The proteasome

How does it work? The proteasome consists of a cylindrical 20 S catalytic chamber (see Figure), capped on one or both ends by a 19 S regulatory complex. The regulatory complex (which is made of at least 18 different subunits, six of which are ATPases; not shown in the Figure) recognizes ubiquitinated proteins and is thought to drive their unfolding, although the mechanism is not understood. The proteins are threaded through the small pores at the ends of the catalytic chamber, where three different protease activities, located on the inside face of the chamber, work to degrade them. The resulting peptides exit from the cylinder but it is not known how this occurs. The proteasome also has a deubiquitinating activity that recycles the ubiquitin chains from the degraded protein (see Figure).