Non-charged amino acids from three different domains contribute to link agonist binding to channel gating in α7 nicotinic acetylcholine receptors

Binding of agonists to nicotinic acetylcholine receptors results in channel opening. Previously, we have shown that several charged residues at three different domains of the alpha 7 nicotinic receptor are involved in coupling binding and gating, probably through a network of electrostatic interactions. This network, however, could also be integrated by other residues. To test this hypothesis, non-charged amino acids were mutated and expression levels and electrophysiological responses of mutant receptors were determined. Mutants at positions Asn47 and GIn48 (loop 2), 11030, Trp134, and GIn140 (loop 7), and Thr264 (M2-M3 linker) showed poor or null functional responses, despite significant membrane expression. By contrast, mutants F137A and S265A exhibited a gain of function effect. In all cases, changes in dose-response relationships were small, EC50 values being between threefold smaller and fivefold larger, arguing against large modifications of agonist binding. Peak currents decayed at the same rate in all receptors except two, excluding large effects on desensitization. Thus, the observed changes could be mostly caused by alterations of the gating characteristics. Moreover, analysis of double mutants showed an interconnection between some residues in these domains, especially Gln48 with lle130, suggesting a potential coupling between agonist binding and channel gating through these amino acids.