Figure 7: A working model of the PAR-mediated Chk1 activation at the replication fork.

In the absence of DNA damage, Chk1 is readily associated with the replication fork. The catalytic cleft (regulatory domain) in the C terminus of Chk1 is closed (inactive form) and substrates cannot access the kinase domain. In response to damaged replication forks, PARP1 rapidly binds to the DNA breaks (for example, stalled replication forks) to synthesize PAR onto itself or other acceptor proteins. Chk1 interacts with PAR, supplied by PARP1 (or other poly(ADP-ribosyl)ated proteins), through its N-terminal PbR motif. Single-stranded DNA is coated by RPA (R), which recruits the 9–1–1 complex, TopBP1 (not indicated) and ATR, as well as adaptors (A) and other essential mediators (M) for Chk1, for example, Claspin. In the vicinity of Chk1, which is stabilized by PAR binding, ATR phosphorylates S/T-Q sites within the Chk1 C terminus. PAR binding to Chk1, stabilizes them at the replication forks until full phosphorylation (activation) is achieved, followed by the release of Chk1 from the replication forks. The activation of Chk1 then transduces the damage signal to its downstream effectors that are involved in the S-phase checkpoint.