We studied the functional effects of single amino acid substitutions in the postulated M4 transmembrane domains of Torpedo californica nicotinic acetylcholine receptors (nAChRs) expressed in Xenopus oocytes at the single-channel level. At low ACh concentrations and cold temperatures, the replacement of wild-type alpha418Cys residues with the large, hydrophobic amino acids tryptophan or phenylalanine increased mean open times 26-fold and 3-fold, respectively. The mutation of a homologous cysteine in the beta subunit (beta447Trp) had similar but smaller effects on mean open time. Coexpression of alpha418Trp and beta447Trp had the largest effect on channel open time, increasing mean open time 58-fold. No changes in conductance or ion selectivity were detected for any of the single subunit amino acid substitutions tested. However, the coexpression of the alpha418Trp and beta447Trp mutated subunits also produced channels with at least two additional conductance levels. Block by acetylcholine was apparent in the current records from alpha418Trp mutants. Burst analysis of the alpha418Trp mutations showed an increase in the channel open probability, due to a decrease in the apparent channel closing rate and a probable increase in the effective opening rate. Our results show that modifications in the primary structure of the alpha- and beta subunit M4 domain, which are postulated to be at the lipid-protein interface, can significantly alter channel gating, and that mutations in multiple subunits act additively to increase channel open time.