DNA damage is a key factor both in the evolution and treatment of cancer. Genomic instability is a common feature of cancer cells, fuelling accumulation of oncogenic mutations, while radiation and diverse genotoxic agents remain important, if imperfect, therapeutic modalities. Cellular responses to DNA damage are coordinated primarily by two distinct kinase signaling cascades, the ATM–Chk2 and ATR–Chk1 pathways, which are activated by DNA double-strand breaks (DSBs) and single-stranded DNA respectively. Historically, these pathways were thought to act in parallel with overlapping functions; however, more recently it has become apparent that their relationship is more complex. In response to DSBs, ATM is required both for ATR–Chk1 activation and to initiate DNA repair via homologous recombination (HRR) by promoting formation of single-stranded DNA at sites of damage through nucleolytic resection. Interestingly, cells and organisms survive with mutations in ATM or other components required for HRR, such as BRCA1 and BRCA2, but at the cost of genomic instability and cancer predisposition. By contrast, the ATR–Chk1 pathway is the principal direct effector of the DNA damage and replication checkpoints and, as such, is essential for the survival of many, although not all, cell types. Remarkably, deficiency for HRR in BRCA1- and BRCA2-deficient tumors confers sensitivity to cisplatin and inhibitors of poly(ADP-ribose) polymerase (PARP), an enzyme required for repair of endogenous DNA damage. In addition, suppressing DNA damage and replication checkpoint responses by inhibiting Chk1 can enhance tumor cell killing by diverse genotoxic agents. Here, we review current understanding of the organization and functions of the ATM–Chk2 and ATR–Chk1 pathways and the prospects for targeting DNA damage signaling processes for therapeutic purposes.

The ATM-Chk2 and ATR-Chk1 pathways in DNA damage signaling and cancer.

http://susantaylorlab.ucsd.edu/Publications_PDF/Recent_Publications_2003_present_by_pubmed/15350212_Nolen_RegulationProtein_04.pdf

Regulation of protein kinases; controlling activity through activation segment conformation.

Chk1 is an evolutionarily conserved protein kinase that regulates cell cycle progression in response to checkpoint activation. In this study, we demonstrated that agents that block DNA replication or cause certain forms of DNA damage induce the phosphorylation of human Chk1. The phosphorylated form of Chk1 possessed higher intrinsic protein kinase activity and eluted more quickly on gel filtration columns. Serines 317 and 345 were identified as sites of phosphorylation in vivo, and ATR (the ATM- and Rad3-related protein kinase) phosphorylated both of these sites in vitro. Furthermore, phosphorylation of Chk1 on serines 317 and 345 in vivo was ATR dependent. Mutants of Chk1 containing alanine in place of serines 317 and 345 were poorly activated in response to replication blocks or genotoxic stress in vivo, were poorly phosphorylated by ATR in vitro, and were not found in faster-eluting fractions by gel filtration. These findings demonstrate that the activation of Chk1 in response to replication blocks and certain forms of genotoxic stress involves phosphorylation of serines 317 and 345. In addition, this study implicates ATR as a direct upstream activator of Chk1 in human cells.

ATR-mediated checkpoint pathways regulate phosphorylation and activation of human Chk1.

https://www.cell.com/cell/pdf/S0092-8674(01)00374-9.pdf

Crystal Structure of Glycogen Synthase Kinase 3β: Structural Basis for Phosphate-Primed Substrate Specificity and Autoinhibition

https://ressources.unisciel.fr/biocell/chap11/res/32phosphorylation-controljohnson.pdf

Structural basis for control by phosphorylation.

https://www.cell.com/cell/pdf/S0092-8674(00)80704-7.pdf

Implications for Chk1 Regulation: The 1.7 Å Crystal Structure of Human Cell Cycle Checkpoint Kinase Chk1

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Implications for Chk1 Regulation: The 1.7 Å Crystal Structure of Human Cell Cycle Checkpoint Kinase Chk1