A single residue in the M2-M3 loop is a major determinant of coupling between binding and gating in neuronal nicotinic receptors

Binding of agonists to nicotinic acetylcholine receptors generates a sequence of changes that activate a cation-selective conductance, By measuring electrophysiological responses in chimeric alpha 7/alpha 3 receptors expressed in Xenopus oocytes, we have showed the involvement of the M2-M3 loop in coupling agonist binding to the channel gate. An aspartate residue therein, Asp-266 in the alpha 7 subunit, was identified by site-directed mutagenesis as crucial, since mutants at this position exhibited very poor functional responses to three different nicotinic agonists, We have extended this investigation to another neuronal nicotinic receptor (alpha 3/beta 4), acid found that a homologous residue in the beta 4 subunit, Asp-268, played a similar role in coupling, These findings are consistent with a hypothesis that the aspartate residue in the M2-M3 loop, which is conserved in all homomer-forming alpha-type subunits and all neuronal beta-type subunits that combine to form functional receptors, is a major determinant of information transmission from binding site to channel gate in all neuronal nicotinic receptors.