The anaphase-promoting complex/cyclosome: APC/C

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Abstract
E3 ubiquitin ligases represent the last step of a three-enzyme cascade that tags substrates with ubiquitin chains. First, in an ATP-dependent reaction, a unique ubiquitin-activating enzyme (E1) binds to and activates ubiquitin. Ubiquitin is then transferred to one of several ubiquitin-conjugating enzymes (E2s) that, together with an E3, transfer the ubiquitin onto a lysine residue of the substrate. This reaction is repeated, such that other ubiquitins are progressively added onto the Lys48 residue of the preceding ubiquitin. In vivo, this reaction needs to be highly processive and targets the substrate for proteasomal degradation. The E2s that act with the APC/C in vitro are UbcH10/E2-C and UbcH5/Ubc4, but it is the E3 that confers most substrate specificity (Farras et al., 2005; Pickart, 2004).