulin; D003840:Deoxycholic Acid; D007473:Ion Channels; D000099211:Phospholamban