Energy-coupled transport and signal transduction through the gram-negative outer membrane via TonB-ExbB-ExbD-dependent receptor proteins

FEMS Microbiol Rev. 1995 Jul;16(4):295-307. doi: 10.1111/j.1574-6976.1995.tb00177.x.


Iron in the form of ferric siderophore complexes and vitamin B12 are transported through the outer membrane of Gram-negative bacteria by a mechanism which consumes energy. There is no known energy source in the outer membrane or in the adjacent periplasmic space so that energy is provided by the electrochemical potential across the cytoplasmic membrane. Energy flows from the cytoplasmic into the outer membrane via a complex consisting of the TonB, ExbB and ExbD proteins which are anchored in the cytoplasmic membrane. It is proposed that the TonB--ExbB--ExbD complex opens--via an energized conformation of the TonB protein--channels in the outer membrane, formed by proteins which serves as highly specific binding sites for the various ferric siderophores and vitamin B12. In addition, outer membrane receptors together with the TonB--ExbB--ExbD complex are directly involved in induction of the transcription of ferric citrate and pseudobactin transport genes of Escherichia coli and Pseudomonas putida, respectively.

Publication types

  • Research Support, Non-U.S. Gov't
  • Review

MeSH terms

  • Amino Acid Sequence
  • Bacterial Proteins / metabolism
  • Biological Transport / physiology
  • Cell Membrane / physiology*
  • Colicins / metabolism
  • Energy Metabolism*
  • Escherichia coli Proteins*
  • Ferric Compounds / metabolism
  • Gram-Negative Bacteria / physiology*
  • Membrane Proteins / metabolism
  • Molecular Sequence Data
  • Receptors, Cell Surface / metabolism*
  • Siderophores / metabolism
  • Signal Transduction / physiology*
  • Vitamin B 12 / metabolism


  • Bacterial Proteins
  • Colicins
  • Escherichia coli Proteins
  • ExbB protein, E coli
  • Ferric Compounds
  • Membrane Proteins
  • Receptors, Cell Surface
  • Siderophores
  • tonB protein, Bacteria
  • tonB protein, E coli
  • exbD protein, E coli
  • Vitamin B 12