Spin-label studies of lipid-protein interactions in sodium-potassium ATPase membranes from rectal glands of Squalus acanthias

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Abstract
Lipid-protein interactions in (Na+, K+)-ATPase-rich membranes from the rectal gland of Squalus acanthias were studied by using spin-labeled lipids in condunction with ESR spectroscopy. Lipid-protein associations are revealed by the presence of a 2nd component in the ESR spectra of the membrane in addition to a component which corresponds very closely to the ESR spectra obtainend from dispersion of the extracted membrane lipids. This 2nd component corresponds to spin-labeled lipids whose motion is very significantly restricted relative to that of the fluid lipids in the membrane or the lipid extract. A stoichiometry of .apprx. 66 lipids/265,000-dalton protein is found for the motionally restricted component of those spin-labeled lipids (e.g., phosphatidylcholine) which show least specificity for the protein. This corresponds approximately to the number of lipids which may be accommodated within the 1st shell around the .alpha.2.beta.2 protein dimer. A selectivity of the various spin-labeled lipids for the motionally restricted component associated with the protein is found in the following order: cardiolipin > phosphatidylserine .simeq. stearic acid .gtoreq. phosphatidic acid > phosphatidylglycerol .simeq. phosphatidylcholine .simeq. phosphatidylethanolamine .simeq. androstanol.

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