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, 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.