Expression, phosphorylation and nuclear localization of the human P1 protein, a homologue of the yeast Mcm 3 replication protein

Drug-Free Approach To Study the Unusual Cell Cycle of Giardia intestinalis

Giardia intestinalis is a protozoan parasite that causes giardiasis, a form of severe and infectious diarrhea. Despite the importance of the cell cycle in the control of proliferation and differentiation during a giardia infection, it has been difficult to study this process due to the absence of a synchronization procedure that would not induce cellular damage resulting in artifacts. We utilized counterflow centrifugal elutriation (CCE), a size-based separation technique, to successfully obtain fractions of giardia cultures enriched in G₁, S, and G₂. Unlike drug-induced synchronization of giardia cultures, CCE did not induce double-stranded DNA damage or endoreplication. We observed increases in the appearance and size of the median body in the cells from elutriation fractions corresponding to the progression of the cell cycle from early G₁ to late G₂. Consequently, CCE could be used to examine the dynamics of the median body and other structures and organelles in the giardia cell cycle. For the cell cycle gene expression studies, the actin-related gene was identified by the program geNorm as the most suitable normalizer for reverse transcription-quantitative PCR (RT-qPCR) analysis of the CCE samples. Ten of 11 suspected cell cycle-regulated genes in the CCE fractions have expression profiles in giardia that resemble those of higher eukaryotes. However, the RNA levels of these genes during the cell cycle differ less than 4-fold to 5-fold, which might indicate that large changes in gene expression are not required by giardia to regulate the cell cycle. IMPORTANCE Giardias are among the most commonly reported intestinal protozoa in the world, with infections seen in humans and over 40 species of animals. The life cycle of giardia alternates between the motile trophozoite and the infectious cyst. The regulation of the cell cycle controls the proliferation of giardia trophozoites during an active infection and contains the restriction point for the differentiation of trophozoite to cyst. Here, we developed counterflow centrifugal elutriation as a drug-free method to obtain fractions of giardia cultures enriched in cells from the G₁, S, and G₂ stages of the cell cycle. Analysis of these fractions showed that the cells do not show side effects associated with the drugs used for synchronization of giardia cultures. Therefore, counterflow centrifugal elutriation would advance studies on key regulatory events during the giardia cell cycle and identify potential drug targets to block giardia proliferation and transmission.

Post-transcriptional homeostasis and regulation of MCM2-7 in mammalian cells

The MiniChromosome Maintenance 2-7 (MCM2-7) complex provides essential replicative helicase function. Insufficient MCMs impair the cell cycle and cause genomic instability (GIN), leading to cancer and developmental defects in mice. Remarkably, depletion or mutation of one Mcm can decrease all Mcm levels. Here, we use mice and cells bearing a GIN-causing hypomophic allele of Mcm4 (Chaos3), in conjunction with disruption alleles of other Mcms, to reveal two new mechanisms that regulate MCM protein levels and pre-RC formation. First, the Mcm4^Chaos3 allele, which disrupts MCM4:MCM6 interaction, triggers a Dicer1 and Drosha-dependent ∼40% reduction in Mcm2–7 mRNAs. The decreases in Mcm mRNAs coincide with up-regulation of the miR-34 family of microRNAs, which is known to be Trp53-regulated and target Mcms. Second, MCM3 acts as a negative regulator of the MCM2–7 helicase in vivo by complexing with MCM5 in a manner dependent upon a nuclear-export signal-like domain, blocking the recruitment of MCMs onto chromatin. Therefore, the stoichiometry of MCM components and their localization is controlled post-transcriptionally at both the mRNA and protein levels. Alterations to these pathways cause significant defects in cell growth reflected by disease phenotypes in mice.

Proteomic analysis of the mouse mammary gland is a powerful tool to identify novel proteins that are differentially expressed during mammary development

After lactation, the mouse mammary gland undergoes apoptosis and tissue remodelling as the gland reverts to its prepregnant state. This complex change was investigated using 2-DE. An integrated database was produced from lactation and involution proteomes. Forty-four molecular cluster indexes (MCIs) that showed altered expression from lactation to involution were selected for MS analysis. Of these, 32 gave protein annotations, 18 of which were unequivocal proteins. Selected proteins were then studied across all of development, including pregnancy, using data integrated from another proteome database. Two proteins, the RNA polymerase B transcription factor 3 (BTF3) and the minichromosome maintenance protein 3 (MCM3), although initially selected on the basis of the lactation/involution criteria, had expression profiles that indicated an additional role in mammary development and were further analysed. BTF3, a transcription factor previously not described in the mammary gland, was up-regulated strongly in pregnancy, indicating an involvement in alveolar growth. MCM3's expression was greatest in pregnancy and late involution, decreasing through lactation. Immunohistochemistry localised MCM3 to the mammary epithelium, where a greater proportion of cells stained than for the proliferation marker Ki67. MCM3 expression during lactation may identify cells that are licensed to repopulate the gland during cell loss in lactation and following involution.

The Regulated Association of Cdt1 with Minichromosome Maintenance Proteins and Cdc6 in Mammalian Cells

Chromosomal DNA replication requires the recruitment of the six-subunit minichromosome maintenance (Mcm) complex to chromatin through the action of Cdc6 and Cdt1. Although considerable work has described the functions of Cdc6 and Cdt1 in yeast and biochemical systems, evidence that their mammalian counterparts are subject to distinct regulation suggests the need to further explore the molecular relationships involving Cdc6 and Cdt1. Here we demonstrate that Cdc6 and Cdt1 are mutually dependent on one another for loading Mcm complexes onto chromatin in mammalian cells. The association of Cdt1 with Mcm2 is regulated by cell growth. Mcm2 prepared from quiescent cells associates very weakly with Cdt1, whereas Mcm2 from serum-stimulated cells associates with Cdt1 much more efficiently. Cdc6, which normally accumulates as cells progress from quiescence into G(1), is capable of inducing the binding of Mcm2 to Cdt1 when ectopically expressed in quiescent cells. We further show that Cdc6 physically associates with Cdt1 via its N-terminal noncatalytic domain, a region we had previously shown to be essential for Cdc6 function. Cdt1 activity is inhibited by the geminin protein, and we provide evidence that the mechanism of this inhibition involves blocking the binding of Cdt1 to both Mcm2 and Cdc6. These results identify novel molecular functions for both Cdc6 and geminin in controlling the association of Cdt1 with other components of the replication apparatus and indicate that the association of Cdt1 with the Mcm complex is controlled as cells exit and reenter the cell cycle.

Eukaryotic MCM proteins: beyond replication initiation

The minichromosome maintenance (or MCM) protein family is composed of six related proteins that are conserved in all eukaryotes. They were first identified by genetic screens in yeast and subsequently analyzed in other experimental systems using molecular and biochemical methods. Early data led to the identification of MCMs as central players in the initiation of DNA replication. More recent studies have shown that MCM proteins also function in replication elongation, probably as a DNA helicase. This is consistent with structural analysis showing that the proteins interact together in a heterohexameric ring. However, MCMs are strikingly abundant and far exceed the stoichiometry of replication origins; they are widely distributed on unreplicated chromatin. Analysis of mcm mutant phenotypes and interactions with other factors have now implicated the MCM proteins in other chromosome transactions including damage response, transcription, and chromatin structure. These experiments indicate that the MCMs are central players in many aspects of genome stability.

Complex protein-DNA dynamics at the latent origin of DNA replication of Epstein-Barr virus

The sequential binding of the origin recognition complex (ORC), Cdc6p and the minichromosome maintenance proteins (MCM2-7) mediates replication competence at eukaryotic origins of DNA replication. The latent origin of Epstein-Barr virus, oriP, is a viral origin known to recruit ORC. OriP also binds EBNA1, a virally encoded protein that lacks any activity predicted to be required for replication initiation. Here, we used chromatin immunoprecipitation and chromatin binding to compare the cell-cycle-dependent binding of pre-RC components and EBNA1 to oriP and to global cellular chromatin. Prereplicative-complex components such as the Mcm2p-Mcm7p proteins and HsOrc1p are regulated in a cell-cycle-dependent fashion, whereas other HsOrc subunits and EBNA1 remain constantly bound. In addition, HsOrc1p becomes sensitive to the 26S proteasome after release from DNA during S phase. These results show that the complex protein-DNA dynamics at the viral oriP are synchronized with the cell division cycle. Chromatin-binding and chromatin-immunoprecipitation experiments on G0 arrested cells indicated that the ORC core complex (ORC2-5) and EBNA1 remain bound to chromatin and oriP. HsOrc6p and the MCM2-7 complex are released in resting cells. HsOrc1p is partly liberated from chromatin. Our data suggest that origins remain marked in resting cells by the ORC core complex to ensure a rapid and regulated reentry into the cell cycle. These findings indicate that HsOrc is a dynamic complex and that its DNA binding activity is regulated differently in the various stages of the cell cycle.

HIV-1 integrase forms stable tetramers and associates with LEDGF/p75 protein in human cells

We studied human immunodeficiency virus, type 1 (HIV-1) integrase (IN) complexes derived from nuclei of human cells stably expressing the viral protein from a synthetic gene. We show that in the nuclear extracts IN exists as part of a large distinct complex with an apparent Stokes radius of 61 A, which dissociates upon dilution yielding a core molecule of 41 A. We isolated the IN complexes from cells expressing FLAG-tagged IN and demonstrated that the 41 A core is a tetramer of IN, whereas 61 A molecules are composed of IN tetramers associated with a cellular protein with an apparent molecular mass of 76 kDa. This novel integrase interacting protein was found to be identical to lens epithelium-derived growth factor (LEDGF/p75), a protein implicated in regulation of gene expression and cellular stress response. HIV-1 IN and LEDGF co-localized in the nuclei of human cells stably expressing IN. Furthermore, recombinant LEDGF robustly enhanced strand transfer activity of HIV-1 IN in vitro. Our findings indicate that the minimal IN molecule in human cells is a homotetramer, suggesting that at least an octamer of IN is required to accomplish coordinated integration of both retroviral long terminal repeats and that LEDGF is a cellular factor involved in this process.

Identification of a binding region for human origin recognition complex proteins 1 and 2 that coincides with an origin of DNA replication

We investigated the binding regions of components of the origin recognition complex (ORC) in the human genome. For this purpose, we performed chromatin immunoprecipitation assays with antibodies against human Orc1 and Orc2 proteins. We identified a binding region for human Orc proteins 1 and 2 in a <1-kbp segment between two divergently transcribed human genes. The region is characterized by CpG tracts and a central sequence rich in AT base pairs. Both, Orc1 and Orc2 proteins are found at the intergenic region in the G(1) phase, but S-phase chromatin contains only Orc2 protein, supporting the notion that Orc1p dissociates from its binding site in the S phase. Sequences corresponding to the intergenic region are highly abundant in a fraction of nascent DNA strands, strongly suggesting that this region not only harbors the binding sites for Orc1 protein and Orc2 protein but also serves as an origin of bidirectional DNA replication.

Glycosylated and phosphorylated proteins--expression in yeast and oocytes of Xenopus: prospects and challenges--relevance to expression of thermostable proteins

Phosphorylation and glycosylation are important posttranslational events in the biosynthesis of proteins. The different degrees of phosphorylation and glycosylation of proteins have been an intriguing phenomenon. Advances in genetic engineering have made it possible to control the degree of glycosylation and phosphorylation of proteins. Structural biology of phosphorylated and glycosylated proteins has been advancing at a much slower pace due to difficulties in using high-resolution NMR studies in solution phase. Major difficulties have arisen from the inherent mobilities of phosphorylated and glycosylated side chains. This paper reviews molecular and structural biology of phosphorylated and glycosylated proteins expressed in eukaryotic expression systems which are especially suited for large-scale production of these proteins. In our laboratory, we have observed that eukaryotic expression systems are particularly suited for the expression of thermostable light-activated proteins, e.g., bacteriorhodopsins and plastocyanins.

Molecular cloning of a mammalian nuclear phosphoprotein NUCKS, which serves as a substrate for Cdk1 in vivo

We have isolated and characterized a cDNA encoding a mammalian nuclear phosphoprotein NUCKS, previously designated P1. Molecular analyses of several overlapping and full-length cDNAs from HeLa cells and rat brain revealed a protein with an apparent molecular mass of 27 kDa in both species. The deduced amino-acid sequences are highly conserved between human and rodents, but show no homology with primary structures in protein databases or with translated sequences of cDNAs in cDNA databanks. Although the protein has some features in common with the high mobility group proteins HMGI/Y, attempts to find a putative protein family by database query using both sequence alignment methods and amino-acid composition have failed. Northern blot analyses revealed that human and rat tissues contain three NUCKS transcripts varying in size from 1.5 to 6.5 kb. All human and rat tissues express the gene, but the level of transcripts varies among different tissues. Circular dichroism analysis and secondary structure predictions based on the amino-acid sequence indicate a low level of alpha helical content and substantial amounts of beta turn structures. The protein is phosphorylated in all phases of the cell cycle and exhibits mitosis-specific phosphorylation of threonine residues. Phosphopeptide mapping and back-phosphorylation experiments employing NUCKS from HeLa interphase and metaphase cells show that the protein is phosphorylated by Cdk1 during mitosis of the cell cycle.

The proliferation-specific human Ki-67 protein is a constituent of compact chromatin

The human nuclear Ki-67 protein (Ki-67p) is expressed in proliferating, but not in quiescent, cells and is therefore widely used as a proliferation marker in histopathological research and practice. However, information regarding its intranuclear location is scarce and controversial. Here we describe the results of cell fractionation and nuclease digestion experiments using nuclei isolated from human HeLa cells in interphase. Ki-67p dissociates at 0.3-0.4 M NaCl from its nuclear binding sites, and gradient centrifugations indicate that the released Ki-67p is most likely a single molecular entity and not complexed to other proteins. In nuclei, prepared under physiological salt conditions, the binding sites are largely resistant against micrococcal nuclease. However, when prepared at very low ionic strengths, chromatin regions with associated Ki-67p become accessible to micococcal-nuclease-producing chromatin fragments that carry bound Ki-67p. We conclude that Ki-67p is a chromatin protein and resides at densely packed regions, probably heterochromatin. Our data provide a useful basis for further biochemical research on this human nuclear protein.

Chromatin association of human origin recognition complex, cdc6, and minichromosome maintenance proteins during the cell cycle: assembly of prereplication complexes in late mitosis

Evidence obtained from studies with yeast and Xenopus indicate that the initiation of DNA replication is a multistep process. The origin recognition complex (ORC), Cdc6p, and minichromosome maintenance (MCM) proteins are required for establishing prereplication complexes, upon which initiation is triggered by the activation of cyclin-dependent kinases and the Dbf4p-dependent kinase Cdc7p. The identification of human homologues of these replication proteins allows investigation of S-phase regulation in mammalian cells. Using centrifugal elutriation of several human cell lines, we demonstrate that whereas human Orc2 (hOrc2p) and hMcm proteins are present throughout the cell cycle, hCdc6p levels vary, being very low in early G(1) and accumulating until cells enter mitosis. hCdc6p can be polyubiquitinated in vivo, and it is stabilized by proteasome inhibitors. Similar to the case for hOrc2p, a significant fraction of hCdc6p is present on chromatin throughout the cell cycle, whereas hMcm proteins alternate between soluble and chromatin-bound forms. Loading of hMcm proteins onto chromatin occurs in late mitosis concomitant with the destruction of cyclin B, indicating that the mitotic kinase activity inhibits prereplication complex formation in human cells.

MCM proteins in DNA replication

The MCM proteins are essential replication initiation factors originally identified as proteins required for minichromosome maintenance in Saccharomyces cerevisiae. The best known among them are a family of six structurally related proteins, MCM2-7, which are evolutionally conserved in all eukaryotes. The MCM2-7 proteins form a hexameric complex. This complex is a key component of the prereplication complex that assembles at replication origins during early G1 phase. New evidence suggests that the MCM2-7 proteins may be involved not only in the initiation but also in the elongation of DNA replication. Orchestration of the functional interactions between the MCM2-7 proteins and other components of the prereplication complex by cell cycle-dependent protein kinases results in initiation of DNA synthesis once every cell cycle.

The roles of the MCM, ORC, and Cdc6 proteins in determining the replication competence of chromatin in quiescent cells

Most eukaryotic cell types can withdraw from proliferative cell cycles and remain quiescent for extended periods. Intact nuclei isolated from quiescent murine NIH3T3 cells fail to replicate in vitro when incubated in Xenopus egg extracts, although intact nuclei from proliferating cells replicate well. Permeabilization of the nuclear envelope rescues the ability of quiescent nuclei to replicate in the extract. We show that origin replication complex (ORC), minichromosome maintenance (MCM), and Cdc6 proteins are all present in early quiescent cells. Immunodepletion of Cdc6 or the MCM complex from Xenopus egg extract inhibits replication of permeable, quiescent, but not proliferating, NIH3T3 nuclei. Immunoblotting results demonstrate that mouse homologues of Mcm2, Mcm5, and Cdc6 are displaced from chromatin in quiescent cells. However, this absence of chromatin-bound Cdc6 and MCM proteins from quiescent cells appears not to be due to the absence of ORC subunits as murine homologues of Orc1 and Orc2 remain chromatin-bound in quiescent cells. Surprisingly, intact quiescent nuclei fail to bind exogenously added XCdc6 or to replicate in Xenopus egg extracts immunodepleted of ORC, even though G1- or S-phase nuclei still replicate in these extracts. Our results identify Cdc6 and the MCM complex as essential replication components absent from quiescent chromatin due to nonfunctional chromatin-bound ORC proteins. These results can explain why quiescent mammalian nuclei are unable to replicate in vivo and in Xenopus egg extracts.

Rereplication phenomenon in fission yeast requires MCM proteins and other S phase genes

The fission yeast Schizosaccharomyces pombe can be induced to perform multiple rounds of DNA replication without intervening mitoses by manipulating the activity of the cyclin-dependent kinase p34(cdc2). We have examined the role in this abnormal rereplication of a large panel of genes known to be involved in normal S phase. The genes analyzed can be grouped into four classes: (1) those that have no effect on rereplication, (2) others that delay DNA accumulation, (3) several that allow a gradual increase in DNA content but not in genome equivalents, and finally, (4) mutations that completely block rereplication. The rereplication induced by overexpression of the CDK inhibitor Rum1p or depletion of the Cdc13p cyclin is essentially the same and requires the activity of two minor B-type cyclins, cig1(+) and cig2(+). In particular, the level, composition, and localization of the MCM protein complex does not alter during rereplication. Thus rereplication in fission yeast mimics the DNA synthesis of normal S phase, and the inability to rereplicate provides an excellent assay for novel S-phase mutants.

Evidence for different MCM subcomplexes with differential binding to chromatin in Xenopus

MCM proteins are molecular components of the DNA replication licensing system in Xenopus. These proteins comprise a conserved family made up of six distinct members which have been found to associate in large protein complexes. We have used a combination of biochemical and cytological methods to study the association of soluble and chromatin-bound Xenopus MCM proteins during the cell cycle. In interphase, soluble MCM proteins are found organized in a core salt-resistant subcomplex that includes MCM subunits which are known to have high affinity for histones. The interphasic complex is modified at mitosis and the subunit composition of the resulting mitotic subcomplexes is distinct, indicating that the stability of the MCM complex is under cell cycle control. Moreover, we provide evidence that the binding of MCM proteins to chromatin may occur in sequential steps involving the loading of distinct MCM subunits. Comparative analysis of the chromatin distribution of MCM2, 3, and 4 shows that the binding of MCM4 is distinct from that of MCM2 and 3. Altogether, these data suggest that licensing of chromatin by MCMs occurs in an ordered fashion involving discrete subcomplexes.

Improved cervical smear assessment using antibodies against proteins that regulate DNA replication

Carcinoma of the cervix is one of the most common malignancies. Papanicolaou (Pap) smear tests have reduced mortality by up to 70%. Nevertheless their interpretation is notoriously difficult with high false-negative rates and frequently fatal consequences. We have addressed this problem by using affinity-purified antibodies against human proteins that regulate DNA replication, namely Cdc6 and Mcm5. These antibodies were applied to sections and smears of normal and diseased uterine cervix by using immunoperoxidase or immunofluorescence to detect abnormal precursor malignant cells. Antibodies against Cdc6 and Mcm5 stain abnormal cells in cervical smears and sections with remarkably high specificity and sensitivity. Proliferation markers Ki-67 and proliferating cell nuclear antigen are much less effective. The majority of abnormal precursor malignant cells are stained in both low-grade and high-grade squamous intraepithelial lesions. Immunostaining of cervical smears can be combined with the conventional Pap stain so that all the morphological information from the conventional method is conserved. Thus antibodies against proteins that regulate DNA replication can reduce the high false-negative rate of the Pap smear test and may facilitate mass automated screening.

Ser727-dependent recruitment of MCM5 by Stat1alpha in IFN-gamma-induced transcriptional activation

Stat1alpha is a latent cytoplasmic transcription factor activated in response to interferon-gamma (IFN-gamma). The C-terminal 38 amino acids of Stat1alpha are required to trigger transcription and therefore may possibly serve as a transcription activation domain (TAD). Here we show that the C-terminus of Stat1alpha is an independent TAD which can interact with a specific group of nuclear proteins. Mutation of the Stat1 Ser727 and Leu724 decreases its transcriptional activity and affinity for the nuclear proteins. One of the interacting proteins was identified as MCM5, a member of the mini-chromosome maintenance (MCM) family involved in DNA replication. Both in vitro and in vivo interaction of Stat1alpha and MCM5 were demonstrated. Furthermore, the in vitro interaction required Ser727 and was enhanced by its phosphorylation. Transient over-expression of MCM5 enhanced transcriptional activation by Stat1alpha in a Ser727-dependent manner. Finally, changes in the level of nuclear localized MCM5 during the cell cycle correlated with the changes in transcriptional response to IFN-gamma acting through Stat1alpha. These results strongly suggest that MCM5 is recruited through interaction with Stat1alpha in a Ser727- and Leu724-dependent manner to play a role in optimal transcriptional activation.

Mobilization of chromatin-bound Mcm proteins by micrococcal nuclease

Mcm (minichromosome maintenance) proteins are important components of the eukaryotic replication initiation apparatus. We investigate the binding of human Mcm proteins to HeLa cell chromatin using micrococcal nuclease as a tool. In previous work we prepared chromatin under low ionic strength conditions. The use of a low salt buffer was necessary to prevent the dissociation of Mcm proteins. Here we use chromatin prepared at more physiological salt concentrations (100 mM NaCl) following the procedure of Fujita et al. (J. Biol. Chem. 272, 10928-10935; 1997) who had shown that ATP stabilizes the interaction of Mcm proteins with chromatin. We show here that micrococcal nuclease released Mcm proteins early during the digestion process suggesting that Mcm proteins reside on chromatin sites which are more open to nuclease attack than bulk chromatin. Released Mcm proteins sedimented through glycerol gradients as a multiprotein complex comprising several of the six known human Mcm proteins.

Nucleosome assembly activity and intracellular localization of human CAF-1 changes during the cell division cycle

We characterized changes of nucleosome assembly activity, intracellular localization, and reversible phosphorylation of the human chromatin assembly factor CAF-1 during the somatic cell division cycle. HeLa cells were synchronized in the G1, S, G2, and M phases of the cell cycle. All three subunits of human CAF-1 (p150, p60, and p48) are present during the entire cell cycle. In interphase, p150 and p60 are bound to the nucleus, but they predominantly dissociate from chromatin during mitosis. During S phase, p150 and p60 are concentrated at sites of intranuclear DNA replication. Only a fraction of total p48 is associated with p150 and p60, and the majority is present in other high molecular weight complexes. The other nucleosome assembly protein, NAP-1, is predominantly cytosolic throughout the cell cycle. Human CAF-1 efficiently mediates nucleosome assembly during complementary DNA strand synthesis in G1, S, and G2 phase cytosolic extracts. Active CAF-1 can be isolated as a 6.5 S complex from G1, S, and G2 phase nuclei. In contrast, CAF-1 isolated from mitotic cytosol does not support nucleosome assembly during DNA synthesis. In mitosis, the p60 subunit of inactive CAF-1 is hyperphosphorylated, whereas active CAF-1 in interphase contains hypophosphorylated and/or phosphorylated forms of p60.

Stability of the replicative Mcm3 protein in proliferating and differentiating human cells

Mcm proteins are abundant nuclear proteins involved in the regulation of genome replication. Previous experiments had shown that levels of Mcm-specific mRNAs increase at the G1/S phase transition of the cell cycle, but that the amounts of Mcm proteins do not change much during the cell cycle. To learn more about the stability of an Mcm protein we performed experiments which showed that: (i) more than 60% of [35S]methionine pulse-labeled Mcm3 protein appears to be degraded during a 24-h chase in HeLa cells; (ii) the amount of Mcm3 protein significantly decreases during the differentiation of HL60 cells in vitro (whereas another replication-initiation protein, hOrc2, remains fairly constant); and (iii) according to immunohistochemical staining, Mcm3 protein is present in nuclei of cells in the proliferating zone of human epidermal tissue, but in decreasing amounts in nuclei of differentiating cells of the upper cell layers. Our interpretation is that Mcm3 protein is no longer synthesized after initiation of differentiation and slowly disappears at a half-life of approximately 24 h.

DNA replication licensing factor

DNA Replication Licensing Factor (RLF) is an essential activity required to restrict the duplication of genomic DNA to precisely once per cell cycle. Recent fractionation of RLF activity from Xenopus egg extracts has resulted in the identification of two essential components, RLF-B and RLF-M. RLF-M has been purified to homogeneity and has been shown to consist of a complex of proteins in the MCM/P1 family. RLF-B is still unidentified, but possible candidates for this activity have been identified in yeast. Elucidation of the RLF mechanism will provide important insights into the way that chromosome replication is controlled.

Human protein MCM6 on HeLa cell chromatin

Minichromosome maintenance (Mcm) proteins perform essential functions regulating the replication of chromatin. Human cells, like other eukaryotic cells, express at least six Mcm proteins conserved in the central region. We have earlier described the primary structures of five human Mcm proteins, but the primary structure of the sixth human Mcm protein, MCM6, was identified only recently. We now use antibodies, specific for the MCM6 protein, to assess its intranuclear distribution. We find that a fraction of MCM6 protein occurs in the nucleosol, forming multiprotein complexes with other Mcm proteins. More importantly, we use for the first time micrococcal nuclease as a tool to investigate the association of MCM6 protein with chromatin. After short digestion times, a considerable fraction of the MCM6 protein is released from chromatin as a multiprotein complex that includes other Mcm proteins as well. In addition, fractions of MCM3 and MCM6 proteins are released by nuclease digestion as monomeric proteins indicating that at least these two Mcm proteins may also occur as single molecules on chromatin. The data also suggest that the chromatin regions with bound Mcm proteins are more vulnerable to nuclease attack than bulk chromatin and may therefore differ in the arrangement of nucleosomes.

Initiation of DNA replication in eukaryotic cells

The recent identification of proteins that recognize origins of DNA replication and control the initiation of eukaryotic DNA replication has provided critical molecular tools to dissect this process. Dynamic changes in the assembly and disassembly of protein complexes at origins are important for the initiation of DNA replication and occur throughout the cell cycle. Herein, we review the key proteins required for the initiation of DNA replication, their involvement in the protein complex assembly at replication origins, and how the cell cycle machinery regulates this process.

Identification of a preinitiation step in DNA replication that is independent of origin recognition complex and cdc6, but dependent on cdk2

Before initiation of DNA replication, origin recognition complex (ORC) proteins, cdc6, and minichromosome maintenance (MCM) proteins bind to chromatin sequentially and form preinitiation complexes. Using Xenopus laevis egg extracts, we find that after the formation of these complexes and before initiation of DNA replication, cdc6 is rapidly removed from chromatin, possibly degraded by a cdk2-activated, ubiquitin-dependent proteolytic pathway. If this displacement is inhibited, DNA replication fails to initiate. We also find that after assembly of MCM proteins into preinitiation complexes, removal of the ORC from DNA does not block the subsequent initiation of replication. Importantly, under conditions in which both ORC and cdc6 protein are absent from preinitiation complexes, DNA replication is still dependent on cdk2 activity. Therefore, the final steps in the process leading to initiation of DNA replication during S phase of the cell cycle are independent of ORC and cdc6 proteins, but dependent on cdk2 activity.

High-molecular-mass complexes of human minichromosome-maintenance proteins in mitotic cells

Minichromosome-maintenance (Mcm) proteins perform essential functions regulating the replication of eukaryotic genomes. In interphase cells they are either bound to a nuclear structure, most probably chromatin, or occur as free multiprotein complexes in the nucleoplasm. Mcm proteins are displaced from their chromatin sites during S phase, and several become highly phosphorylated during mitosis. We investigated whether phosphorylation affects the ability of mitotic Mcm proteins to form multiprotein complexes. Our results clearly show that phosphorylated mitotic Mcm proteins form a 14-15-S complex, probably consisting of one molecule each of the six known human Mcm proteins.

Cell cycle control of replication initiation in eukaryotes

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.

Binding of human minichromosome maintenance proteins with histone H3

Minichromosome maintenance (MCM) proteins play essential roles in eukaryotic DNA replication, but their biochemical properties remain to be determined. We detected in HeLa cell extracts six proteins, CDC47, CDC46/MCM5, Cdc21, P1/MCM3, Mis5, and BM28/MCM2, by their binding to a specific antibody and by partial sequencing. The human homologs of the MCM2 (BM28), Mis5, Cdc21, and CDC47 proteins were tightly bound to a histone-Sepharose column and purified to near homogeneity, whereas the P1/MCM3 and CDC46/MCM5 proteins passed through. Among the four core histones, the human BM28/MCM2, Mis5, Cdc21, and CDC47 proteins had high affinity for histone H3. Immunoprecipitation with anti-Cdc21 antibody revealed that these four MCM proteins form complexes. These results are consistent with the findings that MCM proteins bind with chromatin in vivo.

cDNA cloning and characterisation of a maize homologue of the MCM proteins required for the initiation of DNA replication

A central question in cell cycle regulation is how DNA replication is initiated and executed only once in each cell cycle. The cell cycle-regulated assembly of specific initiation protein complexes at chromosomal origins appears to specify the initial sites and timing of DNA replication, and to restrict this process to only one round in the somatic cell cycle. Among the enzymes involved in origin activation, the MCM proteins play a conserved key role. In particular, MCM3 homologues have been shown to be components of the DNA replication licensing activity in yeast and vertebrates. In spite of our detailed knowledge of the regulation of the initiation of DNA synthesis in yeast, there is virtually no information available on the molecules involved in origin activation in higher plants. We have isolated a cDNA from maize root apices, termed ROA (Replication Origin Activator), encoding a protein which shares a high degree of homology with the MCM3 subfamily of MCM proteins. Analysis of gene organisation by Southern blotting shows 2-4 copies per haploid genome of closely related ROA sequences and the presence of further less related sequences in a multigene family. The steady-state levels of ROA mRNA are under developmental control, being relatively high in proliferative tissues such as the root apex, the developing cob and the coleoptile, and are strongly correlated with that of the histone H4 transcript. In situ hybridisation analysis in the root apex reveals that ROA mRNA expression is limited to specific subpopulations of cycling cells, which is typical of cell cycle-regulated expression. The isolation of nearly identical sequences from barley and Arabidopsis by the polymerase chain reaction indicates that MCM-related proteins are conserved in higher plants.

Mechanisms restricting DNA replication to once per cell cycle: MCMS, pre-replicative complexes and kinases

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.

The role of MCM proteins in the cell cycle control of genome duplication

The regulatory mechanism which ensures that eukaryotic chromosomes replicate precisely once per cell cycle is a basic and essential cellular property of eukaryotes. This fundamental aspect of DNA replication is still poorly understood, but recent advances encourage the view that we may soon have a clearer picture of how this regulation is achieved. This review will discuss in particular the role of proteins in the minichromosome maintenance (MCM) family, which may hold the key to understanding how DNA is replicated once, and only once, per cell cycle.

hCDC47, a human member of the MCM family. Dissociation of the nucleus-bound form during S phase

hCDC47 is a putative human homologue of yeast CDC47 and a member of the MCM protein family, which has been implicated in the regulatory machinery causing DNA to replicate only once in the S phase. In the present study, we performed an initial characterization of hCDC47. We found that hCDC47 protein was present in the nucleus of cultured human cells in two different forms; one extractable by a non-ionic detergent and the other resistant to such extraction and tightly associated with the nucleus. The levels of the nucleus-bound form gradually diminished during S phase progression, although the total amount of nuclear hCDC47 protein remained relatively constant, suggesting that the nucleus-bound form becomes dissociated from the nuclear structure during DNA replication. This behavior of hCDC47 protein is very similar to that of other mammalian MCM proteins reported recently. We also found that expression of hCDC47 mRNA was repressed in quiescent cells but was induced at the late G1 to S phase by growth factor stimulation. Together, these findings indicate that hCDC47 protein together with other MCM proteins participates in the regulation of mammalian DNA replication.

Human replication proteins hCdc21, hCdc46 and P1Mcm3 bind chromatin uniformly before S-phase and are displaced locally during DNA replication

Members of the Mcm-protein family have recently been shown to be involved in restricting DNA replication to a single cycle in Xenopus laevis egg extracts. In this study, we extended these observations to human somatic cells and analysed the localisation of the human Mcm-proteins Cdc21, Cdc46 and P1Mcm3 in replicating HeLa cell nuclei. These Mcm-proteins are entirely nuclear in interphase cells and apparently exist in two populations: a nucleosolic population, and a population bound to a nuclear structure, most likely chromatin. The bound population is detected throughout the nucleus in late G1 and early S, and at discrete subnuclear sites following further progression of S-phase. We use high resolution confocal microscopy to determine the subnuclear sites of chromatin-bound Mcm proteins in comparison to the sites of replicating DNA. Importantly, hCdc21, hCdc46 and P1Mcm3 do not colocalise with replication foci, instead these proteins appear to coincide with subnuclear sites of unreplicated chromatin. During progression of S-phase hCdc21, hCdc46 and P1Mcm3 are displaced from their site on chromatin at the time when this site is replicated. Consequently, early replicating sites do not contain bound hCdc21, hCdc46 or P1Mcm3 during later stages of S-phase. Furthermore, G2 nuclei and condensed chromatin in mitotic cells do not contain bound hCdc21, hCdc46 or P1Mcm3. Thus, the human Mcm-proteins Cdc21, Cdc46 and P1Mcm3 are not concentrated at sites of DNA replication. Instead, they appear to be present only on unreplicated chromatin and are displaced from replicating chromatin, consistent with a role in monitoring unreplicated chromatin and ensuring only a single round of DNA replication per cell cycle.

Properties of the human nuclear protein p85Mcm. Expression, nuclear localization and interaction with other Mcm proteins

Recently we identified a cDNA fragment encoding a conserved part of a new human minichromosome maintenance (Mcm) protein, provisionally termed P1.1Mcm3. Here, we report that the protein is most highly related to a yeast cell-division-cycle protein, Cdc47, encoded by the open reading frame YBR1441 on chromosome 11 of Saccharomyces cerevisiae. The human protein migrates on a polyacrylamide gel with an apparent molecular mass of 85 kDa and shares areas of significant similarity with the Mcm family of replication proteins. It is, therefore, designated as p85Mcm. Microscopic immuno-fluorescence studies revealed that protein p85Mcm is located in the nuclei of interphase cells, but is evenly distributed throughout the cell during mitosis. The amounts of p85Mcm do not significantly change during the cell cycle, but mRNA levels rise with the beginning of the S phase. However, in vitro differentiation of HL60 cells results in a striking decrease of both p85Mcm mRNA and protein levels, suggesting a role for p85Mcm in proliferating, but not in differentiated cells. Under physiological salt conditions, p85Mcm is a component of a high molecular-mass complex including other Mcm proteins. The complex dissociates at high ionic strength giving rise to stable subcomplexes, one of which contains protein p85Mcm together with Mcm proteins hCdc21 and p1O5Mcm.

DNA replication. Almost licensed

Exact duplication of all the DNA in a cell occurs during each S phase, and only once in each cell cycle. Recent results show that conserved proteins of the MCM family contribute to these precisely regulated events.

A human homologue of the yeast replication protein Cdc21. Interactions with other Mcm proteins

We present the amino acid sequence of the human homologue of the yeast replication protein Cdc21, a member of the Mcm family of nuclear proteins. Specific antibodies, raised against protein hCdc21, were used to investigate the expression of the protein through the cell cycle. The protein is highly phosphorylated in mitotic cells. The phosphorylated form of protein hCdc21 appears to be less tightly bound to nuclear structures than the underphosphorylated form suggesting that phosphorylation/dephosphorylation reactions may determine the nuclear distribution of the protein. Protein hCdc21 forms a stable trimeric complex with two novel human Mcm proteins, p85Mcm and p105Mcm. Protein BM28/Mcm2 is more loosely associated with the trimeric hCdc21 complex.