Rotational dynamics of lipid and the Ca-ATPase in sarcoplasmic reticulum. The molecular basis of activation by diethyl ether

We have investigated the role of lipid and protein dynamics in the activation of the Ca²⁺-dependent ATPase in sarcoplasmic reticulum (SR) by diethyl ether. Conventional and saturation-transfer electron paramagnetic resonance (EPR) were used to probe rotational motions of spin labels attached either to fatty acid hydrocarbon chains or to the Ca-ATPase in SR. We confirm previous studies (Salama, G., and Scarpa, A. (1980) J. Biol. Chem. 255, 6525-6528; Salama, G., and Scarpa, A. (1983) Biochem. Pharmacol. 32, 3465-3477; Kidd, A., Scales, D., and Inesi, G. (1981) Biochem. Biophys. Acta 65, 124-131) reporting that addition of diethyl ether to SR results in an approximately 2-fold enzymatic activation, without loss of coupling. Diethyl ether progressively fluidizes the SR membrane with respect to lipid hydrocarbon chain dynamics probed at several depths in the bilayer. Digital subtractions, used to analyze two-component lipid spin label spectra, reveal that a 2-fold mobilization occurs in the population of lipid probes motionally restricted by the protein, while the remaining more mobile population is less affected. The microwave saturation properties of lipid probes also indicate that restricted motions of these probes are mobilized in maximally activated SR membranes. Saturation-transfer EPR, applied to maleimide spin-labeled Ca-ATPase, demonstrates that a 2-fold increase in microsecond rotational motion of the Ca-ATPase correlates with the maximal enzymatic activation. Effects of diethyl ether on both the enzymatic activity and molecular dynamics are completely reversible by dilution with buffer. We propose that ether activates by selectively mobilizing lipid chains adjacent to the enzyme, thus facilitating protein motions that are essential for calcium transport.