How well can molecular modelling predict the crystal structure: the case of the ligand-binding domain of glutamate receptors.

The concept that the ligand-binding domain of vertebrate glutamate receptor channels and bacterial periplasmic substrate-binding proteins (PBPs) share similar three-dimensional (3D) structures has gained increasing support in recent years. On the basis of a dual approach that included computer-assisted molecular modelling and functional studies of site-specific mutants, theoretical 3D models of this domain have been proposed. This article reviews to what extent these models could predict the crystal structure of the ligand-binding domain of an ionotropic glutamate receptor subunit recently determined at high resolution by X-ray diffraction studies.

Oral administration of a dual analog of two myasthenogenic T cell epitopes down-regulates experimental autoimmune myasthenia gravis in mice.

Myasthenia gravis (MG) and experimental autoimmune MG (EAMG) are T cell-regulated, antibody-mediated autoimmune diseases. The major autoantigen in MG is the nicotinic acetylcholine receptor (AChR). Two peptides, representing sequences of the human AChR alpha-subunit, p195-212 and p259-271, were previously shown to be immunodominant T cell epitopes in MG patients as well as, respectively, in SJL and BALB/c mice. A dual analog (termed Lys-262-Ala-207) composed of the tandemly arranged two single amino acid analogs of p195-212 and p259-271 was shown to inhibit, in vitro and in vivo, MG-associated autoimmune responses. Furthermore, the dual analog could down-regulate myasthenogenic manifestations in mice with EAMG that was induced by inoculation of a pathogenic T cell line. In the present study, the ability of the dual analog to treat EAMG induced in susceptible C57BL/6 mice by native Torpedo AChR was evaluated. Mice that were diagnosed to have clinical symptoms of EAMG were treated with the dual analog by oral administration, 500 microg per mouse three times a week for 5-8 weeks. Treatment with the dual analog down-regulated the clinical manifestations of the ongoing disease as assessed by the clinical score, grip strength (measured by a grip strength meter), and electromyography. The effects on the clinical EAMG correlated with a reduced production of anti-AChR antibody as well as a decrease in the secretion of interleukin-2 and, more dramatically, interferon-gamma, in response to AChR triggering. Thus, the dual analog is an efficient immunomodulator of EAMG in mice and might be of specific therapeutic potential for MG.

Phosphorylation of the complement component, C9, by an ecto-protein kinase of human leukemic cells.

Ecto-protein kinases (ecto-PK) are surface constituents of many, if not all, animal cell types; normal, transformed or malignant. The occurrence of ecto-PK on the surface of human leukemia cell lines was described [Paas, Y., Fishelson, Z., 1995. Shedding of tyrosine and serine/threonine ecto-PK from human leukemic cells. Arch. Biochem. Biophys. 316 780-788.]. These ecto-PKs have been shown to phosphorylate several exogenous substrates, including the complement C9 protein, an essential component of the terminal complement system. C9 is phosphorylated by ecto-PK of K562 cells on serine residue(s). Phosphorylation occurs in the N-terminal C9a portion produced by cleavage of phosphorylated C9 with human alpha-thrombin. C9 polymers generated upon incubation of C9 with ZnCl2 do not serve as substrates for the K562 ecto-PK. In contrast, unfolded C9, obtained by reduction and alkylation, serves as a superior substrate for the K562 ecto-PK. Native C9 phosphorylation produced a rather low stoichiometry of incorporated phosphate (around 3%) per C9. Despite that, the phosphorylated C9 expressed reduced hemolytic activity. The complement-sensitive variant of K562 (K562/S) did not express the C9 phosphorylating activity. Various PK inhibitors tested failed to block C9 phosphorylation. Only heparin and 2,3-diphosphoglycerate (dpGA) prevented C9 phosphorylation, indicating that the ecto-PK is related to the casein kinase CK2. C9 can be phosphorylated by ecto-PK from other tumor cells, including Jurkat, SK-OV-3 and BT-474. These results suggest that extracellular phosphorylation of C9 may serve as a protective mechanism against complement in tumor cells.

The macro- and microarchitectures of the ligand-binding domain of glutamate receptors.

Over the last decade, a large body of information regarding the amino acid sequences and tertiary structures of many proteins has accumulated. Subtle similarities in sequence patterns identified between glutamate receptors and bacterial periplasmic substrate-binding proteins have suggested that structural kinship exists between these protein families. Many of the bacterial periplasmic binding proteins but none of the glutamate receptors have been crystallized so far. The following article reviews how the resemblance between these two protein families led to computer-assisted structural models of crucial elements involved in ligand binding by various glutamate receptors. A plausible dynamic model of the molecular mechanism of activation and desensitization of glutamate-receptor channels is also discussed.

Expression of GTP-dependent and GTP-independent tissue-type transglutaminase in cytokine-treated rat brain astrocytes.

Tissue-type transglutaminases (TGases) were recently shown to exert dual enzymatic activities; they catalyze the posttranslational modification of proteins by transamidation, and they also act as guanosine triphosphatase (GTPase). Here we show that a tissue-type TGase is expressed in rat brain astrocytes in vitro, and is induced by the inflammation-associated cytokines interleukin-1beta and to a lesser extent by tumor necrosis factor-alpha. Induction is accompanied by overexpression and appearance of an additional shorter clone, which does not contain the long 3'-untranslated region and encodes for a novel TGase enzyme whose C terminus lacks a site that affects the enzyme's interaction with guanosine triphosphate (GTP). Expression of two clones revealed that the long form is inhibited noncompetitively by GTP, but the short form significantly less so. The different affinities for GTP may account for the difference in physiological function between these two enzymes.

Identification of the amino acid subsets accounting for the ligand binding specificity of a glutamate receptor.

In a situation so far unique among neurotransmitter receptors, glutamate receptors share amino acid sequence similarities with the bacterial periplasmic binding proteins (PBPs). On the basis of the primary structure similarity of two bacterial periplasmic proteins (lysine/arginine/ornithine- and phosphate-binding proteins) with the chick cerebellar kainate-binding protein (KBP), a member of the ionotropic glutamate receptor family, we have generated a three-dimensional model structure of the KBP extracellular domain. By an interplay between homology modeling and site-directed mutagenesis, we have investigated the kainate binding properties of 55 different mutants (corresponding to 43 positions) and studied the interactions of some of these mutants with various glutamatergic ligands. As a result, we present here the subsets of amino acids accounting for the binding free energies and specificities of KBP for kainate, glutamate, and CNQX and propose a three-dimensional model, at the microarchitectural level, of the glutamatergic binding domain.

Identification of an extracellular motif involved in the binding of guanine nucleotides by a glutamate receptor.

The chick cerebellar kainate (KA) binding protein (KBP), a member of the family of ionotropic glutamate receptors, harbours a glycine-rich (GxGxxG) motif known to be involved in the binding of ATP and GTP to kinases and G proteins respectively. Here, we report that guanine, but not adenine, nucleotides interact with KBP by inhibiting [3H]KA binding in a competitive-like manner, displaying IC50 values in the micromolar range. To locate the GTP binding site, KBP was photoaffinity labelled with [alpha-32P]GTP. The reaction was blocked by KA, glutamate, 6-cyano-7-nitroquinoxaline-2,3-dione and antibodies raised against a peptide containing the glycine-rich motif. Site-directed mutagenesis of residues K72 and Y73 within the glycine-rich motif followed by the expression of the KBP mutants at the surface of HEK 293 cells showed a decrease in GTP binding affinity by factors of 10 and 100 respectively. The binding of [3H]KA to the K72A/T KBP mutants was not affected but binding to the Y73I KBP mutant was decreased by a factor of 10. Accordingly, we propose that the glycine-rich motif of KBP forms part of a guanine nucleotide binding site. We further suggest that the glycine-rich motif is the binding site at which guanine nucleotides inhibit the glutamate-mediated responses of various members of the subfamily of glutamate ionotropic receptors.

Shedding of tyrosine and serine/threonine ecto-protein kinases from human leukemic cells.

Ecto-protein kinases (ecto-PK), primarily of the serine/threonine kinase type, have been previously described on the surface of various normal, transformed, and tumor cells. We have found that in the presence of ATP and Mg2+, exogenously added substrates such as phosvitin and poly(Glu4-Tyr) are phosphorylated by intact K562 erythroleukemia, HL60 promyelocytic leukemia, and U937 histiocytic leukemia human cells. Phosphoamino acid analysis indicated that phosvitin, histone H2B, casein, and protamine are phosphorylated on serine and threonine residues, whereas poly(Glu4-Tyr) is phosphorylated on tyrosine. We also present evidence showing that the C9 complement protein, a key component of the membranolytic protein complex of the complement system, is exclusively phosphorylated by the K562 cells on serine residues. Phosphorylation of poly(Glu4-Tyr) is markedly enhanced by Mn2+, whereas C9 phosphorylation is rather inhibited by Mn2+. It is concluded that human leukemic cells express on their surface two types of ecto-PK, one phosphorylating serines and threonines and one specific to tyrosines. The ecto-PKs are spontaneously shed from fully viable cells into the medium in a temperature-dependent manner. Upon sedimentation of cell supernatants at 100,000g, the ecto-PKs are found sedimented with small membrane vesicles. Treatment of intact K562 cells or of released membrane vesicles with bacterial phospholipase C, but not with trypsin or pronase, releases the two types of ecto-PK from the cell or vesicle membrane, respectively. This is accompanied by a marked increase in the released phosphorylating activity. It is, therefore, suggested that these ecto-PKs are either covalently linked to phospholipids or strongly attached to lipid-anchored molecules in the cell surface membrane. Several endogenous proteins in the released membranes are phosphorylated by the ecto-PKs on serines and to a lesser extend on threonines. Two proteins (PTP79 and PTP54) are phosphorylated in a manganese-dependent manner on tyrosines.

A ligand binding study of the interactions of guanine nucleotides with non-NMDA receptors.

The interactions of guanine nucleotides, and particularly GTP, with the [3H]-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) and [3H]-kainate (KA) binding sites present on brain membranes was studied, using the ligand binding methodology and Scatchard analysis, in order to establish the competitive/non competitive nature of the interaction and determine whether guanine nucleotides, KA and AMPA share common binding sites. GTP was found to block [3H]-AMPA and [3H]-KA binding to rat cortical membranes with IC50 values of 0.4 mM and 1 mM respectively and the [3H] KA-binding to chick cerebellar membranes with a IC50 value of 20 microM. Scatchard analysis of [3H]-KA binding performed in the absence or presence of 1 mM GTP or 0.25 mM AMPA reveals that the high affinity [3H]-KA binding component is not affected by GTP but blocked in a non competitive fashion by AMPA while the low affinity [3H]-KA binding component is not affected by AMPA but blocked by GTP. Scatchard analysis of [3H]-KA binding to chick cerebellar membranes performed in the absence or presence of 33 microM GTP reveals a single binding site blocked in a competitive fashion by GTP. Scatchard analysis of [3H]-AMPA binding performed in the absence or presence of 0.5 mM GTP or 30 microM KA reveals that the high affinity [3H]-AMPA binding component is affected in a non competitive fashion by both GTP and KA while the low affinity [3H] AMPA binding component is affected in a competitive fashion by both GTP and KA.(ABSTRACT TRUNCATED AT 250 WORDS)