The Structure of the BfrB–Bfd Complex Reveals Protein–Protein Interactions Enabling Iron Release from Bacterioferritin

Ferritin-like molecules are unique to cellular iron homeostasis because they can store iron at concentrations much higher than those dictated by the solubility of Fe<sup>3+</sup>. Very little is known about the protein interactions that deliver iron for storage or promote the mobilization of stored iron from ferritin-like molecules. Here, we report the X-ray crystal structure of Pseudomonas aeruginosa bacterioferritin (<i>Pa</i>-BfrB) in complex with bacterioferritin-associated ferredoxin (<i>Pa</i>-Bfd) at 2.0 Å resolution. As the first example of a ferritin-like molecule in complex with a cognate partner, the structure provides unprecedented insight into the complementary interface that enables the [2Fe-2S] cluster of <i>Pa</i>-Bfd to promote heme-mediated electron transfer through the BfrB protein dielectric (∼18 Å), a process that is necessary to reduce the core ferric mineral and facilitate mobilization of Fe<sup>2+</sup>. The <i>Pa</i>-BfrB–Bfd complex also revealed the first structure of a Bfd, thus providing a first view to what appears to be a versatile metal binding domain ubiquitous to the large Fer2_BFD family of proteins and enzymes with diverse functions. Residues at the <i>Pa</i>-BfrB–Bfd interface are highly conserved in Bfr and Bfd sequences from a number of pathogenic bacteria, suggesting that the specific recognition between <i>Pa</i>-BfrB and <i>Pa</i>-Bfd is of widespread significance to the understanding of bacterial iron homeostasis.