The human Myt1 kinase preferentially phosphorylates Cdc2 on threonine 14 and localizes to the endoplasmic reticulum and Golgi complex

Cyclin-dependent kinases: engines, clocks, and microprocessors

Cyclin-dependent kinases (Cdks) play a well-established role in the regulation of the eukaryotic cell division cycle and have also been implicated in the control of gene transcription and other processes. Cdk activity is governed by a complex network of regulatory subunits and phosphorylation events whose precise effects on Cdk conformation have been revealed by recent crystallographic studies. In the cell, these regulatory mechanisms generate an interlinked series of Cdk oscillators that trigger the events of cell division.

Analysis of the early embryonic cell cycles of Xenopus; regulation of cell cycle length by Xe-wee1 and Mos

In Xenopus, cdc2 tyrosine phosphorylation is detected in the first 60–75 minute cell cycle but not in the next eleven cell cycles (cycles 2–12) which are only 30 minutes long. Here we report that the wee1/cdc25 ratio increases before the first mitotic interphase. We show that the Xe-wee1 protein is absent in stage VI oocytes and is expressed from meiosis II until gastrulation. A dominant negative form of Xe-wee1 (KM wee1) reduced the level cdc2 tyrosine phosphorylation and length of the first cycle. However, the ratio of wee1/cdc25 did not decrease after the first cycle and therefore did not explain the lack of cdc2 tyrosine phosphorylation in, nor the rapidity of, cycles 2–12. Furthermore, there was no evidence for a wee1/myt1 inhibitor in cycles 2–12. We examined the role of Mos in the first cycle because it is present during the first 20 minutes of this cycle. We arrested the rapid embryonic cell cycle (cycle 2 or 3) with Mos and restarted the cell cycle with calcium ionophore; the 30 minute cycle was converted into a 60 minute cycle, with cdc2 tyrosine phosphorylation. In addition, the injection of a non-degradable Mos (MBP-Mos) into the first cycle resulted in a dramatic elongation of this cycle (to 140 minutes). MBP-Mos did not delay DNA replication or the translation of cyclins A or B; it did, however, result in the marked accumulation of tyrosine phosphorylated cdc2. Thus, while the wee1/cdc25 ratio changes during development, these changes may not be responsible for the variety of cell cycles observed during early Xenopus embryogenesis. Our experiments indicate that Mos/MAPK can also contribute to cell cycle length.

Cell cycle-regulated expression, phosphorylation, and degradation of p55Cdc. A mammalian homolog of CDC20/Fizzy/slp1

p55Cdc is a mammalian protein that shows high homology to the cell cycle proteins Cdc20p of Saccharomyces cerevisiae and the product of the Drosophila fizzy (fzy) gene, both of which contain WD repeats and are thought to be required for the metaphase-anaphase transition. The fzy mutants exhibit a metaphase arrest phenotype, which is accompanied by stabilization of cyclins A and B, leading to the hypothesis that fzy function is required for cell cycle-regulated ubiquitin-mediated proteolysis. p55Cdc expression was initiated at the G1/S transition and steady state levels of p55Cdc were highest at M and lowest in G1. Inhibition of the 26 S proteasome prevented both mitotic exit and loss of p55Cdc at the M/G1 transition, suggesting that p55Cdc degradation was mediated by the cell cycle-regulated proteolytic pathway. Immune complexes of p55Cdc obtained at different cell cycle stages showed a variety of proteins with dramatic differences observed in the pattern of associated proteins during the transition from G2 to M. Immunolocalization of p55Cdc demonstrated dynamic changes in p55Cdc localization as the cells transit mitosis. p55Cdc appears to act as a regulatory protein interacting with several other proteins, perhaps via its seven WD repeats, at multiple points in the cell cycle.

On cyclins, oocytes, and eggs

Oocyte and egg are suitable model systems for studying cell division since meiotic maturation resembles a G2/M transition and early embryonic divisions are precisely timed and occur without zygotic transcription. The analysis of oocytes and eggs from different species provides the opportunity to understand the roles of proteins that the critical to the progression and maintenance of the cell cycle. Among them, cyclins are certainly worthy of investigation. Mitotic cyclins (cyclins A and B) are clearly implicated in meiosis and early embryonic cell cycles. More recent studies have revealed that G1-type cyclins (cyclins E and D) could also play a role in both processes and cyclin H has been suggesed to participate to CAK activity (cdc2-activating kinase) in oocytes. The study of cyclins in oocytes and eggs clearly offer insights into their roles during the cell cycle.

Human Myt1 is a cell cycle-regulated kinase that inhibits Cdc2 but not Cdk2 activity

Activation of the Cdc2.cyclin B kinase is a pivotal step of mitotic initiation. This step is mediated principally by the dephosphorylation of residues threonine 14 (Thr14) and tyrosine 15 (Tyr15) on the Cdc2 catalytic subunit. In several organisms homologs of the Wee1 kinase have been shown to be the major activity responsible for phosphorylating the Tyr15 inhibitory site. A membrane-bound kinase capable of phosphorylating residue Thr14, the Myt1 kinase, has been identified in the frog Xenopus laevis and more recently in human. In this study, we have examined the substrate specificity and cell cycle regulation of the human Myt1 kinase. We find that human Myt1 phosphorylates and inactivates Cdc2-containing cyclin complexes but not complexes containing Cdk2 or Cdk4. Analysis of endogenous Myt1 demonstrates that it remains membrane-bound throughout the cell cycle, but its kinase activity decreased during M phase arrest, when Myt1 became hyperphosphorylated. Further, Cdc2. cyclin B1 was capable of phosphorylating Myt1 in vitro, but this phosphorylation did not affect Myt1 kinase activity. These findings suggest that human Myt1 is negatively regulated by an M phase-activated kinase and that Myt1 inhibits mitosis due to its specificity for Cdc2.cyclin complexes.

Myt1: a Wee1-type kinase that phosphorylates Cdc2 on residue Thr14

Most somatic cell division cycles contain a gap period (G2 phase) between the completion of DNA synthesis and the initiation of mitosis. This delay of mitotic entry is controlled, at least in part, by the repression of Cdc2 kinase activity by the phosphorylation of two conserved residues (Thr14 and Tyr15) within the ATP-binding pocket of the Cdc2 catalytic subunit. The kinases responsible for these two phosphorylation events include the Myt1 and Wee1 kinases, which phosphorylate Cdc2 on Thr14 and Tyr15, respectively. In this discussion, we summarise our current knowledge of the Myt1 kinase and its regulation of Cdc2 kinase activity during the G2-to -M phase transition.