Acetylcholine Receptor Channel Structure Probed in Cysteine-Substitution Mutants

In order to understand the structural bases of ion conduction, ion selectivity, and gating in the nicotinic acetylcholine receptor, mutagenesis and covalent modification were combined to identify the amino acid residues that line the channel. The side chains of alternate residues-Ser$^{248}$, Leu$^{250}$, Ser$^{252}$, and Thr$^{254}$-in M2, a membrane-spanning segment of the α subunit, are exposed in the closed channel. Thus $\alpha^{248-254}$ probably forms a β strand, and the gate is closer to the cytoplasmic end of the channel than any of these residues. On channel opening, Leu$^{251}$ is also exposed. These results lead to a revised view of the closed and open channel structures.