Unknown, though MVP-beta is required for normal vault structure. The vault ribonucleoprotein particle is a huge (400 A x 670 A) cage structure of 12.9 MDa. It consists of a dimer of half-vaults, with each half-vault comprising 39 identical major vault protein (MVP) chains. Dictyostelium is one of the few organisms in which the major component is actually two proteins (alpha and beta).