Phage Capsid-like Structure of Myxococcus xanthus Encapsulin, a Protein Shell That Stores Iron

Iron is both an essential cofactor of many enzymes and a producer of highly reactive hydroxyl radicals that can cause cellular damage. To regulate the supply of intracellular iron, cells have developed protein-based organelles, like ferritins, that act as iron storage containers. Myxococcus xanthus, a soil-dwelling gram-negative myxobacterium, produces another type of protein-based organelle that has been related to iron metabolism, the encapsulin nanocompartment [1]. It has a spherical shell composed of a 32 kDa protein called EncA or encapsulin, and three minor proteins in the 11 kDa to 17 kDa range [1]. To gain insight on the molecular architecture of this complex, we have studied encapsulin nanocompartments purified from M. xanthus and recombinant EncA shells produced in E. coli using single-particle cryo-electron microscopy (cryo-EM). This analysis was supplemented with conventional TEM with and without negative staining and scanning transmission electron microscopy (STEM).