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

1. The contributions of specific residues in gamma- and epsilon-subunits to the developmental changes in conductance and open time of Xenopus muscle a cetylcholine receptors (AChRs) were investigated. This study was directed p rimarily at residues in the M2 domains of gamma- and epsilon-subunits; howe ver, the results of additional mutations in the extracellular region flanki ng M2 and in the amphipathic region between M3 and M4 are also described. 2. The M2 domains of gamma- and epsilon-subunits differ at only three amino acid residues, two of which are adjacent to each other and located near th e narrowest part of the pore. These two residues (NI in gamma, SV in epsilo n) were found to be major determinants of the difference in conductance and open time of AChRs bearing gamma- or epsilon-subunits. 3. Mutation of N to S in the gamma-subunit converted the long open time of receptors bearing the gamma-subunit (gamma-AChRs) to the brief open time ch aracteristic of receptors bearing an epsilon-subunit (epsilon-AChRs). Conve rsely, epsilon-AChRs with SV mutated to NI in the is an element of-subunit exhibited a long open time characteristic of gamma-AChRs. 4. Mutation of N to S in the gamma-subunit increased the conductance of gam ma-AChRs but did not confer the full conductance of wild-type epsilon-AChRs . Conversely, mutation of SV to NI in the E-subunit reduced the conductance of epsilon-AChRs, but not completely to the level of wild-type gamma-AChRs .