Phosphorylation states of phospholamban.
Phospholamban is a small integral membrane protein of cardiac, smooth, and slow-twitch skeletal muscle sarcoplasmic reticulum that interacts with the Ca2+ pump of these organelles and inhibits Ca(2+)-pump activity while in the dephosphorylated form. Three sites of Ser/Thr phosphorylation have been identified in the primary sequence of phospholamban, at Ser-10, Ser-16, and Thr-17. In vitro studies indicate that these residues are phosphorylated by PKC (Ser-10), PKA, PKG or PKC (Ser-16), and CaM kinase II (Thr-17). Phosphorylation of Ser-16 (or Thr-17) is accompanied by an increase in Ca2+ pump activity in direct proportion to the stoichiometry of phosphorylation. Dual phosphorylation of both Ser-16 and Thr-17 does not cause any further stimulation of pump function over that achieved by stoichiometric phosphorylation of a single site. Examination of the pattern of phosphorylation in vivo has been aided by the generation of polyclonal antibodies specific for the phosphorylated forms of phospholamban. beta-Adrenergic stimulation of cardiac muscle results in phosphorylation of both Ser-16 and Thr-17. The time course of Ser-16 phosphorylation precedes Thr-17. The spatial distribution of Ser-16 and Thr-17 phosphorylated forms of phospholamban is not identical; phospholamban located in the nuclear membrane of a cardiac myocyte is phosphorylated exclusively on Ser-16, whereas phospholamban molecules in the SR membrane of the same cell are phosphorylated on Ser-16 and/or Thr-17. Finally, we have identified a novel stimulus for the phosphorylation of phospholamban. Ca2+ store depletion, achieved by exposure of myocytes to SERCA inhibitors, prompts the phosphorylation of phospholamban on Ser-16. This would be expected to increase Ca2+ uptake by the SR in an attempt to achieve the refilling of the SR.
Colyer
J
J
School of Biochemistry and Molecular Biology, University of Leeds, United Kingdom. j.colyer@leeds.ac.uk
D000818:Animals; D002135:Calcium-Binding Proteins / Q000737:chemistry* / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D046911:Macromolecular Substances; D008954:Models, Biological; D010766:Phosphorylation; D011494:Protein Kinases / Q000378:metabolism*; D020836:Protein Structure, Quaternary; D012694:Serine; D013912:Threonine; D000099211:Phospholamban
D002135:Calcium-Binding Proteins; D000252:Calcium-Transporting ATPases; D046911:Macromolecular Substances; D011494:Protein Kinases; D012694:Serine; D013912:Threonine; D000099211:Phospholamban