Proteasome-mediated proteolysis.

Polyubiquitinated proteins are substrates for proteolysis by the 26S proteasome, which is made up of a core 20S particle and two 19S regulatory cap structures at either end. The 20S core is arranged as four stacked rings, with each of the outer rings containing α subunits that serve a structural function, while the two inner rings contain β subunits that encode the proteolytic activities. Ub tags are recognized by subunits of the cap structure and removed by deubiquitinating enzymes for later reuse. Protein targets are then unwound in an ATP-dependent fashion and fed into a central chamber in the proteasome, where the proteases generate oligopeptides that are later further degraded outside of the proteasome into their constituent amino acids. This figure has been adapted from ScienceSlides 2005 (VisiScience Inc.; www.visiscience.com) with permission of the publisher.