By defining the functional defect in a congenital myasthenic syndrome (CMS), we show that the third transmembrane domain (M3) of the muscle acetylcholine receptor governs the speed and efficiency of gating of its channel. The clinical phenotype of this CMS results from the mutation V285I in M3 of the alpha subunit, which attenuates endplate currents, accelerates their decay and causes abnormally brief acetylcholine-induced single-channel currents. Kinetic analysis of engineered alpha V285I receptors demonstrated a predominant effect on channel gating, with abnormally slow opening and rapid closing rates. Analysis of site-directed mutations revealed stereochemical and volume-dependent contributions of alpha V285 to channel gating. Thus, we demonstrate a functional role for the M3 domain as a key component of the nicotinic acetylcholine receptor channel-gating mechanism.

By defining functional defects in a congenital myasthenic syndrome (CMS), we show that two mutant residues, located in a binding site region of the acetylcholine receptor (AChR) epsilon subunit, exert opposite effects on ACh binding and suppress channel gating. Single channel kinetic analysis reveals that the first mutation, ɛN182Y, increases ACh affinity for receptors in the resting closed state, which promotes sequential occupancy of the binding sites and discloses rate constants for ACh occupancy of the nonmutant αδ site. Studies of the analogous mutation in the δ subunit, δN187Y, disclose rate constants for ACh occupancy of the nonmutant αɛ site. The second CMS mutation, ɛD175N, reduces ACh affinity for receptors in the resting closed state; occupancy of the mutant site still promotes gating because a large difference in affinity is maintained between closed and open states. ɛD175N impairs overall gating, however, through an effect independent of ACh occupancy. When mapped on a structural model of the AChR binding site, ɛN182Y localizes to the interface with the α subunit, and ɛD175 to the entrance of the ACh binding cavity. Both ɛN182Y and ɛD175 show state specificity in affecting closed relative to desensitized state affinities, suggesting that the protein chain harboring ɛN182 and ɛD175 rearranges in the course of receptor desensitization. The overall results show that key residues at the ACh binding site differentially stabilize the agonist bound to closed, open and desensitized states, and provide a set point for gating of the channel.

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