The UPS and PROTAC mechanism of action. The ubiquitin-activating (E1) enzymes adenylate ubiquitin in an ATP-dependent reaction and the ubiquitin molecule is translocated to a ubiquitin-conjugating enzyme (E2). Ubiquitin-ligases (E3) bind to the target protein, recruiting the target to the E2-ubiquitin complex and catalyzing E2-driven transfer of ubiquitin to the target protein. Additional ubiquitin molecules may be attached to form a polyubiquitin chain, with variation in linkage configurations conferring different downstream effects on the protein’s fate. Once ubiquitinated, specific proteasomal subunits recognize the tagged protein via ubiquitin receptors or ubiquitin-associated domains. This initiates the unfolding of the protein and its loading into the proteolytic core of the proteasome, in which it is catalytically degraded. Deubiquitinating enzymes then liberate the ubiquitin molecules, enabling their participation in future ubiquitination reactions. ATP, adenosine triphosphate; Ub, ubiquitin molecules; TPr, target protein.