Muscarinic cholinergic receptor of rat brain

1 The muscarinic cholinergic receptor present in synaptosomal membranes of rat brain was covalently labelled with the alkylating muscarinic antagonist, tritiated propylbenzilylcholine mustard. The labelled receptor was then solubilized in sodium deoxycholate and sodium dodecyl sulphate, and its migration in polyacrylamide gel electrophoresis and gel filtration in the presence of sodium dodecyl sulphate analysed. 2 Provided both proteolysis and inter‐chain disulphide bond formation were vigorously prevented, the receptor from rat forebrain (cerebral cortex plus caudate putamen) migrated, in sodium dodecyl sulphate/polyacrylamide gel electrophoresis, as a broad band of apparent M r 66000–76000. Two dominantly labelled polypeptides, of apparent M r 68000 and 73000, could be distinguished as the major components of this band. These multiple species seen in electrophoresis may reflect a structural diversity related to the different binding properties, and modes of action, of this receptor. 3 In electrophoresis using discontinuous buffer systems the labelled receptor readily formed intermolecular disulphide bonds and so aggregated. In particular, if solubilized membranes were reduced with 2‐mercaptoethanol. and reformation of disulphide bonds during electrophoresis not prevented, then formation of a dimeric species (apparent M r 119000–128000) occurred. This probably explains previous reports in the literature of larger‐ M , species seen in electrophoresis. 4 During gel filtration, the receptor formed intra‐chain disulphide bonds which produced conformational heterogeneity, leading to polydisperse migration. In addition, extensive proteolytic degradation of the receptor occurred due to a protease migrating slightly ahead of the receptor. Both effects were eliminated by alkylation of the solubilized membranes with iodoacetamide before gel filtration. 5 Alkylated receptor migrated on Sephacryl S‐300 in 0.5% sodium dodecyl sulphate with an equivalent Stokes' radius of 6.1 nm. This is identical to that of reduced ovalbumin, a molecule with an apparent M r in gel electrophoresis of 43000. On a different gel matrix, TSK HW 55(S). the receptor migrated with a somewhat larger Stokes' radius, eluting just behind reduced bovine serum albumin (Stokes' radius 8.5 nm; apparent M r in electrophoresis 67000). Thus the receptor appears to adsorb to the Sephacryl matrix, although even on the TSK gel the receptor eluted as a somewhat smaller protein than expected from its behaviour in gel electrophoresis. 6 Solubilized, alkylated receptor, partly purified by gel filtration so that it was not degraded by endogenous proteases, was not cleaved by mild hydroxylamine treatment. This does not support suggestions that the receptor is made from two smaller polypeptide chains held together by an hydroxylamine‐labile linkage, such as an ester bond.