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Identification of the ABC protein SapD as the subunit that confers ATP dependence to the K⁺-uptake systems Trk^H and Trk^G from Escherichia coli K-12

Abteilung Mikrobiologie, Barbarastraße 11, Universität Osnabrück, D-49069 Osnabrück, Germany¹
Laboratory of Molecular Cell Biology, Faculty of Pharmaceutical Sciences, Chiba University, Inage-ku, Chiba 263, Japan²

Author for correspondence: Evert P. Bakker. Tel: +49 541 969 3515. Fax: +49 541 969 2870. e-mail: Bakker_E{at}biologie.uni-osnabrueck.de

The activity of the two almost identical K⁺-uptake systems, Trk^H and Trk^G, from Escherichia coli K-12 depends completely and partially on the presence of the trkE gene, respectively. trkE maps inside the sapABCDF operon, which encodes an ATP-binding cassette (ABC) transporter of unknown function from the subgroup of peptide-uptake systems. This study was carried out to clarify the role of sapABCDF gene products in the ATP dependence of the E. coli Trk systems. For this purpose sapABCDF trkG and sapABCDF trkH strains of E. coli containing plasmids with sap genes from either E. coli or Vibrio alginolyticus were used. All five plasmid-encoded E. coli Sap proteins were made in E. coli mini-cells. The presence of the ATP-binding SapD protein from either E. coli or V. alginolyticus alone was sufficient for stimulating the K⁺ transport activity of the Trk^H and Trk^G systems. K⁺-uptake experiments with Escherichia coli cells containing SapD variants with changes in the Walker A box Lys-46 residue, the Walker B box Asp-183 residue and the signature motif residues Gly-162 or Gln-165 suggested that adenine nucleotide binding to SapD rather than ATP hydrolysis by this subunit is required for the activity of the E. coli Trk^H system. K⁺ transport via two plasmid-encoded Trk systems in a sapABCDF E. coli strain remained dependent on both a high membrane potential and a high cytoplasmic ATP concentration, indicating that in E. coli ATP dependence of Trk activity can be independent of Sap proteins. These data are interpreted to mean that Trk systems can interact with an ABC protein other than SapD.

Keywords: ABC system, peptide transporter family, K⁺ transport, Kir-SUR system, regulation by adenine nucleotides

Abbreviations: ABC, ATP-binding cassette; EcTrk, VaTrk, Trk from E. coli and V. alginolyticus, respectively; EcSap and Ecsap, Sap proteins and sap genes, respectively, from E. coli; pmf, proton-motive force; VaSap and Vasap, Sap proteins and sap genes, respectively, from V. alginolyticus

The accession numbers for the nucleotide sequences reported in this paper are X97282 and AB015765.

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