Chk1 regulates the S phase checkpoint by coupling the physiological turnover and ionizing radiation-induced accelerated proteolysis of Cdc25A

Cancer Cell. 2003 Mar;3(3):247-58. doi: 10.1016/s1535-6108(03)00048-5.


Chk1 kinase coordinates cell cycle progression and preserves genome integrity. Here, we show that chemical or genetic ablation of human Chk1 triggered supraphysiological accumulation of the S phase-promoting Cdc25A phosphatase, prevented ionizing radiation (IR)-induced degradation of Cdc25A, and caused radioresistant DNA synthesis (RDS). The basal turnover of Cdc25A operating in unperturbed S phase required Chk1-dependent phosphorylation of serines 123, 178, 278, and 292. IR-induced acceleration of Cdc25A proteolysis correlated with increased phosphate incorporation into these residues generated by a combined action of Chk1 and Chk2 kinases. Finally, phosphorylation of Chk1 by ATM was required to fully accelerate the IR-induced degradation of Cdc25A. Our results provide evidence that the mammalian S phase checkpoint functions via amplification of physiologically operating, Chk1-dependent mechanisms.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Ataxia Telangiectasia Mutated Proteins
  • Cell Cycle / physiology*
  • Cell Cycle / radiation effects
  • Cell Cycle Proteins
  • Checkpoint Kinase 1
  • Checkpoint Kinase 2
  • DNA Replication / radiation effects
  • DNA-Binding Proteins
  • Enzyme Activation
  • HeLa Cells
  • Humans
  • Kinetics
  • Models, Biological
  • Phosphorylation
  • Protein Kinases / metabolism*
  • Protein-Serine-Threonine Kinases / physiology
  • Radiation, Ionizing
  • S Phase / radiation effects
  • Serine / metabolism
  • Signal Transduction
  • Tumor Cells, Cultured
  • Tumor Suppressor Proteins
  • cdc25 Phosphatases / physiology*
  • cdc25 Phosphatases / radiation effects


  • Cell Cycle Proteins
  • DNA-Binding Proteins
  • Tumor Suppressor Proteins
  • Serine
  • Protein Kinases
  • Checkpoint Kinase 2
  • ATM protein, human
  • Ataxia Telangiectasia Mutated Proteins
  • CHEK1 protein, human
  • CHEK2 protein, human
  • Checkpoint Kinase 1
  • Protein-Serine-Threonine Kinases
  • cdc25 Phosphatases