Cyclin-dependent protein kinases CDK

Cyclins and Cyclin-dependent Protein Kinases (CDKs)... 

Progression through the cell cycle is regulated by the cyclin-dependent protein kinases (CDKs), which act at specific points in the cycle, phosphorylating key proteins and modulating their activities. The catalytic subunit of CDKs is inactive unless associated with the regulatory cyclin subunit. 

Cyclin-Cdks were appropriately named the cell cycle s engines . The cyclin-dependent protein kinases (Cdks) are soluble serine/threonine kinases of 34-40 kDa. The Cdks share with other serine/threonine protein kinases sequence similarities, including a subset of residues that is essential for catalytic activity. Cdks contribute the catalytic subunit, whereas the regulatory subunit is contributed by a cyclin. Cyclins control the kinase activity, determine the substrate specificity and the subcellular location of Cdks. Each of these processes is a potential site of regulation. The major substrates of the Cdks are proteins regulating gene transcription. Cdks can be controlled in three major ways ... 

The wild type of p53 protein activates transcription of a number of genes, including the WAFl/CIPl gene. The p21 protein product of the p2l " gene binds to and inhibits the cyclin-dependent protein kinases (cdk) that are active in the Gi phase of the cell cycle. The cell-cycle arrest function of p53 in response to DNA damage is mediated by p21. 

An oscillating system is formed by the interplay of the three protein classes, and the activity of this system makes up the specific biochemical functions of the individual phases of the cycle. The activity of the cyclin-dependent protein kinases (CDKs) is central to the oscillating system. These create a signal that initiates downstream biochemical processes and thus determines the individual phases of the cycle. CDK activity is also the starting point for intrinsic and external control mechanisms. 

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