a b s t r a c t CHK1 regulates the DNA damage-induced checkpoint involving an ATR-or ATM-dependent pathway. In this paper, we focused on the autophosphorylation of Ser296, one of the DNA damageinduced phosphorylation sites. First, we demonstrated that the Ser296 autophosphorylation of CHK1 is mainly regulated by an intramolecular mechanism in response to DNA damage. In examining the relationship between Ser296 and Ser317/Ser345, the other ATR dependent phosphorylation sites, we found that the Ser296 cis-autophosphorylation was dependent on both Ser317 and Ser345 phosphorylation. Our findings suggest that CHK1 mediates cell cycle checkpoint signals by both cisautophosphorylation and trans-phosphorylation of downstream factors. Structured summary of protein interactions:CHK1 phosphorylates CHK1 by protein kinase assay (View Interaction: 1, 2, 3)

CASP and non-CASP family proteases intricately regulate cleavage for up-regulation of CHK1 kinase activity during PCD.