Mitosis: Ubiquitin Protein Ligases

By Levi Clancy for Student Reader on
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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.

ComplexOverview
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.


S.F.SubstrateOverview
NoneSic1SCF degrades phosphorylated Sic1 (inhibitor of S-cyclin+CDK) to initiate S-phase.
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.


S.F.SubstrateOverview
Cdc20SecurinCdc20 directs APC/C to degrade Securin, initiating anaphase. Induces partial degradation of cyclin Bs.
Cdh2Cyclin BsAPC/C-Cdh2 initiates telophase by degrading S-phase and mitotic cyclins, thus allowing prereplication complexes to form at DNA replication origins. Degrades geminin in metazoans. Inactivated by G1 -cyclin+CDK.