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Activity and nature of p21(WAF1) complexes during the cell cycle

K Cai1, B D Dynlacht

  • 1Department of Molecular and Cellular Biology, Harvard University, 16 Divinity Avenue, Cambridge, MA 02138, USA.

Proceedings of the National Academy of Sciences of the United States of America
|October 15, 1998
PubMed

Insights

Monoclonal antibodies reveal that p21 (WAF1) cyclin-dependent kinase inhibitor inactivates cyclin A/Cdk2 kinase activity. This inactivation is crucial for regulating the G1-to-S phase transition in human fibroblasts.

Area of Science:

  • Cell Biology
  • Molecular Biology
  • Biochemistry

Background:

  • Elevated p21(WAF1) (p21) levels induce cell growth arrest.
  • Understanding p21's regulatory interactions is key to cell cycle control.

Purpose of the Study:

  • Characterize monoclonal antibodies against human p21.
  • Investigate p21's role in regulating cyclin A/Cdk2 kinase activity.
  • Elucidate the molecular architecture of p21 complexes.

Main Methods:

  • Monoclonal antibody generation and characterization.
  • Analysis of kinase activity in p21-associated complexes.
  • Western blot analysis of protein levels during cell cycle progression.

Main Results:

  • p21 binding significantly reduces cyclin A/Cdk2 kinase activity, suggesting in vivo inactivation.
  • Multiple antibodies enabled initial studies of p21 complex molecular architecture.
  • Quiescent fibroblasts show high levels of 'free' p21, which decreases as cells enter S phase.
  • Cyclin A levels rise as free p21 decreases, leading to excess active cyclin A/Cdk2.

Conclusions:

  • p21 acts as a critical inhibitor of cyclin A/Cdk2 activity.
  • The G1-to-S phase transition is regulated by the relative concentrations of cyclin A/Cdk2 and p21.

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