title: The substrate-binding protein imposes directionality on an electrochemical sodium gradient-driven TRAP transporter.
abstract: Substrate-binding protein-dependent secondary transporters are widespread in prokaryotes and are represented most frequently by members of the tripartite ATP-independent periplasmic (TRAP) transporter family. Here, we report the membrane reconstitution of a TRAP transporter, the sialic acid-specific SiaPQM system from Haemophilus influenzae, and elucidate its mechanism of energy coupling. Uptake of sialic acid via membrane-reconstituted SiaQM depends on the presence of the sialic acid-binding protein, SiaP, and is driven by the electrochemical sodium gradient. The interaction between SiaP and SiaQM is specific as transport is not reconstituted using the orthologous sialic acid-binding protein VC1779. Importantly, the binding protein also confers directionality on the transporter, and reversal of sialic acid transport from import to export is only possible in the presence of an excess of unliganded SiaP.
authors.affiliation: Department of Biology (Area 10), University of York, P.O. Box 373, York YO10 5YW, United Kingdom.
authors.forename: Christopher
authors.initials: C
authors.lastname: Mulligan
authors.forename: Eric R
authors.initials: ER
authors.lastname: Geertsma
authors.forename: Emmanuele
authors.initials: E
authors.lastname: Severi
authors.forename: David J
authors.initials: DJ
authors.lastname: Kelly
authors.forename: Bert
authors.initials: B
authors.lastname: Poolman
authors.forename: Gavin H
authors.initials: GH
authors.lastname: Thomas
mesh_terms: D001692:Biological Transport; D004734:Energy Metabolism; D006193:Haemophilus influenzae / Q000737:chemistry*; D026901:Membrane Transport Proteins / Q000378:metabolism*; D019158:N-Acetylneuraminic Acid / Q000378:metabolism*; D027361:Organic Anion Transporters / Q000378:metabolism*; D011485:Protein Binding; D013379:Substrate Specificity; D027981:Symporters / Q000378:metabolism*; D014764:Viral Proteins / Q000378:metabolism
chemical_list: D026901:Membrane Transport Proteins; D027361:Organic Anion Transporters; D027981:Symporters; D014764:Viral Proteins; C421224:sialic acid transport proteins; D019158:N-Acetylneuraminic Acid