Encapsulin nanocages: Protein encapsulation and iron sequestration

Compartmentalization is an essential process that allows cells to organize, creating controlled microenvironments for specific metabolic pathways. Therefore, this allows the increase of reaction rates and/or protection of the cell from the effect of harmful substrates or products. Prokaryotes produce protein-based compartments by self-assembly of the protomers. Smaller and less complex than the eukaryotic organelles, they are designated as micro/nanocompartments. Encapsulins, one of the nanocompartments recently described, are protein nanocages that have the ability of sequestering other smaller proteins (cargo proteins) within their inner cavity. Depending on the organism, these porous nanocompartments can have different sizes and can encapsulate more than one type of cargo proteins, native or exogeneous. The physiological function of encapsulins is still unknown but seems to be determined by the type of encapsulated cargo proteins. The involvement of encapsulins in the iron metabolism and defense mechanisms against stress (oxidative and nutrient limitation) is nowadays consensual. Although significant progress has been made to understand the structural properties of encapsulins, much remain to be elucidated such as the mechanism of encapsulation or their physiological function. The present manuscript reviews not only the findings made at the structural level, for both the encapsulin shell and cargo proteins, as well as their functions so far described. The use of encapsulins as nanoreactors, nanomaterials, delivery platforms for diagnosis and therapeutics or imaging probes have been recently boosted and is also reviewed.