Ubiquitin transferases catalyse the transfer of ubiquitin from one protein to another. They include ubiquitin-conjugating (E2) enzymes and ubiquitin ligase (E3) enzymes, which act sequentially to result in covalent attachment of ubiquitin to target proteins. Ubiquitination is required for many cellular processes, including protein degradation by the proteasome, cell cycle progression, transcriptional regulation, DNA repair and signal transduction. (Adapted from
PMID:27015313.)