title: TonB interacts with nonreceptor proteins in the outer membrane of Escherichia coli.
abstract: The Escherichia coli TonB protein serves to couple the cytoplasmic membrane proton motive force to active transport of iron-siderophore complexes and vitamin B(12) across the outer membrane. Consistent with this role, TonB has been demonstrated to participate in strong interactions with both the cytoplasmic and outer membranes. The cytoplasmic membrane determinants for that interaction have been previously characterized in some detail. Here we begin to examine the nature of TonB interactions with the outer membrane. Although the presence of the siderophore enterochelin (also known as enterobactin) greatly enhanced detectable cross-linking between TonB and the outer membrane receptor, FepA, the absence of enterochelin did not prevent the localization of TonB to the outer membrane. Furthermore, the absence of FepA or indeed of all the iron-responsive outer membrane receptors did not alter this association of TonB with the outer membrane. This suggested that TonB interactions with the outer membrane were not limited to the TonB-dependent outer membrane receptors. Hydrolysis of the murein layer with lysozyme did not alter the distribution of TonB, suggesting that peptidoglycan was not responsible for the outer membrane association of TonB. Conversely, the interaction of TonB with the outer membrane was disrupted by the addition of 4 M NaCl, suggesting that these interactions were proteinaceous. Subsequently, two additional contacts of TonB with the outer membrane proteins Lpp and, putatively, OmpA were identified by in vivo cross-linking. These contacts corresponded to the 43-kDa and part of the 77-kDa TonB-specific complexes described previously. Surprisingly, mutations in these proteins individually did not appear to affect TonB phenotypes. These results suggest that there may be multiple redundant sites where TonB can interact with the outer membrane prior to transducing energy to the outer membrane receptors.
authors.affiliation: School of Molecular Biosciences, Washington State University, Pullman, Washington 99164-4233, USA.
authors.forename: Penelope I
authors.initials: PI
authors.lastname: Higgs
authors.forename: Tracy E
authors.initials: TE
authors.lastname: Letain
authors.forename: Kelley K
authors.initials: KK
authors.lastname: Merriam
authors.forename: Neal S
authors.initials: NS
authors.lastname: Burke
authors.forename: HaJeung
authors.initials: H
authors.lastname: Park
authors.forename: ChulHee
authors.initials: C
authors.lastname: Kang
authors.forename: Kathleen
authors.initials: K
authors.lastname: Postle
mesh_terms: D001425:Bacterial Outer Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D001426:Bacterial Proteins / Q000378:metabolism*; D001692:Biological Transport; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism; D004926:Escherichia coli / Q000378:metabolism*; D029968:Escherichia coli Proteins / Q000235:genetics / Q000378:metabolism; D008074:Lipoproteins; D008565:Membrane Proteins / Q000378:metabolism*; D009154:Mutation; D011485:Protein Binding; D012965:Sodium Chloride
chemical_list: D001425:Bacterial Outer Membrane Proteins; D001426:Bacterial Proteins; D002352:Carrier Proteins; D029968:Escherichia coli Proteins; D008074:Lipoproteins; C099932:Lpp protein, E coli; D008565:Membrane Proteins; C057616:tonB protein, Bacteria; C489181:tonB protein, E coli; C078167:OMPA outer membrane proteins; D012965:Sodium Chloride