The ubiquitin (Ub)-dependent pathway is a key regulatory mechanism which controls the degradation of proteins important in various biological processes such as cell cycle, DNA repair, transcriptional regulation, signal transduction, endocytosis and stress responses. It is therefore not surprising that deregulation of ubiquitin pathways has been implicated in the pathogenesis of many human disorders including neurodegeneration, inflammation and cancer.
Ubiquitylation is a covalent attachment of ubiquitin to the lysine residue(s) of a protein substrate which represents an important post-translational regulation of protein activity, half-life and localization. Ubiquitination of proteins requires a multi-enzyme system comprised of ubiquitin-activating enzyme (E1), ubiquitin-conjugating enzyme (E2), and ubiquitin ligase enzyme (E3). Ubiquitin is first activated by binding to E1 and then transferred to an E2 before covalently linked to a protein substrate in a reaction catalyzed by E3 ubiquitin ligase. Each E3 ubiquitin ligase recognizes a set of substrates and controls the specificity in ubiquitin-mediated protein degradation. Thus, targeting specific E3 ubiquitin ligases by using small molecules is a promising strategy to regulate the degradation of specific proteins.