Chk1, one of the downstream protein kinases of ATM/ATR, plays an important role in DNA damage checkpoint, embryonic development and tumor suppression (1). The activation of Chk1 in response to replication blocks and certain forms of genotoxic stress involves phosphorylation of serines 317 and 345 (2). Chk1 is phosphorylated after DNA damage at additional sites, such as serines 280 and 296, but the importance of these sites remains to be discovered. Activated Chk1 can inactivate cdc25C via phosphorylation at serine 216, blocking the activation of cdc2 and transition into M-phase (3). Chk1 can also phosphorylate p53 at serine 20 in vitro (4). Chk2 is the mammalian homologue of the budding yeast Rad53 and fission yeast Cds1 checkpoint kinases (5-7). The amino-terminal domain of Chk2 contains a series of seven serine or threonine residues (Ser19,Thr26, Ser28, Ser33, Ser35, Ser50 and Thr68) followed by glutamine (SQ or TQ motif).
Cell cycle events are regulated by the sequential activation and deactivation of cyclin dependent kinases (Cdks) and by proteolysis of cyclins. Chk1 and Chk2 are involved in these processes as regulators of Cdks. Chk1 and Chk2 both function as essential components in the G2-DNA damage checkpoint by phosphorylating CDC25C in response to DNA damage. CDC25A, CDC25B and CDC25C protein tyrosine phosphatases function as mitotic activators by dephosphorylating CDC2 p34 on regulatory tyrosine residues. Phosphorylation of CDC25C by Chk1 and 2 inhibits its phosphatase activity, thereby blocking mitosis. It has also been shown that Chk1 can phosphorylate Wee1 in vitro, providing evidence that the hyperphosphorylated form of Wee1, seen in cells delayed by Chk1 overexpression, is due to phosphorylation by Chk1.
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