title: Mechanism of substrate recognition and transport by an amino acid antiporter.
abstract: In extremely acidic environments, enteric bacteria such as Escherichia coli rely on the amino acid antiporter AdiC to expel protons by exchanging intracellular agmatine (Agm(2+)) for extracellular arginine (Arg(+)). AdiC is a representative member of the amino acid-polyamine-organocation (APC) superfamily of membrane transporters. The structure of substrate-free AdiC revealed a homodimeric assembly, with each protomer containing 12 transmembrane segments and existing in an outward-open conformation. The overall folding of AdiC is similar to that of the Na(+)-coupled symporters. Despite these advances, it remains unclear how the substrate (arginine or agmatine) is recognized and transported by AdiC. Here we report the crystal structure of an E. coli AdiC variant bound to Arg at 3.0 A resolution. The positively charged Arg is enclosed in an acidic binding chamber, with the head groups of Arg hydrogen-bonded to main chain atoms of AdiC and the aliphatic portion of Arg stacked by hydrophobic side chains of highly conserved residues. Arg binding induces pronounced structural rearrangement in transmembrane helix 6 (TM6) and, to a lesser extent, TM2 and TM10, resulting in an occluded conformation. Structural analysis identified three potential gates, involving four aromatic residues and Glu 208, which may work in concert to differentially regulate the upload and release of Arg and Agm.
authors.affiliation: Ministry of Education Protein Science Laboratory, Tsinghua University, Beijing 100084, China.
authors.forename: Xiang
authors.initials: X
authors.lastname: Gao
authors.forename: Lijun
authors.initials: L
authors.lastname: Zhou
authors.forename: Xuyao
authors.initials: X
authors.lastname: Jiao
authors.forename: Feiran
authors.initials: F
authors.lastname: Lu
authors.forename: Chuangye
authors.initials: C
authors.lastname: Yan
authors.forename: Xin
authors.initials: X
authors.lastname: Zeng
authors.forename: Jiawei
authors.initials: J
authors.lastname: Wang
authors.forename: Yigong
authors.initials: Y
authors.lastname: Shi
mesh_terms: D000376:Agmatine / Q000378:metabolism; D026905:Amino Acid Transport Systems / Q000737:chemistry* / Q000378:metabolism*; D017920:Antiporters / Q000737:chemistry* / Q000378:metabolism*; D001120:Arginine / Q000737:chemistry / Q000378:metabolism*; D001692:Biological Transport; D017124:Conserved Sequence; D018360:Crystallography, X-Ray; D029968:Escherichia coli Proteins / Q000737:chemistry* / Q000378:metabolism*; D006860:Hydrogen Bonding; D006863:Hydrogen-Ion Concentration; D057927:Hydrophobic and Hydrophilic Interactions; D011487:Protein Conformation; D017510:Protein Folding; D055503:Protein Multimerization; D011522:Protons; D055672:Static Electricity; D013329:Structure-Activity Relationship; D013379:Substrate Specificity
chemical_list: C476528:AdiC protein, E coli; D026905:Amino Acid Transport Systems; D017920:Antiporters; D029968:Escherichia coli Proteins; D011522:Protons; D000376:Agmatine; D001120:Arginine