Identification of Functional Domains of the Human Glutamate Transporters EAAT1 and EAAT2*

Glutamate transporters serve the important function of mediating removal of glutamate released at excitatory synapses and maintaining extracellular concentrations below excitotoxic levels. Excitatory amino acid transporter subtypes EAAT1 and EAAT2 have a high degree of sequence homology and similar predicted topology and yet display a number of functional differences. Several recombinant chimeric transporters were generated to identify domains that contribute to functional differences between EAAT1 and EAAT2. Wild-type transporters and chimeric transporters were expressed in Xenopus laevis oocytes, and electrogenic transport was studied under voltage clamp conditions. The differential sensitivity of EAAT1 and EAAT2 to transport blockers, kainate,threo-3-methylglutamate, and (2S,4R)-4-methylglutamate as well asl-serine-O-sulfate transport and chloride permeability were employed to characterize chimeric transporters. One particular region, transmembrane domains 9 and 10, plays an important role in defining these functional differences. The intracellular carboxyl-terminal region may also play a minor role in conferring an effect on chloride permeability. This study provides important insight into the identification of functional domains that determine differences among glutamate transporter subtypes.

Glutamate transporters serve the important function of mediating removal of glutamate released at excitatory synapses and maintaining extracellular concentrations below excitotoxic levels. Excitatory amino acid transporter subtypes EAAT1 and EAAT2 have a high degree of sequence homology and similar predicted topology and yet display a number of functional differences. Several recombinant chimeric transporters were generated to identify domains that contribute to functional differences between EAAT1 and EAAT2. Wild-type transporters and chimeric transporters were expressed in Xenopus laevis oocytes, and electrogenic transport was studied under voltage clamp conditions. The differential sensitivity of EAAT1 and EAAT2 to transport blockers, kainate, threo-3-methylglutamate, and (2S,4R)-4-methylglutamate as well as L-serine-O-sulfate transport and chloride permeability were employed to characterize chimeric transporters. One particular region, transmembrane domains 9 and 10, plays an important role in defining these functional differences. The intracellular carboxyl-terminal region may also play a minor role in conferring an effect on chloride permeability. This study provides important insight into the identification of functional domains that determine differences among glutamate transporter subtypes.
L-Glutamate transport in the central nervous system and also in peripheral organs is mediated by a family of structurally related membrane proteins. In the central nervous system, glutamate transporters play an important role by mediating removal of glutamate released at excitatory synapses and also influencing the kinetics of glutamate receptor activation (1,2). Complementary DNAs encoding five different glutamate transporters have been identified (3)(4)(5)(6)(7). The human clones have been named Excitatory Amino Acid Transporters 1-5 (EAAT1-5) (6)(7)(8). The recent cloning and expression of recombinant glutamate transporter proteins in heterologous expression systems allow a number of questions concerning the molecular basis for transporter function to be addressed. In particular, an important aspect of such studies is the identification of domains involved in substrate recognition and substrate translocation through the membrane. In this study we have constructed a series of chimeric glutamate transporters, using the EAAT1 and EAAT2 subtypes, and exploited the pharmacolog-ical and electrophysiological differences between the two subtypes to map functional domains of these proteins.
To make meaningful predictions about the functional role of various domains, an understanding of the structure of the transporters is particularly useful. The predicted amino acid sequences of EAAT1 and EAAT2 are 65% identical, and if conservative substitutions are allowed the degree of relatedness is increased to 80% (8). These two proteins are likely to form very similar structures and therefore are good candidates for the generation of functional chimeras. Through the use of hydrophobicity analysis of the amino acid sequences it has been predicted that there are six ␣-helical transmembrane segments in the amino-terminal portion of the protein. Tagging amino acid residues between the various transmembrane segments within this region and identifying whether the tags are located intracellularly or extracellularly have confirmed the predictions from the hydrophobicity analysis (9,10). Hydrophobicity analysis of the remaining portion of the proteins has been inconclusive. This region is the most highly conserved part of the protein and has been suggested to form part of the pore of the transporter through which glutamate and various ions pass in the transport process (11). This suggestion has been supported by evidence from point mutation studies (11)(12)(13). Therefore, it is highly likely that at least some of this part of the protein is closely associated with the membrane. In two recent studies, molecular tags have been introduced into the transporters, and the intracellular and extracellular locations of the tags have been used to predict the topology of this region (10,14). Two very different models for the transmembrane topology of the transporters have been predicted from the two studies. In both models, four transmembrane domains have been predicted, but in the model suggested by Slotboom and co-workers (14), the four transmembrane segments are ␣-helical, whereas in the model suggested by Wahle and Stoffel (10), the four transmembrane segments are ␤-sheets. In most membrane proteins the most highly conserved domains are located within the transmembrane regions. Although other topological models are also possible, 1 for the purposes of this study, we tentatively favor the first model, consisting of 10 ␣-helical transmembrane segments, because this model places a larger proportion of the highly conserved domain within the membrane. However, further work is clearly required to resolve this issue.
Despite the high degree of amino acid sequence conservation between EAAT1 and EAAT2, the transporter subtypes show quite distinct pharmacological differences. The most striking pharmacological difference between the transporter subtypes is the selective sensitivity of the EAAT2 subtype to competitive inhibition or blockade by dihydrokainate, kainate, and threo-3-methylglutamate (8,15). These compounds block glutamate transport by EAAT2 in a voltage-independent manner, suggesting that they block transport by competing with glutamate at an extracellular recognition site. Another compound, (2S,4R)-4-methylglutamate, shows some very distinctive differences in its mechanism of action on the transporters. (2S,4R)-4-Methylglutamate is a voltage-independent, competitive blocker of glutamate transport by EAAT2, but it is also a potent substrate of the EAAT1 transporter subtype. As such these compounds provide a set of selective molecular probes for the extracellular glutamate recognition site of the transporters.
In contrast to the competitive blockers of glutamate transporters, a number of other pharmacological agents have been shown to inhibit glutamate transport by competing with glutamate as a substrate for the transporters (8). One pharmacological substrate that shows some clear differences in transport parameters between EAAT1 and EAAT2 is L-serine-O-sulfate. L-Serine-O-sulfate is a substrate for both EAAT1 and EAAT2, but it is a less potent substrate on EAAT2 than EAAT1. In addition, the EC 50 2 values for L-serine-O-sulfate transport by EAAT2 are steeply voltage-dependent but relatively constant for transport by EAAT1 (16). This difference between EAAT1 and EAAT2 in L-serine-O-sulfate transport is likely to reflect a difference in the structure of the pore of the two transporters. Thus, L-serine-O-sulfate may be used as a molecular probe for the pore of the transporters.
In addition to the pharmacological differences, the different transporter subtypes allow a varying degree of a thermodynamically uncoupled chloride flux through the transporter (6,7,(17)(18)(19). Different transporter substrates also appear to be coupled differentially to the activation of chloride fluxes (6,17,19). In particular, the chloride conductance activated by Daspartate transport by EAAT1 is significantly greater than the chloride conductance activated by D-aspartate transport by EAAT2 (17). These differences in substrate-activated chloride conductance between transporter subtypes provide another independent molecular probe for identifying the domain responsible for forming the pore of the transporters.
We have constructed a series of chimeric glutamate transporters with the aim of investigating the importance of transmembrane domains 7-10 in determining functional differences between EAAT1 and EAAT2. The chimeric glutamate transporters were characterized in terms of electrogenic glutamate transport; sensitivity to the transport blockers kainate, threo-3-methylglutamate, and (2S,4R)-4-methylglutamate; L-serine-O-sulfate transport; and chloride permeability.
Construction of Chimeric Transporters-The chimeric glutamate transporters were constructed using cDNAs encoding the EAAT1 and EAAT2 transporter subtypes. Two procedures were used to construct the various chimeras. First, an in vivo recombination procedure was used to construct the chimeras based on the method described by Buck and Amara (20). EAAT1 and EAAT2 cDNAs were cloned in tandem into the pCMV plasmid (21) with a unique XbaI site between the cDNAs. The tandem construct was linearized with XbaI, and 100 ng of DNA was used to transform competent DH5␣ cells. Colonies were screened for recombinant plasmids that contained only a single cDNA and sequenced to determine the junction sites (Sequenase, U. S. Biochemical Corp.). A number of chimeras were isolated, and all of them had junction sites within the most highly conserved domain from serine 366 to glutamine 415 (EAAT1 numbering). One of these chimeras, termed EAAT1-EAAT2-Chimera-7 (E12C7), contained the EAAT1 sequence at the amino terminus and junctions with EAAT2 at leucine 404. The chimera was subcloned into pOTV for synthesis of cRNA and expression in Xenopus oocytes. In all of the remaining chimeras unique restriction sites were engineered into the cDNAs, using the Quickchange sitedirected mutagenesis kit (Stratagene, La Jolla, CA), and then the cDNA fragments of one of the transporters were cut out of the plasmid and ligated into corresponding sites of the other transporter cDNA. More than 20 different chimeric glutamate transporters were constructed in this way, but only the chimeras in Fig. 1 were functional in terms of supporting electrogenic glutamate transport. All of the functional chimeras were sequenced using a Perkin-Elmer 377 sequencer with the Catalyst 800 robotic work station at the Sydney University and Prince Alfred Macromolecular Analysis Center. Selected nonfunctional chimeras were also sequenced to check that polymerase chain reaction-generated errors were not introduced into the chimeric cDNAs. The functional chimeras are named according to the orientation of cDNA fragments, with the numbering referring to the transmembrane closest to the junction site. For example, E12C7 contains EAAT1 sequence from the amino terminus to transmembrane domain 7 and then the EAAT2 sequence to the carboxyl terminus. E121C7-8 contains the EAAT1 sequence from the amino terminus to transmembrane domain 7 followed by the EAAT2 sequence to the end of transmembrane 8 and then EAAT1 sequence to the carboxyl-terminal end.
Electrophysiological Recording-Xenopus laevis were obtained from the African Xenopus Facility, Republic of South Africa. A lobe of the ovary was surgically removed while the frog was anesthetized with 3-aminobenzoic acid ethyl ester methanesulfonate salt (Sigma), and stage V-VI oocytes were isolated after collagenase treatment. 10 -50 ng of cRNAs encoding the various glutamate transporters were injected into oocytes. 2-7 days later transport was measured by two-electrode voltage clamp recording using a Geneclamp 500 amplifier (Axon Instruments) and a MacLab 2e recorder (ADInstruments). In transport experiments the membrane potential of the oocytes expressing the given transporter was clamped at Ϫ60 mV and superfused continuously with frog ringer buffer (96 mM NaCl, 2 mM KCl, 1.8 mM CaCl 2 , 1 mM MgCl 2 , and 5 mM HEPES, pH 7.55). When measuring dose responses for various substrates, this buffer was changed to one containing the indicated concentration of substrate. Voltage-dependent L-glutamate or D-aspartate transport currents were measured by clamping the membrane at various potentials between Ϫ60 and ϩ60 mV and applying the substrate to the bath solution.
Analysis of Kinetic Data-Current (I) as a function of substrate concentration ([S]) was fitted by least squares to I ϭ I max ⅐[S]/(EC 50 ϩ [S]), where I max is the maximal current, and EC 50 is the concentration of substrate which generates half-maximal current. The I max values for L-serine-O-sulfate and, where appropriate, (2S,4R)-4-methylglutamate, are expressed relative to a maximal dose of L-glutamate in the same cell. A selectivity ratio, EC 50 R, represents the EC 50 value for each substrate divided by the EC 50 for glutamate for EAAT1, EAAT2, and each chimeric transporter. I max and EC 50 values are expressed as mean Ϯ S.E. and were determined by fitting data from individual oocytes. Current (I) as a function of blocker concentration ([B]) was fitted by least squares to I ϭ I max Ϫ (I max ⅐[B]/(IC 50 ϩ [B])), where I max is the current generated by 30 M glutamate, and IC 50 is the concentration of the blocker that causes a 50% reduction in the glutamate transport current. The transport blockers, (2S,4R)-4-methylglutamate, threo-3-methylglutamate, and kainate, have all been shown to be competitive blockers of glutamate transport by EAAT2 using Schild analysis (8,15,22), allowing the calculation of inhibition constants (K i ) from IC 50 values using the equation ϭ 30 M, and EC 50 for glutamate was calculated for each transporter as described above. K i values are expressed as mean Ϯ S.E. and were determined by fitting data from individual oocytes.

Generation of Functional Chimeric
Transporters-A number of chimeric glutamate transporters (Ͼ20) were constructed using the EAAT1 and EAAT2 subtypes, but only the five chimeric transporters depicted in Fig. 1 showed high affinity electrogenic glutamate transport in oocytes expressing the corresponding cDNAs. The EC 50 values for glutamate transport by EAAT1 and EAAT2 are 14.7 Ϯ 2.9 M and 23.2 Ϯ 1.2 M, respectively, and the corresponding values for the five func-tional chimeric transporters fall within the range 3.2-37.2 M (Fig. 2 and Table I).
For ease of interpretation the chimeric transporters are effectively dissected into four regions. Region 1 starts at the amino terminus and includes the first six transmembrane do-mains and part of transmembrane domain 7. Region 2 continues through the seventh transmembrane domain to the end of the eighth transmembrane domain. Region 3 includes transmembrane domains 9 and 10. Region 4 consists of the intracellular carboxyl-terminal region (Fig. 1).  Tables I-III).
Functional Domain Conferring Substrate Selectivity-The EC 50 values for glutamate did not differ more than by a factor of 2 for the chimeras, with the exception of E121C7-8, EC 50 3.2 M. To allow meaningful interpretation of EC 50 data a selectivity ratio has been included for each chimera. This ratio represents the EC 50 value for each substrate divided by the EC 50 for glutamate for each chimera, allowing the comparison with wild-type transporters with standardization for the EC 50 differences observed.
The chimeric transporters define a functional domain (region 3) that influences the substrate selectivity of L-serine-O-sulfate observed with EAAT1 and EAAT2. Chimera E21C8, which contains EAAT1 sequence in region 3 and 4, has EAAT1-like L-serine-O-sulfate selectivity. Chimera E12C10 showed EAAT1-like L-serine-O-sulfate selectivity, and the reverse chimera E21C10 had EAAT2-like L-serine-O-sulfate selectivity, implying that the intracellular region 4 does not confer Lserine-O-sulfate selectivity. L-Serine-O-sulfate transport by chimera E12C7, which contains EAAT2 sequence in regions 2, 3, and 4, displayed EAAT2 selectivity, whereas E121C7-8, which contains EAAT1 sequence in regions 3 and 4, displayed EAAT1 selectivity, confirming the importance of region 3. In summary, these results suggest that region 3, which includes the 9th and 10th transmembrane domains, is a major determinant of differences in substrate selectivity between EAAT1 and EAAT2.
Functional Domain Determining Differences in Chloride Permeability-Chloride permeation of both wild-type and chimeric transporters was estimated by measuring voltage-dependent transport currents of D-aspartate and L-glutamate. Differences in chloride permeation between EAAT1 and EAAT2 are most obvious for D-aspartate transport (17) (Fig. 3 and Table II), and the membrane potential at which D-aspartate transport currents reverse direction may be used as an indicator of relative chloride permeability. The chimeric transporters for the most part clearly resembled either EAAT1 or EAAT2. Chimeric transporters E21C8 (E rev D-aspartate ϩ7.1 Ϯ 1.5; E rev L-glutamate Ͼ ϩ50 mV) and E12C10 (E rev D-aspartate ϩ2.3 Ϯ 1.2; E rev L-glutamate ϩ37.0 Ϯ 3.8 mV) both have reversal potentials that are similar to EAAT1 (Fig. 3 and Table II). Chimera E12C7 (E rev D-aspartate Ͼ ϩ50 mV; E rev L-glutamate Ͼ ϩ50 mV) did not show a reversal of current in the membrane potential range up to Ͼ ϩ50 mV, indicative of EAAT2-like chloride permeability ( Fig. 3 and Table II). Chimera E21C10 (E rev D-aspartate ϩ23.9 Ϯ 2.5 mV; E rev L-glutamate Ͼ ϩ50 mV) did not obviously correspond to either EAAT1 or EAAT2 classification ( Fig. 3 and Table II).
The results from the reversal potential measurements of the chimeric transporters suggest that the third region, which is transmembrane domains 9 and 10, is a major determinant of the differences in chloride permeability observed between EAAT1 and EAAT2. Chimera E21C8, which contains EAAT1 sequence in regions 3 and 4, has chloride permeation parameters similar to those of EAAT1 wild-type, suggesting that the region conferring differences in chloride permeation is region 3 and 4, which contains transmembrane domains 9 and 10. The results for E12C7 are also consistent with this conclusion. The transport currents for chimera E121C7-8 showed some unusual characteristics. A comparison of the current-voltage relationships for L-glutamate and D-aspartate of chimera E121C7-8 displays relative maximal currents similar to those of EAAT1, especially at negative membrane potentials (Fig. 3). However, at positive membrane potentials the D-aspartate current does not reverse direction, which is characteristic of EAAT2 (Fig. 3). In addition to the unusual current-voltage curves for L-glutamate and D-aspartate transport, the chimera E121C7-8 supports a leak current because of chloride-modulated fluxes of Na ϩ and K ϩ (23). Leak currents have not been detected in either EAAT1 (24) or EAAT2 (22), which suggests that the functional properties of the E121C7-8 chimera have been partially compromised. The lack of outward currents generated by D-aspartate transport at positive membrane potentials may be related to the compromised ion permeation properties of this chimera. As such, the reliability of any conclusions relating to ion permeation by E121C7-8 must be questioned.
The reversal potential measurements for the chimera E12C10 are similar to those of EAAT1, which is also consistent with region 3 conferring differences in chloride permeability. However, chimera E21C10 (which is the reverse chimera of E12C10 and contains only the carboxyl-terminal region 4 from EAAT1), displayed a lower reversal potential for D-aspartate than observed for EAAT2 (Table II). The current-voltage relationship for the chimera E21C10 resembled EAAT1-like characteristics more than EAAT2 (Fig. 3). The results from this chimera imply that the carboxyl terminus may play an additional role in determining differences in chloride permeation.
It appears that the major determinant of the differences in both chloride permeability and L-serine-O-sulfate substrate se-lectivity between EAAT1 and EAAT2 may be localized to the same region of the transporter, which includes transmembrane domains 9 and 10. In addition, the intracellular carboxyl-terminal region may also play a minor role in chloride permeation.
Although there are a number of similarities among the regions that confer the various functional differences between EAAT1 and EAAT2, there also appear to be some distinct differences in defining regions which confer inhibitor sensitivity compared with substrate selectivity and chloride permeability. Chimera E21C10 displays sensitivity to blockers, whereas chimera E12C10 is insensitive to them, indicating that region 4 does not contribute to the blocker sensitivity. Chimera E21C8, which contains the EAAT1 sequence in regions 3 and 4, is insensitive to kainate/threo-3-methylglutamate blockade, indicative of region 3 (transmembrane domains 9 -10) playing a role in the blocker site.
Chimera E12C7 contains regions 2, 3, and 4 EAAT2 sequence and is insensitive to kainate and threo-3-methylglutamate, indicating that region 3 alone does not account for all of the blocker site. The blocker sensitivity observed in chimera E21C10 is likely to be due to region 3 combined with other regions of the transporter forming the functional blocker site.
Chimera E121C7-8 supported a leak current (23) that was blocked by kainate (IC 50 95.1 Ϯ 23.2 M) and threo-3-methylglutamate (IC 50 22.1 Ϯ 2.9 M) ( Fig. 4 and Table III). However, the IC 50 values for inhibition of [ 3 H]glutamate transport by kainate are considerably higher (IC 50 Ͼ 300 M; data not shown). A possible explanation for this apparent discrepancy may be that kainate (and threo-3-methylglutamate) binds to separate sites on the transporter to block the leak current compared with block of transport.
The kainate/threo-3-methylglutamate-sensitive leak current was not observed in E21C10, the other kainate/threo-3-methylglutamate-sensitive chimeric transporter. The leak current observed in chimera E121C7-8 may have been generated as a consequence of the formation of the chimera and thus may cast doubt on the results obtained with this chimera in terms of defining a site that confers blocker sensitivity. It is possible that a new blocker site has been generated in this chimera through misfolding of the protein and hence may not be a true representation of a region conferring the blocker site.
In summary, the chimeric transporters indicate that region 3 (transmembrane domains 9 -10) is important for blocker sensitivity, but it appears that other regions of the glutamate transporter are also required for blocker activity.

DISCUSSION
Various methods for identifying functional domains of glutamate transporters are possible. We have used the chimeric protein approach because it has a number of advantages over other methods such as site-directed point mutations. The principal advantage is that only fully functional proteins that show characteristics of one or both the parental transporters are used in the analysis. As such the conclusions are based on positive results rather than alteration of function or in some instances loss of function associated with the generation of site-directed mutations. One limitation of this approach is that only information concerning domains that confer functional differences will be obtained. These domains may not necessarily coincide with a region that forms the glutamate recognition site.
Although more than 20 different chimeric glutamate transporters were constructed, electrogenic glutamate transport was detected only in the five chimeras depicted in Fig. 1. This suggests that the formation of functional transporters requires a number of specific interactions among amino acid residues from multiple regions. Subtle differences in these specific interactions between EAAT1 and EAAT2 may explain the functional differences between these two glutamate transporters. In the formation of the chimeric transporters, some of these interactions may be altered, leading to loss of function.
The utility of the chimera approach is based on both the degree of the structural relatedness and the functional differences between the two proteins being studied. This study presents structure-function analyses of chimeric constructs of two subtypes of glutamate transporters, EAAT1 and EAAT2. These two glutamate transporters were chosen because they display a high degree of homology, 65%, and the greatest degree of difference in substrate selectivity, blocker sensitivity, and chloride permeability among transporter subtypes while transporting glutamate with a similar EC 50 (8,15). The pharmacological and electrophysiological differences were exploited to define domains of the transporters which confer particular characteristics.
Three different molecular probes were employed to characterize EAAT1, EAAT2, and chimeric transporters. The first molecular probe, L-serine-O-sulfate, is a substrate for transporters which displays a difference in affinity (8,16,23). Chloride permeability, the second molecular probe, is markedly different between EAAT1 transport of D-aspartate and EAAT2 transport of D-aspartate (17) and was employed to delineate further the differences in EAAT1 and EAAT2 related to the pore of the transporter. Kainate (8), threo-3-methylglutamate, and (2S,4R)-4-methylglutamate (15) provide probes for an extracellular glutamate recognition site on EAAT2. (2S,4R)-4-Methylglutamate provided added information in that it is transported by EAAT1 (15).
The chimeric transporters define a domain including transmembrane regions 9 and 10, which is of primary importance in determining functional differences between EAAT1 and EAAT2. This region consists of 57 amino acids of which 17 are not conserved between EAAT1 and EAAT2. Three of the 17 amino acids have been allocated to extracellular domains, 10 are located in the two transmembrane domains, and 4 are positioned intracellularly according to the model by Slotboom and colleagues (14). This particular region is highly conserved among the five different human glutamate transporters. In this region EAAT1 has 68% identity with EAAT2, 78% identity with EAAT3, 92% identity with EAAT4, and 81% with EAAT5. It is generally accepted that highly conserved amino acid sequences form functionally important structures, and it may be predicted that critical changes in amino acid sequence in such a region will alter the functional properties of the protein. One would predict that one or more of the nonconserved 17 amino acids in transmembrane domains 9 and 10 contribute to differences in L-serine-O-sulfate substrate selectivity, differences in chloride permeability, and differences in blocker sensitivity between EAAT1 and EAAT2.
Although this region is clearly involved in determining functional differences, other regions must also be involved. A recent report, using chimeras generated from GLT-1 (mouse equivalent of EAAT2) and EAAC1 (mouse equivalent of EAAT3) has suggested that an intracellular region between transmembrane domain 6 and transmembrane domain 7, from amino acid residues 301 to 338 of EAAC1, contributes to the formation of a dihydrokainate recognition site (25). The results of Kanai and co-workers (25) are consistent with the observation in this report, that other regions in addition to region 3 contribute to the formation of the kainate recognition site. These results pose an interesting conundrum as to how an intracellular site and two transmembrane domains form an extracellular recognition site for kainate or dihydrokainate. Two possibilities may be considered. First, the topological model is incorrect, and the 38-amino acid residue domain identified by Kanai et al. (25) is not intracellular but within the membrane or extracellular. The second possibility is that the intracellular segment may associate with transmembrane domains 9 and 10 to alter the conformation of an extracellular site to that kainate or dihydrokainate binds. Clearly, finer resolution of the topological model is required before these alternate possibilities may be resolved.
The intracellular carboxyl-terminal domain may also contribute to differences between EAAT1 and EAAT2, defined by region 4. The intracellular carboxyl terminus has very low sequence homology between the two transporter subtypes, and predictions of which amino acid residues are conferring these minor effects is difficult. The intracellular carboxyl terminus appears to confer functional differences to EAAT1 and EAAT2 in conjunction with other regions. The effects on chloride permeability may result from an association of the intracellular carboxyl terminus with other intracellular domains, which in turn influence the conformation of the pore of the transporter.
It has been suggested that the highly conserved region from amino acid residues 383-415 (EAAT1 numbering) forms part of the pore of the transporter, based on point mutations (11,13) and cysteine scanning mutagenesis. 1 Mutation of glutamate 404 in transmembrane domain 7 of GLT-1 (rat equivalent to EAAT2) to aspartate disrupts glutamate transport without altering glutamate binding (13). The mutation of glutamate 404 to aspartate changes substrate selectivity and prevents both forward and reverse transport of potassium ions, whereas sodium transport and chloride ion fluxes are unaffected (11,13). These results suggest that glutamate 404 does not disrupt substrate recognition but alters the permeation pathway and ion coupling of the transporter (11,13). Thus, Kavanaugh and co-workers (11) concluded that this residue is closely associated with the pore of the transporter.
The importance of transmembrane domains 9 and 10 in determining differences in L-serine-O-sulfate substrate selectivity and chloride permeability has been identified in this study. This result implies either that transmembrane domains 9 and 10 form part of the pore region of the transporter or that they have an interactive role with the pore region, influencing how substrates and chloride permeate the pore of the transporter. Hence, an interaction between transmembrane domains 9 and 10 and the highly conserved region which includes transmembrane domain 7 may form the basis of the differences observed in transport parameters between EAAT1 and EAAT2.
In this study we have identified an important region, transmembrane domains 9 and 10, which plays a role in defining functional differences in substrate selectivity, chloride permeability, and blocker sensitivity between EAAT1 and EAAT2. Other regions are also involved in defining the blocker sensitivity of EAAT2. A minor role is identified for the intracellular carboxyl-terminal region conferring an effect on chloride permeability. Further point mutations in these regions will assist in defining better the molecular basis for substrate recognition, substrate and ion translocation through the transporter, and release of substrates at the intracellular surface. As the topology of the excitatory amino acid transporters becomes more conclusively defined, the regions identified in this study will allow more directed predictions of the amino acid residues responsible for the differences in the transport process of EAAT1 and EAAT2.