Structural analysis and functional role of the carbohydrate component of glycine transporter

Elucidating the Mechanism Behind Sodium-Coupled Neurotransmitter Transporters by Reconstitution

Sodium-coupled neurotransmitter transporters play a fundamental role in the termination of synaptic neurotransmission, which makes them a major drug target. The reconstitution of these secondary active transporters into liposomes has shed light on their molecular transport mechanisms. From the earliest days of the reconstitution technique up to today's single-molecule studies, insights from live functioning transporters have been indispensable for our understanding of their physiological impact. The two classes of sodium-coupled neurotransmitter transporters, the neurotransmitter: sodium symporters and the excitatory amino acid transporters, have vastly different molecular structures, but complementary proteoliposome studies have sought to unravel their ion-dependence and transport kinetics. Furthermore, reconstitution experiments have been used on both protein classes to investigate the role of e.g. the lipid environment, of posttranslational modifications, and of specific amino acid residues in transport. Techniques that allow the detection of transport at a single-vesicle resolution have been developed, and single-molecule studies have started to reveal single transporter kinetics, which will expand our understanding of how transport across the membrane is facilitated at protein level. Here, we review a selection of the results and applications where the reconstitution of the two classes of neurotransmitter transporters has been instrumental.

Glycine transporter dimers: evidence for occurrence in the plasma membrane

Different Na(+)/Cl(-)-dependent neurotransmitter transporters of the SLC6a family have been shown to form dimers or oligomers in both intracellular compartments and at the cell surface. In contrast, the glycine transporters (GlyTs) GlyT1 and -2 have been reported to exist as monomers in the plasma membrane based on hydrodynamic and native gel electrophoretic studies. Here, we used cysteine substitution and oxidative cross-linking to show that of GlyT1 and GlyT2 also form dimeric complexes within the plasma membrane. GlyT oligomerization at the cell surface was confirmed for both GlyT1 and GlyT2 by fluorescence resonance energy transfer microscopy. Endoglycosidase treatment and surface biotinylation further revealed that complex-glycosylated GlyTs form dimers located at the cell surface. Furthermore, substitution of tryptophan 469 of GlyT2 by an arginine generated a transporter deficient in dimerization that was retained intracellulary. Based on these results and GlyT structures modeled by using the crystal structure of the bacterial homolog LeuT(Aa), as a template, residues located within the extracellular loop 3 and at the beginning of transmembrane domain 6 are proposed to contribute to the dimerization interface of GlyTs.

Differential distribution of glycine transporters in Müller cells and neurons in amphibian retinas

Amphibian retinas are commonly used for electrophysiological studies on neural function and transduction because they share the same general properties as higher vertebrate retinas. Glycinergic synapses have been well described in amphibian retinas. However, the role of glycine transporters in the synapses is largely unknown. We studied the distribution and function of glycine transporters in the retinas from tiger salamanders, mudpuppies, and leopard frogs by immunofluorescence labeling and whole-cell recording methods. Our results indicated that GlyT1- and GlyT2-like transporters were present in Müller cells and neurons, respectively. GlyT1 labeling was present in Müller glial cells and co-localized with Glial fibrillary acidic protein (GFAP), a Müller cell marker, whereas the GlyT2 immunoreactivity was present in the somas of amacrine cells (ACs) and processes in the inner plexiform layer (IPL) and the outer plexiform layer (OPL). Because the axon processes of glycinergic interplexiform cells (IPCs) are the only source of glycine input in the OPL, GlyT2 staining revealed a spatial pattern of the axon processes of IPCs in the OPL. The function of GlyT2 in the IPCs was studied in tiger salamander retinal horizontal cells (HCs) by whole-cell gramicidin perforated recording. The results demonstrated that inhibition of GlyT2 by a specific inhibitor, amoxapine, increased a tonic glycine input to HCs. Thus, the GlyT2 transporter is responsible for uptake of synaptic glycine in the outer retina. We also compared the distribution of glycine transporters in other amphibian species: salamander, mudpuppy, and frog. The results are consistent with the general pattern that GlyT1-like transporters are present in Müller cells and GlyT2-like transporters in neurons in amphibian retinas.

Mutations within the human GLYT2 (SLC6A5) gene associated with hyperekplexia

Hereditary hyperekplexia is a neuromotor disorder characterized by exaggerated startle reflexes and muscle stiffness in the neonate. The disease has been associated with mutations in the glycine receptor subunit genes GLRA1 and GLRB. Here, we describe mutations within the neuronal glycine transporter 2 gene (GLYT2, or SLC6A5, ) of hyperekplexia patients, whose symptoms cannot be attributed to glycine receptor mutations. One of the GLYT2 mutations identified causes truncation of the transporter protein and a complete loss of transport function. Our results are consistent with GLYT2 being a disease gene in human hyperekplexia.

Lessons from the knocked-out glycine transporters

Glycine has multiple neurotransmitter functions in the central nervous system (CNS). In the spinal cord and brainstem of vertebrates, it serves as a major inhibitory neurotransmitter. In addition, it participates in excitatory neurotransmission by modulating the activity of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptors. The extracellular concentrations of glycine are regulated by Na+/Cl(-)-dependent glycine transporters (GlyTs), which are expressed in neurons and adjacent glial cells. Considerable progress has been made recently towards elucidating the in vivo roles of GlyTs in the CNS. The generation and analysis of animals carrying targeted disruptions of GlyT genes (GlyT knockout mice) have allowed investigators to examine the different contributions of individual GlyT subtypes to synaptic transmission. In addition, they have provided animal models for two hereditary human diseases, glycine encephalopathy and hyperekplexia. Selective GlyT inhibitors have been shown to modulate neurotransmission and might constitute promising therapeutic tools for the treatment of psychiatric and neurological disorders such as schizophrenia and pain. Therefore, pharmacological and genetic studies indicate that GlyTs are key regulators of both glycinergic inhibitory and glutamatergic excitatory neurotransmission. This chapter describes our present understanding of the functions of GlyTs and their involvement in the fine-tuning of neuronal communication.

Nonsynaptic localization of the excitatory amino acid transporter 4 in photoreceptors

Excitatory amino acid transporters (EAATs) are involved in regulating extracellular glutamate levels at synaptic regions in the CNS. EAAT1, 2, 3, and 5 have been found in the mammalian retina, but the presence of EAAT4 has remained controversial. Recently, we found a high level of EAAT4 mRNA in the human retina, and this observation lead us to examine whether EAAT4 was expressed in the mammalian retina. Immunoblotting studies showed the presence of EAAT4-immunoreactive proteins in human and mouse retinas, corresponding to EAAT4 monomers and dimers. Immunohistochemistry revealed that EAAT4 was localized in rod and cone photoreceptor outer segments in the human retina, and in the outer and inner segments of mouse and ground squirrel retinas. In no case was EAAT4 found in the outer plexiform layer or in any other layer in the retina. EAAT4 expression by photoreceptors was confirmed by immunoblotting a purified rod outer segment preparation, which showed the presence of a 50-kDa EAAT4-immunoreactive protein. In addition, the EAAT4-associated protein, GTRAP41, was found in the human, mouse, and squirrel retinas as well as in the rod outer segment preparation. Further immunocytochemical and co-immunoprecipitation experiments demonstrated that GTRAP41 was colocalized and interacted in vivo with EAAT4. Importantly, glutamate uptake and drug inhibition experiments showed that an EAAT4-like glutamate uptake system is present in the rod outer segments. Finally, we examined whether glutamate signaling mediated by EAAT4 can modulate rod outer segment phagocytosis by the retinal pigment epithelium. Results of the present study show that EAAT4 is present in the outer segments, a nonsynaptic region of photoreceptors, where it might provide a feedback mechanism for sensing extracellular glutamate or serve as an outer barrier to prevent glutamate from escaping from the retina.

Structure, function and regulation of glycine neurotransporters

Glycine exerts multiple functions in the central nervous system, as an inhibitory neurotransmitter through activation of specific, Cl--permeable, ligand-gated ionotropic receptors and as an obligatory co-agonist with glutamate on the activation of N-methyl-D-aspartate (NMDA) receptors. In some areas of the central nervous system, glycine seems to be co-released with gamma-aminobutyric acid (GABA), the main inhibitory amino acid neurotransmitter. The synaptic action of glycine ends by active recapture through sodium- and chloride-coupled glycine transporters located in glial and neuronal plasma membranes, whose structure-function relationship is being studied. The trafficking and plasma membrane expressions of these proteins are controlled by regulatory mechanisms. Glycine transporter inhibitors may find application in the treatment of muscle tone defects, epilepsy, schizophrenia, pain and neurodegenerative disorders. This review deals on recent progress on localization, transport mechanisms, structure, regulation and pharmacology of the glycine transporters (GLYTs).

Membrane glycine transport proteins

Structurally, the simplest amino acid is glycine, and it has a number of important yet distinct functions in the body. This review focuses on the different transport systems and the associated carrier proteins for glycine that are responsible for its movement across biological membranes. Transport proteins in the class GLYT appear to be the most specific for glycine. However, the B0+ system also carries significant amounts of glycine. Other amino acid transport systems capable of carrying small amounts of glycine are ASC, asc and system L. In addition, an ATP-dependent transport process exists that takes up glycine into synaptic vesicles at nerve endings. This is known as the vesicular inhibitory amino acid transporter since, in addition to glycine, it can transport possibly two other inhibitory neurotransmitters.

Substrate-induced conformational changes of extracellular loop 1 in the glycine transporter GLYT2

The neurotransmitter glycine is removed from the synaptic cleft by two Na(+)-and Cl(-)-dependent transporters, the glial (GLYT1) and neuronal (GLYT2) glycine transporters. GLYT2 lacks a conserved cysteine in the first hydrophilic loop (EL1) that is reactive to [2-(trimethylammonium)ethyl] methanethiosulfonate (MTSET) in related transporters. A chimeric GLYT2 (GLYT2a-EL1) that contains GLYT1 sequences in this region, including the relevant cysteine, was sensitive to the reagent, and its sensitivity was decreased by co-substrates. We combined cysteine-specific biotinylation to detect transporter-reagent interactions with MTSET inactivation assays and temperature dependence analysis to study the mechanism by which Cl(-), Na(+), and glycine reduce methanethiosulfonate reagent inhibition. We demonstrate a Na(+) protective effect rather than an increased susceptibility to the reagent exerted by Li(+), as reported for the serotonin transporter. The different inhibition, protection, and reactivation properties between GLYT2a-EL1 and serotonin transporter suggest that EL1 is a source of structural heterogeneity involved in the specific effect of lithium on serotonin transport. The protection by Na(+) or Cl(-) on GLYT2a-EL1 was clearly dependent on temperature, suggesting that EL1 is not involved in ion binding but is subjected to ion-induced conformational changes. Na(+) and Cl(-) were required for glycine protection, indicating the necessity of prior ion interaction with the transporter for the binding of glycine. We conclude that EL1 acts as a fluctuating hinge undergoing sequential conformational changes during the transport cycle.

Pharmacological characterization and visualization of the glial serotonin transporter

Astrocytes contain transport systems that are capable of removing various neurotransmitters from the synaptic cleft by transporters present in the plasma membrane. Glial serotonin transporter (SERT) plays an important role in the re-uptake of 5-hydroxytryptamine (5-HT). We examined the pharmacological characterization of 5-HT uptake into rat cortical synaptosomes and cultured rat astrocytes, and the immunodetection of glial SERT proteins using specific site-directed monoclonal antibodies (MoAb). Furthermore, using a reverse transcriptase-polymerase chain reaction (RT-PCR) method, we addressed the expression of SERT mRNA in cultured rat astrocytes. We investigated the inhibitory effects of various monoamine uptake inhibitors on the uptake of [3H]5-HT into cultured astrocytes and cortical synaptosomes. Tricyclic antidepressants (clomipramine and imipramine) as well as selective serotonin re-uptake inhibitors (fluvoxamine, fluoxetine and zimelidine) were very potent inhibitors of [3H]5-HT uptake in both preparations. In contrast, the inhibitory effects of NE uptake inhibitors (nisoxetine and desipramine) and cocaine were weaker than those of 5-HT uptake inhibitors. In addition, dopamine (DA) uptake inhibitors (nomifensine and GBR-12935) exhibited a Ki value in the low micromolar range. The inhibitory potencies were in the order 5-HT uptake inhibitors (clomipramine, fluvoxamine, fluoxetine, imipramine and zimelidine) > NE uptake inhibitors (nisoxetine and desipramine) = cocaine > DA uptake inhibitors (nomifensine and GBR-12935). There was no difference in the order of the inhibitory effects of various monoamine uptake inhibitors between the two preparations. A correlation analysis of the potencies of various monoamine uptake inhibitors in the inhibition of [3H]5-HT into cultured astrocytes and cortical synaptosomes produced a highly significant correlation coefficient of 0.9893 (P < 0.0001). Immunocytochemical staining using anti-SERT MoAb in cultured astrocytes revealed that the plasma membrane, as well as intracellular, perinuclear compartments, presumably endoplasmic reticulum or golgi membranes, showed a considerable level of immunoreactivity. Extracts of astrocytes and synaptosomes from the cortex were immunoblotted with anti-SERT MoAb. SDS-PAGE/Western blots indicate that anti-SERT MoAb recognized two bands of 120 and 73 kDa in both preparations. RT-PCR demonstrated that astrocytes in cultured expressed mRNA for the cloned SERT protein, which has been characterized as the neuronal SERT. These pharmacological experiments indicate that this uptake process takes place through glial SERT that is very similar to neuronal SERT. Furthermore, the present data also indicate that the presence of the mRNA and protein for the neuronal SERT were established in cultured rat astrocytes, and the polypeptide portion of SERT in astrocytes and frontal cortex could be the same gene product.

The role of N-glycosylation in transport to the plasma membrane and sorting of the neuronal glycine transporter GLYT2

Glycine transporter GLYT2 is an axonal glycoprotein involved in the removal of glycine from the synaptic cleft. To elucidate the role of the carbohydrate moiety on GLYT2 function, we analyzed the effect of the disruption of the putative N-glycosylation sites on the transport activity, intracellular traffic in COS cells, and asymmetrical distribution of this protein in polarized Madin-Darby canine kidney (MDCK) cells. Transport activity was reduced by 35-40% after enzymatic deglycosylation of the transporter reconstituted into liposomes. Site-directed mutagenesis of the four glycosylation sites (Asn-345, Asn-355, Asn-360, and Asn-366), located in the large extracellular loop of GLYT2, produced an inactive protein that was retained in intracellular compartments when transiently transfected in COS cells or in nonpolarized MDCK cells. When expressed in polarized MDCK cells, wild type GLYT2 localizes in the apical surface as assessed by transport and biotinylation assays. However, a partially unglycosylated mutant (triple mutant) was distributed in a nonpolarized manner in MDCK cells. The apical localization of GLYT2 occurred by a glycolipid rafts independent pathway.

Transmembrane domain III plays an important role in ion binding and permeation in the glycine transporter GLYT2

The neuronal glycine transporter GLYT2 takes up glycine from the extracellular space by an electrogenic process where this neurotransmitter is co-transported with sodium and chloride ions. We report in this paper that tyrosine at position 289 of GLYT2a is crucial for ion coupling, glycine affinity and sodium selectivity, stressing the essential role played by this residue of transmembrane domain III in the mechanism of transport. Substitution to tryptophan (Y289W), phenylalanine (Y289F), or serine (Y289S), renders transporters unable to catalyze glycine uptake. Measurements of glycine evoked steady-state currents in transfected HEK-293 cells reveal EC(50) values for glycine 17-fold (Y289F) and 45-fold (Y289S) higher than that of the wild type transporter. Sodium dependence is severely altered in tyrosine 289 mutants, both at the level of apparent affinity and cooperativity, with the more dramatic change corresponding to the less conservative substitution (Y289S). Accordingly, sodium selectivity is gradually lost in Y289F and Y289S mutants, and chloride dependence of glycine evoked currents is markedly decreased in Y289F and Y289S mutants. In the absence of three-dimensional information from these transporters, these results provide experimental evidence supporting the hypothesis of transmembrane domain III being part of a common permeation pathway for substrate and co-transported ions.

Biosynthesis, post-translation modification, and functional characterization of Drm/Gremlin

Down-regulated by mos (Drm)/Gremlin is a highly conserved protein whose properties and expression pattern suggest a role in early development, tissue-specific differentiation, and cell transformation. We have investigated the biosynthesis and processing of Drm expressed endogenously in rat fibroblasts or overexpressed following transient or stable transfection. Analysis of metabolically labeled cells revealed that Drm exists in secreted and cell-associated forms that exhibit similar mobilities in SDS-polyacrylamide gel electrophoresis. Protein analysis indicated that Drm is present in two major species: a slow migrating glycosylated form and a nonglycosylated form. Both forms of Drm are able to undergo phosphorylation. Drm is released into the media within 30 min of synthesis and is detectable for up to 4-5 h, whereas the cell-associated form has a half-life of about 1 h. Confocal immunofluorescent microscopy indicates that Drm is present both on the external surface of expressing cells, as well as within the endoplasmic reticulum and the Golgi. Both glycosylated and nonglycosylated forms of Drm exhibit identical distributions and are able to antagonize bone morphogenetic protein signaling. Like the soluble form, the cell-associated forms are capable of binding (125)I-bone morphogenetic protein-4. These properties are consistent with a role for Drm in interfering with signaling and indicate that Drm may act at the cell surface during tissue development and transformation.

Differential effects of ethanol on glycine uptake mediated by the recombinant GLYT1 and GLYT2 glycine transporters

The effects of ethanol on the function of recombinant glycine transporter 1 (GLYT1) and glycine transporter 2 (GLYT2) have been investigated. GLYT1b and GLYT2a isoforms stably expressed in human embryonic kidney 293 (HEK 293) cells showed a differential behaviour in the presence of ethanol; only the GLYT2a isoform was acutely inhibited. The 'cut-off' (alcohols with four carbons) displayed by the n-alkanols on GLYT2a indicates that a specific binding site for ethanol exists on GLYT2a or on a GLYT2a-interacting protein. The non-competitive inhibition of GLYT2a indicates an allosteric modulation by ethanol of GLYT2a activity. Chronic treatment with ethanol caused differential adaptive responses on the activity and the membrane expression levels of these transporters. The neuronal GLYT2a isoform decreased in activity and surface expression and the mainly glial GLYT1b isoform slightly increased in function and surface density. These changes may be involved in some of the modifications of glycinergic or glutamatergic neurotransmitter systems produced by ethanol intoxication.

Differential effects of the tricyclic antidepressant amoxapine on glycine uptake mediated by the recombinant GLYT1 and GLYT2 glycine transporters

We examined the effects of nine different tricyclic antidepressant drugs on the glycine uptake mediated by the glycine transporter 1b (GLYT1b) and glycine transporter 2a (GLYT2a) stably expressed in human embryonic kidney 293 cells. Desipramine, imipramine, clomipramine, nomifensine and mianserin had no effect on the activity of the glycine transporters. Doxepin, amitriptyline and nortriptyline inhibited the two transporter subtypes to a similar extent. Amoxapine displayed a selective inhibition of GLYT2a behaving as a 10 fold more efficient inhibitor of this isoform than of GLYT1b. Kinetic analysis of the initial rates of glycine uptake by GLYT2a as a function of either glycine, chloride or sodium concentration, in the absence and presence of amoxapine indicated that amoxapine behaved as a competitive inhibitor of both glycine and chloride and a mixed-type inhibitor with respect to sodium. A kinetic model was developed which explains adequately these data, and gives information about the order of binding of sodium and chloride ions to GLYT2a. Our results may contribute to the development of the glycine transporter pharmacology. Additionally, the inhibition of the glycine uptake by GLYT2 is suggested to have some role in the sedative and psychomotor side effects of amoxapine. British Journal of Pharmacology (2000) 129, 200 - 206

Molecular biology of mammalian plasma membrane amino acid transporters

Molecular biology entered the field of mammalian amino acid transporters in 1990-1991 with the cloning of the first GABA and cationic amino acid transporters. Since then, cDNA have been isolated for more than 20 mammalian amino acid transporters. All of them belong to four protein families. Here we describe the tissue expression, transport characteristics, structure-function relationship, and the putative physiological roles of these transporters. Wherever possible, the ascription of these transporters to known amino acid transport systems is suggested. Significant contributions have been made to the molecular biology of amino acid transport in mammals in the last 3 years, such as the construction of knockouts for the CAT-1 cationic amino acid transporter and the EAAT2 and EAAT3 glutamate transporters, as well as a growing number of studies aimed to elucidate the structure-function relationship of the amino acid transporter. In addition, the first gene (rBAT) responsible for an inherited disease of amino acid transport (cystinuria) has been identified. Identifying the molecular structure of amino acid transport systems of high physiological relevance (e.g., system A, L, N, and x(c)- and of the genes responsible for other aminoacidurias as well as revealing the key molecular mechanisms of the amino acid transporters are the main challenges of the future in this field.

A product of DAN, a novel candidate tumour suppressor gene, is secreted into culture medium and suppresses DNA synthesis

Our previous studies have shown that the DAN gene product possesses an ability to revert phenotypes of transformed rat fibroblasts and represents a candidate tumour suppressor gene for neuroblastoma. In the present study, characterisation of DAN was carried out using rat fibroblast 3Y1 cells and their DAN-overexpressor counterparts (S-9). The N-terminal region of DAN (amino acids 1-24) was highly hydrophobic and DAN protein was found to be secreted into the culture medium. When DAN was treated with PNGase F, a enzyme that cleaves most N-linked carbohydrate residues, the mobility of both cytoplasmic and secreted DAN was increased in SDS-polyacrylamide gel electrophoresis, suggesting DAN is N-glycosylated, irrespective of its localisation. When partially purified, DAN was able, when added to the culture, to suppress DNA synthesis of Rous sarcoma virus-transformed 3Y1 cells, which lack the expression of DAN.

Molecular biology of glycinergic neurotransmission

Glycine is a major inhibitory neurotransmitter in the spinal cord and brainstem of vertebrates. Glycine is accumulated into synaptic vesicles by a proton-coupled transport system and released to the synaptic cleft after depolarization of the presynaptic terminal. The inhibitory action of glycine is mediated by pentameric glycine receptors (GlyR) that belong to the ligand-gated ion channel superfamily. The synaptic action of glycine is terminated by two sodium- and chloride-coupled transporters, GLYT1 and GLYT2, located in the glial plasma membrane and in the presynaptic terminals, respectively. Dysfunction of inhibitory glycinergic neurotransmission is associated with several forms of inherited mammalian myoclonus. In addition, glycine could participate in excitatory neurotransmission by modulating the activity of the NMDA subtype of glutamate receptor. In this article, we discuss recent progress in our understanding of the molecular mechanisms that underlie the physiology and pathology of glycinergic neurotransmission.

Differential ontogenic regulation of basolateral and canalicular bile acid transport proteins in rat liver

The hepatic transport systems mediating bile acid uptake and excretion undergo independent, stage-specific expression during development in the rat. In this study, the mechanisms underlying ontogenic regulation of both the Na(+)-dependent basolateral bile acid transporter and canalicular bile acid transporter/ecto-ATPase were examined. Steady state mRNA levels for the basolateral transporter were less than 20% of adult values prior to birth, increased to 35% on the first postnatal day, and reached adult levels by 1 week of age. This was paralleled by transcription rates, which were low prior to birth, reached 47% by day 1, and were maximal by 1 week of age. Steady state mRNA levels for ecto-ATPase were 12% of adult values prior to birth and showed a 2-fold increase by the first day of life. Thereafter, there was a gradual increase in mRNA for this transporter, with adult levels being reached at 4 weeks of age. Transcription rates paralleled this increment, although adult levels were reached earlier. Surprisingly, for both transporters, the full complement of protein was present well before adult levels of mRNA were reached. The basolateral protein was expressed at 82% of adult levels on the first day of life but was of lower apparent molecular mass (39 kDa), a difference that persisted until 4 weeks of age. N-Glycanase digestion suggested that this difference could be fully accounted for by N-linked glycosylation. The ecto-ATPase protein was present at 33% of adult levels prior to birth, 77% by 1 day, and 84% of adult levels by 1 week of age. Unlike the basolateral transporter, the apparent molecular weight of this protein did not change during development. In summary, the ontogeny of bile acid transporters on the plasma membrane of the hepatocyte is complex and appears to be regulated at transcriptional, translational, and post-translational levels.

The putative amino-terminal signal peptide of the cloned rat brain Na(+)-Ca2+ exchanger gene (RBE-1) is not mandatory for functional expression

The rat brain Na(+)-Ca2+ exchanger (RBE) gene, as well as other isoforms of this protein family, can be organized into 12 transmembrane alpha helices, the first of which was proposed by Durkin et al. (14) to constitute a cleavable signal peptide. We have prepared three amino-terminal mutants, in which 21, 26, and 31 amino acids beyond the initiating methionine were deleted. The deletions include the hydrophobic core of the putative signal peptide (N21), the entire putative signal peptide and parts of the putative signal peptidase cleavage site (N26), and the entire putative signal peptide and putative signal peptidase cleavage site (N31). All three mutant clones were transiently expressed in HeLa cells. The average Na+ gradient-dependent Ca2+ transport activity of the mutant exchangers was 108% (N21), 37.2% (N26), and 60.06% (N31) of the wild-type clone. Mutation of the putative cleavage site by an exchange of Ala-32 --> Asp, resulted in a decrease in Na(+)-Ca2+ exchange activity to 7.7%, relative to the wild-type exchanger. Functional reconstitution of the proteins that were expressed in the transfected cells, resulted in transport activities of: 60.1% (N21), 26.75% (N26), 85.36% (N31), and 31% (Ala-32 --> Asp) relative to the wild-type exchanger. Western blot analysis of the protein profile of RBE-1, N21, N26, N31 and Ala-32 --> Asp-transfected HeLa cells was carried out by using an antipeptide antibody directed against a pentadecapeptide segment derived from the large putative cytoplasmic loop of the cloned rat exchanger gene. In the total cell extract and in the plasma membrane-enriched fraction, in addition to a major protein band of about 125 kDa, which corresponds to the molecular mass of the mature fully processed Na(+)-Ca2+ exchanger, an additional protein of about 135 kDa is revealed in the profile of N21- and N26-transfected cells. This band is not detected in the protein profile of RBE-1, N31, or Ala-32 -->Asp. The amino-terminal truncated mutants of the cloned Na(+)-Ca2+ exchanger could be expressed and processed also in a reticulocyte lysate supplemented with dog microsomes. Our results suggest that the putative signal peptide of the cloned Na(+)-Ca2+ exchanger gene does not play a mandatory role in functional expression of the protein in HeLa cells.

Mammalian brain-specific L-proline transporter. Neuronal localization of mRNA and enrichment of transporter protein in synaptic plasma membranes

The expression of a high affinity Na(+)- (and Cl-) dependent L-proline transporter (PROT) in subpopulations of putative glutamatergic pathways in rat brain raises the possibility of a specific physiological role(s) for this carrier in excitatory neurotransmission (Fremeau, R. T., Jr., Caron, M. G., and Blakely, R. D. (1992) Neuron 8, 915-926). However, the biochemical properties and regional, cellular, and subcellular distribution of the PROT protein have yet to be elucidated. Here, we document the brain-specific expression and neuronal localization of rat PROT mRNA. We also report the first identification and partial biochemical characterization of the mammalian brain PROT protein. An affinity-purified antipeptide antibody was produced that specifically recognized a single 68-kDa PROT protein on immunoblots of rat and human brain tissues. Deglycosylation of rat hippocampal membranes with peptide-N-glycosidase F reduced the apparent molecular mass of the native PROT protein from 68 to 53 kDa, the size of the primary PROT translation product determined by in vitro translation of the rat PROT cDNA in the absence of microsomes. Subcellular fractionation studies demonstrated that the PROT protein was enriched in synaptic plasma membranes but absent from postsynaptic densities. A differential distribution of PROT mRNA and protein was observed in rat striatum, suggesting that the transporter protein is synthesized in neuronal cell bodies in the cortex and exported to axon terminals in the caudate putamen. These findings warrant the consideration of a novel presynaptic regulatory role for this transporter in excitatory synaptic transmission.

Localization of N-glycosylation sites and functional role of the carbohydrate units of GLAST-1, a cloned rat brain L-glutamate/L-aspartate transporter

The L-glutamate transporter GLAST-1 belongs to the newly discovered family of Na(+)-dependent, high-affinity glutamate transporters, which are involved in the regulation of synaptic excitatory neurotransmitter concentration in mammalian brain. The members of this family have a similar topological organisation with at least six transmembrane helices (TMHs) and two putative N-glycosylation sites located in the extracellular loop connecting TMH 3 and TMH 4. Besides these two conserved N-glycosylation motifs at Asn206 and Asn216, GLAST-1 possesses an additional one at Asn35. The putative N-glycosylation consensus motifs (Asn-Xaa-Ser/Thr) were deleted by replacement of Asn206 and/or Asn216 by Thr using site-directed mutagenesis (mutants N206T, N216T and N206,216T). The cDNAs encoding wild-type GLAST-1 and the three glycosylation-defective transport proteins were expressed in the Xenopus laevis oocyte system. Immunoprecipitation of the [35S]methionine-labeled and glycopeptidase-F-treated transporter molecules indicates that GLAST-1 is glycosylated at Asn206 and Asn216, whereas Asn35 remains unglycosylated. To assess a possible functional role of the two glycosylation sites wild-type and glycosylation-deficient GLAST-1 were expressed in Xenopus oocytes and characterized functionally by using the whole-cell voltage-clamp technique. The results prove that N-glycosylation has no impact on the transport activity of GLAST-1.

Localization of N-glycosylation sites and functional role of the carbohydrate units of GLAST-1, a cloned rat brain L-glutamate/L-aspartate transporter

The L-glutamate transporter GLAST-1 belongs to the newly discovered family of Na(+)-dependent, high-affinity glutamate transporters, which are involved in the regulation of synaptic excitatory neurotransmitter concentration in mammalian brain. The members of this family have a similar topological organisation with at least six transmembrane helices (TMHs) and two putative N-glycosylation sites located in the extracellular loop connecting TMH 3 and TMH 4. Besides these two conserved N-glycosylation motifs at Asn206 and Asn216, GLAST-1 possesses an additional one at Asn35. The putative N-glycosylation consensus motifs (Asn-Xaa-Ser/Thr) were deleted by replacement of Asn206 and/or Asn216 by Thr using site-directed mutagenesis (mutants N206T, N216T and N206,216T). The cDNAs encoding wild-type GLAST-1 and the three glycosylation-defective transport proteins were expressed in the Xenopus laevis oocyte system. Immunoprecipitation of the [35S]methionine-labeled and glycopeptidase-F-treated transporter molecules indicates that GLAST-1 is glycosylated at Asn206 and Asn216, whereas Asn35 remains unglycosylated. To assess a possible functional role of the two glycosylation sites wild-type and glycosylation-deficient GLAST-1 were expressed in Xenopus oocytes and characterized functionally by using the whole-cell voltage-clamp technique. The results prove that N-glycosylation has no impact on the transport activity of GLAST-1.

The role of N-glycosylation in the targeting and activity of the GLYT1 glycine transporter

To elucidate the role of N-glycosylation in the function of the high affinity glycine transporter GLYT1, we have investigated the effect of the glycosylation inhibitor tunicamycin as well as the effect of the disruption of the putative glycosylation sites by site-directed mutagenesis. SDS-polyacrylamide gel electrophoresis of proteins from GLYT1-transfected COS cells reveals a major band of 80-100 kDa and a minor one of 57 kDa. Treatment with tunicamycin produces a 40% inhibition in transport activity and a decrease in the intensity of the 80-100-kDa band, whereas the 57-kDa band decreases in size to yield a 47-kDa protein corresponding to the unglycosylated form of the transporter. Simultaneous mutation of Asn-169, Asn-172, Asn-182, and Asn-188 to Gln also produces the 47-kDa form of the protein, indicating that there are no additional sites for N-glycosylation. Progressive mutation of the potential glycosylation sites produces a progressive decrease in transport activity and in size of the protein, indicating that the four putative glycosylation sites are actually glycosylated. N-Glycosylation of the GLYT1 is not indispensable for the transport activity itself, as demonstrated by enzymatic deglycosylation of the transporter. Analysis of surface proteins by biotinylation and by immunofluorescence demonstrates that a significant portion of the unglycosylated GLYT1 mutant remains in the intracellular compartment. This suggests that the carbohydrate moiety of glycine transporter GLYT1 is necessary for the proper trafficking of the protein to the plasma membrane.