Two serine residues of the glutamate transporter GLT-1 are crucial for coupling the fluxes of sodium and the neurotransmitter

The dual-function glutamate transporters: structure and molecular characterisation of the substrate-binding sites

Glutamate transporters are essential for terminating synaptic excitation and for maintaining extracellular glutamate concentrations below neurotoxic levels. These transporters also mediate a thermodynamically uncoupled chloride flux, activated by two of the molecules they transport, sodium and glutamate. Five eukaryotic glutamate transporters have been cloned and identified. They exhibit approximately 50% identity and this homology is even greater at the carboxyl terminal half, which is predicted to have an unusual topology. Determination of the topology shows that the carboxyl terminal part contains several transmembrane domains separated by two reentrant loops that are in close proximity to each other. We have identified several conserved amino acid residues in the carboxyl terminal half that play crucial roles in the interaction of the transporter with its substrates: sodium, potassium and glutamate. The conformation of the transporter gating the anion conductance is different from that during substrate translocation. However, there exists a dynamic equilibrium between these conformations.

A hydrophobic domain in glutamate transporters forms an extracellular helix associated with the permeation pathway for substrates

Recent work has shown that cysteine residues introduced into domain 10, a highly hydrophobic segment in the excitatory amino acid transporter 1, react readily when hydrophilic sulfhydryl-modifying reagents are applied extracellularly. To investigate the functional contributions of this region, we mutated each residue in domain 10 (Ala(446)-Gly(459)) to cysteine and assessed the transport kinetics and inhibitor sensitivities of the mutant carriers. Modification of the introduced sulfhydryl group with membrane-impermeant methanethiosulfonate derivatives inhibited substrate transport by all but one functional cysteine mutant. Substrates and/or non-transported inhibitors block thiol modification of most mutants within this region, implying that access to the domain becomes restricted as a consequence of the binding of substrates and substrate analogs. An examination of the temperature dependence of substrate protection for one mutant (I453C) indicates that substrates prevent modification at a step prior to the large conformational changes associated with translocation. When superimposed on a helical model, mutants with similar attributes are positioned in close proximity. Our data are consistent with a model in which domain 10 exists as an alpha-helix at an aqueous interface of the translocation pathway, which can be directly occluded by substrates and inhibitors at an early step in the transport cycle.

Cysteine-scanning mutagenesis reveals a conformationally sensitive reentrant pore-loop in the glutamate transporter GLT-1

Removal of glutamate from the synaptic cleft by (Na(+) + K(+))-coupled transporters prevents neurotoxicity due to elevated concentrations of the transmitter. These transporters exhibit an unusual topology, including two reentrant loops. Reentrant loop II plays a pivotal role in coupling ion and glutamate fluxes. Here we used cysteine-scanning mutagenesis of the GLT-1 transporter to test the idea that this loop undergoes conformational changes following sodium and substrate binding. 15 of 22 consecutive single cysteine mutants in the stretch between Gly-422 and Ser-443 exhibited 30-100% of the transport activity of the cysteine-less transporter when expressed in HeLa cells. The transport activity of 11 of the 15 active mutants including five consecutive residues in the ascending limb was inhibited by small hydrophilic methanethiosulfonate reagents. The sensitivity of seven cysteine mutants, including A438C and S440C, to the reagents was significantly reduced by sodium ions, but the opposite was true for A439C. The non-transportable analogue dihydrokainate protected at almost all positions throughout the loop, and at two of the positions, the analogue protected even in the absence of sodium. Our results indicate that reentrant loop II forms part of an aqueous pore, the access of which is blocked by the glutamate analogue dihydrokainate, and that sodium influences the conformation of this pore-loop.

Crystal structure of a dual activity IMPase/FBPase (AF2372) from Archaeoglobus fulgidus. The story of a mobile loop

Several hyperthermophilic organisms contain an unusual phosphatase that has dual activity toward inositol monophosphates and fructose 1,6-bisphosphate. The structure of the second member of this family, an FBPase/IMPase from Archaeoglobus fulgidus (AF2372), has been solved. This enzyme shares many kinetic and structural similarities with that of a previously solved enzyme from Methanococcus jannaschii (MJ0109). It also shows some kinetic differences in divalent metal ion binding as well as structural variations at the dimer interface that correlate with decreased thermal stability. The availability of different crystal forms allowed us to investigate the effect of the presence of ligands on the conformation of a mobile catalytic loop independently of the crystal packing. This conformational variability in AF2372 is compared with that observed in other members of this structural family that are sensitive or insensitive to submillimolar concentrations of Li(+). This analysis provides support for the previously proposed mechanism of catalysis involving three metal ions. A direct correlation of the loop conformation with strength of Li(+) inhibition provides a useful system of classification for this extended family of enzymes.

Dynamic equilibrium between coupled and uncoupled modes of a neuronal glutamate transporter

In the brain, the neurotransmitter glutamate is removed from the synaptic cleft by (Na(+) + K(+))-coupled transporters by an electrogenic process. Moreover, these transporters mediate a sodium- and glutamate-dependent uncoupled chloride conductance. In contrast to the wild type, the uptake of radiolabeled substrate by the I421C mutant is inhibited by the membrane-impermeant [2-(trimethylammonium)ethyl]methanethiosulfonate and also by other sulfhydryl reagents. In the wild-type and the unmodified mutant, substrate-induced currents are inwardly rectifying and reflect the sum of the coupled electrogenic flux and the anion conductance. Remarkably, the I421C mutant modified by sulfhydryl reagents exhibits currents that are non-rectifying and reverse at the equilibrium potential for chloride. Strikingly, almost 10-fold higher concentrations of d-aspartate are required to activate the currents in the modified mutant as compared with untreated I421C. Under conditions in which only the coupled currents are observed, the modified mutant does not exhibit any currents. However, when the uncoupled current is dominant, sulfhydryl reagents cause >4-fold stimulation of this current. Thus, the modification of the cysteine introduced at position 421 impacts the coupled but not the uncoupled fluxes. Although both fluxes are activated by substrate, they behave as independent processes that are in dynamic equilibrium.

Proximity of two oppositely oriented reentrant loops in the glutamate transporter GLT-1 identified by paired cysteine mutagenesis

Sodium- and potassium-coupled transporters clear the excitatory neurotransmitter glutamate from the synaptic cleft. Their function is essential for effective glutamatergic neurotransmission. Glutamate transporters have an unusual topology, containing eight membrane-spanning domains and two reentrant loops of opposite orientation. We have introduced pairwise cysteine substitutions in several structural elements of the GLT-1 transporter. A complete inhibition of transport by Cu(II)(1,10-phenanthroline)(3) is observed in the double mutants A412C/V427C and A364C/S440C, but not in the corresponding single mutants. No inhibition is observed in more then 20 other double cysteine mutants. The Cu(II)(1,10-phenanthroline)(3) inhibition can be partly prevented by the nontransportable glutamate analogue dihydrokainate. Treatment with dithiothreitol restores much of the transport activity. Moreover, micromolar concentrations of cadmium ions reversibly inhibit transport catalyzed by A412C/V427C and A364C/S440C double mutants, but not by the corresponding single mutants. Inhibition by Cu(II)(1,10-phenanthroline)(3) and by cadmium is only observed when the cysteine pairs are introduced in the same polypeptide. Therefore, in both cases the proximity appears to be intra- rather than intermolecular. Positions 364 and 440 are located on reentrant loop I and II, respectively. Our results suggest that these two loops, previously shown to be essential for glutamate transport, come in close proximity.

Sulfhydryl modification of V449C in the glutamate transporter EAAT1 abolishes substrate transport but not the substrate-gated anion conductance

Excitatory amino acid transporters (EAATs) buffer and remove synaptically released L-glutamate and maintain its concentrations below neurotoxic levels. EAATs also mediate a thermodynamically uncoupled substrate-gated anion conductance that may modulate cell excitability. Here, we demonstrate that modification of a cysteine substituted within a C-terminal domain of EAAT1 abolishes transport in both the forward and reverse directions without affecting activation of the anion conductance. EC(50)s for L-glutamate and sodium are significantly lower after modification, consistent with kinetic models of the transport cycle that link anion channel gating to an early step in substrate translocation. Also, decreasing the pH from 7.5 to 6.5 decreases the EC(50) for L-glutamate to activate the anion conductance, without affecting the EC(50) for the entire transport cycle. These findings demonstrate for the first time a structural separation of transport and the uncoupled anion flux. Moreover, they shed light on some controversial aspects of the EAAT transport cycle, including the kinetics of proton binding and anion conductance activation.

Coupled, but not uncoupled, fluxes in a neuronal glutamate transporter can be activated by lithium ions

In the central nervous system a family of related (Na(+)-K(+))-coupled glutamate transporters remove the transmitter from the cleft and prevent its neurotoxic actions. In addition to this coupled uptake, these transporters also mediate a sodium- and glutamate-dependent uncoupled anion conductance. Most models assume that the initial steps for both processes are the same, leading to the anticipation that both may exhibit a similar requirement for cations. In this study we have tested this idea in the neuronal glutamate transporter EAAC-1. We report that in this transporter lithium can replace sodium in the coupled uptake. Strikingly, the glutamate-dependent gating of the uncoupled conductance mediated by EAAC-1 has a strict requirement for sodium; lithium cannot substitute for it. Moreover, we describe two mutants, T370S and G410S, in which the cation selectivity of the two processes is affected differently. In both mutants sodium, but not lithium, can support coupled transport. On the other hand, the sodium selectivity of the gated anion conductance in oocytes expressing the mutant transporters is not affected. Our observations indicate that although both the coupled and the uncoupled fluxes are sodium-dependent, the conformation gating the anion conductance is different from that during substrate translocation.

Glutamate transporters combine transporter- and channel-like features

Glutamate transporters in the mammalian central nervous system have a unique position among secondary transport proteins as they exhibit glutamate-gated chloride-channel activity in addition to glutamate-transport activity. In this article, the available data on the structure of the glutamate transporters are compared with high-resolution crystal structures of channel proteins. In addition, binding-site properties of glutamate transporters, and the ligand-binding site of an ionotropic glutamate receptor of which the crystal structure is known, are compared. Possible structural solutions for the combination of channel and transporter activity in one membrane protein are proposed.

Glutamate uptake

Brain tissue has a remarkable ability to accumulate glutamate. This ability is due to glutamate transporter proteins present in the plasma membranes of both glial cells and neurons. The transporter proteins represent the only (significant) mechanism for removal of glutamate from the extracellular fluid and their importance for the long-term maintenance of low and non-toxic concentrations of glutamate is now well documented. In addition to this simple, but essential glutamate removal role, the glutamate transporters appear to have more sophisticated functions in the modulation of neurotransmission. They may modify the time course of synaptic events, the extent and pattern of activation and desensitization of receptors outside the synaptic cleft and at neighboring synapses (intersynaptic cross-talk). Further, the glutamate transporters provide glutamate for synthesis of e.g. GABA, glutathione and protein, and for energy production. They also play roles in peripheral organs and tissues (e.g. bone, heart, intestine, kidneys, pancreas and placenta). Glutamate uptake appears to be modulated on virtually all possible levels, i.e. DNA transcription, mRNA splicing and degradation, protein synthesis and targeting, and actual amino acid transport activity and associated ion channel activities. A variety of soluble compounds (e.g. glutamate, cytokines and growth factors) influence glutamate transporter expression and activities. Neither the normal functioning of glutamatergic synapses nor the pathogenesis of major neurological diseases (e.g. cerebral ischemia, hypoglycemia, amyotrophic lateral sclerosis, Alzheimer's disease, traumatic brain injury, epilepsy and schizophrenia) as well as non-neurological diseases (e.g. osteoporosis) can be properly understood unless more is learned about these transporter proteins. Like glutamate itself, glutamate transporters are somehow involved in almost all aspects of normal and abnormal brain activity.

Early intermediates in the transport cycle of the neuronal excitatory amino acid carrier EAAC1

Electrogenic glutamate transport by the excitatory amino acid carrier 1 (EAAC1) is associated with multiple charge movements across the membrane that take place on time scales ranging from microseconds to milliseconds. The molecular nature of these charge movements is poorly understood at present and, therefore, was studied in this report in detail by using the technique of laser-pulse photolysis of caged glutamate providing a 100-micros time resolution. In the inward transport mode, the deactivation of the transient component of the glutamate-induced coupled transport current exhibits two exponential components. Similar results were obtained when restricting EAAC1 to Na(+) translocation steps by removing potassium, thus, demonstrating (1) that substrate translocation of EAAC1 is coupled to inward movement of positive charge and, therefore, electrogenic; and (2) the existence of at least two distinct intermediates in the Na(+)-binding and glutamate translocation limb of the EAAC1 transport cycle. Together with the determination of the sodium ion concentration and voltage dependence of the two-exponential charge movement and of the steady-state EAAC1 properties, we developed a kinetic model that is based on sequential binding of Na(+) and glutamate to their extracellular binding sites on EAAC1 explaining our results. In this model, at least one Na(+) ion and thereafter glutamate rapidly bind to the transporter initiating a slower, electroneutral structural change that makes EAAC1 competent for further, voltage-dependent binding of additional sodium ion(s). Once the fully loaded EAAC1 complex is formed, it can undergo a much slower, electrogenic translocation reaction to expose the substrate and ion binding sites to the cytoplasm.

Cysteine-scanning mutagenesis reveals a highly amphipathic, pore-lining membrane-spanning helix in the glutamate transporter GltT

The carboxyl-terminal membrane-spanning segment 8 of the glutamate transporter GltT of Bacillus stearothermophilus was studied by cysteine-scanning mutagenesis. 21 single cysteine mutants were constructed in a stretch ranging from Gly-374 to Gln-404. Two mutants were not expressed, four were inactive, and two showed severely reduced glutamate transport activity. Cysteine mutations at the other positions were well tolerated. Only the two most amino- and carboxyl-terminal mutants (G374C, I375C, S399C, and Q404C) could be labeled with the large thiol reagent fluorescein maleimide, indicating unrestricted access and a location in a loop structure outside the membrane. The labeling pattern of these mutants using membrane- permeable and -impermeable thiol reagents showed that the N and C termini of the mutated stretch are located extra- and intracellularly, respectively. Thus, the location of the membrane-spanning segment was confined to a stretch of 23 residues between Gly-374 and Ser-399. Cysteine residues in three mutants in the central part of the segment (M381C, V388C, and N391C) could be labeled with the small and flexible reagent 2-aminoethyl methanethiosulfonate hydrobromide only, suggesting accessibility via a narrow aqueous pore. When the region was modeled as an alpha-helix, all positions at which cysteine mutations lead to inactive or severely impaired transporters cluster on one face of this helix. The inactive mutants showed neither proton motive force-driven uptake activity nor exchange activity nor glutamate binding. The results indicate that transmembrane segment 8 forms an amphipathic alpha-helix. The hydrophilic face of the helix lines an aqueous pore and contains many residues that are important for activity.

Amyotrophic lateral sclerosis-linked glutamate transporter mutant has impaired glutamate clearance capacity

We have investigated the functional impact of a naturally occurring mutation of the human glutamate transporter GLT1 (EAAT2), which had been detected in a patient with sporadic amyotrophic lateral sclerosis. The mutation involves a substitution of the putative N-linked glycosylation site asparagine 206 by a serine residue (N206S) and results in reduced glycosylation of the transporter and decreased uptake activity. Electrophysiological analysis of N206S revealed a pronounced reduction in transport rate compared with wild-type, but there was no alteration in the apparent affinities for glutamate and sodium. In addition, no change in the sensitivity for the specific transport inhibitor dihydrokainate was observed. However, the decreased rate of transport was associated with a reduction of the N206S transporter in the plasma membrane. Under ionic conditions, which favor the reverse operation mode of the transporter, N206S exhibited an increased reverse transport capacity. Furthermore, if coexpressed in the same cell, N206S manifested a dominant negative effect on the wild-type GLT1 activity, whereas it did not affect wild-type EAAC1. These findings provide evidence for a role of the N-linked glycosylation in both cellular trafficking and transport function. The resulting alteration in glutamate clearance capacity likely contributes to excitotoxicity that participates in motor neuron degeneration in amyotrophic lateral sclerosis.

Neurochemistry of L-Glutamate Transport in the CNS: A Review of Thirty Years of Progress

The review highlights the landmark studies leading from the discovery and initial characterization of the Na+-dependent "high affinity" uptake in the mammalian brain to the cloning of individual transporters and the subsequent expansion of the field into the realm of molecular biology. When the data and hypotheses from 1970's are confronted with the recent developments in the field, we can conclude that the suggestions made nearly thirty years ago were essentially correct: the uptake, mediated by an active transport into neurons and glial cells, serves to control the extracellular concentrations of L-glutamate and prevents the neurotoxicity. The modern techniques of molecular biology may have provided additional data on the nature and location of the transporters but the classical neurochemical approach, using structural analogues of glutamate designed as specific inhibitors or substrates for glutamate transport, has been crucial for the investigations of particular roles that glutamate transport might play in health and disease. Analysis of recent structure/activity data presented in this review has yielded a novel insight into the pharmacological characteristics of L-glutamate transport, suggesting existence of additional heterogeneity in the system, beyond that so far discovered by molecular genetics. More compounds that specifically interact with individual glutamate transporters are urgently needed for more detailed investigations of neurochemical characteristics of glutamatergic transport and its integration into the glutamatergic synapses in the central nervous system. A review with 162 references.

Arginine 447 plays a pivotal role in substrate interactions in a neuronal glutamate transporter

Glutamate transporters from the central nervous system play a crucial role in the clearance of the transmitter from the synaptic cleft. Glutamate is cotransported with sodium ions, and the electrogenic translocation cycle is completed by countertransport of potassium. Mutants that cannot interact with potassium are only capable of catalyzing electroneutral exchange. Here we identify a residue involved in controlling substrate recognition in the neuronal transporter EAAC-1 that transports acidic amino acids as well as cysteine. When arginine 447, a residue conserved in all glutamate transporters, is replaced by cysteine, transport of glutamate or aspartate is abolished, but sodium-dependent cysteine transport is left intact. Analysis of other substitution mutants shows that the replacement of arginine rather than the introduced cysteine is responsible for the observed phenotype. In further contrast to wild type, acidic amino acids are unable to inhibit cysteine transport in R447C-EAAC-1, indicating that the selectivity change is manifested at the binding step. Electrophysiological analysis shows that in the mutant cysteine, transport has become electroneutral, and its interaction with the countertransported potassium is impaired. Thus arginine 447 plays a pivotal role in the sequential interaction of acidic amino acids and potassium with the transporter and, thereby, constitutes one of the molecular determinants of coupling their fluxes.

The accessibility of a novel reentrant loop of the glutamate transporter GLT-1 is restricted by its substrate

The excitatory neurotransmitter glutamate is removed from the synaptic cleft by several related sodium- and potassium-coupled transporters. They thereby restrict the neurotoxicity of this transmitter. Based on the accessibility of single cysteines to the large sulfhydryl reagent 3-N-maleimidyl(propionyl)biocytin, we have proposed a topological model for the astroglial glutamate transporter GLT-1 (Grunewald, M., Bendahan, A. and Kanner, B. I. (1998) Neuron 21, 623-632). Because of several unexpected observations, we have investigated the topological disposition of 19 cysteine residues engineered into a loop proposed to be intracellular. We have probed the accessibility of these cysteines to small and large sulfhydryl reagents. The impermeant hydrophilic sulfhydryl reagent [(2-trimethylammonium)ethyl] methanethiosulfonate inhibits transport activity only at two of these positions, weakly at G365C and potently at A364C. Glutamate and its nontransportable analogue dihydrokainate markedly protect A364C transporters against this impermeant reagent. Using a biotinylated maleimide, we found that, among the 14 mutants tested with it, only A364C is accessible to it from the extracellular side. This, together with our previous observations, indicates that the loop-including amino acid residues 354, 359, 373, and 379-is largely intracellular, but a short region of it forms a reentrant pore-loop-like structure, the accessibility of which is dependent on the conformation of the transporter.

A model for the topology of excitatory amino acid transporters determined by the extracellular accessibility of substituted cysteines

Excitatory amino acid transporters (EAATs) function as both substrate transporters and ligand-gated anion channels. Characterization of the transporter's general topology is the first requisite step in defining the structural bases for these distinct activities. While the first six hydrophobic domains can be readily modeled as conventional transmembrane segments, the organization of the C-terminal hydrophobic domains, which have been implicated in both substrate and ion interactions, has been controversial. Here, we report the results of a comprehensive evaluation of the C-terminal topology of EAAT1 determined by the chemical modification of introduced cysteine residues. Our data support a model in which two membrane-spanning domains flank a central region that is highly accessible to the extracellular milieu and contains at least one reentrant loop domain.

Selective high-affinity transport of aspartate by a Drosophila homologue of the excitatory amino-acid transporters

Excitatory amino-acid transporters (EAATs) are structurally related plasma membrane proteins that mediate the high-affinity uptake of the acidic amino acids glutamate and aspartate released at excitatory synapses, and maintain the extracellular concentrations of these neurotransmitters below excitotoxic levels [1] [2] [3] [4]. Several members of the EAAT family have been described previously. So far, all known EAATs have been reported to transport glutamate and aspartate with a similar affinity. Here, we report that dEAAT2 - a nervous tissue-specific EAAT homologue that we recently identified in the fruit fly Drosophila [5] - is a selective Na(+)-dependent high-affinity aspartate transporter (K(m) = 30 microM). We found that dEAAT2 can also transport L-glutamate but with a much lower affinity (K(m) = 185 microM) and a 10- to 15-fold lower relative efficacy (V(max)/K(m)). Competition experiments showed that the binding of glutamate to this transporter is much weaker than the binding of D- or L-aspartate. As dEAAT2 is the first known EAAT to show this substrate selectivity, it suggests that aspartate may play a specific role in the Drosophila nervous system.

A conserved serine-rich stretch in the glutamate transporter family forms a substrate-sensitive reentrant loop

Neuronal and glial glutamate transporters remove the excitatory neurotransmitter glutamate from the synaptic cleft. The proteins belong to a large family of secondary transporters, which includes bacterial glutamate transporters. The C-terminal half of the glutamate transporters is well conserved and thought to contain the translocation path and the binding sites for substrate and coupling ions. A serine-rich sequence motif in this part of the proteins is located in a putative intracellular loop. Cysteine-scanning mutagenesis was applied to this loop in the glutamate transporter GltT of Bacillus stearothermophilus. The loop was found to be largely intracellular, but three consecutive positions in the conserved serine-rich motif (S269, S270, and E271) are accessible from both sides of the membrane. Single-cysteine mutants in the serine-rich motif were still capable of glutamate transport, but modification with N-ethylmaleimide blocked the transport activity in six mutants (T267C, A268C, S269C, S270C, E271C, and T272C). Two milimolars L-glutamate effectively protected against the modification of the cysteines at position 269-271 from the periplasmic side of the membrane but was unable to protect cysteine modification from the cytoplasmic side of the membrane. The results indicate that the conserved serine-rich motif in the glutamate transporter forms a reentrant loop, a structure that is found in several ion channels but is unusual for transporter proteins. The reentrant loop is of crucial importance for the function of the glutamate transporter.

C-terminal interactions modulate the affinity of GLAST glutamate transporters in salamander retinal glial cells

1. Proteins that interact with the intracellular carboxy termini of neurotransmitter- and voltage-gated ion channels are known to control the subcellular localization of the channels, localize other proteins near those channels, and modulate channel activity. By contrast, little is known about the control of neurotransmitter transporter function by interacting proteins. 2. To competitively disrupt interactions of the C- and N-termini of the GLAST glutamate transporter with other proteins, we dialysed whole-cell patch-clamped retinal glia with peptides identical to the eight amino acids at the C- or N-termini of the transporter, and compared the effect on transporter-mediated currents with dialysis of scrambled versions of the same peptides. 3. Dialysis with the N-terminus peptide had no effect on the maximum glutamate-evoked current nor on the glutamate affinity of the transporter. Dialysis with the C-terminus peptide had no effect on the maximum current, but increased the affinity of the transporter for glutamate (compared with scrambled C-terminus peptide, and with N- and scrambled N-terminus peptides: Km decreased from 16 to 11 microM)). 4. These data suggest that disruption of an interaction between an intracellular protein and the last eight amino acids of the GLAST C-terminus, which have some similarity to the PDZ binding domain of ion channel C-termini, increases the glutamate affinity of GLAST. Thus, the interacting protein decreases the affinity of GLAST transporters. 5. Removing the GLAST C-terminus interaction increases the transporter current by 40 % at low glutamate concentrations. Thus, this interaction may significantly slow the removal of low concentrations of glutamate from the extracellular space, and affect the kinetics of retinal cell light responses.