Structural studies on phospholamban and implications for regulation of the Ca(2+)-ATPase

The cardiac sarcoplasmic reticulum (SR) protein phospholamban (PLB) is an endogenous inhibitor of the SR Ca(2+)-ATPase. Phosphorylation of PLB relieves this inhibition and up-regulates calcium transport. PLB has proved remarkably difficult to study by conventional solution-state nuclear magnetic resonance (NMR) methods, due primarily to the extreme hydrophobic nature of the protein and its propensity to form pentamers. That the C-terminal domain of PLB is helical and membrane spanning is now well established; the structure of the cytoplasmic domain is relatively ill defined. In order to discern the effect of phosphorylation on the structure of the cytoplasmic domain, we have characterized a variety of model peptides in several structure-inducing and/or lipid-mimicking environments using circular dichroism and solution-state NMR. The resolution of peptide structures obtained in aqueous trifluoroethanol was markedly improved by the incorporation of 15N labels into the peptide backbone, allowing a variety of isotope edited, filtered, and resolved techniques to be applied. Molecular dynamics simulations on the full-length protein were combined with an analysis of published data to suggest a revised model for the structure of PLB.