Regulation of Chk1

Abstract

Chk1 is a serine/threonine protein kinase that is the effector of the G2 DNA damage checkpoint. Chk1 homologs have a highly conserved N-terminal kinase domain, and a less conserved Cterminal regulatory domain of ~200 residues. In response to a variety of genomic lesions, a number of proteins collaborate to activate Chk1, which in turn ensures that the mitotic cyclin-dependent kinase Cdc2 remains in an inactive state until DNA repair is completed. Chk1 activation requires the phosphorylation of residues in the C-terminal domain, and this is catalyzed by the ATR protein kinase. How phosphorylation of the C-terminal regulatory domain activates the N-terminal kinase domain has not been elucidated, though some studies have suggested that this phosphorylation relieves an inhibitory intramolecular interaction between the N-and C-termini. However, recent studies in the fission yeast Schizosaccharomyces pombe have revealed that there is more to Chk1 regulation than this auto-inhibition model, and we review these findings and their implication to the biology of this genome integrity determinant. Review A little history: control of entry into mitosis and the identification of chk1The cell cycle is an orderly progression driven by the activities of the cyclin-dependent kinases (CDK) that control the transitions from G1 in S-phase, and from G2 into mitosis. The G2/M transition is particularly ancient in origin and is controlled by a universal mechanism common to virtually all eukaryotes [1]. Cdc2 (also known as Cdk1) is the mitotic CDK, and its activity is reliant upon binding to the cyclically expressed A-and B-type cyclins. To ensure that the transition from G2 into mitosis is a rapid switch, Cdc2 molecules that bind to cyclin partners are rapidly inactivated by inhibitory tyrosine phosphorylation on residue 15 (Y15). This inhibitory phosphorylation is catalyzed by the Wee1-family of kinases. During G2, Y15 phosphorylated Cdc2-Cyclin complexes accumulate, and are maintained in this inactive state until conditions appropriate for mitotic entry are completed. Given the highly mechanical and irreversible nature of mitosis, achieving appropriate cell mass, the completion of DNA replication and the absence of genomic lesions are crucial criteria that must be met for the cell to commit to mitotic entry. Once these conditions are met, Cdc2 is rapidly activated by dephosphorylation of Y15, catalyzed by the Cdc25-family of phosphatases [2]. This becomes a "point of no return", and the cells are then committed to pass through mitosis, where proteolysis of the cyclins resets the system for the subsequent cell cycle [3].To ensure cells do not enter mitosis prematurely, checkpoints overlay the core cell cycle machinery to ultimately

Checkpoint kinase 1 (Chk1)-short is a splice variant and endogenous inhibitor of Chk1 that regulates cell cycle and DNA damage checkpoints

“…Despite recognition of the importance of S345/S317 phosphorylation, it remains unclear how this phosphorylation activates Chk1 (28)(29)(30). We showed that Chk1-S expression did not change appreciably in DDR (SI Appendix, Fig.…”

“…Earlier work suggested that the C-terminal domain may antagonize the N-terminal kinase domain via an intramolecular interaction that can be disrupted by phosphorylation, leading to Chk1 activation (31). This "autoinhibition" model, although supported by some observations (28), has been seriously challenged (30). Alternatively, Chk1 activity may be governed by a repressing factor(s), the dissociation of which from Chk1 leads to Chk1 activation (29).…”

“…Despite these remarkable roles, it is unknown how Chk1 activity is controlled in various phases of the cell cycle. During DNA damage, Chk1 is activated by phosphorylation in its C-terminal domain, but it is unclear how the C-terminal phosphorylation leads to the activation of the N-terminal kinase domain (28)(29)(30)(31). Earlier work suggested that the C-terminal domain may antagonize the N-terminal kinase domain via an intramolecular interaction that can be disrupted by phosphorylation, leading to Chk1 activation (31).…”

Heat induction of a novel Rad9 variant from a cryptic translation initiation site reduces mitotic commitment

“…Chk1 kinase has a basal activity level which increases ,5-10 fold upon phosphorylation of S345 in its Cterminal domain (Capasso et al, 2002;Kosoy and O'Connell, 2008). The details of its activation remain to be resolved, but it is generally believed that modification of S345 by ATR Rad3 releases the C-terminal domain from the N-terminal catalytic domain thereby stimulating kinase activity (Tapia-Alveal et al, 2009). DNA damage-induced phosphorylation at S345 is removed by Dis2 phosphatase in S. pombe (den Elzen and O'Connell, 2004) and by protein phosphatase 2A (PP2A) in human cells (LeungPineda et al, 2006).…”

Transformation/Transcription Domain-Associated Protein (TRRAP)-Mediated Regulation of Wee1

“…However, while deletion of the regulatory domain increases Chk1 activity in vitro (Chen et al 2000), it is essential for Chk1 function in vivo (Kosoy and O'Connell 2008). Further, mutations in the C-terminal domain can either inactivate or super-activate Chk1 function in vivo (Wang and Dunphy 2000;Kosoy and O'Connell 2008;Palermo et al 2008;Pereira et al 2009), suggesting that it contributes more than an inhibitory function to the catalytic domain (Tapia-Alveal et al 2009). …”