title: Structure and mechanism of a glutamate-GABA antiporter.
abstract: Food-borne hemorrhagic Escherichia coli, exemplified by the strains O157:H7 and O104:H4 (refs 1, 2), require elaborate acid-resistance systems (ARs) to survive the extremely acidic environment such as the stomach (pH ≈ 2). AR2 expels intracellular protons through the decarboxylation of L-glutamate (Glu) in the cytoplasm and exchange of the reaction product γ-aminobutyric acid (GABA) with extracellular Glu. The latter process is mediated by the Glu-GABA antiporter GadC, a representative member of the amino-acid-polyamine-organocation superfamily of membrane transporters. The functional mechanism of GadC remains largely unknown. Here we show, with the use of an in vitro proteoliposome-based assay, that GadC transports GABA/Glu only under acidic conditions, with no detectable activity at pH  values higher than 6.5. We determined the crystal structure of E. coli GadC at 3.1 Å resolution under basic conditions. GadC, comprising 12 transmembrane segments (TMs), exists in a closed state, with its carboxy-terminal domain serving as a plug to block an otherwise inward-open conformation. Structural and biochemical analyses reveal the essential transport residues, identify the transport path and suggest a conserved transport mechanism involving the rigid-body rotation of a helical bundle for GadC and other amino acid antiporters.
authors.affiliation: Ministry of Education Protein Science Laboratory, Center for Structural Biology, Tsinghua University, Beijing 100084, China.
authors.forename: Dan
authors.initials: D
authors.lastname: Ma
authors.forename: Peilong
authors.initials: P
authors.lastname: Lu
authors.forename: Chuangye
authors.initials: C
authors.lastname: Yan
authors.forename: Chao
authors.initials: C
authors.lastname: Fan
authors.forename: Ping
authors.initials: P
authors.lastname: Yin
authors.forename: Jiawei
authors.initials: J
authors.lastname: Wang
authors.forename: Yigong
authors.initials: Y
authors.lastname: Shi
mesh_terms: D017920:Antiporters / Q000737:chemistry* / Q000378:metabolism*; D001692:Biological Transport; D018360:Crystallography, X-Ray; D019453:Escherichia coli O157 / Q000737:chemistry*; D029968:Escherichia coli Proteins / Q000737:chemistry* / Q000378:metabolism*; D018698:Glutamic Acid / Q000378:metabolism*; D008565:Membrane Proteins / Q000737:chemistry* / Q000378:metabolism*; D008958:Models, Molecular; D017434:Protein Structure, Tertiary; D011510:Proteolipids / Q000378:metabolism; D012399:Rotation; D013329:Structure-Activity Relationship; D005680:gamma-Aminobutyric Acid / Q000378:metabolism*
chemical_list: D017920:Antiporters; D029968:Escherichia coli Proteins; C491226:GadC protein, E coli; D008565:Membrane Proteins; D011510:Proteolipids; C020485:proteoliposomes; D018698:Glutamic Acid; D005680:gamma-Aminobutyric Acid