Suppression of targeting of Dbf4-dependent kinase to pre-replicative complex in G0 nuclei.

Intact G0 nuclei isolated from quiescent cells are not capable of DNA replication in interphase Xenopus egg extracts, which allow efficient replication of permeabilized G0 nuclei. Previous studies have shown multiple control mechanisms for maintaining the quiescent state, but DNA replication inhibition of intact G0 nuclei in the extracts remains poorly understood. Here, we showed that pre-RC is assembled on chromatin, but its activation is inhibited after incubating G0 nuclei isolated from quiescent NIH3T3 cells in the extracts. Concomitant with the inhibition of replication, Mcm4 phosphorylation mediated by Dbf4-dependent kinase (DDK) as well as chromatin binding of DDK is suppressed in G0 nuclei without affecting the nuclear transport of DDK. We further found that the nuclear extracts of G0 but not proliferating cells inhibit the binding of recombinant DDK to pre-RC assembled plasmids. In addition, we observed rapid activation of checkpoint kinases after incubating G0 nuclei in the egg extracts. However, specific inhibitors of ATR/ATM are unable to promote DNA replication in G0 nuclei in the egg extracts. We suggest that a novel inhibitory mechanism is functional to prevent the targeting of DDK to pre-RC in G0 nuclei, thereby suppressing DNA replication in Xenopus egg extracts.

MCM interference during licensing of DNA replication in Xenopus egg extracts-Possible Role of a C-terminal region of MCM3.

The minichromosome maintenance (MCM) complex, consisting of six subunits, Mcm2-7, is loaded onto replication origins through loading factors (origin recognition complex [ORC], Cdc6, and Cdt1) and forms an MCM double hexamer that licenses the initiation of DNA replication. Previous studies with Xenopus egg extracts showed that loading factors, especially Cdc6, dissociate from chromatin on MCM loading, but the molecular mechanism and physiological significance remain largely unknown. Using a cell-free system for MCM loading onto plasmid DNA in Xenopus egg extracts, we found that MCM loaded onto DNA prevents DNA binding of the loading factors ORC, Cdc6, and Cdt1. We further report that a peptide of the C-terminal region of MCM3 (MCM3-C), previously implicated in the initial association with ORC/Cdc6 in budding yeast, prevents ORC/Cdc6/Cdt1 binding to DNA in the absence of MCM loading. ATP-γ-S suppresses inhibitory activities of both the MCM loaded onto DNA and the MCM3-C peptide. Other soluble factors in the extract, but neither MCM nor Cdt1, are required for the activity. Conservation of the amino acid sequences of MCM3-C and its activity in vertebrates implies a novel negative autoregulatory mechanism that interferes with MCM loading in the vicinity of licensed origins to ensure proper origin licensing.

Characterization of conserved arginine residues on Cdt1 that affect licensing activity and interaction with Geminin or Mcm complex.

All organisms ensure once and only once replication during S phase through a process called replication licensing. Cdt1 is a key component and crucial loading factor of Mcm complex, which is a central component for the eukaryotic replicative helicase. In higher eukaryotes, timely inhibition of Cdt1 by Geminin is essential to prevent rereplication. Here, we address the mechanism of DNA licensing using purified Cdt1, Mcm and Geminin proteins in combination with replication in Xenopus egg extracts. We mutagenized the 223th arginine of mouse Cdt1 (mCdt1) to cysteine or serine (R-S or R-C, respectively) and 342nd and 346th arginines constituting an arginine finger-like structure to alanine (RR-AA). The RR-AA mutant of Cdt1 could not only rescue the DNA replication activity in Cdt1-depleted extracts but also its specific activity for DNA replication and licensing was significantly increased compared to the wild-type protein. In contrast, the R223 mutants were partially defective in rescue of DNA replication and licensing. Biochemical analyses of these mutant Cdt1 proteins indicated that the RR-AA mutation disabled its functional interaction with Geminin, while R223 mutations resulted in ablation in interaction with the Mcm2∼7 complex. Intriguingly, the R223 mutants are more susceptible to the phosphorylation-induced inactivation or chromatin dissociation. Our results show that conserved arginine residues play critical roles in interaction with Geminin and Mcm that are crucial for proper conformation of the complexes and its licensing activity.

RecQ4 promotes the conversion of the pre-initiation complex at a site-specific origin for DNA unwinding in Xenopus egg extracts.

Eukaryotic DNA replication is initiated through stepwise assembly of evolutionarily conserved replication proteins onto replication origins, but how the origin DNA is unwound during the assembly process remains elusive. Here, we established a site-specific origin on a plasmid DNA, using in vitro replication systems derived from Xenopus egg extracts. We found that the pre-replicative complex (pre-RC) was preferentially assembled in the vicinity of GAL4 DNA-binding sites of the plasmid, depending on the binding of Cdc6 fused with a GAL4 DNA-binding domain in Cdc6-depleted extracts. Subsequent addition of nucleoplasmic S-phase extracts to the GAL4-dependent pre-RC promoted initiation of DNA replication from the origin, and components of the pre-initiation complex (pre-IC) and the replisome were recruited to the origin concomitant with origin unwinding. In this replication system, RecQ4 is dispensable for both recruitment of Cdc45 onto the origin and stable binding of Cdc45 and GINS to the pre-RC assembled plasmid. However, both origin binding of DNA polymerase α and unwinding of DNA were diminished upon depletion of RecQ4 from the extracts. These results suggest that RecQ4 plays an important role in the conversion of pre-ICs into active replisomes requiring the unwinding of origin DNA in vertebrates.

Evolutionary diversification of MCM3 genes in Xenopus laevis and Danio rerio.

Embryonic cell cycles of amphibians are rapid and lack zygotic transcription and checkpoint control. At the mid-blastula transition, zygotic transcription is initiated and cell divisions become asynchronous. Several cell cycle-related amphibian genes retain 2 distinct forms, maternal and zygotic, but little is known about the functional differences between these 2 forms of proteins. The minichromosome maintenance (MCM) 2-7 complex, consisting of 6 MCM proteins, plays a central role in the regulation of eukaryotic DNA replication. Almost all eukaryotes retain just a single MCM gene for each subunit. Here we report that Xenopus and zebrafish have 2 copies of MCM3 genes, one of which shows a maternal and the other a zygotic expression pattern. Phylogenetic analysis shows that the Xenopus and zebrafish zygotic MCM3 genes are more similar to their mammalian MCM3 ortholog, suggesting that maternal MCM3 was lost during evolution in most vertebrate lineages. Maternal MCM3 proteins in these 2 species are functionally different from zygotic MCM3 proteins because zygotic, but not maternal, MCM3 possesses an active nuclear localization signal in its C-terminal region, such as mammalian MCM3 orthologs do. mRNA injection experiments in zebrafish embryos show that overexpression of maternal MCM3 impairs proliferation and causes developmental defects, whereas zygotic MCM3 has a much weaker effect. This difference is brought about by the difference in their C-terminal regions, which contain putative nuclear localization signals; swapping the C-terminal region between maternal and zygotic genes diminishes the developmental defects. This study suggests that evolutionary diversification has occurred in MCM3 genes, leading to distinct functions, possibly as an adaption to the rapid DNA replication required for early development of Xenopus and zebrafish.

Efficient initiation of DNA replication in eukaryotes requires Dpb11/TopBP1-GINS interaction.

Dpb11/Cut5/TopBP1 is evolutionarily conserved and is essential for the initiation of DNA replication in eukaryotes. The Dpb11 of the budding yeast Saccharomyces cerevisiae has four BRCT domains (BRCT1 to -4). The N-terminal pair (BRCT1 and -2) and the C-terminal pair (BRCT3 and -4) bind to cyclin-dependent kinase (CDK)-phosphorylated Sld3 and Sld2, respectively. These phosphorylation-dependent interactions trigger the initiation of DNA replication. BRCT1 and -2 and BRCT3 and -4 of Dpb11 are separated by a short stretch of ~100 amino acids. It is unknown whether this inter-BRCT region functions in DNA replication. Here, we showed that the inter-BRCT region is a GINS interaction domain that is essential for cell growth and that mutations in this domain cause replication defects in budding yeast. We found the corresponding region in the vertebrate ortholog, TopBP1, and showed that the corresponding region also interacts with GINS and is required for efficient DNA replication. We propose that the inter-BRCT region of Dpb11 is a functionally conserved GINS interaction domain that is important for the initiation of DNA replication in eukaryotes.

Mcm8 and Mcm9 form a complex that functions in homologous recombination repair induced by DNA interstrand crosslinks.

DNA interstrand crosslinks (ICLs) are highly toxic lesions that stall the replication fork to initiate the repair process during the S phase of vertebrates. Proteins involved in Fanconi anemia (FA), nucleotide excision repair (NER), and translesion synthesis (TS) collaboratively lead to homologous recombination (HR) repair. However, it is not understood how ICL-induced HR repair is carried out and completed. Here, we showed that the replicative helicase-related Mcm family of proteins, Mcm8 and Mcm9, forms a complex required for HR repair induced by ICLs. Chicken DT40 cells lacking MCM8 or MCM9 are viable but highly sensitive to ICL-inducing agents, and exhibit more chromosome aberrations in the presence of mitomycin C compared with wild-type cells. During ICL repair, Mcm8 and Mcm9 form nuclear foci that partly colocalize with Rad51. Mcm8-9 works downstream of the FA and BRCA2/Rad51 pathways, and is required for HR that promotes sister chromatid exchanges, probably as a hexameric ATPase/helicase.

The prereplication complex recruits XEco2 to chromatin to promote cohesin acetylation in Xenopus egg extracts.

BACKGROUND: Sister chromatids are held together by the ring-shaped cohesin complex, which is loaded onto chromosomes before DNA replication. Cohesion between sister chromosomes is established during DNA replication, and it requires acetylation of the Smc3 subunit of cohesin by evolutionally conserved cohesin acetyltransferases (CoATs). However, how CoATs are recruited to chromatin and how cohesin acetylation is regulated remain unclear. RESULTS: We found that cohesin acetylation requires pre-RC-dependent chromatin loading of cohesin, but surprisingly, it is independent of DNA synthesis in Xenopus egg extracts. Immunodepletion experiments revealed that XEco2 is the CoAT responsible for Smc3 acetylation and sister chromatid cohesion. Recruitment of XEco2 onto chromatin was dependent on pre-RC assembly but was independent of cohesin loading and DNA synthesis. Two short N-terminal motifs, PBM-A and PBM-B, which are conserved among vertebrate Esco2/XEco2 homologs, were collectively essential for pre-RC-dependent chromatin association of XEco2, cohesin acetylation, and subsequent sister chromatid cohesion. The conserved PCNA-interacting protein box in XEco2 was largely dispensable for Smc3 acetylation but was partially required for cohesion. Interaction of acetylated cohesin with DNA was stabilized against salt-wash treatments after DNA replication. CONCLUSIONS: Our results demonstrate that pre-RC formation regulates chromatin association of XEco2 in Xenopus egg extracts. We propose that this reaction is critical to acetylate cohesin, whose DNA binding is subsequently stabilized by DNA replication.

Mcm10 plays a role in functioning of the eukaryotic replicative DNA helicase, Cdc45-Mcm-GINS.

Eukaryotic DNA replication is initiated at multiple origins of replication, where many replication proteins assemble under the control of the cell cycle [1]. A key process of replication initiation is to convert inactive Mcm2-7 to active Cdc45-Mcm-GINS (CMG) replicative helicase [2]. However, it is not known whether the CMG assembly would automatically activate its helicase activity and thus assemble the replisome. Mcm10 is an evolutionally conserved essential protein required for the initiation of replication [3, 4]. Although the roles of many proteins involved in the initiation are understood, the role of Mcm10 remains controversial [5-9]. To characterize Mcm10 in more detail, we constructed budding yeast cells bearing a degron-fused Mcm10 protein that can be efficiently degraded in response to auxin. In the absence of Mcm10, a stable CMG complex was assembled at origins. However, subsequent translocation of CMG, replication protein A loading to origins, and the intra-S checkpoint activation were severely diminished, suggesting that origin unwinding is defective. We also found that Mcm10 associates with origins during initiation in an S-cyclin-dependent kinase- and Cdc45-dependent manner. Thus, Mcm10 plays an essential role in functioning of the CMG replicative helicase independent of assembly of a stable CMG complex at origins.

Inter-origin cooperativity of geminin action establishes an all-or-none switch for replication origin licensing.

In metazoans, geminin functions as a molecular switch for preventing re-replication of chromosomal DNA. Geminin binds to and inhibits Cdt1, which is required for replication origin licensing, but little is known about the mechanisms underlying geminin's all-or-none action in licensing inhibition. Using Xenopus egg extract, we found that the all-or-none activity correlated with the formation of Cdt1 foci on chromatin, suggesting that multiple Cdt1-geminin complexes on origins cooperatively inhibit licensing. Based on experimental identification of licensing intermediates targeted by geminin and Cdt1, we developed a mathematical model of the licensing process. The model involves positive feedback owing to the cooperative action of geminin at neighboring origins and accurately accounts for the licensing activity mediated by geminin and Cdt1 in the extracts. The model also predicts that such cooperativity leads to clustering of licensing-inhibited origins, an idea that is supported by the experimentally measured distribution of inter-origin distances. We propose that geminin inhibits licensing through an inter-origin interaction, ensuring strict and coordinated control of multiple replication origins on chromosomes.

An auxin-based degron system for the rapid depletion of proteins in nonplant cells.

Plants have evolved a unique system in which the plant hormone auxin directly induces rapid degradation of the AUX/IAA family of transcription repressors by a specific form of the SCF E3 ubiquitin ligase. Other eukaryotes lack the auxin response but share the SCF degradation pathway, allowing us to transplant the auxin-inducible degron (AID) system into nonplant cells and use a small molecule to conditionally control protein stability. The AID system allowed rapid and reversible degradation of target proteins in response to auxin and enabled us to generate efficient conditional mutants of essential proteins in yeast as well as cell lines derived from chicken, mouse, hamster, monkey and human cells, thus offering a powerful tool to control protein expression and study protein function.

Replisome progression complex links DNA replication to sister chromatid cohesion in Xenopus egg extracts.

Cohesin-mediated sister chromatid cohesion is established during the S-phase, and recent studies demonstrate that a cohesin protein ring concatenates sister DNA molecules. However, little is known about how DNA replication is linked to the establishment of sister chromatid cohesion. Here, we used Xenopus egg extracts to show that AND-1 and Tim1-Tipin, homologues of Saccharomyces cerevisiae Ctf4 and Tof1-Csm3, respectively, are associated with the replisome and are required for proper establishment of the cohesion observed in the M-phase extracts. Immunodepletion of both AND-1 and Tim1-Tipin from the extracts leads to aberrant sister chromatid cohesion, which is similarly induced by the depletion of cohesin. These results demonstrate that AND-1 and Tim1-Tipin are key factors linking DNA replication and establishment of sister chromatid cohesion. On the basis of the physical interactions between AND-1 and DNA polymerases, we discuss a model to describe how replisome progression complex establishes sister chromatid cohesion.

The phosphorylated C-terminal domain of Xenopus Cut5 directly mediates ATR-dependent activation of Chk1.

ATR-dependent activation of the kinase Chk1 is the initial step in signal transduction in the DNA replication checkpoint, which allows a cell to enter mitosis only after the completion of DNA replication. TopBP1-related proteins in higher eukaryotes are implicated in the replication checkpoint, but their exact role remains elusive because of their requirements for replication initiation. Here we report that the initiation function of Xenopus Cut5/TopBP1 could be entirely separated from its checkpoint function: the N-terminal half fragment, a region of Cut5 conserved through evolution, is sufficient for initiation, but is incapable of activating the checkpoint; the C-terminal half fragment, which is unique in metazoan species, is by itself capable of activating the checkpoint response without initiating replication. Upon the activation of Chk1, the Ser1131 within the C-terminal region of Cut5 is phosphorylated, and this phosphorylation is critical for the checkpoint response. Furthermore, Cut5 directly stimulated Chk1 phosphorylation in the in vitro kinase assay reconstituted with recombinant proteins and ATR immunoprecipitated from extracts. On the basis of replication protein A (RPA)-dependent loading of Cut5 on to replicating and replication-arrested chromatin, we propose that Cut5 plays a crucial role in the initial amplification step of the ATR-Chk1 signaling pathway at the stalled replication fork.

The N-terminal noncatalytic region of Xenopus RecQ4 is required for chromatin binding of DNA polymerase alpha in the initiation of DNA replication.

Recruitment of DNA polymerases onto replication origins is a crucial step in the assembly of eukaryotic replication machinery. A previous study in budding yeast suggests that Dpb11 controls the recruitment of DNA polymerases alpha and epsilon onto the origins. Sld2 is an essential replication protein that interacts with Dpb11, but no metazoan homolog has yet been identified. We isolated Xenopus RecQ4 as a candidate Sld2 homolog. RecQ4 is a member of the metazoan RecQ helicase family, and its N-terminal region shows sequence similarity with Sld2. In Xenopus egg extracts, RecQ4 is essential for the initiation of DNA replication, in particular for chromatin binding of DNA polymerase alpha. An N-terminal fragment of RecQ4 devoid of the helicase domain could rescue the replication activity of RecQ4-depleted extracts, and antibody against the fragment inhibited DNA replication and chromatin binding of the polymerase. Further, N-terminal fragments of RecQ4 physically interacted with Cut5, a Xenopus homolog of Dpb11, and their ability to bind to Cut5 closely correlated with their ability to rescue the replication activity of the depleted extracts. Our data suggest that RecQ4 performs an essential role in the assembly of replication machinery through interaction with Cut5 in vertebrates.

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.

Regulation of replication licensing by acetyltransferase Hbo1.

The initiation of DNA replication is tightly regulated in eukaryotic cells to ensure that the genome is precisely duplicated once and only once per cell cycle. This is accomplished by controlling the assembly of a prereplicative complex (pre-RC) which involves the sequential binding to replication origins of the origin recognition complex (ORC), Cdc6/Cdc18, Cdt1, and the minichromosome maintenance complex (Mcm2-Mcm7, or Mcm2-7). Several mechanisms of pre-RC regulation are known, including ATP utilization, cyclin-dependent kinase levels, protein turnover, and Cdt1 binding by geminin. Histone acetylation may also affect the initiation of DNA replication, but at present neither the enzymes nor the steps involved are known. Here, we show that Hbo1, a member of the MYST histone acetyltransferase family, is a previously unrecognized positive regulatory factor for pre-RC assembly. When Hbo1 expression was inhibited in human cells, Mcm2-7 failed to associate with chromatin even though ORC and Cdc6 loading was normal. When Xenopus egg extracts were immunodepleted of Xenopus Hbo1 (XHbo1), chromatin binding of Mcm2-7 was lost, and DNA replication was abolished. The binding of Mcm2-7 to chromatin in XHbo1-depleted extracts could be restored by the addition of recombinant Cdt1.

Intrinsic nuclear import activity of geminin is essential to prevent re-initiation of DNA replication in Xenopus eggs.

Prior to S phase, eukaryotic chromosomes are licensed for initiation of DNA replication, and re-licensing is prohibited after S phase has started until late mitosis, thus ensuring that genomic DNA is duplicated precisely once in each cell cycle. Here, we report that over-expression of Cdt1, an essential licensing protein, induced re-replication in Xenopus egg extracts. Geminin, a metazoan-specific inhibitor of Cdt1, was critical for preventing re-replication induced by Cdt1. Re-replication induced by the addition of recombinant Cdt1 and/or by the depletion of geminin from extracts was enhanced by a proteasome inhibitor, which suppressed the degradation of Cdt1 in the extracts. Furthermore, a nuclear localization sequence identified in Xenopus geminin had a significant role in the suppression of re-replication induced by Cdt1. These results suggest that nuclear accumulation of geminin plays a dominant role in the licensing system of Xenopus eggs.

Xenopus Cut5 is essential for a CDK-dependent process in the initiation of DNA replication.

Fission yeast Cut5/Rad4 and its budding yeast homolog Dpb11 are required for both DNA replication and the S-phase checkpoint. Here, we have investigated the role of the Xenopus homolog of Cut5 in the initiation of DNA replication using Xenopus egg extracts. Xenopus Cut5, which shows sequence similarity to DmMus101 and HsTopBP1, is essential for DNA replication in the egg extracts. It is required for the chromatin binding of Cdc45 and DNA polymerases, but not for the formation of pre-replicative complexes or the elongation stage of DNA replication. The chromatin binding of Cut5 consists of two distinct modes. S-phase cyclin-dependent kinase (S-CDK)-independent binding is sufficient for DNA replication while S-CDK-dependent binding is dispensable. Further, S-CDK acts after the chromatin binding of Cut5 and before the binding of Cdc45. These results demonstrate that the chromatin binding of Cut5 is required for the action of S-CDK, which in turn triggers the formation of pre-initiation complexes of DNA replication.

A novel ring-like complex of Xenopus proteins essential for the initiation of DNA replication.

We have identified Xenopus homologs of the budding yeast Sld5 and its three interacting proteins. These form a novel complex essential for the initiation of DNA replication in Xenopus egg extracts. The complex binds to chromatin in a manner dependent on replication licensing and S-phase CDK. The chromatin binding of the complex and that of Cdc45 are mutually dependent and both bindings require Xenopus Cut5, the yeast homolog of which interacts with Sld5. On replicating chromatin the complex interacts with Cdc45 and MCM, putative components of replication machinery. Electron microscopy further reveals that the complex has a ring-like structure. These results suggest that the complex plays an essential role in the elongation stage of DNA replication as well as the initiation stage.