Membranes rich in acetylcholine receptor: characterization and reconstitution to excitable membranes from exogenous lipids

Abstract

Characterization of acetylcholine-receptor-enriched membranes from Torpedo californica electric tissue by negative-staining electron-microscopy and by lipid analysis is described. The protein/lipid ratio is 70%/30%. The lipids consist of 70% phospholipids (46% phosphatidylcholine, 31% phosphatidylethanolamine, 14% phosphatidylserine, 7% sphingomyelin, 2% phosphatidylinositol of the phospholipids determined) and 20% cholesterol. The acetylcholinesterase-enriched membranes show a similar composition. The only differences are a lower protein/lipid ratio (45%/55%) and a lower phosphatidylcholine/sphingomyelin ratio of 39%/14% as compared to 46%/7% for the receptor-enriched membranes. A method of preparing single-walled phosphatidylcholine vesicles by gel filtration on Sephadex G50 according to Brunner et al. (Biochem. Biophys. Acta, 455, 322--331, 1976) is used to recombine the lipid-depleted receptor complex with artificial lipid vesicles. Starting from a lipid mixture of 46% phosphatidylcholine, 31% phosphatidylethanolamine, 14% phosphatidylserine, 7% sphingomyelin, 2% phosphatidylinositol and 15% cholesterol we obtained vesicles associated with the acetylcholine receptor complex. These receptor vesicles are chemically excitable by 10 micrometer carbamoylcholine as measured by efflux of 22Na+ from the vesicles. The excitability is blocked by preincubation with 0.5 mM alpha-toxin from Naja naja siamensis venom and by reduction with 5 mM dithioerythritol.