wee1 is a component of the cell cycle machinery. It is a kinase phosphorylating two differents sites of cdk2:Tyr15 and Thr14. In S.pombe Thr14 is less often phosphorylated than Tyr15. The phosphorylations have an inhibitory effect on the activity of cdk2. At least three checkpoints exist for which the inhibition of cdk2 by wee1 is important.
- G2/M checkpoint:wee1 phosphorylates the amino acid Thr161 of cdk2, this keeps the kinase activity of Cdk2 low and prevents mitosis entry. During this time in S.pombe a further cell growth is possible. During entry of mitosis the activity of wee1 is decreased by several regulators. This increases the activity of the cdk2 which in leads to a even smaller wee1 activity. The decreased wee1 activity alone is not sufficient for mitotic entry:Synthesis of cyclins and an activating phosphorylation by cdc25 are also important.
- Cell size checkpoint:There is evidence pointing to the existence of a cell size checkpoint [wee1-size for S.pombe and budding yeast], i.e. a checkpoint preventing small cells from entering mitois. wee1 plays a role in this by checkpoint by coordinating cellsize and cell cycle progression.
- DNA damage checkpoint:This checkpoint is also located at the G2/M transition. In S.pombe this checkpoint delays the mitosis entry of cells with DNA damage (for example induced by gamma irradiation). The elongation of the G2 phase depends on wee1, wee1 mutants have accordingly no elongated G2 phase after gamma irradiation. 
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