Phosphorylation and regulation of the Ca(2+)-pumping ATPase in cardiac sarcoplasmic reticulum by calcium/calmodulin-dependent protein kinase.
In cardiac muscle, a membrane-associated Ca2+/calmodulin-dependent protein kinase (CaM kinase) phosphorylates the Ca(2+)-pumping ATPase in addition to its previously characterized substrates, phospholamban and Ca(2+)-release channel (ryanodine receptor). The phosphorylated amino acid in the Ca(2+)-ATPase has been identified as serine. Posphorylation of the Ca(2+)-ATPase is rapid and is reversible by a membrane-associated protein phosphatase, Ca(2+)-ATPase purified from cardiac SR underwent phosphorylation by exogenous CaM kinase, and the phosphorylated enzyme displayed twofold greater catalytic activity without alteration in its Ca(2+)-sensitivity. The phosphorylation of the Ca(2+)-ATPase was found to be isoform-specific in that the cardiac and slow-twitch skeletal muscle isoform (SERCA 2), but not the fast-twitch skeletal muscle isoform (SERCA 1), underwent phosphorylation by CaM kinase. Studies using SERCA 1 and SERCA 2 isoforms and their mutants expressed in a heterelogous cell system have resulted in i) confirmation of the isoform specificity of Ca(2+)-ATPase phosphorylation by CaM kinase, ii) identification of Ser38 as the site in SERCA 2 phosphorylated by CaM kinase, and iii) demonstration of phosphorylation-induced increase in Vmax of Ca2+ transport by the SERCA 2 enzyme. These observations suggest that in cardiac and slow-twitch skeletal muscle direct phosphorylation of the SR Ca(2+)-ATPase by the membrane-bound CaM kinase may serve to stimulate Ca2+ sequestration and therefore, the speed of muscle relaxation.
Narayanan
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N
Department of Physiology, University of Western Ontario London, Canada.
Xu
A
A
D000818:Animals; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000502:physiology*; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D006801:Humans; D007527:Isoenzymes / Q000378:metabolism; D010766:Phosphorylation; D012519:Sarcoplasmic Reticulum / Q000201:enzymology*; D012694:Serine / Q000378:metabolism
D007527:Isoenzymes; D012694:Serine; D017871:Calcium-Calmodulin-Dependent Protein Kinases; D000252:Calcium-Transporting ATPases