Cell cycle-regulated nuclear localization of MCM2 and MCM3, which are required for the initiation of DNA synthesis at chromosomal replication origins in yeast

Mcm3 is polyubiquitinated during mitosis before establishment of the pre-replication complex

To ensure fidelity in genome duplication, eukaryotes restrict DNA synthesis to once every cell division by a cascade of regulated steps. Central to this cascade is the periodic assembly of the hexameric MCM2-7 complex at replication origins. However, in Saccharomyces cerevisiae, only a fraction of each MCM protein is able to assemble into hexamers and associate with replication origins during M phase, suggesting that MCM complex assembly and recruitment may be regulated post-translationally. Here we show that a small fraction of Mcm3p is polyubiquitinated at the onset of MCM complex assembly. Reducing the rate of ubiquitination by uba1-165, a suppressor of mcm3-10, restored the interaction of Mcm3-10p with subunits of the MCM complex and its recruitment to the replication origin. Possible roles for ubiquitinated Mcm3p in the assembly of the MCM complex at replication origins are discussed.

Human Mcm proteins at a replication origin during the G1 to S phase transition

Previous work with yeast cells and with Xenopus egg extracts had shown that eukaryotic pre-replication complexes assemble on chromatin in a step-wise manner whereby specific loading factors promote the recruitment of essential Mcm proteins at pre-bound origin recognition complexes (ORC with proteins Orc1p-Orc6p). While the order of assembly--Mcm binding follows ORC binding--seems to be conserved in cycling mammalian cells in culture, it has not been determined whether mammalian Mcm proteins associate with ORC-bearing chromatin sites. We have used a chromatin immunoprecipitation approach to investigate the site of Mcm binding in a genomic region that has previously been shown to contain an ORC-binding site and an origin of replication. Using chromatin from HeLa cells in G1 phase, antibodies against Orc2p as well as antibodies against Mcm proteins specifically immunoprecipitate chromatin enriched for a DNA region that includes a replication origin. However, with chromatin from cells in S phase, only Orc2p-specific antibodies immunoprecipitate the origin-containing DNA region while Mcm-specific antibodies immunoprecipitate chromatin with DNA from all parts of the genomic region investigated. Thus, human Mcm proteins first assemble at or adjacent to bound ORC and move to other sites during genome replication.

Two mcm3 mutations affect different steps in the initiation of DNA replication

Mcm3 is a subunit of the hexameric MCM2-7 complex required for the initiation and elongation of DNA replication in eukaryotes. We have characterized two mutant alleles, mcm3-1 and mcm3-10, in Saccharomyces cerevisiae and showed that they are defective at different steps of the replication initiation process. Mcm3-10 contains a P118L substitution that compromises its interaction with Mcm5 and the recruitment of Mcm3 and Mcm7 to a replication origin. P118 is conserved between Mcm3, Mcm4, Mcm5, and Mcm7. An identical substitution of this conserved residue in Mcm5 (P83L of mcm5-bob1) strengthens the interaction between Mcm3 and Mcm5 and allows cells to enter S phase independent of Cdc7-Dbf4 kinase (Hardy, C. F., Dryga, O., Pahl, P. M. B., and Sclafani, R. A. (1997) Proc. Natl. Acad. Sci. U. S. A. 94, 3151-3155). Mcm3-1 contains a G246E mutation that diminishes the efficiency of replication initiation (Yan, H., Merchant, A. M., and Tye, B. K. (1993) Genes Dev. 7, 2149-2160) but not its interaction with Mcm5 or recruitment of the MCM2-7 complex to replication origin. These observations indicate that Mcm3-10 is defective in a step before, and Mcm3-1 is defective in a step after the recruitment of the MCM2-7 complex to replication origins.

Noc3p, a bHLH protein, plays an integral role in the initiation of DNA replication in budding yeast

Initiation of eukaryotic DNA replication requires many proteins that interact with one another and with replicators. Using a yeast genetic screen, we have identified Noc3p (nucleolar complex-associated protein) as a novel replication-initiation protein. Noc3p interacts with MCM proteins and ORC and binds to chromatin and replicators throughout the cell cycle. It functions as a critical link between ORC and other initiation proteins to effect chromatin association of Cdc6p and MCM proteins for the establishment and maintenance of prereplication complexes. Noc3p is highly conserved in eukaryotes and is the first identified bHLH (basic helix-loop-helix) protein required for replication initiation. As Noc3p is also required for pre-rRNA processing, Noc3p is a multifunctional protein that plays essential roles in two vital cellular processes.

Genes involved in the initiation of DNA replication in yeast

Replication and segregation of the information contained in genomic DNA are strictly regulated processes that eukaryotic cells alternate to divide successfully. Experimental work on yeast has suggested that this alternation is achieved through oscillations in the activity of a serine/threonine kinase complex, CDK, which ensures the timely activation of DNA synthesis. At the same time, this CDK-mediated activation sets up the basis of the mechanism that ensures ploidy maintenance in eukaryotes. DNA synthesis is initiated at discrete sites of the genome called origins of replication on which a prereplicative complex (pre-RC) of different protein subunits is formed during the G1 phase of the cell division cycle. Only after pre-RCs are formed is the genome competent to be replicated. Several lines of evidence suggest that CDK activity prevents the assembly of pre-RCs ensuring single rounds of genome replication during each cell division cycle. This review offers a descriptive discussion of the main molecular events that a unicellular eukaryote such as the budding yeast Saccharomyces cerevisiae undergoes to initiate DNA replication.

Proteome analysis of nuclear matrix proteins during apoptotic chromatin condensation

The nuclear matrix (NM) is considered a proteinaceous scaffold spatially organizing the interphase nucleus, the integrity of which is affected during apoptosis. Caspase-mediated degradation of NM proteins, such as nuclear lamins, precedes apoptotic chromatin condensation (ACC). Nevertheless, other NM proteins remain unaffected, which most likely maintain a remaining nuclear structure devoid of chromatin. We, therefore, screened various types of apoptotic cells for changes of the nuclear matrix proteome during the process of apoptotic ACC. Expectedly, we observed fundamental alterations of known chromatin-associated proteins, comprising both degradation and translocation to the cytosol. Importantly, a consistent set of abundant NM proteins, some (e.g. hNMP 200) of which displaying structural features, remained unaffected during apoptosis and might therefore represent constituents of an elementary scaffold. In addition, proteins involved in DNA replication and DNA repair were found accumulated in the NM fraction before cells became irreversibly committed to ACC, a time point characterized in detail by inhibitor studies with orthovanadate. In general, protein alterations of a consistent set of NM proteins (67 of which were identified), were reproducibly detectable in Fas-induced Jurkat cells, in UV-light treated U937 cells and also in staurosporine-treated HeLa cells. Our data indicate that substantial alterations of proteins linking chromatin to an elementary nuclear protein scaffold might play an intriguing role for the process of ACC.

Interdependent nuclear accumulation of budding yeast Cdt1 and Mcm2-7 during G1 phase

Cdt1 is essential for loading Mcm2-7 proteins into prereplicative complexes (pre-RCs) during replication licensing and has been found in organisms as diverse as fission yeast and humans. We have identified a homologue of Cdt1 in Saccharomyces cerevisiae, which is required for pre-RC assembly. We show that, like Mcm2-7p, Cdt1p accumulates in the nucleus during G1 phase and is excluded from the nucleus later in the cell cycle by cyclin dependent kinases (cdks). Cdt1p interacts with the Mcm2--7p complex, and the nuclear accumulation of these proteins during G1 is interdependent. This coregulation of Cdt1p and Mcm2-7p represents a novel level of pre-RC control.

Cyclin A- and cyclin E-Cdk complexes shuttle between the nucleus and the cytoplasm

Cyclins A and E and their partner cyclin-dependent kinases (Cdks) are key regulators of DNA synthesis and of mitosis. Immunofluorescence studies have shown that both cyclins are nuclear and that a proportion of cyclin A is localized to sites of DNA replication. However, recently, both cyclin A and cyclin E have been implicated as regulators of centrosome replication, and it is unclear when and where these cyclin-Cdks can interact with cytoplasmic substrates. We have used live cell imaging to study the behavior of cyclin/Cdk complexes. We found that cyclin A and cyclin E are able to regulate both nuclear and cytoplasmic events because they both shuttle between the nucleus and the cytoplasm. However, we found that there are marked differences in their shuttling behavior, which raises the possibility that cyclin/Cdk function could be regulated at the level of nuclear import and export. In the course of these experiments, we have also found that, contrary to published results, mutations in the hydrophobic patch of cyclin A do affect Cdk binding and nuclear import. This has implications for the role of the hydrophobic patch as a substrate selection motif.

Analysis of Cdc6 function in the assembly of mammalian prereplication complexes

Eukaryotic DNA replication requires the previous formation of a prereplication complex containing the ATPase Cdc6 and the minichromosome maintenance (Mcm) complex. Although considerable insight has been gained from in vitro studies and yeast genetics, the functional analysis of replication proteins in intact mammalian cells has been lacking. We have made use of adenoviral vectors to express normal and mutant forms of Cdc6 in quiescent mammalian cells to assess function. We demonstrate that Cdc6 expression alone is sufficient to induce a stable association of endogenous Mcm proteins with chromatin in serum-deprived cells where cyclin-dependent kinase (cdk) activity is low. Moreover, endogenous Cdc6 is sufficient to load Mcm proteins onto chromatin in the absence of cdk activity in p21-arrested cells. Cdc6 synergizes with physiological levels of cyclin E/Cdk2 to induce semiconservative DNA replication in quiescent cells whereas cyclin A/Cdk2 is unable to collaborate with Cdc6. Cdc6 that cannot be phosphorylated by cdks is fully capable of inducing Mcm chromatin association and replication. Mutation of the Cdc6 ATP-binding site severely impairs the ability of Cdc6 to induce Mcm chromatin loading and reduces its ability to induce replication. Nevertheless, the ATPase domain of Cdc6 in the absence of the noncatalytic amino terminus is not sufficient for either Mcm chromatin loading or DNA replication, indicating a requirement for this domain of Cdc6.

MCM2-7 proteins are essential components of prereplicative complexes that accumulate cooperatively in the nucleus during G1-phase and are required to establish, but not maintain, the S-phase checkpoint

A prereplicative complex (pre-RC) of proteins is assembled at budding yeast origins of DNA replication during the G1-phase of the cell cycle, as shown by genomic footprinting. The proteins responsible for this prereplicative footprint have yet to be identified but are likely to be involved in the earliest stages of the initiation step of chromosome replication. Here we show that MCM2-7 proteins are essential for both the formation and maintenance of the pre-RC footprint at the origin ARS305. It is likely that pre-RCs contain heteromeric complexes of MCM2-7 proteins, since degradation of Mcm2, 3, 6, or 7 during G1-phase, after pre-RC formation, causes loss of Mcm4 from the nucleus. It has been suggested that pre-RCs on unreplicated chromatin may generate a checkpoint signal that inhibits premature mitosis during S-phase. We show that, although mitosis does indeed occur in the absence of replication if MCM proteins are degraded during G1-phase, anaphase is prevented if MCMs are degraded during S-phase. Our data indicate that pre-RCs do not play a direct role in checkpoint control during chromosome replication.

c-Jun N-terminal kinase is required for metalloproteinase expression and joint destruction in inflammatory arthritis

Mitogen-activated protein kinase (MAPK) cascades are involved in inflammation and tissue destruction in rheumatoid arthritis (RA). In particular, c-Jun N-terminal kinase (JNK) is highly activated in RA fibroblast-like synoviocytes and synovium. However, defining the precise function of this kinase has been difficult because a selective JNK inhibitor has not been available. We now report the use of a novel selective JNK inhibitor and JNK knockout mice to determine the function of JNK in synoviocyte biology and inflammatory arthritis. The novel JNK inhibitor SP600125 (anthra[1,9-cd]pyrazol-6(2H)-one) completely blocked IL-1--induced accumulation of phospho-Jun and induction of c-Jun transcription in synoviocytes. Furthermore, AP-1 binding and collagenase mRNA accumulation were completely suppressed by SP600125. In contrast, complete inhibition of p38 had no effect, and ERK inhibition had only a modest effect. The essential role of JNK was confirmed in cultured synoviocytes from JNK1 knockout mice and JNK2 knockout mice, each of which had a partial defect in IL-1--induced AP-1 activation and collagenase-3 expression. Administration of SP600125 modestly decreased the rat paw swelling in rat adjuvant-induced arthritis. More striking was the near-complete inhibition of radiographic damage that was associated with decreased AP-1 activity and collagenase-3 gene expression. Therefore, JNK is a critical MAPK pathway for IL-1--induced collagenase gene expression in synoviocytes and in joint arthritis, indicating that JNK is an important therapeutic target for RA.

Replication factors MCM2 and ORC1 interact with the histone acetyltransferase HBO1

The minichromosome maintenance (MCM) proteins, together with the origin recognition complex (ORC) proteins and Cdc6, play an essential role in eukaryotic DNA replication through the formation of a pre-replication complex at origins of replication. We used a yeast two-hybrid screen to identify MCM2-interacting proteins. One of the proteins we identified is identical to the ORC1-interacting protein termed HBO1. HBO1 belongs to the MYST family, characterized by a highly conserved C2HC zinc finger and a putative histone acetyltransferase domain. Biochemical studies confirmed the interaction between MCM2 and HBO1 in vitro and in vivo. An N-terminal domain of MCM2 is necessary for binding to HBO1, and a C2HC zinc finger of HBO1 is essential for binding to MCM2. A reverse yeast two-hybrid selection was performed to isolate an allele of MCM2 that is defective for interaction with HBO1; this allele was then used to isolate a suppressor mutant of HBO1 that restores the interaction with the mutant MCM2. This suppressor mutation was located in the HBO1 zinc finger. Taken together, these findings strongly suggest that the interaction between MCM2 and HBO1 is direct and mediated by the C2HC zinc finger of HBO1. The biochemical and genetic interactions of MYST family protein HBO1 with two components of the replication apparatus, MCM2 and ORC1, suggest that HBO1-associated HAT activity may play a direct role in the process of DNA replication.

Initiating DNA synthesis: from recruiting to activating the MCM complex

The exact duplication of a genome once per cell division is required of every proliferating cell. To achieve this goal, eukaryotes adopt a strategy that limits every replication origin to a single initiation event within a narrow window of the cell cycle by temporally separating the assembly of the pre-replication complex (pre-RC) from the initiation of DNA synthesis. A key component of the pre-RC is the hexameric MCM complex, which is also the presumed helicase of the growing forks. An elaborate mechanism recruits the MCM complex to replication origins, and a regulatory chain reaction converts the poised, but inactive, MCM complex into an enzymatically active helicase. A growing list of proteins, including Mcm10 and Cdt1, are involved in the recruitment process. Two protein kinases, the Cdc7-Dbf4 kinase (DDK) and the cyclin-dependent kinase (CDK), trigger a chain reaction that results in the phosphorylation of the MCM complex and finally in the initiation of DNA synthesis. A composite picture from recent studies suggests that DDK is recruited to the pre-RC during G(1) phase but must wait until S phase to phosphorylate the MCM complex. CDK is required for the recruitment of Cdc45 and other downstream components of the elongation machinery.

Identification of Cdc6 protein domains involved in interaction with Mcm2 protein and Cdc4 protein in budding yeast cells

The Cdc6 protein (Cdc6p) has essential roles in regulating initiation of DNA replication. Cdc6p is recruited to origins of replication by the origin recognition complex (ORC) late in mitosis; Cdc6p in turn recruits minichromosome maintenance (Mcm) proteins to form the pre-replicative complex. Cdc6p is thought to interact with one or more Mcm proteins but this point has not yet been demonstrated. In the present study we observed that Cdc6p interacted significantly only with Mcm2p out of six Mcm proteins in yeast two-hybrid cells. Our results indicate that the interaction of Cdc6p with Mcm2p is specific, although we cannot exclude the possibility that the interaction might not be direct. In attempts to identify domains of Cdc6p important for interaction with Mcm2p, we tested interactions of various deleted versions of Cdc6p with Mcm2p and also with Cdc4p, which was previously known to interact with Cdc6p. The portion of Cdc6p from amino acid residues 51 to 394 was able to interact with Mcm2p. During the course of the studies we also discovered a previously undetected Cdc4p interaction domain between residues 51 and 394. Interestingly, when all six putative Cdc28 phosphorylation sites in Cdc6p were changed to alanine, a 6-7-fold increase in binding to Mcm2p was observed. This result suggests that unphosphorylated Cdc6p has higher affinity than phosphorylated Cdc6p for Mcm2p; this might partly explain the previous observation that Cdc6p failed to load Mcm proteins on replication origins during S phase when the cyclin-dependent protein kinase was active, thus helping to prevent the reinitiation of activated replicons.

Evidence for a replication function of FFA-1, the Xenopus orthologue of Werner syndrome protein

DNA replication in higher eukaryotic cells occurs at a large number of discrete sites called replication foci. We have previously purified a protein, focus-forming activity 1 (FFA-1), which is involved in the assembly of putative prereplication foci in Xenopus egg extracts. FFA-1 is the orthologue of the Werner syndrome gene product (WRN), a member of the RecQ helicase family. In this paper we show that FFA-1 colocalizes with sites of DNA synthesis and the single-stranded DNA binding protein, replication protein A (RPA), in nuclei reconstituted in the egg extract. In addition, we show that two glutathione S-transferase FFA-1 fusion proteins can inhibit DNA replication in a dominant negative manner. The dominant negative effect correlates with the incorporation of the fusion proteins into replication foci to form "hybrid foci," which are unable to engage in DNA replication. At the biochemical level, RPA can interact with FFA-1 and specifically stimulates its DNA helicase activity. However, in the presence of the dominant negative mutant proteins, the stimulation is prevented. These results provide the first direct biochemical evidence of an important role for FFA-1 in DNA replication.

Regulation of chromosome replication

The initiation of DNA replication in eukaryotic cells is tightly controlled to ensure that the genome is faithfully duplicated once each cell cycle. Genetic and biochemical studies in several model systems indicate that initiation is mediated by a common set of proteins, present in all eukaryotic species, and that the activities of these proteins are regulated during the cell cycle by specific protein kinases. Here we review the properties of the initiation proteins, their interactions with each other, and with origins of DNA replication. We also describe recent advances in understanding how the regulatory protein kinases control the progress of the initiation reaction. Finally, we describe the checkpoint mechanisms that function to preserve the integrity of the genome when the normal course of genome duplication is perturbed by factors that damage the DNA or inhibit DNA synthesis.

Interactions between Mcm10p and other replication factors are required for proper initiation and elongation of chromosomal DNA replication in Saccharomyces cerevisiae

BACKGROUND

MCM10 is essential for the initiation of chromosomal DNA replication in Saccharomyces cerevisiae. Previous work showed that Mcm10p interacts with the Mcm2-7 protein complex that may be functioning as the replication-licensing factor. In addition, Mcm10p is required during origin activation and disassembly of the prereplicative complex, which allows smooth passage of replication forks.

RESULTS

We show that an mcm10 mutation causes a slow progression of DNA synthesis and a loss of chromosome integrity during the S phase and prevents entry into mitosis, despite apparent completion of chromosomal DNA replication at nonpermissive temperatures. Furthermore, Mcm10p interacts genetically with the origin recognition complex (ORC) and various replication elongation factors, including a subunit of DNA polymerases epsilon and delta. Mcm10p is an abundant protein (approximately 4 x 10(4) copies per haploid cell) that is almost exclusively localized in the chromatin and/or nuclear matrix fractions during all phases of the cell cycle. When it is visualized by the chromosome-spreading method followed by immunostaining, Mcm10p forms punctate foci on chromatin throughout the cell cycle and these foci mostly overlap with those of Orc1p, a component of ORC.

CONCLUSIONS

These results suggest that Mcm10p, like the Mcm2-7 proteins, is a critical component of the prereplication chromatin and acts together with ORC during the initiation of chromosomal DNA replication; in addition, Mcm10p plays an important role during the elongation of DNA replication.

Characterization of the yeast Cdc7p/Dbf4p complex purified from insect cells. Its protein kinase activity is regulated by Rad53p

The yeast Saccharomyces cerevisiae Cdc7p/Dbf4p protein kinase complex was purified to near homogeneity from insect cells. The complex efficiently phosphorylated yeast Mcm2p and less efficiently the remaining Mcm proteins or other replication proteins. Significantly, when pretreated with alkaline phosphatase, Mcm2p became completely inactive as a substrate, suggesting that it must be phosphorylated by other protein kinase(s) to be a substrate for the Cdc7p/Dbf4p complex. Mutant Cdc7p/Dbf4p complexes containing either Cdc7-1p or Dbf4-1 approximately 5p were also partially purified from insect cells and characterized in vitro. Furthermore, the autonomously replicating sequence binding activity of various dbf4 mutants was also analyzed. These studies suggest that the autonomously replicating sequence-binding and Cdc7p protein kinase activation domains of Dbf4p collaborate to form an active Cdc7p/Dbf4p complex and function during S phase in S. cerevisiae. It is shown that Rad53p phosphorylates the Cdc7p/Dbf4p complex in vitro and that this phosphorylation greatly inhibits the kinase activity of Cdc7p/Dbf4p. This result suggests that Rad53p controls the initiation of chromosomal DNA replication by regulating the protein kinase activity associated with the Cdc7p/Dbf4p complex.

A coordinated interplay: proteins with multiple functions in DNA replication, DNA repair, cell cycle/checkpoint control, and transcription

In eukaryotic cells, DNA transactions such as replication, repair, and transcription require a large set of proteins. In all of these events, complexes of more than 30 polypetides appear to function in highly organized and structurally well-defined machines. We have learned in the past few years that the three essential macromolecular events, replication, repair, and transcription, have common functional entities and are coordinated by complex regulatory mechanisms. This can be documented for replication and repair, for replication and checkpoint control, and for replication and cell cycle control, as well as for replication and transcription. In this review we cover the three different protein classes: DNA polymerases, DNA polymerase accessory proteins, and selected transcription factors. The "common enzyme-different pathway strategy" is fascinating from several points of view: first, it might guarantee that these events are coordinated; second, it can be viewed from an evolutionary angle; and third, this strategy might provide cells with backup mechanisms for essential physiological tasks.

The function of Xenopus Bloom's syndrome protein homolog (xBLM) in DNA replication

The Bloom's syndrome gene (BLM) plays a pivotal role in the maintenance of genomic stability in somatic cells. It encodes a DNA helicase (BLM) of the RecQ family, but the exact function of BLM remains elusive. To study this question, we have cloned the BLM homolog of the frog Xenopus laevis (xBLM) and have raised antibodies to it. Immunodepletion of xBLM from a Xenopus egg extract severely inhibits the replication of DNA in reconstituted nuclei. Moreover, the inhibition can be rescued by the addition of the recombinant xBLM protein. These results provide the first direct evidence that BLM plays an important role in DNA replication, suggesting that Bloom's syndrome may be the consequence of defective DNA replication.

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.

Loss control of Mcm5 interaction with chromatin in cdc6-1 mutated in CDC-NTP motif

Saccharomyces cerevisiae Cdc6 plays an essential role in establishing and maintaining the prereplicative complex (pre-RC) by interacting with the origin recognition complex (ORC) and associating with chromatin origins. These interactions are required to load minichromosome maintenance proteins (MCMs) and other initiator proteins onto replication origins. Although the temperature-sensitive cdc6 mutant, cdc6-1, has been widely used for these studies, the molecular mechanism of the cdc6-1 mutation has been unclear. In this study, we have identified a base substitution at Gly260-->Asp, near the CDC-NTP motif. Using a chromatin immunoprecipitation assay (CHIP), we found that cdc6-1 fails to load Mcm5 onto the replication origins. Chromatin fractions were used to study Mcm5 binding in both the wildtype and mutant background. These studies indicated that Cdc6 is also involved in unloading Mcm5 from chromatin. Specifically, the cdc6-1 mutation protein, cdc6(G260D), which failed to load Mcm5 onto replication origins, also failed to unload the Mcm5 protein. Furthermore, the overexpression of wildtype CDC6 accelerated the unloading of Mcm5 from chromatin fractions. In the absence of functional Cdc6, the Mcm5 protein showed nonorigin binding to chromatin with the cell cycle arrested at the G1S phase transition. Our results suggested that the cdc6(G260D) mutant protein fails to assemble an operational replicative complex and that wildtype Cdc6 plays a role in preventing re-replication by controlling the unloading the MCMs from chromatin origins.

Inhibition of Mcm4,6,7 helicase activity by phosphorylation with cyclin A/Cdk2

A strong body of evidence indicates that cyclin-dependent protein kinases are required not only for the initiation of DNA replication but also for preventing over-replication in eukaryotic cells. Mcm proteins are one of the components of the replication licensing system that permits only a single round of DNA replication per cell cycle. It has been reported that Mcm proteins are phosphorylated by the cyclin-dependent kinases in vivo, suggesting that these two factors are cooperatively involved in the regulation of DNA replication. Our group has reported that a 600-kDa Mcm4,6,7 complex has a DNA helicase activity that is probably necessary for the initiation of DNA replication. Here, we examined the in vitro phosphorylation of the Mcm complexes with cyclin A/Cdk2 to understand the interplay between Mcm proteins and cyclin-dependent kinases. The cyclin A/Cdk2 mainly phosphorylated the amino-terminal region of Mcm4 in the Mcm4,6,7 complex. The phosphorylation was associated with the inactivation of its DNA helicase activity. These results raise the possibility that the inactivation of Mcm4,6,7 helicase activity by Cdk2 is a part of the system for regulating DNA replication.

DNA synthesis at individual replication forks requires the essential initiation factor Cdc45p

Cdc45p assembles at replication origins before initia tion and is required for origin firing in Saccharomyces cerevisiae. A heat-inducible cdc45 degron mutant was constructed that promotes rapid degradation of Cdc45p at the restrictive temperature. Consistent with a role in initiation, loss of Cdc45p in G(1) prevents all detectable DNA replication without preventing subsequent entry into mitosis. Loss of Cdc45p activity during S-phase blocks S-phase completion but not activation of replication checkpoints. Using density substitution, we show that after allowing replication fork establishment, Cdc45p inactivation prevents the subsequent progression of individual replication forks. This provides the first direct functional evidence that Cdc45p plays an essential role during elongation. Thus, like the large T antigen in SV40 replication, Cdc45p plays a central role in both initiation and elongation phases of chromosomal DNA replication.

Mcm10 and the MCM2–7 complex interact to initiate DNA synthesis and to release replication factors from origins

MCM2–7, a complex of six subunits, is an essential component of the prereplication chromatin that is assembled at Saccharomyces cerevisiae replication origins during G1 phase. It is also believed to be the processive helicase at growing forks. To elucidate the action of MCM2–7 during the transition from initiation to elongation replication, we have focused our studies on Mcm10, a replication initiation protein that physically interacts with members of the MCM2–7 complex. We show that Mcm10 is a chromatin-associated protein that mediates the association of the MCM2–7 complex with replication origins. Furthermore, diminished interaction between Mcm10 and Mcm7, a subunit of the MCM2–7 complex, by a mutation in either Mcm10 or Mcm7 inhibits replication initiation. Surprisingly, a double mutant containing both the mcm10-1 and mcm7-1(cdc47-1) alleles restores interaction between Mcm10 and Mcm7 and corrects all of the defects exhibited by each of the single mutants, including the stalling of replication forks at replication origins typically seen in mcm10-1 cells. This mutual compensation of defects between two independently isolated mutations is allele specific. These results suggest that Mcm10, like Mcm7, is a critical component of the prereplication chromatin and that interaction between Mcm10 and Mcm7 is required for proper replication initiation and prompt release of origin-bound factors.

Chromatin binding of the fission yeast replication factor mcm4 occurs during anaphase and requires ORC and cdc18

We describe an in situ technique for studying the chromatin binding of proteins in the fission yeast Schizosaccharomyces pombe. After tagging the protein of interest with green fluorescent protein (GFP), chromatin-associated protein is detected by GFP fluorescence following cell permeabilization and washing with a non-ionic detergent. Cell morphology and nuclear structure are preserved in this procedure, allowing structures such as the mitotic spindle to be detected by indirect immunofluorescence. Cell cycle changes in the chromatin association of proteins can therefore be determined from individual cells in asynchronous cultures. We have applied this method to the DNA replication factor mcm4/cdc21, and find that chromatin association occurs during anaphase B, significantly earlier than is the case in budding yeast. Binding of mcm4 to chromatin requires orc1 and cdc18 (homologous to Cdc6 in budding yeast). Release of mcm4 from chromatin occurs during S phase and requires DNA replication. Upon overexpressing cdc18, we show that mcm4 is required for re-replication of the genome in the absence of mitosis and is associated with chromatin in cells undergoing re-replication.

Association of human origin recognition complex 1 with chromatin DNA and nuclease-resistant nuclear structures

An origin recognition complex (ORC) consisting of six polypeptides has been identified as a DNA replication origin-binding factor in Saccharomyces cerevisiae. Homologues of ORC subunits have been discovered among eukaryotes, and we have prepared monoclonal antibodies against a human homologue of ORC1 (hORC1) to study its localization in human cells. It was thus found to associate with nuclei throughout the cell cycle and to be resistant to nonionic detergent treatment, in contrast to MCM proteins, which are other replication factors, the association of which with nuclei is clearly dependent on the phase of the cell cycle. A characteristic feature of hORC1 is dissociation by NaCl in a narrow concentration range around 0.25 M, suggesting interaction with some specific partner(s) in nuclei. Nuclease treatment experiments and UV cross-linking experiments further indicated interaction with both nuclease-resistant nuclear structures and chromatin DNA. Although its DNA binding was unaffected, some variation in the cell cycle was apparent, the association with nuclear structures being less stable in the M phase. Interestingly, the less stable association occurred concomitantly with hyperphosphorylation of hORC1, suggesting that this hyperphosphorylation may be involved in M phase changes.

Clb/Cdc28 kinases promote nuclear export of the replication initiator proteins Mcm2-7

BACKGROUND

In the budding yeast Saccharomyces cerevisiae, the cyclin-dependent kinases of the Clb/Cdc28 family restrict the initiation of DNA replication to once per cell cycle by preventing the re-assembly of pre-replicative complexes (pre-RCs) at replication origins that have already initiated replication. This assembly involves the Cdc6-dependent loading of six minichromosome maintenance (Mcm) proteins, Mcm2-7, onto origins. How Clb/Cdc28 kinases prevent pre-RC assembly is not understood.

RESULTS

In living cells, the Mcm proteins were found to colocalize in a cell-cycle-regulated manner. Mcm2-4, 6 and 7 were concentrated in the nucleus in G1 phase, gradually exported to the cytoplasm during S phase, and excluded from the nucleus by G2 and M phase. Tagging any single Mcm protein with the SV40 nuclear localization signal made all Mcm proteins constitutively nuclear. In the absence of functional Cdc6, Clb/Cdc28 kinases were necessary and sufficient for efficient net nuclear export of a fusion protein between Mcm7 and the green fluorescent protein (Mcm7-GFP), whereas inactivation of these kinases at the end of mitosis coincided with the net nuclear import of Mcm7-GFP. In contrast, in the presence of functional Cdc6, which loads Mcm proteins onto chromatin, S-phase progression as well as Clb/Cdc28 kinases was required for Mcm-GFP export.

CONCLUSIONS

We propose that Clb/Cdc28 kinases prevent pre-RC reassembly in part by promoting the net nuclear export of Mcm proteins. We further propose that Mcm proteins become refractory to this regulation when they load onto chromatin and must be dislodged by DNA replication before they can be exported. Such an arrangement could ensure that Mcm proteins complete their replication function before they are removed from the nucleus.

Sequential MCM/P1 subcomplex assembly is required to form a heterohexamer with replication licensing activity

Replication licensing factor (RLF) is a multiprotein complex involved in ensuring that chromosomal DNA replicates only once in a single cell cycle. It comprises two components, termed RLF-M and RLF-B. Purified RLF-M consists of a mixture of complexes containing all six members of the MCM/P1 family of minichromosome maintenance proteins. The precise composition of these different complexes and their contribution to RLF-M activity has been unclear. Here we show that in Xenopus extracts, MCM/P1 proteins mainly form heterohexamers containing each of the six proteins. This heterohexamer is readily split into subcomplexes, whose interactions and subunit composition we characterize in detail. We show for the first time an ordered multistep assembly pathway by which the heterohexamer can be reformed from the subcomplexes. Importantly, this novel pathway is essential for DNA replication, since only the full heterohexamer can bind productively to chromatin and provide RLF-M activity.

Dbf4p, an essential S phase-promoting factor, is targeted for degradation by the anaphase-promoting complex

The Dbf4p/Cdc7p protein kinase is essential for the activation of replication origins during S phase. The catalytic subunit, Cdc7p, is present at constant levels throughout the cell cycle. In contrast, we show here that the levels of the regulatory subunit, Dbf4p, oscillate during the cell cycle. Dbf4p is absent from cells during G(1) and accumulates during the S and G(2) phases. Dbf4p is rapidly degraded at the time of chromosome segregation and remains highly unstable during pre-Start G(1) phase. The rapid degradation of Dbf4p during G(1) requires a functional anaphase-promoting complex (APC). Mutation of a sequence in the N terminus of Dbf4p which resembles the cyclin destruction box eliminates this APC-dependent degradation of Dbf4p. We suggest that the coupling of Dbf4p degradation to chromosome separation may play a redundant role in ensuring that prereplicative complexes, which assemble after chromosome segregation, do not immediately refire.

Nuclear localization of Schizosaccharomyces pombe Mcm2/Cdc19p requires MCM complex assembly

The minichromosome maintenance (MCM) proteins MCM2-MCM7 are conserved eukaryotic replication factors that assemble in a heterohexameric complex. In fission yeast, these proteins are nuclear throughout the cell cycle. In studying the mechanism that regulates assembly of the MCM complex, we analyzed the cis and trans elements required for nuclear localization of a single subunit, Mcm2p. Mutation of any single mcm gene leads to redistribution of wild-type MCM subunits to the cytoplasm, and this redistribution depends on an active nuclear export system. We identified the nuclear localization signal sequences of Mcm2p and showed that these are required for nuclear targeting of other MCM subunits. In turn, Mcm2p must associate with other MCM proteins for its proper localization; nuclear localization of MCM proteins thus requires assembly of MCM proteins in a complex. We suggest that coupling complex assembly to nuclear targeting and retention ensures that only intact heterohexameric MCM complexes remain nuclear.

G1-phase and B-type cyclins exclude the DNA-replication factor Mcm4 from the nucleus

Cyclin-dependent kinases (CDKs) activate the firing of replication origins during the S phase of the cell cycle. They also block re-initiation of DNA replication within a single cell cycle, by preventing the assembly of prereplicative complexes at origins. We show here that, in budding yeast, CDKs exclude the essential prereplicative-complex component Mcm4 from the nucleus. Although origin firing can be triggered by the B-type cyclins only, both G1-phase and B-type cyclins cause exit of Mcm4 from the nucleus. These results suggest that G1 cyclins may diminish the cell's capacity to assemble prereplicative complexes before B-type cyclins trigger origin firing during S phase.

The mitotically phosphorylated form of the transcription termination factor TTF-1 is associated with the repressed rDNA transcription machinery

The transcription termination factor TTF-1 exerts two functions in ribosomal gene (rDNA) transcription: facilitating initiation and mediating termination of transcription. Using HeLa cells, we show that TTF-1 protein is colocalized with the active transcription machinery in the nucleolus and also with the inactive machinery present in certain mitotic nucleolar organizer regions (NORs) when rDNA transcription is repressed. We also show that TTF-1 is specifically phosphorylated during mitosis in a manner dependent on the cdc2-cyclin B kinase pathway and on an okadaic acid-sensitive phosphatase. Interestingly, the mitotically phosphorylated form of TTF-1 appearing at the G(2)/M transition phase was more easily solubilized than was the interphase form. This indicates that the chromatin-binding affinity of TTF-1 appears to be different in mitotic chromosomes compared to the interphase nucleolus. Correlated with this, the other DNA-binding factor, UBF, which interferes with chromatin conformation in the rDNA promoter, was more strongly bound to rDNA during mitosis than at interphase. The reorganization of the mitotic rDNA promoter might be induced by phosphorylation of certain components of the rDNA transcription machinery and participate in silencing of rDNA during mitosis.

Cell cycle regulation of human CDC6 protein. Intracellular localization, interaction with the human mcm complex, and CDC2 kinase-mediated hyperphosphorylation

The binding of mammalian MCM complexes to chromatin is cell cycle-regulated and under CDC2 kinase negative control. Here, we investigated the properties of mammalian CDC6 protein, a candidate regulator of MCM. The levels of CDC6 were relatively constant during the HeLa cell cycle. In asynchronous cells, CDC6 was mainly detected in the nuclei with immunostaining, but some CDC6 was not extractable with nonionic detergent. In contrast to the chromatin-bound MCM, this fraction of CDC6 was resistant to DNase I treatment, suggesting that it binds to the detergent- and nuclease-resistant nuclear structure. In S phase cells, CDC6 became detectable in the cytoplasm with immunostaining; however, the level of the bound CDC6 was unchanged. In G(2)/M phase cells, the level of the bound CDC6 was still maintained, which was hyperphosphorylated by CDC2 kinase. These data suggest that some CDC6 protein is associated with the specific nuclear structure throughout the cell cycle and that major binding sites on chromatin differ between MCM and CDC6. However, co-immunoprecipitation assays with chemical cross-linking indicated that a small part of the chromatin-bound MCM is present close to the bound CDC6.

Cell cycle control of Cdc7p kinase activity through regulation of Dbf4p stability

In Saccharomyces cerevisiae, the heteromeric kinase complex Cdc7p-Dbf4p plays a pivotal role at replication origins in triggering the initiation of DNA replication during the S phase. We have assayed the kinase activity of endogenous levels of Cdc7p kinase by using a likely physiological target, Mcm2p, as a substrate. Using this assay, we have confirmed that Cdc7p kinase activity fluctuates during the cell cycle; it is low in the G1 phase, rises as cells enter the S phase, and remains high until cells complete mitosis. These changes in kinase activity cannot be accounted for by changes in the levels of the catalytic subunit Cdc7p, as these levels are constant during the cell cycle. However, the fluctuations in kinase activity do correlate with levels of the regulatory subunit Dbf4p. The regulation of Dbf4p levels can be attributed in part to increased degradation of the protein in G1 cells. This G1-phase instability is cdc16 dependent, suggesting a role of the anaphase-promoting complex in the turnover of Dbf4p. Overexpression of Dbf4p in the G1 phase can partially overcome this elevated turnover and lead to an increase in Cdc7p kinase activity. Thus, the regulation of Dbf4p levels through the control of Dbf4p degradation has an important role in the regulation of Cdc7p kinase activity during the cell cycle.

Minichromosome maintenance as a genetic assay for defects in DNA replication

Minichromosome maintenance (mcm) is an effective genetic assay for mutants defective in DNA replication. Two classes of mcm mutants have been identified using this screen: those that differentially affect the activities of certain autonomously replicating sequences (ARSs) and those that uniformly affect the activities of all ARSs. The ARS-specific MCM genes are essential for the initiation of DNA replication. Among these are members of the MCM2-7 family that encode subunits of the preinitiation complex and MCM10, whose gene product interacts with members of the Mcm2-7 proteins. Among the ARS-nonspecific MCM gene products are chromosome transmission factors. Refinement of this genetic assay as a screening tool and further analysis of existing mcm mutants may reveal new replication initiation proteins.

The role of nucleotide binding and hydrolysis in the function of the fission yeast cdc18(+) gene product

The fission yeast cdc18(+) gene is required for both initiation of DNA replication and the mitotic checkpoint that normally inhibits mitosis in the absence of DNA replication. The cdc18(+) gene product contains conserved Walker A and B box motifs. Studies of other ATPases have shown that these motifs are required for nucleotide binding and hydrolysis, respectively. We have observed that mutant strains in which either of these motifs is disrupted are inviable. The effects of these mutations were examined by determining the phenotypes of mutant strains following depletion of complementing wild-type Cdc18. In both synchronous and asynchronous cultures, the nucleotide-hydrolysis motif mutant (DE286AA) arrests with a 1C-2C DNA content, and thus exhibits no obvious defects in entry into S phase or in the mitotic checkpoint. In contrast, in cultures synchronized by hydroxyurea arrest and release, the nucleotide-binding motif mutant (K205A) exhibits the null phenotype, with 1C and <1C DNA content, indicating a block in entry into S phase and loss of checkpoint control. In asynchronous cultures this mutant exhibits a mixed phenotype: a percentage of the population displays the null phenotype, while the remaining fraction arrests with a 2C DNA content. Thus, the phenotype exhibited by the K205A mutant is dependent on the cell-cycle position at which wild-type Cdc18 is depleted. These data indicate that both nucleotide binding and hydrolysis are required for Cdc18 function. In addition, the difference in the phenotypes exhibited by the nucleotide-binding and hydrolysis motif mutants is consistent with a two-step model for Cdc18 function in which nucleotide binding and hydrolysis are required for distinct aspects of Cdc18 function that may be executed at different points in the cell cycle.

The Cdc6 protein is ubiquitinated in vivo for proteolysis in Saccharomyces cerevisiae

The Saccharomyces cerevisiae Cdc6 protein is necessary for the formation of pre-replicative complexes that are required for firing DNA replication at origins at the beginning of S phase. Cdc6p protein levels oscillate during the cell cycle. In a normal cell cycle the presence of this protein is restricted to G1, partly because the CDC6 gene is transcribed only during G1 and partly because the Cdc6p protein is rapidly degraded at late G1/early S phase. We report here that the Cdc6p protein is degraded in a Cdc4-dependent manner, suggesting that phosphorylated Cdc6 is specifically recognized by the ubiquitin-mediated proteolysis machinery. Indeed, we have found that Cdc6 is ubiquitinated in vivo and degraded by a Cdc4-dependent mechanism. Our data, together with previous observations regarding Cdc6 stability, suggest that under physiological conditions budding yeast cells degrade ubiquitinated Cdc6 every cell cycle at the beginning of S phase.

Fission yeast cdc24 is a replication factor C- and proliferating cell nuclear antigen-interacting factor essential for S-phase completion

At the nonpermissive temperature the fission yeast cdc24-M38 mutant arrests in the cell cycle with incomplete DNA replication as indicated by pulsed-field gel electrophoresis. The cdc24(+) gene encodes a 501-amino-acid protein with no significant homology to any known proteins. The temperature-sensitive cdc24 mutant is effectively rescued by pcn1(+), rfc1(+) (a fission yeast homologue of RFC1), and hhp1(+), which encode the proliferating cell nuclear antigen (PCNA), the large subunit of replication factor C (RFC), and a casein kinase I involved in DNA damage repair, respectively. The Cdc24 protein binds PCNA and RFC1 in vivo, and the domains essential for Cdc24 function and for RFC1 and PCNA binding colocalize in the N-terminal two-thirds of the molecule. In addition, cdc24(+) genetically interacts with the gene encoding the catalytic subunit of DNA polymerase epsilon, which is stimulated by PCNA and RFC, and with those encoding the fission yeast counterparts of Mcm2, Mcm4, and Mcm10. These results indicate that Cdc24 is an RFC- and PCNA-interacting factor required for DNA replication and might serve as a target for regulation.

Phosphorylation of mammalian CDC6 by cyclin A/CDK2 regulates its subcellular localization

Cyclin-dependent kinases (CDKs) are essential for regulating key transitions in the cell cycle, including initiation of DNA replication, mitosis and prevention of re-replication. Here we demonstrate that mammalian CDC6, an essential regulator of initiation of DNA replication, is phosphorylated by CDKs. CDC6 interacts specifically with the active Cyclin A/CDK2 complex in vitro and in vivo, but not with Cyclin E or Cyclin B kinase complexes. The cyclin binding domain of CDC6 was mapped to an N-terminal Cy-motif that is similar to the cyclin binding regions in p21(WAF1/SDI1) and E2F-1. The in vivo phosphorylation of CDC6 was dependent on three N-terminal CDK consensus sites, and the phosphorylation of these sites was shown to regulate the subcellular localization of CDC6. Consistent with this notion, we found that the subcellular localization of CDC6 is cell cycle regulated. In G1, CDC6 is nuclear and it relocalizes to the cytoplasm when Cyclin A/CDK2 is activated. In agreement with CDC6 phosphorylation being specifically mediated by Cyclin A/CDK2, we show that ectopic expression of Cyclin A, but not of Cyclin E, leads to rapid relocalization of CDC6 from the nucleus to the cytoplasm. Based on our data we suggest that the phosphorylation of CDC6 by Cyclin A/CDK2 is a negative regulatory event that could be implicated in preventing re-replication during S phase and G2.

cDNA cloning and expression during development of Drosophila melanogaster MCM3, MCM6 and MCM7

cDNAs encoding three Drosophila melanogaster MCM proteins, DmMCM3, DmMCM6 and DmMCM7, candidates of DNA replication-licensing factors, were cloned and sequenced. The deduced amino-acid sequences displayed 60, 59 and 68% identities with the respective Xenopus laevis homologues, XMCM3, XMCM6 and XMCM7. Six members of the D. melanogaster MCM family were found to share 31-36% identities in their amino-acid sequences, and to possess the five common domains carrying conserved amino-acid sequences as reported with X. laevis MCM proteins. DmMCM3, DmMCM6 and DmMCM7 genes were mapped to the 4F region on the X chromosome, the 6B region on the X chromosome and the 66E region on the third chromosome, respectively, by in situ hybridization. Contents of their mRNAs were proved to be high in unfertilized eggs and early embryos (0-4h after fertilization), then decrease gradually by the 12h time point, with only low levels detected at later stages of development except in adult females. This fluctuation pattern is similar to those of genes for proteins involved in DNA replication, such as DNA polymerase alpha and proliferating cell nuclear antigen, suggesting that expression of DmMCM genes is under the regulatory mechanism which regulates expression of other genes involved in DNA replication.

Replication focus-forming activity 1 and the Werner syndrome gene product

The initiation of DNA replication involves a minimum of four factors: a specific DNA sequence (origin), an initiator protein which binds to the origin, a helicase that unwinds the origin and a protein that binds single-stranded DNA that stabilizes the unwound origin. In eukaryotic cells, the origin recognition complex (ORC) is the initiator protein and replication protein A (RPA; ref. 3) is the single-stranded DNA-binding protein. However, the helicase has not been identified and the nature of origins remains elusive, except in the case of Saccharomyces cerevisiae. A unique feature of eukaryotic DNA replication is that it occurs at a few-hundred discrete foci. It has thus been proposed that a real origin must contain a specific DNA sequence and must be attached to replication foci. Using Xenopus laevis egg extracts, we have identified and purified a 170-kD protein, focus-forming activity 1 (FFA-1), which is required for the formation of replication foci. Here we report that FFA-1 has DNA-helicase activity. Moreover, it is a homologue of the human Werner syndrome gene product WRN, a protein associated with premature ageing in humans.

Mutational effect of fission yeast polalpha on cell cycle events

Polalpha is the principal DNA polymerase for initiation of DNA replication and also functions in postinitiation DNA synthesis. In this study, we investigated the cell cycle responses induced by mutations in polalpha+. Germinating spores carrying either a deletion of polalpha+ (polalphaDelta) or a structurally intact but catalytically dead polalpha mutation proceed to inappropriate mitosis with no DNA synthesis. This suggests that the catalytic function, and not the physical presence of Polalpha, is required to generate the signal that prevents the cells from entering mitosis prematurely. Cells with a polalphats allele arrest the cell cycle near the hydroxyurea arrest point, but, surprisingly, polalphats in cdc20 (polepsilon mutant) background arrested with a cdc phenoytpe, not a polalphats-like phenotype. At 25 degrees C, replication perturbation caused by polalphats alleles induces Cds1 kinase activity and requires the checkpoint Rads, Cds1, and Rqh1, but not Chk1, to maintain cell viability. At 36 degrees C, replication disruption caused by polalphats alleles induces the phosphorylation of Chk1; however, mutant cells arrest with heterogeneous cell sizes with a population of the cells entering aberrant mitosis. Together, our results indicate that the initiation DNA structure synthesized by Polalpha is required to bring about the S phase to mitosis checkpoint, whereas replication defects of different severity caused by polalphats mutations induce differential downstream kinase responses.

Cell cycle- and chromatin binding state-dependent phosphorylation of human MCM heterohexameric complexes. A role for cdc2 kinase

The mammalian MCM protein family, presently with six members, exists in the nuclei in two forms, chromatin-bound and unbound. The former dissociates from chromatin with progression through the S phase. Recently, we have established a procedure to isolate chromatin-bound and unbound complexes containing all six human MCM (hMCM) proteins by immunoprecipitation. In the present study, we applied this procedure to HeLa cells synchronized in each of the G1, S, and G2/M phases and could detect hMCM heterohexameric complexes in all three. In addition, depending on the cell cycle and the state of chromatin association, hMCM2 and 4 in the complexes were found to variously change their phosphorylation states. Concentrating attention on G2/M phase hyperphosphorylation, we found hMCM2 and 4 in the complexes to be good substrates for cdc2/cyclin B in vitro. Furthermore, when cdc2 kinase was inactivated in temperature-sensitive mutant murine FT210 cells, the G2/M hyperphosphorylation of the murine MCM2 and MCM4 and release of the MCMs from chromatin in the G2 phase were severely impaired. Taken together, the data suggest that the six mammalian MCM proteins function and undergo cell cycle-dependent regulation as heterohexameric complexes and that phosphorylation of the complexes by cdc2 kinase may be one of mechanisms negatively regulating the MCM complex-chromatin association.

Cell cycle regulation of S phase entry in Saccharomyces cerevisiae

Eukaryotic DNA replication is restricted to a narrow window of the cell cycle called S phase, and occurs once and only once during each cell cycle. The combination of genetic and biochemical approaches in the budding yeast Saccharomyces cerevisiae has proven extremely helpful for studying the cell cycle regulation of S phase entry. This review will try to summarise the most recent discoveries which led to a new model to explain how entry into S phase is regulated in eukaryotic cells.

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.