What are inhibitory proteins that control CDK activity?

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Cyclin-dependent kinases (CDKs) are a group of enzymes that play a central role in controlling the progression of the cell cycle. These CDKs must be activated at the correct stage of the cell cycle and this activation depends on their association with cyclins. However, to prevent uncontrolled cell division and ensure genomic stability, the activity of CDKs must also be properly regulated. One of the most important regulatory mechanisms involves CDK inhibitory proteins, which bind to CDKs or cyclin-CDK complexes and block their activity. These inhibitors act like "brakes" in the cell cycle machinery and are mainly divided into two major families: INK4 family and Cip/Kip family.

1. INK4 Family (Inhibitors of CDK4)

The INK4 family specifically inhibits CDK4 and CDK6, which are required for the G1 to S phase transition. These inhibitors prevent the binding of cyclin D to CDK4/6 and thereby stop the activation of the kinase.

Members of INK4 family include:

p15 (INK4b)

p16 (INK4a)

p18 (INK4c)

p19 (INK4d)

These proteins play a major role in early G1 phase and are especially important in stopping the cell cycle when there is stress or DNA damage.

2. Cip/Kip Family (CDK Interacting Protein/Kinase Inhibitory Protein)

This family is more versatile and can inhibit a wider range of CDKs, especially CDK2, CDK1 and CDK4/6. They work by binding directly to cyclin-CDK complexes and inhibiting their kinase activity.

Major members of Cip/Kip family include:

p21 (Cip1/Waf1): Induced by p53 in response to DNA damage. It stops CDK2 and CDK1, thereby halting the cell cycle in G1 and G2.

p27 (Kip1): Active in contact inhibition and cellular quiescence. It prevents cell cycle entry by blocking CDK2 activity.

p57 (Kip2): Plays a key role in development and is important in tissues like placenta and nervous system.

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