The metabotropic glutamate receptors (mGluRs) are key receptors in the modulation of excitatory synaptic transmission in the central nervous system. Here we have determined three different crystal structures of the extracellular ligand-binding region of mGluR1--in a complex with glutamate and in two unliganded forms. They all showed disulphide-linked homodimers, whose 'active' and 'resting' conformations are modulated through the dimeric interface by a packed alpha-helical structure. The bi-lobed protomer architectures flexibly change their domain arrangements to form an 'open' or 'closed' conformation. The structures imply that glutamate binding stabilizes both the 'active' dimer and the 'closed' protomer in dynamic equilibrium. Movements of the four domains in the dimer are likely to affect the separation of the transmembrane and intracellular regions, and thereby activate the receptor. This scheme in the initial receptor activation could be applied generally to G-protein-coupled neurotransmitter receptors that possess extracellular ligand-binding sites.

Metabotropic glutamate receptors play major roles in the activation of excitatory synapses in the central nerve system. We determined the crystal structure of the entire extracellular region of the group II receptor and that of the ligand-binding region of the group III receptor. A comparison among groups I, II, and III provides the structural basis that could account for the discrimination of groupspecific agonists. Furthermore, the structure of group II includes the cysteine-rich domain, which is tightly linked to the ligand-binding domain by a disulfide bridge, suggesting a potential role in transmitting a ligand-induced conformational change into the downstream transmembrane region. The structure also reveals the lateral interaction between the two cysteine-rich domains, which could stimulate clustering of the dimeric receptors on the cell surface. We propose a general activation mechanism of the dimeric receptor coupled with both ligand-binding and interprotomer rearrangements.crystallization ͉ cysteine-rich region ͉ ligand binding ͉ G protein-coupled receptor

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The obese (ob) gene, the mutation of which results in severe hereditary obesity and diabetes in mice, has recently been isolated through positional cloning. In this study, we isolated a full-length human ob complementary DNA (cDNA) clone and examined the tissue distribution of ob gene expression in humans. The nucleotide sequences of the human ob cDNA coding region were 83% identical to those of the mouse and rat ob cDNA coding regions. Analysis of the deduced amino acid sequences revealed that the human ob protein is a 166-amino acid polypeptide with a putative signal sequence and is 84 and 83% homologous to the mouse and rat ob proteins, respectively. Northern blot analysis using the cloned human ob cDNA fragment as a probe identified a single messenger RNA (mRNA) species 4.5 kb in size found abundantly in the adipose tissues obtained from the subcutaneous, omental, retroperitoneal, perilymphatic, and mesenteric fat pads. However, no significant amount of ob mRNA was present in the brain, heart, lung, liver, stomach, pancreas, spleen, small intestine, kidney, prostate, testis, colon, or skeletal muscle. The ob mRNA level in the adipose tissue varied from region to region even in the same individual. Furthermore, in the human adipose tissue, ob gene expression occurred in mature adipocytes rather than in stromal-vascular cells. This study is the first report of the elucidation of ob gene expression in human tissues, thereby leading to better understanding of the physiological and clinical implications of the ob gene.

Peroxisome proliferator-activated receptor ␥ (PPAR␥) functions in various biological processes, including macrophage and adipocyte differentiation. Several natural lipid metabolites have been shown to activate PPAR␥. Here, we report that some PPAR␥ ligands, including 15-deoxy-⌬ 12,14 -prostaglandin J 2 , covalently bind to a cysteine residue in the PPAR␥ ligand binding pocket through a Michael addition reaction by an ␣,␤-unsaturated ketone. Using rhodamine-maleimide as well as mass spectroscopy, we showed that the binding of these ligands is covalent and irreversible. Consistently, mutation at the cysteine residue abolished abilities of these ligands to activate PPAR␥, but not of BRL49653, a non-covalent synthetic agonist, indicating that covalent binding of the ␣,␤-unsaturated ketone in the natural ligands was required for their transcriptional activities. Screening of lipid metabolites containing the ␣,␤-unsaturated ketone revealed that several other oxidized metabolites of hydroxyeicosatetraenoic acid, hydroxyeicosadecaenoic acid, and prostaglandins can also function as novel covalent ligands for PPAR␥. We propose that PPAR␥ senses oxidation of fatty acids by recognizing such an ␣,␤-unsaturated ketone as a common moiety.

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Each metabotropic glutamate receptor possesses a large extracellular domain that consists of a sequence homologous to the bacterial periplasmic binding proteins and a cysteine-rich region. Previous experiments have proposed that the extracellular domain is responsible for ligand binding. However, it is currently unknown whether the extracellular ligand binding site can bind ligands without other domains of the receptor. We began by obtaining a sufficient amount of receptor protein on a baculovirus expression system. In addition to the transfer vector that encodes the entire coding region, transfer vectors that encode portions of the extracellular domain were designed. Here, we report a soluble metabotropic glutamate receptor that encodes only the extracellular domain and retains a ligand binding characteristic similar to that of the full-length receptor. The soluble receptor secreted into culture medium showed a dimerized form. Furthermore, we have succeeded in purifying it to homogeneity. Dose-response curves of agonists for the purified soluble receptor were examined. The effective concentration for half-maximal inhibition (IC 50 ) of quisqualate for the soluble receptor was 3.8 ؋ 10 ؊8 M, which was comparable to that for the full-length receptor. The rank order of inhibition of the agonists was quisqualate > > ibotenate > L-glutamate Ϸ (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid. These data demonstrate that a ligand binding event in metabotropic glutamate receptors can be dissociated from the membrane domain.Glutamate receptors are divided into two distinct classes: ionotropic glutamate receptors (iGluRs) 1 and metabotropic glutamate receptors (mGluRs) (1, 2). The iGluRs consist of Nmethyl-D-aspartate receptors and non-N-methyl-D-aspartate receptors. Non-N-methyl-D-aspartate receptors are further subdivided into two groups: ␣-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors and kainate receptors. iGluRs are ligand-gated ion channels that transduce glutamate binding into cation influx. mGluRs that have been discovered most recently comprise eight subtypes, which are divided into three groups according to agonist selectivity, coupling to different effector systems, and sequence homology (3-6). Group I includes mGluR1 and mGluR5, which are coupled to inositol phospholipid metabolism. Group II (mGluR2 and mGluR3) and group III (mGluR4, mGluR6, mGluR7, and mGluR8) are negatively coupled to adenylate cyclase activity. Functional analyses of these mGluRs are now avidly being performed. The evidence is accumulating that mGluRs modulate excitatory synaptic transmission (7) through various neural transduction pathways, such as regulation of neurotransmitter release (8), influences on ion channel activity (9), and modulation of synaptic plasticity (10).mGluRs have a remarkably large extracellular domain that has no homology with the other G protein-coupled receptors (GPCRs) except Ca 2ϩ -sensing receptors (11). Previous experiments (12, 13) have proposed that the ligand binding site resides mainly in t...

The obese (ob) gene has recently been isolated through a positional cloning approach, the mutation of which causes a marked hereditary obesity and diabetes mellitus in mice.