Shell component of a type 1 encapsulin nanocompartment. Assembles into proteinaceous icosahedral shells 42-43 nm in diameter with an iron- and phosphorus-rich core (1Fe:1.1P) which can store over 23,000-35,000 iron atoms (with a calculated maximum of 83,000 Fe). There are 2 types of negatively charged open pores in the cryo-electron structure; a 3-fold pore where 3 hexamers meet with a minimal size of 7.2 Angstroms and a 5-fold pore where pentamers meet with a minimal size of 2.3 Angstroms. The 2-fold pore seen in other encapsulin nanocompartments is closed. Empty compartments can be generated in E.coli (PubMed:28263314, PubMed:31282860, PubMed:31194509). Both types of pore have extra density in their centers in the structure (PubMed:31282860). 2 different cargo proteins have been identified (IMEF and Fer); when both are expressed in E.coli with the shell protein only IMEF is detected within the nanocompartment. E.coli expressing all 3 genes stores the largest amount of iron and is protected from Fe/H2O2-induced oxidative stress (PubMed:28263314). Part of the iron-mineralizing encapsulin-associated Firmicute (IMEF) system (Probable). The empty encapsulin nanocompartment is stable until 86.6 degrees Celsius, when loaded with cargo protein is stable until 88.9 degrees Celsius and when grown in high-iron conditions is stable until 91.8 degrees Celsius. Initially thought to form a 180 subunit shell (PubMed:28263314). Forms hollow shells composed of 240 subunits, making a shell about 42-43 nm in diameter (PubMed:31282860, PubMed:31194509). The monomer is capable of assuming 4 different conformations which allows packaging into the icosahedron. The shell has 12 pentameric and 30 hexameric capsomers which form the vertices and faces of the icosahedral nanocompartment (PubMed:31282860). Formed of E- and discontuous A- and P-loops. A-loops form the main pores while A- and E-loops are the most flexible parts. E- and P-loops of neighboring monomers arrange head-to-tail and form a chainmail topology. The encapsulin and cargo pair 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. Can also be targeted to cell membranes by addition of a farnesylation signal to its C-terminus. Coexpression of this nanocompartment with a smaller nanocompartment from M.xanthus (AC Q1D6H4) allows of expression of different sized iron-rich particles. The encapsulin shell proteins do not seem to mix. Belongs to the encapsulin family. Family 1 subfamily.