Fig. 8. Model for the role of localization in regulation of Chk1 in fission yeast. When Chk1 binds Rad24 the 14-3-3 protein may act to open up the conformation of Chk1 to expose an NLS, while at the same time blocking access of Crm1 to the NES. This would allow Chk1 to enter the nucleus. Subsequent dissociation of 14-3-3 would expose the NES permitting Crm1 to bind and mediate export. When cells incur DNA damage, Chk1 becomes phosphorylated and its interaction with 14-3-3 protein is stimulated approximately tenfold (Chen et al., 1999). Nuclear accumulation of Chk1 would thus result from the stabilized interaction of Chk1 with Rad24 and the consequent reduction in its association with Crm1.