scholarly journals Phosphorylation and activation of the Xenopus Cdc25 phosphatase in the absence of Cdc2 and Cdk2 kinase activity.

1995 ◽  
Vol 6 (2) ◽  
pp. 215-226 ◽  
Author(s):  
T Izumi ◽  
J L Maller
Keyword(s):  
Kinase Activity ◽  
Cdc2 Kinase ◽  
Nuclear Envelope Breakdown ◽  
Dual Specificity ◽  
M Phase ◽  
Egg Extracts ◽  
Xenopus Egg ◽  
Xenopus Egg Extracts

The M-phase inducer, Cdc25C, is a dual-specificity phosphatase that directly phosphorylates and activates the cyclin B/Cdc2 kinase complex, leading to initiation of mitosis. Cdc25 itself is activated at the G2/M transition by phosphorylation on serine and threonine residues. Previously, it was demonstrated that Cdc2 kinase is capable of phosphorylating and activating Cdc25, suggesting the existence of a positive feedback loop. In the present study, kinases other than Cdc2 that can phosphorylate and activate Cdc25 were investigated. Cdc25 was found to be phosphorylated and activated by cyclin A/Cdk2 and cyclin E/Cdk2 in vitro. However, in interphase Xenopus egg extracts with no detectable Cdc2 and Cdk2, treatment with the phosphatase inhibitor microcystin activated a distinct kinase that could phosphorylate and activate Cdc25. Microcystin also induced other mitotic phenomena such as chromosome condensation and nuclear envelope breakdown in extracts containing less than 5% of the mitotic level of Cdc2 kinase activity. These findings implicate a kinase other than Cdc2 and Cdk2 that may initially activate Cdc25 in vivo and suggest that this kinase may also phosphorylate M-phase substrates even in the absence of Cdc2 kinase.

scholarly journals MAP kinase does not inactivate, but rather prevents the cyclin degradation pathway from being turned on in Xenopus egg extracts

1996 ◽  
Vol 109 (1) ◽  
pp. 239-246 ◽  
Author(s):  
A. Abrieu ◽  
T. Lorca ◽  
J.C. Labbe ◽  
N. Morin ◽  
S. Keyse ◽  
...  
Keyword(s):  
Map Kinase ◽  
Degradation Pathway ◽  
Cdc2 Kinase ◽  
Vitro Assay ◽  
Kinase Activation ◽  
Egg Extracts ◽  
Dependent Protein ◽  
Xenopus Egg ◽  
Xenopus Egg Extracts

Unfertilized frog eggs arrest at the second meiotic metaphase, due to cytostatic activity of the c-mos proto-oncogene (CSF). MAP kinase has been proposed to mediate CSF activity in suppressing cyclin degradation. Using an in vitro assay to generate CSF activity, and recombinant CL 100 phosphatase to inactivate MAP kinase, we confirm that the c-mos proto-oncogene blocks cyclin degradation through MAP kinase activation. We further show that for MAP kinase to suppress cyclin degradation, it must be activated before cyclin B-cdc2 kinase has effectively promoted cyclin degradation. Thus MAP kinase does not inactivate, but rather prevents the cyclin degradation pathway from being turned on. Using a constitutively active mutant of Ca2+/calmodulin dependent protein kinase II, which mediates the effects of Ca2+ at fertilization, we further show that the kinase can activate cyclin degradation in the presence of both MPF and the c-mos proto-oncogene without inactivating MAP kinase.

scholarly journals Plk is an M-phase-specific protein kinase and interacts with a kinesin-like protein, CHO1/MKLP-1.

1995 ◽  
Vol 15 (12) ◽  
pp. 7143-7151 ◽  
Author(s):  
K S Lee ◽  
Y L Yuan ◽  
R Kuriyama ◽  
R L Erikson
Keyword(s):  
Protein Kinase ◽  
Kinase Activity ◽  
Maximum Level ◽  
Specific Protein ◽  
Cyclin B ◽  
Cdc2 Kinase ◽  
M Phase ◽  
Nih 3T3

PLK (STPK13) encodes a murine protein kinase closely related to those encoded by the Drosophila melanogaster polo gene and the Saccharomyces cerevisiae CDC5 gene, which are required for normal mitotic and meiotic divisions. Affinity-purified antibody generated against the C-terminal 13 amino acids of Plk specifically recognizes a single polypeptide of 66 kDa in MELC, NIH 3T3, and HeLa cellular extracts. The expression levels of both poly(A)+ PLK mRNA and its encoded protein are most abundant about 17 h after serum stimulation of NIH 3T3 cells. Plk protein begins to accumulate at the S/G2 boundary and reaches the maximum level at the G2/M boundary in continuously cycling cells. Concurrent with cyclin B-associated cdc2 kinase activity, Plk kinase activity sharply peaks at the onset of mitosis. Plk enzymatic activity gradually decreases as M phase proceeds but persists longer than cyclin B-associated cdc2 kinase activity. Plk is localized to the area surrounding the chromosomes in prometaphase, appears condensed as several discrete bands along the spindle axis at the interzone in anaphase, and finally concentrates at the midbody during telophase and cytokinesis. Plk and CHO1/mitotic kinesin-like protein 1 (MKLP-1), which induces microtubule bundling and antiparallel movement in vitro, are colocalized during late M phase. In addition, CHO1/MKLP-1 appears to interact with Plk in vivo and to be phosphorylated by Plk-associated kinase activity in vitro.

Phosphorylation of Xenopus cyclins B1 and B2 is not required for cell cycle transitions

1991 ◽  
Vol 11 (8) ◽  
pp. 3860-3867
Author(s):  
T Izumi ◽  
J L Maller
Keyword(s):  
Map Kinase ◽  
Cyclin B1 ◽  
Site Directed Mutagenesis ◽  
Cyclin B ◽  
Phosphorylation Sites ◽  
Cdc2 Kinase ◽  
M Phase ◽  
Egg Extracts ◽  
Xenopus Egg Extracts

The cdc2 kinase and B-type cyclins are known to be components of maturation- or M-phase-promoting factor (MPF). Phosphorylation of cyclin B has been reported previously and may regulate entry into and exit from mitosis and meiosis. To investigate the role of cyclin B phosphorylation, we replaced putative cdc2 kinase phosphorylation sites in Xenopus cyclins B1 and B2 by using oligonucleotide site-directed mutagenesis. We found that Ser-90 of cyclin B2 and Ser-94 or Ser-96 of cyclin B1 are the main phosphorylation sites both in functional Xenopus egg extracts and after phosphorylation with purified MPF in vitro. Microtubule-associated protein (MAP) kinase from Xenopus eggs phosphorylated cyclin B1 significantly at Ser-94 or Ser-96, whereas it was largely inactive against cyclin B2. The substitutions that ablated phosphorylation at these sites, however, resulted in no functional differences between mutant and wild-type cyclin, as judged by the kinetics of M-phase degradation, induction of mitosis in egg extracts, or induction of oocyte maturation. These results indicate that the phosphorylation of Xenopus B-type cyclins by cdc2 kinase or MAP kinase is not required for the hallmark functions of cyclin.

scholarly journals Cyclin-binding motifs are essential for the function of p21CIP1.

1996 ◽  
Vol 16 (9) ◽  
pp. 4673-4682 ◽  
Author(s):  
J Chen ◽  
P Saha ◽  
S Kornbluth ◽  
B D Dynlacht ◽  
A Dutta
Keyword(s):  
Binding Motif ◽  
Cyclin Dependent Kinase ◽  
Tumor Suppressor P53 ◽  
Binding Motifs ◽  
Cell Extracts ◽  
Egg Extracts ◽  
Xenopus Egg ◽  
Xenopus Egg Extracts

The cyclin-dependent kinase (Cdk) inhibitor p21 is induced by the tumor suppressor p53 and is required for the G1-S block in cells with DNA damage. We report that there are two copies of a cyclin-binding motif in p21, Cy1 and Cy2, which interact with the cyclins independently of Cdk2. The cyclin-binding motifs of p21 are required for optimum inhibition of cyclin-Cdk kinases in vitro and for growth suppression in vivo. Peptides containing only the Cy1 or Cy2 motif partially inhibit cyclin-Cdk kinase activity in vitro and DNA replication in Xenopus egg extracts. A monoclonal antibody which recognizes the Cy1 site of p21 specifically disrupts the association of p21 with cyclin E-Cdk2 and with cyclin D1-Cdk4 in cell extracts. Taken together, these observations suggest that the cyclin-binding motif of p21 is important for kinase inhibition and for formation of p21-cyclin-Cdk complexes in the cell. Finally, we show that the cyclin-Cdk complex is partially active if associated with only the cyclin-binding motif of p21, providing an explanation for how p21 is found associated with active cyclin-Cdk complexes in vivo. The Cy sequences may be general motifs used by Cdk inhibitors or substrates to interact with the cyclin in a cyclin-Cdk complex.

scholarly journals Phosphorylation of Xenopus cyclins B1 and B2 is not required for cell cycle transitions.

1991 ◽  
Vol 11 (8) ◽  
pp. 3860-3867 ◽  
Author(s):  
T Izumi ◽  
J L Maller
Keyword(s):  
Map Kinase ◽  
Cyclin B1 ◽  
Site Directed Mutagenesis ◽  
Cyclin B ◽  
Phosphorylation Sites ◽  
Cdc2 Kinase ◽  
M Phase ◽  
Egg Extracts ◽  
Xenopus Egg Extracts

The cdc2 kinase and B-type cyclins are known to be components of maturation- or M-phase-promoting factor (MPF). Phosphorylation of cyclin B has been reported previously and may regulate entry into and exit from mitosis and meiosis. To investigate the role of cyclin B phosphorylation, we replaced putative cdc2 kinase phosphorylation sites in Xenopus cyclins B1 and B2 by using oligonucleotide site-directed mutagenesis. We found that Ser-90 of cyclin B2 and Ser-94 or Ser-96 of cyclin B1 are the main phosphorylation sites both in functional Xenopus egg extracts and after phosphorylation with purified MPF in vitro. Microtubule-associated protein (MAP) kinase from Xenopus eggs phosphorylated cyclin B1 significantly at Ser-94 or Ser-96, whereas it was largely inactive against cyclin B2. The substitutions that ablated phosphorylation at these sites, however, resulted in no functional differences between mutant and wild-type cyclin, as judged by the kinetics of M-phase degradation, induction of mitosis in egg extracts, or induction of oocyte maturation. These results indicate that the phosphorylation of Xenopus B-type cyclins by cdc2 kinase or MAP kinase is not required for the hallmark functions of cyclin.

scholarly journals A Novel Role for Cdk1/Cyclin B in Regulating B-Raf Activation at Mitosis

2008 ◽  
Vol 19 (7) ◽  
pp. 2907-2915 ◽  
Author(s):  
Sergiy I. Borysov ◽  
Thomas M. Guadagno
Keyword(s):  
Mapk Cascade ◽  
Cyclin B ◽  
Dependent Manner ◽  
Mapk Activity ◽  
M Phase ◽  
Egg Extracts ◽  
Xenopus Egg ◽  
Subsequent Activation ◽  
Xenopus Egg Extracts

MAPK activity is important during mitosis for spindle assembly and maintenance of the spindle checkpoint arrest. We previously identified B-Raf as a critical activator of the MAPK cascade during mitosis in Xenopus egg extracts and showed that B-Raf activation is regulated in an M-phase–dependent manner. The mechanism that mediates B-Raf activation at mitosis has not been elucidated. Interestingly, activation of 95-kDa B-Raf at mitosis does not require phosphorylation of Thr-599 and Ser-602 residues (Thr-633 and Ser-636 in Xenopus B-Raf), previously shown to be essential for B-Raf activation by Ras. Instead, we provide evidence for Cdk1/cyclin B in mediating mitotic activation of B-Raf. In particular, Cdk1/cyclin B complexes associate with B-Raf at mitosis in Xenopus egg extracts and contribute to its phosphorylation. Mutagenesis and in vitro kinase assays demonstrated that Cdk1/cyclin B directly phosphorylates B-Raf at Serine-144, which is part of a conserved Cdk1 preferential consensus site (S144PQK). Importantly, phosphorylation of Ser-144 is absolutely required for mitotic activation of B-Raf and subsequent activation of the MAPK cascade. However, substitution of a phospho-mimicking amino acid at Ser-144 failed to produce a constitutive active B-Raf indicating that, in addition of Ser-144 phosphorylation, other regulatory events may be needed to activate B-Raf at mitosis. Taken together, our data reveal a novel cell cycle mechanism for activating the B-Raf/MEK/MAPK cascade.

scholarly journals Post–Cytochrome c Protection from Apoptosis Conferred by a MAPK Pathway in Xenopus Egg Extracts

2002 ◽  
Vol 13 (2) ◽  
pp. 393-401 ◽  
Author(s):  
Jessica S. Tashker ◽  
Michael Olson ◽  
Sally Kornbluth
Keyword(s):  
Cytochrome C ◽  
Mapk Pathway ◽  
Intermembrane Space ◽  
Caspase Activation ◽  
Cytochrome C Release ◽  
M Phase ◽  
Egg Extracts ◽  
Xenopus Egg ◽  
Xenopus Egg Extracts

In response to many different apoptotic stimuli, cytochrome c is released from the intermembrane space of the mitochondria into the cytoplasm, where it serves as a cofactor in the activation of procaspase 9. Inhibition of this process can occur either by preventing cytochrome c release or by blocking caspase activation or activity. Experiments involving in vitro reconstitution of apoptosis in cell-free extracts of Xenopus laevis eggs have suggested that extracts arrested in interphase are susceptible to an endogenous apoptotic program leading to caspase activation, whereas extracts arrested in meiotic metaphase are not. We report here that Mos/MEK/MAPK pathways active in M phase–arrested eggs are responsible for rendering them refractory to apoptosis. Interestingly, M phase–arrested extracts are competent to release cytochrome c, yet still do not activate caspases. Concomitantly, we have also demonstrated that recombinant Mos, MEK, and ERK are sufficient to block cytochrome c–dependent caspase activation in purified Xenopus cytosol, which lacks both transcription and translation. These data indicate that the MAP kinase pathway can target and inhibit post–cytochrome c release apoptotic events in the absence of new mRNA/protein synthesis and that this biochemical pathway is responsible for the apoptotic inhibition observed in meiotic X. laevis egg extracts.

scholarly journals Stathmin/Op18 Phosphorylation Is Regulated by Microtubule Assembly

2001 ◽  
Vol 12 (2) ◽  
pp. 437-448 ◽  
Author(s):  
Thomas Küntziger ◽  
Olivier Gavet ◽  
Valérie Manceau ◽  
André Sobel ◽  
Michel Bornens
Keyword(s):  
Feedback Loop ◽  
Somatic Cells ◽  
Network Control ◽  
Microtubule Assembly ◽  
Egg Extracts ◽  
Xenopus Egg ◽  
Xenopus Egg Extracts ◽  
Mitotic Chromatin

Stathmin/Op 18 is a microtubule (MT) dynamics-regulating protein that has been shown to have both catastrophe-promoting and tubulin-sequestering activities. The level of stathmin/Op18 phosphorylation was proved both in vitro and in vivo to be important in modulating its MT-destabilizing activity. To understand the in vivo regulation of stathmin/Op18 activity, we investigated whether MT assembly itself could control phosphorylation of stathmin/Op18 and thus its MT-destabilizing activity. We found that MT nucleation by centrosomes from Xenopus sperm or somatic cells and MT assembly promoted by dimethyl sulfoxide or paclitaxel induced stathmin/Op18 hyperphosphorylation in Xenopus egg extracts, leading to new stathmin/Op18 isoforms phosphorylated on Ser 16. The MT-dependent phosphorylation of stathmin/Op18 took place in interphase extracts as well, and was also observed in somatic cells. We show that the MT-dependent phosphorylation of stathmin/Op18 on Ser 16 is mediated by an activity associated to the MTs, and that it is responsible for the stathmin/Op18 hyperphosphorylation reported to be induced by the addition of “mitotic chromatin.” Our results suggest the existence of a positive feedback loop, which could represent a novel mechanism contributing to MT network control.

scholarly journals Regulation of the microtubule nucleating activity of centrosomes in Xenopus egg extracts: role of cyclin A-associated protein kinase.

1992 ◽  
Vol 116 (6) ◽  
pp. 1431-1442 ◽  
Author(s):  
B Buendia ◽  
G Draetta ◽  
E Karsenti
Keyword(s):  
Critical Concentration ◽  
Cyclin A ◽  
Soluble Form ◽  
Egg Extracts ◽  
Xenopus Egg ◽  
Xenopus Egg Extracts ◽  
Polymer Assembly

Isolated centrosomes nucleate microtubules when incubated in pure tubulin solutions well below the critical concentration for spontaneous polymer assembly (approximately 15 microM instead of 60 microM). Treatment with urea (2-3 M) does not severely damage the centriole cylinders but inactivates their ability to nucleate microtubules even at high tubulin concentrations. Here we show that centrosomes inactivated by urea are functionally complemented in frog egg extracts. Centrosomes can then be reisolated on sucrose gradients and assayed in different concentrations of pure tubulin to quantify their nucleating activity. We show that the material that complements centrosomes is stored in a soluble form in the egg. Each frog egg contains enough material to complement greater than 6,000 urea-inactivated centrosomes. The material is heat inactivated above 56 degrees C. One can use this in vitro system to study how the microtubule nucleating activity of centrosomes is regulated. Native centrosomes require approximately 15 microM tubulin to begin nucleating microtubules, whereas centrosomes complemented in interphase extracts begin nucleating microtubules around 7-8 microM tubulin. Therefore, the critical tubulin concentrations for polymer assembly off native centrosomes is higher than that observed for the centrosomes first denatured and then complemented in egg extracts. In vivo, the microtubule nucleating activity of centrosomes seems to be regulated by phosphorylation at the onset of mitosis (Centonze, V. E., and G. G. Borisy. 1990. J. Cell Sci. 95:405-411). Since cyclins are major regulators of mitosis, we tested the effect of adding bacterially produced cyclins to interphase egg extracts. Both cyclin A and B activate an H1 kinase in the extracts. Cyclin A-associated kinase causes an increase in the microtubule nucleating activity of centrosomes complemented in the extract but cyclin B does not. The critical tubulin concentration for polymer assembly off centrosomes complemented in cyclin A-treated extracts is similar to that observed for centrosomes complemented in interphase extracts. However, centrosomes complemented in cyclin A treated extracts nucleate much more microtubules at high tubulin concentration. We define this as the "capacity" of centrosomes to nucleate microtubules. It seems that the microtubule nucleating activity of centrosomes can be defined by two distinct parameters: (a) the critical tubulin concentration at which they begin to nucleate microtubules and (b) their capacity to nucleate microtubules at high tubulin concentrations, the latter being modulated by phosphorylation.

scholarly journals Spindle Checkpoint Protein Bub1 Is Required for Kinetochore Localization of Mad1, Mad2, Bub3, and Cenp-E, Independently of Its Kinase Activity

2001 ◽  
Vol 153 (6) ◽  
pp. 1239-1250 ◽  
Author(s):  
Hilary Sharp-Baker ◽  
Rey-Huei Chen
Keyword(s):  
Mitotic Spindle ◽  
Kinase Activity ◽  
Spindle Checkpoint ◽  
Wild Type ◽  
Checkpoint Proteins ◽  
Egg Extracts ◽  
Xenopus Egg ◽  
Xenopus Egg Extracts ◽  
Checkpoint Signal

The spindle checkpoint inhibits the metaphase to anaphase transition until all the chromosomes are properly attached to the mitotic spindle. We have isolated a Xenopus homologue of the spindle checkpoint component Bub1, and investigated its role in the spindle checkpoint in Xenopus egg extracts. Antibodies raised against Bub1 recognize a 150-kD phosphoprotein at both interphase and mitosis, but the molecular mass is reduced to 140 upon dephosphorylation in vitro. Bub1 is essential for the establishment and maintenance of the checkpoint and is localized to kinetochores, similar to the spindle checkpoint complex Mad1–Mad2. However, Bub1 differs from Mad1–Mad2 in that Bub1 remains on kinetochores that have attached to microtubules; the protein eventually dissociates from the kinetochore during anaphase. Immunodepletion of Bub1 abolishes the spindle checkpoint and the kinetochore binding of the checkpoint proteins Mad1, Mad2, Bub3, and CENP-E. Interestingly, reintroducing either wild-type or kinase-deficient Bub1 protein restores the checkpoint and the kinetochore localization of these proteins. Our studies demonstrate that Bub1 plays a central role in triggering the spindle checkpoint signal from the kinetochore, and that its kinase activity is not necessary for the spindle checkpoint in Xenopus egg extracts.

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