Outer-membrane transporters

The crystal structure of FepA, the outer membrane transporter of Fe3+-enterobactin, has also been determined (Figure 3.3), and has a structure similar to the FhuA structure (Buchanan et al, 1999). In the FepA structure, the TonB box (residues 12-18) can be clearly seen located inside the barrel and extending into the periplasm, as one would expect from its interaction with the periplasmic portion of TonB. 

FhuA and FepA will prove to be the reference structures for a large group of bacterial outer-membrane transporters that take up bacterial Fe3+-siderophores, Fe3+ released from host transferrin and lactoferrin, haem, and haem released from haemoglobin and haemopexin. It is assumed that all iron sources are transported... 

FhuE FhuE Outer membrane transport (coprogen) K-12 (Hantke, 1983)... 

Regulation of Outer Membrane Transporter Synthesis by Phase Variation... 

Wiener MC. TonB-dependent outer membrane transport going for Baroque Curr. Opin. Struct. Biol. 2005 15 394-400. 

Kim M, Xu Q, Murray D, Cahso DS. Solutes alter the conformation of the ligand binding loops in outer membrane transporters. Biochemistry 2008 47 670-679. 

All outer-membrane transporters (OMTs) involved in iron uptake are made up of a 22-stranded p—barrel, which is occluded by an independently folded mixed a P globular cork domain of around 160 amino acid residues. This is illustrated for a vitamin B12 receptor and the ferric siderophore receptors for citrate, enterobactin, ferrichrome, pyochelin, and pyoverdin from E. coli and P. aeruginosa in Fig. 7.6. The ferric siderophore sits on top of the cork domain, as can be seen in Fig. 7.7 for FecA. The binding of the ligand induces a conformational... 

X-ray structures of wild-type N. gonorrhoeae Fbp " and mutant forms of H. influenzae Fbp have been described in which multinuclear oxo-metal clusters are bound to the protein. These surprising results are informative about aspects of the Fbp structure and formation of oxo-metal clusters on a protein surface, but it is not clear whether they have any physiological relevance. Sadler and coworkers present a case for this, noting that the outer membrane transport protein FecA passes diferric dicitrate into the periplasm and so at least one dinuclear iron complex is available to Fbp. 

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