Hydrodynamic properties of the purified glutamate-binding protein subunit of the N-methyl-D-aspartate receptor.

The hydrodynamic properties of the previously purified glutamate-binding protein from rat synaptic membranes were determined in order to estimate the molecular size of the protein in its native state. This protein is apparently a subunit of a multisubunit complex that forms the N-methyl-D-aspartate subtype of glutamate receptor and has a molecular size of approximately 70 kDa based on electrophoretic migration under denaturing conditions. On the basis of results obtained from H2O/D2O sucrose density gradient sedimentation and gel filtration chromatography of the purified glutamate-binding protein we calculated the partial specific volume of the protein-detergent complex to be 0.766 cc3/g, the Stokes radius of the complex as 4.9 nm, the Mc of the complex as 203,000 +/- 22,000 and the Mr of the protein as 182,000 +/- 19,000. These results are indicative of stable self-association of the glutamate-binding protein and are in agreement with recent studies indicating that more than one molecule of glutamate may be required to activate the N-methyl-D-aspartate receptor-associated ion channel.     

Molecular organization of glutamate-sensitive chemoexcitable membranes of nerve cells. Function of glutamate-binding proteins of the central nervous system when incorporated into liposomes

The functioning of the glutamate-binding protein of rat brain cortex synaptic membranes was studied by its incorporation into liposomes. The optimal conditions for the receptor protein incorporation were established and the kinetics of 22Na+ and 86Rb+ incorporation into the liposomes in the presence of L-glutamate were analyzed. Modelling of the CNS glutamate receptor functions was found to be dependent on the lipid composition and amount of the incorporated membrane protein. The selective transport of 22Na+ into the liposomes was stimulated in the presence of 10(-4) M glutamate. Addition of monoclonal antibodies against glutamate-binding proteins blocked the incorporation of Na+ into the liposomes. The experimental results are suggestive of the nativity of the liposome-incorporated membrane protein, which is capable of binding glutamate and regulating selective transport of Na+. It was assumed that the glutamate receptor macromolecule represents an integral complex made up of several low molecular weight subunits of glucoprotein nature that form a selective ionic channel.     

Solubilization, partial purification, and reconstitution of glutamate- and N-methyl-D-aspartate-activated cation channels from brain synaptic membranes.

L-Glutamate-activated cation channel proteins from rat brain synaptic membranes were solubilized, partially purified, and reconstituted into liposomes. Optimal conditions for solubilization and reconstitution included treatment of the membranes with nonionic detergents in the presence of neutral phospholipids plus glycerol. The affinity batch chromatography procedure described previously [Chen et al. (1988) J. Biol. Chem. 263, 417-427] was used to obtain a fraction enriched in glutamate-binding proteins. Quench-flow procedures were developed to characterize the rapid kinetics of ion flux induced by receptor agonists. [14C]Methylamine, a cation that permeates through the open channel of both vertebrate and invertebrate glutamate receptors, was used to measure the activity of glutamate receptor-ion channel complexes in reconstituted liposomes. L-Glutamate caused an increase in the rate of [14C]methylamine influx into liposomes reconstituted with either solubilized membrane proteins or partially purified glutamate-binding proteins. The increase in methylamine influx was dependent on the concentration of L-glutamic acid with an estimated Kact for L-glutamate equal to 0.2 microM for synaptic membrane proteins and 0.32 microM for purified proteins. Of the major glutamate receptor agonists, only N-methyl-D-aspartate activated cation fluxes in liposomes reconstituted with glutamate-binding proteins. Glutamate-activated methylamine flux was completely inhibited by the N-methyl-D-aspartate receptor antagonist 2-amino-5-phosphonopentanoic acid. In liposomes reconstituted with glutamate-binding proteins, N-methyl-D-aspartate- or glutamate-induced influx of Na+ led to a transient increase in the influx of the lipid-permeable anion probe S14CN-. Electrophoretic analysis of partially purified proteins reconstituted in liposomes indicated enrichment of several bands, the most prominent being those of molecular size equal to approximately 69, 60, 35, and 25 kDa. Antibodies raised against the purified 71- and 63-kDa glutamate-binding proteins reacted strongly with the approximately 69-kDa band of reconstituted proteins and markedly decreased the initial rate of glutamate-activated cation flux. These results indicate the functional reconstitution of N-methyl-D-aspartate-sensitive glutamate receptors and the role of the approximately 69-kDa protein in the function of these ion channels.     

Principal role of NR3 subunits in NR1/NR3 excitatory glycine receptor function

Calcium-permeable N-methyl-d-aspartate (NMDA) receptors are tetrameric cation channels composed of glycine-binding NR1 and glutamate-binding NR2 subunits, which require binding of both glutamate and glycine for efficient channel gating. In contrast, receptors assembled from NR1 and NR3 subunits function as calcium-impermeable excitatory glycine receptors that respond to agonist application only with low efficacy. Here, we show that antagonists of and substitutions within the glycine-binding site of NR1 potentiate NR1/NR3 receptor function up to 25-fold, but inhibition or mutation of the NR3 glycine binding site reduces or abolishes receptor activation. Thus, glycine bound to the NR1 subunit causes auto-inhibition of NR1/NR3 receptors whereas glycine binding to the NR3 subunits is required for opening of the ion channel. Our results establish differential roles of the high-affinity NR3 and low-affinity NR1 glycine-binding sites in excitatory glycine receptor function.     

Molecular organization of glutamate-sensitive, chemoexcitatory membranes of nerve cells. Physico-chemical characteristics of the glutamate-binding synaptic membrane proteins of the rat cerebral cortex

Some physico-chemical properties of glutamate-binding proteins solubilized from rat cerebral cortex synaptic membranes and purified by affinity chromatography were studied. Purified proteins were shown to be homogenous during SDS polyacrylamide gel electrophoresis (Mr 14000). The Scatchard plots for L-[3H]glutamate binding to the purified membrane proteins revealed the presence of one type of binding sites with Kd 800-1000 nM and Bmax 180-200 pmol/mg of protein. Ultracentrifugation of the glutamate-binding membrane protein in sucrose linear gradient demonstrated that the position of the protein peak depends on protein concentration, i.e. after dilution of the sample the protein peak is shifted from 28 000-30 000 to 12 000-15 000. The values of sedimentation coefficients decrease correspondingly to 2.1S. Presumably, these processes are due to dissociation of receptor macromolecules. The glutamate receptor is a glycoprotein-lipid complex made up of several low molecular weight subunits.     

Interaction of the D-isomer of gamma-methylene glutamate with an active site thiol of gamma-glutamylcysteine synthetase

gamma-Glutamylcysteine synthetase has a thiol group in the vicinity of its glutamate-binding site. During efforts to find a covalently bound inhibitor, interaction of the enzyme with gamma-methylene glutamate was examined because this analog of glutamate, which has an alpha,beta-unsaturated moiety, would be expected to bind at the glutamate site and might react with an active site thiol. gamma-Methylene glutamate, which is not a significant substrate, inhibits the enzyme competitively toward glutamate. Preincubation of the enzyme with gamma-methylene DL-glutamate led to substantial inactivation which was dependent upon the presence of Mg2+ or Mn2+; glutamate protected against inactivation. Inactivation was observed with the D-isomer of gamma-methylene glutamate, but not with the corresponding L-isomer. The inactivated enzyme contains close to 1 mol of gamma-methylene glutamate/mol of enzyme. Studies in which enzyme inactivated by treatment with [14C]gamma-methylene glutamate was hydrolyzed indicate that gamma-methylene glutamate reacts with an active site thiol.     

Immune labeling and purification of a 71-kDa glutamate-binding protein from brain synaptic membranes. Possible relationship of this protein to physiologic glutamate receptors

Immunoblot studies of synaptic membranes isolated from rat brain using antibodies raised against a previously purified glutamate-binding protein (GBP) indicated labeling of an approximately 70-kDa protein band. Since the antibodies used were raised against a 14-kDa GBP, the present studies were undertaken to explore the possibility that the 14-kDa protein may have been a proteolytic fragment of a larger Mr protein in synaptic membranes. Protease activity during protein purification was prevented by introducing five protease inhibitors, and a three-step purification procedure was developed that yielded a high degree of purification of glutamate-binding proteins. The major protein enriched in the most highly purified fractions was a 71-kDa glycoprotein, but a 63-kDa protein was co-purified during most steps of the isolation procedure. The glutamate-binding characteristics of these isolated protein fractions were very similar to those previously described for the 14-kDa GBP, including estimated dissociation constants for L-glutamate binding of 0.25 and 1 microM, inhibition of glutamate binding by azide and cyanide, and a selectivity of the ligand binding site for L-glutamate and L-aspartate. The neuroexcitatory analogs of L-glutamate and L-aspartate, ibotenate, quisqualate, and D-glutamate, inhibited L-[3H]glutamate binding to the isolated proteins, as did the antagonist of L-glutamate-induced neuronal excitation, L-glutamate diethylester. On the basis of the lack of any detectable glutamate-related enzyme activity associated with the isolated proteins and the presence of distinguishing sensitivities to analogs that inhibit glutamate transport carriers in synaptic membranes, it is proposed that the 71-kDa protein may be a component of a physiologic glutamate receptor complex in neuronal membranes.     

Effect of growth conditions on glutamate transport in the wild-type strain and glutamate-utilizing mutants of Escherichia coli.

The effects of growth conditions on the glutamate transport activity of intact cells and membrane vesicles and on the levels of glutamate-binding protein in wild-type Escherichia coli K-12 CS101 and in two glutamate-utilizing mutants, CS7 and CS2TC, were studied. Growth of CS101 on aspartate as the sole source of carbon or nitrogen resulted in a severalfold increase in glutamate transport activity of intact cells and membrane preparations to levels characteristic of the operator-constitutive mutant CS7. The high glutamate transport activity of mutant CS7 was not depressed further by growth on aspartate. Synthesis of glutamate-binding protein was not enhanced by aspartate in either strain. Mutant CS2TC produces a heat-labile repressor of glutamate permease synthesis and is therefore able to grow on glutamate at 42 C but not at 30 C. CS2TC cells grown in a glycerol-minimal medium at the restrictive temperature (30 C) exhibit low glutamate transport activity. Growth on aspartate at 30 C results in derepressed synthesis of glutamate permease. Cells grown on glycerol at 42 C have high glutamate transport activity. No further derepression is obtained upon growth on aspartate. Growth of CS101 and CS7 in "rich broth" greatly reduces the levels of glutamate-binding protein but does not appreciably affect glutamate transport by whole cells or membrane preparations. The identity of the carrier and the role of the binding protein in glutamate transport are discussed in the light of these findings.     

Target size analysis of opioid receptors. No difference between receptor types, but discrimination between two receptor states

Target size analysis of opioid receptor is complicated by the presence of multi-exponential inactivation curves. Irradiation of intact frozen tissue proved essential to eliminate such artifacts, due to indirect irradiation effects. Upon irradiation condition, opioid binding activity was inactivated in a single mono-exponential manner. Identical inactivation curves were obtained for mu, delta and kappa binding activities in brain membranes from rat, guinea-pig and frog and in NG 108-15 cells: the molecular mass obtained was 98 +/- 2 kDa. However, when opioid binding was assayed in the presence of Na+, Mg2+ and GTP, the molecular mass was found to be only 56 +/- 4.4 kDa. We suggest that the opioid recognition site comprises a unit of 56 kDa and that in the absence of Na+, Mg2+ and GTP an additional membrane component of 40-44 kDa is necessary for high-affinity opioid binding.     

Immunochemical characterization of brain synaptic membrane glutamate-binding proteins

Two glutamate-binding proteins (71 and 63 kDa) were previously purified from synaptic plasma membranes (Chen, J.-W., Cunningham, M.D., Galton, V., and Michaelis, E. K. (1988) J. Biol. Chem. 263, 417-426). These proteins may play a role in glutamate neurotransmission in brain. Polyclonal antibodies were raised against the denatured glutamate-binding proteins in rabbits, including sets of antibodies against each of the binding proteins. The antibodies reacted specifically against both 71- and 63-kDa proteins. The antibodies recognized the denatured form of the proteins in Western blots and the native state of the proteins in enzyme-linked immunosorbent assays and in immunoaffinity chromatography and extraction procedures. All antibodies labeled most strongly the 71-kDa protein in Western blots, but extracted both proteins from solubilized synaptic membrane preparations. These findings indicate that the two proteins are closely related immunologically but the reactivity on Western blots differs between these two proteins. Immunoextraction of the 71- and 63-kDa proteins led to a approximately 60% decrease in L-[3H]glutamate-binding activity associated with synaptic membrane proteins. Of the brain subcellular fractions examined, the isolated synaptic plasma membranes had the strongest reaction in enzyme-linked immunosorbent assays toward the antiglutamate-binding protein antisera. Electron microscopy combined with gold particle immunohistochemistry revealed the sites labeled by the antibodies as entities present either on the surface or within the postsynaptic membranes and the associated densities of brain nerve ending particles (synaptosomes). Immunohistochemical procedures of gold labeling with silver enhancement of labeled sites revealed selective neuronal labeling in brain regions enriched in glutamate neurotransmitter pathways such as the hippocampus. Labeling was along dendrites and around cell bodies of pyramidal neurons. Based on the pattern of histochemical labeling, the distribution of immune reactivity in synaptic membranes, and the extractions of a major component of membrane glutamate-recognizing proteins by the antibodies, the glutamate-binding proteins must play a role in glutamate neurotransmission.     

Reactive amino acid residues involved in glutamate-binding of human glutamate dehydrogenase isozymes

In the present study, the cassette mutagenesis at several putative positions (K94, G96, K118, K130, or D172) was performed to examine the residues involved in the glutamate-binding of the human glutamate dehydrogenase isozymes (hGDH1 and hGDH2). None of the mutations tested affected the expression or stability of the proteins. There was dramatic reduction in the catalytic efficiency in mutant proteins at K94, G96, K118, or K130 site, but not at D172 site. The K(M) values for glutamate were 4-10-fold greater for the mutants at K94, G96, or K118 site than for the wild-type hGDH1 and hGDH2, whereas no differences in the K(M) values for NAD(+) were detected between the mutant and wild-type enzymes. For K130Y mutant, the K(M) value for glutamate increased 1.6-fold, whereas the catalytic efficiency (k(cat)/K(M)) showed only 2-3% of the wild-type. Therefore, the decreased catalytic efficiency of the K130 mutant mainly results from the reduced k(cat) value, suggesting a possibility that the K130Y residue may be involved in the catalysis rather than in the glutamate-binding. The D172Y mutant did not show any changes in k(cat) value and K(M) values for glutamate and NAD(+), indicating that D172Y is not directly involved in catalysis and substrates binding of the hGDH isozymes. For sensitivity to ADP activation, only the D172Y mutant showed a reduced sensitivity to ADP activation. The reduction of ADP activation in D172Y mutant was more profoundly observed in hGDH2 than in hGDH1. There were no differences in their sensitivities to GTP inhibition between the wild-type and mutant GDHs at all positions tested. Our results suggest that K94, G96, and K118 residues play an important role, although at different degrees, in the binding of glutamate to hGDH isozymes.     

Structural relationship between the hexameric and tetrameric family of glutamate dehydrogenases.

The family of glutamate dehydrogenases include a group of hexameric oligomers with a subunit M(r) of around 50,000, which are closely related in amino acid sequence and a smaller group of tetrameric oligomers based on a much larger subunit with M(r) 115,000. Sequence comparisons have indicated a low level of similarity between the C-terminal portion of the tetrameric enzymes and a substantial region of the polypeptide chain for the more widespread hexameric glutamate dehydrogenases. In the light of the solution of the three-dimensional structure of the hexameric NAD(+)-linked glutamate dehydrogenase from Clostridium symbiosum, we have undertaken a detailed examination of the alignment of the sequence for the C-terminal domain of the tetrameric Neurospora crassa glutamate dehydrogenase against the sequence and the molecular structure of that from C. symbiosum. This analysis reveals that the residues conserved between these two families are clustered in the three-dimensional structure and points to a remarkably similar layout of the glutamate-binding site and the active-site pocket, though with some differences in the mode of recognition of the nucleotide cofactor.     

Isolation of soluble yeast beta-glucans that inhibit human monocyte phagocytosis mediated by beta-glucan receptors

The trypsin-sensitive receptor that mediates phagocytosis of unopsonized zymosan particles by human monocytes has been designated as a beta-glucan receptor because of its functional inhibition by specific algal and plant beta-glucans. Soluble ligands that are chemically and structurally identical to beta-glucan constituents of zymosan were isolated from a carbohydrate-enriched fraction of yeast extract by sequential chromatography on DE-cellulose, SP-Sephadex, and Con A-Sepharose. Preincubation of adherent human monocytes with 278, 210, and 2.5 micrograms/ml hexose equivalents in pooled chromatographic fractions from DE-cellulose, SP-Sephadex, and Con A-Sepharose, respectively, effected 50% reductions in subsequent phagocytosis of zymosan particles without affecting Fc-mediated ingestion of IgG-coated sheep erythrocytes (ESIgG). The purified yeast extract-derived beta-glucans, which contained 92% glucose and 8% mannose by gas chromatographic analysis and eluted from a Sephacryl S-200 column as a broad peak with a Kav of 0.39 and estimated molecular sizes of from 20,000 to 70,000 m.w., required only 3.5 +/- 0.9 micrograms/ml (mean +/- SD, n = 6), as compared with 31.5 micrograms/ml of the algal beta-glucan laminarin to achieve 50% decreases in zymosan ingestion. Alternatively, soluble yeast beta-glucans with estimated molecular sizes of from 2 X 10(5) to 2 X 10(6) were prepared from yeast glucan particles, which contained 98% glucose and 0% mannose, by sonication and sequential centrifugation at 15,000 and 100,000 X G for 30 and 60 min, respectively. Monocyte ingestion of zymosan was reduced by 50% by pretreatment with 60 ng/ml of the soluble beta-glucans in 15,000 X G supernatants, whereas ingestion of ESIgG was unaffected by as much as 50 micrograms/ml of this material. Partial acid hydrolysis of soluble glucan-derived beta-glucans in 15,000 X G supernatants followed by gel filtration on Bio-Gel P-4 revealed two well-defined peaks within the inclusion volume of the column with phagocytosis-inhibiting activity. Oligoglucosides that eluted at a Kav of 0.46 had an estimated molecular size of 2,000 m.w. and effected a 48% reduction in zymosan ingestion at inputs of 2 to 5 micrograms/ml, and smaller oligoglucosides with a Kav of 0.82 and an estimated molecular size of 1,000 m.w. effected a 50% reduction at inputs of 25 micrograms/ml. Preincubation of monocytes for 2 min with 25 micrograms/ml of the oligoglucosides with estimated molecular size of 1,000 m.w. and with 50 ng/ml of soluble glucan-derived beta-glucans in 100,000 X G supernatants reduced zymosan ingestion by 41% +/- 4 and 44% +/- 3 (mean +/- SD, n = 3), respectively.(ABSTRACT TRUNCATED AT 400 WORDS)     

Disassembly and gross structure of particulate aminoacyl-tRNA synthetases from rat liver. Isolation and the structural relationship of synthetase complexes.

The major high molecular weight complex of aminoacyl-tRNA synthetases is purified about 1000-fold with 30% yield from rat liver. The synthetase complex sediments at 24 S with a molecular weight of 900,000 +/- 75,000 and contains aminoacylation activities for lysine, arginine, isoleucine, leucine, methionine, glutamine, glutamate, and proline. The 24 S synthetase complex dissociates into 21 S, 18 S, 13 S, 12 S, and 10 S complexes with specific enzymatic activities. Dissociation of the 24 S complex into active free synthetases is achieved by hydrophobic interaction chromatography. The disassembly of the synthetase complex is consistent with the structural model of a heterotypic multienzyme complex and suggests that the complex formation is due to the specific intermolecular interactions among the synthetases.     

Characteristics of a solubilized thyrotropin receptor from bovine thyroid plasma membranes.

The thyrotropin receptor from bovine thyroid plasma membranes has been solubilized using lithium diiodosalicylate, and an assay to measure thyrotropin binding to the solubilized receptor has been developed. Both the solubilized thyrotropin receptor and the thyrotropin receptor on thyroid plasma membranes have effectively identical nonlinear Scatchard plots and negatively sloped Hill plots, i.e. both preparations have receptors which appear to exhibit a similar negatively cooperative relationship. Although the pH optimum of thyrotropin binding to the solubilized receptor is the same as that of the thyroid plasma membrane receptor, pH 6.0, the pH dependency curve of the solubilized receptor is slightly different in its outline. Thyrotropin binding to the solubilized receptor is less sensitive to salt inhibition than is binding to the thyroid plasma membrane receptor; however, optimal binding remains at 0 degrees. The relative affinities of thyrotropin and two glycoprotein hormones which can be considered structural analogs, luteinizing hormone and human chorionic gonadotropin, are 100:10:5, respectively, toward plasma membrane receptors, but 100:25:40 toward the solubilized receptors. The solubilized receptor preparation is heterogeneous in size in that it has binding components with molecular weights of 286,000, 160,000, 75,000, and 15,000 to 30,000. Tryptic digestion converts all three higher molecular weight components to the 15,000 to 30,000 molecular weight species, and the 15,000 to 30,000 molecular weight receptor component has all of the binding properties of the solubilized receptor preparation before tryptic digestion including an identical nonlinear Scatchard plot. It has the same size as and coelutes from Sephadex G-100 with a 15,000 to 30,000 molecular weight receptor released by tryptic digestion of bovine thyroid plasma membranes or tryptic digestion of bovine or dog thyroid cells in culture. The tryptic fragment of the solubilized receptor or preparations has been purified almost 250-fold by affinity chromatography on thyrotropin-Sepharose columns. The binding activity is lost when the solubilized thyrotropin receptor preparation is exposed to beads of neuraminidase-Sepharose or conconavalin A-Sepharose.     

Kinetic and pharmacological analysis of L-[35S]cystine transport into rat brain synaptosomes

The synaptosomal transport of L-[35S]cystine occurs by three mechanisms that are distinguishable on the basis of their ionic dependence, kinetics of transport and the specificity of inhibitors. They are (a) low affinity sodium-dependent transport (Km 463 +/- 86 microM, Vmax 185 +/- 20 nmol mg protein-1 min-1), (b) high affinity sodium-independent transport (Km 6.90 +/- 2.1 microM, Vmax 0.485 +/- 0.060 nmol mg protein(-1) min(-1)) and (c) low affinity sodium-independent transport (Km 327 +/- 29 microM, Vmax 4.18 +/- 0.25 nmol mg protein(-1) min(-1)). The sodium-dependent transport of L-cystine was mediated by the X(AG)- family of glutamate transporters, and accounted for almost 90% of the total quantity of L-[35S]cystine accumulated into synaptosomes. L-glutamate (Ki 11.2 +/- 1.3 microM) was a non-competitive inhibitor of this transporter, and at 100 microM L-glutamate, the Vmax for L-[35S]cystine transport was reduced to 10% of control. L-cystine did not inhibit the high-affinity sodium-dependent transport of D-[3H]aspartate into synaptosomes. L-histidine and glutathione were the most potent inhibitors of the low affinity sodium-independent transport of L-[35S]cystine. L-homocysteate, L-cysteine sulphinate and L-homocysteine sulphinate were also effective inhibitors. 1 mM L-glutamate reduced the sodium-independent transport of L-cystine to 63% of control. These results suggest that the vast majority of the L-cystine transported into synaptosomes occurs by the high-affinity glutamate transporters, but that L-cystine may bind to a site that is distinct from that to which L-glutamate binds. The uptake of L-cystine by this mechanism is sensitive to inhibition by increased extracellular concentrations of L-glutamate. The importance of these results for understanding the mechanism of glutamate-mediated neurotoxicity is discussed.     

A chloride- and calcium-dependent glutamate-binding protein from rat brain. Identification as a ubiquitous constituent of the inner mitochondrial membrane

We have recently solubilized and enriched a chloride- and calcium-dependent glutamate-binding protein from rat brain (Brose, N., Halpain, S., Suchanek, C., and Jahn, R. (1989) J. Biol. Chem. 264, 9619-9625). The partially purified protein fraction, containing two major protein components of 51,000 Da and 105,000 Da, was used to generate a rabbit antiserum. This serum quantitatively precipitated the binding activity from membrane extracts. Small amounts of the antiserum inhibited glutamate binding when chloride was absent from the incubation medium. Three protein bands were labeled by the serum on immunoblots. From the affinity purified antibody fractions contained in the serum, only the antibodies directed against a 51,000-Da protein were able to immunoprecipitate the binding activity, indicating that this protein is an essential component of the binding site. A survey of a variety of rat tissues by immunoblot analysis revealed a ubiquitous distribution of the protein. After subcellular fractionation of liver and brain, the 51,000-Da protein copurified with mitochondrial markers. Furthermore, exclusive labeling of mitochondria was observed by light and electron microscopy immunocytochemistry. Subfractionation of purified liver mitochondria resulted in a selective association of the protein with inner mitochondrial membranes. Pharmacological characterization of glutamate binding to liver mitochondrial membranes revealed a pattern almost identical to that of the chloride- and calcium-dependent glutamate-binding site in rat brain.     

Characterization of calcium binding to brush-border membranes from rat duodenum.

The Ca2+-binding properties of isolated brush-border membranes at physiological ionic strength and pH were examined by rapid Millipore filtration. A comprehensive analysis of the binding data suggested the presence of two types of Ca2+-binding sites. The high-affinity sites, Ka = (6.3 +/- 3.3) X 10(5) M-1 (mean +/- S.E.M.), bound 0.8 +/- 0.1 nmol of Ca2+/mg of protein and the low-affinity sites, Ka = (2.8 +/- 0.3) X 10(2) M-1, bound 33 +/- 3.5 nmol of Ca2+/mg of protein. The high-affinity site exhibited a selectivity for Ca2+, since high concentrations of competing bivalent cations were required to inhibit Ca2+ binding. The relative effectiveness of the competing cations (1 and 10 mM) for the high-affinity site was Mn2+ approximately equal to Sr2+ greater than Ba2+ greater than Mg2+. Data from the pH studies, treatment of the membranes with carbodi-imide and extraction of phospholipids with aqueous acetone and NH3 provided evidence that the low-affinity sites were primarily phospholipids and the high-affinity sites were either phosphoprotein or protein with associated phospholipid. Two possible roles for the high-affinity binding sites are suggested. Either high-affinity Ca2+ binding is involved with specific enzyme activities or Ca2+ transport across the luminal membrane occurs via a Ca2+ channel which contains a high-affinity Ca2+-specific binding site that may regulate the intracellular Ca2+ concentration and gating of the channel.     

Interactions of alkali cations with glutamate transporters

The transport of glutamate is coupled to the co-transport of three Na+ ions and the countertransport of one K+ ion. In addition to this carrier-type exchange behaviour, glutamate transporters also behave as chloride channels. The chloride channel activity is strongly influenced by the cations that are involved in coupled flux, making glutamate transporters representative of the ambiguous interface between carriers and channels. In this paper, we review the interaction of alkali cations with glutamate transporters in terms of these diverse functions. We also present a model derived from electrostatic mapping of the predicted cation-binding sites in the X-ray crystal structure of the Pyrococcus horikoshii transporter GltPh and in its human glutamate transporter homologue EAAT3. Two predicted Na+-binding sites were found to overlap precisely with the Tl+ densities observed in the aspartate-bound complex. A novel third site predicted to favourably bind Na+ (but not Tl+) is formed by interaction with the substrate and the occluding HP2 loop. A fourth predicted site in the apo state exhibits selectivity for K+ over both Na+ and Tl+. Notably, this K+ site partially overlaps the glutamate-binding site, and their binding is mutually exclusive. These results are consistent with kinetic and structural data and suggest a plausible mechanism for the flux coupling of glutamate with Na+ and K+ ions.