Secondary structural analyses of the nicotinic acetylcholine receptor as a test of molecular models

Circular dichroism (CD) spectroscopy was used to determine that the secondary structure of purified nicotinic acetylcholine receptor (AChR) of Torpedo californica in both reconstituted vesicles and a cholate-solubilized state is, on average, 23% alpha-helix, 43% beta-sheet, 6% beta-turn, and 28% random coil. These data can serve to test models by placing limits on the particular types of secondary structural motifs which make up the receptor. A number of models proposed for the AChR are discussed in relation to the experimental data presented. Additionally, the protein in both vesicle and solubilized environments was exposed to an agonist, carbamylcholine, or a competitive antagonist, hexamethonium, to monitor net conformational changes upon ligand binding. The protein secondary structure was not changed upon solubilization, nor was any large net conformational change observed upon ligand binding, although small local or compensatory changes cannot be ruled out.