G Protein-Coupled Receptors: Structure and Function of Signal-Transducing Proteins

This chapter reviews the knowledge of the structure of G protein-coupled receptors and the structural features that are responsible for the various functions ascribed to cell-surface receptors, particularly it emphasizes on the β-adrenergic receptor, because it, along with rhodopsin, is the most extensively studied of the G protein-coupled receptors. Cell-surface receptors that utilize G proteins represent a large and diverse group of proteins. They include many neurotransmitter receptors (e.g, adrenergic, muscarinic cholinergic, serotonergic, and peptidergic receptors); receptors for pituitary protein hormones, eicosanoids, and rhodopsins; and pheromone receptors in yeast and slime molds. All of these receptors are integral membrane glycoproteins that display significant structural and functional homology. The application of molecular cloning techniques to the study of G protein-coupled receptors has led to the elucidation of the primary structure of several of these receptors. The most striking aspect of the primary structure of the G protein-coupled receptors is the presence of seven stretches of hydrophobic amino acids. In the case of rhodopsins, biophysical, proteolytic, immunocytochemical, and chemical modification experiments have shown that the seven hydrophobic regions are largely helical and span the lipid bilayer.