Structural insights into a novel family of integral membrane siderophore reductases

Significance Secretion of siderophores allows most microbes to assimilate ferric ions into their biological processes. Siderophores must be taken up into the cells, and chelated iron must be released. Here, we present the structure of an inner membrane siderophore reductase, FoxB, which is involved in the uptake of iron from ferrioxamine siderophores in Pseudomonas aeruginosa. Our structure reveals FoxB to be a di-heme membrane protein, which is able to reduce the iron in chelated ferric-siderophore complexes. In combination with in vivo uptake studies, these results offer insights into the function of this poorly characterized membrane protein family and its role in iron release from bacterial siderophores. Gram-negative bacteria take up the essential ion Fe3+ as ferric-siderophore complexes through their outer membrane using TonB-dependent transporters. However, the subsequent route through the inner membrane differs across many bacterial species and siderophore chemistries and is not understood in detail. Here, we report the crystal structure of the inner membrane protein FoxB (from Pseudomonas aeruginosa) that is involved in Fe-siderophore uptake. The structure revealed a fold with two tightly bound heme molecules. In combination with in vitro reduction assays and in vivo iron uptake studies, these results establish FoxB as an inner membrane reductase involved in the release of iron from ferrioxamine during Fe-siderophore uptake.