Kapsulin

Kapsulin is a family of protein-based nanocapsules found in bacteria and archaea. These self-assembling shells are built from multiple copies of a single capsid protein and form hollow, polyhedral shells that enclose an interior cavity. The shells are typically tens of nanometers in diameter and feature pores at the symmetry-appropriate positions to allow selective exchange of small molecules.

Kapsulin shells commonly associate with cargo proteins encoded nearby in the genome. Cargo proteins can be enzymes or binding partners that need to be kept away from the cytosol or concentrated within the capsule to enhance reaction efficiency. Targeting relationships are mediated by short sequence tags or fusion to cargo proteins that interact with the shell interior or with assembly factors. This encapsulation strategy is analogous to, yet distinct from, viral capsids and other bacterial encapsulin systems.

Functions attributed to kapsulin systems include protection of reactive enzymatic steps from the cellular milieu, sequestration of metal ions or reactive intermediates, and metabolic channeling. In some contexts, kapsulins are implicated in oxidative stress responses or specialized metabolic pathways such as detoxification of harmful compounds.

Structural studies using X-ray crystallography and cryo-electron microscopy have revealed icosahedral-like shells with defined pore rings that balance substrate access with cargo containment. Bioengineering efforts aim to repurpose kapsulin shells as nanoreactors or delivery platforms by modifying shell pores or cargo-attachment strategies.

Research continues to map the distribution, diversity, and physiological roles of kapsulins across prokaryotes and to explore their potential biotechnological applications.