Cryo-EM Structural Characterizations of EncB, a Ferritin-like Encapsulin Cargo Protein

Cells are compartmentalized to increase the metabolic efficiency and sequester redox-active substances accumulated under oxidative stress [1]. Unlike the membrane-bound organelles in eukaryotes, prokaryotes have protein-based compartments, such as encapsulin or nanocompartments [2, 3]. Encapsulins exist in many bacteria and archaea, including some medically important parasites, such as Mycobacterium tuberculosis and Myxococcus xanthus [4, 5]. However, the physiological roles of encapsulins are still not clearly understood. M. xanthus forms elaborate spore-filled fruiting bodies upon amino acid starvation, making this organism an ideal model to study cellular response to environmental changes [6]. These encapsulins contain four different proteins: the shell-forming protein EncA and three internal cargo proteins, EncB, EncC, and EncD. Sequence alignment suggests EncB and EncC are probably ferritin-like proteins [5]. M. xanthus encapsulins can store an order of magnitude more iron than ferritin suggesting the iron-binding role of EncB and EncC in the core [5].