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p21 contains independent binding sites for cyclin and cdk2: both sites are required to inhibit cdk2 kinase activity
R Fotedar1, P Fitzgerald, T Rousselle
1Institut de Biologie Structurale J-P Ebel, Grenoble, France.
Abstract:
Cyclin dependent kinases regulate the progression of eukaryotic cells through the cell cycle. p21Cip1/Waf1/Sdi1 is an inhibitor of cdk-cyclin kinase activity, and has been shown to form complexes with cdk-cyclins and with PCNA, an accessory protein of DNA polymerase delta. The kinase inhibitory domain maps to the N-terminus (1-82) and contains the cdk2 binding site (28-82). We have generated a panel of deletion mutants of p21. A functional characterization of p21 mutants in the N-terminal domain reveals that cyclins bind to this domain independently of cdk2. Correlating with these results we find that p21 can associate with cyclin-cdk kinases in two functionally distinct forms, one in which the kinase activity is inhibited and the other in which the kinase is still active. The cdk2 and cyclin binding sites on p21 are both required to inhibit kinase activity. The second type of interaction, in which an active cyclin-cdk complex only interacts with p21 either via the cyclin or the cdk2 binding site but not through both, does not lead to inhibition of cyclin kinase activity. These results thus provide a basis for understanding the mechanism by which p21, and perhaps other cdk-cyclin kinase inhibitory proteins, suppress kinase activity.
Insights
p21 Cip1/Waf1/Sdi1 inhibits cyclin-dependent kinases (cdk) by binding to both cyclin and cdk2. This interaction requires both binding sites to be present on p21 to effectively inhibit kinase activity.
Area of Science:
- Molecular Biology
- Cell Biology
Background:
- Cyclin-dependent kinases (cdks) are key regulators of eukaryotic cell cycle progression.
- p21 Cip1/Waf1/Sdi1 functions as a cdk-cyclin kinase inhibitor and interacts with both cdks and PCNA.
- The N-terminal domain of p21 (1-82) contains the kinase inhibitory domain and the cdk2 binding site.
Purpose of the Study:
- To investigate the functional characterization of p21 mutants in its N-terminal domain.
- To elucidate the distinct mechanisms of p21 interaction with cyclin-cdk complexes.
- To understand how p21 suppresses cdk-cyclin kinase activity.
Main Methods:
- Generation of a panel of deletion mutants of p21.
- Functional characterization of p21 mutants.
- Analysis of p21 interactions with cyclin-cdk complexes.
Main Results:
- Cyclins bind to the N-terminal domain of p21 independently of cdk2.
- p21 associates with cyclin-cdk kinases in two distinct forms: one inhibitory and one non-inhibitory.
- Inhibition of kinase activity requires both cdk2 and cyclin binding sites on p21.
Conclusions:
- p21 can associate with cyclin-cdk complexes in both inhibitory and non-inhibitory manners.
- The dual binding of p21 to both cyclin and cdk2 is essential for complete kinase inhibition.
- These findings provide insights into the mechanism of cdk-cyclin kinase inhibition by p21 and related proteins.