Cargo protein of a type 1 encapsulin nanocompartment. A ferritin-like iron-binding protein probably involved in iron mineralization in the encapsulin nanocompartment. Has ferroxidase activity even when encapsulated, the rate is probably controlled by the rate of Fe flux across the nanocompartment pores (PubMed:31282860). Part of the iron-mineralizing encapsulin-associated Firmicute (IMEF) system. 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). 4 Fe(2+) + 4 H(+) + O2 = 4 Fe(3+) + 2 H2O 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. Homodimer, with 2 Fe atoms bound at the subunit interface (without encapsulin), probably also a dimer when encapsulated. 42 electron-dense accretions can be seen inside the nanocompartment which are probably this cargo protein, although perhaps up to one cargo dimer can be bound per shell protein. The C-terminus (targeting peptide) is probably responsible for targeting to the encapsulin nanocompartment; it is separated from the rest of the protein by a flexible 37-residue linker. 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 are not seen to mix.