Cryo-EM Structure of Bacterioferritin Nanocages Provides Insight into the Bio-mineralization of Ferritins

Iron is an essential element involved in various metabolic processes. The ferritin family of proteins forms nanocage assembly and are involved in iron oxidation, storage and mineralization. Although several structures of human ferritin and bacterioferritin subunits have been resolved, there is still no complete structure that shows both the trapped Fe-biomineral cluster along with the nanocage. Furthermore, whereas the mechanism of iron trafficking has been explained using various approaches, an atomic-level description of the pathway and the biomineralization that occurs inside the cavity are lacking. Here, we report three cryo-EM structures of different states of the Streptomyces coelicolor bacterioferritin nanocage (i.e., apo, holo) at 3.4 [A] to 4.6 [A] resolution and the subunit crystal structure at 2.6 [A] resolution. The holo forms show different stages of Fe-biomineral accumulation inside the nanocage and suggest the possibility of a different Fe biomineral accumulation process. The cryo-EM map shows connections between the Fe-biomineral cluster and residues such as Thr157 and Lys42 from the protein shell, which are involved in iron transport. Mutation and truncation of the bacterioferritin residues involved in these connections can significantly reduce iron binding as compared with wild type bacterioferritin. Moreover, S. coelicolor bacterioferritin binds to various DNA fragments, similar to Dps (DNA-binding protein from starved cells) proteins. Collectively, our results represent a prototype for the ferritin nanocage, revealing insight into its biomineralization and the potential channel for ferritin-associated iron trafficking.