PRE9, proteasome subunit α type-3, is a subunit of the proteasome. The core particle (CP) is a cylindrical particle composed of four stacked rings of subunits, with each ring containing seven distinct subunits. The outer rings are composed of seven α subunits, the inner rings of seven β subunits, which contain the active sites. The N-terminal segments of the α3 subunit seals the central channel, which prevents substrate entry by imposing topological closure on the CP. Inhibition by the α3 subunit tail is relieved upon binding of the regulatory particle to the CP to form the proteasome holoenzyme.