Mitosis: Ubiquitin Protein Ligases
By Levi Clancy for Student Reader on
updated
Polyubiquitination marks eukaryotic proteins for degradation by proteasomes. Three enzymes are required for the Ubiquitin to work: E1, the Ubiquitin-activating enzyme; E2, the Ubiquitin-conjugating enzyme; and E3, the Ubiquitin ligase. SCF and APC/C are ubiquitin protein ligase complexes that that control three major transitions in the cell cycle: onset of S-phase through degradation of Sic1 by SCF; initiation of anaphase via degradation of securin by APC-Cdc20; exit from mitosis via degradation of cyclin B's by APC-Cdh2. APC has several substrates that must be degraded at different times in the cycle; thus, its activity is directed by specificity factors that bind it. SCF only degrades Sic1 and thus its activity is regulated only by phosphorylation of its substrate.
| Complex | Overview | |||||||||
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| SCF | Degradation of phosphoryated Sic1 or p27 to activate S-phase cyclin. SCF is a ubiquitin protein ligase needed for polyubiquitination and proteasomal degradation of phosphorylated Sic1. In contrast to the APC/C, the SCF ubiquitin-protein ligase is not regulated by phosphorylation or other modifications of specificity factors, but rather by phosphorylation of its substrate, Sic1.
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| APC/C | The anaphase promoting complex/cyclosome (aka APC/C) is a E3 ubiquitin protein ligase that is bound by various specificity factors that direct it to degrade different substrates at different times in the cycle.
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