Activation of mammalian Chk1 during DNA replication arrest: a role for Chk1 in the intra-S phase checkpoint monitoring replication origin firing

Checkpoints maintain order and fidelity in the cell cycle by blocking late- occurring events when earlier events are improperly executed. Here we descr ibe evidence for the participation of Chk1 in an intra-S phase checkpoint i n mammalian cells. We show that both Chk1 and Chk2 are phosphorylated and a ctivated in a caffeine sensitive signaling pathway during S phase, but only in response to replication blocks, not during normal S phase progression. Replication block-induced activation of Chk1 and Chk2 occurs normally in at axia telangiectasia (AT) cells, which are deficient in the S phase response to ionizing radiation (IR). Resumption of synthesis after removal of repli cation blocks correlates with the inactivation of Chk1 but not Chk2. Using a selective small molecule inhibitor, cells lacking Chk1 function show a pr ogressive change in the global pattern of replication origin firing in the absence of any DNA replication. Thus, Chk1 is apparently necessary for an i ntra-S phase checkpoint, ensuring that activation of late replication origi ns is blocked and arrested replication fork integrity is maintained when DN A synthesis is inhibited.