Death domain-associated protein DAXX regulates noncoding RNA transcription at the centromere through the transcription regulator ZFAT.

The centromere is an essential chromosomal structure for faithful chromosome segregation during cell division. No protein-coding genes exist at the centromeres, but centromeric DNA is actively transcribed into noncoding RNA (ncRNA). This centromeric transcription and its ncRNA products play important roles in centromere functions. We previously reported that the transcriptional regulator ZFAT (zinc-finger protein with AT hook) plays a pivotal role in ncRNA transcription at the centromere; however, it was unclear how ZFAT involvement was regulated. Here, we show that the death domain-associated protein (DAXX) promotes centromeric localization of ZFAT to regulate ncRNA transcription at the centromere. Coimmunoprecipitation analysis of endogenous proteins clearly reveals that DAXX interacts with ZFAT. In addition, we show that ectopic coexpression of ZFAT with DAXX increases the centromeric levels of both ZFAT and ncRNA, compared with ectopic expression of ZFAT alone. On the other hand, we found that siRNA-mediated depletion of DAXX decreases the centromeric levels of both ZFAT and ncRNA in cells ectopically expressing ZFAT. These results suggest that DAXX promotes the centromeric localization of ZFAT and ZFAT-regulated centromeric ncRNA transcription. Furthermore, we demonstrate that depletion of endogenous DAXX protein is sufficient to cause a decrease in the ncRNA levels at the centromeres of chromosomes 17 and X in which ZFAT regulates the transcription, indicating a physiological significance of DAXX in ZFAT-regulated centromeric ncRNA transcription. Taken together, these results demonstrate that DAXX regulates centromeric ncRNA transcription through ZFAT.

TRF2-mediated ORC recruitment underlies telomere stability upon DNA replication stress.

Telomeres are intrinsically difficult-to-replicate region of eukaryotic chromosomes. Telomeric repeat binding factor 2 (TRF2) binds to origin recognition complex (ORC) to facilitate the loading of ORC and the replicative helicase MCM complex onto DNA at telomeres. However, the biological significance of the TRF2-ORC interaction for telomere maintenance remains largely elusive. Here, we employed a TRF2 mutant with mutations in two acidic acid residues (E111A and E112A) that inhibited the TRF2-ORC interaction in human cells. The TRF2 mutant was impaired in ORC recruitment to telomeres and showed increased replication stress-associated telomeric DNA damage and telomere instability. Furthermore, overexpression of an ORC1 fragment (amino acids 244-511), which competitively inhibited the TRF2-ORC interaction, increased telomeric DNA damage under replication stress conditions. Taken together, these findings suggest that TRF2-mediated ORC recruitment contributes to the suppression of telomere instability.

CENP-B promotes the centromeric localization of ZFAT to control transcription of noncoding RNA.

The centromere is a chromosomal locus that is essential for the accurate segregation of chromosomes during cell division. Transcription of noncoding RNA (ncRNA) at the centromere plays a crucial role in centromere function. The zinc-finger transcriptional regulator ZFAT binds to a specific 8-bp DNA sequence at the centromere, named the ZFAT box, to control ncRNA transcription. However, the precise molecular mechanisms by which ZFAT localizes to the centromere remain elusive. Here we show that the centromeric protein CENP-B is required for the centromeric localization of ZFAT to regulate ncRNA transcription. The ectopic expression of CENP-B induces the accumulation of both endogenous and ectopically expressed ZFAT protein at the centromere in human cells, suggesting that the centromeric localization of ZFAT requires the presence of CENP-B. Coimmunoprecipitation analysis reveals that ZFAT interacts with the acidic domain of CENP-B, and depletion of endogenous CENP-B reduces the centromeric levels of ZFAT protein, further supporting that CENP-B is required for the centromeric localization of ZFAT. In addition, knockdown of CENP-B significantly decreased the expression levels of ncRNA at the centromere where ZFAT regulates the transcription, suggesting that CENP-B is involved in the ZFAT-regulated centromeric ncRNA transcription. Thus, we concluded that CENP-B contributes to the establishment of the centromeric localization of ZFAT to regulate ncRNA transcription.

The histone deacetylases sir2 and rpd3 act on ribosomal DNA to control the replication program in budding yeast.

In S. cerevisiae, replication timing is controlled by epigenetic mechanisms restricting the accessibility of origins to limiting initiation factors. About 30% of these origins are located within repetitive DNA sequences such as the ribosomal DNA (rDNA) array, but their regulation is poorly understood. Here, we have investigated how histone deacetylases (HDACs) control the replication program in budding yeast. This analysis revealed that two HDACs, Rpd3 and Sir2, control replication timing in an opposite manner. Whereas Rpd3 delays initiation at late origins, Sir2 is required for the timely activation of early origins. Moreover, Sir2 represses initiation at rDNA origins, whereas Rpd3 counteracts this effect. Remarkably, deletion of SIR2 restored normal replication in rpd3Δ cells by reactivating rDNA origins. Together, these data indicate that HDACs control the replication timing program in budding yeast by modulating the ability of repeated origins to compete with single-copy origins for limiting initiation factors.

DNA damage responses that enhance resilience to replication stress.

During duplication of the genome, eukaryotic cells may experience various exogenous and endogenous replication stresses that impede progression of DNA replication along chromosomes. Chemical alterations in template DNA, imbalances of deoxynucleotide pools, repetitive sequences, tight DNA-protein complexes, and conflict with transcription can negatively affect the replication machineries. If not properly resolved, stalled replication forks can cause chromosome breaks leading to genomic instability and tumor development. Replication stress is enhanced in cancer cells due, for example, to the loss of DNA repair genes or replication-transcription conflict caused by activation of oncogenic pathways. To prevent these serious consequences, cells are equipped with diverse mechanisms that enhance the resilience of replication machineries to replication stresses. This review describes DNA damage responses activated at stressed replication forks and summarizes current knowledge on the pathways that promote faithful chromosome replication and protect chromosome integrity, including ATR-dependent replication checkpoint signaling, DNA cross-link repair, and SLX4-mediated responses to tight DNA-protein complexes that act as barriers. This review also focuses on the relevance of replication stress responses to selective cancer chemotherapies.

GRWD1 regulates ribosomal protein L23 levels via the ubiquitin-proteasome system.

Glutamate-rich WD40 repeat-containing 1 (GRWD1) is a Cdt1-binding protein that promotes mini-chromosome maintenance (MCM) loading through its histone chaperone activity. GRWD1 acts as a tumor-promoting factor by downregulating p53 (also known as TP53) via the RPL11-MDM2-p53 axis. Here, we identified GRWD1-interacting proteins using a proteomics approach and showed that GRWD1 interacts with various proteins involved in transcription, translation, DNA replication and repair, chromatin organization, and ubiquitin-mediated proteolysis. We focused on the ribosomal protein ribosomal protein L23 (RPL23), which positively regulates nucleolar stress responses through MDM2 binding and inhibition, thereby functioning as a tumor suppressor. Overexpression of GRWD1 decreased RPL23 protein levels and stability; this effect was restored upon treatment with the proteasome inhibitor MG132. EDD (also known as UBR5), an E3 ubiquitin ligase that interacts with GRWD1, also downregulated RPL23, and the decrease was further enhanced by co-expression of GRWD1. Conversely, siRNA-mediated GRWD1 knockdown upregulated RPL23. Co-expression of GRWD1 and EDD promoted RPL23 ubiquitylation. These data suggest that GRWD1 acts together with EDD to negatively regulate RPL23 via the ubiquitin-proteasome system. GRWD1 expression reversed the RPL23-mediated inhibition of anchorage-independent growth in cancer cells. Our data suggest that GRWD1-induced RPL23 proteolysis plays a role in downregulation of p53 and tumorigenesis.

Genome-wide analysis of the spatiotemporal regulation of firing and dormant replication origins in human cells.

In metazoan cells, only a limited number of mini chromosome maintenance (MCM) complexes are fired during S phase, while the majority remain dormant. Several methods have been used to map replication origins, but such methods cannot identify dormant origins. Herein, we determined MCM7-binding sites in human cells using ChIP-Seq, classified them into firing and dormant origins using origin data and analysed their association with various chromatin signatures. Firing origins, but not dormant origins, were well correlated with open chromatin regions and were enriched upstream of transcription start sites (TSSs) of transcribed genes. Aggregation plots of MCM7 signals revealed minimal difference in the efficacy of MCM loading between firing and dormant origins. We also analysed common fragile sites (CFSs) and found a low density of origins at these sites. Nevertheless, firing origins were enriched upstream of the TSSs. Based on the results, we propose a model in which excessive MCMs are actively loaded in a genome-wide manner, irrespective of chromatin status, but only a fraction are passively fired in chromatin areas with an accessible open structure, such as regions upstream of TSSs of transcribed genes. This plasticity in the specification of replication origins may minimize collisions between replication and transcription.

Inhibiting the MCM8-9 complex selectively sensitizes cancer cells to cisplatin and olaparib.

MCM8 and MCM9 are paralogues of the MCM2-7 eukaryotic DNA replication helicase proteins and play a crucial role in a homologous recombination-mediated repair process to resolve replication stress by fork stalling. Thus, deficiency of MCM8-9 sensitizes cells to replication stress caused, for example, by platinum compounds that induce interstrand cross-links. It is suggested that cancer cells undergo more replication stress than normal cells due to hyperstimulation of growth. Therefore, it is possible that inhibiting MCM8-9 selectively hypersensitizes cancer cells to platinum compounds and poly(ADP-ribose) polymerase inhibitors, both of which hamper replication fork progression. Here, we inhibited MCM8-9 in transformed and nontransformed cells and examined their sensitivity to cisplatin and olaparib. We found that knockout of MCM9 or knockdown of MCM8 selectively hypersensitized transformed cells to cisplatin and olaparib. In agreement with reported findings, RAS- and human papilloma virus type 16 E7-mediated transformation of human fibroblasts increased replication stress, as indicated by induction of multiple DNA damage responses (including formation of Rad51 foci). Such replication stress induced by oncogenes was further increased by knockdown of MCM8, providing a rationale for cancer-specific hypersensitization to cisplatin and olaparib. Finally, we showed that knocking out MCM9 increased the sensitivity of HCT116 xenograft tumors to cisplatin. Taken together, the data suggest that conceptual MCM8-9 inhibitors will be powerful cancer-specific chemosensitizers for platinum compounds and poly(ADP-ribose) polymerase inhibitors, thereby opening new avenues to the design of novel cancer chemotherapeutic strategies.

GRWD1 negatively regulates p53 via the RPL11-MDM2 pathway and promotes tumorigenesis.

The ribosomal protein L11 (RPL11) binds and inhibits the MDM2 ubiquitin ligase, thereby promoting p53 stability. Thus, RPL11 acts as a tumor suppressor. Here, we show that GRWD1 (glutamate-rich WD40 repeat containing 1) physically and functionally interacts with RPL11. GRWD1 is localized to nucleoli and is released into the nucleoplasm upon nucleolar stress. Silencing of GRWD1 increases p53 induction by nucleolar stress, whereas overexpression of GRWD1 reduces p53 induction. Furthermore, GRWD1 overexpression competitively inhibits the RPL11-MDM2 interaction and alleviates RPL11-mediated suppression of MDM2 ubiquitin ligase activity toward p53. These effects are mediated by the N-terminal region of GRWD1, including the acidic domain. Finally, we show that GRWD1 overexpression in combination with HPV16 E7 and activated KRAS confers anchorage-independent growth and tumorigenic capacity on normal human fibroblasts. Consistent with this, GRWD1 overexpression is associated with poor prognosis in cancer patients. Taken together, our results suggest that GRWD1 is a novel negative regulator of p53 and a potential oncogene.

TRF2 recruits ORC through TRFH domain dimerization.

Telomeres are specialized chromatin structures that prevent the degradation and instability of the ends of linear chromosomes. While telomerase maintains long stretches of the telomeric repeat, the majority of telomeric DNA is duplicated by conventional DNA replication. A fundamental step in eukaryotic DNA replication involves chromatin binding of the origin recognition complex (ORC). In human cells, telomeric repeat binding factor 2 (TRF2) is thought to play a role in the recruitment of ORC onto telomeres. To better understand the mechanism of TRF2-mediated ORC recruitment, we utilized a lacO-LacI protein tethering system in U2OS cells and found that ectopically targeted TRF2, but not TRF1, can recruit ORC onto the lacO array. We further found that the TRF homology (TRFH) dimerization domain of TRF2, but not its mutant defective in dimerization, is sufficient for ORC and minichromosome maintenance (MCM) recruitment. Mutations impairing the dimerization also compromised ORC recruitment by full-length TRF2. Similar results were obtained using immunoprecipitation and GST pull-down assays. Together, these results suggest that dimerized TRF2 recruits ORC and stimulates pre-replication complex (pre-RC) formation at telomeres through the TRFH domain.

Nucleosome assembly and disassembly activity of GRWD1, a novel Cdt1-binding protein that promotes pre-replication complex formation.

GRWD1 was previously identified as a novel Cdt1-binding protein that possesses histone-binding and nucleosome assembly activities and promotes MCM loading, probably by maintaining chromatin openness at replication origins. However, the molecular mechanisms underlying these activities remain unknown. We prepared reconstituted mononucleosomes from recombinant histones and a DNA fragment containing a nucleosome positioning sequence, and investigated the effects of GRWD1 on them. GRWD1 could disassemble these preformed mononucleosomes in vitro in an ATP-independent manner. Thus, our data suggest that GRWD1 facilitates removal of H2A-H2B dimers from nucleosomes, resulting in formation of hexasomes. The activity was compromised by deletion of the acidic domain, which is required for efficient histone binding. In contrast, nucleosome assembly activity of GRWD1 was not affected by deletion of the acidic domain. In HeLa cells, the acidic domain of GRWD1 was necessary to maintain chromatin openness and promote MCM loading at replication origins. Taken together, our results suggest that GRWD1 promotes chromatin fluidity by influencing nucleosome structures, e.g., by transient eviction of H2A-H2B, and thereby promotes efficient MCM loading at replication origins.

Cdt1-binding protein GRWD1 is a novel histone-binding protein that facilitates MCM loading through its influence on chromatin architecture.

Efficient pre-replication complex (pre-RC) formation on chromatin templates is crucial for the maintenance of genome integrity. However, the regulation of chromatin dynamics during this process has remained elusive. We found that a conserved protein, GRWD1 (glutamate-rich WD40 repeat containing 1), binds to two representative replication origins specifically during G1 phase in a CDC6- and Cdt1-dependent manner, and that depletion of GRWD1 reduces loading of MCM but not CDC6 and Cdt1. Furthermore, chromatin immunoprecipitation coupled with high-throughput sequencing (Seq) revealed significant genome-wide co-localization of GRWD1 with CDC6. We found that GRWD1 has histone-binding activity. To investigate the effect of GRWD1 on chromatin architecture, we used formaldehyde-assisted isolation of regulatory elements (FAIRE)-seq or FAIRE-quantitative PCR analyses, and the results suggest that GRWD1 regulates chromatin openness at specific chromatin locations. Taken together, these findings suggest that GRWD1 may be a novel histone-binding protein that regulates chromatin dynamics and MCM loading at replication origins.

A novel anti-microtubule agent with carbazole and benzohydrazide structures suppresses tumor cell growth in vivo.

BACKGROUND: The mitotic spindles are among the most successful targets of anti-cancer chemotherapy, and they still hold promise as targets for novel drugs. The anti-mitotic drugs in current clinical use, including taxanes, epothilones, vinca alkaloids, and halichondrins, are all microtubule-targeting agents. Although these drugs are effective for cancer chemotherapy, they have some critical problems; e.g., neurotoxicity caused by damage to neuronal microtubules, as well as innate or acquired drug resistance. To overcome these problems, a great deal of effort has been expended on development of novel anti-mitotics. METHODS: We identified novel microtubule-targeting agents with carbazole and benzohydrazide structures: N'-[(9-ethyl-9H-carbazol-3-yl)methylene]-2-methylbenzohydrazide (code number HND-007) and its related compounds. We investigated their activities against cancer cells using various methods including cell growth assay, immunofluorescence analysis, cell cycle analysis, tubulin polymerization assay, and tumor inhibition assay in nude mice. RESULTS: HND-007 inhibits tubulin polymerization in vitro and blocks microtubule formation and centrosome separation in cancer cells. Consequently, it suppresses the growth of various cancer cell lines, with IC50 values in the range 1.3-4.6µM. In addition, HND-007 can inhibit the growth of taxane-resistant cancer cells that overexpress P-glycoprotein. Finally, HND-007 can inhibit HeLa cell tumor growth in nude mice. CONCLUSIONS AND GENERAL SIGNIFICANCE: Taken together, these findings suggest that HND-007 is a promising lead compound for development of novel anti-mitotic, anti-microtubule chemotherapeutic agents.

Clinical report of the immediate placement implants in patients aged 80 and over: Five cases and a short review.

With the population aging, the proportion of elderly individuals is expected to increase, and the proportion of individuals over 80 is also likely to increase. Given that the population is aging and life expectancy is increasing, elderly patients who have lost thei teeth will also increase. Impaired masticatory function caused by tooth loss is a risk factor for cognitive decline and frailty, and neglecting to treat tooth loss is a critical issue. In this report, five patients over 80 requested dental implants for new missing teeth and underwent immediate implant placement. All patients were treated by the same surgeon using the same technique, materials, and dental implants. Five elements were measured for the peri-implant bone from the cone-beam computed tomography images. All patients were followed for three or more years and exhibited good progress without any abnormal findings in their oral cavities or notable radiographic abnormalities such as bone absorption or peri-implantitis. Compared to conventional treatment with delayed placement, combining immediate implant placement and regenerative medicine for fixed superstructures may be the better option for missing tooth replacement if the conditions are suitable. Thus, this treatment modality may improve the quality of life in elderly patients, especially super elderly patients.

Association between treatment response and dose of blonanserin transdermal patch in patients with acute schizophrenia: A post hoc cluster analysis based on baseline psychiatric symptoms.

AIM: To explore the optimal dose of blonanserin transdermal patch (BNS-P) based on baseline psychiatric symptomatic characteristics during acute schizophrenia. METHODS: A post hoc cluster analysis was conducted using data from a 6-week randomized, double-blind, placebo-controlled study of BNS-P (40 or 80 mg/day) in acute schizophrenia. We classified patients into three clusters based on baseline psychiatric symptoms. Efficacy was assessed using the change from baseline to week 6 in the PANSS total score. Safety was assessed by the incidence of adverse events. RESULTS: Among 577 patients, three clusters were identified, characterized by severe psychiatric (Cluster-S; n = 122), predominant negative (Cluster-N; n = 191), and predominant positive (Cluster-P; n = 264) symptoms. In Cluster-P, both BNS-P 40 and 80 mg/day reduced PANSS total score significantly more than placebo (p = 0.036, effect size = 0.342; p < 0.001, effect size = 0.687, respectively). In Cluster-S and -N, only BNS-P 80 mg/day reduced PANSS total score significantly more than placebo (p = 0.045, effect size = 0.497; p = 0.034, effect size = 0.393, respectively). The effect size was greater at 80 mg/day than at 40 mg/day across all clusters. The most common treatment-emergent adverse events were akathisia and skin-related adverse events in all clusters. CONCLUSION: BNS-P exhibited a dose-dependent antipsychotic effect in all clusters, particularly highlighting its efficacy in patients with predominant positive symptoms, even at lower doses. These findings provide novel and valuable insights for determining BNS-P dose tailoring to individual symptomatic characteristics in real-world practice.

Inhibitory Effect of Pyra-Metho-Carnil on the Differentiation and Maturation of Macrophages in Normal and Cancer Microenvironments.

BACKGROUND/AIM: In a previous study, we have demonstrated heightened Pyra-Metho-Carnil (PMC) efficacy in nude mice with intact innate immunity that lack T and B cells. This has prompted hypothesizing that PMC may target macrophages that promote cancer growth. In this study, we conducted co-culture experiments with macrophages derived from THP-1 human monocyte cell lines and spheroids representing normal and cancer microenvironments. We then performed RNA sequencing and flow cytometry analysis to elucidate the mechanisms by which PMC affects macrophage differentiation and maturation. MATERIALS AND METHODS: THP-1 cells were differentiated by phorbol 12-myristate 13-acetate (PMA) and matured by PMA and lipopolysaccharide (LPS) either with or without PMC. Co-cultures were performed using stimulated THP-1 cells and HKe3-wild-type KRAS or HKe3-mutant (mt) KRAS spheroids. We then performed RNA-seq analysis of THP-1 cells stimulated by PMA (either with or without PMC) and flow cytometry analysis of mice peripheral blood obtained after PMC administration. RESULTS: THP-1 cells matured by PMA and LPS specifically increased the area of HKe3-mtKRAS cancer spheroids and the addition of PMC to THP-1 cells was found to inhibit cancer spheroid growth. RNA-seq data suggested that PMC treatment of THP-1 cells stimulated with PMA suppressed cell motility regulatory functions via down-regulation of the NF[Formula: see text]B pathway. Furthermore, flow cytometry results showed that PMC treatment suppressed monocyte maturation in B6 mice. CONCLUSION: The high level of in vivo tumor suppression caused by PMC may be due to inhibition of the differentiation and maturation of tumor-associated macrophages via the NF[Formula: see text]B signaling pathway.

Analysis of DNA replication profiles in budding yeast and mammalian cells using DNA combing.

DNA combing is a powerful method developed by Bensimon and colleagues to stretch DNA molecules on silanized glass coverslips. This technique provides a unique way to monitor the activation of replication origins and the progression of replication forks at the level of single DNA molecules, after incorporation of thymidine analogs, such as 5-bromo-2'-deoxyuridine (BrdU), 5-iodo-2'-deoxyuridine (IdU) and 5-chloro-2'-deoxyuridine (CldU) in newly-synthesized DNA. Unlike microarray-based approaches, this assay gives access to the variability of replication profiles in individual cells. It can also be used to monitor the effect of DNA lesions on fork progression, arrest and restart. In this review, we propose standard DNA combing methods to analyze DNA replication in budding yeast and in human cells. We also show that 5-ethynyl-2'-deoxyuridine (EdU) can be used as a good alternative to BrdU for DNA combing analysis, as unlike halogenated nucleotides, it can be detected without prior denaturation of DNA.

Laparoscopic distal pancreatectomy for a pancreatic lymphoepithelial cyst: case report and review of literature.

CONTEXT: Lymphoepithelial cysts of the pancreas are a rare disease of true pancreatic cysts, the cause of which is unknown. The differential diagnosis is broad and includes many benign and malignant cystic lesions of the pancreas and surrounding organs. A combination of imaging modalities and fine needle aspiration might narrow the differential diagnosis. However, the final diagnosis can only be achieved with certainty after resection of the cyst. CASE REPORT: The present case report is a lymphoepithelial cyst of the pancreas that was resected laparoscopically. A 53-year-old man was incidentally found to have a cystic tumor in the tail of the pancreas after undergoing an abdominal ultrasound, which showed a 41x33 mm cystic mass in the pancreatic tail. He had no abdominal symptoms. Laparoscopic distal pancreatectomy and splenectomy were performed. Histologic examination revealed a lymphoepithelial cyst. CONCLUSION: Herein, we discuss the diagnostic difficulties and management decisions that face surgeons treating pancreatic cysts.

Inhibitory Effect of Pyra-Metho-Carnil on Cancer Spheroid Growth Through Decrease in Glycolysis-associated Molecules.

BACKGROUND/AIM: Pyra-Metho-Carnil (PMC) has been identified as a novel candidate compound for treating numerous malignancies; however, its mechanism of action remains unknown. In this study, we conducted RNA-sequencing (RNA-seq) analyses to elucidate the mechanism of PMC against human colorectal cancer cells harboring mutant KRAS (mtKRAS). MATERIALS AND METHODS: RNA-seq analyses of the HKe3-wild-type KRAS and HKe3-mtKRAS spheroids treated with DMSO or PMC for 6 days were performed. RESULTS: RNA-seq data suggested that PMC treatment suppresses the aerobic glycolysis pathway in HKe3-mtKRAS spheroids through the down-regulation of the HIF1 pathway. Indeed, treatment with PMC markedly suppresses the absorption of glucose by spheroids and the secretion of lactate from them. CONCLUSION: PMC suppresses growth of cancer spheroid through down-regulation of cancer-specific glucose metabolism.

CDC6 interaction with ATR regulates activation of a replication checkpoint in higher eukaryotic cells.

CDC6, a replication licensing protein, is partially exported to the cytoplasm in human cells through phosphorylation by Cdk during S phase, but a significant proportion remains in the nucleus. We report here that human CDC6 physically interacts with ATR, a crucial checkpoint kinase, in a manner that is stimulated by phosphorylation by Cdk. CDC6 silencing by siRNAs affected ATR-dependent inhibition of mitotic entry elicited by modest replication stress. Whereas a Cdk-phosphorylation-mimicking CDC6 mutant could rescue the checkpoint defect by CDC6 silencing, a phosphorylation-deficient mutant could not. Furthermore, we found that the CDC6-ATR interaction is conserved in Xenopus. We show that the presence of Xenopus CDC6 during S phase is essential for Xenopus ATR to bind to chromatin in response to replication inhibition. In addition, when human CDC6 amino acid fragment 180-220, which can bind to both human and Xenopus ATR, was added to Xenopus egg extracts after assembly of the pre-replication complex, Xenopus Chk1 phosphorylation was significantly reduced without lowering replication, probably through a sequestration of CDC6-mediated ATR-chromatin interaction. Thus, CDC6 might regulate replication-checkpoint activation through the interaction with ATR in higher eukaryotic cells.