From the Periplasmic Signaling Domain to the Extracellular Face of an Outer Membrane Signal Transducer of Pseudomonas aeruginosa: Crystal Structure of the Ferric Pyoverdine Outer Membrane Receptor

The pyoverdine outer membrane receptor, FpvA, from Pseudomonas aeruginosa translocates ferric pyoverdine across the outer membrane through an energy consuming mechanism using the proton motive force and the TonB–ExbB–ExbD energy transducing complex from the inner membrane. We solved the crystal structure of the full-length FpvA bound to iron-pyoverdine at 2.7 Å resolution. Signal transduction to an anti-sigma protein of the inner membrane and to TonB–ExbB–ExbD involves the periplasmic domain, which displays a β-α-β fold composed of two α-helices sandwiched by two β-sheets. One iron-pyoverdine conformer is bound at the extracellular face of FpvA, revealing the conformer selectivity of the binding site. The loop that contains the TonB box, involved in interactions with TonB, and connects the signaling domain to the plug domain of FpvA is not defined in the electron density following the binding of ferric pyoverdine. The high flexibility of this loop is probably necessary for signal transduction through the outer membrane.