Genetic approaches to the determination of structure-function relationships of G protein-coupled receptors

The beta-adrenergic receptor (beta AR), which has been extensively characterized pharmacologically, serves as a useful model system for the analysis of the structure-function relationships of G protein-coupled receptors. Genetic and biochemical analysis has revealed that the ligand binding domain of the receptor involves residues within the hydrophobic transmembrane core of the protein. Molecular substitution experiments suggest that adrenergic agonists and antagonists are anchored to the receptor through an ionic interaction between Asp113 in the third hydrophobic region of the receptor and the protonated amine group of the ligand. In addition, catecholamine agonists are bound through hydrogen bonding interactions between two serine residues in the fifth hydrophobic domain of the receptor and the catechol hydroxyl groups of the ligand. Agonist-mediated activation of the G protein Gs requires residues within the cytoplasmic loop linking the fifth and sixth transmembrane helices which are predicted to form amphipathic alpha-helices. The strong structural similarities among G protein-coupled receptors imply that the information gained from genetic analysis of the beta AR should be applicable to other hormone and neurotransmitter receptors of this class.