Claspin and the activated form of ATR-ATRIP collaborate in the activation of Chk1

Claspin is necessary for the ATR-dependent activation of Chk1 in Xenopus egg extracts containing incompletely replicated DNA. ATR possesses a regulatory partner called ATRIP. We have studied the respective roles of ATR-ATRIP and Claspin in the activation of Chk1. ATR-ATRIP bound well to various DNA templates in Xenopus egg extracts. ATR-ATRIP bound to a single-stranded DNA template was weakly active. By contrast, the ATR-ATRIP complex on a DNA template containing both single- and double-stranded regions displayed a large increase in kinase activity. This observation suggests that ATR-ATRIP normally undergoes activation upon association with specific nucleic acid structures at DNA replication forks. Without Claspin, activated ATR-ATRIP phosphorylated Chk1 weakly in a cell-free reaction. The addition of Claspin to this reaction strongly stimulated the phosphorylation of Chk1 by ATR-ATRIP. Claspin also induced significant autophosphorylation of Chk1 in the absence of ATR-ATRIP. Taken together, these results indicate that the checkpoint-dependent phosphorylation of Chk1 is a multistep process involving activation of the ATR-ATRIP complex at replication forks and presentation of Chk1 to this complex by Claspin.