CDK-activating Kinase - Catalytic Activity

Catalytic Activity

Cdk activation requires two steps. First, cyclin must bind to the Cdk. In the second step, CAK must phosphorylate the cylin-Cdk complex on the threonine residue 160, which is located in the Cdk activation segment. Since Cdks need to be free of Cdk inhibitor proteins (CKIs) and associated with cyclins in order to be activated, CAK activity is considered to be indirectly regulated by cyclins.

Phosphorylation is generally considered a reversible modification used to change enzyme activity in different conditions. However, activating phosphorylation of Cdk by CAK appears to be an exception to this trend. In fact, CAK activity remains high throughout the cell cycle and is not regulated by any known cell-cycle control mechanism. However compared to normal cells, CAK activity is reduced in quiescent G0 cells and slightly elevated in tumor cells.

In mammals, activating phosphorylation by CAK can only occur once cylin is bound. In budding yeast, activating phosphorylation by CAK can take place before cyclin binding. In both humans and yeast, cyclin binding is the rate limiting step in the activation of Cdk. Therefore, phosphorylation of Cdk by CAK is considered a post-translational modification that is necessary for enzyme activity. Although activating phosphorylation by CAK is not exploited for cell-cycle regulation purposes, it is a highly conserved process because CAK also regulates transcription.