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Ionotropic glutamate receptors mediate most rapid excitatory synaptic transmission in the mammalian central nervous system, and their involvement in neurological diseases has stimulated widespread interest in their structure and function. Despite a large number of agonists developed so far, few display selectivity among (S)-2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl) propionic acid (AMPA)-receptor subtypes. The present study provides X-ray structures of the glutamate receptor 2 (GluR2)-selective partial agonist (S)-2-amino-3-(1,3,5,6,7-pentahydro-2,4-dioxocyclopenta[e] pyrimidin-1-yl) propanoic acid [(S)-CPW399] in complex with the ligand-binding core of GluR2 (GluR2-S1S2J) and with a (Y702F)GluR2-S1S2J mutant. In addition, the structure of the nonselective partial agonist kainate in complex with (Y702F)GluR2-S1S2J was determined. The results show that the selectivity of (S)-CPW399 toward full-length GluR2 relative to GluR3 is reflected in the binding data on the two soluble constructs, allowing the use of (Y702F)GluR2-S1S2J as a model system for studying GluR2/GluR3 selectivity. Structural comparisons suggest that selectivity arises from disruption of a watermediated network between ligand and receptor. A D1-D2 domain closure occurs upon agonist binding. (S)-CPW399 and kainate induce greater domain closure in the Y702F mutant, indicating that these partial agonists here act in a manner more reminiscent of full agonists. Both kainate and (S)-CPW399 exhibited higher efficacy at (Y702F)GluR2(Q) i than at wild-type GluR2(Q) i . Whereas an excellent correlation exists between domain closure and efficacy of a range of agonists at full-length GluR2 determined by electrophysiology in Xenopus laevis oocytes, a direct correlation between agonist induced domain closure of (Y702F)GluR2-S1S2J and efficacy at the GluR3 receptor is not observed. Although it clearly controls selectivity, mutation of this residue alone is insufficient to explain agonist-induced conformational rearrangements occurring in this variant.Binding of (S)-glutamate to ionotropic glutamate receptors (iGluRs) is a key step in the predominant mechanism of rapid excitatory synaptic transmission among nerve cells within the mammalian central nervous system. iGluRs are important in the development and function of the central nervous system and are implicated in learning and memory formation. Furthermore, iGluRs also seem to be associated with certain neurological and psychiatric diseases (e.g., Alzheimer-type diseases, Parkinson's disease, epilepsy, stroke, amyotrophic lateral sclerosis, and schizophrenia). Therefore, the iGluRs are considered potential drug targets (Dingledine et al., 1999;Brä uner-Osborne et al., 2000). The iGluRs have been divided into three different classes: the (S)-2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl) propionic acid (AMPA), The structures reported in this article have been deposited within the Protein Databank with accession numbers 1SYH, 1SYI, and 1XHY.Article, publication date, and citation information can be found at ...

The X-ray structure of a partly self-complementary peptide nucleic acid (PNA) decamer (H-GTAGATCACT-l-Lys-NH(2)) to 2.60 A resolution is reported. The structure is mainly controlled by the canonical Watson-Crick base pairs formed by the self-complementary stretch of four bases in the middle of the decamer (G(4)A(5)T(6)C(7)). One right- and one left-handed Watson-Crick duplex are formed. The two PNA units C(9)T(10) change helical handedness, so that each PNA strand contains both a right- and a left-handed section. The changed handedness in C(9)T(10) allows formation of Hoogsteen hydrogen bonding between C(9)T(10) and G(4)A(5) of a PNA strand in an adjacent Watson-Crick double helix of the same handedness. Thereby, a PNA-PNA-PNA triplex is formed. The PNA unit A(3) forms a noncanonical base pair with A(8) in a symmetry-related strand of opposite handedness; the base pair is of the A-A reverse Hoogsteen type. The structural diversity of this PNA demonstrates how the PNA backbone is able to adapt to structures governed by the stacking and hydrogen-bonding interactions between the nucleobases. The crystal structure further shows how PNA oligomers containing limited sequence complementarity may form complex hydrogen-bonding networks.

Tetranectin is a plasminogen kringle 4-binding protein. The crystal structure has been determined at 2.8 A resolution using molecular replacement. Human tetranectin is a homotrimer forming a triple oc-helical coiled coil. Each monomer consists of a carbohydrate recognition domain (CRD) connected to a long a-helix. Tetranectin has been classified in a distinct group of the C-type lectin superfamily but has structural similarity to the proteins in the group of collectins. Tetranectin has three intramolecular disulfide bridges. Two of these are conserved in the C-type lectin superfamily, whereas the third is present only in long-form CRDs. Tetranectin represents the first structure of a long-form CRD with intact calcium-binding sites. In tetranectin, the third disulfide bridge tethers the CRD to the long helix in the coiled coil. The trimerization of tetranectin as well as the fixation of the CRDs relative to the helices in the coiled coil indicate a demand for high specificity in the recognition and binding of ligands.© 1997 Federation of European Biochemical Societies.Key words: C-type lectin; X-ray crystal structure; Carbohydrate recognition domain; Plasminogen; Kringle 4; a-Helical coiled coilHuman TN is a homotrimeric protein [17], each polypeptide chain consisting of 181 amino acid residues encoded by three exons [18]. TN contains a carbohydrate recognition domain (CRD) and, according to sequence homology studies, belongs to a distinct group of the C-type (calcium dependent) lectin superfamily, which also includes pancreatic stone protein (lithostathine), sea raven antifreeze protein, and snake venom botrocetin, all isolated CRDs without additional domains in contrast to TN [19,20]. The X-ray structure of human lithostathine, which is a C-type lectin homolog without calcium-binding sites, has recently been published [21]. In addition, structure determinations of proteins of two other groups of C-type lectins have been reported: the group of collectins (three mannose-binding proteins, rat MBP from serum (MBP-A), rat MBP from liver (MBP-C), human MBP) [22][23][24][25][26], and of selectins (human E-selectin) [27].We present here the X-ray structure of recombinant human TN. Knowledge on the structure of TN is essential for unraveling the molecular mechanisms of action determining the biological functions of this protein and for the development of drugs interfering with its function.

The X‐ray structure of the ionotropic GluR2 ligand‐binding core (GluR2‐S1S2J) in complex with the bicyclical AMPA analogue (S)‐2‐amino‐3‐(3‐hydroxy‐7,8‐dihydro‐6H‐cyclohepta[d]‐4‐isoxazolyl)propionic acid [(S)‐4‐AHCP] has been determined, as well as the binding pharmacology of this construct and of the full‐length GluR2 receptor. (S)‐4‐AHCP binds with a glutamate‐like binding mode and the ligand adopts two different conformations. The Ki of (S)‐4‐AHCP at GluR2‐S1S2J was determined to be 185 ± 29 nm and at full‐length GluR2(R)o it was 175 ± 8 nm. (S)‐4‐AHCP appears to elicit partial agonism at GluR2 by inducing an intermediate degree of domain closure (17°). Also, functionally (S)‐4‐AHCP has an efficacy of 0.38 at GluR2(Q)i, relative to (S)‐glutamate. The proximity of bound (S)‐4‐AHCP to domain D2 prevents full D1–D2 domain closure, which is limited by steric repulsion, especially between Leu704 and the ligand.

Tetranectin (TN) is a C-type lectin involved in fibrinolysis, being the only endogenous ligand known to bind specifically to the kringle 4 domain of plasminogen. TN was originally isolated from plasma, but shows a wide tissue distribution. Furthermore, TN has been found in the extracellular matrix of certain human carcinomas, whereas none or little is present in the corresponding normal tissue. The crystal structure of full-length trimeric TN (2.8 A resolution) has recently been published [Nielsen et al. (1997). FEBS Lett. 412, 388-396]. The crystal structure of the carbohydrate recognition domain (CRD) of human TN (TN3) has been determined separately at 2.0 A resolution in order to obtain detailed information on the two calcium binding sites. This information is essential for the elucidation of the specificity of TN towards oligosaccharides. TN3 crystallizes as a dimer, whereas it appears as a monomer in solution. The overall fold of TN3 is similar to other known CRDs. Each monomer is built of two distinct regions, one region consisting of six beta-strands and two alpha-helices, and the other region is composed of four loops harboring two calcium ions. The calcium ion at site 1 forms an eightfold coordinated complex and has Asp116, Glu120, Gly147, Glu150, Asn151, and one water molecule as ligands. The calcium ion at site 2, which is believed to be involved in recognition and binding of oligosaccharides, is sevenfold coordinated with ligands Gln143, Asp145, Glu150, Asp165, and two water molecules. One sulfate ion has been located at the surface of TN3, forming contacts to Glu120, Lys148, Asn106 of a symmetry-related molecule, and to an ethanol molecule.

The R2 protein component of mouse ribonucleotide reductase has been obtained from overproducing Escherichia coli bacteria. It has been crystallized using NaCI as precipitant. The crystals are orthorhombic, space grOuP oC222~ with cell dimensions a = 76.9 A, b = 108.9 A, c = 92.7 A and diffract to at least o 2.5 A. The asymmetric unit of the crystals contains one monomer. Rotation and translation function searches using a model based on the weakly homologous E. coil R2 gave one significant peak. Rotation about a crystallographic 2-fold axis parallel to the aaxis produces an R2 dimer with dimer interactions very similar to those found for E. coil R2.