So far, we have discussed the importance of regulating cyclin levels and of CDK phosphorylation in controlling CDK activity. The final layer of control that is critical in the regulation of CDKs is a family of proteins known as CDK inhibitors, or CKIs, which bind directly to the cyclin-CDK complex and inhibit its activity. As we will see in Section 19.4, these proteins play an especially important role in the regulation of the G₁–S phase transition and its integration with extracellular signals.

All eukaryotes harbor CKIs involved in regulating S phase and mitotic CDKs. Although these inhibitors display little sequence similarity, they are all essential to prevent premature activation of S phase and M phase CDKs. Inhibitors of G₁ CDKs play an essential role in mediating a G₁ arrest in response to proliferation inhibitory signals. A class of CKIs called INK4s (inhibitors of kinase 4) includes several small, closely related proteins that interact only with the G₁ CDKs. Binding of INK4s to CDK4 and CDK6 blocks their interaction with cyclin D and hence their protein kinase activity. A second class of CKIs found in metazoan cells consists of three proteins—p21^CIP, p27^KIP1, and p57^KIP2. These CKIs inhibit G₁/S phase CDKs and S phase CDKs and must be degraded before DNA replication can begin. As we will discuss in Section 19.7, p21^CIP plays an important role in the response of metazoan cells to DNA damage. CKIs regulating G₁ CDKs play a critical role in preventing tumor formation. For example, both copies of the INK4 gene that encodes p16 are found inactivated in a large fraction of human cancers (see Chapter 24).

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