Acetylcholine receptor gating is influenced by the polarity of amino acidsat position 9 ' in the M2 domain

Ligand-gated ion channels contain a conserved leucine at position 9' (L9') in the M2 transmembrane domain. We used multiple substitutions at this posi tion in the gamma subunit of the mouse acetylcholine receptor (AChR) (gamma L9') to examine the role of residue polarity at this position in the gatin g process at both the macroscopic and single-channel levels. The midpoint o f the macroscopic dose-response relationship (EC50) and the channel closing rate constant, alpha, decreased as the polarity of the residue at that pos ition increased, suggesting a stabilization of the open state of the channe l. Both parameters showed similar dependencies on the polarity of the subst ituted residue. These data support the notion that during AChR gating, the amino acid at the 9' position moves into a polar environment, and that inte ractions between this residue and the polar environment determine the stabi lity of the open state. Since this residue is conserved in all other member s of the ligand-gated ion channel family, we suggest that a similar mechani sm applies to the other members of the family.