REGIONS OF BETA-2 AND BETA-4 RESPONSIBLE FOR DIFFERENCES BETWEEN THE STEADY-STATE DOSE-RESPONSE RELATIONSHIPS OF THE ALPHA-3-BETA-2 AND ALPHA-3-BETA-4 NEURONAL NICOTINIC RECEPTORS

We constructed chimeras of the rat beta 2 and beta 4 neuronal nicotini c subunits to locate the regions that contribute to differences betwee n the acetylcholine (ACh) dose-response relationships of the alpha 3 b eta 2 and alpha 3 beta 4 receptors. Expressed in Xenopus oocytes, the alpha 3 beta 2 receptor displays an EC(50) for ACh similar to 20-fold less than the EC(50) of the alpha 3 beta 4 receptor. The apparent Hill slope (n(app)) of alpha 3 beta 2 is near one whereas the alpha 3 beta 4 receptor displays an n(app) near two. Substitutions within the firs t 120 residues convert the EC(50) for ACh from one wild-type value to the other. Exchanging just beta 2:104-120 for the corresponding region of beta 4 shifts the EC(50) of ACh dose-response relationship in the expected direction but does not completely convert the EC(50) Of the d ose-response relationship from one wild-type value to the other. Howev er, substitutions in the beta 2:104-120 region do account for the rela tive sensitivity of the alpha 3 beta 2 receptor to cytisine, tetrameth ylammonium, and ACh. The expression of beta 4-1ike (strong) cooperativ ity requires an extensive region of beta 4 (beta 4:1-301). Relatively short beta 2 substitutions (beta 2:104-120) can reduce cooperativity t o beta 2-like values. The results suggest that amino acids within the first 120 residues of beta 2 and the corresponding region of beta 4 co ntribute to an agonist binding site that bridges the alpha and beta su bunits in neuronal nicotinic receptors.