protein ubiquitination protein ubiquitination (ubiquitin transfer) UPa:UPA00833 The final step of the ubiquitinylation cascade generally requires the activity of one of the hundreds of E3 ubiquitin-protein ligases (often termed simply ubiquitin ligase). E3 enzymes function as the substrate recognition modules of the system and are capable of interaction with both E2 and substrate. E3 enzymes possess one of two domains: * The HECT (Homologous to the E6-AP Carboxyl Terminus) domain * The RING domain (or the closely related U-box domain). pathway