P1: NGM/RSK P2: SDA/VKS QC: SDA/BS T1: SDADecember 12, 1996 4:8 Annual Reviews AR024-16 AR24-16Annu. Rev. Neurosci. 1997. 20:399–427Copyrightc1997 by Annual Reviews Inc. All rights reservedFUNCTIONAL AND STRUCTURALCOMPLEXITY OF SIGNALTRANSDUCTION VIAG-PROTEIN-COUPLED RECEPTORSThomas Gudermann, Torsten Sch¨oneberg, and G¨unter SchultzInstitut f¨ur Pharmakologie, Freie Universit¨at Berlin, D-14195 Berlin, GermanyKEY WORDS: heptahelical receptors, receptor conformation, receptor folding and assembly,receptor–G protein interactionABSTRACTA prerequisite for the maintenance of homeostasis in a living organism is fine-tuned communication between different cells. The majority of extracellular sig-naling molecules, such as hormones and neurotransmitters, interact with a three-protein transmembrane signaling system consisting of a receptor, a G protein,and an effector. These single components interact sequentially and reversibly.Considering that hundreds of G-protein-coupled receptors interact with a limitedrepertoire of G proteins, the question of coupling specificity is worth consider-ing. G-protein-mediated signal transduction is a complex signaling network withdiverging and converging transduction steps at each coupling interface. The re-cent realization that classical signaling pathways are intimately intertwined withgrowth-factor–signaling cascades adds another level of complexity. Elaboratestudies have significantly enhanced our knowledge of the functional anatomy ofG-protein-coupled receptors, and the concept has emerged that receptor functioncan be modulated with high specificity by coexpressed receptor fragments. Theseresults may have significant clinical impact in the future.RECEPTORS AND G PROTEINS AS INTEGRALCOMPONENTS OF TRANSMEMBRANESIGNALING PROCESSESFine-tuned communication between individual cells is a prerequisite for main-taining homeostasis within a living organism. Cells have the ability to process3990147-006X/97/0301-0399$08.00P1: NGM/RSK P2: SDA/VKS QC: SDA/BS T1: SDADecember 12, 1996 4:8 Annual Reviews AR024-16 AR24-16400 GUDERMANN, SCH¨ONEBERG & SCHULTZvast amounts of information conveyed to them by extracellular signals (suchas hormones, neurotransmitters, autacoids, growth factors, and odorants) andby physical signals (such as light). Most of these signals do not enter thecell but affect receptors at the cell surface. In principle, transmembrane sig-naling processes are governed by two fundamentally different and seeminglyunrelated mechanisms. Some membrane receptors (e.g. ligand-gated ion chan-nels, protein tyrosine and serine/threonine kinase receptors, phosphoproteinphosphatases or guanylyl cyclase receptors) intramolecularly combine an ex-tracellular ligand-binding domain and an intracellular (in the case of enzymes)or transmembrane effector domain (a pore in the case of ion channels) (F¨ulleet al 1995, Iismaa et al 1995, Yang et al 1995). This first transduction princi-ple is thus characterized by a rigidly coupled receptor-effector system. Mostintercellular signaling molecules, however, bind to membranous receptors thatrepresent one element of a three-component transmembrane signaling systemwhoseindividualelementsinteractsequentiallyand reversibly. Agonistbindingto a specific receptor results in activation of heterotrimeric guanine nucleotide–binding proteins (G proteins) acting as transducers and signal amplifiers. Gproteins subsequently modulate the activity of effectors, such as enzymes,ion channels, and transporters, resulting in rapid alterations of second mes-senger (e.g. cAMP, inositol phosphates, diacylglycerol, arachidonic acid, andcytosolic ion) concentrations (Birnbaumer et al 1990, Hepler & Gilman 1992,Birnbaumer & Birnbaumer 1995, Neer 1995).Basic Architecture of G-Protein-Coupled ReceptorsG-protein-coupled receptors constitute a large and diverse superfamily(Baldwin 1994, Strader et al 1994, Gudermann et al 1995). Family membershave been identified in organismsas evolutionarily distant as yeast and man. Todate, several hundred G-protein-coupled receptors have been cloned. The totalnumber, including an ever growing subgroup of odorant receptors, is assumedto exceed 1000. A comparison of the primary structures of G-protein-coupledreceptors indicates that these proteins are patterned according to a commonstructural principle. They belong to a superfamily of integral membrane pro-teins characterized by seven hydrophobic stretches of amino acids that are pre-dicted to form transmembrane α helices, connected by alternating extracellularand intracellular loops (Dohlman et al 1991, Baldwin 1994). The N terminus ofthese heptahelical receptors is located extracellularly; the C terminus extendsinto the cytoplasm.Amino acid alignments of cloned receptors allow the construction of den-drograms that reflect the evolutionary history of receptors (Donnelly et al 1994,Vernier et al 1995), thus helping researchers decipher the puzzling molecu-lar and pharmacological diversity of these receptors. In general, amino acidP1: NGM/RSK P2: SDA/VKS QC: SDA/BS T1: SDADecember 12, 1996 4:8 Annual Reviews AR024-16 AR24-16G-PROTEIN-MEDIATED SIGNALING 401identities are highest in the predicted transmembrane spanning regions (TM)and fall off dramatically in the N and C termini and in the extra- and intracellu-lar loops. This pattern may suggest a molecular basis for ligand and G-proteinselectivity. The construction and functional expression of receptor chimerasand of receptors with single amino acid exchanges led to the identification ofspecific residues important for ligand binding (Dohlman et al 1991, Straderet al 1994, Gudermann et al 1995, Wess 1995). The most crucial contact pointsbetween catecholamines and the binding pocket of adrenoceptors are thoughtto be an asparagine in TM 3, two serine residues in TM 5, and a phenylalaninein TM 6, all located deep in the lipid bilayer (Strader et al 1994). As potentialcontact points between other monoamine ligands and their respective recep-tors were indentified in corresponding positions, it was proposed that a bindingpocket located deep in the level of the lipid bilayer represents a universal inter-action and receptor activation site for all members of the seven transmembranereceptor family (Trumpp-Kallmeyer et al 1992). This model of agonist bindingand receptor activation has also been applied to peptide hormone receptors. Di-rected mutagenesis experiments and molecular modeling of