title: Solute-binding protein-dependent ABC transporters are responsible for solute efflux in addition to solute uptake.
abstract: The ATP-binding cassette (ABC) transporter superfamily is one of the most widespread of all gene families and currently has in excess of 1100 members in organisms ranging from the Archaea to manQ1. The movement of the diverse solutes of ABC transporters has been accepted as being strictly unidirectional, with recent models indicating that they are irreversible. However, contrary to this paradigm, we show that three solute-binding protein-dependent (SBP) ABC transporters of amino acids, i.e. the general amino acid permease (Aap) and the branched-chain amino acid permease (Bra) of Rhizobium leguminosarum and the histidine permease (His) of Salmonella typhimurium, are bidirectional, being responsible for efflux in addition to the uptake of solutes. The net solute movement measured for an ABC transporter depends on the rates of uptake and efflux, which are independent; a plateau is reached when both are saturated. SBP ABC transporters promote active uptake because, although the Vmax values for uptake and efflux are not significantly different, there is a 103-104 higher affinity for uptake of solute compared with efflux. Therefore, the SBP ABC transporters are able to support a substantial concentration gradient and provide a net uptake of solutes into bacterial cells.
authors.affiliation: School of Animal and Microbial Sciences, University of Reading, Whiteknights, PO Box 228, Reading RG6 6AJ, UK.
authors.forename: A H
authors.initials: AH
authors.lastname: Hosie
authors.forename: D
authors.initials: D
authors.lastname: Allaway
authors.forename: M A
authors.initials: MA
authors.lastname: Jones
authors.forename: D L
authors.initials: DL
authors.lastname: Walshaw
authors.forename: A W
authors.initials: AW
authors.lastname: Johnston
authors.forename: P S
authors.initials: PS
authors.lastname: Poole
mesh_terms: D018528:ATP-Binding Cassette Transporters / Q000235:genetics / Q000378:metabolism*; D026905:Amino Acid Transport Systems; D026923:Amino Acid Transport Systems, Basic; D000596:Amino Acids / Q000378:metabolism*; D000621:Aminoisobutyric Acids / Q000378:metabolism; D001426:Bacterial Proteins; D001692:Biological Transport; D005656:Fungal Proteins; D007700:Kinetics; D026901:Membrane Transport Proteins / Q000235:genetics / Q000378:metabolism; D008969:Molecular Sequence Data; D019897:Periplasm / Q000378:metabolism; D016961:Rhizobium leguminosarum / Q000235:genetics / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins; D012486:Salmonella typhimurium / Q000235:genetics / Q000378:metabolism
chemical_list: D018528:ATP-Binding Cassette Transporters; D026905:Amino Acid Transport Systems; D026923:Amino Acid Transport Systems, Basic; D000596:Amino Acids; D000621:Aminoisobutyric Acids; C429560:BAP3 protein, S cerevisiae; D001426:Bacterial Proteins; D005656:Fungal Proteins; D026901:Membrane Transport Proteins; D029701:Saccharomyces cerevisiae Proteins; C043828:histidine permease, Bacteria