Shell component of a type 2A encapsulin nanocompartment. Forms encapsulin nanocompartments about 24 nm in diameter from 60 monomers. Probably encapsulates at least cysteine desulfurase (CyD, AC O32975) and allows passage of cysteine into its interior, probably involved in sulfur metabolism (By similarity). Expression in M.smegmatis generates a multimeric protein, whereas expression in E.coli does not (PubMed:8945562). Homooligomeric (PubMed:7476185, PubMed:8945562). The encapsulin nanocompartment is formed by 60 subunits; monomers form pentamers which assemble to form shells. There are 12 charged pores where the pentamers meet as well as 3-fold axis channels and dimer channels (By similarity). Soluble or peripheral membrane protein. Has 4 domains; an N-terminal arm not found in the type 1 subfamily, a discontinuous peripheral domain (P), an elongated loop (E) and the discontinuous axial domain (A). The N-terminus is blocked. Dominant antigen for this bacteria, could possibly be used in vaccine formulation and/or for diagnosis (PubMed:7476185, PubMed:8945562). Protein expressed in M.smegmatis stimulates a gamma interferon-secreting T-cell proliferative response (PubMed:8945562). Belongs to the encapsulin family. Family 2A subfamily.