CHANNEL GATING GOVERNED SYMMETRICALLY BY CONSERVED LEUCINE RESIDUES IN THE M2 DOMAIN OF NICOTINIC RECEPTORS

IN nicotinic acetylcholine receptors (nAChR), as well as glycine, GABA (A) (gamma-aminobutyric acid), serotonin (5-HT3), and GluCl glutamate receptors, a leucine residue at the approximate midpoint of the M2 tra nsmembrane domain (the 9' position(1)) is conserved across most known subunits(2). Structural data for the nAChR suggest that the Leu 9' res idues occupy a 'kink' in each of the five M2 helices and point into th e closed channel; in the opening step, the M2 helices rotate so that L eu 9' side chains no longer occlude the conduction pathway(3), Mutatio n of Leu 9' to one of several other residues slows desensitization and increases sensitivity to agonist(4-6). We have exploited the alpha(2) beta gamma delta stoichiometry of muscle nAChR to express receptors w ith m(s) = 0 to 5 Leu 9'Scr mutated subunits. Strikingly, each Leu 9' Ser mutation shifts the dose-response relation for ACh to the left by similar to 10-fold; a nAChR with m(s) = 4 is 10(4)-fold more sensitiv e than the wild type, The results suggest that each of the five Leu 9' residues participates independently and symmetrically in a key step i n the structural transition between the closed and open states.