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Claspin operates downstream of TopBP1 to direct ATR signaling towards Chk1 activation

Shizhou Liu1, Simon Bekker-Jensen, Niels Mailand

  • 1Institute of Cancer Biology and Centre for Genotoxic Stress Research, Danish Cancer Society, Strandboulevarden 49, DK-2100 Copenhagen, Denmark.

Molecular and Cellular Biology
|August 2, 2006
PubMed

Insights

TopBP1 and Claspin regulate DNA damage response by controlling Chk1 phosphorylation. TopBP1 broadly affects ATR targets, while Claspin specifically modulates Chk1 activation, revealing distinct roles in genotoxic stress signaling.

Area of Science:

  • Cellular response to DNA damage
  • DNA repair mechanisms
  • Protein-protein interactions in signaling pathways

Background:

  • TopBP1 and Claspin are known adaptor proteins crucial for Chk1 phosphorylation by ATR kinase during genotoxic stress.
  • The precise mechanisms by which TopBP1 and Claspin control Chk1 phosphorylation remain incompletely understood.

Purpose of the Study:

  • To elucidate the distinct roles of TopBP1 and Claspin in the ATR-Chk1 signaling pathway.
  • To investigate how these proteins regulate Chk1 phosphorylation and the broader genotoxic stress response.

Main Methods:

  • RNA interference (RNAi) to deplete TopBP1 and Claspin.
  • Immunofluorescence to observe protein colocalization with ATR.
  • Assessment of phosphorylation of ATR targets (Chk1, Nbs1, Smc1, H2AX).
  • Analysis of DNA damage induction upon protein downregulation.

Main Results:

  • TopBP1 colocalizes with ATR in distinct nuclear compartments upon DNA damage but does not affect ATR recruitment.
  • TopBP1 depletion impairs phosphorylation of multiple ATR targets, indicating a general regulatory role.
  • Claspin depletion selectively blocks Chk1 phosphorylation by ATR, without affecting other targets.
  • Claspin downregulation leads to spontaneous DNA damage, mimicking Chk1 inactivation.
  • TopBP1 is essential for the DNA damage-induced interaction between Claspin and Chk1.

Conclusions:

  • TopBP1 acts as a general regulator of ATR kinase activity in response to DNA damage.
  • Claspin functions downstream of TopBP1 to specifically control the Chk1-mediated branch of the genotoxic stress response.
  • These findings refine our understanding of the hierarchical regulation within the ATR signaling pathway.

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