Modulation of erythrocyte membrane proteins by membrane cholesterol and lipid fluidity

Abstract

Human erythrocyte membranes were enriched or depleted of cholesterol and effects on membrane proteins assessed with a membrane-impermeant sulfhydryl reagent, [35S]glutathione-maleimide. Reaction of the probe with intact cells quantifies exofacial sulfhydryl groups and reaction with leaky ghost membranes permitted quantification of endofacial sulfhydryl groups. The mean endofacial sulfhydryl titer of cholesterol-depleted membranes by approximately 45 nmol/mg of protein, or 64%. The corresponding exofacial titer of cholesterol-enriched cells was less than that of cholesterol-depleted cells by approximately 0.4 nmol/mg of protein, or 14%. Labeled membranes were examined by autoradiography of sodium dodecyl sulfate-polyacrylamide gel electropherograms to determine the labeling patterns of individual protein bands. Cholesterol enrichment enhanced the surface labeling of Coomassie brilliant blue stained bands 1, 2, 3 and 5, decreased the labeling of band 6, and did not significantly change that of band 4. Changes in membrane cholesterol which influence lipid fluidity can alter the surface labeling of intrinsic and extrinsic membrane proteins.