Shell component of a type 1, iron-storage encapsulin nanocompartment. Encapsulin nanocompartments are 32 nm in diameter with an iron- and phosphorus-rich core (4Fe:1P) about 24 nm in diameter. Upon expression in E.coli most particles are 32 nm, 20% are 18 nm. The core is filled with an average of 14 dense granules, 5-6 nm in diameter that are not evenly distributed. Each nanocompartment is estimated to hold 30,000-35,000 Fe atoms (PubMed:25024436, PubMed:31194509). The minor proteins EncB, EncC and EncD probably lie against the interior face of the nanocompartment (Probable). The 32 nm encapsulin nanocompartment is formed by 180 subunits; monomers form pentamers which assemble to form shells. There are 36 pores where the pentamers meet as well as 3-fold axis channels and dimer channels. The N-terminus of the protein is inside the shell. Part of the MXAN_3553-MXAN_3554-MXAN_3555-encA operon (PubMed:19996678). Expression is induced by amino acid starvation (at protein level). Not present in spores (at protein level) (PubMed:25024436). Has 3 domains; a discontinuous peripheral domain (P), an elongated loop (E) and the discontinuous axial domain (A). Does not form fruiting bodies (PubMed:19996678). Decreased survival when grown under oxidative stress (0.5 mM H2O2) (PubMed:25024436). 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 expression of different sized iron-rich particles. The encapsulin shell proteins do not seem to mix. Shows substantial structural similarity to gp5 of the HK97 viral capsid, and while the sequence homology is weak, it suggests this protein may have evolved from a viral capsid protein. Belongs to the encapsulin family. Family 1 subfamily. Extended N-terminus.