Chk1 is activated transiently and targets Cdc25A for degradation at the Xenopus midblastula transition.
In Xenopus embryos, cell cycle elongation and degradation of Cdc25A (a Cdk2 Tyr15 phosphatase) occur naturally at the midblastula transition (MBT), at which time a physiological DNA replication checkpoint is thought to be activated by the exponentially increased nucleo-cytoplasmic ratio. Here we show that the checkpoint kinase Chk1, but not Cds1 (Chk2), is activated transiently at the MBT in a maternal/zygotic gene product-regulated manner and is essential for cell cycle elongation and Cdc25A degradation at this transition. A constitutively active form of Chk1 can phosphorylate Cdc25A in vitro and can target it rapidly for degradation in pre-MBT embryos. Intriguingly, for this degradation, however, Cdc25A also requires a prior Chk1-independent phosphorylation at Ser73. Ectopically expressed human Cdc25A can be degraded in the same way as Xenopus Cdc25A. Finally, Cdc25A degradation at the MBT is a prerequisite for cell viability at later stages. Thus, the physiological replication checkpoint is activated transiently at the MBT by developmental cues, and activated Chk1, only together with an unknown kinase, targets Cdc25A for degradation to ensure later development.
Department of Biology, Graduate School of Sciences, Kyushu University, Hakozaki 6-10-1, Fukuoka 812-8581, Japan.
Ken
K
Shimuta
Nobushige
N
Nakajo
Katsuhiro
K
Uto
Yoshimasa
Y
Hayano
Kenji
K
Okazaki
Noriyuki
N
Sagata
D000818:Animals; D001755:Blastocyst; D002453:Cell Cycle; D000071877:Checkpoint Kinase 1; D004789:Enzyme Activation; D006868:Hydrolysis; D016297:Mutagenesis, Site-Directed; D010766:Phosphorylation; D011494:Protein Kinases / Q000235:genetics / Q000378:metabolism*; D014981:Xenopus; D029867:Xenopus Proteins; D020687:cdc25 Phosphatases / Q000378:metabolism*
D029867:Xenopus Proteins; D011494:Protein Kinases; C000605867:CHEK1 protein, human; D000071877:Checkpoint Kinase 1; C000605875:Chek1 protein, Xenopus; C517004:CDC25A protein, human; D020687:cdc25 Phosphatases