title: Disulfide cross-linking of transport and trimerization domains of a neuronal glutamate transporter restricts the role of the substrate to the gating of the anion conductance.
abstract: Excitatory amino acid transporters remove synaptically released glutamate and maintain its concentrations below neurotoxic levels. EAATs also mediate a thermodynamically uncoupled substrate-gated anion conductance that may modulate cell excitability. A structure of an archeal homologue, which reflects an early intermediate on the proposed substrate translocation path, has been suggested to be similar to an anion conducting conformation. To probe this idea by functional studies, we have introduced two cysteine residues in the neuronal glutamate transporter EAAC1 at positions predicted to be close enough to form a disulfide bond only in outward-facing and early intermediate conformations of the homologue. Upon treatment of Xenopus laevis oocytes expressing the W441C/K269C double mutant with dithiothreitol, radioactive transport was stimulated >2-fold but potently inhibited by low micromolar concentrations of the oxidizing reagent copper(II)(1,10-phenanthroline)3. The substrate-induced currents by the untreated double mutant, reversed at approximately -20 mV, close to the reversal potential of chloride, but treatment with dithiothreitol resulted in transport currents with the same voltage dependence as the wild type. It appears therefore that in the oocyte expression system the introduced cysteine residues in many of the mutant transporters are already cross-linked and are only capable of mediating the substrate-gated anion conductance. Reduction of the disulfide bond now allows these transporters to execute the full transport cycle. Our functional data support the idea that the anion conducting conformation of the neuronal glutamate transporter is associated with an early step of the transport cycle.
authors.affiliation: Department of Biochemistry and Molecular Biology, Institute for Medical Research Israel-Canada, Hebrew University Hadassah Medical School, Jerusalem 91120, Israel.
authors.forename: Mustafa
authors.initials: M
authors.lastname: Shabaneh
authors.affiliation: Department of Biochemistry and Molecular Biology, Institute for Medical Research Israel-Canada, Hebrew University Hadassah Medical School, Jerusalem 91120, Israel.
authors.forename: Noa
authors.initials: N
authors.lastname: Rosental
authors.affiliation: Department of Biochemistry and Molecular Biology, Institute for Medical Research Israel-Canada, Hebrew University Hadassah Medical School, Jerusalem 91120, Israel. Electronic address: baruch.kanner@mail.huji.ac.il.
authors.forename: Baruch I
authors.initials: BI
authors.lastname: Kanner
mesh_terms: D019943:Amino Acid Substitution; D000818:Animals; D004229:Dithiothreitol / Q000494:pharmacology; D050580:Excitatory Amino Acid Transporter 3 / Q000235:genetics / Q000378:metabolism*; D006367:HeLa Cells; D006801:Humans; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology*; D017136:Ion Transport / Q000187:drug effects / Q000502:physiology; D020125:Mutation, Missense; D009942:Organometallic Compounds / Q000494:pharmacology; D055503:Protein Multimerization / Q000187:drug effects / Q000502:physiology*; D017434:Protein Structure, Tertiary; D011817:Rabbits; D014982:Xenopus laevis
chemical_list: D050580:Excitatory Amino Acid Transporter 3; D009942:Organometallic Compounds; C514288:copper(2)-(10-phenanthroline)2 tetrachloride; D004229:Dithiothreitol
keywords: Gating; Glutamate; Membrane Biophysics; Membrane Energetics; Membrane Transport; Neurotransmitter Transport