Gating of α3β4 neuronal nicotinic receptor can be controlled by the loop M2-M3 of both α3 and β4 subunits

Previous studies have shown that the gating mechanism of alpha(3)beta(4) neuronal nicotinic receptors is affect ed by a residue in the middle of the M2-M3 loop of the beta(4) subunit. We have extended the study of the same location to the alpha(3) subunit, Bovine alpha(3)beta(4) receptors were mutated in position 268, substituting the residue present in wild-type receptors, i.e. leucine in alpha(3) and asparagine in beta(4), for an aspartate, Wild-type and mutated alpha(3) and beta(4) subunits were combined to form four different receptors. We have measured macroscopic currents in Xenopus oocytes elicited by nicotine, and related them to surface receptor expression measured with an epibatidine-binding essay. We also obtained single-channel recordings of the receptors to study their kinetic behaviour. The results were analysed in terms of an allosteric model with three states. We found that the effect of the mutation in the cc, subunit on the gating of the receptor was similar to the corresponding mutation in the beta(4) subunit, The effect when both subunits were mutated was additive, suggesting that the contribution of each subunit to the gating mechanism is independent.