title: Nuclear magnetic resonance solution structure of the periplasmic signalling domain of the TonB-dependent outer membrane transporter FecA from Escherichia coli.
abstract: Gram-negative bacteria possess outer membrane receptors that utilize energy provided by the TonB system to take up iron. Several of these receptors participate in extracytoplasmic factor (ECF) signalling through an N-terminal signalling domain that interacts with a periplasmic transmembrane anti-sigma factor protein and a cytoplasmic sigma factor protein. The structures of the intact TonB-dependent outer membrane receptor FecA from Escherichia coli and FpvA from Pseudomonas aeruginosa have recently been solved by protein crystallography; however, no electron density was detected for their periplasmic signalling domains, suggesting that it was either unfolded or flexible with respect to the remainder of the protein. Here we describe the well-defined solution structure of this domain solved by multidimensional nuclear magnetic resonance (NMR) spectroscopy. The monomeric protein construct contains the 79-residue N-terminal domain as well as the next 17 residues that are part of the receptor's plug domain. These form two clearly distinct regions: a highly structured domain at the N-terminal end followed by an extended flexible tail at the C-terminal end, which includes the 'TonB-box' region, and connects it to the plug domain of the receptor. The structured region consists of two alpha-helices that are positioned side by side and are sandwiched in between two small beta-sheets. This is a novel protein fold which appears to be preserved in all the periplasmic signalling domains of bacterial TonB-dependent outer membrane receptors that are involved in ECF signalling, because the hydrophobic residues that make up the core of the protein domain are highly conserved.
authors.affiliation: Structural Biology Research Group, Department of Biological Sciences, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4.
authors.forename: Alicia
authors.initials: A
authors.lastname: Garcia-Herrero
authors.forename: Hans J
authors.initials: HJ
authors.lastname: Vogel
mesh_terms: D000595:Amino Acid Sequence; D001692:Biological Transport; D048168:Escherichia coli K12; D029968:Escherichia coli Proteins / Q000737:chemistry* / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D008565:Membrane Proteins / Q000737:chemistry / Q000378:metabolism*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D019897:Periplasm / Q000378:metabolism*; D011956:Receptors, Cell Surface / Q000737:chemistry* / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism; D015398:Signal Transduction
chemical_list: D029968:Escherichia coli Proteins; C028589:FecA protein, E coli; D008565:Membrane Proteins; D011956:Receptors, Cell Surface; C489181:tonB protein, E coli