Calcium and lanthanide binding in the sarcoplasmic reticulum ATPase

The interactions of calcium and lathanides with the sarcoplasmic reticulum ATPase, and their respective ability to activate the enzyme, were studied by direct measurements of binding with radioactive tracers, functional effects on the ATPase partial reactions, changes in the quantum yield of tryptophanyl residues and a covalently bound fluorescein label (fluorescein 5-isothiocyanate, FITC), and energy transfer between bound lanthanide and fluorescent labels. We find that: (a) Lanthanides displace calcium from specific ATPase sites with diphasic kinetics that are consistent with sequential exchange. (b) Lanthanides in excess of the calcium stoichiometry are mostly bound to sarcoplasmic reticulum lipids and non-ATPase proteins. (c) Both calcium and lanthanides activate the ATPase and allow formation of the phosphorylated intermediate by utilization of ATP; however, hydrolytic cleavage of the intermediate formed in the presence of lanthanides occurs at a slower rate than the intermediate formed in the presence of calcium. (d) In contrast to a calcium-dependent change in the quantum yield of both the tryptophanyl residues (transmembrane region) and the FITC label (extramembranous region), lanthanides induce only a change in the quantum yield of the FITC label. (e) Measurements of energy transfer between bound lanthanide and fluorescent labels detect lanthanide bound midway between the catalytic site in the globular region of the ATPase outside the membrane, and the transmembrane calcium binding domain which is involved in enzyme activation (Clarke, D. M., Loo, T. W., Inesi, G., and MacLennan, D. H. (1989a) Nature 339, 476-478). It is apparent that cation bound in this midway location controls exchange of calcium bound in the transmembrane region. The possibility that the midway location may provide a domain for binding of a second calcium is discussed.