Printed in Ij.S.A. Lipid Fluidity Directly Modulates the Overall Protein Rotational Mobility of the Ca-ATPase in Sarcoplasmic Reticulum*

We have developed a quantitative and relatively model-independent measure of lipid fluidity using EPR and have applied this method to compare the temper-ature dependence of lipid hydrocarbon chain fluidity, overall protein rotational mobility, and the calcium-dependent enzymatic activity of the Ca-ATPase in sar-coplasmic reticulum. We define membrane lipid fluid-ity to be T/q, where q is the viscosity of a long chain hydrocarbon reference solvent in which a fatty acid spin label gives the same EPR spectrum (quantitated by the order parameter S) as observed for the same probe in the membrane. This measure is independent of the reference solvent used as long as the spectral line shapes in the membrane and the solvent match precisely, indicating that the same type of anisotropi