title: Protonation of glutamate 208 induces the release of agmatine in an outward-facing conformation of an arginine/agmatine antiporter.
abstract: Virulent enteric pathogens have developed several systems that maintain intracellular pH to survive extreme acidic conditions. One such mechanism is the exchange of arginine (Arg(+)) from the extracellular region with its intracellular decarboxylated form, agmatine (Agm(2+)). The net result of this process is the export of a virtual proton from the cytoplasm per antiport cycle. Crystal structures of the arginine/agmatine antiporter from Escherichia coli, AdiC, have been recently resolved in both the apo and Arg(+)-bound outward-facing conformations, which permit us to assess for the first time the time-resolved mechanisms of interactions that enable the specific antiporter functionality of AdiC. Using data from ∼1 μs of molecular dynamics simulations, we show that the protonation of Glu-208 selectively causes the dissociation and release of Agm(2+), but not Arg(+), to the cell exterior. The impact of Glu-208 protonation is transmitted to the substrate binding pocket via the reorientation of Ile-205 carbonyl group at the irregular portion of transmembrane (TM) helix 6. This effect, which takes place only in the subunits where Agm(2+) is released, invites attention to the functional role of the unwound portion of TM helices (TM6 Trp-202-Glu-208 in AdiC) in facilitating substrate translocation, reminiscent of the behavior observed in structurally similar Na(+)-coupled transporters.
authors.affiliation: Department of Computational & Systems Biology, School of Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania 15213, USA.
authors.forename: Elia
authors.initials: E
authors.lastname: Zomot
authors.forename: Ivet
authors.initials: I
authors.lastname: Bahar
mesh_terms: D000376:Agmatine / Q000737:chemistry* / Q000378:metabolism; D026905:Amino Acid Transport Systems / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D017920:Antiporters / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D001665:Binding Sites; D001692:Biological Transport / Q000502:physiology; D018360:Crystallography, X-Ray; D004926:Escherichia coli / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D029968:Escherichia coli Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D018698:Glutamic Acid / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D008958:Models, Molecular; D017433:Protein Structure, Secondary
chemical_list: C476528:AdiC protein, E coli; D026905:Amino Acid Transport Systems; D017920:Antiporters; D029968:Escherichia coli Proteins; D018698:Glutamic Acid; D000376:Agmatine