The 26S proteasome is used for the digestion of ubiquitin-marked proteins. It 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. It is about 150 Å long and 115 Å wide.
Each 19S unit consists of a lid and a base. One 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.