Energy-coupled outer membrane transport proteins and regulatory proteins
- First Online:
FhuA and FecA are two examples of energy-coupled outer membrane import proteins of gram-negative bacteria. FhuA transports iron complexed by the siderophore ferrichrome and serves as a receptor for phages, a toxic bacterial peptide, and a toxic protein. FecA transports diferric dicitrate and regulates transcription of an operon encoding five ferric citrate (Fec) transport genes. Properties of FhuA mutants selected according to the FhuA crystal structure are described. FhuA mutants in the TonB box, the hatch, and the β-barrel are rather robust. TonB box mutants in FhuA FecA, FepA, Cir, and BtuB are compared; some mutations are suppressed by mutations in TonB. Mutant studies have not revealed a ferrichrome diffusion pathway, and tolerance to mutations in the region linking the TonB box to the hatch does not disclose a mechanism for how energy transfer from the cytoplasmic membrane to FhuA changes the conformation of FhuA such that bound substrates are released, the pore is opened, and substrates enter the periplasm, or how surface loops change their conformation such that TonB-dependent phages bind irreversibly and release their DNA into the cells. The FhuA and FecA crystal structures do not disclose the mechanism of these proteins, but they provide important information for specific functional studies. FecA is also a regulatory protein that transduces a signal from the cell surface into the cytoplasm. The interacting subdomains of the proteins in the FecA → FecR → FecI → RNA polymerase signal transduction pathway resulting in fecABCDE transcription have been determined. Energy-coupled transporters transport not only iron and vitamin B12, but also other substrates of very low abundance such as sugars across the outer membrane; transcription regulation of the transport genes may occur similarly to that of the Fec transport genes.
KeywordsBacterial iron and sugar transporters Signaling device ECF sigma factors
- Ferguson AD, Coulton JW, Diederichs K, Welte W (2001) The ferric hydroxamate uptake receptor FhuA and related TonB dependent transporters in the outer membrane of gram-negative bacteria. In: Messerschmidt A, Huber R, Poulos T, Wiegenhardt K (eds) Handbook of Metalloproteins. Chichester Ltd., Wiley, New York, pp 834–849Google Scholar
- Habeck M (1998) Energiekopplung durch TonB im Eisendicitrat-Transportsystem von Escherichia coli K-12. Thesis, University of TübingenGoogle Scholar
- Langenscheid J (2003) Aktivitäten von FhuA Mutanten von Escherichia coli K12. Diploma thesis. University of TübingenGoogle Scholar
- Pawelek PD, Croteau N, Ng-Thow-Hing C et al (2006) Structure of TonB in complex with FhuA, E. coli outer membrane receptor. Science 312:1399–1402Google Scholar