| Abstract
| - In iron limitation conditions, Pseudomonas aeruginosa secretes a major fluorescent siderophorenamed pyoverdin (PaA). PaA has an extremely high affinity for Fe3+ but also chelates other ions such asAl3+ and Ga3+ with a lower affinity. The transfer of PaA−Fe3+ across the outer membrane of the bacteriais mediated by the receptor FpvA, a TonB-dependent outer membrane transport protein. FpvA binds theiron-free and iron-loaded forms of pyoverdin with similar affinities, but only PaA−Fe3+ is taken up bythe cell, suggesting that FpvA adopts different conformations depending on its loading status. We usedtime-resolved fluorescence spectroscopy to characterize the different forms of FpvA−PaA in vitro. Weshowed that the FpvA−PaA complex adopts two different conformations depending on how it was prepared(formed in vitro or in vivo prior to purification). The dihydroquinoline moiety of both conformers is fullyprotonated, or coordinated by protein charged groups, but the polarity of its environment, its solventaccessibility, and its rotational dynamics are much slower when the FpvA−PaA complex is formed invivo than in vitro. In the presence of Ga3+ or Al3+ ions, the solvent accessibility and mobility of thedihydroquinoline moiety in the two FpvA−PaA complexes are intermediate between those observed forthe metal-free ones. In addition, the Förster resonance energy transfer kinetics from FpvA tryptophanresidues to the PaA chromophore differs from one complex to the other, revealing differences in one ormore of the donor−acceptor topologies.
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