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 s keletal muscle sarcoplasmic reticulum at Cys-344. Resonance energy tra nsfer 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 mea sured by resonance energy transfer for the ATPase reconstituted into b ilayers 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 vanadat e. Quenching of the fluorescence of the coumarin-labelled ATPase with KI suggested that the fluorophore is not fully exposed on the ATPase.