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Regulated activating Thr172 phosphorylation of cyclin-dependent kinase 4(CDK4): its relationship with cyclins and CDK

Laurence Bockstaele1, Hugues Kooken, Frederick Libert

  • 1Institute of Interdisciplinary Research, Université Libre de Bruxelles, Campus Erasme, B-1070 Brussels, Belgium.

Molecular and Cellular Biology
|June 20, 2006
PubMed

Insights

Cyclin-dependent kinase 4 (CDK4) phosphorylation at Thr172 requires D-type cyclin binding but also occurs in p16-bound CDK4. CDK4-activating kinases need reevaluation based on these findings.

Area of Science:

  • Cell Biology
  • Molecular Biology
  • Biochemistry

Background:

  • Cyclin-dependent kinase 4 (CDK4) integrates extracellular signals and is vital for oncogenic transformation.
  • Key aspects of CDK4 activation, including cyclin D binding, Thr172 phosphorylation, and the role of CDK inhibitors (CKIs), are not fully understood.

Purpose of the Study:

  • To reinvestigate the regulation of CDK4 activation, focusing on Thr172 phosphorylation and the influence of CKIs.
  • To clarify the interplay between D-type cyclins, CDK4, and CKIs (p21, p27, p16) in controlling CDK4 activity.

Main Methods:

  • Two-dimensional gel electrophoresis was employed across diverse experimental systems (human fibroblasts, canine thyroid cells, transfected cells).
  • Investigated the impact of p27 stoichiometry and nuclear localization on cyclin D3-CDK4 complexes and CDK4 phosphorylation.

Main Results:

  • Thr172 phosphorylation of CDK4 is dependent on D-type cyclin binding but also occurs in p16-bound CDK4.
  • Thr172-phosphorylated CDK4 was found in complexes with p21 and p27, even at inhibitory p27 levels.
  • T-loop phosphorylation of CDK4, but not CDK6, within cyclin D3 complexes is directly regulated, suggesting it's a key control point.

Conclusions:

  • CDK4-activating kinases require reevaluation given that Thr172 phosphorylation is not solely dependent on prior cyclin binding.
  • CDK4 phosphorylation is intricately regulated by CKIs and cyclin binding, with implications for cell cycle control.
  • Extracellular factors directly regulate T-loop phosphorylation of CDK4 within cyclin D complexes, highlighting its role in cell cycle regulation.

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