Reversal of agonist selectivity by mutations of conserved amino acids in the binding site of nicotinic acetylcholine receptors.

Abstract Homomeric α7 and heteromeric α4β2 nicotinic acetylcholine receptors (nAChR) can be distinguished by their pharmacological properties, including agonist specificity. We introduced point mutations of conserved amino acids within the C loop, a region of the receptor critical for agonist binding, and we examined the expression of the mutant receptors in Xenopus oocytes. Mutation of either a conserved C loop tyrosine (188) to phenylalanine or a nearby conserved aspartate (197) to alanine resulted in α7 receptors for which the α7-selective agonist 3-(4-hydroxy, 2-methoxybenzylidene) anabaseine (4OH-GTS-21) had roughly the same potency as for wild-type receptors, whereas the physiologic agonist acetylcholine (ACh) showed drastically reduced potency for these mutant receptors. Corresponding mutations in α4 receptors co-expressed with β2 resulted in α4β2 receptors for which ACh potency was relatively unchanged, although the efficacy of the α7-selective agonist 4OH-GTS-21 was increased greatly relative to that of ACh. We also investigated the significance of a conserved lysine (145 in α7), proposed to form a stable salt bridge with Asp-197 in the resting state of the receptor. Mutations of this residue in both α7 and α4 resulted in receptors that were largely unresponsive to both ACh and 4OH-GTS-21. Our results suggest that initiation of gating depends both on specific interactions between residues in the C loop domain and, depending on receptor subtype, the physiochemical properties of the agonist, so that in the altered environment of the α4Y190F-binding site, large hydrophobic benzylidene anabaseines may close the C loop and initiate channel gating more effectively than the polar agonist ACh.