Interaction of the Acetylcholine (Nicotinic) Receptor Protein from Torpedo marmorata Electric Organ with Monolayers of Pure Lipids

Abstract

Membrane fragments rich in cholinergic (nicotinic) receptor protein were purified from the electric organ of T. marmorata. Their lipid composition was essentially characterized by the prominence of cholesterol, phosphatidylethanolamine and phosphatidylcholine, long-chain fatty acyl constituents, and the absence of sphingomyelin. Solubilized receptor was purified from these fragments and the concentration of sodium cholate lowered by dialysis to 0.01% (wt/vol). When this preparation was injected under a lipid monolayer, an increase of surface pressure developed, which was not observed with the detergent alone nor in the absence of lipid film. When covalently radiolabeled receptor preparations were injected at a constant surface pressure the radioactivity recovered with the film was proportional to the increase in area. Apparently the pressure or area increases are due to the penetration of the cholinergic receptor protein into the lipid film. Incorporation experiments into films formed from various pure lipids showed that the protein interacted more readily with cholesterol than with ergosterol, phosphatidylcholine or other phospholipids. Its affinity was also higher for long-chain phosphatidylcholines than for short-chain ones. The degree of unsaturation and fluidity of the 3-sn-phosphatidylcholine (lecithin) films were of secondary importance. Parallel experiments with covalently and non-covalently labeled receptor preparations showed that part of the protein recovered with the film lost its .alpha.-toxin binding ability during the penetration. Similar data were obtained with the receptor purified from Electrophorus electricus electric organ.