The alpha4beta2 nicotinic acetylcholine receptor (nAChR) is the most abundant nAChR type in the brain, and this receptor type exists in alternate (alpha4beta2)2alpha4 and (alpha4beta2)2beta2 forms, which are activated by agonists with strikingly differing efficacies. Recent breakthroughs have identified an additional operational agonist binding site in the (alpha4beta2)2alpha4 nAChR that is responsible for the signature sensitivity of this receptor to activation by agonists, yet the structural mechanisms determining agonist efficacy at this receptor type are not yet fully understood. In this study, we characterized the ligand selectivity of the individual agonist sites of the (alpha4beta2)2alpha4 nAChR to determine whether differences in agonist selectivity influence agonist efficacy. Applying the substituted cysteine accessibility method to individual agonist sites in concatenated (alpha4beta2)2alpha4 receptors, we determined the agonist selectivity of the agonist sites of the (alpha4beta2)2alpha4 receptor. We show that (a) accessibility of substituted cysteines to covalent modification by methanesulfonate reagent depends on the agonist site at which the modification occurs and (b) that agonists such as sazetidine-A and TC-2559 are excluded from the site at the alpha4/alpha4 interface. Given that additional binding to the agonist site in the alpha4/alpha4 interface increases acetylcholine efficacy and that agonists excluded from the agonist site at the alpha4/alpha4 interface behave as partial agonists, we conclude that the ability to engage all agonist sites in (alpha4beta2)2alpha4 nAChRs is a key determinant of agonist efficacy. The findings add another level of complexity to the structural mechanisms that govern agonist efficacy in heteromeric nAChRs and related ligand-gated ion channels.
        
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Mazzaferro S, Gasparri F, New K, Alcaino C, Faundez M, Iturriaga Vasquez P, Vijayan R, Biggin PC, Bermudez I (2014) Non-equivalent Ligand Selectivity of Agonist Sites in (alpha4beta2)2alpha4 Nicotinic Acetylcholine Receptors: A KEY DETERMINANT OF AGONIST EFFICACY Journal of Biological Chemistry289: 21795-806
Mazzaferro S, Gasparri F, New K, Alcaino C, Faundez M, Iturriaga Vasquez P, Vijayan R, Biggin PC, Bermudez I (2014) Journal of Biological Chemistry289: 21795-806