Schematic diagram of the ubiquitin-proteasome system. The UPS involves at least three enzymes (E1, E2, and E3) that catalyze the addition of ubiquitin to lysine residues on the substrate protein. Polyubiquitinated substrates are then recognized by the proteasome or proteasome associating protein, and the ubiquitin removed by deubiquitinases and the substrate unfolded and translocated in the 20S core for proteolysis.
