S-phase-promoting factor
The S-phase-promoting factor is a CDK-cyclin complex that induces the S-phase of the cell cycle.
The S-phase-promoting factor is inhibited by Sic1.
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.
