Identification of residues required for ligand binding to the beta-adrenergic receptor.

TLDR

The study examined the role of conserved polar residues in the beta‑adrenergic receptor’s transmembrane region by mutating the hamster beta‑AR gene and expressing mutants in COS‑7 cells. Mutations were introduced via oligonucleotide‑directed mutagenesis of the hamster beta‑AR gene, and mutant receptors were expressed in COS‑7 cells. Mutagenesis revealed that D113 is essential for antagonist binding, D79 and N318 are critical for agonist affinity, and P323 is required for proper receptor folding, indicating that the ligand binding site resides within the transmembrane region.

Abstract

The functional significance of conserved polar amino acids within the putative transmembrane region of the beta-adrenergic receptor (beta AR) was examined by oligonucleotide-directed mutagenesis of the hamster gene encoding beta AR and expression of the mutant genes in COS-7 cells. Although a substitution of aspartate at position 113 with an asparagine residue did not affect expression or processing of the protein, the resulting mutant beta AR did not show detectable binding toward the antagonist iodocyanopindolol. Replacement of the aspartate and asparagine residues at positions 79 and 318, respectively, had no effect on the affinity of the receptor toward antagonists but reduced the affinity of the receptor toward agonists by 1 order of magnitude. Furthermore, we observed that substitution of the proline at position 323 with a serine residue resulted in improper or incomplete processing of the beta AR, presumably reflecting a role for this residue in the folding of the receptor. Together with our previous results from deletion mutagenesis studies, these observations indicate that the ligand binding site involves the transmembrane region of the beta AR.