Chk1 is activated transiently and targets Cdc25A for degradation at the Xenopus midblastula transition

Abstract

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.

Cdc25A phosphatase: combinatorial phosphorylation, ubiquitylation and proteolysis

“…Interestingly, Shimuta et al (2002) showed that Cdc25A in Xenopus is phosphorylated on Ser 73 (Ser 76 in human Cdc25A) by an unknown kinase. This phosphorylation is required for Chk1-induced degradation of Cdc25A and primes Cdc25A for degradation both in a normal cell cycle and in response to DNA damage.…”

Cdc25 and the importance of G2control

“…Similar to the regulation of Twine, Cdc25A is abruptly degraded at the MBT in Xenopus, 26 although as with String, the degradation is regulated by Chk1, which is transiently activated coincident with the start of the MBT. 27 In Xenopus, Cdc25A degradation alone does not affect the MBT, since overexpression of a stabilized Cdc25A does not change the cell cycle length at the MBT. 27,28 However, phosphorylation of Cdc25A by Chk1 inactivates it, and this inactivation is essential for establishing the MBT.…”

“…27 In Xenopus, Cdc25A degradation alone does not affect the MBT, since overexpression of a stabilized Cdc25A does not change the cell cycle length at the MBT. 27,28 However, phosphorylation of Cdc25A by Chk1 inactivates it, and this inactivation is essential for establishing the MBT. 28 Thus, both inactivation and degradation are likely to be involved in regulating Cdc25 in vertebrates (Fig.…”

“…Unlike in Drosophila, Chk1 phosphorylation and activation is independent of zygotic transcription in Xenopus. 14,27 Instead, Chk1 activation depends on the depletion of specific DNA replication factors and limiting dNTP levels, which occurs at a precise nucleocytoplasmic ratio. 29 Although we now know quite a bit about the mechanism of the MBT in select model organisms, why does an MBT even exist?…”

Hsp90 Inhibition Depletes Chk1 and Sensitizes Tumor Cells to Replication Stress

“…Second, activated Chk1 slows progression through S phase by blocking the firing of unfired origins of the replication (22)(23)(24)(25). In this checkpoint, Chk1 phosphorylates Cdc25A, leading to Cdc25A degradation (24,26,27). Because Cdc25A is required for activation of Cdk2 complexes, which then control the firing of origins of replication, activation of this pathway blocks S phase progression (28 -31).…”