S-phase-promoting factor
The S-phase-promoting factor (SPF) is a CDK-cyclin complex that induces the S-phase (synthesis phase) of the cell cycle.
The S-phase-promoting factor (SPF) is inhibited by Sic1 (a protein which also is a stoichiometric inhibitor of Cdk1-Clb and B-type cyclins). Sic1 releases SPF from inhibition after it is phosphorylated by G1-cyclin-CDK. The phosphorylation marking SPF for polyubiquitination by the SCF ubiquitin ligase. Then the active SPF trigger initiation of DNA synthesis.
Toward the end of the G1 phase of mitosis, an SPF is phosphorylated, causing a biochemical cascade that results in the activation of DNA replication proteins and the initiation of the G1/S transition.
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