TonB-dependent maltose transport by Caulobacter crescentus.
We have shown previously that Caulobacter crescentus grows on maltodextrins which are actively transported across the outer membrane by the MalA protein. Evidence for energy-coupled transport was obtained by deletion of the exbB exbD genes which abolished transport. However, removal of the TonB protein, which together with the ExbB ExbD proteins is predicted to form an energy-coupling device between the cytoplasmic membrane and the outer membrane, left transport unaffected. Here we identify an additional tonB gene encoded by the cc2334a ORF, which when deleted abolished maltose transport. MalA contains a TonB box that reads EEVVIT and is predicted to interact with TonB. Replacement of valine number 15 in the TonB box by proline abolished maltose transport. Maltose was transported across the cytoplasmic membrane by the MalY protein (CC2283). Maltose transport was induced by maltose and repressed by the MalI protein (CC2284). In addition to MalA, MalY and MalI, the mal locus encodes two predicted cytoplasmic alpha-amylases (CC2285 and CC2286) and a periplasmic glucoamylase (CC2282). The TonB dependence together with the previously described ExbB ExbD dependence demonstrates energy-coupled maltose transport across the outer membrane. MalY is involved in maltose transport across the cytoplasmic membrane by a presumably ion-coupled mechanism.
Max Planck Institute for Developmental Biology, Department of Protein Evolution, Tübingen, Germany.|Microbiology/Membrane Physiology, University of Tübingen, Germany.
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S
Lohmiller
Microbiology/Membrane Physiology, University of Tübingen, Germany.
K
K
Hantke
Max Planck Institute for Developmental Biology, Department of Protein Evolution, Tübingen, Germany.
S I
SI
Patzer
Max Planck Institute for Developmental Biology, Department of Protein Evolution, Tübingen, Germany.
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V
Braun
D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D001693:Biological Transport, Active; D016935:Caulobacter crescentus / Q000235:genetics* / Q000378:metabolism*; D015964:Gene Expression Regulation, Bacterial; D023061:Gene Order; D005798:Genes, Bacterial / Q000235:genetics; D008320:Maltose / Q000378:metabolism*; D008565:Membrane Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D008969:Molecular Sequence Data; D012097:Repressor Proteins / Q000235:genetics / Q000378:metabolism
D001426:Bacterial Proteins; D008565:Membrane Proteins; D012097:Repressor Proteins; C057616:tonB protein, Bacteria; D008320:Maltose