Cytoplasmic interactions between phospholamban residues 1-20 and the calcium-activated ATPase of the sarcoplasmic reticulum

*School of Biosciences, University of Birmingham, Birmingham B15 2TT, U.K., and ‰Department of Physiology, Division of Medical Sciences, School of Medicine,University of Birmingham, Birmingham B15 2TT, U.K.Phospholamban regulates the activity of the calcium-activatedATPase (CaATPase) of cardiac sarcoplasmic reticulum. Equi-librium fluorescence studies have shown that the N-terminalcytoplasmicregionofphospholamban(residues1–20,domain1)causes a decrease in the intrinsic tryptophan fluorescence of theCaATPase. The interaction of phospholamban residues 1–20with the CaATPase also results in spectral changes for theextrinsic chromophore FITC covalently attached to the cyto-plasmic region of the calcium pump. The fluorescence changesforbothreportergroupscorrelatewithadissociationconstantofE40 lM for the complex between phospholamban residues1–20 and the CaATPase. Complex formation is notably weakerwhen phospholamban 1–20 is titrated into the CaATPase in thepresenceofcalcium,withalteredconformationaleffectsresulting