In Search of the Binding Site of the Nicotinic Acetylcholine Receptor

Abstract

It is well accepted that the ligand-binding site of the nicotinic acetylcholine receptor (AChR) is harbored, at least in part, in the receptor α-subunit (Popot and Changeux 1984; Conti-Tronconi and Raftery 1982; Karlin 1983). Probing this site with α-bungarotoxin (BTX) is especially convenient due to its high affinity for it (10^-11 M). The fact that the affinity alkylating analogue of ACh, i.e. maleimido-benzyl trimethyl ammonium bromide (MBTA), can irreversibly block both toxin and ACh binding to AChR, has been taken as proof that indeed the toxin binding site is either in close proximity to, or equal with the cholinergic binding site (Karlin 1983). Moreover, as MBTA binds via a maleimido moiety, a Cys residue should be present no more than 1 nm away from the actual site (Karlin 1983). Thus, much effort has been devoted towards localizing the ligand binding site on the AChR α-subunit and identifying the alkylated Cys residue.