Gating of alpha(3)beta(4) neuronal nicotinic receptor can be controlled bythe loop M2-M3 of both alpha(3) and beta(4) subunits

Previous studies have shown that the gating mechanism of alpha(3)beta(4) ne uronal nicotinic receptors is affect ed by a residue in the middle of the M 2-M3 loop of the beta(4) subunit. We have extended the study of the same lo cation to the alpha(3) subunit, Bovine alpha(3)beta(4) receptors were mutat ed 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, Wil d-type and mutated alpha(3) and beta(4) subunits were combined to form four different receptors. We have measured macroscopic currents in Xenopus oocy tes elicited by nicotine, and related them to surface receptor expression m easured with an epibatidine-binding essay. We also obtained single-channel recordings of the receptors to study their kinetic behaviour. The results w ere analysed in terms of an allosteric model with three states. We found th at the effect of the mutation in the cc, subunit on the gating of the recep tor was similar to the corresponding mutation in the beta(4) subunit, The e ffect when both subunits were mutated was additive, suggesting that the con tribution of each subunit to the gating mechanism is independent.