Figure 2.

Ubiquitin-Proteasome System. A chain of ubiquitin moieties is attached by the action of a series of ubiquitin ligases (E1, E2, E3) to lysine residues on the target protein to be degraded. The ubiquitin-protein complex is transported to the proteasome, where the ubiquitin chain is removed, allowing the target protein to be unfolded and translocated to the interior of the proteasome, where it is degraded by 3 threonine proteases to yield peptide fragments. Adapted from Molineaux [67].