51
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Treisman JE, Follette PJ, O'Farrell PH, Rubin GM. Cell proliferation and DNA replication defects in a Drosophila MCM2 mutant. Genes Dev 1995; 9:1709-15. [PMID: 7622035 DOI: 10.1101/gad.9.14.1709] [Citation(s) in RCA: 74] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Abstract
The yeast MCM2, MCM3, and MCM5/CDC46 genes are required for DNA replication and have been proposed to act as factors that license the DNA for one and only one round of replication per cell cycle. We have identified a Drosophila gene, DmMCM2, that is highly homologous to MCM2. A P-element insertion into this gene, which prevents its transcription, inhibits proliferation of cells in the imaginal discs and central nervous system (CNS) and causes an apparent prolongation of S phase in the embryonic and larval CNS. DmMCM2 is expressed in the embryo in a pattern corresponding to that of S-phase cells. These results suggest that DmMCM2 plays a role in the regulation of DNA replication analogous to that of its yeast counterpart.
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Affiliation(s)
- J E Treisman
- Howard Hughes Medical Institute, University of California, Berkeley 94720, USA
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52
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53
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Chong JP, Mahbubani HM, Khoo CY, Blow JJ. Purification of an MCM-containing complex as a component of the DNA replication licensing system. Nature 1995; 375:418-21. [PMID: 7760937 DOI: 10.1038/375418a0] [Citation(s) in RCA: 287] [Impact Index Per Article: 9.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023]
Abstract
Replication licensing factor (RLF) ensures that eukaryotic chromosomal DNA is replicated exactly once in each cell cycle. On exit from metaphase, RLF is activated and binds to or modifies chromatin. This modification (the 'licence') is required for subsequent DNA replication; the licence is also inactivated in the process of replication. Active RLF is not imported into the nucleus, so further DNA replication cannot occur until the DNA is relicensed by passage throught mitosis. We have developed an assay to purify RLF from Xenopus eggs. Activity resolves into two components, RLF-M and RLF-B, both of which are required for licensing. RLF-M has been purified to apparent homogeneity: it consists of three polypeptides, one of which is a Xenopus homologue of the yeast MCM3 protein. Xenopus Mcm3 associates with chomatin in G1 and is removed during replication, consistent with its being a component of the RLF system.
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Affiliation(s)
- J P Chong
- ICRF Clare Hall Laboratories, South Mimms, Herts, UK
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54
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Madine MA, Khoo CY, Mills AD, Laskey RA. MCM3 complex required for cell cycle regulation of DNA replication in vertebrate cells. Nature 1995; 375:421-4. [PMID: 7760938 DOI: 10.1038/375421a0] [Citation(s) in RCA: 215] [Impact Index Per Article: 7.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023]
Abstract
An intact nuclear membrane restricts DNA replication to only one round in each cell cycle, apparently by excluding an essential replication-licensing factor throughout interphase. A family of related yeast replication proteins, MCM2, 3 and 5 (also called, after cell-division cycle, CDC46), resemble licensing factor, entering the nucleus only during mitosis. We have cloned a Xenopus homologue of MCM3 (XMCM3) and raised antibodies against expressed protein. Immunodepletion of Xenopus egg extracts removes a complex of MCM2, 3 and 5 homologues and inhibits replication of Xenopus sperm nuclei or permeable G2 HeLa nuclei. However, G1 HeLa nuclei still replicate efficiently. Mock-depleted extracts replicate all three templates. XMCM3 accumulates in nuclei before replication but anti-XMCM3 staining decreases during replication. These results can explain why replicated nuclei are unable to reinitiate replication in a single cell cycle.
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55
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Sauer K, Knoblich JA, Richardson H, Lehner CF. Distinct modes of cyclin E/cdc2c kinase regulation and S-phase control in mitotic and endoreduplication cycles of Drosophila embryogenesis. Genes Dev 1995; 9:1327-39. [PMID: 7797073 DOI: 10.1101/gad.9.11.1327] [Citation(s) in RCA: 183] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023]
Abstract
Drosophila cyclin E (DmcycE) is required in embryos for S phase of mitotic and endoreduplication cycles. Here, we describe regulatory differences characteristic for these two cell cycle types. While DmcycE transcript levels decline in DmcycE mutant cells programmed for mitotic proliferation, they are maintained and no longer restricted to transient pulses in DmcycE mutant cells programmed for endoreduplication. Moreover, DmcycE expression in endoreduplicating cells is down-regulated by ectopic expression of a heat-inducible cyclin E transgene. DmcycE expression in endoreduplicating tissues, therefore, is restricted by a negative feedback to the transient pulse triggering entry into S-phase. Conversely, during mitotic cycles, where S phase entry is not only dependent on cyclin E but also on progression through M phase, cyclin E and associated Dmcdc2c kinase activity are present throughout the cell cycle. Reinitiation of DNA replication during the G2 phase of the mitotic cell cycle, therefore, is prevented by cyclin E/Dmcdc2c kinase-independent regulation. Observations in cyclin A mutants implicate G2 cyclins in this regulation. Our results suggest molecular explanations for the different rules governing S phase during mitotic and endoreduplication cycles.
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Affiliation(s)
- K Sauer
- Friedrich-Miescher-Laboratorium der Max-Planck-Gesellschaft, Tübingen, Germany
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56
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Kubota Y, Mimura S, Nishimoto S, Takisawa H, Nojima H. Identification of the yeast MCM3-related protein as a component of Xenopus DNA replication licensing factor. Cell 1995; 81:601-9. [PMID: 7758114 DOI: 10.1016/0092-8674(95)90081-0] [Citation(s) in RCA: 230] [Impact Index Per Article: 7.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023]
Abstract
Replication licensing factor is thought to be involved in the strict control of the initiation of DNA replication in eukaryotes. We identified a 100 kDa protein as a candidate for the licensing factor in Xenopus egg extracts. This protein was required for replication; it bound to sperm DNA before the formation of nuclei and apparently dissociated from the nuclear DNA during the progression of replication without being transported into the nuclei. An immunologically homologous protein in HeLa cells behaved similarly to the Xenopus protein during the cell cycle. Cloning and sequencing of the cDNAs encoding the Xenopus and human proteins revealed that they are homologs of yeast Mcm3, a putative yeast DNA replication licensing factor.
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Affiliation(s)
- Y Kubota
- Department of Biology, Faculty of Science, Osaka University, Japan
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57
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Chevalier S, Tassan JP, Cox R, Philippe M, Ford C. Both cdc2 and cdk2 promote S phase initiation in Xenopus egg extracts. J Cell Sci 1995; 108 ( Pt 5):1831-41. [PMID: 7657707 DOI: 10.1242/jcs.108.5.1831] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Xenopus egg extracts induce S phase DNA replication in added sperm pronuclei in a highly regulated manner, similar to events in vivo. Removal of cyclin-dependant kinases (cdks) or cdk2 from these extracts using affinity matrices severely inhibits initiation of S phase. We have used p13suc1 beads to remove both cdk2 and cdc2 proteins from egg extracts and developed a method to replace either protein alone to assess their capacity to initiate DNA replication. Re-addition of either cdk2 or cdc2 proteins to depleted extracts, through translation of their respective mRNAs, restimulated replication, judged by both total synthesis and labelling index. An ATP-binding-site mutant cdk2 mRNA (cdk2.R33) failed to stimulate replication and inhibited S phase initiation in mock-depleted extracts. Both human and Xenopus cdc2 mRNAs rescued replication in this system. Human mutant mRNAs have been used to show that the stimulation induced requires cdc2 catalytic activity, though not its mitotically active form. Rescue of replication by p34cdc2 is also observed in extracts depleted of cdks with a cdk2 antibody, which still retain much of their endogenous cdc2 protein. We conclude that newly synthesised p34cdc2, but not the inherited ‘old’ form, can induce S phase and in this form may overlap in function with p33cdk2.
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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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58
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Starborg M, Brundell E, Gell K, Larsson C, White I, Daneholt B, Höög C. A murine replication protein accumulates temporarily in the heterochromatic regions of nuclei prior to initiation of DNA replication. J Cell Sci 1995; 108 ( Pt 3):927-34. [PMID: 7622621 DOI: 10.1242/jcs.108.3.927] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
We have analyzed the expression of the murine P1 gene, the mammalian homologue of the yeast MCM3 protein, during the mitotic cell cycle. The MCM3 protein has previously been shown to be of importance for initiation of DNA replication in Saccharomyces cerevisiae. We found that the murine P1 protein was present in the nuclei of mammalian cells throughout interphase of the cell cycle. This is in contrast to the MCM3 protein, which is located in the nuclei of yeast cells only between the M and the S phase of the cell cycle. Detailed analysis of the intranuclear localization of the P1 protein during the cell cycle revealed that it accumulates transiently in the heterochromatic regions towards the end of G1. The accumulation of the P1 protein in the heterochromatic regions prior to activation of DNA replication suggests that the mammalian P1 protein is also of importance for initiation of DNA replication. The MCM2-3.5 proteins have been suggested to represent yeast equivalents of a hypothetical replication licensing factor initially described in Xenopus. Our data support this model and indicate that the murine P1 protein could function as replication licensing factor. The chromosomal localization of the P1 gene was determined by fluorescence in situ hybridization to region 6p12 in human metaphase chromosomes.
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Affiliation(s)
- M Starborg
- Department of Cell and Molecular Biology, Medical Nobel Institute, Karolinska Institute, Stockholm, Sweden
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59
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Murphy J, Crompton CM, Hainey S, Codd GA, Hutchison CJ. The role of protein phosphorylation in the assembly of a replication competent nucleus: investigations in Xenopus egg extracts using the cyanobacterial toxin microcystin-LR. J Cell Sci 1995; 108 ( Pt 1):235-44. [PMID: 7738100 DOI: 10.1242/jcs.108.1.235] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Cell-free extracts of Xenopus eggs support nuclear assembly and DNA replication in vitro. Extracts supplemented with the protein phosphatase inhibitor microcystin-LR displayed various inhibitory effects at different concentrations of the toxin. In the presence of cycloheximide, additions of microcystin did not induce histone H1-kinase activity. Nevertheless, increasing concentrations of microcystin did sequentially prevent DNA replication, nuclear lamina assembly and nuclear envelope assembly. DNA replication was prevented when microcystin was added at 250 nM. Furthermore, this effect could be reversed after the addition of the catalytic sub-unit of protein phosphatase 2A to inhibited extracts. At a concentration of 250 nM microcystin, nuclear membrane assembly, nuclear lamina assembly and nuclear transport all occurred in egg extracts. In addition single-stranded M13 DNA replication was also permitted. However, it appeared that replicase assembly was not completed, since nuclei assembled in microcystin-treated extracts displayed an unusual distribution of proliferating cell nuclear antigen (PCNA). Although PCNA was located at sites that resembled pre-replication foci, this nuclear protein was readily solubilised when nuclei were isolated and extracted sequentially with Triton, nucleases and salts. Despite this, nuclei containing pre-assembled replication forks could synthesise DNA when transferred into microcystin-treated extracts.
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Affiliation(s)
- J Murphy
- Department of Biological Sciences, University of Dundee, Scotland
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60
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Leno GH, Munshi R. Initiation of DNA replication in nuclei from quiescent cells requires permeabilization of the nuclear membrane. J Cell Biol 1994; 127:5-14. [PMID: 7929570 PMCID: PMC2120183 DOI: 10.1083/jcb.127.1.5] [Citation(s) in RCA: 27] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023] Open
Abstract
We have investigated the replication capacity of intact nuclei from quiescent cells using Xenopus egg extract. Nuclei, with intact nuclear membranes, were isolated from both exponentially growing and contact-inhibited BALB/c 3T3 fibroblasts by treatment of the cells with streptolysin-O. Flow cytometry showed that > 90% of all contact-inhibited cells and approximately 50% of the exponential cells were in G0/G1-phase at the time of nuclear isolation. Intact nuclei were assayed for replication in the extract by incorporation of [alpha-32P]dATP or biotin-dUTP into nascent DNA. Most nuclei from exponential cells replicated in the egg extract, consistent with previous results showing that intact G1 nuclei from HeLa cells replicate in this system. In contrast, few nuclei from quiescent cells replicated in parallel incubations. However, when the nuclear membranes of these intact quiescent nuclei were permeabilized with lysophosphatidylcholine prior to addition to the extract, nearly all the nuclei replicated under complete cell cycle control in a subsequent incubation. The ability of LPC-treated quiescent nuclei to undergo DNA replication was reversed by resealing permeable nuclear membranes with Xenopus egg membranes prior to extract incubation demonstrating that the effect of LPC treatment is at the level of the nuclear membrane. These results indicate that nuclei from G1-phase cells lose their capacity to initiate DNA replication following density-dependent growth arrest and suggest that changes in nuclear membrane permeability may be required for the initiation of replication upon re-entry of the quiescent cell into the cell cycle.
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Affiliation(s)
- G H Leno
- Department of Biochemistry, University of Mississippi Medical Center, Jackson, 39216-4505
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61
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Reipert S, Reipert BM, Allen TD. Preparation of isolated nuclei from K 562 haemopoietic cell line for high resolution scanning electron microscopy. Microsc Res Tech 1994; 29:54-61. [PMID: 8000085 DOI: 10.1002/jemt.1070290108] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Abstract
The aim of the work is to visualise nuclear pore complexes (NPCs) in mammalian cells by high resolution scanning electron microscopy. A detergent-free isolation protocol was employed to obtain clean nuclei from the haemopoietic cell line K 562. Nuclear isolation was performed by mechanical homogenisation under hypotonic conditions followed by purification of the nuclear fraction. The isolated nuclei were attached to silicon chips, fixed, critical point dried, and sputter coated with a thin film (3-4 nm) of tantalum. Analysis of the nuclear surface by scanning electron microscopy (SEM) revealed a strong sensitivity of the outer nuclear membrane (ONM) to disruption during the isolation procedure. A significant reduction of the characteristic pattern of damage to the ONM was achieved by means of an isopicnic centrifugation on an isoosmolar balanced Percoll gradient. Analysis of the population of isolated nuclei by flow cytometry showed no signs of cell cycle specific losses of nuclei during isolation. The SEM investigations of the morphology of the nuclear envelope (NE) and of substructural details of NPCs and polyribosomes were performed using an in-lens field emission scanning electron microscope.
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Affiliation(s)
- S Reipert
- CRC Department of Structural Cell Biology, Paterson Institute for Cancer Research, Christie Hospital NHS Trust, Manchester, United Kingdom
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62
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Diffley JF, Cocker JH, Dowell SJ, Rowley A. Two steps in the assembly of complexes at yeast replication origins in vivo. Cell 1994; 78:303-16. [PMID: 8044842 DOI: 10.1016/0092-8674(94)90299-2] [Citation(s) in RCA: 452] [Impact Index Per Article: 15.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Abstract
The integration of chromosomal DNA replication into the eukaryotic cell cycle might involve temporal regulation of interactions between cellular factors and replication origins. We show here that yeast replication origins exist in two chromatin states during the cell cycle. In the postreplicative state, genomic footprints closely resemble those produced in vitro by the purified ORC and ABF1 proteins, indicating that the binding of these proteins to replication origins is not sufficient to drive the initiation of DNA replication. The prereplicative state is characterized by an additional region of protection overlapping the ORC footprint. This prereplicative complex appears near the end of mitosis and persists through G1. After entry into S phase, origins return to the postreplicative state. Similarities in temporal regulation of the prereplicative state and the Xenopus licensing factor suggest that mechanisms limiting DNA replication to once per cell cycle may be conserved among eukaryotes.
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Affiliation(s)
- J F Diffley
- Imperial Cancer Research Fund, Clare Hall Laboratories, South Mimms, Hertfordshire, England
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63
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Abstract
The past year has seen the genetic characterization of a human replication origin as well as the identification and characterization of some key components of replication initiation complexes in budding yeast. These results should provide important information for determining how the initial events in DNA replication are regulated during the eukaryotic cell cycle.
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Affiliation(s)
- J F Diffley
- ICRF Clare Hall Laboratories, South Mimms, UK
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64
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Abstract
DNA replication occurs only once in each normal mitotic cell cycle. To explain this strict control, a 'licensing factor' was proposed to enter the nucleus periodically as the nuclear envelope disintegrates and reassembles at the end of mitosis. Inactivation of licensing factor immediately following initiation of DNA synthesis would prevent reinitiation until after the next mitosis. The MCM2-3-5 proteins of Saccharomyces cerevisiae may be yeast's equivalent of licensing factor: they are present in the nucleus only between M and S phase, bind to chromatin and are important for the initiation of DNA replication.
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Affiliation(s)
- B K Tye
- Section of Biochemistry, Molecular and Cell Biology, Cornell University, Ithaca, NY 14853, USA
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65
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Rowley A, Dowell SJ, Diffley JF. Recent developments in the initiation of chromosomal DNA replication: a complex picture emerges. BIOCHIMICA ET BIOPHYSICA ACTA 1994; 1217:239-56. [PMID: 8148369 DOI: 10.1016/0167-4781(94)90283-6] [Citation(s) in RCA: 47] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/29/2023]
Affiliation(s)
- A Rowley
- ICRF Clare Hall, Chromosome Replication Laboratory, South Mimms, Herts., UK
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66
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67
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Blow JJ. Preventing re-replication of DNA in a single cell cycle: evidence for a replication licensing factor. J Cell Biol 1993; 122:993-1002. [PMID: 8354699 PMCID: PMC2119628 DOI: 10.1083/jcb.122.5.993] [Citation(s) in RCA: 158] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/30/2023] Open
Abstract
Xenopus egg extracts treated with the protein kinase inhibitor 6-dimethylaminopurine (6-DMAP) are unable to support the initiation of DNA replication. Nuclei assembled in 6-DMAP extracts behave as though they are in G2, and will not undergo another round of DNA replication until passage through mitosis. 6-DMAP extracts are functionally devoid of a replication factor that modifies chromatin in early G1 before nuclear envelope assembly, but which is itself incapable of crossing the nuclear envelope. This chromatin modification is capable of supporting only a single round of semiconservative replication. The behavior of this replication factor is sufficient to explain why eukaryotic DNA is replicated once and only once in each cell cycle, and conforms to the previous model of a Replication Licensing Factor. Cell cycle analysis shows that this putative Licensing Factor is inactive during metaphase, but becomes rapidly activated on exit from metaphase when it can modify chromatin before nuclear envelope assembly is complete.
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Affiliation(s)
- J J Blow
- ICRF Clare Hall Laboratories, Potters Bar, Herts, England
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