The 20S proteasome core is composed of 14 alpha and 14 beta subunits that assemble into four stacked heptameric rings, resulting in a barrel-shaped structure. The two inner rings, each composed of seven catalytic beta subunits, are sandwiched by two outer rings, each composed of seven alpha subunits. The catalytic chamber with the active sites is on the inside of the barrel. Has a gated structure, the ends of the cylinder being occluded by the N-termini of the alpha-subunits. Is capped by the proteasome-associated ATPase, ARC (By similarity).