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Complete inhibition of Cdk/cyclin by one molecule of p21(Cip1)

L Hengst1, U Göpfert, H A Lashuel

  • 1Max-Planck-Institut für Biochemie, D-82152 Martinsried, Germany.

Genes & Development
|December 31, 1998
PubMed

Insights

A single molecule of the Cdk inhibitor p21 is sufficient to inactivate cyclin-dependent kinases (Cdks). The extent of Cdk inactivation depends on the proportion of kinases bound by p21, not inhibitor stoichiometry or phosphorylation.

Area of Science:

  • Molecular Biology
  • Cell Biology
  • Biochemistry

Background:

  • Cell-cycle phase transitions are regulated by cyclin-dependent kinases (Cdks).
  • Cdk inhibitors (CKIs) like p21 are crucial for controlling Cdk activity in response to antiproliferative signals and during cell-cycle progression.
  • A prevailing model suggested that multiple p21 molecules were required to inactivate Cdks.

Purpose of the Study:

  • To investigate the stoichiometry and mechanism of Cdk inhibition by p21.
  • To challenge the existing dogma regarding the requirement of multiple p21 molecules for Cdk inactivation.
  • To determine the factors governing the level of Cdk inactivation by p21.

Main Methods:

  • Analytical ultracentrifugation of purified p21/cyclin A/Cdk2 complexes.
  • Biochemical assays to assess kinase activity.
  • Analysis of p21 phosphorylation state.

Main Results:

  • A single molecule of p21 is sufficient to inhibit Cdk2 kinase activity.
  • p21-saturated Cdk2 complexes stably bind only one p21 molecule.
  • Phosphorylated forms of p21 retain potent Cdk inhibitory function.
  • The extent of Cdk inactivation is determined by the fraction of kinase complexes bound by p21.

Conclusions:

  • The current model of Cdk inhibition by p21 requires revision.
  • Cdk inactivation by p21 is regulated by the concentration of available inhibitor, not by inhibitor stoichiometry or phosphorylation.
  • This finding clarifies the mechanism of cell-cycle control by p21.

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