An investigation of the mechanism of inhibition of the Ca2+-ATPase by phospholamban

The Ca2+-ATPase of skeletal muscle sarcoplasmic reticulum has been reconstituted with peptides corresponding to the hydrophobic domain of phospholamban (PLB) with or without the three Cys residues replaced by Ala, and with PLB with the three Cys residues replaced by Ala [PLB(Cys-)(1-52)]. Reconstitution with the hydrophobic domain of PLB [PLB(25-52)] was found to decrease the apparent affinity of the ATPase for Ca2+ with no effect on the maximal rate of ATP hydrolysis observed at saturating concentrations of Ca2+. Reconstitution with PLB(Cys-)(1-52) decreased both the apparent affinity for Ca2+ and the maximal activity; the effect on maximal activity followed from a decrease in the rate of the Ca2+ transport step (E1PCa(2) --> E2P) as observed with the hydrophilic domain PLB(1-25). The concentration dependences of the effects of the hydrophobic domain and of the whole PLB molecule were very similar, suggesting that the hydrophilic domain made little contribution to the affinity of the ATPase for PLB. The effect of PLB on the ATPase was dependent on the molar ratio of phospholipid to ATPase, suggesting partition of the PLB between its binding site on the ATPase and the bulk lipid phase in the membrane. Neither PLB nor its hydrophobic domain affected the rates of phosphorylation or dephosphorylation of the ATPase. Despite their effects on the apparent affinity of the ATPase for Ca2+ neither PLB nor its hydrophobic domain had any effect on the true affinity of the ATPase for Ca2+, as measured from changes in the tryptophan fluorescence of the ATPase. The effects of PLB on the activity of the ATPase are the sum of the effects of its hydrophilic and hydrophobic domains.