Figure 3.

The ubiquitin-proteasome system. The conserved 76-amino-acid polypeptide ubiquitin (Ub) is activated in an ATP-dependent reaction by an E1 ubiquitin-activating enzyme and transferred to an E2 ubiquitin-conjugating enzyme. An E3 ubiquitin ligase binds both the substrate protein and a ubiquitin-charged E2 enzyme for ubiquitin transfer, resulting in the mono-, multi- (not shown) or polyubiquitination of the substrate. The mode of ubiquitination determines whether the substrate protein is degraded by the 26S proteasome or altered in a non-proteolytic manner. See Boxes 1 and 2 for additional information.

Sarikas et al. Genome Biology 2011 12:220   doi:10.1186/gb-2011-12-4-220
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