Affinity−directed cross−linking of membrane−bound acetylcholine receptor polypeptides with photolabile alpha−bungarotoxin derivatives

Photolabile derivatives of [125I]−alpha−bungarotoxin that retain specific binding to Torpedo californica acetylcholine receptor have been utilized as structural probes of the receptor complex of polypeptide components in its membrane−associated form. The derivatized toxins contained aryl azide side chains poised to form covalent cross−links to both associated and adjacent polypeptides following toxin−receptor complex formation. The results demonstrate that, depending on the possible radius of extension of the photoactivated group from the parent toxin, either (1) both the polypeptide to which the toxin derivative binds and an adjacent polypeptide can be derivatized upon photolysis or (2) only the adjacent polypeptide is labeled. The results lend strong support to the notion that the nicotinic receptor from T. california is composed of a complex of different polypeptide