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Affinity labeling of the subunits of the membrane associated cholinergic receptor.

Membranes enriched in acetylcholine receptor have been purified from Torpedo californica electric organs by gradient sedimentation and affinity partitioning. The preparations contain 40,000, 49,000, 60,000, 67,000 and 105,000 dalton peptides; the latter peptide can be virtually removed by the partitioning step. Two-dimensional electrophoresis reveals that the 67,000 and to a lesser extent the 49,000 dalton peptides exist as disulfide linked dimers. p -(Trimethylammonium) benzenediazonium fluoroborate (TDF), a structural analogue of phenyltrimethylammonium, binds covalently to the 40,000 and 105,000 dalton peptides present in the membrane fragments but only the labeling of the 40,000 dalton subunit is blocked by prior association of cholinergic agonists, antagonists and α-toxin. The protectable labeling is saturable and stoichiometric with irreversibly blocked toxin sites; one molecule of [ 3 H]TDF binds per toxin site. TDF thus appears to label specifically the ligand/toxin binding site of the receptor. Procedures which convert the receptor to its high affinity state for agonists such as prolonged incubation with agonist or short term incubation with agonist and local anesthetic enhance the extent of protection against irreversible labeling by TDF. Hence, covalent labeling by TDF appears to distinguish the two interconvertible receptor states identified previously in the membrane preparation by their different affinities for agonists.