Cargo protein of a type 1 encapsulin nanocompartment. May help nucleate Fe atoms in the interior of the encapsulin nanocompartment (Probable). Present in about 36 copies/encapsulin nanocompartment (PubMed:25024436). Probably lies against the interior face of the nanocompartment. Forms wild-type fruiting bodies. The encapsulin and a cargo construct (an encB-encC-encD fusion) can be overexpressed in E.coli and in human HEK293T cells. In HEK293T in the presence of 0.5 M ferrous ammonium sulfate nanocompartments can be detected and used as cell markers. Coexpression of this nanocompartment with a larger nanocompartment from Q.thermotolerans (AC A0A0F5HPP7) allows of expression of different sized iron-rich particles. The encapsulin shell proteins are not seen to mix. Belongs to the ferritin-like superfamily. Extended N-terminus.