Assaying cell cycle checkpoints: activity of the protein kinase Chk1

Activation of the DNA damage checkpoint in mutants defective in DNA replication initiation

In the fission yeast, Schizosaccharomyces pombe, blocks to DNA replication elongation trigger the intra-S phase checkpoint that leads to the activation of the Cds1 kinase. Cds1 is required to both prevent premature entry into mitosis and to stabilize paused replication forks. Interestingly, although Cds1 is essential to maintain the viability of mutants defective in DNA replication elongation, mutants defective in DNA replication initiation require the Chk1 kinase. This suggests that defects in DNA replication initiation can lead to activation of the DNA damage checkpoint independent of the intra-S phase checkpoint. This might result from reduced origin firing that leads to an increase in replication fork stalling or replication fork collapse that activates the G2 DNA damage checkpoint. We refer to the Chk1-dependent, Cds1-independent phenotype as the rid phenotype (for replication initiation defective). Chk1 is active in rid mutants, and rid mutant viability is dependent on the DNA damage checkpoint, and surprisingly Mrc1, a protein required for activation of Cds1. Mutations in Mrc1 that prevent activation of Cds1 have no effect on its ability to support rid mutant viability, suggesting that Mrc1 has a checkpoint-independent role in maintaining the viability of mutants defective in DNA replication initiation.

DNA-dependent phosphorylation of Chk1 and Claspin in a human cell-free system

Cell-cycle checkpoints induced by DNA damage or replication play critical roles in the maintenance of genomic integrity during cell proliferation. Biochemical analysis of checkpoint pathways has been greatly facilitated by the use of cell-free systems made from Xenopus eggs. In the present study, we describe a human cell-free system that reproduces a DNA-dependent checkpoint pathway acting on the Chk1 protein kinase. In this system, double-stranded DNA oligonucleotides induce the phosphorylation of Chk1 at activating sites targeted by ATR [ATM (ataxia telangiectasia mutated)- and Rad3-related] and ATM kinases. Phosphorylation of Chk1 is dependent on the interaction of Claspin, a protein first identified in Xenopus as a Chk1-binding protein. We show that the DNA-dependent binding of Chk1 to Claspin requires two phosphorylation sites, Thr916 and Ser945, which lie within the Chk1-binding domain of Claspin. Using a phosphopeptide derived from the consensus motif of these sites, we show that the interaction of Claspin with Chk1 is required for the ATR/ATM-dependent phosphorylation of Chk1. Using a panel of protein kinase inhibitors, we provide evidence that Chk1 is phosphorylated at an additional site in response to activation of the checkpoint response, probably by autophosphorylation. Claspin is phosphorylated in the Chk1-binding domain in an ATR/ATM-dependent manner and is also targeted by additional kinases in response to double-stranded DNA oligonucleotides. This cell-free system will facilitate further biochemical analysis of the Chk1 pathway in humans.