Abstract
The ferric iron uptake (Fiu) transporter from Escherichia coli functions in the transport of iron–catecholate complexes across the bacterial outer membrane, providing the bacterium with iron, which is essential for growth. Recently it has become clear that Fiu also represents a liability for E. coli because its activity allows import of antimicrobial compounds that mimic catecholate. This inadvertent import suggests the potential utility of antimicrobial catechol siderophore mimetics in managing bacterial infections. However, to fully exploit these compounds, a detailed understanding of the mechanism of transport through Fiu and related transporters is required. To address this question, we determined the crystal structure of Fiu at 2.1–2.9 Å and analyzed its function in E. coli. Through analysis of the Fiuo crystal structure, in combination with in silico docking and mutagenesis, we provide insight into how Fiu and related transporters bind catecholate in a surface-exposed cavity. Moreover, through determination of the structure of Fiu in multiple crystal states, we revealed the presence of a large, selectively gated cavity in the interior of this transporter. This chamber is large enough to accommodate the Fiu substrate and may allow import of substrates via a two-step mechanism. This would avoid channel formation through the transporter and inadvertent import of toxic molecules. As Fiu and its homologs are the targets of substrate-mimicking antibiotics, these results may assist in the development of these compounds.
Highlights
The ferric iron uptake (Fiu) transporter from Escherichia coli functions in the transport of iron– catecholate complexes across the bacterial outer membrane, providing the bacterium with iron, which is essential for growth
It has been demonstrated previously that the archetypical Escherichia coli strain BW25113 possesses three TonB-dependent transporter (TBDT) transporters that function in the uptake of catecholate siderophores: FepA, Cir, and Fiu [25]
We performed phylogenetic analysis of these transporters in the context of a panel of diverse TBDTs of known structure and/or function. This analysis revealed that, Cir and FepA belong to the same clade of the TBDT phylogram, Fiu belongs to a distal clade with the TBDTs PiuA and PiuD that mediate catecholate transport [30, 33] (Fig. 1A)
Summary
The ferric iron uptake (Fiu) transporter from Escherichia coli functions in the transport of iron– catecholate complexes across the bacterial outer membrane, providing the bacterium with iron, which is essential for growth. Aerobic organisms solubilize iron through the formation of iron-chelating chemicals (siderophores) or incorporate it into other organic structures, such as the porphyrin ring of heme or iron-binding proteins [5,6,7] These iron-containing complexes are larger than the diffusion limit of the bacterial outer membrane, and so, to obtain the iron required for growth, bacteria have evolved transporters capable of selectively binding and importing iron-containing complexes [8, 9]. It is thought that the presence of these chemical mimetics leads to their inadvertent import into the bacterial cell via Fiu [27] This Fiu-mediated sensitivity is observed even in the absence of iron because Fiu functions in import of catecholate-containing molecules, seemingly independent of their size or Fe coordination state [18].
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