Fusion of the shell and cargo protein of a type 1 encapsulin nanocompartment (PubMed:17397865). The nanocompartment is probably involved in iron storage (Probable). Expression in E.coli generates spherical particles (PfSPs) about 30 nm in diameter (PubMed:16091594). 180 monomers assemble into 12 pentamers and 20 hexamers which further assemble into an icosahedral particle about 36.6 nm in diameter. The ferritin-like domain N-terminal 109 disordered residues are located inside the organelle in the crystal. The rest of the protein has 3 structural domains; a discontinuous peripheral domain (P, 110-138, 167-210, 304-334), an elongated loop (E, 139-166) and the discontinuous axial domain (A, 211-303 and 335-345). The E loop is highly flexible and allows formation of the pentamers and hexamers from the same protein. 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. In the N-terminal section; belongs to the ferritin-like superfamily. In the C-terminal section; belongs to the encapsulin family. Family 1 subfamily.