BACKGROUND Chk1 kinase acts downstream of ATM/ATR kinase to play an important role in DNA damage checkpoint control, embryonic development and tumor suppression (1). Activation of Chk1 involves phosphorylation of Ser317 and Ser345 and occurs in response to blocked DNA replication and certain forms of genotoxic stress (2). Phosphorylation at Ser 345 serves to localize Chk1 to the nucleus (3) following checkpoint acitvation while recently phosphorylation at Ser 317 along with site-specific phosphorylation of PTEN was shown to allow for reentry into the cell cycle following stalled DNA replication. (4). Chk1 exerts it checkpoint mechanism on the cell cycle in part by regulating the cdc25 family of phosphatases. Chk1 phosphorylation of cdc25A targets it for proteolysis and inhibits it’s activity though 14-3-3 binding. (5). Activated Chk1 can inactivate cdc25C via phosphorylation at Ser216, blocking the activation of cdc2 and transition into mitosis (6). Also, centrosomal Chk1 has been shown to phosphorylate cdc25B inhibiting its activation of CDK1-cyclin B1 and thus mitotic spindle formation and chromatin condensation (7). Furthermore, Chk1 plays a role in spindle checkpoint function through regulation of Aurora B and BubR1 (8). Chk1 has emerged as a drug target for cancer therapy as its inhibition leads to cell death in many cancer cell lines (9).
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4. Martin, S.A. and Ouchi, T. (2008) Mol Cancer Ther 7, 2509-16.
5. Chen, M.S. et al. (2003) Mol Cell Biol 23, 7488-97.
6. Zeng, Y. et al. (1998) Nature 395, 507-10.
7. Löffler, H. et al. (2006) Cell Cycle 5, 2543-7.
8. Zachos, G. et al. (2007) Dev Cell 12, 247-60.
9. Garber, K. (2005) J Natl Cancer Inst 97, 1026-8.
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