1
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Iyalomhe O, Herrick DZ, Cafiso DS, Maloney PC. Closure of the cytoplasmic gate formed by TM5 and TM11 during transport in the oxalate/formate exchanger from Oxalobacter formigenes. Biochemistry 2014; 53:7735-44. [PMID: 25409483 PMCID: PMC4270380 DOI: 10.1021/bi5012173] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/04/2022]
Abstract
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OxlT, the oxalate/formate exchanger
of Oxalobacter
formigenes, is a member of the major facilitator superfamily
of transporters. In the present work, substrate (oxalate) was found
to enhance the reactivity of the cysteine mutant S336C on the cytoplasmic
end of helix 11 to methanethiosulfonate ethyl carboxylate. In addition,
S336C is found to spontaneously cross-link to S143C in TM5 in either
native or reconstituted membranes under conditions that support transport.
Continuous wave EPR measurements are consistent with this result and
indicate that positions 143 and 336 are in close proximity in the
presence of substrate. These two residues are localized within helix
interacting GxxxG-like motifs (G140LASG144 and
S336DIFG340) at the cytoplasmic poles of TM5
and TM11. Pulse EPR measurements were used to determine distances
and distance distributions across the cytoplasmic or periplasmic ends
of OxlT and were compared with the predictions of an inside-open homology
model. The data indicate that a significant population of transporter
is in an outside-open configuration in the presence of substrate;
however, each end of the transporter exhibits significant conformational
heterogeneity, where both inside-open and outside-open configurations
are present. These data indicate that TM5 and TM11, which form part
of the transport pathway, transiently close during transport and that
there is a conformational equilibrium between inside-open and outside-open
states of OxlT in the presence of substrate.
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Affiliation(s)
- Osigbemhe Iyalomhe
- Department of Physiology, The Johns Hopkins University, School of Medicine , 725 North Wolfe Street, Baltimore, Maryland 21205, United States
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2
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Yaffe D, Radestock S, Shuster Y, Forrest LR, Schuldiner S. Identification of molecular hinge points mediating alternating access in the vesicular monoamine transporter VMAT2. Proc Natl Acad Sci U S A 2013; 110:E1332-41. [PMID: 23530208 PMCID: PMC3625309 DOI: 10.1073/pnas.1220497110] [Citation(s) in RCA: 35] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Vesicular monoamine transporter 2 (VMAT2) catalyzes transport of monoamines into storage vesicles in a process that involves exchange of the charged monoamine with two protons. VMAT2 is a member of the DHA12 family of multidrug transporters that belongs to the major facilitator superfamily (MFS) of secondary transporters. Here we present a homology model of VMAT2, which has the standard MFS fold, that is, with two domains of six transmembrane helices each which are related by twofold pseudosymmetry and whose axis runs normal to the membrane and between the two halves. Demonstration of the essential role of a membrane-embedded glutamate and confirmation of the existence of a hydrogen bond probably involved in proton transport provide experimental evidence that validates some of the predictions inherent to the model. Moreover, we show the essential role of residues at two anchor points between the two bundles. These residues appear to function as molecular hinge points about which the two six transmembrane-helix bundles flex and straighten to open and close the pathways on either side of the membrane as required for transport. Polar residues that create a hydrogen bond cluster form one of the anchor points of VMAT2. The other results from hydrophobic interactions. Residues at the anchor points are strongly conserved in other MFS transporters in one way or another, suggesting that interactions at these locations will be critical in most, if not all, MFS transporters.
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Affiliation(s)
- Dana Yaffe
- Department of Biological Chemistry, Alexander A. Silberman Institute of Life Sciences, Hebrew University of Jerusalem, Jerusalem 91904, Israel; and
| | - Sebastian Radestock
- Computational Structural Biology Group, Max Planck Institute of Biophysics, 60438 Frankfurt am Main, Germany
| | - Yonatan Shuster
- Department of Biological Chemistry, Alexander A. Silberman Institute of Life Sciences, Hebrew University of Jerusalem, Jerusalem 91904, Israel; and
| | - Lucy R. Forrest
- Computational Structural Biology Group, Max Planck Institute of Biophysics, 60438 Frankfurt am Main, Germany
| | - Shimon Schuldiner
- Department of Biological Chemistry, Alexander A. Silberman Institute of Life Sciences, Hebrew University of Jerusalem, Jerusalem 91904, Israel; and
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3
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Kang DC, Venkataraman PA, Dumont ME, Maloney PC. Oligomeric state of the oxalate transporter, OxlT. Biochemistry 2011; 50:8445-53. [PMID: 21866906 DOI: 10.1021/bi201175y] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/09/2023]
Abstract
OxlT, the oxalate transporter of Oxalobacter formigenes, was studied to determine its oligomeric state in solution and in the membrane. Three independent approaches were used. First, we used triple-detector (SEC-LS) size exclusion chromatography to analyze purified OxlT in detergent/lipid micelles. These measurements evaluate protein mass in a manner independent of contributions from detergent and lipid; such work shows an average OxlT mass near 47 kDa for detergent-solubilized material, consistent with that expected for monomeric OxlT (46 kDa). A disulfide-linked OxlT mutant was used to verify that it was possible detect dimers under these conditions. A second approach used amino-reactive cross-linkers of varying spacer lengths to study OxlT in detergent/lipid micelles and in natural or artificial membranes, followed by analysis via sodium dodecyl sulfate-polyacrylamide gel electrophoresis. These tests, performed under conditions where the presence of dimers can be documented for either of two known dimeric transporters (AdiC or TetL), indicate that OxlT exists as a monomer in the membrane and retains this status upon detergent solubilization. In a final test, we showed that reconstitution of OxlT into lipid vesicles at variable protein/lipid ratios has no effect on the specific activity of subsequent oxalate transport, as the OxlT content varies between 0.027 and 5.4 OxlT monomers/proteoliposome. We conclude that OxlT is a functional monomer in the membrane and in detergent/lipid micelles.
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Affiliation(s)
- Di-Cody Kang
- Department of Physiology, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, United States
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4
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Castagna M, Soragna A, Mari SA, Santacroce M, Betté S, Mandela PG, Rudnick G, Peres A, Sacchi VF. Interaction between lysine 102 and aspartate 338 in the insect amino acid cotransporter KAAT1. Am J Physiol Cell Physiol 2007; 293:C1286-95. [PMID: 17626242 DOI: 10.1152/ajpcell.00190.2007] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
KAAT1 is a lepidopteran neutral amino acid transporter belonging to the NSS super family (SLC6), which has an unusual cation selectivity, being activated by K(+) and Li(+) in addition to Na(+). We have previously demonstrated that Asp338 is essential for KAAT1 activation by K(+) and for the coupling of amino acid and driver ion fluxes. By comparing sequences of NSS family members, site-directed mutagenesis, and expression in Xenopus laevis oocytes, we identified Lys102 as a residue likely to interact with Asp338. Compared with wild type, the single mutants K102V and D338E each showed altered leucine uptake and transport-associated currents in the presence of both Na(+) and K(+). However, in K102V/D338E double mutant, the K102V mutation reversed both the inhibition of Na(+)-dependent transport and the block in K(+)-dependent transport that characterize the D338E mutant. K(+)-dependent leucine currents were not observed in any mutants with D338E. In the presence of the oxidant Cu(II) (1,10-phenanthroline)(3), we observed specific and reversible inhibition of K102C/D338C mutant, but not of the corresponding single cysteine mutants, suggesting that these residues are sufficiently close to form a disulfide bond. Thus both structural and functional evidence suggests that these two residues interact. Similar results have been obtained mutating the bacterial transporter homolog TnaT. Asp338 corresponds to Asn286, a residue located in the Na(+) binding site in the recently solved crystal structure of the NSS transporter LeuT(Aa) (41). Our results suggest that Lys102, interacting with Asp338, could contribute to the spatial organization of KAAT1 cation binding site and permeation pathway.
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Affiliation(s)
- M Castagna
- Institute of General Physiology and Biological Chemistry G Esposito, Via Trentacoste 2, 20134, Milan, Italy
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5
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Yang Q, Wang X, Ye L, Mentrikoski M, Mohammadi E, Kim YM, Maloney PC. Experimental tests of a homology model for OxlT, the oxalate transporter of Oxalobacter formigenes. Proc Natl Acad Sci U S A 2005; 102:8513-8. [PMID: 15932938 PMCID: PMC1150865 DOI: 10.1073/pnas.0503533102] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/07/2005] [Indexed: 11/18/2022] Open
Abstract
Using the x-ray structure of the glycerol 3-phosphate transporter (GlpT), we devised a model for the distantly related oxalate transporter, OxlT. The model accommodates all earlier biochemical information on OxlT, including the idea that Lys-355 lies on the permeation pathway, and predicts that Lys-355 and a second positive center, Arg-272, comprise the binding site for divalent oxalate. Study of R272K, R272A, and R272Q derivatives verifies that Arg-272 is essential, and comparisons with GlpT show that both anion transporters bind substrates within equivalent domains. In 22 single-cysteine variants in TM7 and TM8, topology as marked by accessibility to Oregon green maleimide is predicted by the model, with similar concordance for 52 positions probed earlier. The model also reconciles cross-linking of a cysteine pair placed near the periplasmic ends of TM2 and TM7, and retrospective study of TM2 and TM11 confirms that positions supporting disulfide trapping lie at a helical interface. Our work describes a pathway to the modeling of OxlT and other transporters in the major facilitator superfamily and outlines simple experimental tests to evaluate such proposals.
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Affiliation(s)
- Qiang Yang
- Department of Physiology, Johns Hopkins Medical School, 725 North Wolfe Street, Baltimore, MD 21205-2185, USA
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6
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Beuming T, Weinstein H. Modeling membrane proteins based on low-resolution electron microscopy maps: a template for the TM domains of the oxalate transporter OxlT. Protein Eng Des Sel 2005; 18:119-25. [PMID: 15820982 DOI: 10.1093/protein/gzi013] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022] Open
Abstract
The availability of both EM and high-resolution crystallographic data for several membrane proteins (MPs) permits a detailed evaluation of the ability of molecular modeling techniques to complement EM data in the development of models of MPs. A protocol for this purpose is presented, consisting of (1) identifying transmembrane (TM) domains from sequence; (2) assigning buried and lipid-exposed faces of the TM domains; and (3) assembling the TM domains into a bundle, based on geometric restraints obtained from the EM data. The protocol is validated by predicting the structures of several 7- and 12-TM MPs to within 3-5 A r.m.s.d. from their crystal structures. The protocol is applied to generate a model of the oxalate transporter OxlT, for which a high-resolution structure is not yet available.
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Affiliation(s)
- Thijs Beuming
- Department of Physiology and Biophysics, Weill Medical College of Cornell University, 1300 York Avenue, New York, NY 10021, USA
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7
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Svedruzić D, Jónsson S, Toyota CG, Reinhardt LA, Ricagno S, Lindqvist Y, Richards NGJ. The enzymes of oxalate metabolism: unexpected structures and mechanisms. Arch Biochem Biophys 2005; 433:176-92. [PMID: 15581576 DOI: 10.1016/j.abb.2004.08.032] [Citation(s) in RCA: 106] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/12/2004] [Revised: 08/31/2004] [Indexed: 10/26/2022]
Abstract
Oxalate degrading enzymes have a number of potential applications, including medical diagnosis and treatments for hyperoxaluria and other oxalate-related diseases, the production of transgenic plants for human consumption, and bioremediation of the environment. This review seeks to provide a brief overview of current knowledge regarding the major classes of enzymes and related proteins that are employed in plants, fungi, and bacteria to convert oxalate into CO(2) and/or formate. Not only do these enzymes employ intriguing chemical strategies for cleaving the chemically unreactive C-C bond in oxalate, but they also offer the prospect of providing new insights into the molecular processes that underpin the evolution of biological catalysts.
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Affiliation(s)
- Drazenka Svedruzić
- Department of Chemistry, University of Florida, Gainesville, FL 32611-7200, USA
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8
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Hirai T, Heymann JAW, Maloney PC, Subramaniam S. Structural model for 12-helix transporters belonging to the major facilitator superfamily. J Bacteriol 2003; 185:1712-8. [PMID: 12591890 PMCID: PMC148079 DOI: 10.1128/jb.185.5.1712-1718.2003] [Citation(s) in RCA: 86] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
The major facilitator superfamily includes a large collection of evolutionarily related proteins that have been implicated in the transport of a variety of solutes and metabolites across the membranes of organisms ranging from bacteria to humans. We have recently reported the three-dimensional structure, at 6.5 A resolution, of the oxalate transporter, OxlT, a representative member of this superfamily. In the oxalate-bound state, 12 helices surround a central cavity to form a remarkably symmetrical structure that displays a well-defined pseudo twofold axis perpendicular to the plane of the membrane as well as two less pronounced, mutually perpendicular pseudo twofold axes in the plane of the membrane. Here, we combined this structural information with sequence information from other members of this protein family to arrive at models for the arrangement of helices in this superfamily of transport proteins. Our analysis narrows down the number of helix arrangements from about a billion starting possibilities to a single probable model for the relative spatial arrangement for the 12 helices, consistent both with our structural findings and with the majority of previous biochemical studies on members of this superfamily.
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Affiliation(s)
- Teruhisa Hirai
- Laboratory of Biochemistry, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892, USA
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9
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Sadlish H, Williams FMR, Flintoff WF. Functional role of arginine 373 in substrate translocation by the reduced folate carrier. J Biol Chem 2002; 277:42105-12. [PMID: 12194981 DOI: 10.1074/jbc.m206459200] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The reduced folate carrier (RFC) plays a critical role in the cellular uptake of folates. However, little is known regarding the mechanism used to transport substrates or the tertiary structure of the protein. Through the analysis of a Chinese hamster ovary cell line deficient in folate uptake, we have identified a single residue in TM10 (Arg-373) of RFC that appears to play a critical role in the translocation of substrate. Replacement of this position with various amino acids (KHQNA) diminished the rate of translocation by 16-50-fold, although substrate binding, protein stability, and localization were unaffected. Furthermore, the translocation capabilities of an R373C mutant in a cysteine-less form of the reduced folate carrier were enhanced 2.5-fold by the positively charged methanethiosulfonate reagent, confirming the essential role of a positive charge at this position. When considering the membrane-impermeable nature of this reagent, the data further suggest that the Arg-373 residue is located within the substrate translocation pathway of the RFC protein. Moreover, cross-linking analysis of the Arg-373 residue demonstrates that it is within 6 A of residue Glu-394 (TM11), providing the first definitive tertiary structural information for this protein.
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Affiliation(s)
- Heather Sadlish
- Department of Microbiology and Immunology, University of Western Ontario, London, Canada
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10
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Soskine M, Steiner-Mordoch S, Schuldiner S. Crosslinking of membrane-embedded cysteines reveals contact points in the EmrE oligomer. Proc Natl Acad Sci U S A 2002; 99:12043-8. [PMID: 12221291 PMCID: PMC129395 DOI: 10.1073/pnas.192392899] [Citation(s) in RCA: 74] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
EmrE is a small multidrug transporter that extrudes various drugs in exchange with protons, thereby rendering Escherichia coli cells resistant to these compounds. In this study, relative helix packing in the EmrE oligomer solubilized in detergent was probed by intermonomer crosslinking analysis. Unique cysteine replacements in transmembrane domains were shown to react with organic mercurials but not with sulfhydryl reagents, such as maleimides and methanethiosulfonates. A new protocol was developed based on the use of HgCl(2), a compound known to react rapidly and selectively with sulfhydryl groups. The reaction can bridge vicinal pairs of cysteines and form an intermolecular mercury-linked dimer. To circumvent problems inherent to mercury chemistry, a second crosslinker, hexamethylene diisocyanate, was used. After the HgCl(2) treatment, excess reagent was removed and the oligomers were dissociated with a strong denaturant. Only those previously crosslinked reacted with hexamethylene diisocyanate. Thus, vicinal cysteine-substituted residues in the EmrE oligomer were identified. It was shown that transmembrane domain (TM)-1 and TM4 in one subunit are in contact with the corresponding TM1 and TM4, respectively, in the other subunit. In addition, TM1 is also in close proximity to TM4 of the neighboring subunit, suggesting possible arrangements in the binding and translocation domain of the EmrE oligomer. This method should be useful for other proteins with cysteine residues in a low-dielectric environment.
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Affiliation(s)
- Misha Soskine
- Alexander Silberman Institute of Life Sciences, Hebrew University of Jerusalem, Jerusalem 91904, Israel
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11
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Ye L, Maloney PC. Structure/function relationships in OxlT, the oxalate/formate antiporter of Oxalobacter formigenes: assignment of transmembrane helix 2 to the translocation pathway. J Biol Chem 2002; 277:20372-8. [PMID: 11919184 DOI: 10.1074/jbc.m111140200] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
We constructed a single cysteine panel encompassing transmembrane helix two (TM2) of OxlT, the oxalate/formate antiporter of Oxalobacter formigenes. Among the 21 positions targeted, cysteine substitution identified one (phenylalanine 59) as essential to OxlT expression and three (glutamine 56, glutamine 66, and serine 69) as potentially critical to OxlT function. By probing membranes with a bulky hydrophilic probe (Oregon Green maleimide) we also located a central inaccessible core of at least eight residues in length, extending from leucine 61 to glycine 68. Functional assays based on reconstitution of crude detergent extracts showed that of single cysteine mutants within the TM2 core only the Q63C variant was substantially (> or =95%) inhibited by thiol-specific agents (carboxyethyl methanethiosulfonate and ethylsulfonate methanethiosulfonate). Subsequent analytical work using the purified Q63C protein showed that inhibition by ethylsulfonate methanethiosulfonate was blocked by substrate and that the concentration dependence of such substrate protection occurred with a binding constant of 0.16 mm oxalate, comparable with the Michaelis constant observed for oxalate transport (0.23 mm). These findings lead us to conclude that position 63 lies on the OxlT translocation pathway. Our conclusion is strengthened by the finding that position 63, along with most other positions relevant to TM2 function, is found on a helical face that can be cross-linked to the pathway-facing surface of TM11 (Fu, D., Sarker, R. I., Bolton, E., and Maloney, P. C. (2001) J. Biol. Chem. 276, 8753-8760).
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Affiliation(s)
- Liwen Ye
- Department of Physiology, Johns Hopkins Medical School, Baltimore, Maryland 21205, USA
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12
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Brocke L, Bendahan A, Grunewald M, Kanner BI. Proximity of two oppositely oriented reentrant loops in the glutamate transporter GLT-1 identified by paired cysteine mutagenesis. J Biol Chem 2002; 277:3985-92. [PMID: 11724778 DOI: 10.1074/jbc.m107735200] [Citation(s) in RCA: 67] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Sodium- and potassium-coupled transporters clear the excitatory neurotransmitter glutamate from the synaptic cleft. Their function is essential for effective glutamatergic neurotransmission. Glutamate transporters have an unusual topology, containing eight membrane-spanning domains and two reentrant loops of opposite orientation. We have introduced pairwise cysteine substitutions in several structural elements of the GLT-1 transporter. A complete inhibition of transport by Cu(II)(1,10-phenanthroline)(3) is observed in the double mutants A412C/V427C and A364C/S440C, but not in the corresponding single mutants. No inhibition is observed in more then 20 other double cysteine mutants. The Cu(II)(1,10-phenanthroline)(3) inhibition can be partly prevented by the nontransportable glutamate analogue dihydrokainate. Treatment with dithiothreitol restores much of the transport activity. Moreover, micromolar concentrations of cadmium ions reversibly inhibit transport catalyzed by A412C/V427C and A364C/S440C double mutants, but not by the corresponding single mutants. Inhibition by Cu(II)(1,10-phenanthroline)(3) and by cadmium is only observed when the cysteine pairs are introduced in the same polypeptide. Therefore, in both cases the proximity appears to be intra- rather than intermolecular. Positions 364 and 440 are located on reentrant loop I and II, respectively. Our results suggest that these two loops, previously shown to be essential for glutamate transport, come in close proximity.
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Affiliation(s)
- Lihi Brocke
- Department of Biochemistry, Hadassah Medical School, Hebrew University, Jerusalem 91120, Israel
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