Cargo protein of a type 1 encapsulin nanocompartment. A ferritin-like protein that probably stores iron in the encapsulin nanocompartment. Probably forms a decamer which binds to the pentameric axis of the interior of the protein shell; as the Flp cargo protein is flexible, packing into the shell is not rigid. The C-terminal 15-30 residues (cargo loading-peptide, CLP, or targeting peptide) are sufficient to target this protein to the nanocompartment in vivo, while the C-terminal 5 residues suffice in vitro. The 15 or 30 C-terminal residues can be used to target foreign proteins to the nanocompartment. Belongs to the ferritin-like superfamily.