title: Cell envelope signaling in Escherichia coli. Ligand binding to the ferrichrome-iron receptor fhua promotes interaction with the energy-transducing protein TonB.
abstract: The ferrichrome-iron receptor of Escherichia coli is FhuA, an outer membrane protein that is dependent upon the energy-coupling protein TonB to enable active transport of specific hydroxamate siderophores, infection by certain phages, and cell killing by the protein antibiotics colicin M and microcin 25. In vivo cross-linking studies were performed to establish at the biochemical level the interaction between FhuA and TonB. In an E. coli strain in which both proteins were expressed from the chromosome, a high molecular mass complex was detected when the ferrichrome homologue ferricrocin was added immediately prior to addition of cross-linker. The complex included both proteins; it was absent from strains of E. coli that were devoid of either FhuA or TonB, and it was detected with anti-FhuA and anti-TonB monoclonal antibodies. These results indicate that, in vivo, the binding of ferricrocin to FhuA enhances complex formation between the receptor and TonB. An in vitro system was established with which to examine the FhuA-TonB interaction. Incubation of TonB with histidine-tagged FhuA followed by addition of Ni2+-nitrilotriacetate-agarose led to the specific recovery of both TonB and FhuA. Addition of ferricrocin or colicin M to FhuA in this system greatly increased the coupling between FhuA and TonB. Conversely, a monoclonal antibody that binds near the N terminus of FhuA reduced the retention of TonB by histidine-tagged FhuA. These studies demonstrate the significance of ligand binding at the external surface of the cell to mediate signal transduction across the outer membrane.
authors.affiliation: Department of Microbiology and Immunology, McGill University, Montreal, Quebec H3A 2B4, Canada.
authors.forename: G S
authors.initials: GS
authors.lastname: Moeck
authors.forename: J W
authors.initials: JW
authors.lastname: Coulton
authors.forename: K
authors.initials: K
authors.lastname: Postle
mesh_terms: D000818:Animals; D001425:Bacterial Outer Membrane Proteins / Q000378:metabolism*; D001426:Bacterial Proteins / Q000378:metabolism*; D001692:Biological Transport; D002352:Carrier Proteins / Q000378:metabolism; D004926:Escherichia coli / Q000378:metabolism*; D029968:Escherichia coli Proteins; D005291:Ferrichrome / Q000031:analogs & derivatives / Q000378:metabolism; D006041:Goats; D008024:Ligands; D008565:Membrane Proteins / Q000378:metabolism*; D051379:Mice; D011485:Protein Binding; D011487:Protein Conformation; D011956:Receptors, Cell Surface; D011991:Receptors, Virus / Q000378:metabolism*; D015398:Signal Transduction
chemical_list: D001425:Bacterial Outer Membrane Proteins; D001426:Bacterial Proteins; D002352:Carrier Proteins; D029968:Escherichia coli Proteins; C050900:FhuA protein, E coli; D008024:Ligands; D008565:Membrane Proteins; D011956:Receptors, Cell Surface; D011991:Receptors, Virus; C029489:enterobactin receptor; C057616:tonB protein, Bacteria; C489181:tonB protein, E coli; D005291:Ferrichrome; C021678:ferricrocin