The ubiquitin-proteasome pathway (UPP) regulates a variety of cellular processes by degradation of targeted proteins.[1] Ubiquitin (Ub) is first activated by ubiquitin-activating enzyme (E1), and the activated ubiquitin is transferred by transacylation to an active cysteine residue of ubiquitinconjugating enzymes (E2s). The E2 enzymes then cooperate with ubiquitin ligases (E3s) to attach Ub to the amino group of lysine residues on protein ...