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Address correspondence to Steven Sine, Receptor Biology Laboratory, Department of Physiology and Biophysics, Mayo Clinic and Foundation, Rochester, MN 55905. Fax: (507) 284-9420; E-mail: sine{at}mayo.edu
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, 
N182Y, 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 
site. Studies of the analogous mutation in the subunit, N187Y, disclose rate constants for ACh occupancy of the nonmutant site. The second CMS mutation, D175N, 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. D175N impairs overall gating, however, through an effect independent of ACh occupancy. When mapped on a structural model of the AChR binding site, N182Y localizes to the interface with the subunit, and D175 to the entrance of the ACh binding cavity. Both N182Y and D175 show state specificity in affecting closed relative to desensitized state affinities, suggesting that the protein chain harboring N182 and D175 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.
Key Words: congenital myasthenic syndrome • single channel kinetics • agonist binding • channel gating • mutation analysis
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