Active transport of biogenic amines in chromaffin granule membrane vesicles

Chromaffin granule membrane vesicles accumulate large amounts of catecholamines against their concentration gradients. This process is ATP-dependent, reserpine, FCCP and nigericin sensitive. Carrier-mediated, reserpine-sensitive accumulation has also been demonstrated in the absence of ATP when a pH gradient (delta pH) is artificially generated across the membrane. Crude preparation of 5-hydroxytryptamine storage vesicles from rat brain or from pig platelets showed similar requirement of a transmembrane pH gradient for accumulation of the amine. The catecholamine transporter from chromaffin granules has been solubilized by the use of detergents in the presence of phospholipids. Removal of the detergent either by Sephadex filtration or by dialysis results in the formation of proteoliposomes which catalyze delta pH-dependent, reserpine-sensitive catecholamine accumulation. Under proper conditions, the solubilized H+-translocating ATPase has been incorporated into the same proteoliposomes with the catecholamine transporter, and ATP-dependent transport has been measured. The reconstituted protein shows specificity and affinity towards catecholamines similar to the native one.

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.

Engineered Zn(2+) switches in the gamma-aminobutyric acid (GABA) transporter-1. Differential effects on GABA uptake and currents

Two high affinity Zn(2+) binding sites were engineered in the otherwise Zn(2+)-insensitive rat gamma-aminobutyric acid (GABA) transporter-1 (rGAT-1) based on structural information derived from Zn(2+) binding sites engineered previously in the homologous dopamine transporter. Introduction of a histidine (T349H) at the extracellular end of transmembrane segment (TM) 7 together with a histidine (E370H) or a cysteine (Q374C) at the extracellular end of TM 8 resulted in potent inhibition of [3H]GABA uptake by Zn(2+) (IC(50) = 35 and 44 microM, respectively). Upon expression in Xenopus laevis oocytes it was similarly observed that Zn(2+) was a potent inhibitor of the GABA-induced current (IC(50) = 21 microM for T349H/E370H and 51 microM for T349H/Q374C), albeit maximum inhibition was only approximately 40% in T349H/E370H versus approximately 90% in T349H/Q374C. In the wild type, Zn(2+) did not affect the Na(+)-dependent transient currents elicited by voltage jumps and thought to reflect capacitive charge movements associated with Na(+) binding. However, in both mutants Zn(2+) caused a reduction of the inward transient currents upon jumping to hyperpolarized potentials as reflected in rightward-shifted Q/V relationships. This suggests that Zn(2+) is inhibiting transporter function by stabilizing the outward-facing Na(+)-bound state. Translocation of lithium by the transporter does not require GABA binding and analysis of this uncoupled Li(+) conductance revealed a potent inhibition by Zn(2+) in T349H/E370H, whereas surprisingly the T349H/Q374C leak was unaffected. This differential effect supports that the leak conductance represents a unique operational mode of the transporter involving conformational changes different from those of the substrate translocation process. Altogether our results support both an evolutionary conserved structural organization of the TM 7/8 domain and a key role of this domain in GABA-dependent and -independent conformational changes of the transporter.

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.

Molecular characterization of substrate-binding sites in the glutamate transporter family

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 that is activated by two of the molecules that 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 in the carboxyl terminal half, which is predicted to have an unusual topology. Determination of the topology shows that the carboxyl terminal part of the molecule contains several transmembrane domains that are separated by at least two re-entrant loops. In these structural elements, we have identified several conserved amino acid residues that play crucial roles in the interaction with the transporter substrates sodium, potassium and glutamate.

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.

Mutation of arginine 44 of GAT-1, a (Na(+) + Cl(-))-coupled gamma-aminobutyric acid transporter from rat brain, impairs net flux but not exchange

The gamma-aminobutyric acid (GABA) transporter GAT-1 is a prototype of a large family of neurotransmitter transporters that includes those of dopamine and serotonin. GAT-1 maintains low synaptic concentrations of neurotransmitter by coupling GABA uptake to the fluxes of sodium and chloride. Here we identify a stretch of four amino acid residues predicted to lie in the juxtamembrane region prior to transmembrane domain 1 in the cytoplasmic amino-terminal tail of GAT-1, which is critical for its function. Two residues, arginine 44 and tryptophan 47, are fully conserved within the transporter family, and their deletion abolishes GABA transport in the HeLa cell expression system used. Tryptophan 47 can be replaced only by aromatic residues without loss of activity. Arginine 44 is essential for activity. Only when it is replaced by lysine, low activity levels (around 15% of those of the wild type) are observed. Using a reconstitution assay, we show that mutants in which this residue is replaced by lysine or histidine exhibit sodium- and chloride-dependent GABA exchange similar to the wild type. This indicates that these mutants are selectively impaired in the reorientation of the unloaded transporter, a step in the translocation cycle by which net flux and exchange differ. The high degree of conservation in the consensus sequence RXXW suggests that this region may influence the reorientation step in related transporters as well.

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.

The reactivity of the gamma-aminobutyric acid transporter GAT-1 toward sulfhydryl reagents is conformationally sensitive. Identification of a major target residue

The gamma-aminobutyric acid (GABA) transporter GAT-1 is a prototype of neurotransmitter transporters that maintain low synaptic levels of the transmitter. Transport by GAT-1 is sensitive to the polar sulfhydryl reagent 2-aminoethyl methanethiosulfonate. Following replacement of endogenous cysteines to other residues by site-directed mutagenesis, we have identified cysteine 399 as the major determinant of the sensitivity of the transporter to sulfhydryl modification. Cysteine-399 is located in the intracellular loop connecting putative transmembrane domains eight and nine. Binding of both sodium and chloride leads to a reduced sensitivity to sulfhydryl reagents, whereas subsequent binding of GABA increases it. Strikingly binding of the nontransportable GABA analogue SKF100330A gives rise to a marked protection against sulfhydryl modification. These effects were not observed in C399S transporters. Under standard conditions GAT-1 is almost insensitive toward the impermeant 2-(trimethylammonium)ethyl methanethiosulfonate. However, in a chloride-free medium, addition of SKF100330A renders wild type GAT-1, but not C399S, very sensitive to this impermeant reagent. These observations indicate that the accessibility of cysteine 399 is highly dependent on the conformation of GAT-1. Consequently, topological assignments based on accessibility of endogeneous or engineered cysteines to small polar sulfhydryl reagents need to be interpreted with extreme caution.

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

The neurotoxicity of glutamate in the central nervous system is restricted by several (Na+ + K+)-coupled transporters for this neurotransmitter. The astroglial transporter GLT-1 is the only subtype that exhibits high sensitivity to the nontransportable glutamate analogue dihydrokainate. A marked reduction in sensitivity to the blocker is observed when serine residues 440 and 443 are mutated to glycine and glutamine, which, respectively, occupy these positions in the other homologous glutamate transporters. They are located in the ascending limb of the recently identified pore-loop-like structure. Strikingly, mutation of serine-440 to glycine enables not only sodium but also lithium ions to drive net influx of acidic amino acids. Moreover, the efficiency of lithium as a driving ion for glutamate transport depends on the nature of the amino acid residue present at position 443. Mutant transporters containing single cysteines at the position of either serine residue become sensitive to positively as well as negatively charged methanethiosulfonate derivatives. In S440C transporters significant protection against this inhibition is provided both by transportable and nontransportable glutamate analogues, but not by sodium alone. Our observations indicate that the pore-loop-like structure plays a pivotal role in coupling ion and glutamate fluxes and suggest that it is close to the glutamate-binding site.

Biotinylation of single cysteine mutants of the glutamate transporter GLT-1 from rat brain reveals its unusual topology

In the central nervous system, (Na+ + K+)-coupled glutamate transporters restrict the neurotoxicity of this transmitter and limit the duration of synaptic excitation at some synapses. The various isotransporters exhibit a particularly high homology in an extended hydrophobic domain of ill-defined topology that contains several determinants involved in ion and transmitter binding. Here, we describe the determination of the membrane topology of the cloned astroglial glutamate transporter GLT-1. A series of functional transporters containing single cysteines was engineered. Their topological disposition was determined by using a biotinylated sulfhydryl reagent. The glutamate transporter has eight transmembrane domains long enough to span the membrane as et heiices. Strikingly, between the seventh and eighth domains, a structure reminiscent of a pore loop and an outward-facing hydrophobic linker are positioned.

Cysteine scanning of the surroundings of an alkali-ion binding site of the glutamate transporter GLT-1 reveals a conformationally sensitive residue

Glutamate transporters remove this transmitter from the extracellular space by cotransport with three sodium ions and a proton. The cycle is completed by translocation of a potassium ion in the opposite direction. Recently we have identified two adjacent amino acid residues of the glutamate transporter GLT-1 that influence potassium coupling. Using the scanning cysteine accessibility method we have now explored the highly conserved region surrounding them. Replacement of each of the five consecutive residues 396-400 by cysteine abolished transport activity but at several other positions the substitution is tolerated. One residue, tyrosine 403, was identified where cysteine substitution renders the transporter sensitive to modification by positively charged methanethiosulfonate derivates in a sodium-protectable fashion. In the presence of sodium, the nontransported glutamate analogue dihydrokainate potentiated the covalent modification, presumably by binding to the glutamate site and locking the protein in a conformation in which tyrosine 403 is accessible from the external bulk medium. In contrast, transported substrates significantly slowed the reaction, suggesting that during the transport cycle residue 403 becomes occluded. On the other hand, transportable substrates are not able to protect Y403C transporters against N-ethylmaleimide, which is highly permeant but unable to modify cysteine residues buried within membrane proteins. These results indicate that tyrosine 403 is alternately accessible from either side of the membrane, consistent with its role as structural determinant of the potassium binding site.

Molecular determinant of ion selectivity of a (Na+ + K+)-coupled rat brain glutamate transporter

Glutamate transporters remove this neurotransmitter from the synaptic cleft by a two-stage electrogenic process, in which glutamate is first cotransported with three sodium ions and a proton. Subsequently, the cycle is completed by translocation of a potassium ion in the opposite direction. Recently, we have identified an amino acid residue of the glutamate transporter GLT-1 (Glu-404) that influences potassium coupling. We have now analyzed the effect of seven other amino acid residues in the highly conserved region surrounding this site. One of these residues, Tyr-403, also proved important for potassium coupling, because mutation to Phe (Y403F) resulted in an electroneutral obligate exchange mode of glutamate transport. This mutation in the transporter also caused an approximately 8-fold increase in the apparent sodium affinity, with no change in the apparent affinity for L-glutamate or D-aspartate. Strikingly, although exchange catalyzed by the wild-type transporter is strictly dependent on sodium, the selectivity of Y403F mutant transporters is altered so that sodium can be replaced by other alkaline metal cations including lithium and cesium. These results indicate the presence of interacting sites in or near the transporter pore that control selectivity for sodium and potassium.

Tyrosine 140 of the gamma-aminobutyric acid transporter GAT-1 plays a critical role in neurotransmitter recognition

The gamma-aminobutyric acid (GABA) transporter GAT-1 is located in nerve terminals and catalyzes the electrogenic reuptake of the neurotransmitter with two sodium ions and one chloride. We now identify a single tyrosine residue that is critical for GABA recognition and transport. It is completely conserved throughout the superfamily, and even substitution to the other aromatic amino acids, phenylalanine (Y140F) and tryptophan (Y140W), results in completely inactive transporters. Electrophysiological characterization reveals that both mutant transporters exhibit the sodium-dependent transient currents associated with sodium binding as well as the chloride-dependent lithium leak currents characteristic of GAT-1. On the other hand, in both mutants GABA is neither able to induce a steady-state transport current nor to block their transient currents. The nontransportable analog SKF 100330A potently inhibits the sodium-dependent transient in the wild type GAT-1 but not in the Y140W transporter. It partly blocks the transient of Y140F. Thus, although sodium and chloride binding are unimpaired in the tyrosine mutants, they have a specific defect in the binding of GABA. The total conservation of the residue throughout the family suggests that tyrosine 140 may be involved in the liganding of the amino group, the moiety common to all of the neurotransmitters.

Mutation of an amino acid residue influencing potassium coupling in the glutamate transporter GLT-1 induces obligate exchange

Glutamate transporters maintain low synaptic concentrations of neurotransmitter by coupling uptake to flux of other ions. After cotransport of glutamic acid with Na+, the cycle is completed by countertransport of K+. We have identified an amino acid residue (glutamate 404) influencing ion coupling in a domain of the transporter implicated previously in kainate binding. Mutation of this residue to aspartate (E404D) prevents both forward and reverse transport induced by K+. Sodium-dependent transmitter exchange and a transporter-mediated chloride conductance are unaffected by the mutation, indicating that this residue selectively influences potassium flux coupling. The results support a kinetic model in which sodium and potassium are translocated in distinct steps and suggest that this highly conserved region of the transporter is intimately associated with the ion permeation pathway.

The membrane topology of GAT-1, a (Na+ + Cl-)-coupled gamma-aminobutyric acid transporter from rat brain

The membrane topology of GAT-1, a sodium- and chloride-coupled gamma-aminobutyric acid transporter from rat brain, has been probed using N-glycosylation scanning mutagenesis. Overall, the results support the theoretical 12-transmembrane segment model. This model (based on hydropathy analysis) was originally proposed for GAT-1 and adopted for all other members of the sodium- and chloride-dependent neurotransmitter transporter superfamily. However, our data indicate that the loop connecting putative transmembrane domains 2 and 3, which was predicted to be located intracellularly, can be glycosylated in vivo. Furthermore, studies with permeant and impermeant methanesulfonate reagents suggest that cysteine 74, located in the hydrophilic loop connecting transmembrane domains 1 and 2, is intracellular rather than extracellular. We present a model in which the topology deviates from the theoretical one in the amino-terminal third of the transporter. It also contains 12 transmembrane segments, but the highly conserved domain 1 does not form a conventional transmembrane alpha-helix.

Ion binding and permeation at the GABA transporter GAT1

This study addresses the binding of ions and the permeation of substrates during function of the GABA transporter GAT1. GAT1 was expressed in Xenopus oocytes and studied electrophysiologically as well as with [3H]GABA flux; GAT1 was also expressed in mammalian cells and studied with [3H]GABA and [3H]tiagabine binding. Voltage jumps, Na+ and Cl- concentration jumps, and exposure to high-affinity blockers (NO-05-711 and SKF-100330A) all produce capacitive charge movements. Occlusive interactions among these three types of perturbations show that they all measure the same population of charges. The concentration dependences of the charge movements reveal (1) that two Na+ ions interact with the transporter even in the absence of GABA, and (2) that Cl- facilitates the binding of Na+. Comparison between the charge movements and the transport-associated current shows that this initial Na(+)-transporter interaction limits the overall transport rate when [GABA] is saturating. However, two classes of manipulation--treatment with high-affinity uptake blockers and the W68L mutation-"lock" Na+ onto the transporter by slowing or preventing the subsequent events that release the substrates to the intracellular medium. The Na+ substitutes Li+ and Cs+ do not support charge movements, but they can permeate the transporter in an uncoupled manner. Our results (1) support the hypothesis that efficient removal of synaptic transmitter by the GABA transporter GAT1 depends on the previous binding of Na+ and Cl-, and (2) indicate the important role of the conserved putative transmembrane domain 1 in interactions with the permeant substrates.

Glutamate-101 is critical for the function of the sodium and chloride-coupled GABA transporter GAT-1

We have investigated the possible role of selected negatively-charged amino acids of the sodium and chloride-coupled GABA transporter GAT-1 on sodium binding. These residues located adjacent to putative transmembrane domains and which are conserved throughout the large superfamily of neurotransmitter transporters were changed by site-directed mutagenesis. The functional consequences were that one of the residues, glutamate-101, was critical for transport. Its replacement by aspartate left only 1% of the activity, and no activity could be detected when it was replaced by other residues. Expression levels and targeting to the plasma membrane of the mutant transporters appeared normal. Transient sodium currents were not observed in the mutants, and increased sodium concentrations did not affect the percentage of wild type transport of the E101D mutant. It is concluded that residue glutamate-101 is critical for one or more of the conformational changes of GAT-1 during its transport cycle.

Glutamate 404 is involved in the substrate discrimination of GLT-1, a (Na+ + K+)-coupled glutamate transporter from rat brain

Sodium-coupled glutamate transporters, located in the plasma membrane of nerve terminals and glial processes, serve to keep its extracellular glutamate concentration below extracellular levels. Moreover, they help in conjunction with diffusion to terminate the transmitter's action in synaptic transmission. We have investigated the role of negatively charged amino acid residues of GLT-1, a cloned (Na+ + K+)-coupled glutamate transporter from rat brain. Using site-directed mutagenesis we modified these negative residues, which are located in hydrophobic surroundings and are highly conserved within the glutamate transporter family. Out of five residues meeting these criteria, three, aspartate 398, glutamate 404, and aspartate 470, are critical for heterologously expressed glutamate transport. This defective transport cannot be attributed to the mere requirement of a negative charge at these positions. After prelabeling of the proteins with [35S]methionine, immunoprecipitation of all mutant transporters indicates that their expression levels are similar to that of wild type. No cryptic activity was revealed by reconstitution experiments aimed to monitor the activity of transporter molecules not located in the plasma membrane. Significantly, whereas all of the mutants at the glutamate 404 position exhibit impaired transport of glutamate, they possess considerable transport of D- and L-aspartate, up to 80% of wild type values. Binding of glutamate is not impaired in these mutants. Our observations indicate that the glutamate 404 residue may be located in the vicinity of the glutamate-aspartate permeation pathway.

The number of amino acid residues in hydrophilic loops connecting transmembrane domains of the GABA transporter GAT-1 is critical for its function

Transporter proteins consist of multiple transmembrane domains connected by hydrophillic loops. As the importance of these loops in transport processes is poorly understood, we have studied this question using the cDNA coding for GAT-1, a Na+/Cl(-)-coupled gamma-aminobutyric acid transporter from rat brain. Deletions of randomly picked non-conserved single amino acids in the loops connecting helices 7 and 8 or 8 and 9 result in inactive transport upon expression in HeLa cells. However, transporters where these amino acids are replaced with glycine retain significant activity. The expression level of the inactive mutant transporters was similar to that of the wild-type, but one of these, delta Val-348, appears to be defectively targetted to the plasma membrane. Our data are compatible with the idea that a minimal length of the loops is required, presumably to enable the transmembrane domains to interact optimally with each other.

Sodium-coupled neurotransmitter transport: structure, function and regulation

The removal of neurotransmitters by their transporters--located in the plasma membranes of nerve terminals and glial cells--plays an important role in the termination of synaptic transmission. In the last 3 years, many neurotransmitter transporters have been cloned. Structurally and functionally they can be divided into two groups: glutamate transporters, of which to date three have been cloned, couple the flow of glutamate to that of sodium and potassium. The second group of transporters includes those for GABA, glycine, taurine, norepinephrine, dopamine and serotonin. They are sodium- and chloride-dependent, but do not require potassium for function. One of these, the GABAA transporter, encoded by GAT-1, is perhaps the best characterized. It has been purified and reconstituted and has a molecular mass of around 80 kDa, of which 10-15 kDa is sugar. Amino and carboxyl termini (around 50 amino acids each) are not required for function. The transporter is protected against proteolysis at multiple sites by GABA, provided that the two cosubstrates--sodium and chloride--are present. Several amino acid residues that are critical for function have been identified in the GABA transporter. These include arginine-69 and tryptophan-222 located in the first and fourth putative transmembrane helices, respectively. The first is possibly involved in the binding of chloride. The tryptophan appears to serve as a binding site for the amino group of GABA.

Histidine 326 is critical for the function of GLT-1, a (Na+ + K+)-coupled glutamate transporter from rat brain

Removal of glutamate from the synaptic cleft is carried out by transporter molecules located in presynaptic nerve terminals and fine glial processes surrounding the cleft. Three such transporters, which are approximately 55% identical to each other, have recently been cloned. They catalyze electrogenic transport of this neurotransmitter, which is coupled to the fluxes of three ions: sodium, potassium, and protons (or hydroxyl). One of these transporters, GLT-1, contains 573 amino acids and 6-10 putative membrane-spanning alpha-helices. These helices contain only two positively charged amino acid residues (lysine 298 and histidine 326) that are fully conserved in the glutamate transporters and two related neutral amino acid transporters. Using site-directed mutagenesis we have investigated the role of these residues, each of which was replaced by small hydrophilic as well as by positively charged amino acids. Expression of all replacement mutants at the histidine 326 position reveals that they are severely impaired in sodium-dependent glutamate transport. On the other hand, mutations at lysine 298 retain significant activity, especially if a positively charged amino acid replaces the lysine. After prelabeling of the proteins with [35S]methionine, immunoprecipitation of all mutant transporters indicates that their expression levels are similar to those of wild type. Reconstitution experiments, aimed to reveal the activity of transporter molecules not located in the plasma membrane, indicate that the lowered activity of the K298T and K298N transporters in intact cells is partly due to a targeting defect. Histidine residue 326 appears to be required for the intrinsic activity of the transporter. As histidine residues have been implicated in the mechanism of H+ transport in several systems, we propose that histidine 326 may be involved in the proton translocation accompanying sodium- and potassium-coupled glutamate transport.

Localization of the glutamate transporter GLT-1 in rat and macaque monkey retinae

Antibodies directed against a glutamate transporter (GLT-1) purified from rat brain were applied to cryostat sections of rat and macaque monkey retinae. In the brain, GLT-1 expression is found mainly in astrocytes, and therefore it has been suggested that GLT-1 may be a glutamate transporter specific to glial cells. However, in the rat retina, cones and two distinct cone bipolar cell types were strongly immunoreactive. In the monkey retina, flat midget bipolars and one diffuse bipolar cell type (DB2)), were found to be labelled. Müller cells or astrocytes, the neuroglial cells of rat and monkey retinae, were not GLT-1-immunoreactive.

Identification of tryptophan residues critical for the function and targeting of the gamma-aminobutyric acid transporter (subtype A)

The gamma-aminobutyric acid transporter is localized in nerve terminals. It catalyzes coupled electrogenic translocation of the neurotransmitter with two or three sodium ions and one chloride ion. The transporter contains 599 amino acids and 12 putative membrane spanning alpha-helices. It is the first described member of a neurotransmitter transporter superfamily. Using site-directed mutagenesis we have investigated the role of all 10 tryptophan residues predicted to reside in these helices. All 10 have been changed to serine as well as to leucine residues. Expression of mutant cDNAs in which the tryptophans, located in positions 68, 222, and 230, are replaced by either of these two amino acids reveals that they are severely impaired in gamma-aminobutyric acid transport. Mutants in which a phenylalanine or a tyrosine residue is introduced, at either position 68 or 230, are active. On the other hand, at the 222 position replacement of the tryptophan by the aromatic amino acids results in inactive transport. After prelabeling of the proteins with [35S]methionine, immunoprecipitation of mutant transporters indicates that their expression levels are similar to those of the wild type. Reconstitution experiments, aimed to reveal the activity of transporter molecules not apparent in the plasma membrane, indicate that the lack of activity of the W230S transporter in intact cells is by and large due to its inefficient targeting to the plasma membrane. Tryptophan residues 68 and 222 appear to be required for the intrinsic activity of the transporter. Based on several observations, including one that tryptophan residue 222 is conserved in all amino acid transporter members of the superfamily, but not in those transporting biogenic amines, we hypothesize that the pi electrons of this tryptophan could be involved in the binding of the amino group of these neurotransmitters.

Phosphorylation and modulation of brain glutamate transporters by protein kinase C

High affinity sodium- and potassium-coupled L-glutamate transport into presynaptic nerve terminals and fine glial processes removes the neurotransmitter from the synaptic cleft, thereby terminating glutamergic transmission. This report describes that the purified L-glutamate transporter from pig brain is phosphorylated by protein kinase C, predominantly at serine residues. Upon exposure of C6 cells, a cell line of glial origin, to 12-O-tetradecanoylphorbol-13-acetate, about a 2-fold stimulation of L-glutamate transport is observed within 30 min. Concomitantly, the level of phosphorylation increases with similar kinetics. The phorbol ester also stimulates L-glutamate transport in HeLa cells infected with a recombinant vaccinia virus expressing T7 RNA polymerase and transfected with pT7-GLT-1. The latter is a recently cloned rat brain glutamate transporter of glial origin. Mutation of serine 113 to asparagine does not affect the levels of expressed transport but abolishes its stimulation by the phorbol ester. To our knowledge, this is the first direct demonstration of the regulation of a neurotransmitter transporter by phosphorylation.

The substrates of a sodium- and chloride-coupled gamma-aminobutyric acid transporter protect multiple sites throughout the protein against proteolytic cleavage

Fragments of the (Na(+) + Cl-)-coupled GABAA transporter were produced by proteolysis of membrane vesicles and reconstituted preparations from rat brain. The former were digested with Pronase, the latter with trypsin. Fragments with different apparent molecular masses were recognized by sequence-directed antibodies raised against this transporter. When GABA was present in the digestion medium, the generation of these fragments was almost entirely blocked. At the same time, the neurotransmitter largely prevented the loss of activity caused by the protease. The effect was specific for GABA; protection was not afforded by other neurotransmitters. It was only observed when the two cosubstrates, sodium and chloride, were present on the same side of the membrane as GABA. The results indicate that the transporter may exist in two conformations. In the absence of one or more of the substrates, multiple sites located throughout the transporter are accessible to the proteases. In the presence of all three substrates--conditions favoring the formation of the translocation complex--the conformation is changed such that these sites become inaccessible to protease action.

Hippocampal gamma-aminobutyric acid and benzodiazepine receptors after early phenobarbital exposure

Mice were exposed to phenobarbital (PhB) prenatally (PreB offspring) by feeding their mothers 3 g/kg PhB in milled food on gestation days 9-18, or neonatally by directly injecting pups of intact mothers with daily dose of 50 mg PhB on postnatal days 2-21 (NeoB offspring). At age 22 or 50 days, the offspring were tested for gamma-aminobutyric acid (GABA) up take in the hippocampus and in the rest of the brain. In addition, [3H]muscimol and [3H]flunitrazepam binding in the hippocampus and cortex were measured in the offspring at age 22 and 50 days. Long-term decrease in GABA uptake was found in the NeoB group. A 23% decrease was found in 22-day-old mice (P < 0.001) and a 22% decrease in 50-day-old mice (P < 0.05). In addition, there was a 22% decrease in GABA uptake in the brain of 22-day-old PreB mice (P < 0.05). An increase of 52% in [3H]muscimol binding (P < 0.001) and 45% (P < 0.001) in [3H]flunitrazepam binding were measured in the hippocampus in the 22-day-old NeoB mice; no differences were found in affinity. The differences were short-term and could no longer be detected at age 50 days. No differences were found in the cortex; unlike NeoB, PreB mice did not differ from controls. The results suggest upregulation of the GABAergic system in early PhB exposed mice.

Glutamate transporters from brain. A novel neurotransmitter transporter family

The removal of neurotransmitters by their transporters in presynaptic nerve terminals and glial cells plays an important role in the termination of synaptic transmission. Many neurotransmitter transporters, which are sodium- and chloride-coupled, have been cloned and shown to constitute a large superfamily. Glutamate is the major excitatory neurotransmitter in the central nervous system. If not efficiently removed, it causes death of neuronal cells. Its transporter couples the flow of glutamate to that of sodium and potassium. Recently three different but related glutamate transporters have been cloned, which have no significant homology to the members of the superfamily.

Identification of domains of a cloned rat brain GABA transporter which are not required for its functional expression

The sodium and chloride coupled gamma-aminobutyric acid (GABA) transporter purified from rat brain, belongs to a superfamily of neurotransmitter transporters. They are involved in the termination of synaptic transmission and are predicted to have 12 membrane spanning alpha-helices with both amino- and carboxyl-termini oriented toward the cytoplasm. In order to define the domains not required for functional expression, we have constructed and expressed a series of deletion mutants in GAT-1, the cDNA clone encoding for the transporter. Transporters truncated at either end until just a few amino-acids distance from the beginning of helix 1 and the end of helix 12, retain their ability to catalyze sodium and chloride-dependent GABA transport.

Only one of the charged amino acids located in the transmembrane alpha-helices of the gamma-aminobutyric acid transporter (subtype A) is essential for its activity

The gamma-aminobutyric acid (GABA) transporter (subtype A) is located in nerve terminals and catalyses coupled electrogenic uptake of the neurotransmitter with two or three sodium and one chloride ions. It contains 599 amino acids and 12 putative membrane spanning alpha-helices and is the first described member of a neurotransmitter transporter superfamily. The membrane domain contains 5 charged amino acids which are basically conserved. Using site-directed mutagenesis, we show that only one of them, arginine 69, is absolutely essential for activity. It is located in a highly conserved region encompassing parts of helices 1 and 2. The three other positively charged amino acids and the only negative charged one, glutamate 467, are not critical. These results suggest that the translocation pathway of the sodium ions through the membrane does not involve charged amino acid residues and underline the importance of the highly conserved stretch between amino acids 66 and 86.

Cloning and expression of a rat brain L-glutamate transporter

Synaptic transmission of most vertebrate synapses is thought to be terminated by rapid transport of the neurotransmitter into presynaptic nerve terminals or neuroglia. L-Glutamate is the major excitatory transmitter in brain and its transport represents the mechanism by which it is removed from the synaptic cleft and kept below toxic levels. Here we use an antibody against a glial L-glutamate transporter from rat brain to isolate a complementary DNA clone encoding this transporter. Expression of this cDNA in transfected HeLa cells indicates that L-glutamate accumulation requires external sodium and internal potassium and transport shows the expected stereospecificity. The cDNA sequence predicts a protein of 573 amino acids with 8-9 putative transmembrane alpha-helices. Database searches indicate that this protein is not homologous to any identified protein of mammalian origin, including the recently described superfamily of neurotransmitter transporters. This protein therefore seems to be a member of a new family of transport molecules.

A monoclonal antibody against a Na(+)-L-glutamate cotransporter from rat brain

A monoclonal mouse IgM antibody (Z8E9) was raised against the Na(+)-L-glutamate cotransporter from rat brain. In a preparation of brain plasma membrane vesicles, Z8E9 binds specifically to a polypeptide with an apparent molecular weight of 70,000 and inhibits Na+ gradient-dependent L-glutamate cotransport (up to 50%) in brain membrane vesicles. In the membrane vesicles, the antibody does not alter the membrane permeability for Na+ and K+ nor the Na+ gradient-dependent uptake of gamma-aminobutyric acid. Kinetic experiments showed that Z8E9 does not alter the K0.5 values for L-glutamate and Na+ activation of L-glutamate transport. However, an apparent cooperativity observed for L-glutamate activation was increased, and the Vmax of L-glutamate transport was decreased. Immunostaining of rat cerebellum identified antigenic sites of Z8E9 in Golgi epithelial cells and astrocytes (by light and electron microscopy), whereas no labeling at nerve terminals was detected. The data suggest that a component of a Na(+)-L-glutamate cotransporter subtype has been identified that is specific for glia cells in brain.

Comparative analysis of sodium-dependent L-glutamate transport of synaptosomal and astroglial membrane vesicles from mouse cortex

Uptake of [3H]L-glutamate into membrane vesicles prepared from either mouse cortical astrocyte cultures or synaptosomes was found to be an electrogenic sodium- and potassium-dependent transport process with saturable uptake kinetics. Pharmacological differences were revealed by using a variety of substrate analogues. L-trans-PDC inhibited the synaptosomal glutamate transport 2-4-fold stronger than the astroglial uptake. The substrate analogues DL-threo-beta-hydroxy-aspartate, DL-aspartate-beta-hydroxamate, L-aspartate and D-aspartate inhibited glutamate transport of astroglial and neuronal membrane vesicles in a distinctive manner, whereas D-glutamate, quisqualate and dihydrokainate had no effect in either case. Immunoblotting and immunocytochemical labeling with antibodies against the rat brain glutamate transporter revealed the selective reaction of a band at about 75 kDa mol. wt. and a specific pattern of astrocyte immunostaining.

An [Na+ + K+]coupled L-glutamate transporter purified from rat brain is located in glial cell processes

Polyclonal antibodies were generated against the major polypeptide (73,000 mol. wt) present in a highly purified preparation of the [Na+ + K+]coupled L-glutamate transporter from rat brain. These antibodies were able to selectively immunoprecipitate the 73,000 mol. wt polypeptide as well as most of the L-glutamate transport activity--as assayed upon reconstitution--from crude detergent extracts of rat brain membranes. The immunoreactivity in the various fractions obtained during the purification procedure [Danbolt et al. (1990) Biochemistry 29, 6734-6740] closely correlated with the L-glutamate transport activity. Immunoblotting of a crude sodium dodecyl sulphate brain extract, separated by two-dimensional isoelectric focusing-sodium dodecyl sulphate-polyacrylamide gel electrophoresis, showed that the antibodies recognized one 73,000 mol. wt protein species only. Deglycosylation of the protein gave a 10,000 reduction in molecular mass, but no reduction in immunoreactivity. These findings establish that the 73,000 mol. wt polypeptide represents the L-glutamate transporter or a subunit thereof. The antibodies also recognize a 73,000 mol. wt polypeptide and immunoprecipitate L-glutamate transport activity in extracts of brain plasma membranes from rabbit, pig, cow, cat and man. Using the antibodies, the immunocytochemical localization of the transporter was studied at the light and electron microscopic levels in rat central nervous system. In all regions examined (including cerebral cortex, caudatoputamen, corpus callosum, hippocampus, cerebellum, spinal cord) it was found to be located in glial cells rather than in neurons. In particular, fine astrocytic processes were strongly stained. Putative glutamatergic axon terminals appeared non-immunoreactive. The uptake of glutamate by such terminals (for which there is strong previous evidence) therefore may be due to a subtype of glutamate transporter different from the glial transporter demonstrated by us.

Neuronal and glial gamma-aminobutyric acid+ transporters are distinct proteins

In the central nervous system, two subtypes of sodium- and chloride-coupled GABA transporter exist. One is sensitive to ACHC, the other to beta-alanine. They are thought to be of neuronal and glial origin, respectively. GABA transport in membrane vesicles derived from astroglial cells was found to be sodium- and chloride-dependent, electrogenic and much more sensitive to beta-alanine than to ACHC. Immunoblotting with antibodies directed against a variety of sequences of the ACHC-sensitive transporter indicated that none of these epitopes was shared by the glial transporter.

Expression of a cloned gamma-aminobutyric acid transporter in mammalian cells

The cDNA clone GAT-1, which encodes a Na(+)- and Cl(-)-coupled GABA transporter from rat brain, has been expressed in mammalian cells using three different systems: (1) transient expression upon transfection of mouse Ltk- cells with a eukaryotic expression vector containing GAT-1; (2) stable expression in L-cells transfected with the same vector; (3) transfection of HeLa cells infected with a recombinant vaccinia virus expressing T7 RNA polymerase. Similar results both qualitatively and quantitatively were obtained with all systems. The GABA transporter expressed in HeLa and L-cells retains all the properties described previously for GABA transport into synaptosomes and synaptic plasma membrane vesicles. It was fully inhibited by cis-3-aminocyclohexanecarboxylic acid (ACHC) and not by beta-alanine. The KM for GABA transport and the IC50 for ACHC inhibition were similar to the presynaptic transporter. Accumulated [3H]GABA was released from transfected cells by dissipating the transmembrane Na+ gradient with nigericin or by exchange with unlabeled external GABA. Accumulation was stimulated by both Na+ and Cl- in the external medium. However, in the absence of external Cl-, a small amount of GABA transport remained which was dependent on GAT-1 transfection. Functional expression of the GABA transporter was abolished by tunicamycin. An antitransporter antibody specifically immunoprecipitates a polypeptide with an apparent molecular mass of about 70 kDa from GAT-1-transfected cells. When cells were grown in the presence of tunicamycin, only a faint band of apparent mass of about 60 kDa was observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Neither amino nor carboxyl termini are required for function of the sodium- and chloride-coupled gamma-aminobutyric acid transporter from rat brain

Antibodies were raised against synthetic peptides corresponding to several regions of the rat brain gamma-aminobutyric acid (GABA) transporter. According to our model, this glycoprotein has 12 transmembrane alpha-helices with both amino and carboxyl termini located in the cytoplasm. The antibodies recognized the intact transporter on Western blots. Upon papain treatment, a reconstitutively active transporter can be isolated upon lectin chromatography (Kanner, B. I., Keynan, S., and Radian, R. (1989) Biochemistry 28, 3722-3728). The papainized transporter runs on sodium dodecyl sulfate-polyacrylamide gels as a broad band with an apparent molecular mass between about 58 and 68 kDa as compared to 80 kDa for the untreated transporter. The transporter fragment was recognized by all the antibodies, except for that raised against the amino terminus. Pronase cleaves the transporter to a relatively sharp 60-kDa band, which reacts with the antibodies against the internal loops but not with either the amino- or the carboxyl-terminal. This pronase-treated transporter, upon isolation by lectin chromatography, was reconstituted. It exhibits full GABA transport activity. This activity exhibits the same features as the intact system including an absolute dependence on sodium and chloride as well as electrogenicity. We conclude that the amino- and carboxyl-terminal parts of the transporter, possibly including transmembrane alpha-helices 1, 2, and 12, are not required for the transport function.

Counterflow of L-glutamate in plasma membrane vesicles and reconstituted preparations from rat brain

Membrane vesicles from rat brain exhibit sodium-dependent uptake of L-[3H]glutamate in the absence of any transmembrane ion gradients. The substrate specificity of the process is identical with (Na+ + K+)-coupled L-glutamate accumulation. Although these vesicles are prepared after osmotic shock and are washed repeatedly, they contain about 1.5 nmol/mg of protein endogenous L-glutamate, apparently located inside the vesicles. The affinity of the process (Km approximately 1 microM) is similar to that of (Na+ + K+)-dependent accumulation by the L-glutamate transporter. Membrane vesicles have been disrupted by the detergent cholate, and the solubilized proteins have been subsequently reconstituted into liposomes. The reconstituted proteoliposomes also exhibit the above uptake--with the same characteristics--provided they contain entrapped cold L-glutamate. Counterflow is optimal when sodium is present on both sides of the membrane, but partial activity is still observed when sodium is present either on the inside or on the outside. Increasing the L-glutamate concentration above the Km results in counterflow completely independent of cis sodium. The initial rate of counterflow is 100-200-fold lower than that of net trans potassium dependent flux. The rate of net flux in the presence of trans sodium or lithium is about 10-fold lower than when choline or Tris are used instead. However, the rate of counterflow (no internal potassium present) was not stimulated by replacing internal sodium or lithium by internal choline. Therefore, optimal functioning of the transporter requires internal potassium while internal sodium and lithium are inhibitory.(ABSTRACT TRUNCATED AT 250 WORDS)

Cloning and expression of a rat brain GABA transporter

A complementary DNA clone (designated GAT-1) encoding a transporter for the neurotransmitter gamma-aminobutyric acid (GABA) has been isolated from rat brain, and its functional properties have been examined in Xenopus oocytes. Oocytes injected with GAT-1 synthetic messenger RNA accumulated [3H]GABA to levels above control values. The transporter encoded by GAT-1 has a high affinity for GABA, is sodium-and chloride-dependent, and is pharmacologically similar to neuronal GABA transporters. The GAT-1 protein shares antigenic determinants with a native rat brain GABA transporter. The nucleotide sequence of GAT-1 predicts a protein of 599 amino acids with a molecular weight of 67 kilodaltons. Hydropathy analysis of the deduced protein suggests multiple transmembrane regions, a feature shared by several cloned transporters; however, database searches indicate that GAT-1 is not homologous to any previously identified proteins. Therefore, GAT-1 appears to be a member of a previously uncharacterized family of transport molecules.

Purification and reconstitution of the sodium- and potassium-coupled glutamate transport glycoprotein from rat brain

The sodium- and potassium-coupled L-glutamate transporter from rat brain has been purified to near homogeneity by reconstitution of transport as an assay, assuming that inactivated and active transporters cochromatograph. The purification steps involve lectin chromatography of the membrane proteins solubilized with 3-[(3-chloramidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS), fractionation on hydroxylapatite, and ion-exchange chromatography. The specific activity is increased 30-fold. The actual purification is higher since 3-5-fold inactivation occurs during the purification. The efficiency of reconstitution was about 20%. The properties of the pure transporter are fully preserved. They include ion dependence, electrogenicity, affinity, substrate specificity, and stereospecificity. Sodium dodecyl sulfate-polyacrylamide electrophoresis revealed one main band with an apparent molecular mass of around 80 kDa and a few minor bands. Comparison of polypeptide composition with L-glutamate transport activity throughout the fractionation procedure reveals that only the 80-kDa band can be correlated with activity. The GABA transporter, which has the same apparent molecular mass (Radian et al., 1986), is separated from it during the last two purification steps. Immunoblot experiments reveal that the antibodies against the GABA transporter only reacted with fractions exhibiting GABA transport activity and not with those containing the glutamate transporter. We conclude that the 80-kDa band represents the functional sodium- and potassium-coupled L-glutamate transporter.

Cholesterol is required for the reconstruction of the sodium- and chloride-coupled, gamma-aminobutyric acid transporter from rat brain

The reconstruction of the purified sodium- and chloride-coupled gamma-aminobutyric acid transporter from rat brain into asolectin liposomes requires the addition of brain lipids (Radian, R., and Kanner, B. I. (1985) J. Biol. Chem. 260, 11859-11865). The reconstitution assay was used to identify the component(s) from brain lipids responsible for the stimulation during the fractionation of brain lipids. The distribution of the active component was found to be similar to that of cholesterol. Furthermore, cholesterol was found to mimic the effect of brain lipids and it stimulated the transport activity up to 20-fold. Optimal reconstituted transport activity was achieved with mixtures of cholesterol and any one of several phospholipids, such as phosphatidylcholine, phosphatidylserine or phosphatidylglycerol. gamma-Aminobutyric acid transport in these liposomes of defined composition exhibited all the properties of the native transporter, such as the absolute dependence on sodium and chloride and electrogenicity. Cholesterol could not be replaced by cholest-4-en-3one and other steroids, and thus its effect is probably not due to effects on membrane fluidity. The requirement was also not due to effects on intactness of the liposomes or incorporation of proteins into them. Furthermore it was found that the reconstitution of the sodium and potassium coupled L-glutamic acid transporter from rat brain also required cholesterol. However, in this case the optimal activity was reached by 4-5-fold lower levels of cholesterol than those necessary for gamma-aminobutyric acid transport. When cholesterol depletion from the transporters was incomplete, addition of exogenous brain lipids was not required. Thus, if the cholesterol was still associated with the transporter proteins, its final concentration, as a fraction of the total lipids present in the reconstitution mixture, was only about 0.01 mol%. Thus, it is likely that the effects of cholesterol are due to direct interactions with the cotransporters and not to an average effect on membrane properties.

Two pharmacologically distinct sodium- and chloride-coupled high-affinity gamma-aminobutyric acid transporters are present in plasma membrane vesicles and reconstituted preparations from rat brain

Electrogenic sodium- and chloride-dependent gamma-aminobutyric acid (GABA) transport in crude synaptosomal membrane vesicles is partly inhibited by saturating levels of either of the substrate analogues cis-3-aminocyclohexanecarboxylic acid (ACHC) or beta-alanine. However, both of them together potently and fully inhibit the process. Transport of beta-alanine, which exhibits an apparent Km of about 44 microM, is also electrogenic and sodium and chloride dependent and competitively inhibited by GABA with a Ki of about 3 microM. This value is very similar to the Km of 2-4 microM found for GABA transport. On the other hand, ACHC does not inhibit beta-alanine transport at all. Upon solubilization of the membrane proteins with cholate and fractionation with ammonium sulfate, a fraction is obtained which upon reconstitution into proteoliposomes exhibits 4- to 10-fold-increased GABA transport. This activity is fully inhibited by low concentrations of ACHC and is not sensitive at all to beta-alanine. GABA transport in this preparation exhibits an apparent Km of about 2.5 microM and it is competitively inhibited by ACHC (Ki approximately 7 microM). These data indicate the presence of two GABA transporter subtypes in the membrane vesicles: the A type, sensitive to ACHC, and the B type, sensitive to beta-alanine.