Acetylcholine receptor M3 domain: stereochemical and volume contributions to channel gating

By defining the functional defect in a congenital myasthenic syndrome (CMS) , we show that the third transmembrane domain (M3) of the muscle acetylchol ine 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 th e alpha subunit, which attenuates endplate currents, accelerates their deca y and causes abnormally brief acetylcholine-induced single-channel currents . Kinetic analysis of engineered alpha V285I receptors demonstrated a predo minant effect on channel gating, with abnormally slow opening and rapid clo sing rates. Analysis of site-directed mutations revealed stereochemical and volume-dependent contributions of alpha V285 to channel gating. Thus, we d emonstrate a functional role for the M3 domain as a key component of the ni cotinic acetylcholine receptor channel-gating mechanism.