The role of conserved leucines in the M2 domain of the acetylcholine receptor in channel gating.

A highly conserved leucine is found in the middle of the porelining (M2) domain of the members of the ligand-gated ion channel family. Two very different roles have been proposed for this leucine. In one model, this residue swings into the lumen of the channel during desensitization to form the nonconducting desensitized state, whereas in the other model, the leucines from each subunit interact with each other to form a constriction in the channel that constitutes the actual gate of the channel. We examined the role of this leucine in the muscle-type acetylcholine receptor by replacing it with the polar amino acid threonine. Replacement of the leucine in any one subunit slows desensitization and shifts the dose-response relationship toward lower concentrations. Replacement of leucines in additional subunits leads to progressively larger shifts in the dose-response curves. The shift depends only on the number of leucines replaced, not on which particular subunits contain the mutation; in other words, the mutations act independently. At the single-channel level, the mutation greatly increases the channel mean open time. We conclude that the role of the conserved leucine is to set the mean open time of the channel through interactions with other regions of the receptor rather than to serve as the gate per se of the ion channel.