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

Molecular basis for substrate recognition and transport of human GABA transporter GAT1

γ-Aminobutyric acid (GABA), an important inhibitory neurotransmitter in the central nervous system, is recycled through specific GABA transporters (GATs). GAT1, which is mainly expressed in the presynaptic terminals of axons, is a potential drug target of neurological disorders due to its essential role in GABA transport. Here we report four cryogenic electron microscopy structures of human GAT1, at resolutions of 2.2-3.2 Å. GAT1 in substrate-free form or in complex with the antiepileptic drug tiagabine exhibits an inward-open conformation. In the presence of GABA or nipecotic acid, inward-occluded structures are captured. The GABA-bound structure reveals an interaction network bridged by hydrogen bonds and ion coordination for GABA recognition. The substrate-free structure unwinds the last helical turn of transmembrane helix TM1a to release sodium ions and substrate. Complemented by structure-guided biochemical analyses, our studies reveal detailed mechanism of GABA recognition and transport, and elucidate mode of action of the inhibitors, nipecotic acid and tiagabine.

4-Phenylbutyrate restored γ-aminobutyric acid uptake and reduced seizures in SLC6A1 patient variant-bearing cell and mouse models

We have studied the molecular mechanisms of variants in solute carrier Family 6 Member 1 associated with neurodevelopmental disorders, including various epilepsy syndromes, autism and intellectual disability. Based on functional assays of solute carrier Family 6 Member 1 variants, we conclude that partial or complete loss of γ-amino butyric acid uptake due to reduced membrane γ-amino butyric acid transporter 1 trafficking is the primary aetiology. Importantly, we identified common patterns of the mutant γ-amino butyric acid transporter 1 protein trafficking from biogenesis, oligomerization, glycosylation and translocation to the cell membrane across variants in different cell types such as astrocytes and neurons. We hypothesize that therapeutic approaches to facilitate membrane trafficking would increase γ-amino butyric acid transporter 1 protein membrane expression and function. 4-Phenylbutyrate is a Food and Drug Administration-approved drug for paediatric use and is orally bioavailable. 4-Phenylbutyrate shows promise in the treatment of cystic fibrosis. The common cellular mechanisms shared by the mutant γ-amino butyric acid transporter 1 and cystic fibrosis transmembrane conductance regulator led us to hypothesize that 4-phenylbutyrate could be a potential treatment option for solute carrier Family 6 Member 1 mutations. We examined the impact of 4-phenylbutyrate across a library of variants in cell and knockin mouse models. Because γ-amino butyric acid transporter 1 is expressed in both neurons and astrocytes, and γ-amino butyric acid transporter 1 deficiency in astrocytes has been hypothesized to underlie seizure generation, we tested the effect of 4-phenylbutyrate in both neurons and astrocytes with a focus on astrocytes. We demonstrated existence of the mutant γ-amino butyric acid transporter 1 retaining wildtype γ-amino butyric acid transporter 1, suggesting the mutant protein causes aberrant protein oligomerization and trafficking. 4-Phenylbutyrate increased γ-amino butyric acid uptake in both mouse and human astrocytes and neurons bearing the variants. Importantly, 4-phenylbutyrate alone increased γ-amino butyric acid transporter 1 expression and suppressed spike wave discharges in heterozygous knockin mice. Although the mechanisms of action for 4-phenylbutyrate are still unclear, with multiple possibly being involved, it is likely that 4-phenylbutyrate can facilitate the forward trafficking of the wildtype γ-amino butyric acid transporter 1 regardless of rescuing the mutant γ-amino butyric acid transporter 1, thus increasing γ-amino butyric acid uptake. All patients with solute carrier Family 6 Member 1 variants are heterozygous and carry one wildtype allele, suggesting a great opportunity for treatment development leveraging wildtype protein trafficking. The study opens a novel avenue of treatment development for genetic epilepsy via drug repurposing.

Structural basis of GABA reuptake inhibition

γ-Aminobutyric acid (GABA) transporter 1 (GAT1)¹ regulates neuronal excitation of the central nervous system by clearing the synaptic cleft of the inhibitory neurotransmitter GABA upon its release from synaptic vesicles. Elevating the levels of GABA in the synaptic cleft, by inhibiting GABA reuptake transporters, is an established strategy to treat neurological disorders, such as epilepsy². Here we determined the cryo-electron microscopy structure of full-length, wild-type human GAT1 in complex with its clinically used inhibitor tiagabine³, with an ordered part of only 60 kDa. Our structure reveals that tiagabine locks GAT1 in the inward-open conformation, by blocking the intracellular gate of the GABA release pathway, and thus suppresses neurotransmitter uptake. Our results provide insights into the mixed-type inhibition of GAT1 by tiagabine, which is an important anticonvulsant medication. Its pharmacodynamic profile, confirmed by our experimental data, suggests initial binding of tiagabine to the substrate-binding site in the outward-open conformation, whereas our structure presents the drug stalling the transporter in the inward-open conformation, consistent with a two-step mechanism of inhibition⁴. The presented structure of GAT1 gives crucial insights into the biology and pharmacology of this important neurotransmitter transporter and provides blueprints for the rational design of neuromodulators, as well as moving the boundaries of what is considered possible in single-particle cryo-electron microscopy of challenging membrane proteins.

Common molecular mechanisms of SLC6A1 variant-mediated neurodevelopmental disorders in astrocytes and neurons

Solute carrier family 6 member 1 (SLC6A1) is abundantly expressed in the developing brain even before the CNS is formed. Its encoded GABA transporter 1 (GAT-1) is responsible for the reuptake of GABA into presynaptic neurons and glia, thereby modulating neurotransmission. GAT-1 is expressed globally in the brain, in both astrocytes and neurons. The GABA uptake function of GAT-1 in neurons cannot be compensated for by other GABA transporters, while the function in glia can be partially replaced by GABA transporter 3. Recently, many variants in SLC6A1 have been associated with a spectrum of epilepsy syndromes and neurodevelopmental disorders, including myoclonic atonic epilepsy, childhood absence epilepsy, autism, and intellectual disability, but the pathomechanisms associated with these phenotypes remain unclear. The presence of GAT-1 in both neurons and astrocytes further obscures the role of abnormal GAT-1 in the heterogeneous disease phenotype manifestations. Here we examine the impact on transporter trafficking and function of 22 SLC6A1 variants identified in patients with a broad spectrum of phenotypes. We also evaluate changes in protein expression and subcellular localization of the variant GAT-1 in various cell types, including neurons and astrocytes derived from human patient induced pluripotent stem cells. We found that a partial or complete loss-of-function represents a common disease mechanism, although the extent of GABA uptake reduction is variable. The reduced GABA uptake appears to be due to reduced cell surface expression of the variant transporter caused by variant protein misfolding, endoplasmic reticulum retention, and subsequent degradation. Although the extent of reduction of the total protein, surface protein, and the GABA uptake level of the variant transporters is variable, the loss of GABA uptake function and endoplasmic reticulum retention is consistent across induced pluripotent stem cell-derived cell types, including astrocytes and neurons, for the surveyed variants. Interestingly, we did not find a clear correlation of GABA uptake function and the disease phenotypes, such as myoclonic atonic epilepsy versus developmental delay, in this study. Together, our study suggests that impaired transporter protein trafficking and surface expression are the major disease-associated mechanisms associated with pathogenic SLC6A1 variants. Our results resemble findings from pathogenic variants in other genes affecting the GABA pathway, such as GABAA receptors. This study provides critical insight into therapeutic developments for SLC6A1 variant-mediated disorders and implicates that boosting transporter function by either genetic or pharmacological approaches would be beneficial.

Genetic mosaicism, intrafamilial phenotypic heterogeneity, and molecular defects of a novel missense SLC6A1 mutation associated with epilepsy and ADHD

BACKGROUND

Mutations in SLC6A1, encoding γ-aminobutyric acid (GABA) transporter 1 (GAT-1), have been recently associated with a spectrum of neurodevelopmental disorders ranging from variable epilepsy syndromes, intellectual disability (ID), autism and others. To date, most identified mutations are de novo. We here report a pedigree of two siblings associated with myoclonic astatic epilepsy, attention deficit hyperactivity disorder (ADHD), and ID.

METHODS

Next-generation sequencing identified a missense mutation in the SLC6A1 gene (c.373G > A(p.Val125Met)) in the sisters but not in their shared mother who is also asymptomatic, suggesting gonadal mosaicism. We have thoroughly characterized the clinical phenotypes: EEG recordings identified features for absence seizures and prominent bursts of occipital intermittent rhythmic delta activity (OIRDA). The molecular pathophysiology underlying the clinical phenotypes was assessed using a multidisciplinary approach including machine learning, confocal microscopy, and high-throughput ³H radio-labeled GABA uptake assays in mouse astrocytes and neurons.

RESULTS

The GAT-1(Val125Met) mutation destabilizes the global protein conformation and reduces transporter protein expression at total and cell surface. The mutant transporter protein was localized intracellularly inside the endoplasmic reticulum (ER) in both HEK293T cells and astrocytes which may directly contribute to seizures in patients. Radioactive ³H-labeled GABA uptake assay indicated the mutation reduced the function of the mutant GAT-1(Val125Met) to ~30% of the wildtype.

CONCLUSIONS

The seizure phenotypes, ADHD, and impaired cognition are likely caused by a partial loss-of-function of GAT-1 due to protein destabilization resulting from the mutation. Reduced GAT-1 function in astrocytes and neurons may consequently alter brain network activities such as increased seizures and reduced attention.

Forty Four Years With Baruch Kanner and The Chloride Ion

Baruch Kanner and this author have had parallel careers investigating neurotransmitter transporters. At multiple times during their careers, they have found themselves collaborating or competing, but always learning from each other. This commentary elaborates on the interactions between the Kanner and Rudnick laboratories, with a focus on transporters in the Neurotransmitter: Sodium Symporter (NSS) family of amino acid and amine transporters. A key focus of these interactions is the mechanism by which chloride ions activate and drive transport.

Endoplasmic reticulum retention and degradation of a mutation in SLC6A1 associated with epilepsy and autism

Mutations in SLC6A1, encoding γ-aminobutyric acid (GABA) transporter 1 (GAT-1), have been recently associated with a spectrum of epilepsy syndromes, intellectual disability and autism in clinic. However, the pathophysiology of the gene mutations is far from clear. Here we report a novel SLC6A1 missense mutation in a patient with epilepsy and autism spectrum disorder and characterized the molecular defects of the mutant GAT-1, from transporter protein trafficking to GABA uptake function in heterologous cells and neurons. The heterozygous missense mutation (c1081C to A (P361T)) in SLC6A1 was identified by exome sequencing. We have thoroughly characterized the molecular pathophysiology underlying the clinical phenotypes. We performed EEG recordings and autism diagnostic interview. The patient had neurodevelopmental delay, absence epilepsy, generalized epilepsy, and 2.5–3 Hz generalized spike and slow waves on EEG recordings. The impact of the mutation on GAT-1 function and trafficking was evaluated by ³H GABA uptake, structural simulation with machine learning tools, live cell confocal microscopy and protein expression in mouse neurons and nonneuronal cells. We demonstrated that the GAT-1(P361T) mutation destabilizes the global protein conformation and reduces total protein expression. The mutant transporter protein was localized intracellularly inside the endoplasmic reticulum (ER) with a pattern of expression very similar to the cells treated with tunicamycin, an ER stress inducer. Radioactive ³H-labeled GABA uptake assay indicated the mutation reduced the function of the mutant GAT-1(P361T), to a level that is similar to the cells treated with GAT-1 inhibitors. In summary, this mutation destabilizes the mutant transporter protein, which results in retention of the mutant protein inside cells and reduction of total transporter expression, likely via excessive endoplasmic reticulum associated degradation. This thus likely causes reduced functional transporter number on the cell surface, which then could cause the observed reduced GABA uptake function. Consequently, malfunctioning GABA signaling may cause altered neurodevelopment and neurotransmission, such as enhanced tonic inhibition and altered cell proliferation in vivo. The pathophysiology due to severely impaired GAT-1 function may give rise to a wide spectrum of neurodevelopmental phenotypes including autism and epilepsy.

γ-Aminobutyric acid transporters as relevant biological target: Their function, structure, inhibitors and role in the therapy of different diseases

γ-Aminobutyric acid (GABA) is a major inhibitory neurotransmitter in the nervous system. It plays a crucial role in many physiological processes. Upon release from the presynaptic element, it is removed from the synaptic cleft by reuptake due to the action of GABA transporters (GATs). GATs belong to a large SLC6 protein family whose characteristic feature is sodium-dependent relocation of neurotransmitters through the cell membrane. GABA transporters are characterized in many contexts, but their spatial structure is not fully known. They are divided into four types, which differ in occurrence and role. Herein, the special attention was paid to these transporting proteins. This comprehensive review presents the current knowledge about GABA transporters. Their distribution in the body, physiological functions and possible utilization in the therapy of different diseases were fully discussed. The important structural features were described based on published data, including sequence analysis, mutagenesis studies, and comparison with known SLC6 transporters for leucine (LeuT), dopamine (DAT) and serotonin (SERT). Moreover, the most important inhibitors of GABA transporters of various basic scaffolds, diverse selectivity and potency were presented.

A Reappraisal of GAT-1 Localization in Neocortex

γ-Aminobutyric acid (GABA) transporter (GAT)-1, the major GABA transporter in the brain, plays a key role in modulating GABA signaling and is involved in the pathophysiology of several neuropsychiatric diseases, including epilepsy. The original description of GAT-1 as a neuronal transporter has guided the interpretation of the findings of all physiological, pharmacological, genetic, or clinical studies. However, evidence published in the past few years, some of which is briefly reviewed herein, does not seem to be consistent with a neurocentric view of GAT-1 function and calls for more detailed analysis of its localization. We therefore performed a thorough systematic assessment of GAT-1 localization in neocortex and subcortical white matter. In line with earlier work, we found that GAT-1 was robustly expressed in axon terminals forming symmetric synapses and in astrocytic processes, whereas its astrocytic expression was more diffuse than expected and, even more surprisingly, immature and mature oligodendrocytes and microglial cells also expressed the transporter. These data indicate that the era of “neuronal” and “glial” GABA transporters has finally come to a close and provide a wider perspective from which to view GABA-mediated physiological phenomena. In addition, given the well-known involvement of astrocytes, oligodendrocytes, and microglial cells in physiological as well as pathological conditions, the demonstration of functional GAT-1 in these cells is expected to provide greater insight into the phenomena occurring in the diseased brain as well as to prompt a reassessment of earlier findings.

A missense mutation in SLC6A1 associated with Lennox-Gastaut syndrome impairs GABA transporter 1 protein trafficking and function

BACKGROUND

Mutations in SLC6A1 have been associated mainly with myoclonic atonic epilepsy (MAE) and intellectual disability. We identified a novel missense mutation in a patient with Lennox-Gastaut syndrome (LGS) characterized by severe seizures and developmental delay.

METHODS

Exome Sequencing was performed in an epilepsy patient cohort. The impact of the mutation was evaluated by ³H γ-aminobutyric acid (GABA) uptake, structural modeling, live cell microscopy, cell surface biotinylation and a high-throughput assay flow cytometry in both neurons and non neuronal cells.

RESULTS

We discovered a heterozygous missense mutation (c700G to A [pG234S) in the SLC6A1 encoding GABA transporter 1 (GAT-1). Structural modeling suggests the mutation destabilizes the global protein conformation. With transient expression of enhanced yellow fluorescence protein (YFP) tagged rat GAT-1 cDNAs, we demonstrated that the mutant GAT-1(G234S) transporter had reduced total protein expression in both rat cortical neurons and HEK 293 T cells. With a high-throughput flow cytometry assay and live cell surface biotinylation, we demonstrated that the mutant GAT-1(G234S) had reduced cell surface expression. ³H radioactive labeling GABA uptake assay in HeLa cells indicated a reduced function of the mutant GAT-1(G234S).

CONCLUSIONS

This mutation caused instability of the mutant transporter protein, which resulted in reduced cell surface and total protein levels. The mutation also caused reduced GABA uptake in addition to reduced protein expression, leading to reduced GABA clearance, and altered GABAergic signaling in the brain. The impaired trafficking and reduced GABA uptake function may explain the epilepsy phenotype in the patient.

Internal gate mutants of the GABA transporter GAT1 are capable of substrate exchange

GAT1 is a member of the neurotransmitter:sodium: symporter family and mediates transport of GABA together with sodium and chloride in an electrogenic process enabling efficient synaptic transmission. Biochemical and modelling studies based on the structure of the bacterial homologue LeuT are consistent with a transport mechanism whereby the binding pocket is alternately accessible to either side of the membrane. This is achieved by the sequential opening and closing of extracellular and intracellular gates. The amino acid residues participating in the formation of these gates are highly conserved within the neurotransmitter:sodium: symporter family. Net flux requires that the gating mechanism is operative regardless if the binding pocket is loaded with substrate or empty. On the other hand, exchange of labelled for non-labelled substrate across the membrane only requires gating in the presence of substrate. To address the question if the gating requirements of the substrate-bound and empty transporters are similar or different, we analyzed the impact of mutation of intra- and extra-cellular gate residues on net GABA influx and on exchange by liposomes inlaid with the mutant transporters. Whereas net flux by all four internal gate mutants tested was severely abrogated, each exhibited significant levels of exchange. In contrast, two external gate mutants were impaired in both processes. Our results indicate that perturbation of the internal gate of GAT1 selectively impairs the gating mechanism of the empty transporter. This article is part of the issue entitled 'Special Issue on Neurotransmitter Transporters'.

SLC6A1 variants identified in epilepsy patients reduce γ-aminobutyric acid transport

Previous reports have identified SLC6A1 variants in patients with generalized epilepsies, such as myoclonic-atonic epilepsy and childhood absence epilepsy. However, to date, none of the identified SLC6A1 variants has been functionally tested for an effect on GAT-1 transporter activity. The purpose of this study was to determine the incidence of SLC6A1 variants in 460 unselected epilepsy patients and to evaluate the impact of the identified variants on γ-aminobutyric acid (GABA)transport. Targeted resequencing was used to screen 460 unselected epilepsy patients for variants in SLC6A1. Five missense variants, one in-frame deletion, one nonsense variant, and one intronic splice-site variant were identified, representing a 1.7% diagnostic yield. Using a [³ H]-GABA transport assay, the seven identified exonic variants were found to reduce GABA transport activity. A minigene splicing assay revealed that the splice-site variant disrupted canonical splicing of exon 9 in the mRNA transcript, leading to premature protein truncation. These findings demonstrate that SLC6A1 is an important contributor to childhood epilepsy and that reduced GAT-1 function is a common consequence of epilepsy-causing SLC6A1 variants.

Both reentrant loops of the sodium-coupled glutamate transporters contain molecular determinants of cation selectivity

In the brain, glutamate transporters terminate excitatory neurotransmission by removing this neurotransmitter from the synapse via cotransport with three sodium ions into the surrounding cells. Structural studies have identified the binding sites of the three sodium ions in glutamate transporters. The residue side-chains directly interact with the sodium ions at the Na1 and Na3 sites and are fully conserved from archaeal to eukaryotic glutamate transporters. The Na2 site is formed by three main-chain oxygens on the extracellular reentrant hairpin loop HP2 and one on transmembrane helix 7. A glycine residue on HP2 is located closely to the three main-chain oxygens in all glutamate transporters, except for the astroglial transporter GLT-1, which has a serine residue at that position. Unlike for WT GLT-1, substitution of the serine residue to glycine enables sustained glutamate transport also when sodium is replaced by lithium. Here, using functional and simulation studies, we studied the role of this serine/glycine switch on cation selectivity of substrate transport. Our results indicate that the side-chain oxygen of the serine residues can form a hydrogen bond with a main-chain oxygen on transmembrane helix 7. This leads to an expansion of the Na2 site such that water can participate in sodium coordination at Na2. Furthermore, we found other molecular determinants of cation selectivity on the nearby HP1 loop. We conclude that subtle changes in the composition of the two reentrant hairpin loops determine the cation specificity of acidic amino acid transport by glutamate transporters.

An Extra Amino Acid Residue in Transmembrane Domain 10 of the γ-Aminobutyric Acid (GABA) Transporter GAT-1 Is Required for Efficient Ion-coupled Transport

The GABA transporter GAT-1 mediates electrogenic transport of its substrate together with sodium and chloride. It is a member of the neurotransmitter:sodium:symporters, which are crucial for synaptic transmission. Compared with all other neurotransmitter:sodium:symporters, GAT-1 and other members of the GABA transporter subfamily all contain an extra amino acid residue at or near a conserved glycine in transmembrane segment 10. Therefore, we studied the functional impact of deletion and replacement mutants of Gly-457 and its two adjacent residues in GAT-1. The glycine replacement mutants were devoid of transport activity, but remarkably the deletion mutant was active, as were mutants obtained by deleting positions on either side of Gly-457. However, the inward rectification of GABA-induced transport currents by all three deletion mutants was diminished, and the charge-to-flux ratio was increased by more than 2.5-fold, both of which indicate substantial uncoupled transport. These observations suggest that the deletions render the transporters less tightly packed. Consistent with this interpretation, the inactive G457A mutant was partially rescued by removing the adjacent serine residue. Moreover, the activity of several gating mutants was also partially rescued upon deletion of Gly-457. Structural modeling showed that the stretch surrounding Gly-457 is likely to form a π-helix. Our data indicate that the "extra" residue in transmembrane domain 10 of the GABA transporter GAT-1 provides extra bulk, probably in the form of a π-helix, which is required for stringent gating and tight coupling of ion and substrate fluxes in the GABA transporter family.

GAT-1 mediated GABA uptake in rat oligodendrocytes

Stimulated by the results of a recent paper on the effects of tiagabine, a selective inhibitor of the main GABA transporter GAT-1, on oligodendrogenesis, we verified the possibility that GAT-1 may be expressed in oligodendrocytes using immunocytochemical methods and functional assays. Light microscopic analysis of the subcortical white matter of all animals revealed the presence of numerous GAT-1+ cells of different size (from 3 to 29 µm) and morphology. An electron microscope analysis revealed that, besides fibrous astrocytes and interstitial neurons, GAT-1 immunoreactivity was present in immature and mature oligodendrocytes. Co-localization studies between GAT-1 and markers specific for oligodendrocytes (NG2 and RIP) showed that about 12% of GAT-1 positive cells in the white matter were immature oligodendrocytes, while about 15% were mature oligodendrocytes. In vitro functional assays showed that oligodendrocytes exhibit tiagabine-sensitive Na⁺ -dependent GABA uptake. Although relationships between GABA and oligodendrocytes have been known for many years, this is the first demonstration that GAT-1 is expressed in oligodendrocytes. The present results on the one hand definitely closes the era of "neuronal" and "glial" GABA transporters, on the other they suggest that oligodendrocytes may contribute to pathophysiology of the several diseases in which GAT-1 have been implicated to date. GLIA 2017;65:514-522.

Revised Ion/Substrate Coupling Stoichiometry of GABA Transporters

The purpose of this review is to highlight recent evidence in support of a 3 Na⁺: 1 Cl^-: 1 GABA coupling stoichiometry for plasma membrane GABA transporters (SLC6A1 , SLC6A11 , SLC6A12 , SLC6A13 ) and how the revised stoichiometry impacts our understanding of the contribution of GABA transporters to GABA homeostasis in synaptic and extrasynaptic regions in the brain under physiological and pathophysiological states. Recently, our laboratory probed the GABA transporter stoichiometry by analyzing the results of six independent measurements, which included the shifts in the thermodynamic transporter reversal potential caused by changes in the extracellular Na⁺, Cl^-, and GABA concentrations, as well as the ratio of charge flux to substrate flux for Na⁺, Cl^-, and GABA under voltage-clamp conditions. The shifts in the transporter reversal potential for a tenfold change in the external concentration of Na⁺, Cl^-, and GABA were 84 ± 4, 30 ± 1, and 29 ± 1 mV, respectively. Charge flux to substrate flux ratios were 0.7 ± 0.1 charges/Na⁺, 2.0 ± 0.2 charges/Cl^-, and 2.1 ± 0.1 charges/GABA. We then compared these experimental results with the predictions of 150 different transporter stoichiometry models, which included 1-5 Na⁺, 0-5 Cl^-, and 1-5 GABA per transport cycle. Only the 3 Na⁺: 1 Cl^-: 1 GABA stoichiometry model correctly predicts the results of all six experimental measurements. Using the revised 3 Na⁺: 1 Cl^-: 1 GABA stoichiometry, we propose that the GABA transporters mediate GABA uptake under most physiological conditions. Transporter-mediated GABA release likely takes place under pathophysiological or extreme physiological conditions.

Investigating substrate-induced motion between the scaffold and transport domains in the glutamate transporter EAAT1

Excitatory amino acid transporter 1 plays an important role in keeping the synaptic glutamate concentration below neurotoxic levels by translocating this neurotransmitter into the cell. Both reentrant hairpin loops, HP1 and -2, have been shown to take part in binding the substrate and the more deeply buried sodium ion, and might therefore be a part of the intra- or extracellular gate of the transporter. However, the shape of the motion of either loop relative to transmembrane domain (TM) 4 during the transport cycle has not yet been fully resolved. Using copper(II) (1,10-phenanthroline)3 (CuPh) for cross-linking cysteine pairs, we found strong inhibition of transport when A243C (TM4) was combined with S366C (HP1), I453C (HP2), or T456C (HP2). These findings were reinforced by the impact of cadmium on transport activity, and both approaches consistently showed that proximity was exclusively intramonomeric. Under conditions that promote the inward-facing state, inhibition by CuPh in A243C/S366C was reduced, while the opposite was seen when the outward-facing one was stabilized, suggesting that the two positions are farther apart in the former conformation than in the latter. Surprisingly, maximal cross-linking of A243C with I453C or T456C was not observed under conditions that promote the inward-facing state. Altogether, our data suggest that the transporter may undergo complex relative movement between these positions on TM4 and HP1/HP2 during the transport cycle.

Conformationally sensitive proximity of extracellular loops 2 and 4 of the γ-aminobutyric acid (GABA) transporter GAT-1 inferred from paired cysteine mutagenesis

The sodium- and chloride-coupled GABA transporter GAT-1 is a member of the neurotransmitter:sodium:symporters, which are crucial for synaptic transmission. Structural work on the bacterial homologue LeuT suggests that extracellular loop 4 closes the extracellular solvent pathway when the transporter becomes inward-facing. To test whether this model can be extrapolated to GAT-1, cysteine residues were introduced at positions 359 and 448 of extracellular loop 4 and transmembrane helix 10, respectively. Treatment of HeLa cells, expressing the double cysteine mutant S359C/K448C with the oxidizing reagent copper(II)(1,10-phenantroline)3, resulted in a significant inhibition of [(3)H]GABA transport. However, transport by the single cysteine mutant S359C was also inhibited by the oxidant, whereas its activity was almost 4-fold stimulated by dithiothreitol. Both effects were attenuated when the conserved cysteine residues, Cys-164 and/or Cys-173, were replaced by serine. These cysteines are located in extracellular loop 2, the role of which in the structure and function of the eukaryotic neurotransmitter:sodium:symporters remains unknown. The inhibition of transport of S359C by the oxidant was markedly reduced under conditions expected to increase the proportion of inward-facing transporters, whereas the reactivity of the mutants to a membrane-impermeant sulfhydryl reagent was not conformationally sensitive. Our data suggest that extracellular loops 2 and 4 come into close proximity to each other in the outward-facing conformation of GAT-1.

The aromatic and charge pairs of the thin extracellular gate of the γ-aminobutyric acid transporter GAT-1 are differently impacted by mutation

GAT-1 is a sodium- and chloride-coupled GABA transporter and a member of the neurotransmitter:sodium:symporters, which are crucial for synaptic transmission. The structure of bacterial homologue LeuT shows a thin extracellular gate consisting of a charge and an aromatic pair. Here we addressed the question of whether mutation of the aromatic and charge pair residues of GAT-1 has similar consequences. In contrast to charge pair mutants, significant radioactive GABA transport was retained by mutants of the aromatic pair residue Phe-294. Moreover, the magnitude of maximal transport currents induced by GABA by these mutants was comparable with those by wild type GAT-1. However, the apparent affinity of the nonconserved mutants for GABA was reduced up to 20-fold relative to wild type. The voltage dependence of the sodium-dependent transient currents of the Phe-294 mutants was similar to that of the wild type. On the other hand, the conserved charge pair mutant D451E exhibited a right-shifted voltage dependence, indicating an increased apparent affinity for sodium. In further contrast to D451E, whereas the extracellular aqueous accessibility of an endogenous cysteine residue to a membrane-impermeant sulfhydryl reagent was increased relative to wild type, this was not the case for the aromatic pair mutants. Our data indicate that, in contrast to the charge pair, the aromatic pair is not essential for gating. Instead they are compatible with the idea that they serve to diminish dissociation of the substrate from the binding pocket.

Cysteine mutagenesis reveals alternate proximity between transmembrane domain 2 and hairpin loop 1 of the glutamate transporter EAAT1

Excitatory amino acid transporter 1 (EAAT1) plays an important role in restricting the neurotoxicity of glutamate. Previous structure-function studies have provided evidence that reentrant helical hairpin loop (HP) 1 has predominant function during the transport cycle. The proposed internal gate HP1 is packed against transmembrane domain (TM) 2 and TM5 in its closed state, and two residues located in TM2 and HP2 of EAAT1 are in close proximity. However, the spatial relationship between TM2 and HP1 during the transport cycle remains unknown. In this study, we used chemical cross-linking of introduced cysteine pair (V96C and S366C) in a cysteine-less version of EAAT1 to assess the proximity of TM2 and HP1. Here, we show that inhibition of transport by copper(II)(1,10-phenanthroline)3 (CuPh) and cadmium ion (Cd(2+)) were observed in the V96C/S366C mutant. Glutamate or potassium significantly protected against the inhibition of transport activity of V96C/S366C by CuPh, while TBOA potentiated the inhibition of transport activity of V96C/S366C by CuPh. We also checked the kinetic parameters of V96C/S366C treated with or without CuPh in the presence of NaCl, NaCl + L-glutamate, NaCl + TBOA, and KCl, respectively. The sensitivity of V96C and S366C to membrane-impermeable sulfhydryl reagent MTSET [(2-trimethylammonium) methanethiosulfonate] was attenuated by glutamate or potassium. TBOA had no effect on the sensitivity of V96C and S366C to MTSET. These data suggest that the spatial relationship between Val-96 of TM2 and Ser-366 of HP1 is altered in the transport cycle.

Disulfide cross-linking of transport and trimerization domains of a neuronal glutamate transporter restricts the role of the substrate to the gating of the anion conductance

Excitatory amino acid transporters remove synaptically released glutamate and maintain its concentrations below neurotoxic levels. EAATs also mediate a thermodynamically uncoupled substrate-gated anion conductance that may modulate cell excitability. A structure of an archeal homologue, which reflects an early intermediate on the proposed substrate translocation path, has been suggested to be similar to an anion conducting conformation. To probe this idea by functional studies, we have introduced two cysteine residues in the neuronal glutamate transporter EAAC1 at positions predicted to be close enough to form a disulfide bond only in outward-facing and early intermediate conformations of the homologue. Upon treatment of Xenopus laevis oocytes expressing the W441C/K269C double mutant with dithiothreitol, radioactive transport was stimulated >2-fold but potently inhibited by low micromolar concentrations of the oxidizing reagent copper(II)(1,10-phenanthroline)3. The substrate-induced currents by the untreated double mutant, reversed at approximately -20 mV, close to the reversal potential of chloride, but treatment with dithiothreitol resulted in transport currents with the same voltage dependence as the wild type. It appears therefore that in the oocyte expression system the introduced cysteine residues in many of the mutant transporters are already cross-linked and are only capable of mediating the substrate-gated anion conductance. Reduction of the disulfide bond now allows these transporters to execute the full transport cycle. Our functional data support the idea that the anion conducting conformation of the neuronal glutamate transporter is associated with an early step of the transport cycle.

Functional defects in the external and internal thin gates of the γ-aminobutyric acid (GABA) transporter GAT-1 can compensate each other

The GABA transporter GAT-1 belongs to the neurotransmitter:sodium:symporters which are crucial for synaptic transmission. GAT-1 mediates electrogenic transport of GABA together with sodium and chloride. Structure-function studies indicate that the bacterial homologue LeuT, which possess extra- and intracellular thin gates, is an excellent model for this class of neurotransmitter transporters. We recently showed that a conserved aspartate residue of GAT-1, Asp-451, whose LeuT equivalent participates in its thin extracellular gate, is functionally irreplaceable in GAT-1. Only the D451E mutant exhibited residual transport activity but with an elevated apparent sodium affinity as a consequence of an increased proportion of outward-facing transporters. Because during transport the opening and closing of external and internal gates should be tightly coupled, we have addressed the question of whether mutations of the intracellular thin gate residues Arg-44 and Asp-410 can compensate for the effects of their extracellular counterparts. Mutation of Asp-410 to glutamate resulted in impaired transport activity and a reduced apparent affinity for sodium. However, the transport activity of the double mutant D410E/D451E was increased by approximately 10-fold of that of each of the single mutants. Similar compensatory effects were also seen when other combinations of intra- and extracellular thin gate mutants were analyzed. Moreover, the introduction of D410E into the D451E background resulted in lower apparent sodium affinity than that of D451E alone. Our results indicate that a functional interaction of the external and internal gates of GAT-1 is essential for transport.

The accessibility in the external part of the TM5 of the glutamate transporter EAAT1 is conformationally sensitive during the transport cycle

Background

Excitatory amino acid transporter 1 (EAAT1) is a glutamate transporter which is a key element in the termination of the synaptic actions of glutamate. It serves to keep the extracellular glutamate concentration below neurotoxic level. However the functional significance and the change of accessibility of residues in transmembrane domain (TM) 5 of the EAAT1 are not clear yet.

Methodology/Principal Findings

We used cysteine mutagenesis with treatments with membrane-impermeable sulfhydryl reagent MTSET [(2-trimethylammonium) methanethiosulfonate] to investigate the change of accessibility of TM5. Cysteine mutants were introduced from position 291 to 300 of the cysteine-less version of EAAT1. We checked the activity and kinetic parameters of the mutants before and after treatments with MTSET, furthermore we analyzed the effect of the substrate and blocker on the inhibition of the cysteine mutants by MTSET. Inhibition of transport by MTSET was observed in the mutants L296C, I297C and G299C, while the activity of K300C got higher after exposure to MTSET. V_max of L296C and G299C got lower while that of K300C got higher after treated by MTSET. The L296C, G299C, K300C single cysteine mutants showed a conformationally sensitive reactivity pattern. The sensitivity of L296C to MTSET was potentiated by glutamate and TBOA,but the sensitivity of G299C to MTSET was potentiated only by TBOA.

Conclusions/Significance

All these facts suggest that the accessibility of some positions of the external part of the TM5 is conformationally sensitive during the transport cycle. Our results indicate that some residues of TM5 take part in the transport pathway during the transport cycle.

An acidic amino acid transmembrane helix 10 residue conserved in the neurotransmitter:sodium:symporters is essential for the formation of the extracellular gate of the γ-aminobutyric acid (GABA) transporter GAT-1

GAT-1 mediates transport of GABA together with sodium and chloride in an electrogenic process enabling efficient GABAergic transmission. Biochemical and modeling studies based on the structure of the bacterial homologue LeuT are consistent with a mechanism whereby the binding pocket is alternately accessible to either side of the membrane and which predicts that the extracellular part of transmembrane domain 10 (TM10) exhibits aqueous accessibility in the outward-facing conformation only. In this study we have engineered cysteine residues in the extracellular half of TM10 of GAT-1 and probed their state-dependent accessibility to sulfhydryl reagents. In three out of four of the accessible cysteine mutants, the inhibition of transport by a membrane impermeant sulfhydryl reagent was diminished under conditions expected to increase the proportion of inward-facing transporters, such as the presence of GABA together with the cotransported ions. A conserved TM10 aspartate residue, whose LeuT counterpart participates in a "thin" extracellular gate, was found to be essential for transport and only the D451E mutant exhibited residual transport activity. D451E exhibited robust sodium-dependent transient currents with a voltage-dependence indicative of an increased apparent affinity for sodium. Moreover the accessibility of an endogenous cysteine to a membrane impermeant sulfhydryl reagent was enhanced by the D451E mutation, suggesting that sodium binding promotes an outward-facing conformation of the transporter. Our results support the idea that TM10 of GAT-1 lines an accessibility pathway from the extracellular space into the binding pocket and plays a role in the opening and closing of the extracellular transporter gate.

A conserved aspartate residue located at the extracellular end of the binding pocket controls cation interactions in brain glutamate transporters

In the brain, transporters of the major excitatory neurotransmitter glutamate remove their substrate from the synaptic cleft to allow optimal glutamatergic neurotransmission. Their transport cycle consists of two sequential translocation steps, namely cotransport of glutamic acid with three Na(+) ions, followed by countertransport of K(+). Recent studies, based on several crystal structures of the archeal homologue Glt(Ph), indicate that glutamate translocation occurs by an elevator-like mechanism. The resolution of these structures was not sufficiently high to unambiguously identify the sites of Na(+) binding, but functional and computational studies suggest some candidate sites. In the Glt(Ph) structure, a conserved aspartate residue (Asp-390) is located adjacent to a conserved tyrosine residue, previously shown to be a molecular determinant of ion selectivity in the brain glutamate transporter GLT-1. In this study, we characterize mutants of Asp-440 of the neuronal transporter EAAC1, which is the counterpart of Asp-390 of Glt(Ph). Except for substitution by glutamate, this residue is functionally irreplaceable. Using biochemical and electrophysiological approaches, we conclude that although D440E is intrinsically capable of net flux, this mutant behaves as an exchanger under physiological conditions, due to increased and decreased apparent affinities for Na(+) and K(+), respectively. Our present and previous data are compatible with the idea that the conserved tyrosine and aspartate residues, located at the external end of the binding pocket, may serve as a transient or stable cation binding site in the glutamate transporters.

Proximity of transmembrane segments 5 and 8 of the glutamate transporter GLT-1 inferred from paired cysteine mutagenesis

Background

GLT-1 is a glial glutamate transporter which maintains low synaptic concentrations of the excitatory neurotransmitter enabling efficient synaptic transmission. Based on the crystal structure of the bacterial homologue Glt_Ph, it has been proposed that the reentrant loop HP2, which connects transmembrane domains (TM) 7 and 8, moves to open and close access to the binding pocket from the extracellular medium. However the conformation change between TM5 and TM8 during the transport cycle is not clear yet. We used paired cysteine mutagenesis in conjunction with treatments with Copper(II)(1,10-Phenanthroline)₃ (CuPh), to verify the predicted proximity of residues located at these structural elements of GLT-1.

Methodology/Principal Findings

To assess the proximity of transmembrane domain (TM) 5 relative to TM8 during transport by the glial glutamate transporter GLT-1/EAAT2, cysteine pairs were introduced at the extracellular ends of these structural elements. A complete inhibition of transport by Copper(II)(1,10-Phenanthroline)₃ is observed in the double mutants I295C/I463C and G297C/I463C, but not in the corresponding single mutants. Glutamate and potassium, both expected to increase the proportion of inward-facing transporters, significantly protected against the inhibition of transport activity of I295C/I463C and G297C/I463C by CuPh. Transport by the double mutants I295C/I463C and G297C/I463C also was inhibited by Cd²⁺.

Conclusions/Significance

Our results suggest that TM5 (Ile-295, Gly-297) is in close proximity to TM8 (Ile-463) in the mammalian transporter, and that the spatial relationship between these domains is altered during the transport cycle.

A glutamine residue conserved in the neurotransmitter:sodium:symporters is essential for the interaction of chloride with the GABA transporter GAT-1

Neurotransmitter:sodium symporters are crucial for efficient synaptic transmission. The transporter GAT-1 mediates electrogenic cotransport of GABA, sodium, and chloride. The presence of chloride enables the transporter to couple the transport of the neurotransmitter to multiple sodium ions, thereby enabling its accumulation against steep concentration gradients. Here we study the functional impact of mutations of the putative chloride-binding residues on transport by GAT-1, with the emphasis on a conserved glutamine residue. In contrast to another putative chloride coordinating residue, Ser-331, where mutation to glutamate led to chloride-independent GABA transport, the Q291E mutant was devoid of any transport activity, despite substantial expression at the plasma membrane. Low but significant transport activity was observed with substitution mutants with small side chains such as Q291S/A/G. Remarkably, when these mutations were combined with the S331E mutation, transport was increased significantly, even though the activity of the S331E single mutant was only ∼25% of that of wild type GAT-1. Transport by these double mutants was largely chloride-independent. Like mutants of other putative chloride coordinating residues, the apparent affinity of the active Gln-291 single mutants for chloride was markedly reduced along with a change their anion selectivity. In addition to the interaction of the transporter with chloride, Gln-291 is also required at an additional step during transport. Electrophysiological analysis of the Q291N and Q291S mutants, expressed in Xenopus laevis oocytes, is consistent with the idea that this additional step is associated with the gating of the transporter.

Involvement of sialic acid in the regulation of γ--aminobutyric acid uptake activity of γ-aminobutyric acid transporter 1

The γ-aminobutyric acid (GABA) transporters (GATs) have long been recognized for their key role in the uptake of neurotransmitters. The GAT1 belongs to the family of Na(+)- and Cl(-)-coupled transport proteins, which possess 12 putative transmembrane (TM) domains and three N-glycosylation sites on the extracellular loop between TM domains 3 and 4. Previously, we demonstrated that terminal trimming of N-glycans is important for the GABA uptake activity of GAT1. In this work, we examined the effect of deficiency, removal or oxidation of surface sialic acid residues on GABA uptake activity to investigate their role in the GABA uptake of GAT1. We found that the reduced concentration of sialic acid on N-glycans was paralleled by a decreased GABA uptake activity of GAT1 in Chinese hamster ovary (CHO) Lec3 cells (mutant defective in sialic acid biosynthesis) in comparison to CHO cells. Likewise, either enzymatic removal or chemical oxidation of terminal sialic acids using sialidase or sodium periodate, respectively, resulted in a strong reduction in GAT1 activity. Kinetic analysis revealed that deficiency, removal or oxidation of terminal sialic acids did not affect the K(m) GABA values. However, deficiency and removal of terminal sialic acids of GAT1 reduced the V(max) GABA values with a reduced apparent affinity for extracellular Na(+). Oxidation of cell surface sialic acids also strongly reduced V(max) without affecting both affinities of GAT1 for GABA and Na(+), respectively. These results demonstrated for the first time that the terminal sialic acid of N-linked oligosaccharides of GAT1 plays a crucial role in the GABA transport process.

A conserved methionine residue controls the substrate selectivity of a neuronal glutamate transporter

Glutamate transporters located in the brain maintain low synaptic concentrations of the neurotransmitter by coupling its flux to that of sodium and other cations. In the binding pocket of the archeal homologue Glt(Ph), a conserved methionine residue has been implicated in the binding of the benzyl moiety of the nontransportable substrate analogue threo-beta-benzyloxyaspartate. To determine whether the corresponding methionine residue of the neuronal glutamate transporter EAAC1, Met-367, fulfills a similar role, M367L, M367C, and M367S mutants were expressed in HeLa cells and Xenopus laevis oocytes to monitor radioactive transport and transport currents, respectively. The apparent affinity of the Met-367 mutants for D-aspartate and L-glutamate, but not for L-aspartate, was 10-20-fold reduced as compared with wild type. Unlike wild type, the magnitude of I(max) was different for each of the three substrates. D-glutamate, which is also a transportable substrate of EAAC1, did not elicit any detectable response with M367C and M367S but acted as a nontransportable substrate analogue in M367L. In the mutants, substrates inhibited the anion conductance as opposed to the stimulation observed with wild type. Remarkably, the apparent affinity of the blocker D,L-threo-beta-benzyloxyaspartate in the mutants was similar to that of wild type EAAC1. Our results are consistent with the idea that the side chain of Met-367 fulfills a steric role in the positioning of the substrate in the binding pocket in a step subsequent to its initial binding.

Mechanism of cation binding to the glutamate transporter EAAC1 probed with mutation of the conserved amino acid residue Thr101

The glutamate transporter excitatory amino acid carrier 1 (EAAC1) catalyzes the co-transport of three Na(+) ions, one H(+) ion, and one glutamate molecule into the cell, in exchange for one K(+) ion. Na(+) binding to the glutamate-free form of the transporter generates a high affinity binding site for glutamate and is thus required for transport. Moreover, sodium binding to the transporters induces a basal anion conductance, which is further activated by glutamate. Here, we used the [Na(+)] dependence of this conductance as a read-out of Na(+) binding to the substrate-free transporter to study the impact of a highly conserved amino acid residue, Thr(101), in transmembrane domain 3. The apparent affinity of substrate-free EAAC1 for Na(+) was dramatically decreased by the T101A but not by the T101S mutation. Interestingly, in further contrast to EAAC1(WT), in the T101A mutant this [Na(+)] dependence was biphasic. This behavior can be explained by assuming that the binding of two Na(+) ions prior to glutamate binding is required to generate a high affinity substrate binding site. In contrast to the dramatic effect of the T101A mutation on Na(+) binding, other properties of the transporter, such as its ability to transport glutamate, were impaired but not eliminated. Our results are consistent with the existence of a cation binding site deeply buried in the membrane and involving interactions with the side chain oxygens of Thr(101) and Asp(367). A theoretical valence screening approach confirms that the predicted site of cation interaction has the potential to be a novel, so far undetected sodium binding site.

The equivalent of a thallium binding residue from an archeal homolog controls cation interactions in brain glutamate transporters

Glutamate transporters maintain low synaptic concentrations of neurotransmitter by coupling uptake to flux of other ions. Their transport cycle consists of two separate translocation steps, namely cotransport of glutamic acid with three Na(+) followed by countertransport of K(+). Two Tl(+) binding sites, presumed to serve as sodium sites, were observed in the crystal structure of a related archeal homolog and the side chain of a conserved aspartate residue contributed to one of these sites. We have mutated the corresponding residue of the eukaryotic glutamate transporters GLT-1 and EAAC1 to asparagine, serine, and cysteine. Remarkably, these mutants exhibited significant sodium-dependent radioactive acidic amino acid uptake when expressed in HeLa cells. Reconstitution experiments revealed that net uptake by the mutants in K(+)-loaded liposomes was impaired. However, with Na(+) and unlabeled L-aspartate inside the liposomes, exchange levels were around 50-90% of those by wild-type. In further contrast to wild-type, where either substrate or K(+) stimulated the anion conductance by the transporter, substrate but not K(+) modulated the anion conductance of the mutants expressed in oocytes. Both with wild-type EAAC1 and EAAC1-D455N, not only sodium but also lithium could support radioactive acidic amino acid uptake. In contrast, with D455S and D455C, radioactive uptake was only observed in the presence of sodium. Thus the conserved aspartate is required for transporter-cation interactions in each of the two separate translocation steps and likely participates in an overlapping sodium and potassium binding site.

Inhibitors of the gamma-aminobutyric acid transporter 1 (GAT1) do not reveal a channel mode of conduction

We expressed the gamma-aminobutyric acid (GABA) transporter GAT1 (SLC6A1) in Xenopus laevis oocytes and performed GABA uptake experiments under voltage clamp at different membrane potentials as well as in the presence of the specific GAT1 inhibitors SKF-89976A and NO-711. In the absence of the inhibitors, GAT1 mediated the inward translocation of 2 net positive charges across the plasma membrane for every GABA molecule transported into the cell. This 2:1 charge flux/GABA flux ratio was the same over a wide range of membrane potentials from -110 mV to +10 mV. Moreover, when GABA-evoked (500 microM) currents were measured at -50 and -90 mV, neither SKF-89976A (5 and 25 microM) nor NO-711 (2 microM) altered the 2:1 charge flux/GABA flux ratio. The results are not consistent with previous hypotheses that (i) GABA evokes an uncoupled channel-mediated current in GAT1, and (ii) GAT1 inhibitors block the putative uncoupled current gated by GABA. Rather, the results suggest tight coupling of GAT1-mediated charge flux and GABA flux.

Transmembrane domain 8 of the {gamma}-aminobutyric acid transporter GAT-1 lines a cytoplasmic accessibility pathway into its binding pocket

GAT-1 is a sodium- and chloride-coupled gamma-aminobutyric acid (GABA) transporter, which fulfills an essential role in the synaptic transmission by this neurotransmitter. Cysteine-399 is the major site of inhibition of GAT-1 by membrane-permeant sulfhydryl reagents. This cysteine residue was previously thought to reside on a cytoplasmic loop connecting transmembrane domains (TMs) 8 and 9. However, the crystal structure of LeuT, a bacterial homologue of the mammalian neurotransmitter:sodium symporters, revealed that the residue corresponding to Cys-399 is in fact located in the middle of TM 8. This residue is located to the cytoplasmic side of Asp-395 and Ser-396, whose side chains are thought to ligand one of the two cotransported sodium ions. To determine how the sulfhydryl reagents approach cysteine-399, a cysteine scan of all 35 residues of TM 8 was performed. Sulfhydryl reagents inhibited transport when a cysteine residue was present at either of the positions 399, 402, 406, and 410. SKF-89976A and other non-transportable analogues, which are expected to lock the transporter in a conformation facing the extracellular medium, protected against the sulfhydryl modification at positions 399, 402, and 406. Such a protection was not seen by GABA itself, which actually modestly potentiated the modification at positions 399 and 402. Our results point to an alpha-helical stripe on TM8 lining an aqueous access pathway from the cytoplasm into the binding pocket, which gets occluded in the conformation of the transporter where the binding pocket is exposed to the extracellular medium.

Turnover rate of the gamma-aminobutyric acid transporter GAT1

We combined electrophysiological and freeze-fracture methods to estimate the unitary turnover rate of the gamma-aminobutyric acid (GABA) transporter GAT1. Human GAT1 was expressed in Xenopus laevis oocytes, and individual cells were used to measure and correlate the macroscopic rate of GABA transport and the total number of transporters in the plasma membrane. The two-electrode voltage-clamp method was used to measure the transporter-mediated macroscopic current evoked by GABA (I(NaCl)(GABA)), macroscopic charge movements (Q (NaCl)) evoked by voltage pulses and whole-cell capacitance. The same cells were then examined by freeze-fracture and electron microscopy in order to estimate the total number of GAT1 copies in the plasma membrane. GAT1 expression in the plasma membrane led to the appearance of a distinct population of 9-nm freeze-fracture particles which represented GAT1 dimers. There was a direct correlation between Q (NaCl) and the total number of transporters in the plasma membrane. This relationship yielded an apparent valence of 8 +/- 1 elementary charges per GAT1 particle. Assuming that the monomer is the functional unit, we obtained 4 +/- 1 elementary charges per GAT1 monomer. This information and the relationship between I(NaCl)(GABA) and Q (NaCl) were used to estimate a GAT1 unitary turnover rate of 15 +/- 2 s(-1) (21 degrees C, -50 mV). The temperature and voltage dependence of GAT1 were used to estimate the physiological turnover rate to be 79-93 s(-1) (37 degrees C, -50 to -90 mV).

Mechanism of chloride interaction with neurotransmitter:sodium symporters

Neurotransmitter:sodium symporters (NSS) have a critical role in regulating neurotransmission and are targets for psychostimulants, anti-depressants and other drugs. Whereas the non-homologous glutamate transporters mediate chloride conductance, in the eukaryotic NSS chloride is transported together with the neurotransmitter. In contrast, transport by the bacterial NSS family members LeuT, Tyt1 and TnaT is chloride independent. The crystal structure of LeuT reveals an occluded binding pocket containing leucine and two sodium ions, and is highly relevant for the neurotransmitter transporters. However, the precise role of chloride in neurotransmitter transport and the location of its binding site remain elusive. Here we show that introduction of a negatively charged amino acid at or near one of the two putative sodium-binding sites of the GABA (gamma-aminobutyric acid) transporter GAT-1 from rat brain (also called SLC6A1) renders both net flux and exchange of GABA largely chloride independent. In contrast to wild-type GAT-1, a marked stimulation of the rate of net flux, but not of exchange, was observed when the internal pH was lowered. Equivalent mutations introduced in the mouse GABA transporter GAT4 (SLC6A11) and the human dopamine transporter DAT (SLC6A3) also result in chloride-independent transport, whereas the reciprocal mutations in LeuT and Tyt1 render substrate binding and/or uptake by these bacterial NSS chloride dependent. Our data indicate that the negative charge, provided either by chloride or by the transporter itself, is required during binding and translocation of the neurotransmitter, probably to counterbalance the charge of the co-transported sodium ions.

Structural Genomics

Drug discovery based on structural knowledge has proven useful as several structure-based medicines are already on the market. Structural genomics aims at studying a large number of gene products including whole genomes, topologically similar proteins, protein families and protein subtypes in parallel. Particularly, therapeutically relevant targets have been selected for structural genomics initiatives. In this context, integral membrane proteins, which represent 60–70% of the current drug targets, have been of major interest. Paradoxically, membrane proteins present the last frontier to conquer in structural biology as some 100 high resolution structures among the 30,000 entries in public structural databases are available. The modest success rate on membrane proteins relates to the difficulties in their expression, purification and crystallography. To facilitate technology development large networks providing expertise in molecular biology, protein biochemistry and structural biology have been established. The privately funded MePNet program has studied 100 G protein-coupled receptors, which resulted in high level expression of a large number of receptors at structural biology compatible levels. Currently, selected GPCRs have been purified and subjected to crystallization attempts

The substrate specificity of a neuronal glutamate transporter is determined by the nature of the coupling ion

Glutamate transporters are essential for terminating synaptic transmission. Glutamate is translocated together with three sodium ions. In the neuronal glutamate transporter EAAC1, lithium can replace sodium. To address the question of whether the coupling ion interacts with the 'driven' substrate during co-transport, the kinetic parameters of transport of the three substrates, L-glutamate and D- and L-aspartate by EAAC-1 in sodium- and lithium-containing media were compared. The major effect of the substitution of sodium by lithium was on Km. In the presence of sodium, the values for Km and Imax of these substrates were similar. In the presence of lithium, the Km for L-aspartate was increased around 13-fold. Remarkably, the corresponding increase for L-glutamate and D-aspartate was much larger, around 130-fold. In marked contrast, the Ki values for a non-transportable substrate analogue were similar in the presence of either sodium or lithium. The preference for L-aspartate in the presence of lithium was also observed when electrogenic transport of radioactive substrates was monitored in EAAC1-containing proteoliposomes. Our results indicate that, subsequent to substrate binding, the co-transported solutes interact functionally in the binding pocket of the transporter.

Multiple Consequences of Mutating Two Conserved β-Bridge Forming Residues in the Translocation Cycle of a Neuronal Glutamate Transporter

Glutamate transporters remove this neurotransmitter from the synapse in an electrogenic process. After sodium-coupled glutamate translocation, the cycle is completed by obligatory outward translocation of potassium. In the crystal structure of an archaeal homologue, two conserved residues form a beta-bridge, which points away from the binding pocket. In the neuronal glutamate transporter EAAC1, the equivalent residues are asparagine 366 and aspartate 368. Substitution mutants N366Q and D368E, but not N366D and D368N, show glutamate-induced inwardly rectifying steady-state currents, but their apparent substrate affinity is dramatically decreased. Such currents, which reflect electrogenic net uptake of substrate are not observed with the reciprocal double mutant N366D/D368N. Remarkably, the double mutant exhibits slow substrate-induced voltage-dependent capacitative transient currents. These currents apparently reflect the reversible sodium-coupled glutamate translocation step, because the interaction of the double mutant with potassium is largely impaired. Moreover, when the analogous double mutant in the glutamate transporter GLT-1 is reconstituted into liposomes, a slow exchange of radioactive and unlabeled acidic amino acids is observed. Our results suggest that it is the interaction of asparagine 366 and aspartate 368 that is important during the glutamate translocation step. On the other hand, the side chains of these residues themselves are required for the subsequent potassium relocation step.

Identification of a Lithium Interaction Site in the γ-Aminobutyric Acid (GABA) Transporter GAT-1

The sodium- and chloride-dependent electrogenic gamma-aminobutyric acid (GABA) transporter GAT-1, which transports two sodium ions together with GABA, is essential for synaptic transmission by this neurotransmitter. Although lithium by itself does not support GABA transport, it has been proposed that lithium can replace sodium at one of the binding sites but not at the other. To identify putative lithium selectivity determinants, we have mutated the five GAT-1 residues corresponding to those whose side chains participate in the sodium binding sites Na1 and Na2 of the bacterial leucine-transporting homologue LeuT(Aa). In GAT-1 and in most other neurotransmitter transporter family members, four of these residues are conserved, but aspartate 395 replaces the Na2 residue threonine 354. At varying extracellular sodium, lithium stimulated sodium-dependent transport currents as well as [3H]GABA uptake in wild type GAT-1. The extent of this stimulation was dependent on the GABA concentration. In mutants in which aspartate 395 was replaced by threonine or serine, the stimulation of transport by lithium was abolished. Moreover, these mutants were unable to mediate the lithium leak currents. This phenotype was not observed in mutants at the four other positions, although their transport properties were severely impacted. Thus at saturating GABA, the site corresponding to Na2 behaves as a low affinity sodium binding site where lithium can replace sodium. We propose that GABA participates in the other sodium binding site, just like leucine does in the Na1 site, and that at limiting GABA, this site determines the apparent sodium affinity of GABA transport.

Novel properties of a mouse gamma-aminobutyric acid transporter (GAT4)

We expressed the mouse gamma-aminobutyric acid (GABA) transporter GAT4 (homologous to rat/ human GAT-3) in Xenopus laevis oocytes and examined its functional and pharmacological properties by using electrophysiological and tracer uptake methods. In the coupled mode of transport (Na+/ Cl-/GABA cotransport), there was tight coupling between charge flux and GABA flux across the plasma membrane (2 charges/GABA). Transport was highly temperature-dependent with a temperature coefficient (Q10) of 4.3. The GAT4 turnover rate (1.5 s(-l); -50 mV, 21 degrees C) and temperature dependence suggest physiological turnover rates of 15-20 s(-1). No uncoupled current was observed in the presence of Na+. In the absence of external Na+, GAT4 exhibited two distinct uncoupled currents. (i) A Cl- leak current (ICl(leak)) was observed when Na+ was replaced with choline or tetraethylammonium. The reversal potential of (ICl(leak)) followed the Cl- Nernst potential. (ii) A Li+ leak current (ILi(leak)) was observed when Na+ was replaced with Li+. Both leak currents were inhibited by Na+, and both were temperature-independent (Q10 approximately 1). The two leak modes appeared not to coexist, as Li+ inhibited (ICl(leak)). The results suggest the existence of cation- and anion-selective channel-like pathways in GAT4. Flufenamic acid inhibited GAT4 Na+/Cl-/GABA cotransport, ILi(leak), and ICl(leak), (Ki approximately 30 microM), and the voltage-induced presteady-state charge movements (Ki approximately 440 microM). Flufenamic acid exhibited little or no selectivity for GAT1, GAT2, or GAT3. Sodium and GABA concentration jicroumps revealed that slow Na+ binding to the transporter is followed by rapid GABA-induced translocation of the ligands across the plasma membrane. Thus, Na+ binding and associated conformational changes constitute the rate-limiting steps in the transport cycle.

The role of N-glycosylation in the stability, trafficking and GABA-uptake of GABA-transporter 1. Terminal N-glycans facilitate efficient GABA-uptake activity of the GABA transporter

Neurotransmitter transporters play a major role in achieving low concentrations of their respective transmitter in the synaptic cleft. The GABA transporter GAT1 belongs to the family of Na(+)- and Cl(-)-coupled transport proteins which possess 12 putative transmembrane domains and three N-glycosylation sites in the extracellular loop between transmembrane domain 3 and 4. To study the significance of N-glycosylation, green fluorescence protein (GFP)-tagged wild type GAT1 (NNN) and N-glycosylation defective mutants (DDQ, DGN, DDN and DDG) were expressed in CHO cells. Compared with the wild type, all N-glycosylation mutants showed strongly reduced protein stability and trafficking to the plasma membrane, which however were not affected by 1-deoxymannojirimycin (dMM). This indicates that N-glycosylation, but not terminal trimming of the N-glycans is involved in the attainment of a correctly folded and stable conformation of GAT1. All N-glycosylation mutants were expressed on the plasma membrane, but they displayed markedly reduced GABA-uptake activity. Also, inhibition of oligosaccharide processing by dMM led to reduction of this activity. Further experiments showed that both N-glycosylation mutations and dMM reduced the V(max) value, while not increasing the K(m) value for GABA uptake. Electrical measurements revealed that the reduced transport activity can be partially attributed to a reduced apparent affinity for extracellular Na+ and slowed kinetics of the transport cycle. This indicates that N-glycans, in particular their terminal trimming, are important for the GABA-uptake activity of GAT1. They play a regulatory role in the GABA translocation by affecting the affinity and the reaction steps associated with the sodium ion binding.

Proximity of transmembrane domains 1 and 3 of the gamma-aminobutyric acid transporter GAT-1 inferred from paired cysteine mutagenesis

GAT-1 is a sodium- and chloride-dependent gamma-aminobutyric acid transporter and is the first identified member of a family of transporters that maintain low synaptic neurotransmitter levels and thereby enable efficient synaptic transmission. Because transmembrane domains 1 and 3 contain amino acid residues important for transport activity, we hypothesized that these domains may participate in the formation of the binding pocket of the transporter. Pairwise substitutions have been introduced in several predicted transmembrane domains and in the first extracellular loop of GAT-1. In the double mutant W68C/I143C, in which the cysteines were introduced at locations at the extracellular part of transmembrane domains 1 and 3, respectively, approximately 70% inhibition of transport was observed by cadmium with an IC50 of approximately 10 microm. This inhibition was not observed in the corresponding single mutants and also not in > 10 other double mutants, except for V67C/I143C, where the half-maximal effect was obtained at approximately 50 microm. The inhibition by cadmium was only observed when the cysteine pairs were introduced in the same polypeptide. Our results suggest that transmembrane domains 1 and 3 come in close proximity within the transporter monomer.

Rapid substrate-induced charge movements of the GABA transporter GAT1

The GABA transporter GAT1 removes the neurotransmitter GABA from the synaptic cleft by coupling of GABA uptake to the co-transport of two sodium ions and one chloride ion. The aim of this work was to investigate the individual reaction steps of GAT1 after a GABA concentration jump. GAT1 was transiently expressed in HEK293 cells and its pre-steady-state kinetics were studied by combining the patch-clamp technique with the laser-pulse photolysis of caged GABA, which allowed us to generate GABA concentration jumps within <100 micros. Recordings of transport currents generated by GAT1, both in forward and exchange transport modes, showed multiple charge movements that can be separated along the time axis. The individual reactions associated with these charge movements differ from the well-characterized electrogenic "sodium-occlusion" reaction by GAT1. One of the observed electrogenic reactions is shown to be associated with the GABA-translocating half-cycle of the transporter, in contradiction to previous studies that showed no charge movements associated with these reactions. Interestingly, reactions of the GABA-bound transporter were not affected by the absence of extracellular chloride, suggesting that Cl- may not be co-translocated with GABA. Based on the results, a new alternating access sequential-binding model is proposed for GAT1's transport cycle that describes the results presented here and those by others.

Transporter-associated currents in the gamma-aminobutyric acid transporter GAT-1 are conditionally impaired by mutations of a conserved glycine residue

To determine whether glycine residues play a role in the conformational changes during neurotransmitter transport, we have analyzed site-directed mutants of the gamma-aminobutyric acid (GABA) transporter GAT-1 in a domain containing three consecutive glycines conserved throughout the sodium- and chloride-dependent neurotransmitter transporter family. Only cysteine replacement of glycine 80 resulted in the complete loss of [(3)H]GABA uptake, but oocytes expressing this mutant exhibited the sodium-dependent transient currents thought to reflect a charge-moving conformational change. When sodium was removed and subsequently added back, the transients by G80C did not recover, as opposed to wild type, where recovery was almost complete. Remarkably, the transients by G80C could be restored after exposure of the oocytes to either GABA or a depolarizing pre-pulse. These treatments also resulted in a full recovery of the transients by the wild type. Whereas in wild type lithium leak currents are observed after prior sodium depletion, this was not the case for the glycine 80 mutants unless GABA was added or the oocytes were subjected to a depolarizing pre-pulse. Thus, glycine 80 appears essential for conformational transitions in GAT-1. When this residue is mutated, removal of sodium results in "freezing" the transporter in one conformation from which it can only exit by compensatory changes induced by GABA or depolarization. Our results can be explained by a model invoking two outward-facing states of the empty transporter and a defective transition between these states in the glycine 80 mutants.

The Mutation F227I Increases the Coupling of Metal Ion Transport in DCT1

Metal ion transport by DCT1, a member of the natural resistance-associated macrophage protein family, is driven by protons. The stoichiometry of the proton to metal ion is variable, and under optimal transport conditions, more than 10 protons are co-transported with a single metal ion. To understand this phenomenon better, we used site-directed mutagenesis of DCT1 and analyzed the mutants by complementation of yeast suppressor of mitochondria import function-null mutants and electrophysiology with Xenopus oocytes. The mutation F227I resulted in an increase of up to 14-fold in the ratio between metal ions to protons transported. This observation suggests that low metal ion to proton transport of DCT1 resulting from a proton slippage is not a necessity of the transport mechanism in which positively charged protons are driving two positive charges of the metal ion in the same direction. It supports the idea that the proton slippage has a physiological advantage, and the proton slip was positively selected during the evolution of DCT1.

Transmembrane domains I and II of the gamma-aminobutyric acid transporter GAT-4 contain molecular determinants of substrate specificity

The sodium- and chloride-dependent GABA transporters GABA transporter (GAT) 1 to 4 in the central nervous system enable efficient synaptic transmission by removing the neurotransmitter from the cleft. Taurine interacts only weakly with the GABA transporter GAT-4 (IC50 approximately 1.6 mM). Glutamate-61 is located in the conserved transmembrane domain I of GAT-4, whereas in the related taurine-transporter taurine transporter (TAUT), glycine occupies the equivalent position. [3H]GABA uptake by the GAT-4 E61G mutant becomes markedly more sensitive to inhibition by taurine (IC50 approximately 0.26 mM). Replacement of cysteine-94, located in the conserved transmembrane domain II of GAT-4, to its TAUT counterpart serine, results only in a modest increase in the ability of taurine to inhibit GABA uptake. However, introduction of glycine at this position decreases the IC50 for taurine by approximately 8-fold (IC50 approximately 0.20 mM). The inhibitory potency of taurine is inversely correlated with the volume of the side chain of the amino acid residue introduced at positions 61 and 94. It is striking that the IC50 for taurine of the E61G/C94G double mutant is decreased by approximately 35-fold (IC50 approximately 0.05 mM), and this inhibition of GABA transport is competitive. Changes in the inhibitory potency of the mutants described are also observed with beta-ala-nine and GABA, although they are much less pronounced. Our results suggest that determinants on transmembrane domains I and II can influence the specificity of the substrate binding pocket. The size of the side chain at positions 61 and 94 seems to determine the ability of substrate and substrate analogs to interact with the transporter.