Restoration of 125I-alpha-bungarotoxin binding activity to the alpha subunit of Torpedo acetylcholine receptor isolated by gel electrophoresis in sodium dodecyl sulfate.

The four subunits (alpha, beta, gamma, delta) of the acetylcholine receptor from Torpedo californica have been isolated by preparative gel electrophoresis in sodium dodecyl sulfate. After removal of the sodium dodecyl sulfate by dialysis of the polypeptides against a cholate-containing buffer, the alpha subunit, but not the other chains, binds 125I-alpha-bungarotoxin in a saturable manner. The binding affinity, 0.1-0.2 microM, is approximately 10(4)-fold lower than that observed for native acetylcholine receptor. For three preparations of alpha subunit, 1 mol of subunit bound 0.87, 0.38, and 0.33 mol of 125I-alpha-bungarotoxin at saturation. The binding was inhibited by cholinergic ligands, although the apparent affinities of these ligands for alpha were 50-100-fold lower than that found for the native receptor. These results indicate that at least part of the alpha-bungarotoxin binding site resides on the alpha subunit.