Conserved Aspartic Acid Residues 79 and 113 of the Beta-adrenergic Receptor Have Different Roles in Receptor Function

Deletion mutagenesis experiments have demonstrated that the binding site of the beta-adrenergic receptor involves the hydrophobic core of the protein (Dixon, R. A. F., Sigal, I. S., Rands, E., Register, R. B., Candelore, M. R., Blake, A. D., and Strader, C. D. (1987) Nature 326, 73-77). Single amino acid replacements for the conserved Asp79 and Asp113 within this putative transmembrane region had profound effects on the ability of the receptor to bind radiolabeled ligands (Strader, C. D., Sigal, I. S., Register, R. B., Candelore, M. R., Rands, E., and Dixon, R. A. F. (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 4384-4388). In this report we have analyzed the ability of these mutant receptors to stimulate adenylyl cyclase in the presence of agonists. The substitution of Asp79 with Ala caused 10-fold increases in both the Kd for isoproterenol binding and the Kact for adenylyl cyclase stimulation. The substitution of Asp113 by Asn or Glu resulted in 8,000-40,000 and 300-1,500-fold increases, respectively, in the Kact values for agonist stimulation of adenylyl cyclase without altering the maximum level of stimulation. Whereas the binding of antagonists to the receptor was not affected by substitution of Asp79, substitution of Asp113 decreased the affinity for the antagonist propranolol by 10,000-fold. These data are consistent with overlapping but not identical binding sites for agonists and antagonists on the beta-adrenergic receptor, in which the carboxylate group of Asp113 interacts with the amino group of the ligand. The sequence similarity among the family of G-protein-linked receptors suggests that the presence of an Asp residue at the analogous position of one of these receptors is predictive of the ability of the receptor to bind amines as ligands.