The Ferric Iron Uptake (Fiu) transporter is an integral membrane β-barrel protein, responsible for the import catecholate containing molecules across the bacterial outer-membrane (Nikaido, 2003). Catecholate groups have a strong affinity for iron and are often present in siderophores; iron scavenging molecules secreted by bacteria and fungi. As such, Fiu represents an important iron acquisition system in bacteria, through the import of catecholate containing iron-siderophore complexes. In addition to its role in iron uptake, Fiu has also been shown to be important for sensitivity to a diverse group of antibiotics which share the common feature of a catecholate functional group (Critchley et al., 1991). This sensitivity is observed even in the absence of iron, meaning that Fiu functions in the import of catecholate containing molecules independently of their size, charge or coordination state (Destoumieux-Garzón et al., 2006).
Until now, how Fiu achieved this substrate promiscuity was unknown. To answer this question we solved the crystal structure of Fiu in both the presence and absence of a catecholate-iron complex. Analysis of these structures show that while Fiu recognizes the catecholate group through a specific and conserved binding site, the remainder of the binding pocket is defined by large flexible loops. These unconstrained loops account of the flexibility in substrate binding and potentially allow Fiu to access chemically diverse siderophores, through piracy from other microorganisms. We have combined this structural analysis with functional mutagenesis. Through this analysis we define the amino acids crucial for catecholate binding and import, as well as highlight the importance of Fiu for bacterial growth under iron-limiting conditions.