Localization and synthesis of the acetylcholine-binding site in the alpha-chain of the Torpedo californica acetylcholine receptor.

The sequence of the alpha-chain of the acetylcholine receptor of T. californica has been determined by recent cloning studies. The integrity of the disulphide bond between Cys-128 and cys-142 has been shown to be important for the maintenance of the binding activity of the receptor, thus implicating the regions around the disulphide bridge in binding with acetylcholine. In the present work, a synthetic peptide containing this loop region (residues 125-147) was synthesized. Solid-phase radiometric binding assays demonstrated a high binding of 125I-labelled alpha-bungarotoxin to the synthetic peptide. It was further shown that the free peptide bound well to [3H]acetylcholine. Additional experiments demonstrated that pretreatment of peptide 125-147 with 2-mercaptoethanol destroyed its binding activity, clearly showing that the integrity of the disulphide structure was essential for binding. Unlabelled acetylcholine also inhibited the binding of labelled acetylcholine to the synthetic peptide. The region 125-147, therefore, contains essential elements of the acetylcholine binding site of the Torpedo receptor.