Modular organization of the Sulfolobus solfataricus mini-chromosome maintenance protein

Comparative transcriptome analysis of different developmental stage of Bactrocera minax (Diptera: Tephritidae): Implication of the molecular basis of its obligatory diapause induction

The Chinese citrus fruit fly, Bactrocera minax (Enderlein), a devastating pest in citrus, has an obligatory diapause at the pupal stage. Although the physiology of obligatory diapause has been well described, the molecular mechanisms underlying the process remain unknow. Here we investigated the molecular mechanisms of obligatory diapause induction in B. minax using high-throughput RNA-Seq data from second-instar larva (2L), third-instar larva (3L) and pupa (P) stages. A total of 116,402 unigenes were obtained, of which 54,781 unigenes were successfully annotated in public databases, and the differentially expressed genes in the 3L vs 2L, P vs 2L, and P vs 3L comparisons were identified. The cluster co-expression patterns of the differentially expressed genes revealed that significantly differentially-expressed genes in the pupal stage were predicted to be related to diapause induction. All differentially expressed genes were investigated by GO functional and KEGG pathway analysis, and the results showed that genes involved in processes such as 20-hydroxyecdysone (20E) biosynthesis, cell cycle and metabolic pathways are likely related to obligatory diapause induction in B. minax. These results provide important information on the transcriptome of the Chinese citrus fruit fly that can be used for further functional studies as well as contributing to our understanding of the molecular basis of obligatory diapause induction and suggesting potential molecular targets for the control of this pest.

Transcriptome sequencing for identification of diapause-associated genes in fall webworm, Hyphantria cunea Drury

Fall webworm, Hyphantria cunea Drury (Lepidoptera: Arctiidae) is extremely adaptable and highly invasive in China as a defoliator of ornamental and forest trees. Both voltinism and diapause strategies of fall webworm in China are variable, and this variability contributes to it invasiveness. Little is known about molecular regulation of diapause in fall webworm. To gain insight into possible mechanisms of diapause induction, high-throughput RNA-seq data were generated from non-diapause pupae (NDP) and diapause pupae (DP). A total of 58,151 unigenes were assembled and researched against nine public databases. In total, 29,013 up-regulated and 3451 down-regulated unigenes were differentially expressed by DP when compared with those of NDP. Genes encoding proteins such as UDP-glycosyl transferase (UGT), cytochrome P450 and Hsp70 were predicted to be involved in diapause. Moreover, GO function and KEGG pathway enrichments were performed on all differentially expressed genes (DEGs) and showed that cell cycle and insulin signaling pathways may be related to the diapause of the fall webworm. This study provides valuable information about the fall webworm transcriptome for future gene function research, especially as it relates to diapause.

Eukaryotic Replicative Helicase Subunit Interaction with DNA and Its Role in DNA Replication

The replicative helicase unwinds parental double-stranded DNA at a replication fork to provide single-stranded DNA templates for the replicative polymerases. In eukaryotes, the replicative helicase is composed of the Cdc45 protein, the heterohexameric ring-shaped Mcm2-7 complex, and the tetrameric GINS complex (CMG). The CMG proteins bind directly to DNA, as demonstrated by experiments with purified proteins. The mechanism and function of these DNA-protein interactions are presently being investigated, and a number of important discoveries relating to how the helicase proteins interact with DNA have been reported recently. While some of the protein-DNA interactions directly relate to the unwinding function of the enzyme complex, other protein-DNA interactions may be important for minichromosome maintenance (MCM) loading, origin melting or replication stress. This review describes our current understanding of how the eukaryotic replicative helicase subunits interact with DNA structures in vitro, and proposed models for the in vivo functions of replicative helicase-DNA interactions are also described.

Mechanisms and regulation of DNA replication initiation in eukaryotes

Cellular DNA replication is initiated through the action of multiprotein complexes that recognize replication start sites in the chromosome (termed origins) and facilitate duplex DNA melting within these regions. In a typical cell cycle, initiation occurs only once per origin and each round of replication is tightly coupled to cell division. To avoid aberrant origin firing and re-replication, eukaryotes tightly regulate two events in the initiation process: loading of the replicative helicase, MCM2-7, onto chromatin by the origin recognition complex (ORC), and subsequent activation of the helicase by its incorporation into a complex known as the CMG. Recent work has begun to reveal the details of an orchestrated and sequential exchange of initiation factors on DNA that give rise to a replication-competent complex, the replisome. Here, we review the molecular mechanisms that underpin eukaryotic DNA replication initiation - from selecting replication start sites to replicative helicase loading and activation - and describe how these events are often distinctly regulated across different eukaryotic model organisms.

Fundamental Characteristics of AAA+ Protein Family Structure and Function

Many complex cellular events depend on multiprotein complexes known as molecular machines to efficiently couple the energy derived from adenosine triphosphate hydrolysis to the generation of mechanical force. Members of the AAA+ ATPase superfamily (ATPases Associated with various cellular Activities) are critical components of many molecular machines. AAA+ proteins are defined by conserved modules that precisely position the active site elements of two adjacent subunits to catalyze ATP hydrolysis. In many cases, AAA+ proteins form a ring structure that translocates a polymeric substrate through the central channel using specialized loops that project into the central channel. We discuss the major features of AAA+ protein structure and function with an emphasis on pivotal aspects elucidated with archaeal proteins.

New Insights into the Mechanism of DNA Duplication by the Eukaryotic Replisome

The DNA replication machinery, or replisome, is a macromolecular complex that combines DNA unwinding, priming and synthesis activities. In eukaryotic cells, the helicase and polymerases are multi-subunit, highly-dynamic assemblies whose structural characterization requires an integrated approach. Recent studies have combined single-particle electron cryo-microscopy and protein crystallography to gain insights into the mechanism of DNA duplication by the eukaryotic replisome. We review current understanding of how replication fork unwinding by the CMG helicase is coupled to leading-strand synthesis by polymerase (Pol) ɛ and lagging-strand priming by Pol α/primase, and discuss emerging principles of replisome organization.

The Helicase Activity of Hyperthermophilic Archaeal MCM is Enhanced at High Temperatures by Lysine Methylation

Lysine methylation and methyltransferases are widespread in the third domain of life, archaea. Nevertheless, the effects of methylation on archaeal proteins wait to be defined. Here, we report that recombinant sisMCM, an archaeal homolog of Mcm2-7 eukaryotic replicative helicase, is methylated by aKMT4 in vitro. Mono-methylation of these lysine residues occurs coincidently in the endogenous sisMCM protein purified from the hyperthermophilic Sulfolobus islandicus cells as indicated by mass spectra. The helicase activity of mini-chromosome maintenance (MCM) is stimulated by methylation, particularly at temperatures over 70°C. The methylated MCM shows optimal DNA unwinding activity after heat-treatment between 76 and 82°C, which correlates well with the typical growth temperatures of hyperthermophilic Sulfolobus. After methylation, the half life of MCM helicase is dramatically extended at 80°C. The methylated sites are located on the accessible protein surface, which might modulate the intra- and inter- molecular interactions through changing the hydrophobicity and surface charge. Furthermore, the methylation-mimic mutants of MCM show heat resistance helicase activity comparable to the methylated MCM. These data provide the biochemical evidence that posttranslational modifications such as methylation may enhance kinetic stability of proteins under the elevated growth temperatures of hyperthermophilic archaea.

MCM ring hexamerization is a prerequisite for DNA-binding

The hexameric Minichromosome Maintenance (MCM) protein complex forms a ring that unwinds DNA at the replication fork in eukaryotes and archaea. Our recent crystal structure of an archaeal MCM N-terminal domain bound to single-stranded DNA (ssDNA) revealed ssDNA associating across tight subunit interfaces but not at the loose interfaces, indicating that DNA-binding is governed not only by the DNA-binding residues of the subunits (MCM ssDNA-binding motif, MSSB) but also by the relative orientation of the subunits. We now extend these findings by showing that DNA-binding by the MCM N-terminal domain of the archaeal organism Pyrococcus furiosus occurs specifically in the hexameric oligomeric form. We show that mutants defective for hexamerization are defective in binding ssDNA despite retaining all the residues observed to interact with ssDNA in the crystal structure. One mutation that exhibits severely defective hexamerization and ssDNA-binding is at a conserved phenylalanine that aligns with the mouse Mcm4(Chaos3) mutation associated with chromosomal instability, cancer, and decreased intersubunit association.

Mcm2-7 Is an Active Player in the DNA Replication Checkpoint Signaling Cascade via Proposed Modulation of Its DNA Gate

The DNA replication checkpoint (DRC) monitors and responds to stalled replication forks to prevent genomic instability. How core replication factors integrate into this phosphorylation cascade is incompletely understood. Here, through analysis of a unique mcm allele targeting a specific ATPase active site (mcm2DENQ), we show that the Mcm2-7 replicative helicase has a novel DRC function as part of the signal transduction cascade. This allele exhibits normal downstream mediator (Mrc1) phosphorylation, implying DRC sensor kinase activation. However, the mutant also exhibits defective effector kinase (Rad53) activation and classic DRC phenotypes. Our previous in vitro analysis showed that the mcm2DENQ mutation prevents a specific conformational change in the Mcm2-7 hexamer. We infer that this conformational change is required for its DRC role and propose that it allosterically facilitates Rad53 activation to ensure a replication-specific checkpoint response.

Characterization of the MCM homohexamer from the thermoacidophilic euryarchaeon Picrophilus torridus

The typical archaeal MCM exhibits helicase activity independently invitro. This study characterizes MCM from the euryarchaeon Picrophilus torridus. While PtMCM hydrolyzes ATP in DNA-independent manner, it displays very poor ability to unwind DNA independently, and then too only under acidic conditions. The protein exists stably in complex with PtGINS in whole cell lysates, interacting directly with PtGINS under neutral and acidic conditions. GINS strongly activates MCM helicase activity, but only at low pH. In consonance with this, PtGINS activates PtMCM-mediated ATP hydrolysis only at low pH, with the amount of ATP hydrolyzed during the helicase reaction increasing more than fifty-fold in the presence of GINS. While the stimulation of MCM-mediated helicase activity by GINS has been reported in MCMs from P.furiosus, T.kodakarensis, and very recently, T.acidophilum, to the best of our knowledge, this is the first report of an MCM helicase demonstrating DNA unwinding activity only at such acidic pH, across all archaea and eukaryotes. PtGINS may induce/stabilize a conducive conformation of PtMCM under acidic conditions, favouring PtMCM-mediated DNA unwinding coupled to ATP hydrolysis. Our findings underscore the existence of divergent modes of replication regulation among archaea and the importance of investigating replication events in more archaeal organisms.

Structure and regulatory role of the C-terminal winged helix domain of the archaeal minichromosome maintenance complex

The minichromosome maintenance complex (MCM) represents the replicative DNA helicase both in eukaryotes and archaea. Here, we describe the solution structure of the C-terminal domains of the archaeal MCMs of Sulfolobus solfataricus (Sso) and Methanothermobacter thermautotrophicus (Mth). Those domains consist of a structurally conserved truncated winged helix (WH) domain lacking the two typical ‘wings’ of canonical WH domains. A less conserved N-terminal extension links this WH module to the MCM AAA+ domain forming the ATPase center. In the Sso MCM this linker contains a short α-helical element. Using Sso MCM mutants, including chimeric constructs containing Mth C-terminal domain elements, we show that the ATPase and helicase activity of the Sso MCM is significantly modulated by the short α-helical linker element and by N-terminal residues of the first α-helix of the truncated WH module. Finally, based on our structural and functional data, we present a docking-derived model of the Sso MCM, which implies an allosteric control of the ATPase center by the C-terminal domain.

Analysis of the crystal structure of an active MCM hexamer

In a previous Research article (Froelich et al., 2014), we suggested an MCM helicase activation mechanism, but were limited in discussing the ATPase domain because it was absent from the crystal structure. Here we present the crystal structure of a nearly full-length MCM hexamer that is helicase-active and thus has all features essential for unwinding DNA. The structure is a chimera of Sulfolobus solfataricus N-terminal domain and Pyrococcus furiosus ATPase domain. We discuss three major findings: 1) a novel conformation for the A-subdomain that could play a role in MCM regulation; 2) interaction of a universally conserved glutamine in the N-terminal Allosteric Communication Loop with the AAA+ domain helix-2-insert (h2i); and 3) a recessed binding pocket for the MCM ssDNA-binding motif influenced by the h2i. We suggest that during helicase activation, the h2i clamps down on the leading strand to facilitate strand retention and regulate ATP hydrolysis.

DOI:http://dx.doi.org/10.7554/eLife.03433.001

A conserved MCM single-stranded DNA binding element is essential for replication initiation

The ring-shaped MCM helicase is essential to all phases of DNA replication. The complex loads at replication origins as an inactive double-hexamer encircling duplex DNA. Helicase activation converts this species to two active single hexamers that encircle single-stranded DNA (ssDNA). The molecular details of MCM DNA interactions during these events are unknown. We determined the crystal structure of the Pyrococcus furiosus MCM N-terminal domain hexamer bound to ssDNA and define a conserved MCM-ssDNA binding motif (MSSB). Intriguingly, ssDNA binds the MCM ring interior perpendicular to the central channel with defined polarity. In eukaryotes, the MSSB is conserved in several Mcm2-7 subunits, and MSSB mutant combinations in S. cerevisiae Mcm2-7 are not viable. Mutant Mcm2-7 complexes assemble and are recruited to replication origins, but are defective in helicase loading and activation. Our findings identify an important MCM-ssDNA interaction and suggest it functions during helicase activation to select the strand for translocation.

DOI:http://dx.doi.org/10.7554/eLife.01993.001

When DNA was first recognised to be a double helix, it was clear that this structure could easily explain how DNA could be replicated. Each strand was made of bases—represented by the letters ‘A’, ‘C’, ‘G’ and ‘T’—and the two strands were held together by bonds between pairs of bases, one from each strand. Moreover, ‘A’ always paired with ‘T’, and ‘C’ always paired with ‘G’. Therefore, if the two strands were separated, each could be used as a template to guide the synthesis of a new complementary strand and thus create two copies of the original double-stranded molecule. One of the first steps in this replication process involves a ring-shaped complex of six proteins, called an MCM helicase, separating the two strands.

To prepare for DNA replication, two MCM helicase rings wrap around the double-stranded DNA. Then, after the helicase has been activated, the bonds between the DNA base pairs break, and the two rings separate with one ring encircling each DNA strand. However, the details of the interactions between the helicase and the DNA during these events are not fully understood.

Now Froelich, Kang et al. have solved the three-dimensional structure of an MCM helicase ring—taken from a microbe originally found at deep ocean vents—on its own and also when bound to a short piece of single-stranded DNA. The helicase ring becomes more oval when the DNA binds to it. Moreover, rather than passing straight through the ring, the DNA wraps part of the way around the inside of the ring.

Specific amino acids—the building blocks of proteins—on the inside of the ring interact with the single-stranded DNA, and these amino acids are also found in MCM proteins in many other organisms. Furthermore, swapping these amino acids for different amino acids significantly reduced the ability of the ring to bind to single-stranded DNA, but its ability to bind to double-stranded DNA was only slightly affected. Engineering similar changes into the ring complexes of yeast cells was lethal, and the mutant complexes were less able to be loaded onto the DNA, or to be activated and separate the two strands ready for replication.

These insights into how helicases are loaded onto double-stranded DNA, and select one DNA strand to encircle, have improved our understanding of how DNA replication is initiated: a process that is vital for living things.

DOI:http://dx.doi.org/10.7554/eLife.01993.002

ATP-dependent conformational dynamics underlie the functional asymmetry of the replicative helicase from a minimalist eukaryote

The heterohexameric minichromosome maintenance (MCM2-7) complex is an ATPase that serves as the central replicative helicase in eukaryotes. During initiation, the ring-shaped MCM2-7 particle is thought to open to facilitate loading onto DNA. The conformational state accessed during ring opening, the interplay between ATP binding and MCM2-7 architecture, and the use of these events in the regulation of DNA unwinding are poorly understood. To address these issues in isolation from the regulatory complexity of existing eukaryotic model systems, we investigated the structure/function relationships of a naturally minimized MCM2-7 complex from the microsporidian parasite Encephalitozoon cuniculi. Electron microscopy and small-angle X-ray scattering studies show that, in the absence of ATP, MCM2-7 spontaneously adopts a left-handed, open-ring structure. Nucleotide binding does not promote ring closure but does cause the particle to constrict in a two-step process that correlates with the filling of high- and low-affinity ATPase sites. Our findings support the idea that an open ring forms the default conformational state of the isolated MCM2-7 complex, and they provide a structural framework for understanding the multiphasic ATPase kinetics observed in different MCM2-7 systems.

MCM structure and mechanics: what we have learned from archaeal MCM

Minichromosome maintenance (MCM) complexes have been identified as the primary replicative helicases responsible for unwinding DNA for genome replication. The focus of this chapter is to discuss the current structural and functional understanding of MCMs and their role at origins of replication, which are based mostly on the studies of MCM proteins and MCM complexes from archaeal genomes.

Insights into the MCM functional mechanism: lessons learned from the archaeal MCM complex

The helicase function of the minichromosome maintenance protein (MCM) is essential for genomic DNA replication in archaea and eukaryotes. There has been rapid progress in studies of the structure and function of MCM proteins from different organisms, leading to better understanding of the MCM helicase mechanism. Because there are a number of excellent reviews on this topic, we will use this review to summarize some of the recent progress, with particular focus on the structural aspects of MCM and their implications for helicase function. Given the hexameric and double hexameric architecture observed by X-ray crystallography and electron microscopy of MCMs from archaeal and eukaryotic cells, we summarize and discuss possible unwinding modes by either a hexameric or a double hexameric helicase. Additionally, our recent crystal structure of a full length archaeal MCM has provided structural information on an intact, multi-domain MCM protein, which includes the salient features of four unusual beta-hairpins from each monomer, and the side channels of a hexamer/double hexamer. These new structural data enable a closer examination of the structural basis of the unwinding mechanisms by MCM.

The Mcm complex: unwinding the mechanism of a replicative helicase

The Mcm2-7 complex serves as the eukaryotic replicative helicase, the molecular motor that both unwinds duplex DNA and powers fork progression during DNA replication. Consistent with its central role in this process, much prior work has illustrated that Mcm2-7 loading and activation are landmark events in the regulation of DNA replication. Unlike any other hexameric helicase, Mcm2-7 is composed of six unique and essential subunits. Although the unusual oligomeric nature of this complex has long hampered biochemical investigations, recent advances with both the eukaryotic as well as the simpler archaeal Mcm complexes provide mechanistic insight into their function. In contrast to better-studied homohexameric helicases, evidence suggests that the six Mcm2-7 complex ATPase active sites are functionally distinct and are likely specialized to accommodate the regulatory constraints of the eukaryotic process.

Structural biology of MCM helicases

The eukaryotic MCM2-7 complex is recruited onto origins of replication during the G1 phase of the cell cycle and acts as the main helicase at the replication fork during the S phase. Over the last few years a number of structural reports on MCM proteins using both electron microscopy and protein crystallography have been published. The crystal structures of two (almost) full-length archaeal homologs provide the first atomic pictures of a MCM helicase. However one of the structures is at low resolution and the other is of an inactive MCM. Moreover, both proteins are monomeric in the crystal, whereas the activity of the complex is critically dependent on oligomerization. Lower resolution structures derived from electron microscopy studies are therefore crucial to complement the crystallographic analysis and to assemble the multimeric complex that is active in the cell. A critical analysis of all the structural results elucidates the potential conformational changes and dynamic behavior of MCM helicase to provide a first insight into the gamut of molecular configurations adopted during the processes of DNA melting and unwinding.

A biochemically active MCM-like helicase in Bacillus cereus

The mini-chromosome maintenance (MCM) proteins serve as the replicative helicases in archaea and eukaryotes. Interestingly, an MCM homolog was identified, by BLAST analysis, within a phage integrated in the bacterium Bacillus cereus (Bc). BcMCM is only related to the AAA region of MCM-helicases; the typical amino-terminus is missing and is replaced by a segment with weak homology to primases. We show that BcMCM displays 3′→5′ helicase and ssDNA-stimulated ATPase activity, properties that arise from its conserved AAA domain. Isolated BcMCM is a monomer in solution but likely forms the functional oligomer in vivo. We found that the BcMCM amino-terminus can bind ssDNA and harbors a zinc atom, both hallmarks of the typical MCM amino-terminus. No BcMCM-catalyzed primase activity could be detected. We propose that the divergent amino-terminus of BcMCM is a paralog of the corresponding region of MCM-helicases. A divergent amino terminus makes BcMCM a useful model for typical MCM-helicases since it accomplishes the same function using an apparently unrelated structure.

Crystal structure of a near-full-length archaeal MCM: functional insights for an AAA+ hexameric helicase

The minichromosome maintenance protein (MCM) complex is an essential replicative helicase for DNA replication in Archaea and Eukaryotes. Whereas the eukaryotic complex consists of 6 homologous proteins (MCM2-7), the archaeon Sulfolobus solfataricus has only 1 MCM protein (ssoMCM), 6 subunits of which form a homohexamer. Here, we report a 4.35-A crystal structure of the near-full-length ssoMCM. The structure shows an elongated fold, with 5 subdomains that are organized into 2 large N- and C-terminal domains. A near-full-length ssoMCM hexamer generated based on the 6-fold symmetry of the N-terminal Methanothermobacter thermautotrophicus (mtMCM) hexamer shows intersubunit distances suitable for bonding contacts, including the interface around the ATP pocket. Four unusual beta-hairpins of each subunit are located inside the central channel or around the side channels in the hexamer. Additionally, the hexamer fits well into the double-hexamer EM map of mtMCM. Our mutational analysis of residues at the intersubunit interfaces and around the side channels demonstrates their critical roles for hexamerization and helicase function. These structural and biochemical results provide a basis for future study of the helicase mechanisms of the archaeal and eukaryotic MCM complexes in DNA replication.

Cryo-electron microscopy reveals a novel DNA-binding site on the MCM helicase

The eukaryotic MCM2-7 complex is recruited at origins of replication during the G1 phase and acts as the main helicase at the replication fork during the S phase of the cell cycle. To characterize the interplay between the MCM helicase and DNA prior to the melting of the double helix, we determined the structure of an archaeal MCM orthologue bound to a 5.6-kb double-stranded DNA segment, using cryo-electron microscopy. DNA wraps around the N-terminal face of a single hexameric ring. This interaction requires a conformational change within the outer belt of the MCM N-terminal domain, exposing a previously unrecognized helix-turn-helix DNA-binding motif. Our findings provide novel insights into the role of the MCM complex during the initiation step of DNA replication.

Structural analysis of the Sulfolobus solfataricus MCM protein N-terminal domain

The Mini-Chromosome Maintenance (MCM) proteins are candidates of replicative DNA helicase in eukarya and archaea. Here we report a 2.8 Å crystal structure of the N-terminal domain (residues 1–268) of the Sulfolobus solfataricus MCM (Sso MCM) protein. The structure reveals single-hexameric ring-like architecture, at variance from the protein of Methanothermobacter thermoautotrophicus (Mth). Moreover, the central channel in Sso MCM seems significantly narrower than the Mth counterpart, which appears to more favorably accommodate single-stranded DNA than double-stranded DNA, as supported by DNA-binding assays. Structural analysis also highlights the essential role played by the zinc-binding domain in the interaction with nucleic acids and allows us to speculate that the Sso MCM N-ter domain may function as a molecular clamp to grasp the single-stranded DNA passing through the central channel. On this basis possible DNA unwinding mechanisms are discussed.

The MCM complex: (just) a replicative helicase?

The MCM2-MCM7 (minichromosome maintenance 2-7) complex is involved both in the initiation and the elongation step of eukaryotic DNA replication and is believed to be the replicative helicase. Whereas the mechanism of DNA unwinding at the replication fork has been extensively investigated, the role of the MCM2-MCM7 complex during initiation has not yet been characterized by biochemical studies. Here we summarize the in vivo evidence which supports a role for the MCM complex in origin melting. In addition, we present an overview of the mechanism of action of a number of AAA+ (ATPase associated with various cellular activities) initiators and hexameric helicases, which can be used in turn as models for the steps of recognition, duplex melting, loading and nucleic acid translocation of the MCM helicase.

Biochemical characterization of fragmented human MCM2

The molecular dissection of human MCM2, a constituent of MCM2-7 licensing factor complex, was performed to identify the region responsible for its biochemical activities. Partial digestion with trypsin dissected the MCM2 protein into a central region (148-676) containing ATPase motifs and a C-terminal region (677-895). These two fragments, along with three other fragments (148-441, 442-676 and 442-895), were produced using the wheat germ cell-free system and were examined for their ability to inhibit MCM4/6/7 helicase activity. Two fragments (442-895 and 677-895) containing the C-terminus were partly inhibitory to the activity. Further dissection revealed that one fragment (713-895) has strong inhibitory activity. The inhibitory activity of the smaller fragments derived from the C-terminal region correlated with their ability to inhibit SV40 T antigen helicase activity and also with their ability to bind to ssDNA, which has been shown by gel mobility shift analysis. These results strongly suggest that the MCM2 fragments derived from the C-terminal region inhibit DNA helicase activity through their ability to bind to ssDNA. In contrast, two fragments (148-441 and 442-676) from the central region were mainly responsible for the interaction between MCM2 and MCM4, and this was revealed by a pulldown analysis using MCM4 protein beads. Finally, only complete MCM2, not the smaller fragments, could disassemble the MCM4/6/7 hexamer into the MCM2/4/6/7 tetramer.