Regions of β2 and β4 responsible for differences between the steady state dose-response relationships of the α3β2 and α3β4 neuronal nicotinic receptors

Abstract

We constructed chimeras of the rat β2 and β4 neuronal nicotinic subunits to locate the regions that contribute to differences between the acetylcholine (ACh) dose-response relationships of the α3β2 and α3β4 receptors. Expressed in Xenopus oocytes, the α3β2 receptor displays an EC50 for ACh ~20-fold less than the EC50 of the α3β4 receptor. The apparent Hill slope (n(app)) of α3β2 is near one whereas the α3β4 receptor displays an n(app) near two. Substitutions within the first 120 residues convert the EC50 for ACh from one wild-type value to the other. Exchanging just β2:104-120 for the corresponding region of β4 shifts the EC50 of ACh dose-response relationship in the expected direction but does not completely convert the EC50 of the dose-response relationship from one wild-type value to the other. However, substitutions in the β2:104-120 region do account for the relative sensitivity of the α3β2 receptor to cytisine, tetramethylammonium, and ACh. The expression of β4-like (strong) cooperativity requires an extensive region of β4 (β4:1-301). Relatively short β2 substitutions (β2:104-120) can reduce cooperativity to β2-like values. The results suggest that amino acids within the first 120 residues of β2 and the corresponding region of β4 contribute to an agonist binding site that bridges the α and β subunits in neuronal nicotinic receptors.