Agonist Interactions with Chimeric and Mutant β₁- and β₃-Adrenergic Receptors: Involvement of the Seventh Transmembrane Region in Conferring Subtype Specificity

Abstract

β₁- and β₃-adrenergic receptors (AR) are the predominant β-AR subtypes in adipocytes, and analysis of native and recombinant β-AR has revealed several pharmacological and biochemical differences between these subtypes. This study used chimeric and mutated rat β-AR expressed in Chinese hamster ovary cells to examine the basis of certain characteristic differences in the agonist properties of catecholamines and prototypic β₃-AR agonists. The exchange of sequence beyond transmembrane (TM) region 6 between the β-AR subtypes had dramatic and reciprocal effects on the affinity and efficacy of the prototypic β₃-AR agonists BRL 37,344 and CL 316,243, without affecting the interactions with catecholamines. Mutation of Phe350 and Phe351 in TM7 of the β₁-AR to Ala and Leu found in the β₃-AR was sufficient to allow activation by prototypic β₃-AR agonists. Interestingly, this mutation did not affect catecholamine action and it did not impair the ability of propranolol to block the actions of isoproterenol or the selective β₃-AR agonists. β₁-AR containing β₃-AR sequence from predicted TM5 through TM6 exhibited reduced affinity for catecholamines without altering agonist potency, suggesting enhanced coupling efficiency. Inclusion of the homologous β₁-AR sequence in the β₃-AR, however, did not produce reciprocal effects. These results are the first to define a major determinant of β₃-AR subtype-selective agonism in TM7 and demonstrate that the determinants of selective phenethanolamines, catecholamines, and propranolol action are distinct.