151
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Royzman I, Whittaker AJ, Orr-Weaver TL. Mutations in Drosophila DP and E2F distinguish G1-S progression from an associated transcriptional program. Genes Dev 1997; 11:1999-2011. [PMID: 9271122 PMCID: PMC316409 DOI: 10.1101/gad.11.15.1999] [Citation(s) in RCA: 138] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
The E2F transcription factor, a heterodimer of E2F and DP subunits, is capable of driving the G1-S transition of the cell cycle. However, mice in which the E2F-1 gene had been disrupted developed tumors, suggesting a negative role for E2F in controlling cell proliferation in some tissues. The consequences of disrupting the DP genes have not been reported. We screened for mutations that disrupt G1-S transcription late in Drosophila embryogenesis and identified five mutations in the dDP gene. Although mutations in dDP or dE2F nearly eliminate E2F-dependent G1-S transcription, S-phase still occurs. Cyclin E has been shown to be essential for S-phase in late embryogenesis, but in dDP and dE2F mutants the peaks of G1-S transcription of cyclin E are missing. Thus, greatly reduced levels of cyclin E transcript suffice for DNA replication until late in development. Both dDP and dE2F are necessary for viability, and mutations in the genes cause lethality at the late larval/pupal stage. The mutant phenotypes reveal that both genes promote progression of the cell cycle.
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Affiliation(s)
- I Royzman
- Department of Biology, Massachusetts Institute of Technology, and Whitehead Institute, Cambridge 02142, USA
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152
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Sprenger F, Yakubovich N, O'Farrell PH. S-phase function of Drosophila cyclin A and its downregulation in G1 phase. Curr Biol 1997; 7:488-99. [PMID: 9210381 PMCID: PMC2754254 DOI: 10.1016/s0960-9822(06)00220-x] [Citation(s) in RCA: 85] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023]
Abstract
BACKGROUND Cyclin E is the normal inducer of S phase in G1 cells of Drosophila embryos. Stable G1 quiescence requires the downregulation both of cyclin E and of other factors that can bypass the normal regulation of cell cycle progression. RESULTS High-level expression of cyclin A triggered the G1/S transition in wild-type embryos and in mutant embryos lacking cyclin E. Three types of control downregulated this activity of cyclin A. First, cyclin destruction limited the accumulation of cyclin A protein in G1. Second, inhibitory phosphorylation of cdc2, the kinase partner of cyclin A, reduced the S-phase promoting activity of cyclin A in G1. Third, rux, a protein with unknown biochemical function, limited cyclin A function in G1. Overexpression of rux blocked S phase induction by coexpressed cyclin A and promoted the degradation of cyclin A. Rux also prevented a stable cyclin A mutant from inducing S phase, indicating that inhibition does not require cyclin destruction, and drove the nuclear localization of cyclin A. CONCLUSIONS Cyclin A can drive the G1/S transition, but this function is suppressed by three types of control: cyclin A destruction, inhibitory phosphorylation of cdc2, and inhibition by rux. The partly redundant contributions of these three inhibitory mechanisms safeguard the stability of G1 quiescence until the induction of cyclin E. The action of rux during G1 resembles the action of inhibitors of mitotic kinases present during G1 in yeast, although no obvious sequence similarity exists.
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Affiliation(s)
- Frank Sprenger
- Department of Genetics, University of Cologne, Weyertal 121, 50931 Cologne, Germany
| | - Nikita Yakubovich
- Department of Biochemistry and Biophysics, University of California, San Francisco, California 94143-0448, USA
| | - Patrick H. O'Farrell
- Department of Biochemistry and Biophysics, University of California, San Francisco, California 94143-0448, USA
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153
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Polyploidization and Functional Maturation Are Two Distinct Processes During Megakaryocytic Differentiation: Involvement of Cyclin-Dependent Kinase Inhibitor p21 in Polyploidization. Blood 1997. [DOI: 10.1182/blood.v89.11.3980] [Citation(s) in RCA: 66] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
AbstractThe mechanism of megakaryocytic differentiation was investigated using human megakaryocytic leukemia cell line UT-7. Polyploidization of UT-7 cells was induced by the microtubule-depolymerizing agent, nocodazole, and 12-O-tetradecanoylphorbol-13-acetate (TPA), but the effect was much more striking with nocodazole. By contrast, induction of cytoplasmic maturation, as judged by β-thromboglobulin production and platelet factor 4 expression, was more prominent in TPA-treated cells than in nocodazole-treated cells. Nocodazole and TPA could act synergistically to increase ploidy and to enhance the expression of mature phenotypes. Human thrombopoietin induced functional maturation but not polyploidization in UT-7 cells and also acts synergistically with nocodazole. Cyclin-dependent kinase inhibitor p21 was upregulated at the early stage of megakaryocytic differentiation, and overexpression of p21 resulted in an increase in ploidy of UT-7 cells. This suggests that p21 is implicated in polyploidization via suppression of CDC2 activity at mitosis. UT-7 but not HL-60 cells could incorporate [3H]thymidine in the presence of TPA, indicating the presence of megakaryocyte-specific licensing factor to allow DNA replication during differentiation. Taking these data together, we propose that megakaryocytic differentiation consists of two distinct processes, polyploidization and functional maturation, and that these two processes are independently regulated.
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154
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Jallepalli PV, Kelly TJ. Cyclin-dependent kinase and initiation at eukaryotic origins: a replication switch? Curr Opin Cell Biol 1997; 9:358-63. [PMID: 9159077 DOI: 10.1016/s0955-0674(97)80008-7] [Citation(s) in RCA: 43] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023]
Abstract
A growing body of evidence indicates that cyclin-dependent kinases (CDKs) regulate the activity of eukaryotic origins of replication both positively and negatively. Although the details of this control remain unclear, recent work suggests that CDKs act directly at origins, where they associate with and phosphorylate several key initiator proteins. These data suggest that a CDK-regulated replication switch operates at each origin to ensure that initiation occurs precisely once per cell cycle.
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Affiliation(s)
- P V Jallepalli
- Department of Molecular Biology and Genetics, Johns Hopkins University School of Medicine, 725 N Wolfe Street, 601 Pre-Clinical Teaching Building, Baltimore, MD 21205, USA.
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155
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Thomas BJ, Zavitz KH, Dong X, Lane ME, Weigmann K, Finley RL, Brent R, Lehner CF, Zipursky SL. roughex down-regulates G2 cyclins in G1. Genes Dev 1997; 11:1289-98. [PMID: 9171373 DOI: 10.1101/gad.11.10.1289] [Citation(s) in RCA: 67] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023]
Abstract
Cell cycle arrest in G1 at the onset of patterning in the Drosophila eye is mediated by roughex. In roughex mutants, cells accumulate Cyclin A protein in early G1 and progress into S phase precociously. When Roughex is overexpressed in S/G2 cells, Cyclin A is mislocalized to the nucleus and degraded, preventing mitosis. Whereas Roughex inhibits Cyclin A accumulation, Cyclin E down-regulates Roughex protein in vivo. Roughex binds to Cyclin E and is a substrate for a Cyclin E-Cdk complex in vitro. These data argue that Roughex inhibits Cyclin A accumulation in early G1 by targeting Cyclin A for destruction. In late G1, Roughex is destabilized in a Cyclin E-dependent process, releasing Cyclin A for its role in S/G2.
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Affiliation(s)
- B J Thomas
- Laboratory of Biochemistry, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892, USA.
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156
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Hua XH, Yan H, Newport J. A role for Cdk2 kinase in negatively regulating DNA replication during S phase of the cell cycle. J Cell Biol 1997; 137:183-92. [PMID: 9105046 PMCID: PMC2139856 DOI: 10.1083/jcb.137.1.183] [Citation(s) in RCA: 125] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023] Open
Abstract
Using cell-free extracts made from Xenopus eggs, we show that cdk2-cyclin E and A kinases play an important role in negatively regulating DNA replication. Specifically, we demonstrate that the cdk2 kinase concentration surrounding chromatin in extracts increases 200-fold once the chromatin is assembled into nuclei. Further, we find that if the cdk2-cyclin E or A concentration in egg cytosol is increased 16-fold before the addition of sperm chromatin, the chromatin fails to initiate DNA replication once assembled into nuclei. This demonstrates that cdk2-cyclin E or A can negatively regulate DNA replication. With respect to how this negative regulation occurs, we show that high levels of cdk2-cyclin E do not block the association of the protein complex ORC with sperm chromatin but do prevent association of MCM3, a protein essential for replication. Importantly, we find that MCM3 that is prebound to chromatin does not dissociate when cdk2-cyclin E levels are increased. Taken together our results strongly suggest that during the embryonic cell cycle, the low concentrations of cdk2-cyclin E present in the cytosol after mitosis and before nuclear formation allow proteins essential for potentiating DNA replication to bind to chromatin, and that the high concentration of cdk2-cyclin E within nuclei prevents MCM from reassociating with chromatin after replication. This situation could serve, in part, to limit DNA replication to a single round per cell cycle.
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Affiliation(s)
- X H Hua
- Biology Department, University of California, San Diego, La Jolla 92093-0347, USA
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157
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Abstract
The eukaryotic cell cycle is regulated by the temporal activation of different cyclin-dependent kinase (CDK)/cyclin complexes. Whilst the level of the catalytic subunit of the complex, the CDK, remains relatively constant through the cycle, the level of the cyclin subunit generally oscillates. Cyclins are synthesized, bind and activate the CDK and are then destroyed. In this review, we summarize the current knowledge of the regulation of the cell cycle by CDK/cyclin complexes with special emphasis on new developments in cyclin biosynthesis and destruction, the structural analysis of the CDK/cyclin complexes and the role of a set of inhibitors of CDK/cyclin complexes that are important for the coordination of the different stages of the cell cycle.
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Affiliation(s)
- M Arellano
- Departamento de Microbiología y Genética, Universidad de Salamanca, Spain
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158
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Lehner CF, Lane ME. Cell cycle regulators in Drosophila: downstream and part of developmental decisions. J Cell Sci 1997; 110 ( Pt 5):523-8. [PMID: 9092934 DOI: 10.1242/jcs.110.5.523] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
The molecular identification of an evolutionarily conserved set of cell cycle regulators in yeast, Xenopus egg extracts, and vertebrate cell culture has opened up a new perspective for understanding the mechanisms that regulate cell proliferation during metazoan development. Now we can study how the crucial regulators of eukaryotic cell cycle progression, the various cyclin/cdk complexes (for a recent review see Nigg (1995) BioEssays 17, 471–480), are turned on or off during development. In Drosophila, this analysis is most advanced, in particular in the case of the rather rigidly programmed embryonic cell cycles that generate the cells of the larvae. In addition, this analysis has revealed how the mitotic cycle is transformed into an endocycle which allows the extensive growth of larvae and oocytes. In contrast, we know little about cyclin/cdk regulation during the imaginal proliferation that generates the cells of the adult. Nevertheless, we will also consider this second developmental phase with its conspicuous regulative character, because it will be of great interest for the analysis of the molecular mechanisms that integrate growth and proliferation during development.
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Affiliation(s)
- C F Lehner
- Department of Genetics, University of Bayreuth, Germany.
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159
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Dayton JS, Sumi M, Nanthakumar NN, Means AR. Expression of a constitutively active Ca2+/calmodulin-dependent kinase in Aspergillus nidulans spores prevents germination and entry into the cell cycle. J Biol Chem 1997; 272:3223-30. [PMID: 9013558 DOI: 10.1074/jbc.272.6.3223] [Citation(s) in RCA: 22] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023] Open
Abstract
The unique gene for Ca2+/calmodulin-dependent protein kinase (CaMK) has been shown to be essential in Aspergillus nidulans. Disruption of the gene prevents entry of spores into the nuclear division cycle. Here we show that expression of a constitutively active form of CaMK also prevents spores from entering the first S phase in response to a germinating stimulus. Expression of the constitutively active kinase induces premature activation of NIMEcyclin B/NIMXcdc2 in G0/G1. As NIMXcdc2 is present in spores, the elevation of maturation promotion factor activity may be secondary to the early production of NIMEcyclin B or post-translation modification of maturation promotion factor. The expression of the constitutively active CaMK also results in the appearance of NIMA kinase activity within 1 h of the germinating signal. These results support the contention that the activities of maturation promotion factor and NIMA are coincidentally regulated in A. nidulans and suggest that the unscheduled appearance of one or both of these activities may be sufficient to prevent A. nidulans spores from entering into DNA synthesis.
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Affiliation(s)
- J S Dayton
- Department of Pharmacology, Duke University Medical Center, Durham, North Carolina 27710, USA
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160
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Abstract
Cyclin-dependent kinases play essential roles in driving the cell cycle. Much progress has been made in Drosophila over the past year in identifying the specific requirements for individual cyclins in particular cell cycle events. These studies encompass many aspects of the cell cycle, from the addition of a G1 phase to the cell cycle during embryogenesis to the role of cyclin degradation in progression through anaphase.
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Affiliation(s)
- P J Follette
- Department of Biochemistry & Biophysics, University of California San Francisco, California 94143-0448 USA.
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161
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Frattini MG, Hurst SD, Lim HB, Swaminathan S, Laimins LA. Abrogation of a mitotic checkpoint by E2 proteins from oncogenic human papillomaviruses correlates with increased turnover of the p53 tumor suppressor protein. EMBO J 1997; 16:318-31. [PMID: 9029152 PMCID: PMC1169638 DOI: 10.1093/emboj/16.2.318] [Citation(s) in RCA: 61] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023] Open
Abstract
Human papillomavirus (HPV) E2 and E1 proteins are required for the replication of viral genomes in vivo. We have examined the effects of increasing the level of E2 on viral and cellular replication using recombinant adenoviruses. Infection of cells which maintain HPV 31 DNA episomally with E2 recombinant adenoviruses resulted in a 5-fold increase in genome copy number as well as an S phase arrest allowing for the continued replication of cellular DNA. Similar effects on cell cycle progression were seen following infection of normal human foreskin keratinocytes, the natural host cell. The DNA content of these cells increased beyond 4N indicating that multiple rounds of replication had occurred without an intervening mitotic event. In addition, increased cyclin A and E associated kinase activity was observed, while no change was detected in cyclin B associated kinase activity or in the activation state of cdc2 kinase. Interestingly, the levels of the p53 tumor suppresser protein were dramatically reduced through a post-transcriptional mechanism following infection. These data suggest a role for E2 in regulating viral and cellular replication by abrogation of a mitotic checkpoint, which is, at least in part, controlled by p53.
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Affiliation(s)
- M G Frattini
- Department of Microbiology-Immunology, Northwestern University, Chicago, IL 60611, USA
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162
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Steinbeck RG. Proliferation and DNA aneuploidy in mild dysplasia imply early steps of cervical carcinogenesis. Acta Oncol 1997; 36:3-12. [PMID: 9090961 DOI: 10.3109/02841869709100723] [Citation(s) in RCA: 22] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023]
Abstract
This project was focused on cellular proliferation relative to the onset of endoreplication and effects of DNA aneuploidy during carcinogenesis in cervical mucosa. Proliferation was monitored with MIB1 antibody, whereas nuclear DNA content was quantified using an image processing microphotometer. For the later procedure, 8 microm sections were of adequate depth with interphases, and lymphocyte nuclei provided an internal standard of the diploid (2c) DNA content. Results from 95 cervical biopsies displaying different types of dysplasia and carcinoma were supplemented with those of cervical smears from 319 cases. The later specimens had been selected from about 30000 consecutive cases in 1993/94. MIB1-traced proliferation was found in the second cell layer, whereas the bulk of basal cells remained quiescent in normal mucosa. However, predominant MIB1 immunoreactivity was observed with endoreplicated nuclei. A critical 30% of the cases exhibited DNA aneuploidy already in mild dysplasia, which was found in cytological smears and histological sections. The nuclear DNA content of basal cells increased progressively by endoreplication corresponding to the degree of dysplasia. Cases of carcinoma in situ displayed some 18% of non-proliferating diploid cells despite overwhelming endoreplication and DNA aneuploidy. High MIB1 levels combined with DNA aneuploidy unambiguously indicate the beginning of cervical carcinogenesis. The limits of the Bethesda system were discussed.
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Affiliation(s)
- R G Steinbeck
- Department of Tumor Pathology, Karolinska Institute and Hospital, Stockholm, Sweden
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163
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Dong X, Zavitz KH, Thomas BJ, Lin M, Campbell S, Zipursky SL. Control of G1 in the developing Drosophila eye: rca1 regulates Cyclin A. Genes Dev 1997; 11:94-105. [PMID: 9000053 DOI: 10.1101/gad.11.1.94] [Citation(s) in RCA: 97] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023]
Abstract
In the developing eye of Drosophila melanogaster, cells become synchronized in the G1 phase of the cell cycle just prior to the onset of cellular differentiation and morphogenesis. In roughex (rux) mutants, cells enter S phase precociously because of ectopic activation of a Cyclin A/Cdk complex in early G1. This leads to defects in cell fate and pattern formation, and results in abnormalities in the morphology of the adult eye. A screen for dominant suppressors of the rux eye phenotype led to the identification of mutations in cyclin A, string (cdc25), and new cell cycle genes. One of these genes, regulator of cyclin A (rca1), encodes a novel protein required for both mitotic and meiotic cell cycle progression. rca1 mutants arrest in G2 of embryonic cell cycle 16 with a phenotype very similar to cyclin A loss of function mutants. Expression of rca1 transgenes in G1 or in postmitotic neurons promotes Cyclin A protein accumulation and drives cells into S phase in a Cyclin A-dependent fashion.
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Affiliation(s)
- X Dong
- Department of Biological Chemistry, Molecular Biology Institute, University of California, Los Angeles 90095, USA
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164
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Lane ME, Sauer K, Wallace K, Jan YN, Lehner CF, Vaessin H. Dacapo, a cyclin-dependent kinase inhibitor, stops cell proliferation during Drosophila development. Cell 1996; 87:1225-35. [PMID: 8980229 DOI: 10.1016/s0092-8674(00)81818-8] [Citation(s) in RCA: 284] [Impact Index Per Article: 10.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023]
Abstract
Most cell types in multicellular eukaryotes exit from the mitotic cell cycle before terminal differentiation. We show that the dacapo gene is required to arrest the epidermal cell proliferation at the correct developmental stage during Drosophila embryogenesis. dacapo encodes an inhibitor of cyclin E/cdk2 complexes with similarity to the vertebrate Cip/Kip inhibitors. dacapo is transiently expressed beginning late in the G2 phase preceding the terminal division (mitosis 16). Mutants unable to express the inhibitor fail to arrest cell proliferation after mitosis 16 and progress through an extra division cycle. Conversely, premature dacapo expression in transgenic embryos results in a precocious G1 arrest.
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Affiliation(s)
- M E Lane
- Friedrich-Miescher-Laboratorium der Max-Planck-Gesellschaft, Tübingen,Federal Republic of Germany
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165
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Abstract
During early development in many species, maternally supplied gene products permit the cell cycle to run at maximum velocity, subdividing the fertilized egg into smaller and smaller cells. As development proceeds, zygotic controls are activated that first limit divisions to defined spatial and temporal domains, coordinating them with morphogenesis, and then halt proliferation altogether, to allow cell differentiation. Analysis of the regulation of cyclin-dependent kinases (Cdks) in Drosophila has provided insights into how this embryonic program of cell proliferation is controlled at the molecular level and how it is linked to developmental cues. Recent studies have also begun to reveal how cell proliferation is controlled during the second phase of Drosophila development, which occurs in imaginal tissues. In contrast to their embryonic progenitors, imaginal cells proliferate with a cycle that requires cell growth and is linked to patterning processes controlled by secreted cell signaling molecules. The functions of these signaling molecules appear to be nearly as conserved between vertebrates and invertebrates as the cell cycle control apparatus itself, suggesting that the mechanisms that coordinate growth, patterning, and cell proliferation in developing tissues have ancient origins.
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Affiliation(s)
- B A Edgar
- Division of Basic Sciences, Fred Hutchinson Cancer Research Center, Seattle, WA 98109, USA
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166
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Abstract
Studies on the initiation of DNA replication in eukaryotes have progressed recently through different approaches that promise to converge. Proteins interacting with the origin recognition complex form a prereplicative complex early in the cell cycle. The regulation of the binding of MCM/P1 proteins to chromatin plays a key role in the replication licensing system which prevents re-replication in a single cell cycle. Cyclin-dependent kinases provide an overall control of the cell cycle by stimulating S-phase entry and possibly by preventing re-establishment of prereplicative complexes in G2 phase.
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Affiliation(s)
- S Chevalier
- Imperial Cancer Research Fund, Clare Hall Laboratories, South Mimms, Hertfordshire, EN6 3LD, UK.
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167
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Diffley JF. Once and only once upon a time: specifying and regulating origins of DNA replication in eukaryotic cells. Genes Dev 1996; 10:2819-30. [PMID: 8918884 DOI: 10.1101/gad.10.22.2819] [Citation(s) in RCA: 208] [Impact Index Per Article: 7.4] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023]
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168
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Sauer K, Weigmann K, Sigrist S, Lehner CF. Novel members of the cdc2-related kinase family in Drosophila: cdk4/6, cdk5, PFTAIRE, and PITSLRE kinase. Mol Biol Cell 1996; 7:1759-69. [PMID: 8930898 PMCID: PMC276024 DOI: 10.1091/mbc.7.11.1759] [Citation(s) in RCA: 54] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023] Open
Abstract
In addition to the previously identified Drosophila cdc2 and cdc2c genes, we have identified four additional cdc2-related genes with low stringency and polymerase chain reaction approaches. Sequence comparisons suggest that the four putative kinases represent the Drosophila homologues of vertebrate cdk4/6, cdk5, PCTAIRE, and PITSLRE kinases. Although the similarity between human and Drosophila homologues is extensive in the case of cdk5, PCTAIRE, and PITSLRE kinases (78%, 58%, and 65% identity in the kinase domain), only limited conservation is observed for Drosophila cdk4/6 (47% identity). However, like vertebrate cdk4 and cdk6, Drosophila cdk4/6 binds also to a D-type cyclin according to the results of two-hybrid experiments in yeast. Northern blot analysis indicated that the four Drosophila kinases are expressed throughout embryogenesis. Expression in early embryogenesis appeared to be ubiquitous according to in situ hybridization. Abundant expression already at the start of embryogenesis and long before neuron differentiation was also observed in the case of cdk5 protein, which has been described as predominantly neuron specific in mice. Sequence conservation and expression pattern, therefore, suggest that all of these kinases perform important cellular functions.
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Affiliation(s)
- K Sauer
- Friedrich-Miescher-Laboratorium, Max-Planck-Gesellschaft, Tübingen, Germany
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169
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Lilly MA, Spradling AC. The Drosophila endocycle is controlled by Cyclin E and lacks a checkpoint ensuring S-phase completion. Genes Dev 1996; 10:2514-26. [PMID: 8843202 DOI: 10.1101/gad.10.19.2514] [Citation(s) in RCA: 233] [Impact Index Per Article: 8.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023]
Abstract
Early during Drosophila oogenesis the 16 interconnected cells of each germ-line cyst choose between two alternative fates. The single future oocyte enters meiosis, arrests, and becomes transcriptionally quiescent. The remaining 15 cells initiate a series of polyploid cell cycles to prepare for their role as nurse cells. Like many other polyploid and polytene cells, during nurse cell growth the major satellite DNAs become highly under-represented by a mechanism that has remained obscure. We implicate the cell-cycle regulator cyclin E in DNA under-representation by identifying a hypomorphic, female sterile cycE mutation, cycE01672, that increases the amount of satellite DNA propagated in nurse cells. In mutant but not wild-type endomitotic nurse cells, "late S" patterns of bromodeoxyuridine incorporation are observed similar to those in mitotic cells. CycE protein still cycles in cycE01672 germ-line cysts but at reduced levels, and it is found throughout a longer fraction of the cell cycle. Our experiments support the view that oscillating levels of CycE control the polyploid S phase. Moreover, they indicate that a checkpoint linking the presence of unreplicated DNA to the CycE oscillator is lacking, leading to incomplete replication of late-replicating sequences such as satellite DNAs. Unexpectedly, two to three of the 16 cells in cycE01672 cysts frequently differentiate as oocytes, implicating cell-cycle programming in oocyte determination.
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Affiliation(s)
- M A Lilly
- Howard Hughes Medical Institute Research Laboratories, Carnegie Institution of Washington, Baltimore, Maryland 21210, USA
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170
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Abstract
Both the heterodimeric transcription factor, E2F, and the G1 cyclin, cyclin E, are required for the G1-S transition at the start of the metazoan cell cycle. It has been established that cyclin E can act as an upstream activator of E2F. In addition to this action, we show here that cyclin E has an essential role in DNA replication distinct from activating E2F. We have created transgenic Drosophila capable of inducible, ectopic production of E2F activity. Simultaneous overexpression of both Drosophila E2F subunits, dE2F and dDP, in embryos stimulated the expression of multiple E2F-target genes including cyclin E, and also caused the initiation of S phase. Mutation of cyclin E prevented the initiation of S phase after overexpression of dE2F/dDP without affecting induction of target gene expression. Thus, E2F-directed transcription cannot bypass loss of cyclin E in Drosophila embryos.
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Affiliation(s)
- R J Duronio
- Department of Biochemistry and Biophysics, University of California, San Francisco 94143, USA
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171
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Stern B, Nurse P. A quantitative model for the cdc2 control of S phase and mitosis in fission yeast. Trends Genet 1996. [DOI: 10.1016/s0168-9525(96)80016-3] [Citation(s) in RCA: 157] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/01/2022]
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172
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Edgar BA, Datar SA. Zygotic degradation of two maternal Cdc25 mRNAs terminates Drosophila's early cell cycle program. Genes Dev 1996; 10:1966-77. [PMID: 8756353 DOI: 10.1101/gad.10.15.1966] [Citation(s) in RCA: 140] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023]
Abstract
In Drosophila embryos the maternal/zygotic transition (MZT) in cell cycle control normally follows mitosis 13. Here we show that this transition requires degradation of two maternal mRNAs, string and twine, which encode Cdc25 phosphatases. Although twine is essential for meiosis and string is essential for most mitotic cycles, the two genes have mutually complementing, overlapping functions in the female germ line and the early embryo. Deletion of both gene products from the female germ line arrests germ-line development. Reducing the maternal dose of both products can lower the number of early embryonic mitoses to 12, whereas increasing maternal Cdc25(twine) can increase the number of early mitoses to 14. Blocking the activation of zygotic transcription stabilizes maternal string and twine mRNAs and also allows an extra maternal mitosis, which is Cdc25 dependent. We propose that Drosophila's MZT comprises a chain reaction in which (1) proliferating nuclei deplete factors (probably mitotic cyclins) required for cell cycle progression; (2) this depletion causes the elongation of interphases and allows zygotic transcription; (3) new gene products accumulate that promote degradation of maternal mRNAs, including string and twine; and (4) consequent loss of Cdc25 phosphatase activity allows inhibitory phosphorylation of Cdc2 by Dwee1 kinase, effecting G2 arrest. Unlike timing or counting mechanisms, this mechanism can compensate for losses or additions of nuclei by altering the timing and number of the maternal cycles and thus will always generate the correct cell density at the MZT.
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Affiliation(s)
- B A Edgar
- Division of Basic Sciences, Fred Hutchinson Cancer Research Center, Seattle, Washington 98104, USA
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173
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Chevalier S, Couturier A, Chartrain I, Le Guellec R, Beckhelling C, Le Guellec K, Philippe M, Ford CC. Xenopus cyclin E, a nuclear phosphoprotein, accumulates when oocytes gain the ability to initiate DNA replication. J Cell Sci 1996; 109 ( Pt 6):1173-84. [PMID: 8799808 DOI: 10.1242/jcs.109.6.1173] [Citation(s) in RCA: 36] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
The capacity to initiate DNA replication appears during oocyte maturation in Xenopus. Initiation of S phase is driven by several components which include active cyclin/cdk complexes. We have identified three Xenopus cyclin E clones showing 59% amino acid identity with human cyclin E. The recruitment of cyclin E mRNA, like cdk2 mRNA, into the polysomal fraction during oocyte maturation, results in the accumulation of the corresponding proteins in unfertilized eggs. Cyclin E mRNA remains polyadenylated during cleavage and anti-cyclin E antibodies detect Xlcyclin E in embryonic nuclei at this time. Cdk2 protein is necessary for the phosphorylation of radiolabelled cyclin E added to egg extracts. Radiolabelled Xlcyclin E enters interphase nuclei and, though stable through interphase and mitosis, is not associated with condensed mitotic chromatin. In egg extracts, endogenous Xlcyclin E rapidly associates with nuclei before S phase and remains nuclear throughout interphase, becoming nucleoplasmic in G2/prophase. Under conditions where initiation of replication is limiting in extracts, Xlcyclin E associates only with those nuclei that undergo S phase. These features are entirely consistent with the view that Xlcyclin E is required for initiation of S phase.
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Affiliation(s)
- S Chevalier
- Département de Biologie et Génétique du développement, CNRS URA 256, Université de Rennes I, France
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174
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Du W, Vidal M, Xie JE, Dyson N. RBF, a novel RB-related gene that regulates E2F activity and interacts with cyclin E in Drosophila. Genes Dev 1996; 10:1206-18. [PMID: 8675008 DOI: 10.1101/gad.10.10.1206] [Citation(s) in RCA: 189] [Impact Index Per Article: 6.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023]
Abstract
Genetic studies have shown that cyclin E and dE2F are critical regulators of S-phase entry during Drosophila embryogenesis. Whereas the ectopic expression of cyclin E activates dE2F-dependent transcription, it has been proposed that cyclin E does not act directly on dE2F but targets a negative regulator of E2F activity. Such a regulator might be analogous to the family of RB-related proteins (pRB, p107, and p130) that associate with E2F in humans; however, extensive efforts have failed to find such homologs in Drosophila. We have developed a two-hybrid approach that allows transcription activators to be used as bait for interacting proteins. From a screen using Drosophila E2F (dE2F and dDP) as bait, we identified a novel gene, RBF. RBF combines several of the structural features of pRB, p107, and p130, suggesting that it may have evolved from a common ancestor to the three human genes. RBF associates with dE2F and dDP in vivo and is a stoichiometric component of E2F DNA-binding complexes. RBF specifically repressed E2F-dependent transcription and suppressed the phenotype generated by ectopic expression of dE2F and dDP in the developing Drosophila eye. RBF was phosphorylated by a cyclin E-associated kinase in vitro, and loss-of-function cyclin E mutations enhanced an RBF overexpression phenotype, consistent with the idea that the biological activity of RBF is negatively regulated by endogenous cyclin E. The properties of RBF suggest that it is the intermediary factor that was proposed to allow cyclin E induction of E2F activity. These findings indicate that RBF plays a critical role in the regulation of cell proliferation in Drosophila and show that analogous pathways regulate S-phase entry in a diverse range of species.
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Affiliation(s)
- W Du
- Massachusetts General Hospital, Cancer Center, Charlestown 02129, USA
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175
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Romanowski P, Madine MA. Mechanisms restricting DNA replication to once per cell cycle: MCMS, pre-replicative complexes and kinases. Trends Cell Biol 1996; 6:184-8. [PMID: 15157470 DOI: 10.1016/0962-8924(96)10015-5] [Citation(s) in RCA: 43] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
An important aspect of cell behaviour is that DNA replication happens only once per cell cycle. Replicated DNA is unable to re-replicate until cell division has occurred. Unreplicated DNA is in a replication-competent or 'licensed' state. The ability to replicate is lost in S phase and regained following passage through mitosis. Recent evidence has implicated an MCM (minichromosome maintenance) protein complex and the Cdc6 protein in determining replication competence. Regeneration of replication competence upon passage through mitosis entails changes in protein kinase activity, of which the MCMs are a likely target. Features of the mechanism that restricts DNA replication to once per cell cycle appear to be conserved throughout eukaryotes.
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Affiliation(s)
- P Romanowski
- Wellcome/CRC Institute, Tennis Court Road, Cambridge, UK CB2 1QR
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176
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Abstract
A clearer picture of replication control is emerging through the characterization of proteins, such as cdc18/Cdc6 and members of the mini-chromosome maintenance (MCM) protein family, that are involved in the initiation step. Cyclin B dependent kinases have conserved roles in both Saccharomyces cerevisiae and Schizosaccharomyces pombe, switching on DNA replication in G1 and preventing re-replication in G2. A model is suggested where MCMs and CDKs play complementary roles to ensure 'once-per-cell-cycle' replication, with CDKs maintaining a G1 or G2 state, whereas MCMs provide a cis-acting control on chromatin to prevent reinitiation during a single S phase.
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Affiliation(s)
- S E Kearsey
- Department of Zoology, University of Oxford, UK.
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177
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Abstract
DNA replication in G2 does not normally occur due to the checkpoint control. To elucidate its mechanism, the functions of the escargot and Dmcdc2 genes of Drosophila were studied. When escargot function was eliminated, diploid imaginal cells that were arrested in G2 lost Cyclin A, a regulatory subunit of G2/M cdk, and entered an endocycle. escargot genetically interacted with Dmcdc2 which encodes a catalytic subunit of G2/M cdk. The mutant phenotypes of Dmcdc2 itself was similar to those of escargot: many diploid cells in imaginal discs, salivary glands and the central nervous system entered an endocycle and sometimes formed polytene chromosomes. Since mitotically quiescent abdominal histoblasts still required Dmcdc2 to remain diploid, the inhibitory activity of G2/M cdk on DNA replication appeared to be separable from its activity as the mitosis promoting factor. These results suggest that in G2, escargot is required to maintain a high level of G2/M cdk that actively inhibits the entry into S phase.
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Affiliation(s)
- S Hayashi
- National Institute of Genetics, Mishima, Shizuoka-ken, Japan
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178
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Jallepalli PV, Kelly TJ. Rum1 and Cdc18 link inhibition of cyclin-dependent kinase to the initiation of DNA replication in Schizosaccharomyces pombe. Genes Dev 1996; 10:541-52. [PMID: 8598285 DOI: 10.1101/gad.10.5.541] [Citation(s) in RCA: 81] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
Abstract
Eukaryotic cells have evolved regulatory mechanisms to ensure the strict alternation of DNA replication and mitosis. Recent work has suggested that the mitotic form of cyclin-dependent kinase (Cdc2/cyclin B) has a role in preventing re-replication of the genome before mitosis, but the relevant targets of this inhibition are unknown. In this report we present evidence that the mitotic cyclin-dependent kinase affects DNA replication by inhibiting the accumulation and function of Cdc18, a critical regulator of S-phase entry. We found that the ruml+ gene efficiently suppresses the lethality of a conditional cdc18 mutant. Conversely, deletion of ruml+ increases the severity of the cdc18 mutant phenotype, resulting in inappropriate cell division and a rapid loss of viability. Biochemical experiments indicate that Ruml potently inhibits Cdc2 phosphorylation of histone H1 or a Cdc18 fusion protein by directly interacting with the Cdc2/cyclin B complex. Overexpression of Ruml under conditions that promote re-replication of the genome induces a striking accumulation of Cdc18 protein by a largely post-transcriptional mechanism. Overexpression of SIC1, an unrelated cyclin-dependent kinase inhibitor from budding yeast, causes a similar accumulation of Cdc18 and also leads to re-replication. Our data link a potent inhibitor of Cdc2 kinase to a key protein required for the initiation of DNA replication and strongly suggest that inhibition of Cdc18 by cyclin-dependent kinases has an important role in ensuring that the genome is duplicated precisely once each cell cycle.
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Affiliation(s)
- P V Jallepalli
- Department of Molecular Biology and Genetics, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, USA
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179
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Abstract
In all eukaryotes, entry into mitosis from G2 phase is initiated by a complex of the cdc2 kinase and a B-type cyclin. It has now been shown that, in fission yeast, B-type cyclins also activate cdc2 in G1, thus governing cell-cycle commitment, as well as the onset of S phase. In this article, Karim Labib and Sergio Moreno review the evidence that ruml inhibits the kinase activity of cdc2 associated with B-type cyclins and is an important regulator o f G1 progression in fission yeast.
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Affiliation(s)
- K Labib
- Dept of Zoology, University of Oxford, South Parks Road, Oxford, UK OX1 3PS
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180
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Correa-Bordes J, Nurse P. p25rum1 orders S phase and mitosis by acting as an inhibitor of the p34cdc2 mitotic kinase. Cell 1995; 83:1001-9. [PMID: 8521500 DOI: 10.1016/0092-8674(95)90215-5] [Citation(s) in RCA: 164] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
Abstract
p25rum1 from the fission yeast S. pombe is shown to act as a specific in vitro inhibitor of the p34cdc2/p56cdc13 mitotic kinase. It is also shown that early G1 cells contain p25rum1, which associates with and inhibits the mitotic kinase, and maintains p56cdc13 mitotic B cyclin at a low level, ensuring that these cells do not undergo a premature lethal entry into mitosis. A high level of p25rum1 in G2 cells inhibits the p34cdc2/p56cdc13 kinase that removes the block preventing a further S phase and leads to repeated rounds of DNA replication. Thus, the cyclin-dependent kinase inhibitor p25rum1, acting on the p34cdc2 mitotic kinase, plays an important role in ensuring the correct sequence of S phase and mitosis during the cell cycle.
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181
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Abstract
During embryogenesis, a genetic program coordinates cell proliferation with morphogenesis and cell differentiation. Recent studies using Drosophila have shown how, as development proceeds, this program directs different cell types to acquire unique modes of cell cycle regulation. As maternal cell cycle factors are exhausted and replaced by differentially expressed zygotic factors, an increasing repertoire of gene products become potential regulators of the cycle. Cyclin B, Cdc25, and Cyclin E each act as limiting regulators in Drosophila in specific cell types at particular developmental stages. The genes encoding these and many other candidate regulators have been cloned from mice, but their roles in vivo have yet to be understood.
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Affiliation(s)
- B Edgar
- Fred Hutchinson Cancer Research Center, Seattle, USA.
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182
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Dahmann C, Diffley JF, Nasmyth KA. S-phase-promoting cyclin-dependent kinases prevent re-replication by inhibiting the transition of replication origins to a pre-replicative state. Curr Biol 1995; 5:1257-69. [PMID: 8574583 DOI: 10.1016/s0960-9822(95)00252-1] [Citation(s) in RCA: 310] [Impact Index Per Article: 10.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
Abstract
BACKGROUND DNA replication and mitosis are triggered by activation of kinase complexes, each made up of a cyclin and a cyclin-dependent kinase (Cdk). It had seemed possible that the association of Cdks with different classes of cyclins specifies whether S phase (replication) or M phase (mitosis) will occur. The recent finding that individual B-type cyclins (encoded by the genes CLB1-CLB6) can have functions in both processes in the budding yeast Saccharomyces cerevisiae casts doubt on this notion. RESULTS S. cerevisiae strains lacking C1b1-C1b4 undergo DNA replication once but fail to enter mitosis. We have isolated mutations in two genes, SIM1 and SIM2 (SIM2 is identical to SEC72), which allow such cells to undergo an extra round of DNA replication without mitosis. The Clb5 kinase, which promotes S phase, remains active during the G2-phase arrest of cells of the parental strain, but its activity declines rapidly in sim mutants. Increased expression of the CLB5 gene prevents re-replication. Thus, a cyclin B-kinase that promotes DNA replication in G1-phase cells can prevent re-replication in G2-phase cells. Inactivation of C1b kinases by expression of the specific C1b-Cdk1 inhibitor p40SIC1 is sufficient to induce a prereplicative state at origins of replication in cells blocked in G2/M phase by nocodazole. Re-activation of C1b-Cdk1 kinases induces a second round of DNA replication. CONCLUSIONS We propose that S-phase-promoting cyclin B--Cdk complexes prevent re-replication during S, G2 and M phases by inhibiting the transition of replication origins to a pre-replicative state. This model can explain both why origins 'fire' only once per S phase and why S phase is dependent on completion of the preceding M phase.
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Affiliation(s)
- C Dahmann
- I.M.P. Research Institute of Molecular Pathology, Vienna, Austria
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183
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Affiliation(s)
- T T Su
- Department of Biochemistry and Biophysics University of California, San Francisco 94143-0448, USA
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