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

Abstract

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 3+ . Very little is known about the protein interactions that deliver iron for storage or promote the mobilization of stored iron from ferritinlike molecules. Here, we report the X-ray crystal structure of Pseudomonas aeruginosa bacterioferritin (Pa-BfrB) in complex with bacterioferritin-associated ferredoxin (Pa-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 Pa-Bfd to promote hememediated 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 2+ . The Pa-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 Pa-BfrB−Bfd interface are highly conserved in Bfr and Bfd sequences from a number of pathogenic bacteria, suggesting that the specific recognition between Pa-BfrB and Pa-Bfd is of widespread significance to the understanding of bacterial iron homeostasis.

Bacterioferritin: Structure Function and Protein–Protein Interactions

“…To completely understand the participation of apo-Bfd in the release of iron, apo-Bfd was prepared and isolated in a subsequent study. 110 As expected, the apo-protein shows only a 280 nm band (red in Figure 17(B)), which differs from the spectrum of holo Pa Bfd (black trace) in that peaks at 334 nm, 424 nm, and 465 nm characteristic of an iron sulfur cluster are missing. The influence of apoBfd on iron release from Pa BfrB was then probed by addition of NADPH to a mixture of Pa BfrB, Pa FPR, and apo-Bfd in concentrations very similar to those used to study the effect of Pa Bfd.…”

“…To distinguish between these possibilities, the conditions resulting from preparing apo-Bfd in situ were emulated in a solution containing apo-Bfd, Pa BfrB, Pa FPR, Fe 2+ , S 2− , and bipy. 110 Addition of NADPH to this solution promotes iron release from Pa BfrB (black plot in Figure 17(C)). In contrast, iron release from a solution containing Pa BfrB, Pa FPR, Fe 2+ , S 2− , and bipy, but lacking apo-Bfd is inefficient (red in Figure 17(C)).…”

Ferritin iron uptake and release in the presence of metals and metalloproteins: Chemical implications in the brain

“…10). Recent theoretical studies [82] and experimental observations [138] commented above give nowadays further support to this proposal. The interaction of ferritin with multiple biomolecules, not strictly metalloproteins, has also been reported in the literature.…”

Unity in the Biochemistry of the Iron-Storage Proteins Ferritin and Bacterioferritin

“…202 A recent X-ray crystallography study using Pseudomonas aeroginosa bacterioferritin (PaBFR) showed that PaBfd associates with the bacterioferritin surface at the 2-fold symmetry axis on the top of the heme group ( Figure 17). 203 As a result, the edge-to-edge distance between the [2Fe-2S] cluster in PaBfd and the heme group in PaBFR is ca. 18 Å, suggesting long distance electron transfer 204 (Figure 17).…”