Protein structural effects of agonist binding to the nicotinic acetylcholine receptor.

The effects on the protein structure produced by binding of cholinergic agonists to purified acetylcholine receptor (AcChR) reconstituted into lipid vesicles, has been studied by Fourier-transform infrared spectroscopy and differential scanning calorimetry. Spectral changes in the conformationally sensitive amide I infrared band indicates that the exposure of the AcChR to the agonist carbamylcholine, under conditions which drive the AcChR into the desensitized state, produces alterations in the protein secondary structure. Quantitative estimation of these agonist-induced alterations by band-fitting analysis of the amide I spectral band reveals no appreciable changes in the percent of alpha-helix, but a decrease in beta-sheet structure, concomitant with an increase in less ordered structures. Additionally, agonist binding results in a concentration-dependent increase in the protein thermal stability, as indicated by the temperature dependence of the protein infrared spectrum and by calorimetric analysis, which further suggest that AcChR desensitization induced by the cholinergic agonist implies significant rearrangements in the protein structure.