Involvement of histidine 134 in the binding of alpha-bungarotoxin to the nicotinic acetylcholine receptor.

Peptides corresponding to the sequence alpha 124-147 of the Torpedo californica and Homo sapiens nicotinic cholinergic receptors were synthesized. The His residue at position 134 was ethoxyformylated or substituted by Ala. Effects of such modifications were studied by: (a) a toxin blot assay and (b) a competition assay between each peptide and the Discopyge Ischudii receptor for 125I alpha-bungarotoxin, in solution. Apparent Kd values were 0.1 and 0.8 microM for Torpedo californica and Homo sapiens native peptides, respectively, and no binding was observed when the His residue was modified or substituted by Ala. ic50 values for the Torpedo californica and Homo sapiens fragments were 1.0 and 0.8 microM, respectively, and no significant displacement occurred when His 134 was ethoxyformylated or substituted by Ala. Hydroxylamine treatment restored 80-100% of their binding ability. Results strongly support the involvement of His 134 in the binding of alpha-bungarotoxin either to the Torpedo californica or the Homo sapiens receptor.