Recognition of iron-free siderophores by TonB-dependent iron transporters.
TonB-dependent iron transporters reside in the outer membranes of Gram-negative bacteria, transporting ferric-complexes into the periplasm by a mechanism requiring proton motive force and an integral inner membrane complex, TonB-ExbB-ExbD. Certain TonB-dependent transporters contain an additional domain at the N-terminus, which interacts with an inner membrane regulatory protein and a cytoplasmic sigma factor to induce transcription of iron transport genes when a ferric-ligand is bound at the extracellular surface of the transporter. Transport of the ferric-ligand is apparently not necessary for transcription induction. Recent biophysical and crystallographic experiments have shown that this subclass of TonB-dependent iron transporters can bind iron-free ligands, whereas only the ferric-ligands are transported into the periplasm. This review focuses on the ligand binding properties of these transporters and includes a discussion of the biological function of the additional domain, the mechanism of transcription induction and the mechanism of ferric-ligand transport.
Schalk
Isabelle J
IJ
Département des Récepteurs et Protéines Membranaires, UPR 9050, CNRS, ESBS, Bld Sébastien Brant, F-67 400 Illkirch, Strasbourg, France. schalk@esbs.u-strasbg.fr
Yue
Wyatt W
WW
Buchanan
Susan K
SK
D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D001692:Biological Transport; D015964:Gene Expression Regulation, Bacterial; D006090:Gram-Negative Bacteria / Q000378:metabolism*; D007501:Iron / Q000378:metabolism*; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D008969:Molecular Sequence Data; D017262:Siderophores / Q000737:chemistry / Q000378:metabolism*
D001426:Bacterial Proteins; D008565:Membrane Proteins; D017262:Siderophores; C057616:tonB protein, Bacteria; D007501:Iron