Structures of the extracellular regions of the group II/III metabotropic glutamate receptors

Homology modeling of GPCRs

Over 1,000 sequences likely to encode G protein-coupled receptors (GPCRs) are currently available in publicly accessible and proprietary databases and this number may grow with the refinement of a number of different genomes. However, despite recent efforts in the crystallization of these proteins, homology modeling approaches are becoming widely used as a method for obtaining quantitative and qualitative information for structure-based drug design as well as the interpretation of experimental data.

Optical measurement of mGluR1 conformational changes reveals fast activation, slow deactivation, and sensitization

Metabotropic glutamate receptor (mGluR) activation has been extensively studied under steady-state conditions. However, at central synapses, mGluRs are exposed to brief submillisecond glutamate transients and may not reach steady-state. The lack of information on the kinetics of mGluR activation impairs accurate predictions of their operation during synaptic transmission. Here, we report experiments designed to investigate mGluR kinetics in real-time. We inserted either CFP or YFP into the second intracellular loop of mGluR1beta. When these constructs were coexpressed in PC12 cells, glutamate application induced a conformational change that could be monitored, using fluorescence resonance energy transfer (FRET), with an EC(50) of 7.5 microM. The FRET response was mimicked by the agonist DHPG, abolished by the competitive antagonist MCPG, and partially inhibited by mGluR1-selective allosteric modulators. These results suggest that the FRET response reports active conformations of mGluR1 dimers. The solution exchange at the cell membrane was optimized for voltage-clamped cells by recording the current induced by co-application of 30 mM potassium. When glutamate was applied at increasing concentrations up to 2 mM, the activation time course decreased to a minimum of approximately 10 ms, whereas the deactivation time course remained constant (approximately 50 ms). During long-lasting applications, no desensitization was observed. In contrast, we observed a robust sensitization of the FRET response that developed over approximately 400 ms. Activation, deactivation, and sensitization time courses and amplitudes were used to derive a kinetic scheme and rate constants, from which we inferred the EC(50) and frequency dependence of mGluR1 activation under non-steady-state conditions, as occurs during synaptic transmission.

The N-terminal domain of GluR6-subtype glutamate receptor ion channels

The amino-terminal domain (ATD) of glutamate receptor ion channels, which controls their selective assembly into AMPA, kainate and NMDA receptor subtypes, is also the site of action of NMDA receptor allosteric modulators. Here we report the crystal structure of the ATD from the kainate receptor GluR6. The ATD forms dimers in solution at micromolar protein concentrations and crystallizes as a dimer. Unexpectedly, each subunit adopts an intermediate extent of domain closure compared to the apo and ligand-bound complexes of LIVBP and G protein-coupled glutamate receptors (mGluRs), and the dimer assembly has a markedly different conformation from that found in mGluRs. This conformation is stabilized by contacts between large hydrophobic patches in the R2 domain that are absent in NMDA receptors, suggesting that the ATDs of individual glutamate receptor ion channels have evolved into functionally distinct families.

Molecular pharmacology of promiscuous seven transmembrane receptors sensing organic nutrients

A number of highly promiscuous seven transmembrane (7TM) receptors have been cloned and characterized within the last few years. It is noteworthy that many of these receptors are activated broadly by amino acids, proteolytic degradation products, carbohydrates, or free fatty acids and are expressed in taste tissue, the gastrointestinal tract, endocrine glands, adipose tissue, and/or kidney. These receptors thus hold the potential to act as sensors of food intake, regulating, for example, release of incretin hormones from the gut, insulin/glucagon from the pancreas, and leptin from adipose tissue. The promiscuous tendency in ligand recognition of these receptors is in contrast to the typical specific interaction with one physiological agonist seen for most receptors, which challenges the classic "lock-and-key" concept. We here review the molecular mechanisms of nutrient sensing of the calcium-sensing receptor, the G protein-coupled receptor family C, group 6, subtype A (GPRC6A), and the taste1 receptor T1R1/T1R3, which are sensing L-alpha-amino acids, the carbohydrate-sensing T1R2/T1R3 receptor, the proteolytic degradation product sensor GPR93 (also termed GPR92), and the free fatty acid (FFA) sensing receptors FFA1, FFA2, FFA3, GPR84, and GPR120. The involvement of the individual receptors in sensing of food intake has been validated to different degrees because of limited availability of specific pharmacological tools and/or receptor knockout mice. However, as a group, the receptors represent potential drug targets, to treat, for example, type II diabetes by mimicking food intake by potent agonists or positive allosteric modulators. The ligand-receptor interactions of the promiscuous receptors of organic nutrients thus remain an interesting subject of emerging functional importance.

Crystal structure and association behaviour of the GluR2 amino-terminal domain

Fast excitatory neurotransmission is mediated largely by ionotropic glutamate receptors (iGluRs), tetrameric, ligand-gated ion channel proteins comprised of three subfamilies, AMPA, kainate and NMDA receptors, with each subfamily sharing a common, modular-domain architecture. For all receptor subfamilies, active channels are exclusively formed by assemblages of subunits within the same subfamily, a molecular process principally encoded by the amino-terminal domain (ATD). However, the molecular basis by which the ATD guides subfamily-specific receptor assembly is not known. Here we show that AMPA receptor GluR1- and GluR2-ATDs form tightly associated dimers and, by the analysis of crystal structures of the GluR2-ATD, propose mechanisms by which the ATD guides subfamily-specific receptor assembly.

Activation switch in the transmembrane domain of metabotropic glutamate receptor

Metabotropic glutamate receptors (mGluRs), members of family 3 G protein-coupled receptors, play pivotal roles in the modulation of synaptic transmission and are important drug targets for various neurological diseases. The structures of the extracellular ligand-binding domain (ECD) of mGluRs and its changes upon ligand binding have been well studied by various techniques, including X-ray crystallography. In contrast, little is known about the structure and structural changes of the seven-transmembrane domain (TMD). Here we searched for constitutively active mutation (CAM) sites in the TMD of mGluR8 to get insight into the epicenter of the structural changes in TMD, a potential target for allosteric ligands. Mutational analyses based on the knowledge of activating mutations of calcium-sensing receptor showed the presence of several CAM sites in the TMD of mGluR8. Among them, the site at position 764 in helix V, where threonine is present, is unique in that any substitution resulted in elevation of the basal activity, and some substitutions caused a loss of responsiveness to agonist. Further comprehensive mutational analyses indicated that the additional mutation of the CAM site at position 663 in helix III, where a tyrosine residue is present, generated a revertant phenotype. Mutations at these sites also increased the agonist binding affinity, although these sites are far from the ECD. These results indicated that the specific pair of amino acids at these CAM sites forms an activation switch that stabilizes the inactive state of mGluR8 and mediates the signal flow from the ECD to the cytoplasmic G protein-interacting site.

Pharmacochaperone-mediated rescue of calcium-sensing receptor loss-of-function mutants

The calcium sensing receptor (CaSR) is a Family C/3 G protein-coupled receptor that translates changes in extracellular Ca(2+) into diverse intracellular signals. Loss-of-function mutations in human CaSR cause familial hypocalciuric hypercalcemia and neonatal severe hyperparathyroidism. CaSR must navigate a number of endoplasmic reticulum quality control checkpoints during biosynthesis, including a conformational/functional checkpoint. Here we examine the biosynthesis of 25 CaSR mutations causing familial hypocalciuric hypercalcemia /neonatal severe hyperparathyroidism using immunoprecipitation, biotinylation, and functional assays. We define classes of CaSR mutants based on their biosynthetic profile. Class I CaSR mutants are not rescued to the plasma membrane. To dissect the organellar compartments that class I mutants can access, we engineered a cleavage site for the proprotein convertase furin into the extracellular domain of wild-type CaSR and class I mutants. Based on absence or presence of cleavage fragments, we find most mutants are degraded from the endoplasmic reticulum (no furin-mediated cleavage), whereas others access the Golgi (furin-mediated cleavage) before degradation. Class II CaSR mutants show increased expression and/or enhanced plasma membrane localization upon treatment with MG132 or the pharmacochaperone NPS R-568, permitting assay of functional activity. Of the 10 CaSR mutants that exhibit plasma membrane localization, only two did not show enhanced functional activity after rescue with NPS R-568. The established approaches can be used with current and newly identified CaSR mutations to identify the location of biosynthetic block and to determine the likelihood of rescue by allosteric agonists.

Modeling of glutamate GluR6 receptor and its interactions with novel noncompetitive antagonists

The study proposes the first complete model of an ionotropic glutamate receptor (GluR6). The model is in accordance with available experimental data from single-particle electron microscopy images and exhibits correct shape and dimensions and the appropriate symmetry: 2-fold in the N-terminal domain (NTD), ligand-binding domain (LBD), and external part of the transmembrane region, whereas it is 4-fold deeper in the channel. The methodology applied for GluR6 receptor model building was validated in the docking procedure of competitive and uncompetitive antagonists. The constructed model was used to study molecular interactions of novel noncompetitive GluR6 antagonists with their molecular target. A new binding site in the GluR6 receptor transduction domain has been identified. It is situated between two subunits in the receptor dimer. The following residues were recognized as crucial for interactions: Arg663A, Arg663B (M3-S2 linker), Ser809B (S2-M4 linker), and Phe553A (S1-M1 linker).

Sweet taste in man: a review

A greater understanding of the molecular mechanisms of sweet taste has profound significance for the food industry as well as for consumers. Understanding the mechanism by which sweet taste is elicited by saccharides, peptides, and proteins will assist science and industry in their search for sweet substances with fewer negative health effects. The original AH-B theories have been supplanted by detailed structural models. Recent identification of the human sweet receptor as a dimeric G-protein coupled receptor comprising T1R2 and T1R3 subunits has greatly increased the understanding of the mechanisms involved in sweet molecule binding and sweet taste transduction. This review discusses early theories of the sweet receptor, recent research of sweetener chemoreception of nonprotein and protein ligands, homology modeling, the transduction pathway, the possibility of the sweet receptor functioning allosterically, as well as the implications of allelic variation.

Site-specific unglycosylation to improve crystallization of the metabotropic glutamate receptor 3 extracellular domain

Metabotropic glutamate receptors (mGluRs) are involved in the regulation of many physiological and pathological processes in the central nervous system. The extracellular domain (ECD) of mGluR subtype 3 (mGluR3) was produced using the baculovirus expression system and purified from the culture medium. However, the recombinant protein showed heterogeneity in molecular weight on SDS-PAGE analysis. It was found that the unglycosylation of Asn414 significantly reduced the heterogeneity. Consequently, three site-specifically unglycosylated mutant proteins of mGluR3 ECD, replacing Asn414 only or replacing Asn414 in combination with other glycosylation sites, were successfully crystallized in the presence of L-glutamate. Among them, crystals of the N414/439Q mutant diffracted X-rays to 2.35 A resolution using synchrotron radiation. The crystal belonged to the monoclinic space group P2(1), with unit-cell parameters a = 84.0, b = 97.5, c = 108.1 A, beta = 93.0 degrees . Assuming the presence of two protomers per crystallographic asymmetric unit, the Matthews coefficient V(M) was calculated to be 3.5 A(3) Da(-1) and the solvent content was 65%.

Molecular mechanism for the umami taste synergism

Umami is one of the 5 basic taste qualities. The umami taste of L-glutamate can be drastically enhanced by 5' ribonucleotides and the synergy is a hallmark of this taste quality. The umami taste receptor is a heteromeric complex of 2 class C G-protein-coupled receptors, T1R1 and T1R3. Here we elucidate the molecular mechanism of the synergy using chimeric T1R receptors, site-directed mutagenesis, and molecular modeling. We propose a cooperative ligand-binding model involving the Venus flytrap domain of T1R1, where L-glutamate binds close to the hinge region, and 5' ribonucleotides bind to an adjacent site close to the opening of the flytrap to further stabilize the closed conformation. This unique mechanism may apply to other class C G-protein-coupled receptors.

Structural diversity of G protein-coupled receptors and significance for drug discovery

G protein-coupled receptors (GPCRs) are the largest family of membrane-bound receptors and also the targets of many drugs. Understanding of the functional significance of the wide structural diversity of GPCRs has been aided considerably in recent years by the sequencing of the human genome and by structural studies, and has important implications for the future therapeutic potential of targeting this receptor family. This article aims to provide a comprehensive overview of the five main human GPCR families--Rhodopsin, Secretin, Adhesion, Glutamate and Frizzled/Taste2--with a focus on gene repertoire, general ligand preference, common and unique structural features, and the potential for future drug discovery.

Preferential binding of allosteric modulators to active and inactive conformational states of metabotropic glutamate receptors

Metabotropic glutamate receptors (mGluRs) are G protein coupled receptors that play important roles in synaptic plasticity and other neuro-physiological and pathological processes. Allosteric mGluR ligands are particularly promising drug targets because of their modulatory effects--enhancing or suppressing the response of mGluRs to glutamate. The mechanism by which this modulation occurs is not known. Here, we propose the hypothesis that positive and negative modulators will differentially stabilize the active and inactive conformations of the receptors, respectively. To test this hypothesis, we have generated computational models of the transmembrane regions of different mGluR subtypes in two different conformations. The inactive conformation was modeled using the crystal structure of the inactive, dark state of rhodopsin as template and the active conformation was created based on a recent model of the light-activated state of rhodopsin. Ligands for which the nature of their allosteric effects on mGluRs is experimentally known were docked to the modeled mGluR structures using ArgusLab and Autodock softwares. We find that the allosteric ligand binding pockets of mGluRs are overlapping with the retinal binding pocket of rhodopsin, and that ligands have strong preferences for the active and inactive states depending on their modulatory nature. In 8 out of 14 cases (57%), the negative modulators bound the inactive conformations with significant preference using both docking programs, and 6 out of 9 cases (67%), the positive modulators bound the active conformations. Considering results by the individual programs only, even higher correlations were observed: 12/14 (86%) and 8/9 (89%) for ArgusLab and 10/14 (71%) and 7/9 (78%) for AutoDock. These findings strongly support the hypothesis that mGluR allosteric modulation occurs via stabilization of different conformations analogous to those identified in rhodopsin where they are induced by photochemical isomerization of the retinal ligand--despite the extensive differences in sequences between mGluRs and rhodopsin.

Functioning of the dimeric GABA(B) receptor extracellular domain revealed by glycan wedge scanning

The G-protein-coupled receptor (GPCR) activated by the neurotransmitter GABA is made up of two subunits, GABA(B1) and GABA(B2). GABA(B1) binds agonists, whereas GABA(B2) is required for trafficking GABA(B1) to the cell surface, increasing agonist affinity to GABA(B1), and activating associated G proteins. These subunits each comprise two domains, a Venus flytrap domain (VFT) and a heptahelical transmembrane domain (7TM). How agonist binding to the GABA(B1) VFT leads to GABA(B2) 7TM activation remains unknown. Here, we used a glycan wedge scanning approach to investigate how the GABA(B) VFT dimer controls receptor activity. We first identified the dimerization interface using a bioinformatics approach and then showed that introducing an N-glycan at this interface prevents the association of the two subunits and abolishes all activities of GABA(B2), including agonist activation of the G protein. We also identified a second region in the VFT where insertion of an N-glycan does not prevent dimerization, but blocks agonist activation of the receptor. These data provide new insight into the function of this prototypical GPCR and demonstrate that a change in the dimerization interface is required for receptor activation.

How and why do GPCRs dimerize?

Dimerization is fairly common in the G-protein-coupled receptor (GPCR) superfamily. First attempts to rationalize this phenomenon gave rise to an idea that two receptors in a dimer could be necessary to bind a single molecule of G protein or arrestin. Although GPCRs, G proteins and arrestins were crystallized only in their inactive conformations (in which they do not interact), the structures appeared temptingly compatible with this beautiful model. However, it did not survive the rigors of experimental testing: several recent studies unambiguously demonstrated that one receptor molecule is sufficient to activate a G protein and bind arrestin. Thus, to figure out the biological role of receptor self-association we must focus on other functions of GPCRs at different stages of their functional cycle.

Activation of G protein-coupled receptors: beyond two-state models and tertiary conformational changes

Transformation of G protein-coupled receptors (GPCRs) from a quiescent to an active state initiates signal transduction. All GPCRs share a common architecture comprising seven transmembrane-spanning alpha-helices, which accommodates signal propagation from a diverse repertoire of external stimuli across biological membranes to a heterotrimeric G protein. Signal propagation through the transmembrane helices likely involves mechanistic features common to all GPCRs. The structure of the light receptor rhodopsin may serve as a prototype for the transmembrane architecture of GPCRs. Early biochemical, biophysical, and pharmacological studies led to the conceptualization of receptor activation based on the context of two-state equilibrium models and conformational changes in protein structure. More recent studies indicate a need to move beyond these classical paradigms and to consider additional aspects of the molecular character of GPCRs, such as the oligomerization and dynamics of the receptor.

A day in the life of a G protein-coupled receptor: the contribution to function of G protein-coupled receptor dimerization

G protein-coupled receptors are one of the most actively studied families of proteins. However, despite the ubiquity of protein dimerization and oligomerization as a structural and functional motif in biology, until the last decade they were generally considered as monomeric, non-interacting polypeptides. For the metabotropic glutamate-like group of G protein-coupled receptors, it is now firmly established that they exist and function as dimers or, potentially, even within higher-order structures. Despite some evidence continuing to support the view that rhodopsin-like G protein-coupled receptors are predominantly monomers, many recent studies are consistent with the dimerization/oligomerization of such receptors. Key roles suggested for dimerization of G protein-coupled receptors include control of protein maturation and cell surface delivery and providing the correct framework for interactions with both hetero-trimeric G proteins and arrestins to allow signal generation and its termination. As G protein-coupled receptors are the most targeted group of proteins for the development of therapeutic small molecule medicines, recent indications that hetero-dimerization between co-expressed G protein-coupled receptors may be a common process offers the potential for the development of more selective and tissue restricted medicines. However, many of the key experiments have, so far, been limited to model cell systems. Priorities for the future include the generation of tools and reagents able to identify unequivocally potential G protein-coupled receptor hetero-dimers in native tissues and detailed analyses of the influence of hetero-dimerization on receptor function and pharmacology.

Structure and function of the human calcium-sensing receptor: insights from natural and engineered mutations and allosteric modulators

The human extracellular Ca(2+)-sensing receptor (CaR), a member of the G protein-coupled receptor family 3, plays a key role in the regulation of extracellular calcium homeostasis. It is one of just a few G protein-coupled receptors with a large number of naturally occurring mutations identified in patients. In contrast to the small sizes of its agonists, this large dimeric receptor consists of domains with topologically distinctive orthosteric and allosteric sites. Information derived from studies of naturally occurring mutations, engineered mutations, allosteric modulators and crystal structures of the agonist-binding domain of homologous type 1 metabotropic glutamate receptor and G protein-coupled rhodopsin offers new insights into the structure and function of the CaR.

Direct detection of the interaction between recombinant soluble extracellular regions in the heterodimeric metabotropic gamma-aminobutyric acid receptor

The gamma-aminobutyric acid, type B (GABAB) receptor is a heterodimeric receptor consisting of two complementary subunits, GABAB1 receptor (GBR1) and GABAB2 receptor (GBR2). GBR1 is responsible for GABA binding, whereas GBR2 is considered to perform a critical role in signal transduction toward downstream targets. Therefore, precise communication between GBR1 and GBR2 is thought to be essential for the proper signal transduction process. However, biochemical data describing the interaction of the two subunits, especially for the extracellular regions, are not sufficient. Thus we began by developing a protein expression system of the soluble extracellular regions. One of the soluble recombinant GBR1 proteins exhibited a ligand binding ability, which is similar to that of the full-length GBR1, and thus the ligand-binding domain was determined. Direct interaction between GBR1 and GBR2 extracellular soluble fragments was confirmed by co-expression followed by affinity column chromatography and a sucrose density gradient sedimentation. In addition, we also found homo-oligomeric states of these soluble extracellular regions. The interaction between the two soluble extracellular regions caused the enhancement of the agonist affinity for GBR1 as previously reported in a cell-based assay. These results not only open the way to future structural studies but also highlight the role of the interaction between the extracellular regions, which controls agonist affinity to the heterodimeric receptor.

Activation of a dimeric metabotropic glutamate receptor by intersubunit rearrangement

Although many G protein-coupled receptors (GPCRs) can form dimers, a possible role of this phenomenon in their activation remains elusive. A recent and exciting proposal is that a dynamic intersubunit interplay may contribute to GPCR activation. Here, we examined this possibility using dimeric metabotropic glutamate receptors (mGluRs). We first developed a system to perfectly control their subunit composition and show that mGluR dimers do not form larger oligomers. We then examined an mGluR dimer containing one subunit in which the extracellular agonist-binding domain was uncoupled from the G protein-activating transmembrane domain. Despite this uncoupling in one protomer, agonist stimulation resulted in symmetric activation of either transmembrane domain in the dimer with the same efficiency. This, plus other data, can only be explained by an intersubunit rearrangement as the activation mechanism. Although well established for other types of receptors such as tyrosine kinase and guanylate cyclase receptors, this is the first clear demonstration that such a mechanism may also apply to GPCRs.

Expression, purification, crystallization and preliminary X-ray analysis of the ligand-binding domain of metabotropic glutamate receptor 7

Glutamate is the major excitatory neurotransmitter and its metabotropic glutamate receptor (mGluR) plays an important role in the central nervous system. The ligand-binding domain (LBD) of mGluR subtype 7 (mGluR7) was produced using the baculovirus expression system and purified from the culture medium. The purified protein was characterized by gel-filtration chromatography, SDS-PAGE and a ligand-binding assay. Crystals of mGluR7 LBD were grown at 293 K by the hanging-drop vapour-diffusion method. The crystals diffracted X-rays to 3.30 A resolution using synchrotron radiation and belong to the trigonal space group P3(1)21, with unit-cell parameters a = b = 92.4, c = 114.3 A. Assuming the presence of one protomer per crystallographic asymmetric unit, the Matthews coefficient V(M) was calculated to be 2.5 A3 Da(-1) and the solvent content was 51%.