In situ labeling of Torpedo and rat muscle acetylcholine receptor by a photoaffinity derivative of alpha-bungarotoxin

We synthesized a photoaffinity derivative of 125I-alpha-bungarotoxin that binds tightly and specifically to the acetylcholine receptor. We used this reagent to label the receptor in membranes from Torpedo electric organ and rat muscle and have analyzed the labeled polypeptides by immunoprecipitation and sodium dodecyl sulfate-gel electrophoresis. Reaction of the modified toxin with the Torpedo receptor in situ covalently labels the same four polypeptides as seen in Torpedo receptor preparations after solubilization and purification. The toxin derivative labels five polypeptides in muscle membranes that correspond in molecular weight to the subunits of the purified muscle receptor. These results provide evidence that, for both Torpedo and muscle receptors, the polypeptides associated with the purified protein also form part of the receptor in situ.