Regulation of Chk1 includes chromatin association and 14-3-3 binding following phosphorylation on Ser345.

Checkpoints are biochemical pathways that provide the cell with mechanisms to detect DNA damage and respond by arresting the cell cycle to allow DNA repair. The conserved checkpoint kinase Chk1 regulates mitotic progression in response to DNA damage and replication interference by blocking the activation of Cdk1/CyclinB. Chk1 is phosphorylated on Ser317 and Ser345 following a checkpoint signal, a process that is regulated by Atr, and by the sensor complexes containing Rad17 and Hus1. We show that Chk1 is associated with chromatin in cycling cells and that the chromatin-associated Chk1 is phosphorylated in the absence of exogenous DNA damage. The UV-induced Ser345 phosphorylated forms of Chk1 that appear minutes after treatment are predominantly associated with chromatin. The Ser345 site is in a 14-3-3 consensus-binding motif and is required for nuclear retention of Chk1 following an HU-induced checkpoint signal; nonetheless, Ser345 or Ser317 are not required for the chromatin association of Chk1. Hus1, a member of the PCNA-like damage recognition complex plays a role in the phosphorylation of Chk1 on Ser345, however Hus1 is not required for phosphorylation on Ser317 or for Chk1 localization to chromatin. These results indicate that there is more than one step in Chk1 activation and that the regulation of this checkpoint signaling is achieved at least in part through phosphorylation of Ser345, which serves to localize Chk1 in the nucleus presumably by blocking Crm1-dependent nuclear export.