Animation 17.4 Cyclins, Cdk’s, and the Cell Cycle
Textbook Reference: Regulators of Cell Cycle Progression, p. 663
A protein kinase, known as cyclin-dependent kinase 1 (or Cdk1), is a key cell cycle regulator in all eukaryotes. When activated, it phosphorylates a variety of target proteins that initiate the events of M phase, or mitosis.
Activation of enough Cdk1 molecules for transition into mitosis requires a threshold level of synthesis of another protein, called cyclin B, which occurs during G2 of interphase. The two proteins form a complex, called maturation promoting factor, or MPF.
Cyclin B is the main determinant for Cdk1 activity, but Cdk1 also requires phosphorylation on one of its amino acids: threonine-161. However, during interphase, other phosphorylation events—at Cdk1’s threonine-14 and tyrosine-15—also occur, and these are inhibitory, preventing Cdk1 from launching the cell into premature mitosis.
The transition from G2 to M phase is brought about by the dephosphorylation of threonine-14 and tyrosine-15 by a protein phosphatase called Cdc25. This phosphatase can be considered a mitotic trigger, which itself is regulated by a host of other factors.
Once activated, the Cdk1 protein kinase phosphorylates a variety of target proteins that initiate the events of M phase.
Another activity of Cdk1 is to trigger the degradation of cyclin B, which occurs as a result of ubiquitin-mediated proteolysis. The inactivation of Cdk1 leads the cell to exit mitosis, undergo cytokinesis, and return to interphase.