Shell component of a type 1 encapsulin nanocompartment. Assembles into proteinaceous shells 23-24 nm in diameter with 2-2.5 nm thick walls. Cargo protein Flp (ferritin-like protein, may store iron) is targeted to the interior via its C-terminal extension; empty intact shells can be isolated in the absence of cargo protein (PubMed:19172747, PubMed:27224728, PubMed:32961724, PubMed:30376298, PubMed:33769792, PubMed:33953921, PubMed:34815415). Fe(2+) may be able to pass though the 5-fold and dimer channels in the protein shell (Probable). Protease that exhibits activity toward chymotrypsin and trypsin substrates (PubMed:9872409, PubMed:11210524). Probably does not have antibacterial activity (Probable). A flavin ligand is bound near the 3-fold axis channel; FMN is consistent with the observed density, absorbance data and mass spectrometry. Proteolysis activated by calcium and cobalt. Optimum pH for protease activity is 7.1 (PubMed:11210524, PubMed:9872409). Nanocompartments are stable from pH 4-12, they reversibly disassemble at pH 1 and pH 13 (PubMed:27224728). Optimum temperature for protease activity is 90-93 degrees Celsius. Homomultimeric (PubMed:9872409). This encapsulin nanocompartment is formed by 60 subunits; monomers form 12 pentamers which assemble to form shells. There are 12 pores where the pentamers meet as well as 3-fold axis channels and dimer channels; none are larger than 3-4 Angstroms in diameter. The N-terminus of the protein is inside the shell, the C-terminus is outside. Globular particles 19-20 nm, with 60-80 nm long tails (Probable). Shells are 23-24 nm in diameter, walls are 2.0-2.5 nm thick (PubMed:19172747). Has 3 domains; a discontinuous peripheral domain (P, 13-39, 76-133, 221-254), an elongated loop (E, 53-73) and the discontinuous axial domain (A, 137-216 and 259-263). The E-loop forms contacts between two subunits, while the A domain mediates contacts in the 5-fold interface (PubMed:19172747). Pores are formed by residues 184-189, pore size can be modified by mutagenesis (PubMed:30376298, PubMed:33769792). Foreign proteins can be targeted to ectopic nanocompartments in E.coli upon coexpression with a construct using the 15 C-terminal residues of Flp (AC Q9WZP3), tested with GFP. There are at most 20 GFP per compartment. Empty nanocompartments can be disassembled by extreme pH or by 7 M guanidine hydrochloride (GuHCl); only reassembly from GuHCl-dissociated compartments allows incorporation of targeted cargo (PubMed:27224728). The central pore in the pentamer can be enlarged by modifying the pore-forming loop (residues 184-189), which allows transport of larger metabolites than wild-type pores (PubMed:30376298). Artificial metabolons made by targeting proteins to the outside and inside of the nanocompartment have also been made in E.coli; increasing pore size increases metabolic flux (PubMed:33769792). Foreign proteins targeted to nanocompartments have been purified and characterized from insect cells. By C-terminally tagging only IDM1 of A.thaliana, all 6 components of the IDM complex were targeted to the nanocompartment (PubMed:32961724). Shows substantial structural similarity to gp5 of the HK97 viral capsid; 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.