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Jonathon Pines - One of the best experts on this subject based on the ideXlab platform.
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cdc20 And cks direct the spindle checkpoint independent destruction of Cyclin A
Molecular Cell, 2008Co-Authors: René H. Medema, Rob M F Wolthuis, Lori Clayfarrace, Wouter Van Zon, Mona Yekezare, Lars Koop, Janneke Ogink, Jonathon PinesAbstract:Successful mitosis requires the right protein be degrAded At the right time. CentrAl to this is the spindle checkpoint thAt prevents the destruction of securin And Cyclin B1 when there Are improperly AttAched chromosomes. The principAl tArget of the checkpoint is Cdc20, which ActivAtes the AnAphAse-promoting complex/cyclosome (APC/C). A DrosophilA Cdc20/fizzy mutAnt Arrests in mitosis with high levels of Cyclins A And B, but pArAdoxicAlly the spindle checkpoint does not stAbilize Cyclin A. Here, we investigAted this pArAdox And found thAt Cdc20 is rAte limiting for Cyclin A destruction. Indeed, Cdc20 binds efficiently to Cyclin A before And in mitosis, And this complex hAs little AssociAted MAd2. Furthermore, the Cyclin A complex must bind to A Cks protein to be degrAded independently of the checkpoint. Thus, we identify A cruciAl role for the Cks proteins in mitosis And one mechAnism by which the APC/C cAn tArget substrAtes independently of the spindle checkpoint.
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Cyclin A- And Cyclin E-Cdk complexes shuttle between the nucleus And the cytoplAsm.
Molecular Biology of the Cell, 2002Co-Authors: Mark Jackman, Anja Hagting, Nicole Den Elzen, Yumiko Kubota, Jonathon PinesAbstract:Cyclins A And E And their pArtner Cyclin-dependent kinAses (Cdks) Are key regulAtors of DNA synthesis And of mitosis. Immunofluorescence studies hAve shown thAt both Cyclins Are nucleAr And thAt A proportion of Cyclin A is locAlized to sites of DNA replicAtion. However, recently, both Cyclin A And Cyclin E hAve been implicAted As regulAtors of centrosome replicAtion, And it is uncleAr when And where these Cyclin-Cdks cAn interAct with cytoplAsmic substrAtes. We hAve used live cell imAging to study the behAvior of Cyclin/Cdk complexes. We found thAt Cyclin A And Cyclin E Are Able to regulAte both nucleAr And cytoplAsmic events becAuse they both shuttle between the nucleus And the cytoplAsm. However, we found thAt there Are mArked differences in their shuttling behAvior, which rAises the possibility thAt Cyclin/Cdk function could be regulAted At the level of nucleAr import And export. In the course of these experiments, we hAve Also found thAt, contrAry to published results, mutAtions in the hydrophobic pAtch of Cyclin A do Affect Cdk binding And nucleAr import. This hAs implicAtions for the role of the hydrophobic pAtch As A substrAte selection motif.
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Cyclin A is destroyed in prometAphAse And cAn delAy chromosome Alignment And AnAphAse
Journal of Cell Biology, 2001Co-Authors: Nicole Den Elzen, Jonathon PinesAbstract:Mitosis is controlled by the specific And timely degrAdAtion of key regulAtory proteins, notAbly the mitotic Cyclins thAt bind And ActivAte the Cyclin-dependent kinAses (Cdks). In AnimAl cells, Cyclin A is AlwAys degrAded before Cyclin B, but the exAct timing And the mechAnism underlying this Are not known. Here we use live cell imAging to show thAt Cyclin A begins to be degrAded just After nucleAr envelope breAkdown. This degrAdAtion requires the 26S proteAsome, but is not Affected by the spindle checkpoint. Neither deletion of its destruction box nor disrupting Cdk binding prevents Cyclin A proteolysis, but Cdk binding is necessAry for degrAdAtion At the correct time. We Also show thAt increAsing the levels of Cyclin A delAys chromosome Alignment And sister chromAtid segregAtion. This delAy depends on the proteolysis of Cyclin A And is not cAused by A lAg in the bipolAr AttAchment of chromosomes to the mitotic spindle, nor is it mediAted viA the spindle checkpoint. Thus, proteolysis thAt is not under the control of the spindle checkpoint is required for chromosome Alignment And AnAphAse.
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HumAn Cyclin A is required for mitosis until mid prophAse.
Journal of Cell Biology, 1999Co-Authors: Nobuaki Furuno, Nicole Den Elzen, Jonathon PinesAbstract:We hAve used microinjection And time-lApse video microscopy to study the role of Cyclin A in mitosis. We hAve injected purified, Active Cyclin A/Cyclin-dependent kinAse 2 (CDK2) into synchronized cells At specific points in the cell cycle And AssAyed its effect on cell division. We find thAt Cyclin A/CDK2 will drive G2 phAse cells into mitosis within 30 min of microinjection, up to 4 h before control cells enter mitosis. Often this premAture mitosis is AbnormAl; the chromosomes do not completely condense And dAughter cells fuse. RemArkAbly, microinjecting Cyclin A/CDK2 into S phAse cells hAs no effect on progress through the following G2 phAse or mitosis. In complementAry experiments we hAve microinjected the Amino terminus of p21Cip1/WAf1/Sdi1 (p21N) into cells to inhibit Cyclin A/CDK2 Activity. We find thAt p21N will prevent S phAse or G2 phAse cells from entering mitosis, And will cAuse eArly prophAse cells to return to interphAse. These results suggest thAt Cyclin A/CDK2 is A rAte-limiting component required for entry into mitosis, And for progress through mitosis until lAte prophAse. They Also suggest thAt Cyclin A/CDK2 mAy be the tArget of the recently described prophAse checkpoint.
Randy Yat Choi Poon - One of the best experts on this subject based on the ideXlab platform.
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speciAlized roles of the two mitotic Cyclins in somAtic cells Cyclin A As An ActivAtor of m phAse promoting fActor
Molecular Biology of the Cell, 2007Co-Authors: Tsz Kan Fung, Hoi Tang, Randy Yat Choi PoonAbstract:The role of Cyclin B-CDC2 As M phAse-promoting fActor (MPF) is well estAblished, but the precise functions of Cyclin A remAin A cruciAl outstAnding issue. Here we show thAt down-regulAtion of Cyclin A induces A G2 phAse Arrest through A checkpoint-independent inActivAtion of Cyclin B-CDC2 by inhibitory phosphorylAtion. The phenotype is rescued by expressing Cyclin A resistAnt to the RNA interference. In contrAst, down-regulAtion of Cyclin B disrupts mitosis without inActivAting Cyclin A-CDK, indicAting thAt Cyclin A-CDK Acts upstreAm of Cyclin B-CDC2. Even when ectopicAlly expressed, Cyclin A cAnnot replAce Cyclin B in driving mitosis, indicAting the specific role of Cyclin B As A component of MPF. DeregulAtion of WEE1, but not the PLK1-CDC25 Axis, cAn override the Arrest cAused by Cyclin A knockdown, suggesting thAt Cyclin A-CDK mAy tip the bAlAnce of the Cyclin B-CDC2 bistAble system by initiAting the inActivAtion of WEE1. These observAtions show thAt Cyclin A cAnnot form MPF independent of Cyclin B And underscore A criticAl role of Cyclin A As A trigger for MPF ActivAtion.
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Cyclin A in cell cycle control And cAncer
Cellular and Molecular Life Sciences, 2002Co-Authors: C H Yam, Tsz Kan Fung, Randy Yat Choi PoonAbstract:Cyclin A is pArticulArly interesting Among the Cyclin fAmily becAuse it cAn ActivAte two different Cyclin-dependent kinAses (CDKs) And functions in both S phAse And mitosis. An embryonic form of Cyclin A thAt is only essentiAl for spermAtogenesis is Also present in some orgAnisms. In S phAse, phosphorylAtion of components of the DNA replicAtion mAchinery such As CDC6 by Cyclin A-CDK is believed to be importAnt for initiAtion of DNA replicAtion And to restrict the initiAtion to only once per cell cycle. In mitosis, the precise role of Cyclin A is still obscure, but it mAy contribute to the control of Cyclin B stAbility. Cyclin A stArts to AccumulAte during S phAse And is Abruptly destroyed before metAphAse. The synthesis of Cyclin A is mAinly controlled At the trAnscription level, involving E2F And other trAnscription fActors. RemovAl of Cyclin A is cArried out by ubiquitin-mediAted proteolysis, but whether the sAme AnAphAse-promoting complex/cyclosome tArgeting subunits Are used As for Cyclin B is debAtAble. Consistent with its role As A key cell cycle regulAtor, expression of Cyclin A is found to be elevAted in A vAriety of tumors.
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RegulAtion of Cyclin A-Cdk2 by SCF Component Skp1 And F-Box Protein Skp2
Molecular and cellular biology, 1999Co-Authors: Cain H. Yam, Anita Wan Sze Lau, Wai Yi Siu, Randy Yat Choi PoonAbstract:The cell cycle is driven by A fAmily of protein kinAses cAlled Cyclin-dependent kinAses (CDKs) (21). The Activity of CDKs is tightly regulAted by An intricAte system of protein-protein interAction And phosphorylAtion (24). ActivAtion of CDKs requires binding to A Cyclin subunit And phosphorylAtion on the Thr161 residue. PhosphorylAtion of the Thr14 And Tyr15 residues And binding to protein inhibitors of the p21Cip1/WAF1 And p16INK4A fAmily inhibit the Activity of CDKs. CompArison of the compositions of the proteins thAt AssociAte with Cyclin-CDK complexes in normAl And cAncer cells reveAls proteins thAt mAy be importAnt for the deregulAtion of Cyclin-CDK during tumorigenesis (38). ExAmples of these CDK regulAtors Are the CDK inhibitors p21Cip1/WAF1 And p16INK4A, which Are found in Cyclin-CDK complexes in normAl cells but not in mAny trAnsformed cells. p21Cip1/WAF1 is induced by the tumor suppressor p53 And is responsible for the inhibition of Cdk2 After DNA dAmAge (27). LAck of these CDK inhibitors in trAnsformed cells mAy contribute to the loss of normAl inhibition of cell cycle progression following DNA dAmAge. In normAl humAn fibroblAsts, Cyclin A-Cdk2 exists in A quAternAry complex thAt contAins p21Cip1/WAF1 And PCNA (proliferAting cell nucleAr Antigen) (41). But in mAny trAnsformed cells, p21Cip1/WAf1 And PCNA disAppeAr from the Cyclin A-Cdk2 complexes, And insteAd A substAntiAl frAction of Cyclin A-Cdk2 complexes Are AssociAted with A 19-kDA protein And A 45-kDA protein (38). The 19- And 45-kDA proteins were subsequently identified And renAmed Skp1 And Skp2 (for S-phAse kinAse-AssociAted protein), respectively (40). IdentificAtion of Skp1 homologs from CAenorhAbditis elegAns, ArAbidopsis thAliAnA, And SAcchAromyces cerevisiAe indicAtes thAt Skp1 is evolutionArily highly conserved (6). Cyclin A-Cdk2 complexes AppeAr to bind to Skp1 indirectly through Skp2 (4, 40), And Skp1 binds to Skp2 viA A novel structurAl motif in Skp2 cAlled the F box (4), which is found in A lArge number of diverse proteins including Cyclin F, Grr1, And Cdc4. The mRNA level of Skp2 is most AbundAnt during the S phAse (40). Microinjection of Anti-Skp2 Antibodies or Skp2 Antisense oligonucleotides into normAl humAn fibroblAsts or HeLA cells inhibits entry into S phAse (but not S-phAse progression) (40), suggesting thAt Skp2 is An importAnt regulAtor of S phAse. The SKP2 gene hAs been mApped to chromosome position 5p13, And the SKP1 gene hAs been mApped to 7q11.2 (7). A close homolog or A pseudogene of SKP1 (Skp1B) wAs mApped to 12p12 (7). All three of these loci Are AssociAted with kAryotypic AlterAtions, known AmplificAtions, or suspected tumor suppressor genes. From A different perspective, Skp1 wAs identified As A suppressor of cdc4 mutAnts And As A Cyclin F-binding protein (4). In S. cerevisiAe, Cdc4, Acting in concert with Cdc34 And Cdc53, is involved in the ubiquitin-dependent degrAdAtion of multiple cell cycle regulAtors, including Cln2, Cln3, FAr1, And Sic1 (20, 31, 36). Cyclin F wAs isolAted As A suppressor of the G1/S deficiency of A cdc4 mutAnt (3), suggesting Cyclin F mAy Also be involved in the destruction of other cell cycle regulAtors. The fAct thAt Skp1 is AssociAted with Cyclin F And Cdc4 suggests thAt Skp1 mAy Also be involved in the ubiquitin-dependent destruction of cell cycle regulAtors. Indeed, Skp1 is required for ubiquitin-mediAted proteolysis of Cln2, Clb5, And the CDK inhibitor Sic1 (4). Skp1, Cdc4, And Cdc53 Assemble into A ubiquitin-ligAse complex, nAmed SCFCdc4, thAt cAn reconstitute ubiquitinAtion of phosphorylAted Sic1 in the presence of E1 enzyme, the E2 enzyme Cdc34, And ubiquitin (8, 17, 19, 33). Different skp1 temperAture-sensitive mutAnts Arrest cells in either G1 or G2, suggesting A connection between regulAtion of proteolysis in different stAges of the cycle (4, 6). Skp1 wAs Also identified As A subunit of CBF3, A multiprotein complex thAt binds centromere DNA in vitro (6). Skp1 therefore represents An intrinsic kinetochore protein conserved throughout eukAryotic evolution And mAy be directly involved in linking kinetochore function with the cell cycle-regulAtory mAchinery. In this study, we investigAted the involvement of Skp1 And Skp2 in regulAting Cyclin A-Cdk2 Activity. We showed thAt Skp2 cAn inhibit the kinAse Activity of Cyclin A-Cdk2 And found thAt Skp2 cAn block the phosphorylAtion of Cdk2 by CDK-ActivAting kinAse (CAK) And Wee1. Skp2 cAn Also inhibit the kinAse Activity AssociAted with Cyclin A-Cdk2, Cyclin E-Cdk2, And Skp2 isolAted from mAmmAliAn cell extrActs. Skp2 wAs phosphorylAted by Cyclin A-Cdk2 on residue Ser76, but mutAtion of nonphosphorylAtAble mutAnts of Skp2 cAn still inhibit the kinAse Activity of Cyclin A-Cdk2 towArd histone H1. Consistent with the biochemicAl dAtA, overexpression of Skp2, but not Skp1, in mAmmAliAn cells resulted in cell cycle Arrest. The F-box region of Skp2 is importAnt in binding to Skp1, And both the N- And C-terminAl regions of Skp2 Are involved in binding to Cyclin A-Cdk2. These dAtA suggest thAt ApArt from the potentiAl Action As mediAtors of ubiquitin-dependent proteolysis of Cyclin A, Skp1 And Skp2 mAy Also directly regulAte the kinAse Activity of Cyclin A-Cdk2.
Sherilyn Goldstone - One of the best experts on this subject based on the ideXlab platform.
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cdc25 dependent ActivAtion of Cyclin A cdk2 is blocked in g2 phAse Arrested cells independently of Atm Atr
Oncogene, 2001Co-Authors: Sherilyn Goldstone, J Sinnamon, Sandra Pavey, Alistair R. R. Forrest, Brian GabrielliAbstract:Cyclin A/cdk2 is Active during S And G2 phAses of the cell cycle, but its regulAtion And function during G2 phAse is poorly understood. In this study we hAve exAmined the regulAtion of Cyclin A/cdk2 Activity during normAl G2 phAse progression And in genotoxin-induced G2 Arrest. We show thAt Cyclin A/cdk2 is ActivAted in eArly G2 phAse by A cdc25 Activity. In the G2 phAse checkpoint Arrest initiAted in response to vArious forms of DNA dAmAge, the cdc25-dependent ActivAtion of both Cyclin A/cdk2 And Cyclin B1/cdc2 is blocked. Ectopic expression of cdc25B, but not cdc25C, in G2 phAse Arrested cells efficiently ActivAted both Cyclin A/cdk2 And Cyclin B1/cdc2. FinAlly, we demonstrAte thAt the block in Cyclin A/cdk2 ActivAtion in the G2 checkpoint Arrest is independent of ATM/ATR. We speculAte thAt the ATM/ ATR-independent block in G2 phAse Cyclin A/cdk2 ActivAtion mAy Act As A further lAyer of checkpoint control, And thAt blocking G2 phAse Cyclin A/cdk2 ActivAtion contributes to the G2 phAse checkpoint Arrest.
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cdc25 dependent ActivAtion of Cyclin A cdk2 is blocked in g2 phAse Arrested cells independently of Atm Atr
Oncogene, 2001Co-Authors: Sherilyn Goldstone, J Sinnamon, Sandra Pavey, Alistair R. R. Forrest, Brian GabrielliAbstract:Cyclin A/cdk2 is Active during S And G2 phAses of the cell cycle, but its regulAtion And function during G2 phAse is poorly understood. In this study we hAve exAmined the regulAtion of Cyclin A/cdk2 Activity during normAl G2 phAse progression And in genotoxin-induced G2 Arrest. We show thAt Cyclin A/cdk2 is ActivAted in eArly G2 phAse by A cdc25 Activity. In the G2 phAse checkpoint Arrest initiAted in response to vArious forms of DNA dAmAge, the cdc25-dependent ActivAtion of both Cyclin A/cdk2 And Cyclin B1/cdc2 is blocked. Ectopic expression of cdc25B, but not cdc25C, in G2 phAse Arrested cells efficiently ActivAted both Cyclin A/cdk2 And Cyclin B1/cdc2. FinAlly, we demonstrAte thAt the block in Cyclin A/cdk2 ActivAtion in the G2 checkpoint Arrest is independent of ATM/ATR. We speculAte thAt the ATM/ ATR-independent block in G2 phAse Cyclin A/cdk2 ActivAtion mAy Act As A further lAyer of checkpoint control, And thAt blocking G2 phAse Cyclin A/cdk2 ActivAtion contributes to the G2 phAse checkpoint Arrest.
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Cdc25-dependent ActivAtion of Cyclin A/cdk2 is blocked in G2 phAse Arrested cells independently of ATM/ATR
Oncogene, 2001Co-Authors: Sherilyn Goldstone, J Sinnamon, Sandra Pavey, A Forrest, Brian GabrielliAbstract:Cyclin A/cdk2 is Active during S And G2 phAses of the cell cycle, but its regulAtion And function during G2 phAse is poorly understood. In this study we hAve exAmined the regulAtion of Cyclin A/cdk2 Activity during normAl G2 phAse progression And in genotoxin-induced G2 Arrest. We show thAt Cyclin A/cdk2 is ActivAted in eArly G2 phAse by A cdc25 Activity. In the G2 phAse checkpoint Arrest initiAted in response to vArious forms of DNA dAmAge, the cdc25-dependent ActivAtion of both Cyclin A/cdk2 And Cyclin B1/cdc2 is blocked. Ectopic expression of cdc25B, but not cdc25C, in G2 phAse Arrested cells efficiently ActivAted both Cyclin A/cdk2 And Cyclin B1/cdc2. FinAlly, we demonstrAte thAt the block in Cyclin A/cdk2 ActivAtion in the G2 checkpoint Arrest is independent of ATM/ATR. We speculAte thAt the ATM/ATR-independent block in G2 phAse Cyclin A/cdk2 ActivAtion mAy Act As A further lAyer of checkpoint control, And thAt blocking G2 phAse Cyclin A/cdk2 ActivAtion contributes to the G2 phAse checkpoint Arrest.
Tze-sing Huang - One of the best experts on this subject based on the ideXlab platform.
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Inhibition of DNA synthesis by downregulAtion of Cyclin A but not Skp 2 overexpression in humAn hepAtocellulAr cArcinomA cells
Cancer Letters, 1999Co-Authors: Yee Chao, Yung-luen Shih, Hsin-ju Chen, Shou-dong Lee, Tze-sing HuangAbstract:Cyclin A is An S And G2/M phAse regulAtory protein And AssociAtes with Skp 2 in mAny trAnsformed cells. Our previous results showed thAt 12 (39%) And 17 (55%) out of 31 hepAtocellulAr cArcinomA (HCC) pAtients exhibited higher protein expression levels of Cyclin A And Skp 2, respectively, in their tumorous compAred to non-tumorous tissues. In the present study, we used Western blot AnAlysis to show thAt 3 out of 6 HCC cell lines, HA59T, HA22T And HCC36, exhibited overexpression of Cyclin A, Among which the HCC36 cell line Also expressed A higher Skp 2 protein level. Moreover, we used the Antisense oligonucleotide phosphorothioAtes to down regulAte the overexpression of Cyclin A And Skp 2 proteins to determine whether or not these two proteins Are involved in the mitogenesis of HCC36 cells. After treAtment with Antisense oligonucleotide phosphorothioAtes, the gene product of Cyclin A or Skp 2 wAs suppressed dose-dependently As reveAled by Western blot AnAlyses. By [3H]thymidine incorporAtion AssAy, we found thAt downregulAtion of Cyclin A but not Skp 2 overexpression could inhibit the DNA synthesis Ability of HCC36 cells, suggesting thAt AbnormAl Skp 2 expression is not directly correlAted with the HCC proliferAtion.
Brian Gabrielli - One of the best experts on this subject based on the ideXlab platform.
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cdc25 dependent ActivAtion of Cyclin A cdk2 is blocked in g2 phAse Arrested cells independently of Atm Atr
Oncogene, 2001Co-Authors: Sherilyn Goldstone, J Sinnamon, Sandra Pavey, Alistair R. R. Forrest, Brian GabrielliAbstract:Cyclin A/cdk2 is Active during S And G2 phAses of the cell cycle, but its regulAtion And function during G2 phAse is poorly understood. In this study we hAve exAmined the regulAtion of Cyclin A/cdk2 Activity during normAl G2 phAse progression And in genotoxin-induced G2 Arrest. We show thAt Cyclin A/cdk2 is ActivAted in eArly G2 phAse by A cdc25 Activity. In the G2 phAse checkpoint Arrest initiAted in response to vArious forms of DNA dAmAge, the cdc25-dependent ActivAtion of both Cyclin A/cdk2 And Cyclin B1/cdc2 is blocked. Ectopic expression of cdc25B, but not cdc25C, in G2 phAse Arrested cells efficiently ActivAted both Cyclin A/cdk2 And Cyclin B1/cdc2. FinAlly, we demonstrAte thAt the block in Cyclin A/cdk2 ActivAtion in the G2 checkpoint Arrest is independent of ATM/ATR. We speculAte thAt the ATM/ ATR-independent block in G2 phAse Cyclin A/cdk2 ActivAtion mAy Act As A further lAyer of checkpoint control, And thAt blocking G2 phAse Cyclin A/cdk2 ActivAtion contributes to the G2 phAse checkpoint Arrest.
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cdc25 dependent ActivAtion of Cyclin A cdk2 is blocked in g2 phAse Arrested cells independently of Atm Atr
Oncogene, 2001Co-Authors: Sherilyn Goldstone, J Sinnamon, Sandra Pavey, Alistair R. R. Forrest, Brian GabrielliAbstract:Cyclin A/cdk2 is Active during S And G2 phAses of the cell cycle, but its regulAtion And function during G2 phAse is poorly understood. In this study we hAve exAmined the regulAtion of Cyclin A/cdk2 Activity during normAl G2 phAse progression And in genotoxin-induced G2 Arrest. We show thAt Cyclin A/cdk2 is ActivAted in eArly G2 phAse by A cdc25 Activity. In the G2 phAse checkpoint Arrest initiAted in response to vArious forms of DNA dAmAge, the cdc25-dependent ActivAtion of both Cyclin A/cdk2 And Cyclin B1/cdc2 is blocked. Ectopic expression of cdc25B, but not cdc25C, in G2 phAse Arrested cells efficiently ActivAted both Cyclin A/cdk2 And Cyclin B1/cdc2. FinAlly, we demonstrAte thAt the block in Cyclin A/cdk2 ActivAtion in the G2 checkpoint Arrest is independent of ATM/ATR. We speculAte thAt the ATM/ ATR-independent block in G2 phAse Cyclin A/cdk2 ActivAtion mAy Act As A further lAyer of checkpoint control, And thAt blocking G2 phAse Cyclin A/cdk2 ActivAtion contributes to the G2 phAse checkpoint Arrest.
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Cdc25-dependent ActivAtion of Cyclin A/cdk2 is blocked in G2 phAse Arrested cells independently of ATM/ATR
Oncogene, 2001Co-Authors: Sherilyn Goldstone, J Sinnamon, Sandra Pavey, A Forrest, Brian GabrielliAbstract:Cyclin A/cdk2 is Active during S And G2 phAses of the cell cycle, but its regulAtion And function during G2 phAse is poorly understood. In this study we hAve exAmined the regulAtion of Cyclin A/cdk2 Activity during normAl G2 phAse progression And in genotoxin-induced G2 Arrest. We show thAt Cyclin A/cdk2 is ActivAted in eArly G2 phAse by A cdc25 Activity. In the G2 phAse checkpoint Arrest initiAted in response to vArious forms of DNA dAmAge, the cdc25-dependent ActivAtion of both Cyclin A/cdk2 And Cyclin B1/cdc2 is blocked. Ectopic expression of cdc25B, but not cdc25C, in G2 phAse Arrested cells efficiently ActivAted both Cyclin A/cdk2 And Cyclin B1/cdc2. FinAlly, we demonstrAte thAt the block in Cyclin A/cdk2 ActivAtion in the G2 checkpoint Arrest is independent of ATM/ATR. We speculAte thAt the ATM/ATR-independent block in G2 phAse Cyclin A/cdk2 ActivAtion mAy Act As A further lAyer of checkpoint control, And thAt blocking G2 phAse Cyclin A/cdk2 ActivAtion contributes to the G2 phAse checkpoint Arrest.
