The proteasome is a multicatalytic proteinase complex which is characterized by its ability to cleave peptides with Arg, Phe, Tyr, Leu and Glu adjacent to the cleavage site at neutral or slightly basic pH. The proteasome has an ATP-dependent proteolytic activity. The 26S proteasome consists of a 20S proteasome core and two 19S regulatory subunits. The 20S proteasome core is composed of 28 subunits that are arranged in four stacked rings resulting in a barrel-shaped structure. The 20S core is part of the clastosomes, cellular bodies that contain ubiquitin conjugates, the proteolytically active 20S core and 19S regulatory complexes, and protein substrates of the proteasome.