Activation and Inactivation of CDKs by Phosphorylation

Phosphorylation of the CycA-CDC2 complex at ThrlhO leads to a near 300-fold increase in protein kinase activity. Thrl60 of CDK2 hes in the activation segment (also known as the T loop) that blocks the access to the substrate binding site in the inactive 

Phosphorylation at Thrl4 and TyrlS leads to inactivation of the CDKs. In the fission yeast, the weel kinase is responsible for this phosphorylation and in mammals, there are enzymes homologous to wee-1 kinase. It is not clear whether this kinase performs both phosphorylations. Phosphorylation at Thrl4 and TyrlS is of particular importance for regulation of CDK activity in mitosis. The CDC2-cychn B complex is maintained in an inactive state imtil the end of G2 phase by the phosphorylation of Thrl4 and TyrlS. At the G2/M transition, the inactive state is ended by the action of CDC2S phosphatase, which cleaves off the inhibitory phosphate residues. 

The inactivating phosphorylation at Thrl4 and TyrlS can be reversed by specific phosphatases in a regulated marmer. The dephosphorylation is performed by CDC2S phosphatase. This enzyme, first described for S. pombe, is a protein phosphatase with twofold specificity that can cleave phosphate residues from phosphoserine and phosphoty-rosine residues of CDKs. CDC2S phosphatases have also been observed in higher eucaryotes where they have a similar fimction. 

Furthermore, there is evidence that the protein phosphatases PPl and PP2A dephosphorylate the activating Ser-phosphates of CDC25 phosphatase and thus have a negative, suppressing effect on the function of CDC25 phosphatase. 

The activity of CDC25 phosphatase is also central to the DNA damage and DNA replication checkpoints of the cell cycle. Upon activation of these checkpoints by DNA damage or the presence of unreplicated DNA, the CDC25 phosphatase is inactivated 

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