Ubiquitin ligase
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A Ubiquitin ligase is a protein which covalently attaches ubiquitin to a lysine residue on a target protein. The ubiquitin ligase is typically involved in polyubiquitylation: a second ubiquitin is attached to the first, a third is attached to the second, and so forth. Polyubiquitylation marks proteins for degradation by the proteasome.
The ubiquitin ligase is referred to as an E3 and operates in conjunction with an E1 ubiquitin-activating enzyme and an E2 ubiquitin-conjugating enzyme. There is one major E1 enzyme, shared by all ubiquitin ligases, which uses ATP to activate ubiquitin for conjugation and transfers it to an E2 enzyme. The E2 enzyme interacts with a specific E3 partner and transfers the ubiquitin to the target protein. The E3, which may be a multi-protein complex, is generally responsible for targeting ubiquitylation to specific substrate proteins. In some cases it receives the ubiquitin from the E2 enzyme and transfers it to the target protein; in other cases it acts by interacting with both the E2 enzyme and the substrate but never itself receives the ubiquitin.