Role of M2 domain residues in conductance and gating of acetylcholine receptors in developing Xenopus muscle

The contributions of specific residues in γ- and ɛ-subunits to the developmental changes in conductance and open time of Xenopus muscle acetylcholine receptors (AChRs) were investigated. This study was directed primarily at residues in the M2 domains of γ- and ɛ-subunits; however, the results of additional mutations in the extracellular region flanking M2 and in the amphipathic region between M3 and M4 are also described. The M2 domains of γ- and ɛ-subunits differ at only three amino acid residues, two of which are adjacent to each other and located near the narrowest part of the pore. These two residues (NI in γ, SV in ɛ) were found to be major determinants of the difference in conductance and open time of AChRs bearing γ- or ɛ-subunits. Mutation of N to S in the γ-subunit converted the long open time of receptors bearing the γ-subunit (γ-AChRs) to the brief open time characteristic of receptors bearing an ɛ-subunit (ɛ-AChRs). Conversely, ɛ-AChRs with SV mutated to NI in the ɛ-subunit exhibited a long open time characteristic of γ-AChRs. Mutation of N to S in the γ-subunit increased the conductance of γ-AChRs but did not confer the full conductance of wild-type ɛ-AChRs. Conversely, mutation of SV to NI in the ɛ-subunit reduced the conductance of ɛ-AChRs, but not completely to the level of wild-type γ-AChRs.