Drosophila Wee1 Kinase Regulates Cdk1 and Mitotic Entry during Embryogenesis

Abstract

Cyclin-dependent kinases (Cdks) are the central regulators of the cell division cycle. Inhibitors of Cdks ensure proper coordination of cell cycle events and help regulate cell proliferation in the context of tissues and organs. Wee1 homologs phosphorylate a conserved tyrosine to inhibit the mitotic cyclin-dependent kinase Cdk1. Loss of Wee1 function in fission or budding yeast causes premature entry into mitosis. The importance of metazoan Wee1 homologs for timing mitosis, however, has been demonstrated only in Xenopus egg extracts and via ectopic Cdk1 activation . Here, we report that Drosophila Wee1 (dWee1) regulates Cdk1 via phosphorylation of tyrosine 15 and times mitotic entry during the cortical nuclear cycles of syncytial blastoderm embryos, which lack gap phases. Loss of maternal dwee1 leads to premature entry into mitosis, mitotic spindle defects, chromosome condensation problems, and a Chk2-dependent block of subsequent development, and then embryonic lethality. These findings modify previous models about cell cycle regulation in syncytial embryos and demonstrate that Wee1 kinases can regulate mitotic entry in vivo during metazoan development even in cycles that lack a G2 phase.

Death-effector domain-containing protein DEDD is an inhibitor of mitotic Cdk1/cyclin B1

“…The functional inhibition of Cdk1/cyclin B1 activity by DEDD appears to be specific for mammalian cells, because DEDD (or DEDD homologues) have not been found in databases for lower eukaryotes. This is also consistent with the fact that cell growth before cell division is not significantly affected by the loss of Wee1-related kinases, responsible for phosphorylation of Thr-14/Tyr-15, in higher mammalian cells (4,5,9,10,(14)(15)(16)(17)(18). It is interesting that, in DEDD Ϫ/Ϫ cells, the length was decreased not only in mitosis but also in the G 1 phase (where Cdk1/cyclin B1 is not involved) during cell cycle.…”

“…In contrast, an inadequate duration before division, due to aberrant phosphorylation of Tyr-15, causes cells to enter mitosis before sufficient growth, resulting in decreased size of the daughter cells (14-17). Interestingly, the loss of Wee1-related kinases, responsible for phosphorylation of Thr-14/Tyr-15, causes premature cell division in yeast, Xenopus, or Drosophila cells, but not in higher mammalian cells (4,5,9,10,(15)(16)(17)(18). This suggests the presence of alternative mechanism(s), which may also influence cell size, particularly in mammalian cells.…”

Drosophila Myt1 Is the Major Cdk1 Inhibitory Kinase for Wing Imaginal Disc Development

“…Previous studies of Drosophila Wee1 and Myt1 revealed that these conserved Cdk1 inhibitory kinases were required during early embryogenesis and gametogenesis, respectively (Price et al 2000;Stumpff et al 2004;Jin et al 2005). We have now characterized imaginal and adult developmental defects caused by loss of dMyt1 activity (and to a much lesser extent, dWee1), that confirm the importance of Cdk1 inhibitory phosphorylation for coordinating cell-cycle events with critical developmental processes.…”

“…In contrast, Wee1 kinases appear to phosphorylate Cdk1 exclusively on the Y15 site (Parker et al 1995). Drosophila dWee1 also functions as a Y15-specific Cdk1 inhibitory kinase, in vitro (Campbell et al 1995) and early embryos (Price et al 2000;Stumpff et al 2004).…”

“…We showed previously that dWee1 regulation of Cdk1 is essential for a premitotic checkpoint mechanism that prevents mitotic catastrophe during the rapid S/M nuclear cycles of early embryogenesis (Price et al 2000;Stumpff et al 2004). Zygotic wee1 mutants are viable with no obvious developmental defects although they are sensitive to the DNA replication inhibitor hydroxyurea, suggesting they are impaired for a DNA replication checkpoint (Price et al 2000).…”