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The p21 inhibitor of cyclin-dependent kinases controls DNA replication by interaction with PCNA
1Cold Spring Harbor Laboratory, New York 11724.
Abstract:
The p53 tumour-suppressor protein controls the expression of a gene encoding the p21 cyclin-dependent protein kinase (CDK) regulator. Levels of p21 protein are increased in senescent cells and p21 overexpression blocks the growth of tumour cells. In normal human cells, but not in many tumour cells, p21 exists in a quaternary complex with a cyclin, a CDK, and the proliferating-cell nuclear antigen (PCNA). p21 controls CDK activity, thereby affecting cell-cycle control, whereas PCNA functions in both DNA replication and repair. Here we use simian virus 40 DNA replication in vitro to show than p21 directly inhibits PCNA-dependent DNA replication in the absence of a cyclin/CDK. Furthermore, p21 blocks the ability of PCNA to activate DNA polymerase delta, the principal replicative DNA polymerase. This regulation results from a direct interaction between p21 and PCNA. Thus, during p53-mediated suppression of cell proliferation, p21 and PCNA may be important for coordinating cell-cycle progression, DNA replication and repair of damaged DNA.
Insights
The p21 protein directly inhibits DNA replication by interacting with PCNA, a key factor in cell proliferation. This finding reveals a new mechanism for p53-mediated tumor suppression and cell cycle control.
Area of Science:
- Molecular Biology
- Cell Biology
- Biochemistry
Background:
- The p53 tumor-suppressor protein regulates p21, a cyclin-dependent kinase (CDK) inhibitor.
- p21 levels rise in senescent cells and inhibit tumor cell growth.
- In normal cells, p21 forms a complex with cyclin, CDK, and proliferating-cell nuclear antigen (PCNA), regulating cell-cycle control and DNA replication/repair.
Purpose of the Study:
- To investigate the direct role of p21 in inhibiting DNA replication.
- To elucidate the interaction between p21 and PCNA in DNA replication.
- To understand the mechanism of p53-mediated cell proliferation suppression.
Main Methods:
- In vitro DNA replication assays using simian virus 40.
- Analysis of protein-protein interactions between p21 and PCNA.
- Assessing the effect of p21 on PCNA's ability to activate DNA polymerase delta.
Main Results:
- p21 directly inhibits PCNA-dependent DNA replication independently of cyclin/CDK.
- p21 physically interacts with PCNA.
- p21 blocks PCNA-mediated activation of DNA polymerase delta, the main replicative polymerase.
Conclusions:
- p21 directly inhibits DNA replication by interacting with PCNA.
- This interaction is crucial for p53-mediated suppression of cell proliferation.
- p21 and PCNA may coordinate cell-cycle progression, DNA replication, and DNA repair.