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

Overexpression of the plastidial pseudo-protease AtFtsHi3 enhances drought tolerance while sustaining plant growth

With climate change, droughts are expected to be more frequent and severe, severely impacting plant biomass and quality. Here, we show that overexpressing the Arabidopsis gene AtFtsHi3 (FtsHi3OE) enhances drought-tolerant phenotypes without compromising plant growth. AtFtsHi3 encodes a chloroplast envelope pseudo-protease; knock-down mutants (ftshi3-1) are found to be drought tolerant but exhibit stunted growth. Altered AtFtsHi3 expression therefore leads to drought tolerance, while only diminished expression of this gene leads to growth retardation. To understand the underlying mechanisms of the enhanced drought tolerance, we compared the proteomes of ftshi3-1 and pFtsHi3-FtsHi3OE (pFtsHi3-OE) to wild-type plants under well-watered and drought conditions. Drought-related processes like osmotic stress, water transport, and abscisic acid response were enriched in pFtsHi3-OE and ftshi3-1 mutants following their enhanced drought response compared to wild-type. The knock-down mutant ftshi3-1 showed an increased abundance of HSP90, HSP93, and TIC110 proteins, hinting at a potential downstream role of AtFtsHi3 in chloroplast pre-protein import. Mathematical modeling was performed to understand how variation in the transcript abundance of AtFtsHi3 can, on the one hand, lead to drought tolerance in both overexpression and knock-down lines, yet, on the other hand, affect plant growth so differently. The results led us to hypothesize that AtFtsHi3 may form complexes with at least two other protease subunits, either as homo- or heteromeric structures. Enriched amounts of AtFtsH7/9, AtFtsH11, AtFtsH12, and AtFtsHi4 in ftshi3-1 suggest a possible compensation mechanism for these proteases in the hexamer.

DONSON facilitates Cdc45 and GINS chromatin association and is essential for DNA replication initiation

Faithful cell division is the basis for the propagation of life and DNA replication must be precisely regulated. DNA replication stress is a prominent endogenous source of genome instability that not only leads to ageing, but also neuropathology and cancer development in humans. Specifically, the issues of how vertebrate cells select and activate origins of replication are of importance as, for example, insufficient origin firing leads to genomic instability and mutations in replication initiation factors lead to the rare human disease Meier-Gorlin syndrome. The mechanism of origin activation has been well characterised and reconstituted in yeast, however, an equal understanding of this process in higher eukaryotes is lacking. The firing of replication origins is driven by S-phase kinases (CDKs and DDK) and results in the activation of the replicative helicase and generation of two bi-directional replication forks. Our data, generated from cell-free Xenopus laevis egg extracts, show that DONSON is required for assembly of the active replicative helicase (CMG complex) at origins during replication initiation. DONSON has previously been shown to be essential during DNA replication, both in human cells and in Drosophila, but the mechanism of DONSON's action was unknown. Here we show that DONSON's presence is essential for replication initiation as it is required for Cdc45 and GINS association with Mcm2-7 complexes and helicase activation. To fulfil this role, DONSON interacts with the initiation factor, TopBP1, in a CDK-dependent manner. Following its initiation role, DONSON also forms a part of the replisome during the elongation stage of DNA replication. Mutations in DONSON have recently been shown to lead to the Meier-Gorlin syndrome; this novel replication initiation role of DONSON therefore provides the explanation for the phenotypes caused by DONSON mutations in patients.

The assembly of the MCM2-7 hetero-hexamer and its significance in DNA replication

The mini-chromosome maintenance proteins 2-7 (MCM2-7) hexamer is a protein complex that is key for eukaryotic DNA replication, which occurs only once per cell cycle. To achieve DNA replication, eukaryotic cells developed multiple mechanisms that control the timing of the loading of the hexamer onto chromatin and its activation as the replicative helicase. MCM2-7 is highly abundant in proliferating cells, which confers resistance to replication stress. Thus, the presence of an excess of MCM2-7 is important for maintaining genome integrity. However, the mechanism via which high MCM2-7 levels are achieved, other than the transcriptional upregulation of the MCM genes in the G1 phase, remained unknown. Recently, we and others reported that the MCM-binding protein (MCMBP) plays a role in the maintenance of high MCM2-7 levels and hypothesized that MCMBP functions as a chaperone in the assembly of the MCM2-7 hexamer. In this review, we discuss the roles of MCMBP in the control of MCM proteins and propose a model of the assembly of the MCM2-7 hexamer. Furthermore, we discuss a potential mechanism of the licensing checkpoint, which arrests the cells in the G1 phase when the levels of chromatin-bound MCM2-7 are reduced, and the possibility of targeting MCMBP as a chemotherapy for cancer.

MCMBP promotes the assembly of the MCM2-7 hetero-hexamer to ensure robust DNA replication in human cells

The MCM2–7 hetero-hexamer is the replicative DNA helicase that plays a central role in eukaryotic DNA replication. In proliferating cells, the expression level of the MCM2–7 hexamer is kept high, which safeguards the integrity of the genome. However, how the MCM2–7 hexamer is assembled in living cells remains unknown. Here, we revealed that the MCM-binding protein (MCMBP) plays a critical role in the assembly of this hexamer in human cells. MCMBP associates with MCM3 which is essential for maintaining the level of the MCM2–7 hexamer. Acute depletion of MCMBP demonstrated that it contributes to MCM2–7 assembly using nascent MCM3. Cells depleted of MCMBP gradually ceased to proliferate because of reduced replication licensing. Under this condition, p53-positive cells exhibited arrest in the G1 phase, whereas p53-null cells entered the S phase and lost their viability because of the accumulation of DNA damage, suggesting that MCMBP is a potential target for killing p53-deficient cancers.

The role of DDK and Treslin-MTBP in coordinating replication licensing and pre-initiation complex formation

Treslin/Ticrr is required for the initiation of DNA replication and binds to MTBP (Mdm2 Binding Protein). Here, we show that in Xenopus egg extract, MTBP forms an elongated tetramer with Treslin containing two molecules of each protein. Immunodepletion and add-back experiments show that Treslin-MTBP is rate limiting for replication initiation. It is recruited onto chromatin before S phase starts and recruitment continues during S phase. We show that DDK activity both increases and strengthens the interaction of Treslin-MTBP with licensed chromatin. We also show that DDK activity cooperates with CDK activity to drive the interaction of Treslin-MTBP with TopBP1 which is a regulated crucial step in pre-initiation complex formation. These results suggest how DDK works together with CDKs to regulate Treslin-MTBP and plays a crucial in selecting which origins will undergo initiation.

Regulation of the Minichromosome Maintenance Protein 3 (MCM3) Chromatin Binding by the Prolyl Isomerase Pin1

Human Pin1 is a peptidyl prolyl cis/trans isomerase with a unique preference for phosphorylated Ser/Thr-Pro substrate motifs. Here we report that MCM3 (minichromosome maintenance complex component 3) is a novel target of Pin1. MCM3 interacts directly with the WW domain of Pin1. Proline-directed phosphorylation of MCM3 at S112 and T722 are crucial for the interaction with Pin1. MCM3 as a subunit of the minichromosome maintenance heterocomplex MCM2-7 is part of the pre-replication complex responsible for replication licensing and is implicated in the formation of the replicative helicase during progression of replication. Our data suggest that Pin1 coordinates phosphorylation-dependently MCM3 loading onto chromatin and its unloading from chromatin, thereby mediating S phase control.

The Human Replicative Helicase, the CMG Complex, as a Target for Anti-cancer Therapy

DNA helicases unwind or rearrange duplex DNA during replication, recombination and repair. Helicases of many pathogenic organisms such as viruses, bacteria, and protozoa have been studied as potential therapeutic targets to treat infectious diseases, and human DNA helicases as potential targets for anti-cancer therapy. DNA replication machineries perform essential tasks duplicating genome in every cell cycle, and one of the important functions of these machineries are played by DNA helicases. Replicative helicases are usually multi-subunit protein complexes, and the minimal complex active as eukaryotic replicative helicase is composed of 11 subunits, requiring a functional assembly of two subcomplexes and one protein. The hetero-hexameric MCM2-7 helicase is activated by forming a complex with Cdc45 and the hetero-tetrameric GINS complex; the Cdc45-Mcm2-7-GINS (CMG) complex. The CMG complex can be a potential target for a treatment of cancer and the feasibility of this replicative helicase as a therapeutic target has been tested recently. Several different strategies have been implemented and are under active investigations to interfere with helicase activity of the CMG complex. This review focuses on the molecular function of the CMG helicase during DNA replication and its relevance to cancers based on data published in the literature. In addition, current efforts made to identify small molecules inhibiting the CMG helicase to develop anti-cancer therapeutic strategies were summarized, with new perspectives to advance the discovery of the CMG-targeting drugs.

The High-Affinity Interaction between ORC and DNA that Is Required for Replication Licensing Is Inhibited by 2-Arylquinolin-4-Amines

In late mitosis and G1, origins of DNA replication must be "licensed" for use in the upcoming S phase by being encircled by double hexamers of the minichromosome maintenance proteins MCM2-7. A "licensing checkpoint" delays cells in G1 until sufficient origins have been licensed, but this checkpoint is lost in cancer cells. Inhibition of licensing can therefore kill cancer cells while only delaying normal cells in G1. In a high-throughput cell-based screen for licensing inhibitors we identified a family of 2-arylquinolin-4-amines, the most potent of which we call RL5a. The binding of the origin recognition complex (ORC) to origin DNA is the first step of the licensing reaction. We show that RL5a prevents ORC forming a tight complex with DNA that is required for MCM2-7 loading. Formation of this ORC-DNA complex requires ATP, and we show that RL5a inhibits ORC allosterically to mimic a lack of ATP.

CUL-2LRR-1 and UBXN-3 drive replisome disassembly during DNA replication termination and mitosis

Replisome disassembly is the final step of DNA replication in eukaryotes, involving the ubiquitylation and CDC48-dependent dissolution of the CMG helicase (CDC45-MCM-GINS). Using Caenorhabditis elegans early embryos and Xenopus laevis egg extracts, we show that the E3 ligase CUL-2^LRR-1 associates with the replisome and drives ubiquitylation and disassembly of CMG, together with the CDC-48 cofactors UFD-1 and NPL-4. Removal of CMG from chromatin in frog egg extracts requires CUL2 neddylation, and our data identify chromatin recruitment of CUL2^LRR1 as a key regulated step during DNA replication termination. Interestingly, however, CMG persists on chromatin until prophase in worms that lack CUL-2^LRR-1, but is then removed by a mitotic pathway that requires the CDC-48 cofactor UBXN-3, orthologous to the human tumour suppressor FAF1. Partial inactivation of lrr-1 and ubxn-3 leads to synthetic lethality, suggesting future approaches by which a deeper understanding of CMG disassembly in metazoa could be exploited therapeutically.

Reversal of DDK-Mediated MCM Phosphorylation by Rif1-PP1 Regulates Replication Initiation and Replisome Stability Independently of ATR/Chk1

Dbf4-dependent kinases (DDKs) are required for the initiation of DNA replication, their essential targets being the MCM2-7 proteins. We show that, in Xenopus laevis egg extracts and human cells, hyper-phosphorylation of DNA-bound Mcm4, but not phosphorylation of Mcm2, correlates with DNA replication. These phosphorylations are differentially affected by the DDK inhibitors PHA-767491 and XL413. We show that DDK-dependent MCM phosphorylation is reversed by protein phosphatase 1 (PP1) targeted to chromatin by Rif1. Loss of Rif1 increased MCM phosphorylation and the rate of replication initiation and also compromised the ability of cells to block initiation when challenged with replication inhibitors. We also provide evidence that Rif1 can mediate MCM dephosphorylation at replication forks and that the stability of dephosphorylated replisomes strongly depends on Chk1 activity. We propose that both replication initiation and replisome stability depend on MCM phosphorylation, which is maintained by a balance of DDK-dependent phosphorylation and Rif1-mediated dephosphorylation.

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.

Minichromosome maintenance protein 7 and geminin expression: Prognostic value in laryngeal squamous cell carcinoma in patients treated with radiotherapy and cetuximab

BACKGROUND

Minichromosome maintenance protein 7 (MCM7) is a downstream of human epidermal growth receptor (HER1) signaling. We examined MCM7, geminin, and HER1 expression in patients with laryngeal squamous cell carcinoma (SCC) treated with radiotherapy and cetuximab.

METHODS

MCM7, geminin, and HER1 were evaluated by immunohistochemistry on 61 patients with laryngeal SCC. The follow-up (median, 32.1 months; range, 2-139 months) went from the beginning of therapy to tumor progression-free survival (PFS) and death (overall survival [OS]).

RESULTS

MCM7, but not geminin, was associated only with HER1 expression, whereas no association was found with other clinicopathological characteristics. Patients with MCM7 high - geminin high and MCM7 high - geminin low tumor status had a risk of progression 3.1 times and 17.7 times greater, respectively, than patients with MCM7 low - geminin high tumor status. Tumor site, MCM7, and geminin were independent determinants of PFS, whereas MCM7 was an independent prognostic marker of OS.

CONCLUSION

MCM7-geminin tumor status may be prognostic for patients with laryngeal SCC treated with cetuximab and radiotherapy. © 2016 Wiley Periodicals, Inc. Head Neck 39: 684-693, 2017.

Xenopus Mcm10 is a CDK-substrate required for replication fork stability

During S phase, following activation of the S phase CDKs and the DBF4-dependent kinases (DDK), double hexamers of Mcm2-7 at licensed replication origins are activated to form the core replicative helicase. Mcm10 is one of several proteins that have been implicated from work in yeasts to play a role in forming a mature replisome during the initiation process. Mcm10 has also been proposed to play a role in promoting replisome stability after initiation has taken place. The role of Mcm10 is particularly unclear in metazoans, where conflicting data has been presented. Here, we investigate the role and regulation of Mcm10 in Xenopus egg extracts. We show that Xenopus Mcm10 is recruited to chromatin late in the process of replication initiation and this requires prior action of DDKs and CDKs. We also provide evidence that Mcm10 is a CDK substrate but does not need to be phosphorylated in order to associate with chromatin. We show that in extracts depleted of more than 99% of Mcm10, the bulk of DNA replication still occurs, suggesting that Mcm10 is not required for the process of replication initiation. However, in extracts depleted of Mcm10, the replication fork elongation rate is reduced. Furthermore, the absence of Mcm10 or its phosphorylation by CDK results in instability of replisome proteins on DNA, which is particularly important under conditions of replication stress.

Cross-linking immunoprecipitation-MS (xIP-MS): Topological Analysis of Chromatin-associated Protein Complexes Using Single Affinity Purification

In recent years, cross-linking mass spectrometry has proven to be a robust and effective method of interrogating macromolecular protein complex topologies at peptide resolution. Traditionally, cross-linking mass spectrometry workflows have utilized homogenous complexes obtained through time-limiting reconstitution, tandem affinity purification, and conventional chromatography workflows. Here, we present cross-linking immunoprecipitation-MS (xIP-MS), a simple, rapid, and efficient method for structurally probing chromatin-associated protein complexes using small volumes of mammalian whole cell lysates, single affinity purification, and on-bead cross-linking followed by LC-MS/MS analysis. We first benchmarked xIP-MS using the structurally well-characterized phosphoribosyl pyrophosphate synthetase complex. We then applied xIP-MS to the chromatin-associated cohesin (SMC1A/3), XRCC5/6 (Ku70/86), and MCM complexes, and we provide novel structural and biological insights into their architectures and molecular function. Of note, we use xIP-MS to perform topological studies under cell cycle perturbations, showing that the xIP-MS protocol is sufficiently straightforward and efficient to allow comparative cross-linking experiments. This work, therefore, demonstrates that xIP-MS is a robust, flexible, and widely applicable methodology for interrogating chromatin-associated protein complex architectures.

Polyubiquitylation drives replisome disassembly at the termination of DNA replication

Resolution of replication forks during termination of DNA replication is essential for accurate duplication of eukaryotic genomes. Here we present evidence consistent with the idea that polyubiquitylation of a replisome component (Mcm7) leads to its disassembly at the converging terminating forks because of the action of the p97/VCP/Cdc48 protein remodeler. Using Xenopus laevis egg extract, we have shown that blocking polyubiquitylation results in the prolonged association of the active helicase with replicating chromatin. The Mcm7 subunit is the only component of the active helicase that we find polyubiquitylated during replication termination. The observed polyubiquitylation is followed by disassembly of the active helicase dependent on p97/VCP/Cdc48. Altogether, our data provide insight into the mechanism of replisome disassembly during eukaryotic DNA replication termination.

Xenopus Cdc7 executes its essential function early in S phase and is counteracted by checkpoint-regulated protein phosphatase 1

The initiation of DNA replication requires two protein kinases: cyclin-dependent kinase (Cdk) and Cdc7. Although S phase Cdk activity has been intensively studied, relatively little is known about how Cdc7 regulates progression through S phase. We have used a Cdc7 inhibitor, PHA-767491, to dissect the role of Cdc7 in Xenopus egg extracts. We show that hyperphosphorylation of mini-chromosome maintenance (MCM) proteins by Cdc7 is required for the initiation, but not for the elongation, of replication forks. Unlike Cdks, we demonstrate that Cdc7 executes its essential functions by phosphorylating MCM proteins at virtually all replication origins early in S phase and is not limiting for progression through the Xenopus replication timing programme. We demonstrate that protein phosphatase 1 (PP1) is recruited to chromatin and rapidly reverses Cdc7-mediated MCM hyperphosphorylation. Checkpoint kinases induced by DNA damage or replication inhibition promote the association of PP1 with chromatin and increase the rate of MCM dephosphorylation, thereby counteracting the previously completed Cdc7 functions and inhibiting replication initiation. This novel mechanism for regulating Cdc7 function provides an explanation for previous contradictory results concerning the control of Cdc7 by checkpoint kinases and has implications for the use of Cdc7 inhibitors as anti-cancer agents.

Microarray gene expression analysis of tumorigenesis and regional lymph node metastasis in laryngeal squamous cell carcinoma

BACKGROUND

Laryngeal squamous cell carcinoma (LSCC) is the most common type in head and neck squamous cell carcinoma (HNSCC), and the development and progression of LSCC are multistep processes accompanied by changes of molecular biology.

OBJECTIVE

The purpose of this study was to investigate the molecular basis of tumorigenesis and regional lymph node metastasis in LSCC, and provide a set of genes that may be useful for the development of novel diagnostic markers and/or more effective therapeutic strategies.

METHODS

A total number of 10 patients who underwent surgery for primary laryngeal squamous cell carcinoma were recruited for microarray analysis. LSCC tissues compared with corresponding adjacent non-neoplastic tissues were analysed by Illumina mRNA microarrays, and LSCC tissues with regional lymph node metastasis and LSCC tissues without regional lymph node metastasis were analyzed in the same manner. The most frequently differently expressed genes screened by microarrays were also validated by qRT-PCR in another 42 patients diagnosed for LSCC.

RESULTS

Analysed by Illumina mRNA microarrays, there were 361 genes significantly related to tumorigenesis while 246 genes significantly related to regional lymph node metastasis in LSCC. We found that the six genes (CDK1, CDK2, CDK4, MCM2, MCM3, MCM4) were most frequently differently expressed functional genes related to tumorigenesis while eIF3a and RPN2 were most frequently differently expressed functional genes related to regional lymph node metastasis in LSCC. The expressions of these genes were also validated by qRT-PCR.

CONCLUSIONS

The research revealed a gene expression signature of tumorigenesis and regional lymph node metastasis in laryngeal squamous cell carcinoma. Of the total, the deregulation of several genes (CDK1, CDK2, CDK4, MCM2, MCM3, MCM4, EIF3a and RPN2) were potentially associated with disease development and progression. The result will contribute to the understanding of the molecular basis of LSCC and help to improve diagnosis and treatment.

Norcantharidin inhibits pre-replicative complexes assembly of HepG2 cells

Norcantharidin (NCTD) is currently used for anticancer therapy but the exact mechanism of action remains unknown. Pre-replicative complexes (pre-RCs) are essential for cell DNA replication and highly related to malignant proliferation. Here, we examined the inhibitory effect of NCTD on pre-RC components in HepG2 cells. We showed that NCTD induced degradation of Cdc6 and Mcm2 in a dose-dependent manner. Under 100 μM NCTD concentration, about 70% of Cdc6 and 50% of Mcm2 were degraded. In addition, the nuclear translocation of Mcm6 was inhibited by NCTD. Further studies aiming at G1 synchronous cells showed that, NCTD reduced the chromatin-bound Cdc6, Mcm2 and Mcm6. Moreover, the cells were blocked from entering the S phase and accumulated at the G1 phase when released synchronously into the cell cycle. Consistently, the DNA replication was inhibited by NCTD. Finally, the combination NCTD with Cdc6 depletion lead to more severe cytotoxicity (88%) than NCTD (52%) and Cdc6 depletion (39%) alone. A synergic cytotoxicity was observed between Cdc6 depletion and NCTD. In conclusion, our results demonstrate that NCTD inhibits pre-RC assembly; subsequently blocks the G1 to S transition; and inhibits DNA replication in HepG2 cells. Pre-RCs are an intriguing target for cancer therapy, which merits further investigations for anticancer development.

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.

Interactions of the human MCM-BP protein with MCM complex components and Dbf4

MCM-BP was discovered as a protein that co-purified from human cells with MCM proteins 3 through 7; results which were recapitulated in frogs, yeast and plants. Evidence in all of these organisms supports an important role for MCM-BP in DNA replication, including contributions to MCM complex unloading. However the mechanisms by which MCM-BP functions and associates with MCM complexes are not well understood. Here we show that human MCM-BP is capable of interacting with individual MCM proteins 2 through 7 when co-expressed in insect cells and can greatly increase the recovery of some recombinant MCM proteins. Glycerol gradient sedimentation analysis indicated that MCM-BP interacts most strongly with MCM4 and MCM7. Similar gradient analyses of human cell lysates showed that only a small amount of MCM-BP overlapped with the migration of MCM complexes and that MCM complexes were disrupted by exogenous MCM-BP. In addition, large complexes containing MCM-BP and MCM proteins were detected at mid to late S phase, suggesting that the formation of specific MCM-BP complexes is cell cycle regulated. We also identified an interaction between MCM-BP and the Dbf4 regulatory component of the DDK kinase in both yeast 2-hybrid and insect cell co-expression assays, and this interaction was verified by co-immunoprecipitation of endogenous proteins from human cells. In vitro kinase assays showed that MCM-BP was not a substrate for DDK but could inhibit DDK phosphorylation of MCM4,6,7 within MCM4,6,7 or MCM2-7 complexes, with little effect on DDK phosphorylation of MCM2. Since DDK is known to activate DNA replication through phosphorylation of these MCM proteins, our results suggest that MCM-BP may affect DNA replication in part by regulating MCM phosphorylation by DDK.

The dynamics of replication licensing in live Caenorhabditis elegans embryos

Accurate DNA replication requires proper regulation of replication licensing, which entails loading MCM-2-7 onto replication origins. In this paper, we provide the first comprehensive view of replication licensing in vivo, using video microscopy of Caenorhabditis elegans embryos. As expected, MCM-2-7 loading in late M phase depended on the prereplicative complex (pre-RC) proteins: origin recognition complex (ORC), CDC-6, and CDT-1. However, many features we observed have not been described before: GFP-ORC-1 bound chromatin independently of ORC-2-5, and CDC-6 bound chromatin independently of ORC, whereas CDT-1 and MCM-2-7 DNA binding was interdependent. MCM-3 chromatin loading was irreversible, but CDC-6 and ORC turned over rapidly, consistent with ORC/CDC-6 loading multiple MCM-2-7 complexes. MCM-2-7 chromatin loading further reduced ORC and CDC-6 DNA binding. This dynamic behavior creates a feedback loop allowing ORC/CDC-6 to repeatedly load MCM-2-7 and distribute licensed origins along chromosomal DNA. During S phase, ORC and CDC-6 were excluded from nuclei, and DNA was overreplicated in export-defective cells. Thus, nucleocytoplasmic compartmentalization of licensing factors ensures that DNA replication occurs only once.

The MCM-associated protein MCM-BP is important for human nuclear morphology

Mini-chromosome maintenance complex-binding protein (MCM-BP) was discovered as a protein that is strongly associated with human MCM proteins, known to be crucial for DNA replication in providing DNA helicase activity. The Xenopus MCM-BP homologue appears to play a role in unloading MCM complexes from chromatin after DNA synthesis; however, the importance of MCM-BP and its functional contribution to human cells has been unclear. Here we show that depletion of MCM-BP by sustained expression of short hairpin RNA (shRNA) results in highly abnormal nuclear morphology and centrosome amplification. The abnormal nuclear morphology was not seen with depletion of other MCM proteins and was rescued with shRNA-resistant MCM-BP. MCM-BP depletion was also found to result in transient activation of the G2 checkpoint, slowed progression through G2 and increased replication protein A foci, indicative of replication stress. In addition, MCM-BP depletion led to increased cellular levels of MCM proteins throughout the cell cycle including soluble MCM pools. The results suggest that MCM-BP makes multiple contributions to human cells that are not limited to unloading of the MCM complex.

The eukaryotic Mcm2-7 replicative helicase

In eukaryotes, the Mcm2-7 complex forms the core of the replicative helicase - the molecular motor that uses ATP binding and hydrolysis to fuel the unwinding of double-stranded DNA at the replication fork. Although it is a toroidal hexameric helicase superficially resembling better-studied homohexameric helicases from prokaryotes and viruses, Mcm2-7 is the only known helicase formed from six unique and essential subunits. Recent biochemical and structural analyses of both Mcm2-7 and a higher-order complex containing additional activator proteins (the CMG complex) shed light on the reason behind this unique subunit assembly: whereas only a limited number of specific ATPase active sites are needed for DNA unwinding, one particular ATPase active site has evolved to form a reversible discontinuity (gate) in the toroidal complex. The activation of Mcm2-7 helicase during S-phase requires physical association of the accessory proteins Cdc45 and GINS; structural data suggest that these accessory factors activate DNA unwinding through closure of the Mcm2-7 gate. Moreover, studies capitalizing on advances in the biochemical reconstitution of eukaryotic DNA replication demonstrate that Mcm2-7 loads onto origins during initiation as a double hexamer, yet does not act as a double-stranded DNA pump during elongation.

Evidence for a mammalian late-G1 phase inhibitor of replication licensing distinct from geminin or Cdk activity

Pre-replication complexes (pre-RCs) are assembled onto DNA during late mitosis and G1 to license replication origins for use in S phase. In order to prevent re-replication of DNA, licensing must be completely shutdown prior to entry into S phase. While mechanisms preventing re-replication during S phase and mitosis have been elucidated, the means by which cells first prevent licensing during late G1 phase are poorly understood. We have employed a hybrid mammalian / Xenopus egg extract replication system to dissect activities that inhibit replication licensing at different stages of the cell cycle in Chinese Hamster Ovary (CHO) cells. We find that soluble extracts from mitotic cells inhibit licensing through a combination of geminin and Cdk activities, while extracts from S-phase cells inhibit licensing predominantly through geminin alone. Surprisingly however, geminin did not accumulate until after cells enter S phase. Unlike extracts from cells in early G1 phase, extracts from late G1 phase and early S phase cells contained an inhibitor of licensing that could not be accounted for by either geminin or Cdk. Moreover, inhibiting cyclin and geminin protein synthesis or inhibiting Cdk activity early in G1 phase did not prevent the appearance of inhibitory activity. These results suggest that a soluble inhibitor of replication licensing appears prior to entry into S phase that is distinct from either geminin or Cdk activity. Our hybrid system should permit the identification of this and other novel cell cycle regulatory activities.

Regulatory pathways coordinating cell cycle progression in early Xenopus development

The African clawed frog, Xenopus laevis, is used extensively as a model organism for studying both cell development and cell cycle regulation. For over 20 years now, this model organism has contributed to answering fundamental questions concerning the mechanisms that underlie cell cycle transitions--the cellular components that synthesize, modify, repair, and degrade nucleic acids and proteins, the signaling pathways that allow cells to communicate, and the regulatory pathways that lead to selective expression of subsets of genes. In addition, the remarkable simplicity of the Xenopus early cell cycle allows for tractable manipulation and dissection of the basic components driving each transition. In this organism, early cell divisions are characterized by rapid cycles alternating phases of DNA synthesis and division. The post-blastula stages incorporate gap phases, lengthening progression, and allowing more time for DNA repair. Various cyclin/Cdk complexes are differentially expressed during the early cycles with orderly progression being driven by both the combined action of cyclin synthesis and degradation and the appropriate selection of specific substrates by their Cdk components. Like other multicellular organisms, chief developmental events in early Xenopus embryogenesis coincide with profound remodeling of the cell cycle, suggesting that cell proliferation and differentiation events are linked and coordinated through crosstalk mechanisms acting on signaling pathways involving the expression of cell cycle control genes.

MCM2-7 form double hexamers at licensed origins in Xenopus egg extract

In late mitosis and G1, Mcm2-7 are assembled onto replication origins to license them for initiation in the upcoming S phase. After initiation, Mcm2-7 provide helicase activity to unwind DNA at the replication fork. Here we examine the structure of Mcm2-7 on chromatin in Xenopus egg extracts. We show that prior to replication initiation, Mcm2-7 is present at licensed replication origins in a complex with a molecular mass close to double that of the Mcm2-7 hexamer. This complex has approximately stoichiometric quantities of the 6 Mcm2-7 proteins and we conclude that it consists of a double heterohexamer. This provides a configuration potentially capable of initiating a pair of bidirectional replication forks in S phase. We also show that after initiation, Mcm2-7 associate with Cdc45 and GINS to form a relatively stable CMG (Cdc45-MCM-GINS) complex. The CMG proteins also associate less strongly with other replication proteins, consistent with the idea that a single CMG complex forms the core of the replisome.

MCM-BP regulates unloading of the MCM2-7 helicase in late S phase

Origins of DNA replication are licensed by recruiting MCM2-7 to assemble the prereplicative complex (pre-RC). How MCM2-7 is inactivated or removed from chromatin at the end of S phase is still unclear. Here, we show that MCM-BP can disassemble the MCM2-7 complex and might function as an unloader of MCM2-7 from chromatin. In Xenopus egg extracts, MCM-BP exists in a stable complex with MCM7, but is not associated with the MCM2-7 hexameric complex. MCM-BP accumulates in nuclei in late S phase, well after the loading of MCM2-7 onto chromatin. MCM-BP immunodepletion in Xenopus egg extracts inhibits replication-dependent MCM dissociation without affecting pre-RC formation and DNA replication. When excess MCM-BP is incubated with Xenopus egg extracts or immunopurified MCM2-7, it binds to MCM proteins and promotes disassembly of the MCM2-7 complex. Recombinant MCM-BP also releases MCM2-7 from isolated late-S-phase chromatin, but this activity is abolished when DNA replication is blocked. MCM-BP silencing in human cells also delays MCM dissociation in late S phase. We propose that MCM-BP plays a key role in the mechanism by which pre-RC is cleared from replicated DNA in vertebrate cells.

The effects of oligomerization on Saccharomyces cerevisiae Mcm4/6/7 function

BACKGROUND

Minichromosome maintenance proteins (Mcm) 2, 3, 4, 5, 6 and 7 are related by sequence and form a variety of complexes that unwind DNA, including Mcm4/6/7. A Mcm4/6/7 trimer forms one half of the Mcm2-7 hexameric ring and can be thought of as the catalytic core of Mcm2-7, the replicative helicase in eukaryotic cells. Oligomeric analysis of Mcm4/6/7 suggests that it forms a hexamer containing two Mcm4/6/7 trimers, however, under certain conditions trimeric Mcm4/6/7 has also been observed. The functional significance of the different Mcm4/6/7 oligomeric states has not been assessed. The results of such an assessment would have implications for studies of both Mcm4/6/7 and Mcm2-7.

RESULTS

Here, we show that Saccharomyces cerevisiae Mcm4/6/7 reconstituted from individual subunits exists in an equilibrium of oligomeric forms in which smaller oligomers predominate in the absence of ATP. In addition, we found that ATP, which is required for Mcm4/6/7 activity, shifts the equilibrium towards larger oligomers, likely hexamers of Mcm4/6/7. ATPγS and to a lesser extent ADP also shift the equilibrium towards hexamers. Study of Mcm4/6/7 complexes containing mutations that interfere with the formation of inter-subunit ATP sites (arginine finger mutants) indicates that full activity of Mcm4/6/7 requires all of its ATP sites, which are formed in a hexamer and not a trimer. In keeping with this observation, Mcm4/6/7 binds DNA as a hexamer.

CONCLUSIONS

The minimal functional unit of Mcm4/6/7 is a hexamer. One of the roles of ATP binding by Mcm4/6/7 may be to stabilize formation of hexamers.

Turning placenta into brain: placental mesenchymal stem cells differentiate into neurons and oligodendrocytes

OBJECTIVE

We aimed to induce neural stem (NSC) and progenitor cells (NPC) from human placental tissues.

STUDY DESIGN

Placental stem cells from first-trimester placental chorionic villi and term chorion were isolated. Neural differentiation was initiated with plating on collagen, retinoic acid, and/or human brain-derived neurotrophic factor and epidermal and fibroblast growth factor. Differentiation into neurons, oligodendrocytes, and astrocytes was monitored by immunohistochemistry. Two-dimensional polyacrylamide gel electrophoresis, high-performance liquid chromatography, and tandem mass spectrometry were used to identify proteins involved in the differentiation.

RESULTS

Differentiated cells were mostly immediately postmitotic with some more but not fully mature postmitotic neurons. Neurons had dopaminergic or serotonergic character. Some cells differentiated into predominantly immature oligodendrocytes. Upon differentiation, neuron-specific proteins were up-regulated, whereas placental proteins were reduced.

CONCLUSION

Stem cells derived from human placenta can be differentiated into neural progenitors.

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.

An archaeal order with multiple minichromosome maintenance genes

In eukaryotes, a complex of six highly related minichromosome maintenance (MCM) proteins is believed to function as the replicative helicase. Until recently, systems for exploring the molecular mechanisms underlying eukaryotic MCM function have been biochemically intractable. To overcome this, molecular studies of MCM function have been carried out using MCM homologues from the archaea. Archaeal MCM systems studied to date possess a single functional MCM, which forms a homohexameric complex that displays DNA binding, ATPase and helicase activities. We have identified an archaeal order that possesses multiple MCM homologues. blast searches of available Methanococcales genomes reveal that members of this order possess between two and eight MCM homologues. Phylogenetic analysis suggests that an ancient duplication in the Methanococcales gave rise to two major groups of MCMs. One group contains Methanococcus maripaludis S2 McmD and possesses a conserved C-terminal insert similar to one observed in eukaryotic MCM3, while the other group contains McmA, -B and -C. Analysis of the genome context of MCMs in the latter group indicates that these genes could have arisen from phage-mediated events. When co-expressed in Escherichia coli, the four MCMs from M. maripaludis co-purify, indicating the formation of heteromeric complexes in vitro. The presence of homologues from both groups in all Methanococcales indicates that there could be functionally important differences between these proteins and that Methanococcales MCMs may therefore provide an interesting additional model for eukaryotic MCM function.

Dynamic localization of the DNA replication proteins MCM5 and MCM7 in plants

Genome integrity in eukaryotes depends on licensing mechanisms that prevent loading of the minichromosome maintenance complex (MCM2-7) onto replicated DNA during S phase. Although the principle of licensing appears to be conserved across all eukaryotes, the mechanisms that control it vary, and it is not clear how licensing is regulated in plants. In this work, we demonstrate that subunits of the MCM2-7 complex are coordinately expressed during Arabidopsis (Arabidopsis thaliana) development and are abundant in proliferating and endocycling tissues, indicative of a role in DNA replication. We show that endogenous MCM5 and MCM7 proteins are localized in the nucleus during G1, S, and G2 phases of the cell cycle and are released into the cytoplasmic compartment during mitosis. We also show that MCM5 and MCM7 are topologically constrained on DNA and that the MCM complex is stable under high-salt conditions. Our results are consistent with a conserved replicative helicase function for the MCM complex in plants but not with the idea that plants resemble budding yeast by actively exporting the MCM complex from the nucleus to prevent unauthorized origin licensing and rereplication during S phase. Instead, our data show that, like other higher eukaryotes, the MCM complex in plants remains in the nucleus throughout most of the cell cycle and is only dispersed in mitotic cells.

Effect of heliquinomycin on the activity of human minichromosome maintenance 4/6/7 helicase

The antibiotic heliquinomycin, which inhibits cellular DNA replication at a half-maximal inhibitory concentration (IC(50)) of 1.4-4 microM, was found to inhibit the DNA helicase activity of the human minichromosome maintenance (MCM) 4/6/7 complex at an IC(50) value of 2.4 microM. In contrast, 14 microM heliquinomycin did not inhibit significantly either the DNA helicase activity of the SV40 T antigen and Werner protein or the oligonucleotide displacement activity of human replication protein A. At IC(50) values of 25 and 6.5 microM, heliquinomycin inhibited the RNA priming and DNA polymerization activities, respectively, of human DNA polymerase-alpha/primase. Thus, of the enzymes studied, the MCM4/6/7 complex was the most sensitive to heliquinomycin; this suggests that MCM helicase is one of the main targets of heliquinomycin in vivo. It was observed that heliquinomycin did not inhibit the ATPase activity of the MCM4/6/7 complex to a great extent in the absence of single-stranded DNA. In contrast, heliquinomycin at an IC(50) value of 5.2 microM inhibited the ATPase activity of the MCM4/6/7 complex in the presence of single-stranded DNA. This suggests that heliquinomycin interferes with the interaction of the MCM4/6/7 complex with single-stranded DNA.

The minichromosome maintenance proteins 2-7 (MCM2-7) are necessary for RNA polymerase II (Pol II)-mediated transcription

The MCM2-7 (minichromosome maintenance) proteins are a family of evolutionarily highly conserved proteins. They are essential for DNA replication in yeast and are considered to function as DNA helicases. However, it has long been shown that there is an overabundance of the MCM2-7 proteins when compared with the number of DNA replication origins in chromatin. It has been suggested that the MCM2-7 proteins may function in other biological processes that require the unwinding of the DNA helix. In this report, we show that RNA polymerase II (Pol II)-mediated transcription is dependent on MCM5 and MCM2 proteins. Furthermore, the MCM2-7 proteins are co-localized with RNA Pol II on chromatins of constitutively transcribing genes, and MCM5 is required for transcription elongation of RNA Pol II. Finally, we demonstrate that the integrity of the MCM2-7 hexamer complex and the DNA helicase domain in MCM5 are essential for the process of transcription.

Mutations in subdomain B of the minichromosome maintenance (MCM) helicase affect DNA binding and modulate conformational transitions

Minichromosome maintenance (MCM) proteins are believed to provide the replicative helicase activity in eukaryotes and archaea. The single MCM orthologue from Methanothermobacter thermautotrophicus (MthMCM) has been extensively characterized as a model of the eukaryotic heterohexameric MCM complex. MthMCM forms high molecular weight complexes in solution consistent with a dodecamer. Visualization of this complex by electron microscopy suggests that single and double heptameric or hexameric rings can form. We have mutated two arginine residues (Arg-137, Arg-160) in the N-terminal subdomain B of MthMCM based on their apparent potential to form inter-ring hydrogen bonds. Both the single R137A and the double R137A,R160A mutants were characterized by a combination of biophysical, biochemical, and electron microscopy techniques. Biophysical analysis coupled with electron microscopy studies shows that the R137A mutant forms a double heptameric ring, whereas the R137A,R160A protein assembles as a single heptamer. They both show a defect in DNA binding and a concomitant conformational change in subdomain A, with the double mutant displaying significant defects in helicase activity as well. We propose a model in which MCM loading and the subsequent activation of the helicase activity involve a conformational transition that is connected to a DNA binding event.

Mcm subunits can assemble into two different active unwinding complexes

The replication fork helicase in eukaryotes is a large complex that is composed of Mcm2-7, Cdc45, and GINS. The Mcm2-7 proteins form a heterohexameric ring that hydrolyzes ATP and provide the motor function for this unwinding complex. A comprehensive study of how individual Mcm subunit biochemical activities relate to unwinding function has not been accomplished. We studied the mechanism of the Mcm4-Mcm6-Mcm7 complex, a useful model system because this complex has helicase activity in vitro. We separately purified each of three Mcm subunits until they were each nuclease-free, and we then examined the biochemical properties of different combinations of Mcm subunits. We found that Mcm4 and Mcm7 form an active unwinding assembly. The addition of Mcm6 to Mcm4/Mcm7 results in the formation of an active Mcm4/Mcm6/Mcm7 helicase assembly. The Mcm4-Mcm7 complex forms a ringed-shaped hexamer that unwinds DNA with 3' to 5' polarity by a steric exclusion mechanism, similar to Mcm4/Mcm6/Mcm7. The Mcm4-Mcm7 complex has a high level of ATPase activity that is further stimulated by DNA. The ability of different Mcm mixtures to form rings or exhibit DNA stimulation of ATPase activity correlates with the ability of these complexes to unwind DNA. The Mcm4/Mcm7 and Mcm4/Mcm6/Mcm7 assemblies can open to load onto circular DNA to initiate unwinding. We conclude that the Mcm subunits are surprisingly flexible and dynamic in their ability to interact with one another to form active unwinding complexes.

Dormant origins licensed by excess Mcm2-7 are required for human cells to survive replicative stress

In late mitosis and early G1, Mcm2-7 complexes are loaded onto DNA to license replication origins for use in the upcoming S phase. However, the amount of Mcm2-7 loaded is in significant excess over the number of origins normally used. We show here that in human cells, excess chromatin-bound Mcm2-7 license dormant replication origins that do not fire during normal DNA replication, in part due to checkpoint activity. Dormant origins were activated within active replicon clusters if replication fork progression was inhibited, despite the activation of S-phase checkpoints. After lowering levels of chromatin-bound Mcm2-7 in human cells by RNA interference (RNAi), the use of dormant origins was suppressed in response to replicative stress. Although cells with lowered chromatin-bound Mcm2-7 replicated at normal rates, when challenged with replication inhibitors they had dramatically reduced rates of DNA synthesis and reduced viability. These results suggest that the use of dormant origins licensed by excess Mcm2-7 is a new and physiologically important mechanism that cells utilize to maintain DNA replication rates under conditions of replicative stress. We propose that checkpoint kinase activity can preferentially suppress initiation within inactive replicon clusters, thereby directing new initiation events toward active clusters that are experiencing replication problems.

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.

Differential requirements for MCM proteins in DNA replication in Drosophila S2 cells

BACKGROUND

The MCM2-7 proteins are crucial components of the pre replication complex (preRC) in eukaryotes. Since they are significantly more abundant than other preRC components, we were interested in determining whether the entire cellular content was necessary for DNA replication in vivo.

METHODOLOGY/PRINCIPLE FINDINGS

We performed a systematic depletion of the MCM proteins in Drosophila S2 cells using dsRNA-interference. Reducing MCM2-6 levels by >95-99% had no significant effect on cell cycle distribution or viability. Depletion of MCM7 however caused an S-phase arrest. MCM2-7 depletion produced no change in the number of replication forks as measured by PCNA loading. We also depleted MCM8. This caused a 30% reduction in fork number, but no significant effect on cell cycle distribution or viability. No additive effects were observed by co-depleting MCM8 and MCM5.

CONCLUSIONS/SIGNIFICANCE

These studies suggest that, in agreement with what has previously been observed for Xenopus in vitro, not all of the cellular content of MCM2-6 proteins is needed for normal cell cycling. They also reveal an unexpected unique role for MCM7. Finally they suggest that MCM8 has a role in DNA replication in S2 cells.

Reduced Mcm2 expression results in severe stem/progenitor cell deficiency and cancer

Mcm2 is a component of the DNA replication licensing complex that marks DNA replication origins during G1 of the cell cycle for use in the subsequent S-phase. It is expressed in stem/progenitor cells in a variety of regenerative tissues in mammals. Here, we have used the Mcm2 gene to develop a transgenic mouse in which somatic stem/progenitor cells can be genetically modified in the adult. In these mice, a tamoxifen-inducible form of Cre recombinase is integrated 3' to the Mcm2 coding sequence and expressed via an internal ribosome entry site (IRES). Heterozygous Mcm2(IRES-CreERT2/wild-type (wt)) mice are phenotypically indistinguishable from wild-type at least through 1 year of age. In bigenic Mcm2(IRES-CreERT2/wt); Z/EG reporter mice, tamoxifen-dependent enhanced green fluorescence protein expression is inducible in a wide variety of somatic stem cells and their progeny. However, in Mcm2(IRES-CreERT2/IRES-CreERT2) homozygous embryos or mouse embryonic fibroblasts, Mcm2 is reduced to approximately one-third of wild-type levels. Despite the fact that these mice develop normally and are asymptomatic as young adults, life span is greatly reduced, with most surviving to only approximately 10-12 weeks of age. They demonstrate severe deficiencies in the proliferative cell compartments of a variety of tissues, including the subventricular zone of the brain, muscle, and intestinal crypts. However, the immediate cause of death in most of these animals is cancer, where the majority develop lymphomas. These studies directly demonstrate that deficiencies in the function of the core DNA replication machinery that are compatible with development and survival nonetheless result in a chronic phenotype leading to stem cell deficiency in multiple tissues and cancer. Disclosure of potential conflicts of interest is found at the end of this article.

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.

Analysis of Mcm2–7 chromatin binding during anaphase and in the transition to quiescence in fission yeast

Mcm2-7 proteins are generally considered to function as a heterohexameric complex, providing helicase activity for the elongation step of DNA replication. These proteins are loaded onto replication origins in M-G1 phase in a process termed licensing or pre-replicative complex formation. It is likely that Mcm2-7 proteins are loaded onto chromatin simultaneously as a pre-formed hexamer although some studies suggest that subcomplexes are recruited sequentially. To analyze this process in fission yeast, we have compared the levels and chromatin binding of Mcm2-7 proteins during the fission yeast cell cycle. Mcm subunits are present at approximately 1 x 10(4) molecules/cell and are bound with approximately equal stoichiometry on chromatin in G1/S phase cells. Using a single cell assay, we have correlated the timing of chromatin association of individual Mcm subunits with progression through mitosis. This showed that Mcm2, 4 and 7 associate with chromatin at about the same stage of anaphase, suggesting that licensing involves the simultaneous binding of these subunits. We also examined Mcm2-7 chromatin association when cells enter a G0-like quiescent state. Chromatin binding is lost in this transition in a process that does not require DNA replication or the selective degradation of specific subunits.

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.

The medulloblastoma cell line DAOY but not eleven other tumor cell lines expresses minichromosome maintenance protein 4

Minichromosome maintenance proteins (MCM) are required for initiation and elongation of chromosomal DNA, ensuring that DNA replication takes place only once. Although MCMs are considered of utmost importance in tumor biology and as potential marker proteins, they were not unambiguously identified at the protein level and we therefore aimed to characterize MCM 4 in a medulloblastoma cell line and provide a protein chemical analytical tool. In addition, we searched for this protein in twelve tumor cell lines and a series of non-tumor cells. The DAOY medulloblastoma cell line was cultivated, lysed, proteins extracted and run on two-dimensional gel electrophoresis with subsequent in-gel digestion and mass spectrometrical (MS/MS) analysis of protein spots. One spot at pI 6.2 with an observed molecular weight of 98 kDa was identified as minichromosome maintenance protein 4 by peptide fingerprinting. Sequence coverage of 35% along with 25 matched peptides and MS/MS analyses of three matching peptides warranted unambiguous identification. The use of mass spectrometrical identification unequivocally allowed determination of MCM 4 expression in a medulloblastoma cell line exclusively. Given the biological and probable clinical importance of this molecule as a tentative marker protein, a fair analytical tool, independent of antibody availability and specificity is mandatory and determinations at the transcriptional level cannot be extrapolated to protein levels per se, as there is a long and unpredictable way from mRNA to protein.

Minichromosome maintenance helicase activity is controlled by N- and C-terminal motifs and requires the ATPase domain helix-2 insert

The minichromosome maintenance (MCM) proteins are essential conserved proteins required for DNA replication in archaea and eukaryotes. MCM proteins are believed to provide the replicative helicase activity that unwinds template DNA ahead of the replication fork. Consistent with this hypothesis, MCM proteins can form hexameric complexes that possess ATP-dependent DNA unwinding activity. The molecular mechanism by which the energy of ATP hydrolysis is harnessed to DNA unwinding is unknown, although the ATPase activity has been attributed to a highly conserved AAA+ family ATPase domain. Here we show that changes to N- and C-terminal motifs in the single MCM protein from the archaeon Methanothermobacter thermautotrophicus (MthMCM) can modulate ATP hydrolysis, DNA binding, and duplex unwinding. Furthermore, these motifs appear to influence the movement of the beta-alpha-beta insert in helix-2 of the MCM ATPase domain. Removal of this motif from MthMCM increased dsDNA-stimulated ATP hydrolysis and increased the affinity of the mutant complex for ssDNA and dsDNA. Deletion of the helix-2 insert additionally resulted in the abrogation of DNA unwinding. Our results provide significant insight into the molecular mechanisms used by the MCM helicase to both regulate and execute DNA unwinding.

Structural studies of the archaeal MCM complex in different functional states

The primary candidate for the eukaryotic replicative helicase is the MCM2-7 complex, a hetero-oligomer formed by six AAA+ paralogous polypeptides. A simplified model for structure-function studies is the homo-oligomeric orthologue from the archaeon Methanothermobacter thermoautotrophicus. The crystal structure of the DNA-interacting N-terminal domain of this homo-oligomer revealed a double hexamer in a head-to-head configuration; single-particle electron microscopy studies have shown that the full-length protein complex can form both single and double rings, in which each ring can consist of a cyclical arrangement of six or seven subunits. Using single-particle techniques and especially multivariate statistical symmetry analysis, we have assessed the changes in stoichiometry that the complex undergoes when treated with various nucleotide analogues or when binding a double-stranded DNA fragment. We found that the binding of nucleotides or of double-stranded DNA leads to the preferred formation of double-ring structures. Specifically, the protein complex is present as a double heptamer when treated with a nucleotide analogue, but it is rather found as a double hexamer when complexed with double-stranded DNA. The possible physiological role of the various stoichiometries of the complex is discussed in the light of the proposed mechanisms of helicase activity.

Localization of MCM2-7, Cdc45, and GINS to the site of DNA unwinding during eukaryotic DNA replication

Little is known about the architecture and biochemical composition of the eukaryotic DNA replication fork. To study this problem, we used biotin-streptavidin-modified plasmids to induce sequence-specific replication fork pausing in Xenopus egg extracts. Chromatin immunoprecipitation was employed to identify factors associated with the paused fork. This approach identifies DNA pol alpha, DNA pol delta, DNA pol epsilon, MCM2-7, Cdc45, GINS, and Mcm10 as components of the vertebrate replisome. In the presence of the DNA polymerase inhibitor aphidicolin, which causes uncoupling of a highly processive DNA helicase from the stalled replisome, only Cdc45, GINS, and MCM2-7 are enriched at the pause site. The data suggest the existence of a large molecular machine, the "unwindosome," which separates DNA strands at the replication fork and contains Cdc45, GINS, and the MCM2-7 holocomplex.

Pre-replication complex organization in the atypical DNA replication cycle of Plasmodium falciparum: Characterization of the mini-chromosome maintenance (MCM) complex formation

The overall organization of cell division in Plasmodium is unique compared to that observed in model organisms because DNA replicates more than once per cell cycle at several points of its life cycle. The sequencing of the Plasmodium genome has also revealed the apparent absence of many key components (e.g. Cdt1, DDK and Cdc45) of the eukaryotic cell cycle machinery that are responsible for the formation of the pre-replication complex (pre-RC). We have characterized the Plasmodium falciparum minichromosome maintenance complex (MCM) that plays a key role in the transition of pre-RC to the RC. Similar to other eukaryotes, the Plasmodium genome encodes six MCM subunits. Here, we show that expression levels of at least three of the PfMCM subunits, the homologues of MCM2, MCM6 and MCM7, change during the intraerythrocytic development cycle, peaking in schizont and decreasing in the ring and trophozoite stages. PfMCM2, 6 and 7 subunits interact with each other to form a developmentally regulated complex: these interactions are detectable in rings and schizonts, but not in trophozoites. PfMCM2, 6 and 7 subunits are localized in both cytosolic and nucleosolic fractions during all intraerythrocytic stages of P. falciparum development, with increased nuclear localization in schizonts. Only PfMCM6 is associated with the chromatin fraction at all stages of growth. No phosphorylation of PfMCM2, 6 and 7 was detected, but two as yet unidentified threonine-phosphosphorylated proteins were present in the complex, whose pattern of phosphorylation varied during parasite development.

The ATPase activity of MCM2-7 is dispensable for pre-RC assembly but is required for DNA unwinding

Eukaryotes have six minichromosome maintenance (MCM) proteins that are essential for DNA replication. The contribution of ATPase activity of MCM complexes to their function in replication is poorly understood. We have established a cell-free system competent for replication in which all MCM proteins are supplied by purified recombinant Xenopus MCM complexes. Recombinant MCM2-7 complex was able to assemble onto chromatin, load Cdc45 onto chromatin, and restore DNA replication in MCM-depleted extracts. Using mutational analysis in the Walker A motif of MCM6 and MCM7 of MCM2-7, we show that ATP binding and/or hydrolysis by MCM proteins is dispensable for chromatin loading and pre-replicative complex (pre-RC) assembly, but is required for origin unwinding during DNA replication. Moreover, this ATPase-deficient mutant complex did not support DNA replication in MCM-depleted extracts. Altogether, these results both demonstrate the ability of recombinant MCM proteins to perform all replication roles of MCM complexes, and further support the model that MCM2-7 is the replicative helicase. These data establish that mutations affecting the ATPase activity of the MCM complex uncouple its role in pre-RC assembly from DNA replication.

Identification of a novel cell-cycle-induced MCM family protein MCM9

The minichromosome maintenance (MCM) proteins are essential for DNA replication initiation and elongation in eukaryotes. In mammalian cells, MCM2-MCM7 complexes are believed to unwind DNA during chromosomal DNA replication. Here we identified a novel MCM family gene, MCM9, by using bioinformatics. Human, mouse, and rat MCM9 showed approximately 90-91% total-amino acid identity. MCM9 showed 24-31% total-amino acid identity with MCM2-MCM8 protein. Phylogenetic analysis on MCM family members revealed that MCM9 was most closely related to MCM8. Human, mouse, and rat MCM9 gene, consisting of 7, 8, and 7 exons, was mapped to 6q22.1-22.33, 10B3, and 20q11, respectively. We identified transcription factor E2F-binding motifs in the vicinity of the transcription start site among human, mouse, and rat MCM9 gene. MCM9 mRNA was upregulated by transcription factor E2E1 and serum stimulation in NIH3T3 cells.