A careful examination of the location and biochemical properties of the tryptic peptides identified by site-specific labelling of the muscarinic cholinergic receptor (mAChR) of rat cerebral cortex has been carried out. In brain synaptosomal membranes and isolated neuronal perikarya, mAChR labelled with [3H]propylbenzilylcholine mustard (PrBCM) was tryptically cleaved to peptides of Mr 50,000, 30,000. 18,000 and a limiting fragment of Mr 8000. All of these binding site-carrying fragments, characterized in terms of their content of carbohydrates and thiol groups, were quantitatively recovered as membrane-bound peptides. The delipidated [3H]PrBCM-labelled tryptic limiting fragment was found to be highly hydrophobic and insoluble in aqueous media. Experiments performed with proteinase on the tryptic limiting fragment suggest the existence of an ester linkage between the ligand and the peptide. The results strongly support the hydropathicity profile which predicts the location of the muscarinic receptor protein with respect to the membrane bilayer.

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