A proteasome is a barrel-shaped multi-protein complex that can digest other proteins into short polypeptides and amino acids in an ATP-driven reaction. The proteasome is hollow, providing an enclosed space for protein digestion, and has openings at the two ends to allow entry of the targeted protein.
The 26S proteasome is used for the digestion of ubiquitin-marked proteins. It is located on both sides of a cell's nuclear membrane and consists of a 20S core protease particle and two 19S regulatory particles. The 20S unit consists of 2 rings of α subunits and 2 rings of β subunits, stacked in the order αββα as a series of heptomeric rings. It is about 15 nm long and 11.5 nm wide. Each 19S unit consists of a lid and a base with a 19S regulatory particle is attached to each end of the 20S core particle via its base. Some of the subunits in the base are ATPases.