Half ferritin, half virus: Bacterial subversion of a bacteriophage capsid as an iron megastore - Jon Marles-Wright (University of Newcastle)

Encapsulin nanocompartments are intracellular protein cages that are structurally related to the capsids of the HK97 bacteriophage and are distributed across many bacterial and archaeal species. Encapsulins sequester a number of different enzymes within their lumen through specific interactions with short localisation peptides on their cargo proteins. These cargoes include the well-characterised dye-dependent peroxidases, a new class of decameric ferritin, and a number of recently identified proteins with unknown functions. In the case of the dye-dependent peroxidases, the encapsulin cage protects the cell from oxidative damage resulting from the peroxide-radical mechanism these proteins employ to cleave carbon-carbon bonds in their substrates. Using X-ray crystallography in concert with structural Mass spectrometry, we have determined the structure of the decameric ferritin from Rhodospirillum rubrum and show that it possesses the ferroxidase activity found in other members of the ferritin family, but due to its open annular structure it is unable to directly sequester iron. We show that the encapsulin shell associated with these ferritins acts as a cage to store the ferric iron as ferrihydrite minerals. The encapsulin cages are between 25 and 35 nm in diameter, and have more interior space than the 8 nm ferritin nanocages; they are consequently able to store an order of magnitude more iron than classical ferritins. Our recent results on these decameric ferritins have identified the key residues responsible for both their multimerisation and ferroxidase activity.