Tribbles: a family of kinase-like proteins with potent signalling regulatory function

The recent identification of tribbles as regulators of signal processing systems and physiological processes, including development, together with their potential involvement in diabetes and cancer, has generated considerable interest in these proteins. Tribbles have been reported to regulate activation of a number of intracellular signalling pathways with roles extending from mitosis and cell activation to apoptosis and modulation of gene expression. The current review summarises our current understanding of interactions between tribbles and various other proteins. Since our understanding on the molecular basis of tribbles function is far from complete, we also describe a bioinformatic analysis of various segments of tribbles proteins, which has revealed a number of highly conserved peptide motifs with potentially important functional roles.     

Mother-daughter precursor cell fate transformation after Cdc2 down-regulation in the Drosophila bristle lineage

The Drosophila bristle lineage is an excellent system in which to study how cell cycle and fate determination are synchronized in invariant cell lineages. In this model, five different cells arise from a single precursor cell, pI, after four asymmetric cell divisions. Cell diversity is achieved by the asymmetric segregation of cell determinants, such as Numb and Neuralized (Neur), resulting in differential activation of the Notch (N) pathway. We show that down-regulation of Cdc2, by over-expressing Tribbles, Dwee1, and Dmyt1 (three negative regulators of Cdc2) or by using thermo-sensitive Cdc2 mutant flies, delayed pI mitosis, and altered the polarity and the number of subsequent cell divisions. These modifications were associated with a mother-daughter cell fate transformation as the pI cell acquired the identity of the secondary precursor cell, pIIb. This type of change in cell identity only occurred when the N signaling pathway was inactive since ectopic N signaling transformed pI to pIIa-progeny fate. These transformations in cell identity suggest that, although synchronized, cell cycle and fate determination are independent phenomena in the bristle lineage.     

Tribbles, a cell-cycle brake that coordinates proliferation and morphogenesis during Drosophila gastrulation

BACKGROUND: The final shape and size of an organism is determined by both morphogenetic processes and cell proliferation and it is essential that these processes be properly coordinated. In particular, cell division is incompatible with certain types of morphogenetic cell behaviour, such as migration, adhesion and changes in cell shape. Mechanisms must therefore exist to ensure that one does not interfere with the other. RESULTS: We address here the coordination of proliferation and morphogenesis during the development of the mesoderm in Drosophila. We show that it is essential that mitosis be blocked in the mesoderm during early gastrulation, and identify the putative serine/threonine kinase Tribbles as controlling this block. In its absence, the mitotic block is lifted, resulting in severe defects during early gastrulation. Tribbles, a homologue of a group of vertebrate proteins of unknown function, acts in concert with another, as yet unidentified, factor to counteract the activity of the protein phosphatase Cdc25/String. CONCLUSIONS: In a finely tuned balance with Cdc25/String, Tribbles controls the timing of mitosis in the prospective mesoderm, allowing cell-shape changes to be completed. This mechanism for coordinating cell division and cell-shape changes may have helped Drosophila to evolve its mode of rapid early development.     

The trouble with tribbles

Cell division and cell movements must be coordinated during development. A novel inhibitor of cell division, Tribbles, has been identified that blocks mitosis at a critical point in Drosophila morphogenesis. The data support a role for Tribbles in promoting proteolytic degradation of String/Cdc25, a key regulator of mitosis.     

Kizuna is a novel mitotic substrate for CDC25B phosphatase

CDC25 dual-specificity phosphatases play a central role in cell cycle control through the activation of Cyclin-Dependent Kinases (CDKs). Expression during mitosis of a stabilized CDC25B mutant (CDC25B-DDA), which cannot interact with the F-box protein βTrCP for proteasome-dependent degradation, causes mitotic defects and chromosome segregation errors in mammalian cells. We found, using the same CDC25B mutant, that stabilization and failure to degrade CDC25B during mitosis lead to the appearance of multipolar spindle cells resulting from a fragmentation of pericentriolar material (PCM) and abolish mitotic Plk1-dependent phosphorylation of Kizuna (Kiz), which is essential for the function of Kiz in maintaining spindle pole integrity. Thus, in mitosis Kiz is a new substrate of CDC25B whose dephosphorylation following CDC25B stabilization leads to the formation of multipolar spindles. Furthermore, endogenous Kiz and CDC25B interact only in mitosis, suggesting that Kiz phosphorylation depends on a balance between CDC25B and Plk1 activities. Our data identify a novel mitotic substrate of CDC25B phosphatase that plays a key role in mitosis control.     

Coordination of mitosis and morphogenesis: role of a prolonged G2 phase during chordate neurulation

Chordates undergo a characteristic morphogenetic process during neurulation to form a dorsal hollow neural tube. Neurulation begins with the formation of the neural plate and ends when the left epidermis and right epidermis overlying the neural tube fuse to close the neural fold. During these processes, mitosis and the various morphogenetic movements need to be coordinated. In this study, we investigated the epidermal cell cycle in Ciona intestinalis embryos in vivo using a fluorescent ubiquitination-based cell cycle indicator (Fucci). Epidermal cells of Ciona undergo 11 divisions as the embryos progress from fertilization to the tadpole larval stage. We detected a long G2 phase between the tenth and eleventh cell divisions, during which fusion of the left and right epidermis occurred. Characteristic cell shape change and actin filament regulation were observed during the G2 phase. CDC25 is probably a key regulator of the cell cycle progression of epidermal cells. Artificially shortening this G2 phase by overexpressing CDC25 caused precocious cell division before or during neural tube closure, thereby disrupting the characteristic morphogenetic movement. Delaying the precocious cell division by prolonging the S phase with aphidicolin ameliorated the effects of CDC25. These results suggest that the long interphase during the eleventh epidermal cell cycle is required for neurulation.     

Nuclear localization of CDC25B1 and serine 146 integrity are required for induction of mitosis

CDC25B phosphatases are essential regulators that control cyclin-dependent kinases activities at the entry into mitosis. In this study, we demonstrate that serine 146 is required for two crucial features of CDC25B1. It is essential for CDC25B1 to function as a mitotic inducer and to prevent CDC25B1 export from the nucleus. We also show that serine 146 is phosphorylated in vitro by CDK1-cyclin B. However, phosphorylation of CDC25B does not stimulate its phosphatase activity, and mutation of serine 146 had no effect on its catalytic activity. Serine 146 phosphorylation is proposed to be a key event in the regulation of the CDC25B function in the initiation of mammalian mitosis.     

CDC25B Phosphorylation by Aurora A Occurs at the G2/M Transition and is Inhibited by DNA Damage

CDC25B is one of the three human dual-specificity phosphatases involved in the activation ofcyclin-dependent kinases at key stages of the cell division cycle. CDC25B that is responsiblefor the activation of CDK1-cyclin B1 is regulated by phosphorylation. The STK15/Aurora-Akinase locally phosphorylates CDC25B on serine 353 at the centrosome during the G2/Mtransition. Here we have investigated this phosphorylation event during the cell cycle, and inresponse to activation of the G2 DNA damage checkpoint. We show that accumulation of theS353-phosphorylated form of CDC25B at the centrosome correlates with the relocalisation ofcyclin B1 to the nucleus and the activation of CDK1 at entry into mitosis. Upon activation ofthe G2/M checkpoint by DNA damage, we demonstrate that Aurora-A is not activated andconsequently CDC25B is not phosphorylated. We show that ectopic expression of Aurora-Aresults in a bypass of the checkpoint that partially overcome by a S353A mutant of CDC25B.Finally, we show that bypass of the G2/M checkpoint by the CHK1 kinase inhibitor UCN-01results in the activation of Aurora-A and phosphorylation of CDC25B on S353. These resultsstrongly suggest that Aurora-A-mediated phosphorylation of CDC25B at the centrosome is animportant step contributing to the earliest events inducing mitosis, upstream of CDK1-cyclinB1 activation.     

CDC25B Phosphorylation by p38 and MK-2

CDC25B is one of the three human phosphatases that are involved in the control of the activation of cyclin-dependent kinases. CDC25B participates in regulating entry into mitosis and appears to play a key role in the checkpoint response to DNA injury.CDC25B has been reported to be regulated by a number of kinases and controversial evidence suggests that it is phosphorylated by p38SAPK and/or MAPKAP Kinase-2. In this report, we clarify this issue using an approach combining mass spectrometry andthe use of specific antibodies against phosphorylated CDC25B residues. We report that MAPKAP Kinase-2 phosphorylates CDC25B on multiple sites including S169, S323, S353 and S375, while p38 phosphorylates CDC25B on S249. We show that theS323-phosphorylated form of CDC25B is detected at the centrosome during a normal cell cycle. Since most of these sites are also phosphorylated by several other kinases, our observations highlight the difficulty in characterising and understanding in vivo phosphorylation patterns.     

The "starter" and "gas pedal" of mitosis reside at the centrosome: Commentary on "Characterization of centrosomal localization and dynamics of CDC25C phosphatase in mitosis" by Bonnet et al.

The CDC25 phosphatases play an essential role in the spatial and temporal regulation of the control of entry into mitosis. These enzymes dephosphorylate and activate the CDK-cyclin complexes, in particular CDK1-cyclin B1, the master regulator of mitosis. Three CDC25 genes in exist in humans (CDC25A, CDC25B and CDC25C), and the original model of their function proposed that they acted sequentially at discrete cell cycle transitions, i.e., that CDC25A was dedicated to the activation of the G1/S progression-associated CDKs, CDC25B controlled early prophase events, while CDC25C was thought to achieve the full activation of CDK1-cyclin B1 at entry into mitosis. Indeed, the situation appears much more complicated than this, and current evidence shows that all three CDC25 phosphatases act at a variety of mitotic stages, with and considerable experimental evidence to indicate that all three are involved in orchestrating cell cycle progression in mitosis.1 Previous work has led to the proposal that CDC25B acts as the starter of mitosis. Additionally, a number of recent studies have shown that CDC25B also localizes to the centrosome where its activating role on CDK-cyclin complexes appears to be regulated by multiple activatory and inhibitory kinases.2-5 As such, it has been proposed that CDC25B might act as a central centrosomal integrator and a trigger for the initial events that set up the sequence of events leading to mitosis.6 As a target of the first small pool of activated CDK1-cyclin B1 that translocates to the nucleus, CDC25C was thought to subsequently be responsible for the massive activation of the nuclear pool of CDK1-cyclin B1 that occurs at entry into mitosis. A report from the group headed by May Morris presented in this issue of Cell Cycle (Bonnet et al., pp. 1990–7) provides new insight into the dynamics of these events and in the understanding of the involvement of both CDC25B and CDC25C in the earliest stages of the G2/M transition. Bonnet and collaborators show for the first time, as has long been suspected but until now never observed, the localization of a fraction of CDC25C at the centrosome during interphase. This centrosomal localization occurs from S-phase onward and is also present during mitosis. Using FRAP analysis, their study elegantly shows that this centrosomal population of CDC25C is highly dynamic. Furthermore, the authors show that mutations of CDC25C that impair its catalytic activity or its binding to its CDK-cyclin substrates promote its centrosomal accumulation, thus suggesting an active role in the dephosphorylation and activation of CDK-cyclins at this location. Together with previous reports showing that the activity of CDC25C is amplified following its mitotic phosphorylation by CDK1-cyclin B1 while the activity of CDC25B is not,7 these new findings lead to the proposition of an alternative regulatory model for the control of the G2/M transition. In this model, the CDK1-cyclin B1 complex is activated at the centrosomal level both by the initial action of CDC25B (as has already been suggested8) as well as by the centrosomal pool of activated CDC25C that subsequently amplifies the process through its own phosphorylation and activation (Fig. 1). While CDC25B can be considered as a “starter”, CDC25C plays the role of the “gas pedal” that speeds up entry into mitosis by amplifying the signaling cascade from the centrosome and finally increasing nuclear levels. This model is certainly too simplistic and does not integrate many major issues that remain to be investigated. Among these unsolved questions is the role that the multiple splice variants of the CDC25 phosphatases might play. There are at least five variants for both CDC25B and CDC25C whose specific regulation and roles in the dephosphorylation of individual CDK-cyclins substrates is still unknown.5 Likely related to this question is the issue of the presence of both CDC25B and CDC25C until late stages of mitosis. Why is CDC25C associated with the centrosome when, according to the dogma, the entire pool of CDK1-cyclin B1 has been fully activated? An attractive hypothesis is to speculate that the CDC25 phosphatases might continue to play discrete roles in the dephosphorylation and the activation of sub-populations of CDK-cyclins throughout the entire process of mitosis to ensure a fine tuning of the kinase activities that are involved in the many architectural and functional aspects of the mitotic figure. Centrosomes are made up of numerous proteins whose amino acid sequence suggests a coiled-coil tertiary structure. Increasing evidence indicates that this molecular structure may be well-designed for the organization of multiprotein scaffolds that can anchor a diversity of activities ranging from protein complexes involved in microtubule nucleation to multicomponent pathways for cellular regulation.9 By physically linking components of a common pathway, molecular scaffolds can increase the local concentration of components, limit nonspecific interactions, and provide spatial control for regulatory pathways by positioning by positioning them at specific sites in proximity to downstream targets or upstream modulators. On the basis of the increasing number of regulatory molecules anchored at the centrosome, it is likely that this organelle serves as a centralized control center for regulating a diversity of cellular activities. Recent studies have provided some of the first functional links between centrosomes and regulatory networks in cell cycle transitions from G1 to S-phase, G2 to M-phase and metaphase to anaphase. The findings by Bonnet et al. support this line of evidence.

References

Boutros R, Dozier C, Ducommun B. The when and wheres of CDC25 phosphatases. Curr Opin Cell Biol 2006; 18:185-91. Dutertre S, Cazales M, Quaranta M, Froment C, Trabut V, Dozier C, Mirey G, Bouche J, Theis-Febvre N, Schmitt E, Monsarrat B, Prigent C, Ducommun B. Phosphorylation of CDC25B by Aurora-A at the centrosome contributes to the G2/M transition. J Cell Science 2004; 117:2523-31. Schmitt E, Boutros R, Froment C, Monsarrat B, Ducommun B, Dozier C. CHK1 phosphorylates CDC25B during the cell cycle in the absence of DNA damage. J Cell Sci 2006; 119:4269-75. Boutros R, Ducommun B. Asymmetric localization of the CDC25B phosphatase to the mother centrosome during interphase. Cell Cycle 2008; 7:401-6. Boutros R, Lobjois V, Ducommun B. CDC25 phosphatases in cancer cells: key players? Good targets? Nat Rev Cancer 2007; 7:495-507. Lindqvist A, Kallstrom H, Lundgren A, Barsoum E, Rosenthal CK. Cdc25B cooperates with Cdc25A to induce mitosis but has a unique role in activating cyclin B1-Cdk1 at the centrosome. J Cell Biol 2005; 171:35-45. Baldin V, Pelpel K, Cazales M, Cans C, Ducommun B. Nuclear Localization of CDC25B1 and Serine 146 Integrity Are Required for Induction of Mitosis. J Biol Chem 2002; 277:35176-82. Jackman M, Lindon C, Nigg EA, Pines J. Active cyclin B1-Cdk1 first appears on centrosomes in prophase. Nat Cell Biol 2003; 5:143-8. Kramer A, Lukas J, Bartek J. Checking out the centrosome. Cell Cycle 2004; 3:1390-3.     

Study of the cytolethal distending toxin-induced cell cycle arrest in HeLa cells: involvement of the CDC25 phosphatase

HeLa cells exposed to Escherichia coli cytolethal distending toxins (CDT) arrest their cell cycle at the G2/M transition. We have shown previously that in these cells the CDK1/cyclin B complex is inactive and can be reactivated in vitro using recombinant CDC25 phosphatase. Here we have investigated in vivo the effects of CDC25 on this cell cycle checkpoint. We report that overexpression of CDC25B or CDC25C overrides an established CDT-induced G2 cell cycle arrest and leads the cells to accumulate in an abnormal mitotic stage with condensed chromatin and high CDK1 activity. This effect can be counteracted by coexpression of the WEE1 kinase. In contrast, overexpression of CDC25B or C prior to CDT treatment prevents G2 arrest and allows most of the cells to progress through mitosis with only a low percentage of cells arrested in abnormal mitosis. The implications of these results on the biochemical nature of the CDT-induced cell cycle arrest are discussed.     

CDC25B Phosphorylated by pEg3 Localizes to the Centrosome and the Spindle Poles at Mitosis

The phosphatase CDC25B is one of the key regulators that control entry into mitosis throughthe dephosphorylation and subsequent activation of the cyclin-dependent kinases. Here westudy the phosphorylation of CDC25B at mitosis by the kinase pEg3, a member of theKIN1/PAR-1/MARK family. Using mass spectrometry analysis we demonstrate thatCDC25B is phosphorylated in vitro by pEg3 on serine 169, a residue that lies within the Bdomain. Moreover, using phosphoepitope-specific antibodies we show that serine 169 isphosphorylated in vivo, that this phosphorylated form of CDC25B accumulates duringmitosis, and is localized to the centrosomes. This labelling is abrogated when pEg3expression is repressed by RNA interference. Taken together, these results support a model inwhich pEg3 contributes to the control of progression through mitosis by phosphorylation ofthe CDC25 phosphatases.     

Moderate variations in CDC25B protein levels modulate the response to DNA damaging agents

CDC25B, one of the three members of the CDC25 dual-specificity phosphatase family, plays a critical role in the control of the cell cycle and in the checkpoint response to DNA damage. CDC25B is responsible for the initial dephosphorylation and activation of the cyclin-dependent kinases, thus initiating the train of events leading to entry into mitosis.1 The critical role played by CDC25B is illustrated by the fact that it is specifically required for checkpoint recovery2, 3 and that unscheduled accumulation of CDC25B is responsible for illegitimate entry into mitosis.3-5 Here, we report that in p53-/- colon carcinoma cells, a moderate increase in the CDC25B level is sufficient to impair the DNA damage checkpoint, to increase spontaneous mutagenesis, and to sensitize cells to ionising radiation and genotoxic agents. Using a tumour cell spheroid assay as an alternative to animal studies, we demonstrate that the level of CDC25B expression modulates growth inhibition and apoptotic death. Since CDC25B overexpression has been observed in a significant number of human cancers, including colon carcinoma, and is often associated with high grade tumours and poor prognosis1, our work suggests that the expression level of CDC25B might be a potential key parameter of the cellular response to cancer therapy.     

CDC25B is involved in the centrosomal microtubule nucleation in two‐cell stage mouse embryos

CDC25B has been demonstrated to activate the complex of CDK1/Cyclin B and trigger mitosis. We have recently demonstrated that p‐CDC25B‐Ser351 is located at the centrosomes of mouse oocytes and contributes to the release of mouse oocytes from prophase I arrest. But much less is known about CDC25B function at the centrosome in two‐cell stage mouse embryos. Here we investigate the effect of CDC25B regulating the microtubules nucleation. Microinjection of anti‐CDC25B antibody caused aberrant microtubule nucleation. In addition, embryos injected with anti‐CDC25B antibody showed the marked absence of microtubule repolymerization and Nek2 foci after nocodazole washout. CDC25B overexpression caused microtubule‐organizing center (MTOC) overduplication. Moreover, overexpression of CDC25B–?65 mutant resulted in the loss of CDC25B localization in the perinuclear region and made CDC25B less efficient in inducing mitosis. We additionally identified that CDC25B is responsible for the pericentrin localization to the MTOC. Our data suggest an important role of CDC25B for microtubule nucleation and organization. N‐terminal of CDC25B is required for regulating the microtubule dynamics and mitotic function.     

A Xenopus tribbles orthologue is required for the progression of mitosis and for development of the nervous system

The product of the Drosophila gene tribbles inhibits cell division in the ventral furrow of the embryo and thereby allows the normal prosecution of gastrulation. Cell division is also absent in involuting dorsal mesoderm during gastrulation in Xenopus, and to ask whether the two species employ similar mechanisms to coordinate morphogenesis and the cell cycle, we isolated a putative Xenopus homologue of tribbles which we call Xtrb2. Extensive cDNA cloning identified long and short forms of Xtrb2, termed Xtrb2-L and Xtrb2-S, respectively. Xtrb2 is expressed maternally and in mesoderm and ectoderm at blastula and gastrula stages. Later, it is expressed in dorsal neural tube, eyes, and cephalic neural crest. Time-lapse imaging of GFP-tagged Xtrb2-L suggests that during cell division, it is associated with mitotic spindles. Knockdown of Xtrb2 by antisense morpholino oligonucleotides (MOs) disrupted synchronous cell divisions during blastula stages, apparently as a result of delayed progression through mitosis and cytokinesis. At later stages, tissues expressing the highest levels of Xtrb2 were most markedly affected by morpholino knockdown, with perturbation of neural crest and eye development.     

UV-induced downregulation of the CDC25B protein in human cells

The CDC25B phosphatase regulates the activation of CDK1-Cyclin B at the onset of mitosis, being a key target of the checkpoint pathways activated by cellular stress and DNA damage. Previous work has reported that checkpoint activation induces the sequestration of CDC25B in the cytoplasm. Here we show that in response to UV irradiation, the levels of CDC25B protein can be downregulated independently of classical checkpoints pathways such as p53, ATM/ATR and p38 MAPK. We also show that translational repression mediated by eIF2α phosphorylation regulates CDC25B expression levels. Taken together, our results illustrate a new mechanism of CDC25B regulation in response to stress.     

Study of the docking-dependent PLK1 phosphorylation of the CDC25B phosphatase

CDC25 (A, B and C) phosphatases control cell cycle progression through the timely dephosphorylation and activation of cyclin-dependent kinases (CDK). At mitosis the CDC25B phosphatase activity is dependent on its phosphorylation by multiple kinases impinging on its localisation, stability and catalytic activity. Here we report that prior phosphorylation of CDC25B by CDK1 enhances its substrate properties for PLK1 in vitro, and we also show that phosphorylated S50 serves as a docking site for PLK1. Using a sophisticated strategy based on the sequential phosphorylation of CDC25B with ¹⁶O and ¹⁸O ATP prior to nanoLC–MS/MS analysis we identified 13 sites phosphorylated by PLK1. This study illustrates the complexity of the phosphorylation pattern and of the subsequent regulation of CDC25B activity.     

βTrCP-dependent degradation of CDC25B phosphatase at the metaphase-anaphase transition is a pre-requisite for correct mitotic exit

The dual-specificity phosphatase CDC25B, a key regulator of CDK/Cyclin complexes, is considered as the starter of mitosis. It is an unstable protein, degraded by the proteasome, but often over-expressed in various human cancers. Based on experiments carried out in Xenopus eggs, and on video microscopy studies in mammalian cells, it has been proposed that human CDC25B degradation is dependent of the F-box protein bTrCP, but the involvement of this latter protein was not formally demonstrated yet. Here, we show that indeed, in mammalian cells, bTrCP participates to CDC25B turn-over, and is required for the complete degradation of CDC25B at the metaphase-anaphase transition. Using a stabilized mutant of CDC25B, which cannot interact anymore with bTrCP, we further show that, during late phases of mitosis, reduced degradation of CDC25B leads to an extended window of expression of the protein, which in turn induces a delay in mitosis exit and entails mitotic defects such as chromosomes missegregation. These findings show that a dysfunction in the rapid and precisely controlled degradation of CDC25B at the metaphase-anaphase transition is sufficient to cause genomic instability and suggest that, in human tissues, pathologic stabilization or untimed expression of CDC25B could contribute to tumorigenesis.     

CDC25B acts as a potential target of PRKACA in fertilized mouse eggs

Protein kinase A (PRKACA) has been documented as a pivotal regulator in meiosis and mitosis arrest. Although our previous work has established that PRKACA regulates cell cycle progression of mouse fertilized eggs by inhibiting M-phase promoting factor (MPF), little is known about the intermediate factor between PRKACA and MPF in the mitotic cell cycle. In this study, we investigated the role of the PRKACA/CDC25B pathway on the early development of mouse fertilized eggs. Overexpression of unphosphorylatable CDC25B mutant (Cdc25b-S321A or Cdc25b-S229A/S321A) rapidly caused G2-phase eggs to enter mitosis. Microinjection of either Cdc25b-WT or Cdc25b-S229A mRNA also promoted G2/M transition, but much less efficiently than Cdc25b-S321A and Cdc25b-S229A/S321A. Moreover, mouse fertilized eggs overrode the G2 arrest by microinjection of either Cdc25b-S321A or Cdc25b-S229A/S321A mRNA, which efficiently resulted in MPF activation by directly dephosphorylating CDC2A-Tyr15, despite culture under conditions that maintained exogenous dibutyryl cAMP. Using a highly specific antibody against phospho-Ser321 of CDC25B in Western blotting, we showed that CDC25B-Ser321 was phosphorylated at the G1 and S phases, whereas Ser321 was dephosphorylated at the G2 and M phases in vivo. Our findings identify CDC25B as a potential target of PRKACA and show that PRKACA regulates G2/M transition by phosphorylating CDC25B-Ser321 but not CDC25B-Ser229 on the first mitotic division of mouse fertilized eggs.