Ordered assembly of Sld3, GINS and Cdc45 is distinctly regulated by DDK and CDK for activation of replication origins

The Shu complex interacts with the replicative helicase to prevent mutations and aberrant recombination

Homologous recombination (HR) is important for DNA damage tolerance during replication. The yeast Shu complex, a conserved homologous recombination factor, prevents replication-associated mutagenesis. Here we examine how yeast cells require the Shu complex for coping with MMS-induced lesions during DNA replication. We find that Csm2, a subunit of the Shu complex, binds to autonomous-replicating sequences (ARS) in yeast. Further evolutionary studies reveal that the yeast and human Shu complexes have co-evolved with specific replication-initiation factors. The connection between the Shu complex and replication is underlined by the finding that the Shu complex interacts with the ORC and MCM complexes. For example, the Shu complex interacts, independent of other HR proteins, with the replication initiation complexes through the N-terminus of Psy3. Lastly, we show interactions between the Shu complex and the replication initiation complexes are essential for resistance to DNA damage, to prevent mutations and aberrant recombination events. In our model, the Shu complex interacts with the replication machinery to enable error-free bypass of DNA damage.

Interactions between Fkh1 monomers stabilize its binding to DNA replication origins

Eukaryotic DNA replication is initiated from multiple genomic origins, which can be broadly categorized as firing early or late in the S phase. Several factors can influence the temporal usage of origins to determine the timing of their firing. In budding yeast, the Forkhead family proteins Fkh1 and Fkh2 bind to a subset of replication origins and activate them at the beginning of the S phase. In these origins, the Fkh1/2 binding sites are arranged in a strict configuration, suggesting that Forkhead factors must bind the origins in a specific manner. To explore these binding mechanisms in more detail, we mapped the domains of Fkh1 that were required for its role in DNA replication regulation. We found that a short region of Fkh1 near its DNA binding domain was essential for the protein to bind and activate replication origins. Analysis of purified Fkh1 proteins revealed that this region mediates dimerization of Fkh1, suggesting that intramolecular contacts of Fkh1 are required for efficient binding and regulation of DNA replication origins. We also show that the Sld3-Sld7-Cdc45 complex is recruited to Forkhead-regulated origins already in the G1 phase and that Fkh1 is constantly required to keep these factors bound on origins before the onset of the S phase. Together, our results suggest that dimerization-mediated stabilization of DNA binding by Fkh1 is crucial for its ability to activate DNA replication origins.

Transcription-associated cyclin-dependent kinase 12 (CDK12) as a potential target for cancer therapy

Cyclin-dependent kinase 12 (CDK12), a transcription-related cyclin dependent kinase (CDK), plays a momentous part in multitudinous biological functions, such as replication, transcription initiation to elongation and termination, precursor mRNA (pre-mRNA) splicing, intron polyadenylation (IPA), and translation. CDK12 can act as a tumour suppressor or oncogene in disparate cellular environments, and its dysregulation likely provokes tumorigenesis. A comprehensive understanding of CDK12 will tremendously facilitate the exploitation of novel tactics for the treatment and precaution of cancer. Currently, CDK12 inhibitors are nonspecific and nonselective, which profoundly hinders the pharmacological target validation and drug exploitation process. Herein, we summarize the newly comprehension of the biological functions of CDK12 with a focus on recently emerged advancements of CDK12-associated therapeutic approaches in cancers.

Chimeric nanoparticles for targeting mitochondria in cancer cells

Mitochondrial dysfunction is implicated in myriad diseases, including cancer. Subsequently, targeting mitochondrial DNA (mt-DNA) in cancer cells has emerged as an unorthodox strategy for anti-cancer therapy. However, approaches targeting only one component of the mitochondrial "central dogma" can be evaded by cancer cells through various mechanisms. To address this, herein, we have engineered mitochondria-targeting cholesterol-based chimeric nanoparticles (mt-CNPs) consisting of cisplatin, camptothecin, and tigecycline, which can simultaneously impair mt-DNA, mitochondrial topoisomerase I (mt-Top1), and mitochondrial ribosomes. mt-CNPs were characterized as being positively charged, spherical in shape, and 187 nm in diameter. Confocal microscopy confirmed that mt-CNPs efficiently localized into the mitochondria of A549 lung cancer cells within 6 h, followed by mitochondrial morphology damage and the subsequent generation of reactive oxygen species (ROS). mt-CNPs showed remarkable cancer-cell killing abilities compared to free-drug combinations in A549 (lung), HeLa (cervical), and MCF7 (breast) cancer cells. These mitochondria-targeting lipidic chimeric nanoparticles could be explored further to impair multiple targets in mitochondria, helping researchers to gain an understanding of mitochondrial translational and transcriptional machinery and to develop new strategies for cancer therapy.

DDK/Hsk1 phosphorylates and targets fission yeast histone deacetylase Hst4 for degradation to stabilize stalled DNA replication forks

In eukaryotes, paused replication forks are prone to collapse, which leads to genomic instability, a hallmark of cancer. Dbf4-dependent kinase (DDK)/Hsk1^Cdc7 is a conserved replication initiator kinase with conflicting roles in replication stress response. Here, we show that fission yeast DDK/Hsk1 phosphorylates sirtuin, Hst4 upon replication stress at C-terminal serine residues. Phosphorylation of Hst4 by DDK marks it for degradation via the ubiquitin ligase SCF^pof3. Phosphorylation-defective hst4 mutant (4SA-hst4) displays defective recovery from replication stress, faulty fork restart, slow S-phase progression and decreased viability. The highly conserved fork protection complex (FPC) stabilizes stalled replication forks. We found that the recruitment of FPC components, Swi1 and Mcl1 to the chromatin is compromised in the 4SA-hst4 mutant, although whole cell levels increased. These defects are dependent upon H3K56ac and independent of intra S-phase checkpoint activation. Finally, we show conservation of H3K56ac-dependent regulation of Timeless, Tipin, and And-1 in human cells. We propose that degradation of Hst4 via DDK increases H3K56ac, changing the chromatin state in the vicinity of stalled forks facilitating recruitment and function of FPC. Overall, this study identified a crucial role of DDK and FPC in the regulation of replication stress response with implications in cancer therapeutics.

Whole-transcriptomic Profile of SK-MEL-3 Melanoma Cells Treated with the Histone Deacetylase Inhibitor: Trichostatin A

BACKGROUND

Malignant melanoma cells can rapidly acquire phenotypic properties making them resistant to radiation and mainline chemotherapies such as decarbonize or kinase inhibitors that target RAS-proto-oncogene independent auto-activated mitogen-activated protein kinases (MAPK)/through dual specificity mitogen-activated protein kinase (MEK). Both drug resistance and inherent transition from melanocytic nevi to malignant melanoma involve the overexpression of histone deacetylases (HDACs) and a B-Raf proto-oncogene (BRAF) mutation.

MATERIALS AND METHODS

In this work, the effects of an HDAC class I and II inhibitor trichostatin A (TSA) on the whole transcriptome of SK-MEL-3 cells carrying a BRAF mutation was examined.

RESULTS

The data obtained show that TSA was an extremely potent HDAC inhibitor within SK-MEL-3 nuclear lysates, where TSA was then optimized for appropriate sub-lethal concentrations for in vitro testing. The whole-transcriptome profile shows a basic phenotype dominance in the SK-MEL-3 cell line for i) synthesis of melanin, ii) phagosome acidification, iii) ATP hydrolysis-coupled proton pumps and iv) iron transport systems. While TSA did not affect the aforementioned major systems, it evoked a dramatic change to the transcriptome: reflected by a down-regulation of 810 transcripts and up-regulation of 833, with fold-change from -15.27 to +31.1 FC (p<0.00001). Largest differentials were found for the following transcripts: Up-regulated: Tetraspanin 13 (TSPAN13), serpin family i member 1 (SERPINI1), ATPase Na+/K+ transporting subunit beta 2 (ATP1B2), nicotinamide nucleotide adenylyl transferase 2 (NMNAT2), platelet-derived growth factor receptor-like (PDGFRL), cytochrome P450 family 1 subfamily A member 1 (CYP1A1), prostate androgen-regulated mucin-like protein 1 (PARM1), secretogranin II (SCG2), SYT11 (synaptotagmin 11), rhophilin associated tail protein 1 like (ROPN1L); down-regulated: polypeptide N-acetylgalactosaminyltransferase 3 (GALNT3), carbonic anhydrase 14 (CAXIV), BCL2-related protein A1 (BCL2A1), protein kinase C delta (PRKCD), transient receptor potential cation channel subfamily M member 1 (TRPM1), ubiquitin associated protein 1 like (UBAP1L), glutathione peroxidase 8 (GPX8), interleukin 16 (IL16), tumor protein p53 (TP53), and serpin family H member 1 (SERPINH1). There was no change to any of the HDAC transcripts (class I, II and IV), the sirtuin HDAC family (1-6) or the BRAF proto-oncogene v 599 transcripts. However, the data showed that TSA down-regulated influential transcripts that drive the BRAF-extracellular signal-regulated kinase (ERK)1/2 oncogenic pathway (namely PRKCD and MYC proto-oncogene which negatively affected the cell-cycle distribution. Mitotic inhibition was corroborated by functional pathway analysis and flow cytometry confirming halt at the G₂ phase, occurring in the absence of toxicity.

CONCLUSION

TSA does not alter HDAC transcripts nor BRAF itself, but down-regulates critical components of the MAPK/MEK/BRAF oncogenic pathway, initiating a mitotic arrest.

GINS2 regulates cell proliferation and apoptosis in human epithelial ovarian cancer

Go-Ichi-Ni-San 2 (GINS2), also known as partner of Sld five 2, is involved in the initiation of DNA replication and cell cycle progression. GINS2 is abundantly expressed in a number of malignant solid tumors, including breast cancer, melanoma and hepatic carcinoma. However, the functions of GINS2 in epithelial ovarian cancer (EOC) remain unclear. The aim of the present study was to investigate these functions. GINS2 expression was detected in EOC and normal ovarian tissues using immunohistochemistry. To investigate the functions of GINS2 in EOC, GINS2 expression was stably knocked down in SKOV-3 cells using lentiviral short hairpin RNA (shRNA). The expression of GINS2 mRNA and protein in SKOV-3 cells was examined using reverse-transcription quantitative polymerase chain reaction (RT-qPCR) and western blot analyses, respectively. Cell proliferation was determined using high-content screening and MTT assays. Cell cycle progression and apoptosis were detected using flow cytometry. Compared with normal ovarian tissues, EOC tissues expressed increased levels of GINS2 expression (16.7 vs. 58.3%). Increased expression of GINS2 mRNA was also observed in SKOV-3 and OVCAR3 cells. In the investigation of GINS2 functions in EOC, GINS2 expression at the mRNA and protein levels was significantly inhibited by specific GINS2 shRNA. GINS2 knockdown significantly inhibited the proliferation and viability of SKOV-3 cells and induced cell cycle arrest in S phase. Furthermore, GINS2 knockdown in SKOV-3 cells significantly increased cell apoptosis. GINS2 is markedly expressed in EOC tissues and cell lines. Stable GINS2 knockdown in SKOV-3 cells significantly inhibited cell proliferation and induced cell cycle arrest and cell apoptosis. Therefore, GINS2 may be involved in EOC progression.

The drinking water contaminant dibromoacetonitrile delays G1-S transition and suppresses Chk1 activation at broken replication forks

Chlorination of drinking water protects humans from water-born pathogens, but it also produces low concentrations of dibromoacetonitrile (DBAN), a common disinfectant by-product found in many water supply systems. DBAN is not mutagenic but causes DNA breaks and elevates sister chromatid exchange in mammalian cells. The WHO issued guidelines for DBAN after it was linked with cancer of the liver and stomach in rodents. How this haloacetonitrile promotes malignant cell transformation is unknown. Using fission yeast as a model, we report here that DBAN delays G1-S transition. DBAN does not hinder ongoing DNA replication, but specifically blocks the serine 345 phosphorylation of the DNA damage checkpoint kinase Chk1 by Rad3 (ATR) at broken replication forks. DBAN is particularly damaging for cells with defects in the lagging-strand DNA polymerase delta. This sensitivity can be explained by the dependency of pol delta mutants on Chk1 activation for survival. We conclude that DBAN targets a process or protein that acts at the start of S phase and is required for Chk1 phosphorylation. Taken together, DBAN may precipitate cancer by perturbing S phase and by blocking the Chk1-dependent response to replication fork damage.

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.

Mechanisms Governing DDK Regulation of the Initiation of DNA Replication

The budding yeast Dbf4-dependent kinase (DDK) complex—comprised of cell division cycle (Cdc7) kinase and its regulatory subunit dumbbell former 4 (Dbf4)—is required to trigger the initiation of DNA replication through the phosphorylation of multiple minichromosome maintenance complex subunits 2-7 (Mcm2-7). DDK is also a target of the radiation sensitive 53 (Rad53) checkpoint kinase in response to replication stress. Numerous investigations have determined mechanistic details, including the regions of Mcm2, Mcm4, and Mcm6 phosphorylated by DDK, and a number of DDK docking sites. Similarly, the way in which the Rad53 forkhead-associated 1 (FHA1) domain binds to DDK—involving both canonical and non-canonical interactions—has been elucidated. Recent work has revealed mutual promotion of DDK and synthetic lethal with dpb11-1 3 (Sld3) roles. While DDK phosphorylation of Mcm2-7 subunits facilitates their interaction with Sld3 at origins, Sld3 in turn stimulates DDK phosphorylation of Mcm2. Details of a mutually antagonistic relationship between DDK and Rap1-interacting factor 1 (Rif1) have also recently come to light. While Rif1 is able to reverse DDK-mediated Mcm2-7 complex phosphorylation by targeting the protein phosphatase glycogen 7 (Glc7) to origins, there is evidence to suggest that DDK can counteract this activity by binding to and phosphorylating Rif1.

Regulation of SLD5 gene expression by miR-370 during acute growth of cancer cells

SLD5 is a member of the GINS complex, essential for DNA replication in eukaryotes. It has been reported that SLD5 is involved in early embryogenesis in the mouse, and cell cycle progression and genome integrity in Drosophila. SLD5 may be involved in malignant tumor progression, but its relevance in human cancer has not been determined. Here, we found strong SLD5 expression in both human bladder cancer tissues from patients and cell lines. Knockdown of SLD5 using small interfering RNA resulted in reduction of cell growth both in vitro and an in vivo xenograft model. Moreover, we found that high levels of SLD5 in bladder cancer cells result from downregulation of microRNA (miR)-370 that otherwise suppresses its expression. High level expression of DNA-methyltransferase (DNMT) 1 and IL-6 were also observed in bladder cancer cells. Knockdown of IL-6 led to downregulation of DNMT1 and SLD5 expression, suggesting that IL-6-induced overexpression of DNMT1 suppresses miR-370, resulting in high SLD5 expression. Our findings could contribute to understanding tumorigenic processes and progression of human bladder cancer, whereby inhibition of SLD5 could represent a novel strategy to prevent tumor growth.

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

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

MCM: one ring to rule them all

Precise replication of the eukaryotic genome is achieved primarily through strict regulation of the enzyme responsible for DNA unwinding, the replicative helicase. The motor of this helicase is a hexameric AAA+ ATPase called MCM. The loading of MCM onto DNA and its subsequent activation and disassembly are each restricted to separate cell cycle phases; this ensures that a functional replisome is only built once at any replication origin. In recent years, biochemical and structural studies have shown that distinct conformational changes in MCM, each requiring post-translational modifications and/or the activity of other replication proteins, define the various stages of the chromosome replication cycle. Here, we review recent progress in this area.

Shugoshin forms a specialized chromatin domain at subtelomeres that regulates transcription and replication timing

A chromosome is composed of structurally and functionally distinct domains. However, the molecular mechanisms underlying the formation of chromatin structure and the function of subtelomeres, the telomere-adjacent regions, remain obscure. Here we report the roles of the conserved centromeric protein Shugoshin 2 (Sgo2) in defining chromatin structure and functions of the subtelomeres in the fission yeast Schizosaccharomyces pombe. We show that Sgo2 localizes at the subtelomeres preferentially during G2 phase and is essential for the formation of a highly condensed subtelomeric chromatin body 'knob'. Furthermore, the absence of Sgo2 leads to the derepression of the subtelomeric genes and premature DNA replication at the subtelomeric late origins. Thus, the subtelomeric specialized chromatin domain organized by Sgo2 represses both transcription and replication to ensure proper gene expression and replication timing.

RecQL4 is required for the association of Mcm10 and Ctf4 with replication origins in human cells

Though RecQL4 was shown to be essential for the initiation of DNA replication in mammalian cells, its role in initiation is poorly understood. Here, we show that RecQL4 is required for the origin binding of Mcm10 and Ctf4, and their physical interactions and association with replication origins are controlled by the concerted action of both CDK and DDK activities. Although RecQL4-dependent binding of Mcm10 and Ctf4 to chromatin can occur in the absence of pre-replicative complex, their association with replication origins requires the presence of the pre-replicative complex and CDK and DDK activities. Their association with replication origins and physical interactions are also targets of the DNA damage checkpoint pathways which prevent initiation of DNA replication at replication origins. Taken together, the RecQL4-dependent association of Mcm10 and Ctf4 with replication origins appears to be the first important step controlled by S phase promoting kinases and checkpoint pathways for the initiation of DNA replication in human cells.

A reconstituted system reveals how activating and inhibitory interactions control DDK dependent assembly of the eukaryotic replicative helicase

During G1-phase of the cell-cycle the replicative MCM2–7 helicase becomes loaded onto DNA into pre-replicative complexes (pre-RCs), resulting in MCM2–7 double-hexamers on DNA. In S-phase, Dbf4-dependent kinase (DDK) and cyclin-dependent-kinase (CDK) direct with the help of a large number of helicase-activation factors the assembly of a Cdc45–MCM2–7–GINS (CMG) complex. However, in the absence of S-phase kinases complex assembly is inhibited, which is unexpected, as the MCM2–7 double-hexamer represents a very large interaction surface. Currently it is unclear what mechanisms restricts complex assembly and how DDK can overcome this inhibition to promote CMG-assembly. We developed an advanced reconstituted-system to study helicase activation in-solution and discovered that individual factors like Sld3 and Sld2 can bind directly to the pre-RC, while Cdc45 cannot. When Sld3 and Sld2 were incubated together with the pre-RC, we observed that competitive interactions restrict complex assembly. DDK stabilizes the Sld3/Sld2–pre-RC complex, but the complex is only short-lived, indicating an anti-cooperative mechanism. Yet, a Sld3/Cdc45–pre-RC can form in the presence of DDK and the addition of Sld2 enhances complex stability. Our results indicate that helicase activation is regulated by competitive and cooperative interactions, which restrict illegitimate complex formation and direct limiting helicase-activation factors into pre-initiation complexes.

The quaternary structure of the eukaryotic DNA replication proteins Sld7 and Sld3

The initiation of eukaryotic chromosomal DNA replication requires the formation of an active replicative helicase at the replication origins of chromosomes. Yeast Sld3 and its metazoan counterpart treslin are the hub proteins mediating protein associations critical for formation of the helicase. The Sld7 protein interacts with Sld3, and the complex formed is thought to regulate the function of Sld3. Although Sld7 is a non-essential DNA replication protein that is found in only a limited range of yeasts, its depletion slowed the growth of cells and caused a delay in the S phase. Recently, the Mdm2-binding protein was found to bind to treslin in humans, and its depletion causes defects in cells similar to the depletion of Sld7 in yeast, suggesting their functional relatedness and importance during the initiation step of DNA replication. Here, the crystal structure of Sld7 in complex with Sld3 is presented. Sld7 comprises two structural domains. The N-terminal domain of Sld7 binds to Sld3, and the C-terminal domains connect two Sld7 molecules in an antiparallel manner. The quaternary structure of the Sld3-Sld7 complex shown from the crystal structures appears to be suitable to activate two helicase molecules loaded onto replication origins in a head-to-head manner.

Evolutionary conservation of the CDK targets in eukaryotic DNA replication initiation

A fundamental requirement for all organisms is the faithful duplication and transmission of the genetic material. Failure to accurately copy and segregate the genome during cell division leads to loss of genetic information and chromosomal abnormalities. Such genome instability is the hallmark of the earliest stages of tumour formation. Cyclin-dependent kinase (CDK) plays a vital role in regulating the duplication of the genome within the eukaryotic cell cycle. Importantly, this kinase is deregulated in many cancer types and is an emerging target of chemotherapeutics. In this review, I will consider recent advances concerning the role of CDK in replication initiation across eukaryotes. The implications for strict CDK-dependent regulation of genome duplication in the context of the cell cycle will be discussed.

Switch on the engine: how the eukaryotic replicative helicase MCM2-7 becomes activated

A crucial step during eukaryotic initiation of DNA replication is the correct loading and activation of the replicative DNA helicase, which ensures that each replication origin fires only once. Unregulated DNA helicase loading and activation, as it occurs in cancer, can cause severe DNA damage and genomic instability. The essential mini-chromosome maintenance proteins 2-7 (MCM2-7) represent the core of the eukaryotic replicative helicase that is loaded at DNA replication origins during G1-phase of the cell cycle. The MCM2-7 helicase activity, however, is only triggered during S-phase once the holo-helicase Cdc45-MCM2-7-GINS (CMG) has been formed. A large number of factors and several kinases interact and contribute to CMG formation and helicase activation, though the exact mechanisms remain unclear. Crucially, upon DNA damage, this reaction is temporarily halted to ensure genome integrity. Here, we review the current understanding of helicase activation; we focus on protein interactions during CMG formation, discuss structural changes during helicase activation, and outline similarities and differences of the prokaryotic and eukaryotic helicase activation process.

TopBP1: A BRCT-scaffold protein functioning in multiple cellular pathways

Human TopBP1 contains nine BRCT domains and functions in DNA replication initiation, checkpoint signalling, DNA repair and influences transcriptional control. TopBP1 and its homologues have been the subject of numerous scientific publications since the last comprehensive review in 2005, emerging as a key scaffold protein that links crucial components within these distinct cellular processes. This review focuses on recently published work, with particular emphasis on structural insights into TopBP1 function and the binding partners identified for DNA replication initiation, DNA-dependent checkpoints, DNA repair and transcription. We further summarise what is known about TopBP1 and links to human disease.

Crystal structure of the homology domain of the eukaryotic DNA replication proteins Sld3/Treslin

The initiation of eukaryotic chromosomal DNA replication requires the formation of an active replicative helicase at the replication origins of chromosomal DNA. Yeast Sld3 and its metazoan counterpart Treslin are the hub proteins mediating protein associations critical for the helicase formation. Here, we show the crystal structure of the central domain of Sld3 that is conserved in Sld3/Treslin family of proteins. The domain consists of two segments with 12 helices and is sufficient to bind to Cdc45, the essential helicase component. The structure model of the Sld3-Cdc45 complex, which is crucial for the formation of the active helicase, is proposed.

MYC and the control of DNA replication

The MYC oncogene is a multifunctional protein that is aberrantly expressed in a significant fraction of tumors from diverse tissue origins. Because of its multifunctional nature, it has been difficult to delineate the exact contributions of MYC's diverse roles to tumorigenesis. Here, we review the normal role of MYC in regulating DNA replication as well as its ability to generate DNA replication stress when overexpressed. Finally, we discuss the possible mechanisms by which replication stress induced by aberrant MYC expression could contribute to genomic instability and cancer.

Helicase activation and establishment of replication forks at chromosomal origins of replication

Many replication proteins assemble on the pre-RC-formed replication origins and constitute the pre-initiation complex (pre-IC). This complex formation facilitates the conversion of Mcm2-7 in the pre-RC to an active DNA helicase, the Cdc45-Mcm-GINS (CMG) complex. Two protein kinases, cyclin-dependent kinase (CDK) and Dbf4-dependent kinase (DDK), work to complete the formation of the pre-IC. Each kinase is responsible for a distinct step of the process in yeast; Cdc45 associates with origins in a DDK-dependent manner, whereas the association of GINS with origins depends on CDK. These associations with origins also require specific initiation proteins: Sld3 for Cdc45; and Dpb11, Sld2, and Sld3 for GINS. Functional homologs of these proteins exist in metazoa, although pre-IC formation cannot be separated by requirement of DDK and CDK because of experimental limitations. Once the replicative helicase is activated, the origin DNA is unwound, and bidirectional replication forks are established.

Requirement of SLD5 for early embryogenesis

SLD5 forms a GINS complex with PSF1, PSF2 and PSF3, which is essential for the initiation of DNA replication in lower eukaryotes. Although these components are conserved in mammals, their biological function is unclear. We show here that targeted disruption of SLD5 in mice causes a defect in cell proliferation in the inner cell mass, resulting in embryonic lethality at the peri-implantation stage, indicating that SLD5 is essential for embryogenesis. Moreover, this phenotype of SLD5 mutant mice is quite similar compared with that of PSF1 mutant mice. We have previously reported that haploinsufficiency of PSF1 resulted in failure of acute proliferation of bone marrow hematopoietic stem cells (HSCs) during reconstitution of bone marrow ablated by 5-FU treatment. Since SLD5 was highly expressed in bone marrow, we investigated its involvement in bone marrow reconstitution after bone marrow ablation as observed in PSF1 heterozygous mutant mice. However, heterozygous deletion of the SLD5 gene was found not to significantly affect bone marrow reconstitution. On the other hand, abundant SLD5 expression was observed in human cancer cell lines and heterozygous deletion of the gene attenuated tumor progression in a murine model of spontaneous gastric cancer. These indicated that requirement and dependency of SLD5 for cell proliferation is different in different cell types.

Functional analysis of Drosophila DNA polymerase ε p58 subunit

DNA polymerase ε (polε) plays a central role in DNA replication in eukaryotic cells, and has been suggested to the main synthetic polymerase on the leading strand. It is a hetero-tetrameric enzyme, comprising a large catalytic subunit (the A subunit ~250 kDa), a B subunit of ~60 kDa in most species (~80 kDa in budding yeast) and two smaller subunits (each ~20 kDa). In Drosophila, two subunits of polε (dpolε) have been identified. One is the 255 kDa catalytic subunit (dpolεp255), and the other is the 58 kDa subunit (dpolεp58). The functions of the B subunit have been mainly studied in budding yeast and mammalian cell culture, few studies have been performed in the context of an intact multicellular organism and therefore its functions in this context remain poorly understood. To address this we examined the in vivo role of dpolεp58 in Drosophila. A homozygous dpolεp58 mutant is pupal lethal, and the imaginal discs are less developed in the third instar larvae. In the eye discs of this mutant S phases, as measured by BrdU incorporation assays, were significantly reduced. In addition staining with an anti-phospho histone H3 (PH3) antibody, (a marker of M phase), was increased in the posterior region of eye discs, where usually cells stop replicating and start differentiation. These results indicate that dpolεp58 is essential for Drosophila development and plays an important role in progression of S phase in mitotic cell cycles. We also observed that the size of nuclei in salivary gland cells were decreased in dpolεp58 mutant, indicating that dpolεp58 also plays a role in endoreplication. Furthermore we detect a putative functional interaction between dpolε and ORC2 in discs suggesting that polε plays a role in the initiation of DNA replication in Drosophila.

Regulating DNA replication in eukarya

DNA replication is tightly controlled in eukaryotic cells to ensure that an exact copy of the genetic material is inherited by both daughter cells. Oscillating waves of cyclin-dependent kinase (CDK) and anaphase-promoting complex/cyclosome (APC/C) activities provide a binary switch that permits the replication of each chromosome exactly once per cell cycle. Work from several organisms has revealed a conserved strategy whereby inactive replication complexes are assembled onto DNA during periods of low CDK and high APC activity but are competent to execute genome duplication only when these activities are reversed. Periods of high CDK and low APC/C serve an essential function by blocking reassembly of replication complexes, thereby preventing rereplication. Higher eukaryotes have evolved additional CDK-independent mechanisms for preventing rereplication.

Identification of a heteromeric complex that promotes DNA replication origin firing in human cells

Treslin/TICRR (TopBP1-interacting, replication stimulating protein/TopBP1-interacting, checkpoint, and replication regulator), the human ortholog of the yeast Sld3 protein, is an essential DNA replication factor that is regulated by cyclin-dependent kinases and the DNA damage checkpoint. We identified MDM two binding protein (MTBP) as a factor that interacts with Treslin/TICRR throughout the cell cycle. We show that MTBP depletion by means of small interfering RNA inhibits DNA replication by preventing assembly of the CMG (Cdc45-MCM-GINS) holohelicase during origin firing. Although MTBP has been implicated in the function of the p53 tumor suppressor, we found MTBP is required for DNA replication irrespective of a cell's p53 status. We propose that MTBP acts with Treslin/TICRR to integrate signals from cell cycle and DNA damage response pathways to control the initiation of DNA replication in human cells.

Controlling DNA replication origins in response to DNA damage - inhibit globally, activate locally

DNA replication in eukaryotic cells initiates from multiple replication origins that are distributed throughout the genome. Coordinating the usage of these origins is crucial to ensure complete and timely replication of the entire genome precisely once in each cell cycle. Replication origins fire according to a cell-type-specific temporal programme, which is established in the G1 phase of each cell cycle. In response to conditions causing the slowing or stalling of DNA replication forks, the programme of origin firing is altered in two contrasting ways, depending on chromosomal context. First, inactive or 'dormant' replication origins in the vicinity of the stalled replication fork become activated and, second, the S phase checkpoint induces a global shutdown of further origin firing throughout the genome. Here, we review our current understanding on the role of dormant origins and the S phase checkpoint in the rescue of stalled forks and the completion of DNA replication in the presence of replicative stress.

Coordinated degradation of replisome components ensures genome stability upon replication stress in the absence of the replication fork protection complex

The stabilization of the replisome complex is essential in order to achieve highly processive DNA replication and preserve genomic integrity. Conversely, it would also be advantageous for the cell to abrogate replisome functions to prevent inappropriate replication when fork progression is adversely perturbed. However, such mechanisms remain elusive. Here we report that replicative DNA polymerases and helicases, the major components of the replisome, are degraded in concert in the absence of Swi1, a subunit of the replication fork protection complex. In sharp contrast, ORC and PCNA, which are also required for DNA replication, were stably maintained. We demonstrate that this degradation of DNA polymerases and helicases is dependent on the ubiquitin-proteasome system, in which the SCF(Pof3) ubiquitin ligase is involved. Consistently, we show that Pof3 interacts with DNA polymerase ε. Remarkably, forced accumulation of replisome components leads to abnormal DNA replication and mitotic catastrophes in the absence of Swi1. Swi1 is known to prevent fork collapse at natural replication block sites throughout the genome. Therefore, our results suggest that the cell elicits a program to degrade replisomes upon replication stress in the absence of Swi1. We also suggest that this program prevents inappropriate duplication of the genome, which in turn contributes to the preservation of genomic integrity.

Telomere-binding protein Taz1 controls global replication timing through its localization near late replication origins in fission yeast

In eukaryotes, the replication of chromosome DNA is coordinated by a replication timing program that temporally regulates the firing of individual replication origins. However, the molecular mechanism underlying the program remains elusive. Here, we report that the telomere-binding protein Taz1 plays a crucial role in the control of replication timing in fission yeast. A DNA element located proximal to a late origin in the chromosome arm represses initiation from the origin in early S phase. Systematic deletion and substitution experiments demonstrated that two tandem telomeric repeats are essential for this repression. The telomeric repeats recruit Taz1, a counterpart of human TRF1 and TRF2, to the locus. Genome-wide analysis revealed that Taz1 regulates about half of chromosomal late origins, including those in subtelomeres. The Taz1-mediated mechanism prevents Dbf4-dependent kinase (DDK)-dependent Sld3 loading onto the origins. Our results demonstrate that the replication timing program in fission yeast uses the internal telomeric repeats and binding of Taz1.

DNA polymerization-independent functions of DNA polymerase epsilon in assembly and progression of the replisome in fission yeast

DNA Pol ε synthesizes the leading strands, following the CMG (Cdc45/Mcm2-7/GINS) helicase, although the N-terminal polymerase domain of the catalytic subunit, Cdc20 in fission yeast, is dispensable for viability. We show that the C-terminal domain of Cdc20 plays the noncatalytic essential roles in both the assembly and progression of CMG helicase.

DNA polymerase epsilon (Pol ε) synthesizes the leading strands, following the CMG (Cdc45, Mcm2-7, and GINS [Go-Ichi-Nii-San]) helicase that translocates on the leading-strand template at eukaryotic replication forks. Although Pol ε is essential for the viability of fission and budding yeasts, the N-terminal polymerase domain of the catalytic subunit, Cdc20/Pol2, is dispensable for viability, leaving the following question: what is the essential role(s) of Pol ε? In this study, we investigated the essential roles of Pol ε using a temperature-sensitive mutant and a recently developed protein-depletion (off-aid) system in fission yeast. In cdc20-ct1 cells carrying mutations in the C-terminal domain of Cdc20, the CMG components, RPA, Pol α, and Pol δ were loaded onto replication origins, but Cdc45 did not translocate from the origins, suggesting that Pol ε is required for CMG helicase progression. In contrast, depletion of Cdc20 abolished the loading of GINS and Cdc45 onto origins, indicating that Pol ε is essential for assembly of the CMG complex. These results demonstrate that Pol ε plays essential roles in both the assembly and progression of CMG helicase.

Activation of the replicative DNA helicase: breaking up is hard to do

The precise duplication of the eukaryotic genome is accomplished by carefully coordinating the loading and activation of the replicative DNA helicase so that each replication origin is unwound and assembles functional bi-directional replisomes just once in each cell cycle. The essential Minichromosome Maintenance 2-7 (Mcm2-7) proteins, comprising the core of the replicative DNA helicase, are first loaded at replication origins in an inactive form. The helicase is then activated by recruitment of the Cdc45 and GINS proteins into a holo-helicase known as CMG (Cdc45, Mcm2-7, GINS). These steps are regulated by multiple mechanisms to ensure that Mcm2-7 loading can only occur during G1 phase, whilst activation of Mcm2-7 cannot occur during G1 phase. Here we review recent progress in understanding these critical reactions focusing on the mechanism of helicase loading and activation.

Conditional inactivation of replication proteins in fission yeast using hormone-binding domains

The fission yeast Schizosaccharomyces pombe is a useful model for analysing DNA replication as genetic methods to allow conditional inactivation of relevant proteins can provide important information about S-phase execution. A number of strategies are available to allow regulation of protein level or activity but there are disadvantages specific to each method and this may have limitations for particular proteins or experiments. We have investigated the utility of the inducible hormone-binding domain (HBD) system, which has been described in other organisms but little used in fission yeast, for the creation of conditional-lethal replication mutants. In this method, proteins are tagged with HBD and can be regulated with β-estradiol. In this article, we describe the application of this method in fission yeast, specifically with regard to analysis of the function of GINS, an essential component of the eukaryotic replicative helicase, the CMG complex.

Mcm10 plays an essential role in origin DNA unwinding after loading of the CMG components

The CMG complex composed of Mcm2-7, Cdc45 and GINS is postulated to be the eukaryotic replicative DNA helicase, whose activation requires sequential recruitment of replication proteins onto Mcm2-7. Current models suggest that Mcm10 is involved in assembly of the CMG complex, and in tethering of DNA polymerase α at replication forks. Here, we report that Mcm10 is required for origin DNA unwinding after association of the CMG components with replication origins in fission yeast. A combination of promoter shut-off and the auxin-inducible protein degradation (off-aid) system efficiently depleted cellular Mcm10 to <0.5% of the wild-type level. Depletion of Mcm10 did not affect origin loading of Mcm2-7, Cdc45 or GINS, but impaired recruitment of RPA and DNA polymerases. Mutations in a conserved zinc finger of Mcm10 abolished RPA loading after recruitment of Mcm10. These results show that Mcm10, together with the CMG components, plays a novel essential role in origin DNA unwinding through its zinc-finger function.

Rif1 is a global regulator of timing of replication origin firing in fission yeast

One of the long-standing questions in eukaryotic DNA replication is the mechanisms that determine where and when a particular segment of the genome is replicated. Cdc7/Hsk1 is a conserved kinase required for initiation of DNA replication and may affect the site selection and timing of origin firing. We identified rif1Δ, a null mutant of rif1(+), a conserved telomere-binding factor, as an efficient bypass mutant of fission yeast hsk1. Extensive deregulation of dormant origins over a wide range of the chromosomes occurs in rif1Δ in the presence or absence of hydroxyurea (HU). At the same time, many early-firing, efficient origins are suppressed or delayed in firing timing in rif1Δ. Rif1 binds not only to telomeres, but also to many specific locations on the arm segments that only partially overlap with the prereplicative complex assembly sites, although Rif1 tends to bind in the vicinity of the late/dormant origins activated in rif1Δ. The binding to the arm segments occurs through M to G1 phase in a manner independent of Taz1 and appears to be essential for the replication timing program during the normal cell cycle. Our data demonstrate that Rif1 is a critical determinant of the origin activation program on the fission yeast chromosomes.

Initiation of chromosomal DNA replication in eukaryotic cells; contribution of yeast genetics to the elucidation

Chromosomal DNA replication is a fundamental process in the transmission of genetic information through generations. While the molecular mechanism of DNA replication has been studied for a long time, knowledge regarding this process in eukaryotic cells has advanced rapidly in the past 20 years. Yeast genetics contributed profoundly to this rapid advancement. Reverse genetics and genetic screenings identified all genes encoding replication proteins in budding yeast. Moreover, the genetic interactions that were used in screenings and analyses provided an insight into the molecular mechanism of chromosomal DNA replication. Further studies showed that complicated but sophisticated mechanisms govern chromosomal DNA replication. The retrospective view of the genetic approaches used to elucidate DNA replication in eukaryotes, together with current knowledge, tell us the reasons why some of the genetic screenings are successful, and also provide ideas for future directions.

Differential requirement for the N-terminal catalytic domain of the DNA polymerase ε p255 subunit in the mitotic cell cycle and the endocycle

In Drosophila, the 255kDa catalytic subunit (dpolεp255) and the 58kDa subunit of DNA polymerase ε (dpolεp58) have been identified. The N-terminus of dpolεp255 carries well-conserved six DNA polymerase subdomains and five 3'→5' exonuclease motifs as observed with Polε in other species. We here examined roles of dpolεp255 during Drosophila development using transgenic fly lines expressing double stranded RNA (dsRNA). Expression of dpolεp255 dsRNA in eye discs induced a small eye phenotype and inhibited DNA synthesis, indicating a role in the G1-S transition and/or S-phase progression of the mitotic cycle. Similarly, expression of dpolεp255 dsRNA in the salivary glands resulted in small size and endoreplication defects, demonstrating a critical role in endocycle progression. In the eye disc, defects induced by knockdown of dpolεp255 were rescued by overexpression of the C-terminal region of dpolεp255, indicating that the function of this non-catalytic domain is conserved between yeast and Drosophila. However, this was not the case for the salivary gland, suggesting that the catalytic N-terminal region is crucial for endoreplication and its defect cannot be complemented by other DNA polymerases. In addition, several genetic interactants with dpolεp255 including genes related to DNA replication such as RFC, DNA primase, DNA polη, Mcm10 and Psf2 and chromatin remodeling such as Iswi were also identified.

The GINS complex: structure and function

Eukaryotic chromosomal DNA replication is controlled by a highly ordered series of steps involving multiple proteins at replication origins. The eukaryotic GINS complex is essential for the establishment of DNA replication forks and replisome progression. GINS is one of the core components of the eukaryotic replicative helicase, the CMG (Cdc45-MCM-GINS) complex, which unwinds duplex DNA ahead of the moving replication fork. Eukaryotic GINS also links with other key proteins at the fork to maintain an active replisome progression complex. Archaeal GINS homologues play a central role in chromosome replication by associating with other replisome components. This chapter focuses on the molecular events related with DNA replication initiation, and summarizes our current understanding of the function, structure and evolution of the GINS complex in eukaryotes and archaea.

Origin association of Sld3, Sld7, and Cdc45 proteins is a key step for determination of origin-firing timing

BACKGROUND

Chromosomal DNA replication in eukaryotes initiates from multiple origins of replication, and because of this multiplicity, activation of replication origins is likely to be highly coordinated; origins fire at characteristic times, with some origins firing on average earlier (early-firing origins) and others later (late-firing origins) in the S phase of the budding yeast cell cycle. However, the molecular basis for such temporal regulation is poorly understood.

RESULTS

We show that origin association of the low-abundance replication proteins Sld3, Sld7, and Cdc45 is the key to determining the temporal order of origin firing. These proteins form a complex and associate with the early-firing origins in G1 phase in a manner that depends on Dbf4-dependent kinase (DDK), which is essential for the initiation of DNA replication. An increased dosage of Sld3, Sld7, and Cdc45 allows the late-firing origins to fire earlier in S phase. Additionally, an increased dosage of DDK also allows the late-firing origins to fire earlier.

CONCLUSIONS

The DDK-dependent limited association between origins and Sld3-Sld7-Cdc45 is a key step for determining the timing of origin firing.

Highly organized DnaA-oriC complexes recruit the single-stranded DNA for replication initiation

In Escherichia coli, the replication origin oriC consists of two functional regions: the duplex unwinding element (DUE) and its flanking DnaA-assembly region (DAR). ATP-DnaA molecules multimerize on DAR, unwinding DUE for DnaB helicase loading. However, DUE-unwinding mechanisms and functional structures in DnaA–oriC complexes supporting those remain unclear. Here, using various in vitro reconstituted systems, we identify functionally distinct DnaA sub-complexes formed on DAR and reveal novel mechanisms in DUE unwinding. The DUE-flanking left-half DAR carrying high-affinity DnaA box R1 and the ATP-DnaA-preferential DnaA box R5, τ1-2 and I1-2 sites formed a DnaA sub-complex competent in DUE unwinding and ssDUE binding, thereby supporting basal DnaB loading activity. This sub-complex is further subdivided into two; the DUE-distal DnaA sub-complex formed on the ATP–DnaA-preferential sites binds ssDUE. Notably, the DUE-flanking, DnaA box R1–DnaA sub-complex recruits DUE to the DUE-distal DnaA sub-complex in concert with a DNA-bending nucleoid protein IHF, thereby promoting DUE unwinding and binding of ssDUE. The right-half DAR–DnaA sub-complex stimulated DnaB loading, consistent with in vivo analyses. Similar features are seen in DUE unwinding of the hyperthermophile, Thermotoga maritima, indicating evolutional conservation of those mechanisms.

Multiple pathways can bypass the essential role of fission yeast Hsk1 kinase in DNA replication initiation

A number of different genetic backgrounds and growth conditions bypass DNA replication defects caused by the absence of yeast Hsk1 kinase, demonstrating the plasticity of the eukaryotic DNA replication program.

Cdc7/Hsk1 is a conserved kinase required for initiation of DNA replication that potentially regulates timing and locations of replication origin firing. Here, we show that viability of fission yeast hsk1Δ cells can be restored by loss of mrc1, which is required for maintenance of replication fork integrity, by cds1Δ, or by a checkpoint-deficient mutant of mrc1. In these mutants, normally inactive origins are activated in the presence of hydroxyurea and binding of Cdc45 to MCM is stimulated. mrc1Δ bypasses hsk1Δ more efficiently because of its checkpoint-independent inhibitory functions. Unexpectedly, hsk1Δ is viable at 37°C. More DNA is synthesized, and some dormant origins fire in the presence of hydroxyurea at 37°C. Furthermore, hsk1Δ bypass strains grow poorly at 25°C compared with higher temperatures. Our results show that Hsk1 functions for DNA replication can be bypassed by different genetic backgrounds as well as under varied physiological conditions, providing additional evidence for plasticity of the replication program in eukaryotes.

CDC-48/p97 coordinates CDT-1 degradation with GINS chromatin dissociation to ensure faithful DNA replication

Faithful transmission of genomic information requires tight spatiotemporal regulation of DNA replication factors. In the licensing step of DNA replication, CDT-1 is loaded onto chromatin to subsequently promote the recruitment of additional replication factors, including CDC-45 and GINS. During the elongation step, the CDC-45/GINS complex moves with the replication fork; however, it is largely unknown how its chromatin association is regulated. Here, we show that the chaperone-like ATPase CDC-48/p97 coordinates degradation of CDT-1 with release of the CDC-45/GINS complex. C. elegans embryos lacking CDC-48 or its cofactors UFD-1/NPL-4 accumulate CDT-1 on mitotic chromatin, indicating a critical role of CDC-48 in CDT-1 turnover. Strikingly, CDC-48(UFD-1/NPL-4)-deficient embryos show persistent chromatin association of CDC-45/GINS, which is a consequence of CDT-1 stabilization. Moreover, our data confirmed a similar regulation in Xenopus egg extracts, emphasizing a conserved coordination of licensing and elongation events during eukaryotic DNA replication by CDC-48/p97.

Mcm10 interacts with Rad4/Cut5(TopBP1) and its association with origins of DNA replication is dependent on Rad4/Cut5(TopBP1)

Initiation of DNA replication in eukaryotes is a highly conserved and ordered process involving the co-ordinated, stepwise association of distinct proteins at multiple origins of replication throughout the genome. Here, taking Schizosaccharomyces pombe as a model, the role of Rad4(TopBP1) in the assembly of the replication complex has been examined. Quantitative chromatin immunoprecipitation experiments confirm that Rad4(TopBP1) associates with origins of DNA replication and, in addition, demonstrate that the protein is not present within the active replisome. A direct interaction between Rad4(TopBP1) and Mcm10 is shown and this is reflected in the Rad4(TopBP1)-dependent origin association of Mcm10. Rad4(TopBP1) is also shown to interact with Sld2 and Sld3 and to be required for the stable origin association of these two proteins. Rad4(TopBP1) chromatin association at stalled replication forks was found to be dependent upon the checkpoint protein Rad9, which was not required for Rad4(TopBP1) origin association. Comparison of the levels of chromatin association at origins of replication and stalled replication forks and the differential requirement for Rad9 suggest functional differences for Rad4(TopBP1) at these distinct sites.

Eukaryotic origin-dependent DNA replication in vitro reveals sequential action of DDK and S-CDK kinases

Proper eukaryotic DNA replication requires temporal separation of helicase loading from helicase activation and replisome assembly. Using an in vitro assay for eukaryotic origin-dependent replication initiation, we investigated the control of these events. After helicase loading, we found that the Dbf4-dependent Cdc7 kinase (DDK) but not S phase cyclin-dependent kinase (S-CDK) is required for the initial origin recruitment of Sld3 and the Cdc45 helicase-activating protein. Likewise, in vivo, DDK drives early-firing-origin recruitment of Cdc45 before activation of S-CDK. After S-CDK activation, a second helicase-activating protein (GINS) and the remainder of the replisome are recruited to the origin. Finally, recruitment of lagging but not leading strand DNA polymerases depends on Mcm10 and DNA unwinding. Our studies identify distinct roles for DDK and S-CDK during helicase activation and support a model in which the leading strand DNA polymerase is recruited prior to origin DNA unwinding and RNA primer synthesis.

Direct regulation of Treslin by cyclin-dependent kinase is essential for the onset of DNA replication

Treslin, a TopBP1-interacting protein, is necessary for deoxyribonucleic acid (DNA) replication in vertebrates. Association between Treslin and TopBP1 requires cyclin-dependent kinase (Cdk) activity in Xenopus laevis egg extracts. We investigated the mechanism and functional importance of Cdk for this interaction using both X. laevis egg extracts and human cells. We found that Treslin also associated with TopBP1 in a Cdk-regulated manner in human cells and that Treslin was phosphorylated within a conserved Cdk consensus target sequence (on S976 in X. laevis and S1000 in humans). Recombinant human Cdk2-cyclin E also phosphorylated this residue of Treslin in vitro very effectively. Moreover, a mutant of Treslin that cannot undergo phosphorylation on this site showed significantly diminished binding to TopBP1. Finally, human cells harboring this mutant were severely deficient in DNA replication. Collectively, these results indicate that Cdk-mediated phosphorylation of Treslin during S phase is necessary for both its effective association with TopBP1 and its ability to promote DNA replication in human cells.

CDK promotes interactions of Sld3 and Drc1 with Cut5 for initiation of DNA replication in fission yeast

Study of the essential roles of CDK in initiation of DNA replication in fission yeast indicates that CDK phosphorylates Sld3 and Drc1/Sld2 and promotes their interactions with Cut5, which are required for origin loading of Cut5. Thus CDK regulates assembly of replication factors onto origins by promoting ternary Sld3–Cut5–Drc1 complex formation.

Cyclin-dependent kinase (CDK) plays essential roles in the initiation of DNA replication in eukaryotes. Although interactions of CDK-phosphorylated Sld2/Drc1 and Sld3 with Dpb11 have been shown to be essential in budding yeast, it is not known whether the mechanism is conserved. In this study, we investigated how CDK promotes the assembly of replication proteins onto replication origins in fission yeast. Phosphorylation of Sld3 was found to be dependent on CDK in S phase. Alanine substitutions at CDK sites decreased the interaction with Cut5/Dpb11 at the N-terminal BRCT motifs and decreased the loading of Cut5 onto replication origins. This defect was suppressed by overexpression of drc1⁺. Phosphorylation of a conserved CDK site, Thr-111, in Drc1 was critical for interaction with Cut5 at the C-terminal BRCT motifs and was required for loading of Cut5. In a yeast three-hybrid assay, Sld3, Cut5, and Drc1 were found to form a ternary complex dependent on the CDK sites of Sld3 and Drc1, and Drc1–Cut5 binding enhanced the Sld3–Cut5 interaction. These results show that the mechanism of CDK-dependent loading of Cut5 is conserved in fission yeast in a manner similar to that elucidated in budding yeast.

Mrc1 marks early-firing origins and coordinates timing and efficiency of initiation in fission yeast

How early- and late-firing origins are selected on eukaryotic chromosomes is largely unknown. Here, we show that Mrc1, a conserved factor required for stabilization of stalled replication forks, selectively binds to the early-firing origins in a manner independent of Cdc45 and Hsk1 kinase in the fission yeast Schizosaccharomyces pombe. In mrc1Δ cells (and in swi1Δ cells to some extent), efficiency of firing is stimulated, and its timing is advanced selectively at those origins that are normally bound by Mrc1. In contrast, the late or inefficient origins which are not bound by Mrc1 are not activated in mrc1Δ cells. The enhanced firing and precocious Cdc45 loading at Mrc1-bound early-firing origins are not observed in a checkpoint mutant of mrc1, suggesting that non-checkpoint function is involved in maintaining the normal program of early-firing origins. We propose that prefiring binding of Mrc1 is an important marker of early-firing origins which are precociously activated by the absence of this protein.

Multiple functions of the S-phase checkpoint mediator

There is mounting evidence that replication defects are the major source of spontaneous genomic instability in cells, and that S-phase checkpoints are the principal defense against such instability. The S-phase checkpoint mediator protein Mrc1/Claspin mediates the checkpoint response to replication stress by facilitating phosphorylation of effector kinase by a sensor kinase. In this review, the multiple functions and the regulation of the S-phase checkpoint mediator are discussed.

Hsk1 kinase and Cdc45 regulate replication stress-induced checkpoint responses in fission yeast

In fission yeast, replication fork arrest activates the replication checkpoint effector kinase Cds1(Chk2/Rad53) through the Rad3(ATR/Mec1)-Mrc1(Claspin) pathway. Hsk1, the Cdc7 homologue of fission yeast required for efficient initiation of DNA replication, is also required for Cds1 activation. Hsk1 kinase activity is required for induction and maintenance of Mrc1 hyperphosphorylation, which is induced by replication fork block and mediated by Rad3. Rad3 kinase activity does not change in an hsk1 temperature-sensitive mutant, and Hsk1 kinase activity is not affected by rad3 mutation. Hsk1 kinase vigorously phosphorylates Mrc1 in vitro, predominantly at non-SQ/TQ sites, but this phosphorylation does not seem to affect the Rad3 action on Mrc1. Interestingly, the replication stress-induced activation of Cds1 and hyperphosphorylation of Mrc1 is almost completely abrogated in an initiation-defective mutant of cdc45, but not in an mcm2 or polε mutant. The results suggest that Hsk1-mediated loading of Cdc45 onto replication origins may play important roles in replication stress-induced checkpoint.