Microsecond motions of the lipids associated with trypsinized Na,K-ATPase membranes. Progressive saturation spin-label electron spin resonance studies

The microsecond motions of spin-labeled lipids associated with the Na+/K+-t ransporting ATP hydrolase (Na,K-ATPase) in native and tryptically shaved me mbranes from Squalus acanthias have been studied by progressive saturation electron spin resonance (ESR). This includes both the segmental mobility of the lipid chains and the exchange dynamics of the lipids interacting direc tly with the protein. The lipids at the protein interface display a tempera ture-dependent chain mobility on the submicrosecond time scale. Exchange of these lipids with those in the bulk bilayer regions of the membrane takes place on the time scale of the nitroxide spin-lattice relaxation, i.e., in the microsecond regime. The off-rates for exchange directly reflect the spe cificity of ionized fatty acids relative to protonated fatty acids for inte raction with the Na,K-ATPase. These essential features of the lipid dynamic s at the intramembranous protein surface, namely, a temperature-dependent e xchange on the microsecond time scale that reflects the Lipid selectivity, are preserved on removing the extramembranous parts of the Na,K-ATPase by e xtensive trypsinization.