The effect of structural alterations of the M4 transmembrane segment in the Torpedo californica AChR has shown that substitution of specific residues can be critical to the channel gating (Lasalde et al., 1996). In a previous study we found that phenylalanine and tryptophan substitutions at the alphaC418 residue in the M4 transmembrane segment of the Torpedo californica AChR significantly altered ion channel function (Lee et al., 1994; Ortiz-Miranda et al., 1997). Cassette mutagenesis was used to mutate the Cys residue at the corresponding C418 position in the alpha subunit of mouse AChR. A total of nine mutations on the mouse alphaC418 position were tested, including the alphaC418A, alphaC418V, alphaC418L, alphaC418S, alphaC418M, alphaC418W, alphaC418H, alphaC418E and alphaC418G mutants. All the mutants tested were functional except the alphaC418G which was not expressed on the surface of the oocyte. The data obtained from macroscopic and single channel currents demonstrate that different types of amino acids can be accommodated at this presumably lipid-exposed position without loss of ion-channel function. As with the Torpedo AChR, the mutation of Cys to Trp dramatically decreased the EC(50) for acetylcholine and increased channel open time. The lack of expression of the mouse alphaC418G suggest that there are some differences in folding, oligomerization and perhaps transport to the surface membrane for this mutant between the Torpedo and the mammalian AChR.