Crystal Structure at High Resolution of Ferric-pyochelin and its Membrane Receptor FptA from Pseudomonas aeruginosa

Pyochelin is a siderophore and virulence factor common to Burkholderia cepacia and several Pseudomonas strains. We describe at 2.0 Å resolution the crystal structure of the pyochelin outer membrane receptor FptA bound to the iron-pyochelin isolated from Pseudomonas aeruginosa. One pyochelin molecule bound to iron is found in the protein structure, providing the first three-dimensional structure at the atomic level of this siderophore. The pyochelin molecule provides a tetra-dentate coordination of iron, while the remaining bi-dentate coordination is ensured by another molecule not specifically recognized by the protein. The overall structure of the pyochelin receptor is typical of the TonB-dependent transporter superfamily, which uses the proton motive force from the cytoplasmic membrane through the TonB–ExbB–ExbD energy transducing complex to transport ferric ions across the bacterial outer membrane: a transmembrane 22 β-stranded barrel occluded by a N-terminal domain that contains a mixed four-stranded β-sheet. The N-terminal TonB box is disordered in two crystal forms, and loop L8 is found to point towards the iron–pyochelin complex, suggesting that the receptor is in a transport-competent conformation.