Chk1-mediated Cdc25A degradation as a critical mechanism for normal cell cycle progression.

Chk1 (encoded by CHEK1 in mammals) is an evolutionarily conserved protein kinase that transduces checkpoint signals from ATR to Cdc25A during the DNA damage response (DDR). In mammals, Chk1 also controls cellular proliferation even in the absence of exogenous DNA damage. However, little is known about how Chk1 regulates unperturbed cell cycle progression, and how this effect under physiological conditions differs from its regulatory role in DDR. Here, we have established near-diploid HCT116 cell lines containing endogenous Chk1 protein tagged with a minimum auxin-inducible degron (mAID) through CRISPR/Cas9-based gene editing. Establishment of these cells enabled us to induce specific and rapid depletion of the endogenous Chk1 protein, which resulted in aberrant accumulation of DNA damage factors that induced cell cycle arrest at S or G2 phase. Cdc25A was stabilized upon Chk1 depletion before the accumulation of DNA damage factors. Simultaneous depletion of Chk1 and Cdc25A partially suppressed the defects caused by Chk1 single depletion. These results indicate that, similar to its function in DDR, Chk1 controls normal cell cycle progression mainly by inducing Cdc25A degradation.