Purification of acetylcholine receptors, reconstitution into lipid vesicles, and study of agonist-induced cation channel regulation.

We report the purification of acetylcholine receptors with active agonist-regulated cation channels from Torpedo californica electric organ tissue by five methods. In one method, previously used by others, contaminating proteins were removed from partially purified membranes by alkaline extraction, preserving membrane integrity throughout the procedure. In the other four methods, acetylcholine receptors were purified after solubalization with sodium cholate. The continual presence of soybean lipid in mixed micelles with cholate was required to prevent irreversible inactivation of the cation channel. Solubilized receptors were purified by affinity chromatography using either Naga naja siamensis toxin III or concanavalin A coupled to agarose. Sucrose gradient centrifugation was also used to purify solubilized receptors. The best method combined affinity chromatography on toxin-agarose and concanavalin A agarose. Receptors purified by all five methods were incorporated into soybean lipid vesicles by the cholate dialysis technique. The agonist-regulated cation channels of the receptors were equally active after reconstitution, independent of the method used for purification. All reconstituted vesicle preparations were similar in preferential orientation of acetylcholine receptor toward the external surface, dose-response to carbamylcholine, carbamylcholine-induced desensitization, and carbamycholine-induced influx of 22Na+ per mol of receptor. Carbamylcholine-induced 22Na+ influx/receptor was greater after reconstitution than in native vesicles. This was because, in native vesicles, carbamylcholine-induced 22Na+ influx was limited by equilibration of the internal volume of the vesicles with the external 22Na+ concentration, whereas in reconstituted vesicles 22Na+ influx was limited by desensitization of the receptor molecule. We demonstrate that only one of the two toxin binding sites on the receptor monomer, the one which can be affinity alkylated with 4-(N-maleimido)benzyltrimethylammonium, controls the carbamylcholine-induced opening of the cation channel.