Lateral mobility of phospholipid and cholesterol in the human erythrocyte membrane: effects of protein-lipid interactions

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Abstract
The phospholipid and cholesterol derivatives N-(7-nitro-2,1,3-benzoxadiazol-4-yl)phosphatidylethanolamine (NBD-PE) and N1-cholesterylcarbamoyl-N8-(7-nitro-2,1,3-benzoxadiazol-4-yl)-3,6-dioxaoctane-1,8-diamine (NBD-Chol), respectively, were incorporated into egg phosphatidylcholine/cholesterol multilamellar liposomes, human erythrocyte ghost membranes and multilamellar liposomes derived from extracted human erythrocyte membrane lipids. The lateral mobility of these probes in the plane of the various membranes was measured by using the fluorescence photobleaching recovery technique. NBD-PE and NBD-Chol manifested identical lateral mobilities in egg phosphatidylcholine/cholesterol multilamellar liposomes over the range of tempertures from 10-37.degree. C and the range of cholesterol mole fractions from 0.0-0.5 and in erythrocyte ghost membranes and erythrocyte membrane lipid-derived multilamellar liposomes over the range of temperatures from 15-37.degree. C. The weak temperature dependence of the lateral diffusion coefficients of the lipid probes in both artificial and erythrocyte ghost membranes is consistent with the lack of a phase transition in any of these systems over the temperature range studied. Both NBD-PE and NBD-Chol diffuse 4-fold faster in liposomes derived from extracted erythrocyte membrane lipids (D = 8.0 .times. 10-9 cm2 s-1 at 37.degree. C) than in the ghost membranes themselves (D = 2.1 .times. 10-9 cm2 s-1 at 37.degree. C), suggesting a significant restriction of lipid lateral mobility by membrane protein in the human erythrocyte membrane.