title: TonB-dependent outer membrane transport: going for Baroque?
abstract: The import of essential organometallic micronutrients (such as iron-siderophores and vitamin B(12)) across the outer membrane of Gram-negative bacteria proceeds via TonB-dependent outer membrane transporters (TBDTs). The TBDT couples to the TonB protein, which is part of a multiprotein complex in the plasma (inner) membrane. Five crystal structures of TBDTs illustrate clearly the architecture of the protein in energy-independent substrate-free and substrate-bound states. In each of the TBDT structures, an N-terminal hatch (or plug or cork) domain occludes the lumen of a 22-stranded beta barrel. The manner by which substrate passes through the transporter (the "hatch-barrel problem") is currently unknown. Solution NMR and X-ray crystallographic structures of various TonB domains indicate a striking structural plasticity of this protein. Thermodynamic, biochemical and bacteriological studies of TonB and TBDTs indicate further that existing structures do not yet capture critical energy-dependent and in vivo conformations of the transport cycle. The reconciliation of structural and non-structural experimental data, and the unambiguous experimental elucidation of a detailed molecular mechanism of transport are current challenges for this field.
authors.affiliation: Department of Molecular Physiology and Biological Physics, University of Virginia, PO Box 800736, Charlottesville, VA 22908-0736, USA. mwiener@virginia.edu
authors.forename: Michael C
authors.initials: MC
authors.lastname: Wiener
mesh_terms: D001425:Bacterial Outer Membrane Proteins / Q000737:chemistry / Q000378:metabolism; D001426:Bacterial Proteins / Q000737:chemistry / Q000378:metabolism; D001692:Biological Transport / Q000502:physiology; D018360:Crystallography, X-Ray; D008565:Membrane Proteins / Q000737:chemistry / Q000378:metabolism; D008958:Models, Molecular; D046912:Multiprotein Complexes; D011487:Protein Conformation
chemical_list: D001425:Bacterial Outer Membrane Proteins; D001426:Bacterial Proteins; D008565:Membrane Proteins; D046912:Multiprotein Complexes; C057616:tonB protein, Bacteria