Posttranslational Control of Cdc25 Degradation Terminates Drosophila’s Early Cell-Cycle Program

Abstract

SUMMARY In most metazoans, early embryonic development is characterized by rapid mitotic divisions that are controlled by maternal mRNAs and proteins that accumulate during oogenesis [1]. These rapid divisions pause at the Mid-Blastula Transition (MBT), coinciding with a dramatic increase in gene transcription and the degradation of a subset of maternal mRNAs [2, 3]. In Drosophila, the cell cycle pause is controlled by inhibitory phosphorylation of Cdk1, which in turn is driven by down-regulation of the activating Cdc25 phosphatases [4, 5]. Here, we show that the two Drosophila Cdc25 homologues, String and Twine, differ in their dynamics and that, contrary to current models [4], their down-regulations are not controlled by mRNA degradation but through different post-translational mechanisms. The degradation rate of String protein gradually increases during the late syncytial cycles in a manner dependent on the nuclear-to-cytoplasmic ratio and on the DNA replication checkpoints. Twine, on the other hand, is targeted for degradation at the onset of the MBT through a switch-like mechanism controlled like String by the nuclear-to-cytoplasmic ratio, but not requiring the DNA replication checkpoints. We demonstrate that post-translational control of Twine degradation ensures that the proper number of mitoses precede the MBT.

Cdc25 and the importance of G2control

“…Mutation of the consensus Chk1 phosphorylation sites in Twine does not affect its stability, demonstrating that String and Twine are not regulated the same way. 17 While this provides a satisfying mechanistic explanation of the regulation of the MBT in Drosophila, it is not clear why 2 Cdc25 factors are used in the early embryo, especially since Twine seems to be the key factor in timing G 2 at the MBT.…”

“…At each cell cycle the amount of DNA doubles, and at a set number of cycles, which differs among organisms, the embryo senses through a still unclear mechanism that a correct ratio has been reached, and this initiates the degradation of Twine. 17,18 But Twine degradation is only part of the story. The other Cdc25 ortholog, String, also must be degraded for the MBT to occur.…”

“…Like Twine, the String half-life also changes from 20 min to 5 min between the early cleavage embryo and the MBT, but this change is gradual and begins well before the MBT. 17,18,23 Key to this degradation is the Chk1 kinase ortholog Grapes, 24 which appears to require zygotic transcription to be activated (Fig. 2).…”

“…2). 17,18 This rapid degradation, which introduces a cell cycle delay, is controlled by at least 2 factors, including a potential adaptor protein named Tribbles. 17,19 But how does the embryo know precisely when to degrade Twine?…”

“…17,18 This rapid degradation, which introduces a cell cycle delay, is controlled by at least 2 factors, including a potential adaptor protein named Tribbles. 17,19 But how does the embryo know precisely when to degrade Twine? Part of the answer involves transcription of Twine-degrading factors, 18 most likely regulated by Vielfältig/Zelda, the key transcriptional activator of the zygotic genome at the MBT.…”