Identification and characterization of a novel component of the human minichromosome maintenance complex

Minichromosome maintenance (MCM) complex replicative helicase complexes play essential roles in DNA replication in all eukaryotes. Using a tandem affinity purification-tagging approach in human cells, we discovered a form of the MCM complex that contains a previously unstudied protein, MCM binding protein (MCM-BP). MCM-BP is conserved in multicellular eukaryotes and shares limited homology with MCM proteins. MCM-BP formed a complex with MCM3 to MCM7, which excluded MCM2; and, conversely, hexameric complexes of MCM2 to MCM7 lacked MCM-BP, indicating that MCM-BP can replace MCM2 in the MCM complex. MCM-BP-containing complexes exhibited increased stability under experimental conditions relative to those containing MCM2. MCM-BP also formed a complex with the MCM4/6/7 core helicase in vitro, but, unlike MCM2, did not inhibit this helicase activity. A proportion of MCM-BP bound to cellular chromatin in a cell cycle-dependent manner typical of MCM proteins, and, like other MCM subunits, preferentially associated with a cellular origin in G(1) but not in S phase. In addition, down-regulation of MCM-BP decreased the association of MCM4 with chromatin, and the chromatin association of MCM-BP was at least partially dependent on MCM4 and cdc6. The results indicate that multicellular eukaryotes contain two types of hexameric MCM complexes with unique properties and functions.     

A Role for USP7 in DNA Replication

The minichromosome maintenance (MCM) complex, which plays multiple important roles in DNA replication, is loaded onto chromatin following mitosis, remains on chromatin until the completion of DNA synthesis, and then is unloaded by a poorly defined mechanism that involves the MCM binding protein (MCM-BP). Here we show that MCM-BP directly interacts with the ubiquitin-specific protease USP7, that this interaction occurs predominantly on chromatin, and that MCM-BP can tether USP7 to MCM proteins. Detailed biochemical and structure analyses of the USP7–MCM-BP interaction showed that the ¹⁵⁵PSTS¹⁵⁸ MCM-BP sequence mediates critical interactions with the TRAF domain binding pocket of USP7. Analysis of the effects of USP7 knockout on DNA replication revealed that lack of USP7 results in slowed progression through late S phase without globally affecting the fork rate or origin usage. Lack of USP7 also resulted in increased levels of MCM proteins on chromatin, and investigation of the cause of this increase revealed a defect in the dissociation of MCM proteins from chromatin in mid- to late S phase. This role of USP7 mirrors the previously described role for MCM-BP in MCM complex unloading and suggests that USP7 works with MCM-BP to unload MCM complexes from chromatin at the end of S phase.     

Purification and functional inactivation of the fission yeast MCM(MCM-BP) complex

The MCM (mini-chromosome maintenance) complex is the core of the eukaryotic replicative helicase and comprises six proteins, Mcm2-Mcm7. In humans, a variant form of the complex has Mcm2 replaced by the MCM-BP protein. Recent results suggest that a similar complex exists in fission yeast with an essential role in DNA replication and cell cycle progression. Here, we describe the purification and subunit composition of the fission yeast MCM(Mcb1) complex. Using newly generated temperature-sensitive alleles, we show that loss of MCM(Mcb1) function leads to accumulation of DNA damage, checkpoint activation and cell cycle arrest, and provide evidence for a role for MCM(Mcb1) in meiosis.     

Biochemical analysis of the intrinsic Mcm4-Mcm6-mcm7 DNA helicase activity

Mcm proteins play an essential role in eukaryotic DNA replication, but their biochemical functions are poorly understood. Recently, we reported that a DNA helicase activity is associated with an Mcm4-Mcm6-Mcm7 (Mcm4,6,7) complex, suggesting that this complex is involved in the initiation of DNA replication as a DNA-unwinding enzyme. In this study, we have expressed and isolated the mouse Mcm2, 4,6,7 proteins from insect cells and characterized various mutant Mcm4,6,7 complexes in which the conserved ATPase motifs of the Mcm4 and Mcm6 proteins were mutated. The activities associated with such preparations demonstrated that the DNA helicase activity is intrinsically associated with the Mcm4,6,7 complex. Biochemical analyses of these mutant Mcm4,6,7 complexes indicated that the ATP binding activity of the Mcm6 protein in the complex is critical for DNA helicase activity and that the Mcm4 protein may play a role in the single-stranded DNA binding activity of the complex. The results also indicated that the two activities of DNA helicase and single-stranded DNA binding can be separated.     

The Fission Yeast Minichromosome Maintenance (MCM)-binding Protein (MCM-BP), Mcb1, Regulates MCM Function during Prereplicative Complex Formation in DNA Replication

The minichromosome maintenance (MCM) complex is a replicative helicase, which is essential for chromosome DNA replication. In recent years, the identification of a novel MCM-binding protein (MCM-BP) in most eukaryotes has led to numerous studies investigating its function and its relationship to the MCM complex. However, the mechanisms by which MCM-BP functions and associates with MCM complexes are not well understood; in addition, the functional role of MCM-BP remains controversial and may vary between model organisms. The present study aims to elucidate the nature and biological function of the MCM-BP ortholog, Mcb1, in fission yeast. The Mcb1 protein continuously interacts with MCM proteins during the cell cycle in vivo and can interact with any individual MCM subunit in vitro. To understand the detailed characteristics of mcb1⁺, two temperature-sensitive mcb1 gene mutants (mcb1^ts) were isolated. Extensive genetic analysis showed that the mcb1^ts mutants were suppressed by a mcm5⁺ multicopy plasmid and displayed synthetic defects with many S-phase-related gene mutants. Moreover, cyclin-dependent kinase modulation by Cig2 repression or Rum1 overproduction suppressed the mcb1^ts mutants, suggesting the involvement of Mcb1 in pre-RC formation during DNA replication. These data are consistent with the observation that Mcm7 loading onto replication origins is reduced and S-phase progression is delayed in mcb1^ts mutants. Furthermore, the mcb1^ts mutation led to the redistribution of MCM subunits to the cytoplasm, and this redistribution was dependent on an active nuclear export system. These results strongly suggest that Mcb1 promotes efficient pre-RC formation during DNA replication by regulating the MCM complex.     

Cdc7-Dbf4 phosphorylates MCM proteins via a docking site-mediated mechanism to promote S phase progression

Origins of DNA replication are licensed in G1 by recruiting the minichromosome maintenance (MCM) proteins to form a prereplicative complex (pre-RC). Prior to initiation of DNA synthesis from each origin, a preinitiation complex (pre-IC) containing Cdc45 and other proteins is formed. We report that Cdc7-Dbf4 protein kinase (DDK) promotes assembly of a stable Cdc45-MCM complex exclusively on chromatin in S phase. In this complex, Mcm4 is hyperphosphorylated. Studies in vitro using purified DDK and Mcm4 demonstrate that hyperphosphorylation occurs at the Mcm4 N terminus. However, the DDK substrate specificity is conferred by an adjacent DDK-docking domain (DDD), sufficient for facilitating efficient phosphorylation of artificial phosphoacceptors in cis. Genetic evidence suggests that phosphorylation of Mcm4 by DDK is important for timely S phase progression and for cell viability upon overproduction of Cdc45. We suggest that DDK docks on and phosphorylates MCM proteins at licensed origins to promote proper assembly of pre-IC.     

Prereplicative complexes assembled in vitro support origin‐dependent and independent DNA replication

Eukaryotic DNA replication initiates from multiple replication origins. To ensure each origin fires just once per cell cycle, initiation is divided into two biochemically discrete steps: the Mcm2‐7 helicase is first loaded into prereplicative complexes (pre‐RCs) as an inactive double hexamer by the origin recognition complex (ORC), Cdt1 and Cdc6; the helicase is then activated by a set of “firing factors.” Here, we show that plasmids containing pre‐RCs assembled with purified proteins support complete and semi‐conservative replication in extracts from budding yeast cells overexpressing firing factors. Replication requires cyclin‐dependent kinase (CDK) and Dbf4‐dependent kinase (DDK). DDK phosphorylation of Mcm2‐7 does not by itself promote separation of the double hexamer, but is required for the recruitment of firing factors and replisome components in the extract. Plasmid replication does not require a functional replication origin; however, in the presence of competitor DNA and limiting ORC concentrations, replication becomes origin‐dependent in this system. These experiments indicate that Mcm2‐7 double hexamers can be precursors of replication and provide insight into the nature of eukaryotic DNA replication 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.     

Phosphorylation of Minichromosome Maintenance Protein 7 (MCM7) by Cyclin/Cyclin-dependent Kinase Affects Its Function in Cell Cycle Regulation

MCM7 is one of the subunits of the MCM2–7 complex that plays a critical role in DNA replication initiation and cell proliferation of eukaryotic cells. After forming the pre-replication complex (pre-RC) with other components, the MCM2–7 complex is activated by DDK/cyclin-dependent kinase to initiate DNA replication. Each subunit of the MCM2–7 complex functions differently under regulation of various kinases on the specific site, which needs to be investigated in detail. In this study, we demonstrated that MCM7 is a substrate of cyclin E/Cdk2 and can be phosphorylated on Ser-121. We found that the distribution of MCM7-S121A is different from wild-type MCM7 and that the MCM7-S121A mutant is much less efficient to form a pre-RC complex with MCM3/MCM5/cdc45 compared with wild-type MCM7. By using the Tet-On inducible HeLa cell line, we revealed that overexpression of wild-type MCM7 but not MCM7-S121A can block S phase entry, suggesting that an excess of the pre-RC complex may activate the cell cycle checkpoint. Further analysis indicates that the Chk1 pathway is activated in MCM7-overexpressed cells in a p53-dependent manner. We performed experiments with the human normal cell line HL-7702 and also observed that overexpression of MCM7 can cause S phase block through checkpoint activation. In addition, we found that MCM7 could also be phosphorylated by cyclin B/Cdk1 on Ser-121 both in vitro and in vivo. Furthermore, overexpression of MCM7-S121A causes an obvious M phase exit delay, which suggests that phosphorylation of MCM7 on Ser-121 in M phase is very important for a proper mitotic exit. These data suggest that the phosphorylation of MCM7 on Ser-121 by cyclin/Cdks is involved in preventing DNA rereplication as well as in regulation of the mitotic exit.     

Negative Regulation of DNA Replication by the Retinoblastoma Protein Is Mediated by Its Association with MCM7

A yeast two-hybrid screen was employed to identify human proteins that specifically bind the amino-terminal 400 amino acids of the retinoblastoma (Rb) protein. Two independent cDNAs resulting from this screen were found to encode the carboxy-terminal 137 amino acids of MCM7, a member of a family of proteins that comprise replication licensing factor. Full-length Rb and MCM7 form protein complexes in vitro, and the amino termini of two Rb-related proteins, p107 and p130, also bind MCM7. Protein complexes between Rb and MCM7 were also detected in anti-Rb immunoprecipitates prepared from human cells. The amino-termini of Rb and p130 strongly inhibited DNA replication in an MCM7-dependent fashion in a Xenopus in vitro DNA replication assay system. These data provide the first evidence that Rb and Rb-related proteins can directly regulate DNA replication and that components of licensing factor are targets of the products of tumor suppressor genes.     

Dbf4 and Cdc7 Proteins Promote DNA Replication through Interactions with Distinct Mcm2–7 Protein Subunits

The essential cell cycle target of the Dbf4/Cdc7 kinase (DDK) is the Mcm2–7 helicase complex. Although Mcm4 has been identified as the critical DDK phosphorylation target for DNA replication, it is not well understood which of the six Mcm2–7 subunits actually mediate(s) docking of this kinase complex. We systematically examined the interaction between each Mcm2–7 subunit with Dbf4 and Cdc7 through two-hybrid and co-immunoprecipitation analyses. Strikingly different binding patterns were observed, as Dbf4 interacted most strongly with Mcm2, whereas Cdc7 displayed association with both Mcm4 and Mcm5. We identified an N-terminal Mcm2 region required for interaction with Dbf4. Cells expressing either an Mcm2 mutant lacking this docking domain (Mcm2ΔDDD) or an Mcm4 mutant lacking a previously identified DDK docking domain (Mcm4ΔDDD) displayed modest DNA replication and growth defects. In contrast, combining these two mutations resulted in synthetic lethality, suggesting that Mcm2 and Mcm4 play overlapping roles in the association of DDK with MCM rings at replication origins. Consistent with this model, growth inhibition could be induced in Mcm4ΔDDD cells through Mcm2 overexpression as a means of titrating the Dbf4-MCM ring interaction. This growth inhibition was exacerbated by exposing the cells to either hydroxyurea or methyl methanesulfonate, lending support for a DDK role in stabilizing or restarting replication forks under S phase checkpoint conditions. Finally, constitutive overexpression of each individual MCM subunit was examined, and genotoxic sensitivity was found to be specific to Mcm2 or Mcm4 overexpression, further pointing to the importance of the DDK-MCM ring interaction.     

Phosphopeptide binding by Sld3 links Dbf4‐dependent kinase to MCM replicative helicase activation

The initiation of eukaryotic DNA replication requires the assembly of active CMG (Cdc45‐MCM‐GINS) helicases at replication origins by a set of conserved and essential firing factors. This process is controlled during the cell cycle by cyclin‐dependent kinase (CDK) and Dbf4‐dependent kinase (DDK), and in response to DNA damage by the checkpoint kinase Rad53/Chk1. Here we show that Sld3, previously shown to be an essential CDK and Rad53 substrate, is recruited to the inactive MCM double hexamer in a DDK‐dependent manner. Sld3 binds specifically to DDK‐phosphorylated peptides from two MCM subunits (Mcm4, 6) and then recruits Cdc45. MCM mutants that cannot bind Sld3 or Sld3 mutants that cannot bind phospho‐MCM or Cdc45 do not support replication. Moreover, phosphomimicking mutants in Mcm4 and Mcm6 bind Sld3 without DDK and facilitate DDK‐independent replication. Thus, Sld3 is an essential “reader” of DDK phosphorylation, integrating signals from three distinct protein kinase pathways to coordinate DNA replication during S phase.     

Pairwise interactions of the six human MCM protein subunits

The eukaryotic minichromosome maintenance (MCM) proteins have six subunits, Mcm2 to 7p. Together they play essential roles in the initiation and elongation of DNA replication, and the human MCM proteins present attractive targets for potential anticancer drugs. The six MCM subunits interact and form a ring-shaped heterohexameric complex containing one of each subunit in a variety of eukaryotes, and subcomplexes have also been observed. However, the architecture of the human MCM heterohexameric complex is still unknown. We systematically studied pairwise interactions of individual human MCM subunits by using the yeast two-hybrid system and in vivo protein-protein crosslinking with a non-cleavable crosslinker in human cells followed by co-immunoprecipitation. In the yeast two-hybrid assays, we revealed multiple binary interactions among the six human MCM proteins, and a subset of these interactions was also detected as direct interactions in human cells. Based on our results, we propose a model for the architecture of the human MCM protein heterohexameric complex. We also propose models for the structures of subcomplexes. Thus, this study may serve as a foundation for understanding the overall architecture and function of eukaryotic MCM protein complexes and as clues for developing anticancer drugs targeted to the human MCM proteins.     

A potential role for mini-chromosome maintenance (MCM) proteins in initiation at the dihydrofolate reductase replication origin.

Mini-chromosome maintenance (MCM) proteins were originally identified in yeast, and homologues have been identified in several other eukaryotic organisms, including mammals. These findings suggest that the mechanisms by which eukaryotic cells initiate and regulate DNA replication have been conserved throughout evolution. However, it is clear that many mammalian origins are much more complex than those of yeast. An example is the Chinese hamster dihydrofolate reductase (DHFR) origin, which resides in the spacer between the DHFR and 2BE2121 genes. This origin consists of a broad zone of potential sites scattered throughout the 55-kb spacer, with several subregions (e.g. ori-beta, ori-beta', and ori-gamma) being preferred. We show here that antibodies to human MCMs 2-7 recognize counterparts in extracts prepared from hamster cells; furthermore, co-immunoprecipitation data demonstrate the presence of an MCM2-3-5 subcomplex as observed in other species. To determine whether MCM proteins play a role in initiation and/or elongation in Chinese hamster cells, we have examined in vivo protein-DNA interactions between the MCMs and chromatin in the DHFR locus using a chromatin immunoprecipitation (ChIP) approach. In synchronized cultures, MCM complexes associate preferentially with DNA in the intergenic initiation zone early in S-phase during the time that replication initiates. However, significant amounts of MCMs were also detected over the two genes, in agreement with recent observations that the MCM complex co-purifies with RNA polymerase II. As cells progress through S-phase, the MCMs redistribute throughout the DHFR domain, suggesting a dynamic interaction with DNA. In asynchronous cultures, in which replication forks should be found at any position in the genome, MCM proteins were distributed relatively evenly throughout the DHFR locus. Altogether, these data are consistent with studies in yeast showing that MCM subunits localize to origins during initiation and then migrate outward with the replication forks. This constitutes the first evidence that mammalian MCM complexes perform a critical role during the initiation and elongation phases of replication at the DHFR origin in hamster cells.     

Identification of MCM4 as a target of the DNA replication block checkpoint system

Inhibition of the progression of DNA replication prevents further initiation of DNA replication and allows cells to maintain arrested replication forks, but the proteins that are targets of the replication checkpoint system remain to be identified. We report here that human MCM4, a subunit of the putative DNA replicative helicase, is extensively phosphorylated in HeLa cells when they are incubated in the presence of inhibitors of DNA synthesis or are exposed to UV irradiation. The data presented here indicate that the consecutive actions of ATR-CHK1 and CDK2 kinases are involved in this phosphorylation in the presence of hydroxyurea. The phosphorylation sites in MCM4 were identified using specific anti-phosphoantibodies. Based on results that showed that the DNA helicase activity of the MCM4-6-7 complex is negatively regulated by CDK2 phosphorylation, we suggest that the phosphorylation of MCM4 in the checkpoint control inhibits DNA replication, which includes blockage of DNA fork progression, through inactivation of the MCM complex.     

Phosphorylation of MCM4 by Cdc7 kinase facilitates its interaction with Cdc45 on the chromatin

Cdc7 kinase, conserved from yeasts to human, plays important roles in DNA replication. However, the mechanisms by which it stimulates initiation of DNA replication remain largely unclear. We have analyzed phosphorylation of MCM subunits during cell cycle by examining mobility shift on SDS-PAGE. MCM4 on the chromatin undergoes specific phosphorylation during S phase. Cdc7 phosphorylates MCM4 in the MCM complexes as well as the MCM4 N-terminal polypeptide. Experiments with phospho-amino acid-specific antibodies indicate that the S phase-specific mobility shift is due to the phosphorylation at specific N-terminal (S/T)(S/T)P residues of the MCM4 protein. These specific phosphorylation events are not observed in mouse ES cells deficient in Cdc7 or are reduced in the cells treated with siRNA specific to Cdc7, suggesting that they are mediated by Cdc7 kinase. The N-terminal phosphorylation of MCM4 stimulates association of Cdc45 with the chromatin, suggesting that it may be an important phosphorylation event by Cdc7 for activation of replication origins. Deletion of the N-terminal non-conserved 150 amino acids of MCM4 results in growth inhibition, and addition of amino acids carrying putative Cdc7 target sequences partially restores the growth. Furthermore, combination of MCM4 N-terminal deletion with alanine substitution and deletion of the N-terminal segments of MCM2 and MCM6, respectively, which contain clusters of serine/threonine and are also likely targets of Cdc7, led to an apparent nonviable phenotype. These results are consistent with the notion that the N-terminal phosphorylation of MCM2, MCM4, and MCM6 may play functionally redundant but essential roles in initiation of DNA replication.     

Essential role of phosphorylation of MCM2 by Cdc7/Dbf4 in the initiation of DNA replication in mammalian cells

We report the identification of Cdc7/Dbf4 phosphorylation sites in human MCM2 and the determination of the role of Cdc7/Dbf4 phosphorylation of MCM2 in the initiation of DNA replication. Using immunoblotting, immunofluorescence, and high-speed automated cell-imaging analyses with antibodies specific against MCM2 and Cdc7/Dbf4 phosphorylated MCM2, we show that the chromatin recruitment and phosphorylation of MCM2 are regulated during the cell cycle in HeLa cells. Chromatin-bound MCM2 is phosphorylated by Cdc7/Dbf4 during G1/S, which coincides with the initiation of DNA replication. Moreover, we show that baculovirus-expressed purified MCM2-7 complex and its phosphomimetic MCM2E-7 complex display higher ATPase activity when compared with the nonphosphorylatable MCM2A-7 complex in vitro. Furthermore, suppression of MCM2 expression in HeLa cells by siRNA results in the inhibition of DNA replication. The inhibition can be rescued by the coexpression of wild type MCM2 or MCM2E but not MCM2A. Taken together, these results indicate that Cdc7/Dbf4 phosphorylation of MCM2 is essential for the initiation of DNA replication in mammalian cells.     

Essential role of MCM proteins in premeiotic DNA replication

A critical event in eukaryotic DNA replication involves association of minichromosome maintenance (MCM2-7) proteins with origins, to form prereplicative complexes (pre-RCs) that are competent for initiation. The ability of mutants defective in MCM2-7 function to complete meiosis had suggested that pre-RC components could be irrelevant to premeiotic S phase. We show here that MCM2-7 proteins bind to chromatin in fission yeast cells preparing for meiosis and during premeiotic S phase in a manner suggesting they in fact are required for DNA replication in the meiotic cycle. This is confirmed by analysis of a degron mcm4 mutant, which cannot carry out premeiotic DNA replication. Later in meiosis, Mcm4 chromatin association is blocked between meiotic nuclear divisions, presumably accounting for the absence of a second round of DNA replication. Together, these results emphasize similarity between replication mechanisms in mitotic and meiotic cell cycles.     

Minichromosome maintenance proteins interact with checkpoint and recombination proteins to promote s-phase genome stability

The minichromosome maintenance (MCM) complex plays essential, conserved roles throughout DNA synthesis: first, as a component of the prereplication complex at origins and, then, as a helicase associated with replication forks. Here we use fission yeast (Schizosaccharomyces pombe) as a model to demonstrate a role for the MCM complex in protecting replication fork structure and promoting recovery from replication arrest. Loss of MCM function generates lethal double-strand breaks at sites of DNA synthesis during replication elongation, suggesting replication fork collapse. MCM function also maintains the stability of forks stalled by hydroxyurea that activate the replication checkpoint. In cells where the checkpoint is activated, Mcm4 binds the Cds1 kinase and undergoes Cds1-dependent phosphorylation. MCM proteins also interact with proteins involved in homologous recombination, which promotes recovery from arrest by ensuring normal mitosis. We suggest that the MCM complex links replication fork stabilization with checkpoint arrest and recovery through direct interactions with checkpoint and recombination proteins and that this role in S-phase genome stability is conserved from yeast to human cells.     

Electron microscopic observation and single-stranded DNA binding activity of the Mcm4,6,7 complex

Mcm2-7 proteins that play an essential role in eukaryotic DNA replication contain DNA-dependent ATPase motifs in a central domain that, from yeast to mammals, is highly conserved. Our group has reported that a DNA helicase activity is associated with a 600 kDa human Mcm4, 6 and 7 complex. The structure of the Mcm4,6,7 complex was visualized by electron microscopy after negative staining with uranyl acetate. The complex contained toroidal forms with a central channel and also contained structures with a slit. Gel-shift analysis indicated that the level of affinity of the Mcm4,6,7 complex for single-stranded DNA was comparable to that of SV40 T antigen, although the Mcm4,6,7 complex required longer single-stranded DNA for the binding than did SV40 T antigen. The nucleoprotein complexes of Mcm4,6,7 and single-stranded DNA were visualized as beads in a queue or beads on string-like structures. The formation of these nucleoprotein complexes was erased by Mcm2 that is a potential inhibitor of the Mcm4,6,7 helicase. We also found that the DNA helicase activity of Mcm4,6,7 complex was inhibited by the binding of Mcm3,5 complex. These results support the notion that the Mcm4,6,7 complex functions as a DNA helicase and the formation of 600 kDa complex is essential for the activity.