Cdc7-Dbf4 and the human S checkpoint response to UVC

Area of Science:

• Molecular Biology
• Cell Cycle Regulation
• DNA Damage Response

Background:

• The S checkpoint prevents DNA replication following UV damage, involving ATR and Chk1 kinases.
• Downstream effectors of this UV-induced S checkpoint response remain poorly understood.

Purpose of the Study:

• To investigate the downstream effectors of the UV-induced S checkpoint pathway that inhibits DNA replicon initiation.
• To determine the roles of Cdc25A and the Cdc7-Dbf4 complex in this response.

Main Methods:

• Utilized human cells exposed to sublethal and cytotoxic doses of UVC radiation.
• Assessed Cdc25A protein levels and Cdk2-cyclin E kinase activity.
• Examined interactions between Chk1, Cdc7, and Dbf4 using in vitro and in vivo assays.
• Investigated the effect of Dbf4 overexpression on the S checkpoint response.

Main Results:

• Cdc25A degradation and Cdk2-cyclin E inhibition were not intrinsic to the UVC-induced inhibition of replicon initiation at sublethal doses.
• Cdc25A degradation occurred only at cytotoxic UVC fluences.
• Dbf4 interacted with Cdc7 and was phosphorylated by Chk1 in vitro.
• Dbf4 overexpression abrogated the S checkpoint response to UVC but not ionizing radiation.

Conclusions:

• Cdc25A degradation and Cdk2-cyclin E inhibition are not early effectors of the UVC-induced S checkpoint.
• The findings implicate a Dbf4-dependent kinase as a potential target in the ATR- and Chk1-dependent S checkpoint response to UVC radiation.