Subunit composition of bovine muscle acetylcholine receptor.

Acetylcholine receptors from fetal calf muscle were purified to homogeneity (specific activity up to 7500 nmol/g of protein), in reasonable yields (20-50%), and near-milligram quantity. Purification was by affinity chromatography on Naja naja siamensis toxin coupled to agarose by using methods similar to those for receptors from fish electric organs, but with modifications to account for the low concentration of receptor in muscle and the high probability of proteolysis. Immunochemical methods are described for approximating the extent of proteolysis in receptor preparations. Bovine acetylcholine receptor is composed of four glycoprotein subunits designated alpha (Mr congruent to 41 000), beta (Mr congruent to 50 000), gamma (Mr congruent to 53 000), and delta (Mr congruent to 56 000) which correspond immunochemically to the four glycoprotein subunits of fish electric organ acetylcholine receptors of the same designations. Electron micrographs of purified bovine receptor show that it has the same size and shape as receptors from fish electric organs. Immunization of rats with receptor from bovine and human muscle is very effective at inducing experimental autoimmune myasthenia gravis. Acetylcholine receptors purified from rat muscle are composed of subunits which correspond immunochemically to the alpha, beta, gamma, and delta subunits of receptor from Torpedo californica. The evidence presented strongly suggests that acetylcholine receptors from fish electric organ tissue and mammalian muscle share a fundamentally similar shape, antigenic structure, and alpha 2 beta gamma delta subunit structure.