REVIEWS OLIGOMERIZATION OF G-PROTEIN- COUPLED TRANSMITTER RECEPTORS Michel Bouvier Examples of G-protein-coupled receptors that can be biochemically detected in homo- or heteromeric complexes are emerging at an accelerated rate. Biophysical approaches have confirmed the existence of several such complexes in living cells and there is strong evidence to support the idea that dimerization is important in different aspects of receptor biogenesis and function. While the existence of G-protein-coupled-receptor homodimers raises fundamental questions about the molecular mechanisms involved in transmitter recognition and signal transduction, the formation of heterodimers raises fascinating combinatorial possibilities that could underlie an unexpected level of pharmacological diversity, and contribute to cross-talk regulation between transmission systems. Because G-protein-coupled receptors are major pharmacological targets, the existence of dimers could have important implications for the development and screening of new drugs. Here, we review the evidence supporting the existence of G-protein-coupled-receptor dimerization and discuss its functional importance. BIOGENIC AMINES As one of the largest gene families, G-protein-coupled a wealth of information on the biochemical events A series of molecules that can receptors (GPCRs) represent the most commonly used underlying cellular signalling by GPCRs. act as neurotransmitters and signal-transduction system in the animal kingdom. In The proposed membrane topology of the receptors include noradrenaline and humans, it is estimated that ~1,000 distinct members consists of a hydrophobic core of seven transmembrane adrenaline. direct responses to a wide variety of chemical transmit- α-helices that interact to form a three-dimensional bar- 1 ALLOSTERIC ters, including BIOGENIC AMINES, amino acids, peptides, rel within the cytoplasmic membrane , an extracellular A term to describe proteins that lipids, nucleosides and large polypeptides. These trans- amino-terminal segment bearing amino-linked glyco- have two or more receptor sites, membrane receptors are key controllers of such diverse sylation sites and a cytoplasmic carboxy-terminal tail. one of which (the active site) binds the principal substrate, physiological processes as neurotransmission, cellular Their binding to specific ligands involves multiple inter- whereas the other(s) bind(s) metabolism, secretion, cellular differentiation and actions between functional groups on the ligands and effector molecules that can growth, as well as inflammatory and immune responses. specific amino acids within the extracellular domains influence its biological activity. The GPCRs therefore represent important targets for the and/or the hydrophobic transmembrane core of the development of new drug candidates with potential receptor2. Classically, the basic transduction unit com- applications in all clinical fields. Many therapeutic agents prises two elements in addition to the receptor: first, a Department of Biochemistry used at present act by either activating (agonists) or trimeric (αβγ) G protein; and second, an effector com- and Groupe de Recherche sur blocking (antagonists) GPCRs; widely used examples ponent. Binding of a transmitter promotes ALLOSTERIC le système Nerveux β Autonome, Faculté de are -adrenergic receptor agonists for asthma and interactions between the receptor and the trimeric G Médecine, Université de antagonists for hypertension and heart failure, hista- protein, leading to the release of GDP and the binding Montréal, α mine H1- and H2-receptor antagonists for allergies and of GTP to the -subunit. This destabilizes the trimeric P.O. Box 6128, Down-Town duodenal ulcers, opioid receptor agonists as analgesics, complex, allowing dissociation of the Gα•GTP and βγ Station, Montréal, Quebec, • H3C 3J7 Canada. dopamine receptor antagonists as antipsychotics and dimer . The ‘activated’ G protein, through its Gα GTP e-mail: Michel.Bouvier@ serotonin receptor agonists for migraine. The results of chain, the βγ dimer or both, in turn interacts with and umontreal.ca studies pursued over the past two decades have provided modulates the effector component. Termination of the 274 | APRIL 2001 | VOLUME 2 www.nature.com/reviews/neuro © 2001 Macmillan Magazines Ltd REVIEWS signal is achieved via hydrolysis of GTP to GDP by a by site–site interactions among receptors within dimeric 16,18–22 GTPase activity intrinsic to Gα. or oligomeric complexes . Atypical binding proper- Effector systems known to be modulated by GPCRs ties of dopamine antagonists such as [3H]-spiperone, using the scheme described above include enzymes such which detects only half of the maximal binding sites seen as adenylyl cyclase, phospholipases C and D and cyclic by ligands of the benzamide family, have also been inter- GMP phosphodiesterase, as well as ion channels and preted as evidence for receptor dimers23. Biochemical antiporters such as the calcium and potassium channels studies, including photo-affinity labelling of the mus- and the Na+/H+ exchanger. Recently, additional effector carinic receptor24, radiation inactivation of α-adrenergic systems that were classically believed to be activated by and β-adrenergic25–27, gonadotropin28, gonadotropin- growth-factor receptors via tyrosine kinase activation releasing hormone29, dopamine30 and adenosine A1 were also shown to be modulated by GPCRs3–8. In par- (REF.31) receptors, crosslinking of the glucagon receptor32, ticular, the ERK, p38 and JNK MAP (mitogen-activated and hydrodynamic properties of cardiac muscarinic protein) kinase signalling pathways were shown to be receptors33, also supported the idea that GPCRs might activated by stimulation of G proteins of the Gq,Gi and form oligomeric structures. Gs families. Depending on the system considered, tyro- Despite these observations, the idea that GPCRs sine kinases9, phosphatidylinositol-3-OH kinase10, could function as dimers or oligomers never gained gen- Akt/protein kinase B (REF.7),Src11 and Ras12 have all been eral acceptance and the prevailing model remained that implicated in this pathway. In addition, G-protein-inde- of a single receptor molecule interacting with a single G pendent signalling has been documented. For instance, protein. This dogma remained unchallenged until the β direct interaction of the 2-adrenergic receptor with the mid-1990s, when trans-complementation studies and Na+/H+-exchanger regulatory factor, NHERF, was new biochemical data reopened the question of GPCR shown to modulate the activity of the Na+/H+ exchanger dimerization. Most of the evidence taken to support the type-3 (REF.13). Other examples include the chemokine14 existence of GPCR dimers would also be consistent with and angiotensin II 15 receptors, where direct recruitment the existence of higher-order oligomers. As available and activation of the Janus kinases (JAK) was suggested. techniques do not allow these possibilities to be readily Generally, it was believed that distinct sets of intra- distinguished, the term dimer is often used, being the molecular interactions within the receptors would char- simplest form of oligomer that can explain the observa- acterize the active and inactive conformations upon tions. It is in this context that we use the word dimer binding of the ligands. However, recent data indicate throughout this review. that, in addition to the specific intramolecular interac- tions that could define the activation states of the recep- Complementation and immunoprecipitation tor, intermolecular interactions might also be important. One of the first studies that renewed interest in the pos- Receptor dimerization as well as interactions with acces- sibility that GPCRs could function as dimers was the 34 α sory proteins have been documented and proposed as elegant study by Maggio et al. , using chimeric 2- important determinants of GPCR activity. The following adrenergic/M3 muscarinic receptors composed of the sections review the biochemical and biophysical evi- first five transmembrane domains of one receptor and dence supporting the existence of GPCR homo- and the last two transmembrane domains of the other. heterodimers. The potential roles and implications of the When either chimera was expressed alone, no binding formation of such receptor dimers are discussed in the or signalling could be detected, but coexpression of the light of the most recent data, which indicate that dimer- two chimeras restored binding and signalling to both ization and oligomeric assemblies might represent the muscarinic and adrenergic ligands. Similarly, coexpres- rule rather than the exception for this important class of sion of two binding-defective angiotensin II receptor receptors. In several instances, the formation of oligo- point mutants rescued the binding affinity for the pep- meric complexes larger than dimers could explain the tide35, whereas coexpression of calcium receptors har- data as well as, or in some cases even better16 than, dimers. bouring inactivating mutations in distinct domains was shown to partially rescue calcium-mediated signalling36. History of GPCR dimerization Such functional trans-complementations were inter- The concept that dimerization participates in the activa- preted as intermolecular interactions between inactive tion of transmembrane receptors is well accepted for receptors in a way that restored both ligand-binding and many growth-factor and cytokine receptors, such as the signalling domains within a dimeric complex. epidermal growth