Positions of the sites labeled by N-cyclohexyl-N'-(4-dimethylamino-1-naphthyl)carbodiimide on the (Ca2+ + Mg2+)-ATPase.

N-Cyclohexyl-N'-(4-dimethylamino-1-naphthyl)carbodiimide (NCD-4) labels (Ca2+ + Mg2+)-ATPase at Ca2+-protectable sites, believed to be at or near the two Ca2+ binding sites on the ATPase, and at nonspecific sites. The labeled ATPase has been reconstituted into lipid bilayers containing phosphatidylethanolamine labeled with fluorescein isothiocyanate. The distance between NCD-4 and fluorescein groups was measured using Forster energy transfer and the NCD-4 labels were found to be approx. 20 A from the lipid/water interface suggesting that the Ca2+ binding sites on the ATPase are also 20 A from the lipid/water interface. Addition of vanadate causes no change in the efficiency of energy transfer, suggesting that the Ca2+ binding sites on the E1 conformation of the ATPase do not move significantly with respect to the lipid/water interface in the E1-E2 transition.