LOCALIZATION OF CYS-344 ON THE (CA-2+-MG-2+)-ATPASE OF SARCOPLASMIC-RETICULUM USING RESONANCE ENERGY-TRANSFER

4-Bromomethyl-6,7-dimethoxycoumarin labels the (Ca2+-Mg2+)-ATPase of skeletal muscle sarcoplasmic reticulum at Cys-344. Resonance energy transfer has been used to measure the distance between this site and Lys-515 labelled with fluorescein isothiocyanate as about 37 angstrom. The height of Cys-344 above the phospholipid/water interface has been measured by resonance energy transfer for the ATPase reconstituted into bilayers containing fluorescein-labelled phosphatidylethanolamine; the height was found to be about 45 angstrom. None of these distances was found to alter on changing pH, or on addition of Mg2+, Ca2+ or vanadate. Quenching of the fluorescence of the coumarin-labelled ATPase with KI suggested that the fluorophore is not fully exposed on the ATPase.