Fig. 1. Overview of the ubiquitin-proteasome system.

Ubiquitination is a highly regulated process catalyzed by three types of enzymes, namely E1 (ubiquitin-activating enzyme), E2 (ubiquitin-conjugating enzyme), and E3 (ubiquitin-protein ligase). Ubiquitinated proteins are degraded in the proteasome. Deubiquitinase (DUB) can remove the ubiquitin (Ub) from substrates, thus preventing protein degradation and recycling of ubiquitin.