Meta-Biol

• Pathways

Chk1 is a checkpoint kinase activated during genotoxic stress. Like ATR, Chk1 is essential for viability in mammals. Targeted gene disruption in mice shows that loss of Chk1 causes peri-implantation embryonic lethality. Even though ATR-ATRIP not bound to ssDNA can phosphorylate Chk1, Chk1 activation is greatly enhanced when recruited to stalled replication forks by physical interaction with a modified form of claspin and the Rad9-Hus1-Rad1 sliding clamp. Activation of Chk1 occurs following phosphorylation of two sites (serine 317 and serine 345). Mutational analysis indicates that modification of both sites is essential for maximal kinase activity, while phosphorylation of only a single site causes only weak activation of Chk1. Following phosphorylation, Chk1 can diffuse away from the complex to further amplify the checkpoint signal. ATR appears to be the primary kinase activating Chk1 as conditions that activate ATR (ultraviolet irradiation or treatment with hydroxyurea) also activate Chk1. Stresses that activate ATM, e.g., ionizing irradiation, do not cause significant Chk1 activation. While the ATR and ATM pathways are distinct, there is interplay between the two. For example, double-strand DNA breaks can be processed in an ATM-dependent manner to generate structures that can cause ATR and hence Chk1 activation. The ATR and ATM pathways also have mechanistic similarities. Analogous to the Chk1 kinase existing downstream of ATR, the Chk2 checkpoint kinase is modified and activated by ATM. Although having distinct structures, Chk1 and Chk2 also have overlapping targets with some substrate sites phosphorylatable by both kinases (e.g., serine 20 of p53).