Cell cycle regulation of the yeast Cdc7 protein kinase by association with the Dbf4 protein

The prognostic significance and potential mechanism of DBF4 zinc finger in hepatocellular carcinoma

DBF4 zinc finger (DBF4) is a critical component involved in DNA replication and cell proliferation. It acts as a positive regulator of the cell division cycle 7 kinase. In this study, our investigation encompassed the impact of DBF4 on hepatocellular carcinoma (HCC) progression and delved into the potential mechanisms. We utilized open-access databases like TCGA and GEO to analyze the association between DBF4 and 33 different tumor types. We also conducted immunohistochemistry experiments to validate the expression of DBF4 in HCC, STAD, COAD, READ, PAAD, and LGG. Furthermore, we employed lentiviral transduction to knockdown DBF4 in HLF and SMMC cells, as well as to overexpress DBF4 in Huh7 cells. Subsequently, we evaluated the impact of DBF4 on proliferation, migration, and invasion of hepatocellular carcinoma cells. RNA sequencing and KEGG pathway enrichment analysis were also conducted to identify potential pathways, which were further validated through WB experiments. Finally, pathway inhibitor was utilized in rescue experiments to confirm whether DBF4 exerts its effects on tumor cells via the implicated pathway. Our findings revealed that DBF4 exhibited significant expression levels in nearly all examined tumors, which were further substantiated by the results of immunohistochemistry analysis. High DBF4 expression was correlated with poor overall survival (OS), disease-specific survival (DSS), progression-free interval (PFI), disease-free interval (DFI), relapse-free interval (RFI) in majority of tumor types, particularly in patients with HCC. In vitro experiments demonstrated that inhibition of DBF4 impaired the proliferative, migratory, and invasive abilities of HCC cells, whereas overexpression of DBF4 promoted these phenotypes. Sequencing results indicated that DBF4 may induce these changes through the ERBB signaling pathway. Further experimental validation revealed that DBF4 activates the ERBB signaling pathway, leading to alterations in the JNK/STAT, MAPK, and PI3K/AKT signaling pathways, thereby impacting the proliferative, migratory, and invasive abilities of tumor cells. Lastly, treatment of Huh7 cells overexpressing DBF4 with the ERBB2 inhibitor dacomitinib demonstrated the ability of ERBB2 inhibition to reverse the promoting effect of DBF4 overexpression on the proliferative, migratory, and invasive abilities of HCC cells. DBF4 plays a pivotal oncogenic role in HCC by promoting the ERBB signaling pathway and activating its downstream PI3K/AKT, JNK/STAT3, and MAPK signaling pathways. DBF4 may serve as a prognostic biomarker for patients with HCC.

Snapshot imprinting as a tool for surface mapping and identification of novel biomarkers of senescent cells

Cellular senescence has proved to be a strong contributor to ageing and age-related diseases, such as cancer and atherosclerosis. Therefore, the protein content of senescent cells is highly relevant to drug discovery, diagnostics and therapeutic applications. However, current technologies for the analysis of proteins are based on a combination of separation techniques and mass spectrometry, which require handling large sample sizes and a large volume of data and are time-consuming. This limits their application in personalised medicine. An easy, quick and inexpensive procedure is needed for qualitative and quantitative analysis of proteins expressed by a cell or tissue. Here, we describe the use of the "snapshot imprinting" approach for the identification of proteins differentially expressed by senescent cells. Molecularly imprinted polymer nanoparticles (MIPs) were formed in the presence of whole cells. Following trypsinolysis, protein epitopes protected by complex with MIPs were eluted from the nanoparticles and analysed by LC-MS/MS. In this work, "snapshot imprinting" was performed parallel to a standard proteomic "shaving approach", showing similar results. The analysis by "snapshot imprinting" identified three senescent-specific proteins: cell division cycle 7-related protein kinase, partitioning defective three homolog B and putative ATP-dependent RNA helicase DHX57, the abundance of which could potentially make them specific markers of senescence. Identifying biomarkers for the future elimination of senescent cells grants the potential for developing therapeutics for age-related diseases.

Cdc7-mediated phosphorylation of Cse4 regulates high-fidelity chromosome segregation in budding yeast

Faithful chromosome segregation maintains chromosomal stability as errors in this process contribute to chromosomal instability (CIN), which has been observed in many diseases including cancer. Epigenetic regulation of kinetochore proteins such as Cse4 (CENP-A in humans) plays a critical role in high-fidelity chromosome segregation. Here we show that Cse4 is a substrate of evolutionarily conserved Cdc7 kinase, and that Cdc7-mediated phosphorylation of Cse4 prevents CIN. We determined that Cdc7 phosphorylates Cse4 in vitro and interacts with Cse4 in vivo in a cell cycle-dependent manner. Cdc7 is required for kinetochore integrity as reduced levels of CEN-associated Cse4, a faster exchange of Cse4 at the metaphase kinetochores, and defects in chromosome segregation, are observed in a cdc7-7 strain. Phosphorylation of Cse4 by Cdc7 is important for cell survival as constitutive association of a kinase-dead variant of Cdc7 (cdc7-kd) with Cse4 at the kinetochore leads to growth defects. Moreover, phospho-deficient mutations of Cse4 for consensus Cdc7 target sites contribute to CIN phenotype. In summary, our results have defined a role for Cdc7-mediated phosphorylation of Cse4 in faithful chromosome segregation.

miR-30d-5p represses the proliferation, migration, and invasion of lung squamous cell carcinoma via targeting DBF4

OBJECTIVE

This study aims to explore the mechanism of miR-30d-5p in regulating the development of lung squamous cell carcinoma (LUSC) via targeting DBF4.

METHODS

Bioinformatics methods were employed to analyze the differentially expressed genes in LUSC tissue microarray. qRT-PCR was employed to detect the expression of miR-30d-5p and DBF4 mRNA in normal human bronchial epithelial cells and LUSC cells. CCK-8 was used to detect LUSC cell activity. Wound healing assay was employed to detect the migratory ability of LUSC cells. Transwell was employed to detect invasive ability. Dual-luciferase reporter assay was used to detect the targeting relationship between miR-30d-5p and DBF4. Western blot was used to detect the protein expression of marker molecules associated with epithelial-mesenchymal transition (EMT).

RESULTS

In this study, the expression of miR-30d-5p in LUSC cell lines was found to be obviously low compared with that in normal human bronchial epithelial cell line, which was opposite to the expression of DBF4. Dual-luciferase reporter assay verified that miR-30d-5p could target DBF4 and the overexpression of miR-30d-5p downregulated the expression of DBF4. Overexpression of DBF4 promoted the proliferation, migration, invasion, and EMT of LUSC, whereas over-expression of miR-30d-5p could weaken the promotion of DBF4 on cancer cells.

CONCLUSION

miR-30d-5p downregulates the expression of DBF4 to regulate the development of LUSC.

MYBL2 and ATM suppress replication stress in pluripotent stem cells

Replication stress, a major cause of genome instability in cycling cells, is mainly prevented by the ATR-dependent replication stress response pathway in somatic cells. However, the replication stress response pathway in embryonic stem cells (ESCs) may be different due to alterations in cell cycle phase length. The transcription factor MYBL2, which is implicated in cell cycle regulation, is expressed a hundred to a thousand-fold more in ESCs compared with somatic cells. Here we show that MYBL2 activates ATM and suppresses replication stress in ESCs. Consequently, loss of MYBL2 or inhibition of ATM or Mre11 in ESCs results in replication fork slowing, increased fork stalling and elevated origin firing. Additionally, we demonstrate that inhibition of CDC7 activity rescues replication stress induced by MYBL2 loss and ATM inhibition, suggesting that uncontrolled new origin firing may underlie the replication stress phenotype resulting from loss/inhibition of MYBL2 and ATM. Overall, our study proposes that in addition to ATR, a MYBL2-MRN-ATM replication stress response pathway functions in ESCs to control DNA replication initiation and prevent genome instability.

Resurrection from lethal knockouts: Bypass of gene essentiality

Understanding genotype-phenotype relationships is a central pursuit in biology. Gene knockout generates a complete loss-of-function genotype and is a commonly used approach for probing gene functions. The most severe phenotypic consequence of gene knockout is lethality. Genes with a lethal knockout phenotype are called essential genes. Based on genome-wide knockout analyses in yeasts, up to approximately a quarter of genes in a genome can be essential. Like other genotype-phenotype relationships, gene essentiality is subject to background effects and can vary due to gene-gene interactions. In particular, for some essential genes, lethality caused by knockout can be rescued by extragenic suppressors. Such "bypass of essentiality" (BOE) gene-gene interactions have been an understudied type of genetic suppression. A recent systematic analysis revealed that, remarkably, the essentiality of nearly 30% of essential genes in the fission yeast Schizosaccharomyces pombe can be bypassed by BOE interactions. Here, I review the history and recent progress on uncovering and understanding the bypass of gene essentiality.

Structural Basis for the Activation and Target Site Specificity of CDC7 Kinase

CDC7 is an essential Ser/Thr kinase that acts upon the replicative helicase throughout the S phase of the cell cycle and is activated by DBF4. Here, we present crystal structures of a highly active human CDC7-DBF4 construct. The structures reveal a zinc-finger domain at the end of the kinase insert 2 that pins the CDC7 activation loop to motif M of DBF4 and the C lobe of CDC7. These interactions lead to ordering of the substrate-binding platform and full opening of the kinase active site. In a co-crystal structure with a mimic of MCM2 Ser40 phosphorylation target, the invariant CDC7 residues Arg373 and Arg380 engage phospho-Ser41 at substrate P+1 position, explaining the selectivity of the S-phase kinase for Ser/Thr residues followed by a pre-phosphorylated or an acidic residue. Our results clarify the role of DBF4 in activation of CDC7 and elucidate the structural basis for recognition of its preferred substrates.

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DBF4 activates CDC7 kinase via a two-step mechanism

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Zinc-finger domain in CDC7 KI2 interacts with DBF4 motif M

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Invariant CDC7 residues Arg373 and Arg380 engage P+1 substrate site

Dbf4-Dependent Kinase (DDK)-Mediated Proteolysis of CENP-A Prevents Mislocalization of CENP-A in Saccharomyces cerevisiae

The evolutionarily conserved centromeric histone H3 variant (Cse4 in budding yeast, CENP-A in humans) is essential for faithful chromosome segregation. Mislocalization of CENP-A to non-centromeric chromatin contributes to chromosomal instability (CIN) in yeast, fly, and human cells and CENP-A is highly expressed and mislocalized in cancers. Defining mechanisms that prevent mislocalization of CENP-A is an area of active investigation. Ubiquitin-mediated proteolysis of overexpressed Cse4 (GALCSE4) by E3 ubiquitin ligases such as Psh1 prevents mislocalization of Cse4, and psh1 Δ strains display synthetic dosage lethality (SDL) with GALCSE4 We previously performed a genome-wide screen and identified five alleles of CDC7 and DBF4 that encode the Dbf4-dependent kinase (DDK) complex, which regulates DNA replication initiation, among the top twelve hits that displayed SDL with GALCSE4 We determined that cdc7 -7 strains exhibit defects in ubiquitin-mediated proteolysis of Cse4 and show mislocalization of Cse4 Mutation of MCM5 (mcm5 -bob1) bypasses the requirement of Cdc7 for replication initiation and rescues replication defects in a cdc7 -7 strain. We determined that mcm5 -bob1 does not rescue the SDL and defects in proteolysis of GALCSE4 in a cdc7 -7 strain, suggesting a DNA replication-independent role for Cdc7 in Cse4 proteolysis. The SDL phenotype, defects in ubiquitin-mediated proteolysis, and the mislocalization pattern of Cse4 in a cdc7 -7 psh1 Δ strain were similar to that of cdc7 -7 and psh1 Δ strains, suggesting that Cdc7 regulates Cse4 in a pathway that overlaps with Psh1 Our results define a DNA replication initiation-independent role of DDK as a regulator of Psh1-mediated proteolysis of Cse4 to prevent mislocalization of Cse4.

Distinct surfaces on Cdc5/PLK Polo-box domain orchestrate combinatorial substrate recognition during cell division

Polo-like kinases (Plks) are key cell cycle regulators. They contain a kinase domain followed by a polo-box domain that recognizes phosphorylated substrates and enhances their phosphorylation. The regulatory subunit of the Dbf4-dependent kinase complex interacts with the polo-box domain of Cdc5 (the sole Plk in Saccharomyces cerevisiae) in a phosphorylation-independent manner. We have solved the crystal structures of the polo-box domain of Cdc5 on its own and in the presence of peptides derived from Dbf4 and a canonical phosphorylated substrate. The structure bound to the Dbf4-peptide reveals an additional density on the surface opposite to the phospho-peptide binding site that allowed us to propose a model for the interaction. We found that the two peptides can bind simultaneously and non-competitively to the polo-box domain in solution. Furthermore, point mutations on the surface opposite to the phosphopeptide binding site of the polo-box domain disrupt the interaction with the Dbf4 peptide in solution and cause an early anaphase arrest phenotype distinct from the mitotic exit defect typically observed in cdc5 mutants. Collectively, our data illustrates the importance of non-canonical interactions mediated by the polo-box domain and provide key mechanistic insights into the combinatorial recognition of substrates by Polo-like kinases.

The effects of manipulating levels of replication initiation factors on origin firing efficiency in yeast

Chromosome replication in Saccharomyces cerevisiae is initiated from ~300 origins that are regulated by DNA sequence and by the limited abundance of six trans-acting initiation proteins (Sld2, Sld3, Dpb11, Dbf4, Sld7 and Cdc45). We set out to determine how the levels of individual factors contribute to time of origin activation and/or origin efficiency using induced depletion of single factors and overexpression of sets of multiple factors. Depletion of Sld2 or Sld3 slows growth and S phase progression, decreases origin efficiency across the genome and impairs viability as a result of incomplete replication of the rDNA. We find that the most efficient early origins are relatively unaffected by depletion of either Sld2 or Sld3. However, Sld3 levels, and to a lesser extent Sld2 levels, are critical for firing of the less efficient early origins. Overexpression of Sld3 simultaneously with Sld2, Dpb11 and Dbf4 preserves the relative efficiency of origins. Only when Cdc45 and Sld7 are also overexpressed is origin efficiency equalized between early- and late-firing origins. Our data support a model in which Sld3 together with Cdc45 (and/or Sld7) is responsible for the differential efficiencies of origins across the yeast genome.

Eukaryotic chromosome duplication begins at sites called origins of replication along the chromosomal DNA. A conserved property of eukaryotic origins is that they vary in efficiency—the proportion of cells in a population in which they “fire”—and in the average time of activation within S phase, but the molecular details underlying this variation are not well understood. Previous work has shown that limiting concentrations of a set of conserved replication initiation proteins referred to as “SSDDCS” (Sld2, Sld3, Dbf4, Dpb11, Cdc45, and Sld7) are rate limiting for origin activation in budding yeast and other eukaryotes; combined overexpression of these proteins increases and/or advances origin firing. However, it remained possible that different factors affect different aspects of origin activation (e.g., timing vs. efficiency). Here, by depleting individual factors or by overexpressing sets of factors in budding yeast, we demonstrate that it is levels of Sld3, Cdc45 and/or Sld7 levels are primarily responsible for modulating the differences in relative origin efficiency and timing. This work gives further insights into what shapes the landscape of genome duplication.

Inhibition of spindle extension through the yeast S phase checkpoint is coupled to replication fork stability and the integrity of centromeric DNA

Budding yeast treated with hydroxyurea (HU) activate the S phase checkpoint kinase Rad53, which prevents DNA replication forks from undergoing aberrant structural transitions and nuclease processing. Rad53 is also required to prevent premature extension of the mitotic spindle that assembles during a HU-extended S phase. Here we present evidence that checkpoint restraint of spindle extension is directly coupled to Rad53 control of replication fork stability. In budding yeast, centromeres are flanked by replication origins that fire in early S phase. Mutations affecting the Zn²⁺-finger of Dbf4, an origin activator, preferentially reduce centromere-proximal origin firing in HU, corresponding with suppression of rad53 spindle extension. Inactivating Exo1 nuclease or displacing centromeres from origins provides a similar suppression. Conversely, short-circuiting Rad53 targeting of Dbf4, Sld3, and Dun1, substrates contributing to fork stability, induces spindle extension. These results reveal spindle extension in HU-treated rad53 mutants is a consequence of replication fork catastrophes at centromeres. When such catastrophes occur, centromeres become susceptible to nucleases, disrupting kinetochore function and spindle force balancing mechanisms. At the same time, our data indicate centromere duplication is not required to stabilize S phase spindle structure, leading us to propose a model for how monopolar kinetochore-spindle attachments may contribute to spindle force balance in HU.

A Dual Inhibitor of Cdc7/Cdk9 Potently Suppresses T Cell Activation

T cell activation is mediated by signaling pathways originating from the T cell receptor (TCR). Propagation of signals downstream of the TCR involves a cascade of numerous kinases, some of which have yet to be identified. Through a screening strategy that we have previously introduced, PHA-767491, an inhibitor of the kinases Cdc7 and Cdk9, was identified to impede TCR signaling. PHA-767491 suppressed several T cell activation phenomena, including the expression of activation markers, proliferation, and effector functions. We also observed a defect in TCR signaling pathways upon PHA-767491 treatment. Inhibition of Cdc7/Cdk9 impairs T cell responses, which could potentially be detrimental for the immune response to tumors, and also compromises the ability to resist infections. The Cdc7/Cdk9 inhibitor is a strong candidate as a cancer therapeutic, but its effect on the immune system poses a problem for clinical applications.

Mapping Post-Translational Modifications of de Novo Purine Biosynthetic Enzymes: Implications for Pathway Regulation

Purines represent a class of essential metabolites produced by the cell to maintain cellular homeostasis and facilitate cell proliferation. In times of high purine demand, the de novo purine biosynthetic pathway is activated; however, the mechanisms that facilitate this process are largely unknown. One plausible mechanism is through intracellular signaling, which results in enzymes within the pathway becoming post-translationally modified to enhance their individual enzyme activities and the overall pathway metabolic flux. Here, we employ a proteomic strategy to investigate the extent to which de novo purine biosynthetic pathway enzymes are post-translationally modified in 293T cells. We identified 7 post-translational modifications on 135 residues across the 6 human pathway enzymes. We further asked whether there were differences in the post-translational modification state of each pathway enzyme isolated from cells cultured in the presence or absence of purines. Of the 174 assigned modifications, 67% of them were only detected in one experimental growth condition in which a significant number of serine and threonine phosphorylations were noted. A survey of the most-probable kinases responsible for these phosphorylation events uncovered a likely AKT phosphorylation site at residue Thr397 of PPAT, which was only detected in cells under purine-supplemented growth conditions. These data suggest that this modification might alter enzyme activity or modulate its interaction(s) with downstream pathway enzymes. Together, these findings propose a role for post-translational modifications in pathway regulation and activation to meet intracellular purine demand.

Modulation of Gene Silencing by Cdc7p via H4 K16 Acetylation and Phosphorylation of Chromatin Assembly Factor CAF-1 in Saccharomyces cerevisiae

CAF-1 is an evolutionarily conserved H3/H4 histone chaperone that plays a key role in replication-coupled chromatin assembly and is targeted to the replication fork via interactions with PCNA, which, if disrupted, leads to epigenetic defects. In Saccharomyces cerevisiae, when the silent mating-type locus HMR contains point mutations within the E silencer, Sir protein association and silencing is lost. However, mutation of CDC7, encoding an S-phase-specific kinase, or subunits of the H4 K16-specific acetyltransferase complex SAS-I, restore silencing to this crippled HMR, HMR a e** Here, we observed that loss of Cac1p, the largest subunit of CAF-1, also restores silencing at HMR a e**, and silencing in both cac1Δ and cdc7 mutants is suppressed by overexpression of SAS2 We demonstrate Cdc7p and Cac1p interact in vivo in S phase, but not in G1, consistent with observed cell cycle-dependent phosphorylation of Cac1p, and hypoacetylation of chromatin at H4 K16 in both cdc7 and cac1Δ mutants. Moreover, silencing at HMR a e** is restored in cells expressing cac1p mutants lacking Cdc7p phosphorylation sites. We also discovered that cac1Δ and cdc7-90 synthetically interact negatively in the presence of DNA damage, but that Cdc7p phosphorylation sites on Cac1p are not required for responses to DNA damage. Combined, our results support a model in which Cdc7p regulates replication-coupled histone modification via a CAC1-dependent mechanism involving H4 K16ac deposition, and thereby silencing, while CAF-1-dependent replication- and repair-coupled chromatin assembly per se are functional in the absence of phosphorylation of Cdc7p consensus sites on CAF-1.

Control of Eukaryotic DNA Replication Initiation-Mechanisms to Ensure Smooth Transitions

DNA replication differs from most other processes in biology in that any error will irreversibly change the nature of the cellular progeny. DNA replication initiation, therefore, is exquisitely controlled. Deregulation of this control can result in over-replication characterized by repeated initiation events at the same replication origin. Over-replication induces DNA damage and causes genomic instability. The principal mechanism counteracting over-replication in eukaryotes is a division of replication initiation into two steps—licensing and firing—which are temporally separated and occur at distinct cell cycle phases. Here, we review this temporal replication control with a specific focus on mechanisms ensuring the faultless transition between licensing and firing phases.

Replication stress: Driver and therapeutic target in genomically instable cancers

Genomically instable cancers are characterized by progressive loss and gain of chromosomal fragments, and the acquisition of complex genomic rearrangements. Such cancers, including triple-negative breast cancers and high-grade serous ovarian cancers, typically show aggressive behavior and lack actionable driver oncogenes. Increasingly, oncogene-induced replication stress or defective replication fork maintenance is considered an important driver of genomic instability. Paradoxically, while replication stress causes chromosomal instability and thereby promotes cancer development, it intrinsically poses a threat to cellular viability. Apparently, tumor cells harboring high levels of replication stress have evolved ways to cope with replication stress. As a consequence, therapeutic targeting of such compensatory mechanisms is likely to preferentially target cancers with high levels of replication stress and may prove useful in potentiating chemotherapeutic approaches that exert their effects by interfering with DNA replication. Here, we discuss how replication stress drives chromosomal instability, and the cell cycle-regulated mechanisms that cancer cells employ to deal with replication stress. Importantly, we discuss how mechanisms involving DNA structure-specific resolvases, cell cycle checkpoint kinases and mitotic processing of replication intermediates offer possibilities in developing treatments for difficult-to-treat genomically instable cancers.

Dbf4 recruitment by forkhead transcription factors defines an upstream rate-limiting step in determining origin firing timing

Fang et al. show that Dbf4 is enriched at early origins through its interaction with forkhead transcription factors Fkh1 and Fkh2. Dbf4 interacts directly with Sld3 and promotes the recruitment of downstream limiting factors.

Initiation of eukaryotic chromosome replication follows a spatiotemporal program. The current model suggests that replication origins compete for a limited pool of initiation factors. However, it remains to be answered how these limiting factors are preferentially recruited to early origins. Here, we report that Dbf4 is enriched at early origins through its interaction with forkhead transcription factors Fkh1 and Fkh2. This interaction is mediated by the Dbf4 C terminus and was successfully reconstituted in vitro. An interaction-defective mutant, dbf4ΔC, phenocopies fkh alleles in terms of origin firing. Remarkably, genome-wide replication profiles reveal that the direct fusion of the DNA-binding domain (DBD) of Fkh1 to Dbf4 restores the Fkh-dependent origin firing but interferes specifically with the pericentromeric origin activation. Furthermore, Dbf4 interacts directly with Sld3 and promotes the recruitment of downstream limiting factors. These data suggest that Fkh1 targets Dbf4 to a subset of noncentromeric origins to promote early replication in a manner that is reminiscent of the recruitment of Dbf4 to pericentromeric origins by Ctf19.

O Cdc7 kinase where art thou?

Although Cdc7 protein kinase is important for regulating DNA replication in all eukaryotes and is a target for cancer therapy, it has never been localized in cells. Recently, a novel molecular genomic method used by our laboratory to localize Cdc7 to regions of chromosomes. Originally, mutations in the CDC7 gene were found in the classic cdc mutant collection of Hartwell et al. (Genetics 74:267-286, 1973). The CDC7 gene was found to encode a protein kinase called DDK that has been studied for many years, establishing its precise role in the initiation of DNA replication at origins. Recently, clinical studies are underway with DDK inhibitors against DDK in cancer patients. However, the conundrum is that Cdc7 has never been detected at origins of replication even though many studies have suggested it should be there. We used "Calling Card" system in which DNA binding proteins are localized to the genome via retrotransposon insertion and deep-sequencing methods. We have shown that Cdc7 localizes at many regions of the genome and was enriched at functional origins of replication. These results are consistent with DDK's role in many additional genomic processes including mutagenesis, chromatid cohesion, and meiotic recombination. Thus, the main conclusion from our studies is that Cdc7 kinase is found at many locations in the genome, but is enriched at functional origins of replication. Furthermore, we propose that application of the Calling Card system to other eukaryotes should be useful in identification of functional origins in other eukaryotic cells.

Localization of Cdc7 Protein Kinase During DNA Replication in Saccharomyces cerevisiae

DDK, a conserved serine-threonine protein kinase composed of a regulatory subunit, Dbf4, and a catalytic subunit, Cdc7, is essential for DNA replication initiation during S phase of the cell cycle through MCM2-7 helicase phosphorylation. The biological significance of DDK is well characterized, but the full mechanism of how DDK associates with substrates remains unclear. Cdc7 is bound to chromatin in the Saccharomyces cerevisiae genome throughout the cell cycle, but there is little empirical evidence as to specific Cdc7 binding locations. Using biochemical and genetic techniques, this study investigated the specific localization of Cdc7 on chromatin. The Calling Cards method, using Ty5 retrotransposons as a marker for DNA–protein binding, suggests Cdc7 kinase is preferentially bound to genomic DNA known to replicate early in S phase, including centromeres and origins of replication. We also discovered Cdc7 binding throughout the genome, which may be necessary to initiate other cellular processes, including meiotic recombination and translesion synthesis. A kinase dead Cdc7 point mutation increases the Ty5 retrotransposon integration efficiency and a 55-amino acid C-terminal truncation of Cdc7, unable to bind Dbf4, reduces Cdc7 binding suggesting a requirement for Dbf4 to stabilize Cdc7 on chromatin during S phase. Chromatin immunoprecipitation demonstrates that Cdc7 binding near specific origins changes during S phase. Our results suggest a model where Cdc7 is loosely bound to chromatin during G₁. At the G₁/S transition, Cdc7 binding to chromatin is increased and stabilized, preferentially at sites that may become origins, in order to carry out a variety of cellular processes.

The Kinetochore Receptor for the Cohesin Loading Complex

The ring-shaped cohesin complex brings together distant DNA domains to maintain, express, and segregate the genome. Establishing specific chromosomal linkages depends on cohesin recruitment to defined loci. One such locus is the budding yeast centromere, which is a paradigm for targeted cohesin loading. The kinetochore, a multiprotein complex that connects centromeres to microtubules, drives the recruitment of high levels of cohesin to link sister chromatids together. We have exploited this system to determine the mechanism of specific cohesin recruitment. We show that phosphorylation of the Ctf19 kinetochore protein by a conserved kinase, DDK, provides a binding site for the Scc2/4 cohesin loading complex, thereby directing cohesin loading to centromeres. A similar mechanism targets cohesin to chromosomes in vertebrates. These findings represent a complete molecular description of targeted cohesin loading, a phenomenon with wide-ranging importance in chromosome segregation and, in multicellular organisms, transcription regulation.

Roles of CDK and DDK in Genome Duplication and Maintenance: Meiotic Singularities

Cells reproduce using two types of divisions: mitosis, which generates two daughter cells each with the same genomic content as the mother cell, and meiosis, which reduces the number of chromosomes of the parent cell by half and gives rise to four gametes. The mechanisms that promote the proper progression of the mitotic and meiotic cycles are highly conserved and controlled. They require the activities of two types of serine-threonine kinases, the cyclin-dependent kinases (CDKs) and the Dbf4-dependent kinase (DDK). CDK and DDK are essential for genome duplication and maintenance in both mitotic and meiotic divisions. In this review, we aim to highlight how these kinases cooperate to orchestrate diverse processes during cellular reproduction, focusing on meiosis-specific adaptions of their regulation and functions in DNA metabolism.

The plant cell cycle: Pre-Replication complex formation and controls

The multiplication of cells in all living organisms requires a tight regulation of DNA replication. Several mechanisms take place to ensure that the DNA is replicated faithfully and just once per cell cycle in order to originate through mitoses two new daughter cells that contain exactly the same information from the previous one. A key control mechanism that occurs before cells enter S phase is the formation of a pre-replication complex (pre-RC) that is assembled at replication origins by the sequential association of the origin recognition complex, followed by Cdt1, Cdc6 and finally MCMs, licensing DNA to start replication. The identification of pre-RC members in all animal and plant species shows that this complex is conserved in eukaryotes and, more importantly, the differences between kingdoms might reflect their divergence in strategies on cell cycle regulation, as it must be integrated and adapted to the niche, ecosystem, and the organism peculiarities. Here, we provide an overview of the knowledge generated so far on the formation and the developmental controls of the pre-RC mechanism in plants, analyzing some particular aspects in comparison to other eukaryotes.

The Intra-S Checkpoint Responses to DNA Damage

Faithful duplication of the genome is a challenge because DNA is susceptible to damage by a number of intrinsic and extrinsic genotoxins, such as free radicals and UV light. Cells activate the intra-S checkpoint in response to damage during S phase to protect genomic integrity and ensure replication fidelity. The checkpoint prevents genomic instability mainly by regulating origin firing, fork progression, and transcription of G1/S genes in response to DNA damage. Several studies hint that regulation of forks is perhaps the most critical function of the intra-S checkpoint. However, the exact role of the checkpoint at replication forks has remained elusive and controversial. Is the checkpoint required for fork stability, or fork restart, or to prevent fork reversal or fork collapse, or activate repair at replication forks? What are the factors that the checkpoint targets at stalled replication forks? In this review, we will discuss the various pathways activated by the intra-S checkpoint in response to damage to prevent genomic instability.

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.

Candida albicans Dbf4-dependent Cdc7 kinase plays a novel role in the inhibition of hyphal development

Candida albicans is an opportunistic human fungal pathogen. The ability to switch among multiple cellular forms is key to its pathogenesis. The Dbf4-dependent protein kinase gene CDC7 is conserved due to its role in initiating DNA replication. Because a C. albicans Cdc7 (Cacdc7) homozygous null was not viable, we generated a C. albicans strain with a deleted C. albicans CDC7 (CaCDC7) allele and an expression-repressible allele. Surprisingly, cells of the strain grew as hyphae under the repressed conditions. The in vitro kinase assays confirmed that CaCdc7 (K232) and CaCdc7 (T437) are critical for catalytic and phosphoacceptor of activation activity, respectively. C. albicans cells formed hyphae when expressing either the catalytically inactive CaCdc7 (K232R) or the phosphoacceptor-deficient CaCdc7 (T437A). While CaCdc7 interacted with CaDbf4, cells of the strain in which CaCDC7 was repressed were not rescued by constitutively expressing C. albicans DBF4 or vice versa. We conclude that CaDBF4-dependent CaCDC7 is an essential gene suppressing the hyphal development.

Skp1, a component of E3 ubiquitin ligase, is necessary for growth, sporulation, development and pathogenicity in rice blast fungus (Magnaporthe oryzae)

Ubiqitination is an important process in eukaryotic cells involving E3 ubiquitin ligase, which co-ordinates with cell cycle proteins and controls various cell functions. Skp1 (S-phase kinase-associated protein 1) is a core component of the SCF (Skp1-Cullin 1-F-box) E3 ubiquitin ligase complex necessary for protein degradation by the 26S proteasomal pathway. The rice blast fungus Magnaporthe oryzae has a single MoSKP1(MGG_04978) required for viability. Skp1 has multiple functions; however, its roles in growth, sporulation and appressorial development are not understood. MoSKP1 complements Skp1 function in the fission yeast temperature-sensitive mutant skp1 A7, restoring the normal length of yeast cells at restrictive temperature. The MoSkp1 protein in M. oryzae is present in spores and germ tubes, and is abundantly expressed in appressoria. Various RNA interference (RNAi) and antisense transformants of MoSKP1 in B157 show reduced sporulation, defective spore morphology, lesser septation and diffuse nuclei. Further, they show elongated germ tubes and are unable to form appressoria. Transformants arrested in G1/S stage during initial spore germination show a similar phenotype to wild-type spores treated with hydroxyurea (HU). Reduced MoSkp1 transcript and protein levels in knockdown transformants result in atypical germ tube development. MoSkp1 interacts with the putative F-box protein (MGG_06351) revealing the ability to form protein complexes. Our investigation of the role of MoSKP1 suggests that a decrease in MoSkp1 manifests in decreased total protein ubiquitination and, consequently, defective cell cycle and appressorial development. Thus, MoSKP1 plays important roles in growth, sporulation, appressorial development and pathogenicity of M. oryzae.

Cdk1-mediated phosphorylation of Cdc7 suppresses DNA re-replication

To maintain genetic stability, the entire mammalian genome must replicate only once per cell cycle. This is largely achieved by strictly regulating the stepwise formation of the pre-replication complex (pre-RC), followed by the activation of individual origins of DNA replication by Cdc7/Dbf4 kinase. However, the mechanism how Cdc7 itself is regulated in the context of cell cycle progression is poorly understood. Here we report that Cdc7 is phosphorylated by a Cdk1-dependent manner during prometaphase on multiple sites, resulting in its dissociation from origins. In contrast, Dbf4 is not removed from origins in prometaphase, nor is it degraded as cells exit mitosis. Our data thus demonstrates that constitutive phosphorylation of Cdc7 at Cdk1 recognition sites, but not the regulation of Dbf4, prevents the initiation of DNA replication in normally cycling cells and under conditions that promote re-replication in G2/M. As cells exit mitosis, PP1α associates with and dephosphorylates Cdc7. Together, our data support a model where Cdc7 (de)phosphorylation is the molecular switch for the activation and inactivation of DNA replication in mitosis, directly connecting Cdc7 and PP1α/Cdk1 to the regulation of once-per-cell cycle DNA replication in mammalian cells.

Candida albicans DBF4 gene inducibly duplicated by the mini-Ura-blaster is involved in hypha-suppression

The opportunistic human fungal pathogen Candida albicans is a natural diploid that does not have a complete sexual cycle. The ability to switch between diverse cellular forms is important to its virulence. Here, we describe the characterization of the C. albicans DBF4 gene, a Saccharomyces cerevisiae homolog that encodes a regulatory subunit of Cdc7 kinase that is known to initiate DNA replication. We made a C. albicans strain, with one DBF4 allele deleted by the mini-Ura-blaster and the other controlled by a repressible promoter. We also found a third CaDBF4 copy that was later verified to be inducibly duplicated by targeted recombination with the min-Ura-blaster. Surprisingly, the strain deleted with the third CaDBF4 copy exhibited hyphal growth under repressed conditions. We conclude that the CaDBF4 gene is prone to being duplicated by the mini-Ura-blaster and that it suppresses hyphal growth in C. albicans.

Phosphorylation and dephosphorylation regulate APC/C(Cdh1) substrate degradation

The Anaphase Promoting Complex/Cyclosome (APC/C) ubiquitin ligase activated by its G1 specific adaptor protein Cdh1 is a major regulator of the cell cycle. The APC/C(Cdh1) mediates degradation of dozens of proteins, however, the kinetics and requirements for their degradation are largely unknown. We demonstrate that overexpression of the constitutive active CDH1(m11) mutant that is not inhibited by phosphorylation results in mitotic exit in the absence of the FEAR and MEN pathways, and DNA re-replication in the absence of Cdc7 activity. This mode of mitotic exit also reveals additional requirements for APC/C(Cdh1) substrate degradation, which for some substrates such as Pds1 or Clb5 is dephosphorylation, but for others such as Cdc5 is phosphorylation.

Replication-Coupled PCNA Unloading by the Elg1 Complex Occurs Genome-wide and Requires Okazaki Fragment Ligation

The sliding clamp PCNA is a crucial component of the DNA replication machinery. Timely PCNA loading and unloading are central for genome integrity and must be strictly coordinated with other DNA processing steps during replication. Here, we show that the S. cerevisiae Elg1 replication factor C-like complex (Elg1-RLC) unloads PCNA genome-wide following Okazaki fragment ligation. In the absence of Elg1, PCNA is retained on chromosomes in the wake of replication forks, rather than at specific sites. Degradation of the Okazaki fragment ligase Cdc9 leads to PCNA accumulation on chromatin, similar to the accumulation caused by lack of Elg1. We demonstrate that Okazaki fragment ligation is the critical prerequisite for PCNA unloading, since Chlorella virus DNA ligase can substitute for Cdc9 in yeast and simultaneously promotes PCNA unloading. Our results suggest that Elg1-RLC acts as a general PCNA unloader and is dependent upon DNA ligation during chromosome replication.

Regulation of chromosome dynamics by Hsk1/Cdc7 kinase

Hsk1 (homologue of Cdc7 kinase 1) of the fission yeast is a member of the conserved Cdc7 (cell division cycle 7) kinase family, and promotes initiation of chromosome replication by phosphorylating Mcm (minichromosome maintenance) subunits, essential components for the replicative helicase. Recent studies, however, indicate more diverse roles for Hsk1/Cdc7 in regulation of various chromosome dynamics, including initiation of meiotic recombination, meiotic chromosome segregation, DNA repair, replication checkpoints, centromeric heterochromatin formation and so forth. Hsk1/Cdc7, with its unique target specificity, can now be regarded as an important modulator of various chromosome transactions.

DNA binding polarity, dimerization, and ATPase ring remodeling in the CMG helicase of the eukaryotic replisome

The Cdc45/Mcm2-7/GINS (CMG) helicase separates DNA strands during replication in eukaryotes. How the CMG is assembled and engages DNA substrates remains unclear. Using electron microscopy, we have determined the structure of the CMG in the presence of ATPγS and a DNA duplex bearing a 3' single-stranded tail. The structure shows that the MCM subunits of the CMG bind preferentially to single-stranded DNA, establishes the polarity by which DNA enters into the Mcm2-7 pore, and explains how Cdc45 helps prevent DNA from dissociating from the helicase. The Mcm2-7 subcomplex forms a cracked-ring, right-handed spiral when DNA and nucleotide are bound, revealing unexpected congruencies between the CMG and both bacterial DnaB helicases and the AAA+ motor of the eukaryotic proteasome. The existence of a subpopulation of dimeric CMGs establishes the subunit register of Mcm2-7 double hexamers and together with the spiral form highlights how Mcm2-7 transitions through different conformational and assembly states as it matures into a functional helicase.

Regulation and roles of Cdc7 kinase under replication stress

Cdc7 (cell division cycle 7) kinase together with its activation subunit ASK (also known as Dbf4) play pivotal roles in DNA replication and contribute also to other aspects of DNA metabolism such as DNA repair and recombination. While the biological significance of Cdc7 is widely appreciated, the molecular mechanisms through which Cdc7 kinase regulates these various DNA transactions remain largely obscure, including the role of Cdc7-ASK/Dbf4 under replication stress, a condition associated with diverse (patho)physiological scenarios. In this review, we first highlight the recent findings on a novel pathway that regulates the stability of the human Cdc7-ASK/Dbf4 complex under replication stress, its interplay with ATR-Chk1 signaling, and significance in the RAD18-dependent DNA damage bypass pathway. We also consider Cdc7 function in a broader context, considering both physiological conditions and pathologies associated with enhanced replication stress, particularly oncogenic transformation and tumorigenesis. Furthermore, we integrate the emerging evidence and propose a concept of Cdc7-ASK/Dbf4 contributing to genome integrity maintenance, through interplay with RAD18 that can serve as a molecular switch to dictate DNA repair pathway choice. Finally, we discuss the possibility of targeting Cdc7, particularly in the context of the Cdc7/RAD18-dependent translesion synthesis, as a potential innovative strategy for treatment of cancer.

Dbf4: the whole is greater than the sum of its parts

Together with cyclin-dependent kinases, the Dbf4-dependent kinase (DDK) is essential to activate the Mcm2-7 helicase and, hence, initiate DNA replication in eukaryotes. Beyond its role as the regulatory subunit of the DDK complex, the Dbf4 protein also regulates the activity of other cell cycle kinases to mediate the checkpoint response and prevent premature mitotic exit under stress. Two features that are unusual in DNA replication proteins characterize Dbf4. The first is its evolutionary divergence; the second is how its conserved motifs are combined to form distinct functional units. This structural plasticity appears to be at odds with the conserved functions of Dbf4. In this review, we summarize recent genetic, biochemical and structural work delineating the multiple interactions mediated by Dbf4 and its various functions during the cell cycle. We also discuss how the limited sequence conservation of Dbf4 may be an advantage to regulate the activities of multiple cell cycle kinases.

Sequential primed kinases create a damage-responsive phosphodegron on Eco1

Sister-chromatid cohesion is established during S phase when Eco1 acetylates cohesin. In budding yeast, Eco1 activity falls after S phase due to Cdk1-dependent phosphorylation, which triggers ubiquitination by SCF(Cdc4). We show here that Eco1 degradation requires the sequential actions of Cdk1 and two additional kinases, Cdc7-Dbf4 and the GSK-3 homolog Mck1. These kinases recognize motifs primed by previous phosphorylation, resulting in an ordered sequence of three phosphorylation events on Eco1. Only the latter two phosphorylation sites are spaced correctly to bind Cdc4, resulting in strict discrimination between phosphates added by Cdk1 and by Cdc7. Inhibition of Cdc7 by the DNA damage response prevents Eco1 destruction, allowing establishment of cohesion after S phase. This elaborate regulatory system, involving three independent kinases and stringent substrate selection by a ubiquitin ligase, enables robust control of cohesion establishment during normal growth and after stress.

Crystal structure of human CDC7 kinase in complex with its activator DBF4

CDC7 is a serine/threonine kinase that is essential for the initiation of eukaryotic DNA replication. CDC7 activity is controlled by its activator, DBF4. Here we present crystal structures of human CDC7-DBF4 in complex with a nucleotide or ATP-competing small molecules, revealing the active and inhibited forms of the kinase, respectively. DBF4 wraps around CDC7, burying approximately 6,000 Å(2) of hydrophobic molecular surface in a bipartite interface. The effector domain of DBF4, containing conserved motif C, is essential and sufficient to support CDC7 kinase activity by binding to the kinase N-terminal lobe and stabilizing its canonical αC helix. DBF4 motif M latches onto the C-terminal lobe of the kinase, acting as a tethering domain. Our results elucidate the structural basis for binding to and activation of CDC7 by DBF4 and provide a framework for the design of more potent and specific CDC7 inhibitors.

Control over DNA replication in time and space

DNA replication is precisely regulated in time and space, thereby safeguarding genomic integrity. In eukaryotes, replication initiates from multiple sites along the genome, termed origins of replication, and propagates bidirectionally. Dynamic origin bound complexes dictate where and when replication should initiate. During late mitosis and G1 phase, putative origins are recognized and become "licensed" through the assembly of pre-replicative complexes (pre-RCs) that include the MCM2-7 helicases. Subsequently, at the G1/S phase transition, a fraction of pre-RCs are activated giving rise to the establishment of replication forks. Origin location is influenced by chromatin and nuclear organization and origin selection exhibits stochastic features. The regulatory mechanisms that govern these cell cycle events rely on the periodic fluctuation of cyclin dependent kinase (CDK) activity through the cell cycle.

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.

A synthetic human kinase can control cell cycle progression in budding yeast

The DDK kinase complex, composed of Cdc7 and Dbf4, is required for S-phase progression. The two component proteins show different degrees of sequence conservation between human and yeast. Here, we determine that Saccharomyces cerevisiae bearing human CDC7 and DBF4 grows comparably to cells with yeast DDK under standard growth conditions. HsDrf1 (a second human Dbf4-like protein) does not support growth, suggesting that HsDbf4 is the true ortholog of ScDbf4. Both human subunits are required to complement yeast cdc7Δ or dbf4Δ due to the inability of human Cdc7 or Dbf4 to interact with the corresponding yeast protein. Flow cytometry indicates normal cell cycle progression for yeast containing human DDK. However, yeast containing human DDK is sensitive to long-term exposure to hydroxyurea and fails to sporulate, suggesting that human DDK substitutes for some, but not all, of yeast DDK's functions. We mapped the region of Cdc7 required for species-specific function of DDK to the C-terminus of Cdc7 by substituting the yeast C-terminal 55 amino acid residues in place of the equivalent human residues. The resulting hybrid protein supported growth of a cdc7Δ strain only in the presence of ScDBF4. The strain supported by the hybrid CDC7 was not sensitive to HU and formed tetrads. Together, our data indicate that DDK's targeting of its essential substrate is conserved between species, whereas the interactions within DDK are species specific.

Deregulated Cdc6 inhibits DNA replication and suppresses Cdc7-mediated phosphorylation of Mcm2-7 complex

Mcm2–7 is recruited to eukaryotic origins of DNA replication by origin recognition complex, Cdc6 and Cdt1 thereby licensing the origins. Cdc6 is essential for origin licensing during DNA replication and is readily destabilized from chromatin after Mcm2–7 loading. Here, we show that after origin licensing, deregulation of Cdc6 suppresses DNA replication in Xenopus egg extracts without the involvement of ATM/ATR-dependent checkpoint pathways. DNA replication is arrested specifically after chromatin binding of Cdc7, but before Cdk2-dependent pathways and deregulating Cdc6 after this step does not impair activation of origin firing or elongation. Detailed analyses revealed that Cdc6 deregulation leads to strong suppression of Cdc7-mediated hyperphosphorylation of Mcm4 and subsequent chromatin loading of Cdc45, Sld5 and DNA polymerase α. Mcm2 phosphorylation is also repressed although to a lesser extent. Remarkably, Cdc6 itself does not directly inhibit Cdc7 kinase activity towards Mcm2–4–6–7 in purified systems, rather modulates Mcm2–7 phosphorylation on chromatin context. Taken together, we propose that Cdc6 on chromatin acts as a modulator of Cdc7-mediated phosphorylation of Mcm2–7, and thus destabilization of Cdc6 from chromatin after licensing is a key event ensuring proper transition to the initiation of DNA replication.

Histone H3 Thr 45 phosphorylation is a replication-associated post-translational modification in S. cerevisiae

Post-translational histone modifications are crucial for the regulation of numerous DNA-templated processes, and are thought to mediate both alteration of chromatin dynamics and recruitment of effector proteins to specific regions of the genome. In particular, histone Ser/Thr phosphorylation regulates multiple nuclear functions in the budding yeast Saccharomyces cerevisiae, including transcription, DNA damage repair, mitosis, apoptosis and sporulation. Although modifications to chromatin during replication remain poorly understood, a number of recent studies have described acetylation of the histone H3 N-terminal alpha-helix (alphaN helix) at Lys 56 as a modification that is important for maintenance of genomic integrity during DNA replication and repair. Here, we report phosphorylation of H3 Thr 45 (H3-T45), a histone modification also located within the H3 alphaN helix in S. cerevisiae. Thr 45 phosphorylation peaks during DNA replication, and is mediated by the S phase kinase Cdc7-Dbf4 as part of a multiprotein complex identified in this study. Furthermore, loss of phosphorylated H3-T45 causes phenotypes consistent with replicative defects, and prolonged replication stress results in H3-T45 phosphorylation accumulation over time. Notably, the phenotypes described here are independent of Lys 56 acetylation status, and combinatorial mutations to both Thr 45 and Lys 56 of H3 cause synthetic growth defects. Together, these data identify and characterize H3-T45 phosphorylation as a replication-associated histone modification in budding yeast.

Cell cycle-mediated regulation of plant infection by the rice blast fungus

To gain entry to plants, many pathogenic fungi develop specialized infection structures called appressoria. Here, we demonstrate that appressorium morphogenesis in the rice blast fungus Magnaporthe oryzae is tightly regulated by the cell cycle. Shortly after a fungus spore lands on the rice (Oryza sativa) leaf surface, a single round of mitosis always occurs in the germ tube. We found that initiation of infection structure development is regulated by a DNA replication-dependent checkpoint. Genetic intervention in DNA synthesis, by conditional mutation of the Never-in-Mitosis 1 gene, prevented germ tubes from developing nascent infection structures. Cellular differentiation of appressoria, however, required entry into mitosis because nimA temperature-sensitive mutants, blocked at mitotic entry, were unable to develop functional appressoria. Arresting the cell cycle after mitotic entry, by conditional inactivation of the Blocked-in-Mitosis 1 gene or expression of stabilized cyclinB-encoding alleles, did not impair appressorium differentiation, but instead prevented these cells from invading plant tissue. When considered together, these data suggest that appressorium-mediated plant infection is coordinated by three distinct cell cycle checkpoints that are necessary for establishment of plant disease.

Cdc7p-Dbf4p regulates mitotic exit by inhibiting Polo kinase

Cdc7p-Dbf4p is a conserved protein kinase required for the initiation of DNA replication. The Dbf4p regulatory subunit binds Cdc7p and is essential for Cdc7p kinase activation, however, the N-terminal third of Dbf4p is dispensable for its essential replication activities. Here, we define a short N-terminal Dbf4p region that targets Cdc7p-Dbf4p kinase to Cdc5p, the single Polo kinase in budding yeast that regulates mitotic progression and cytokinesis. Dbf4p mediates an interaction with the Polo substrate-binding domain to inhibit its essential role during mitosis. Although Dbf4p does not inhibit Polo kinase activity, it nonetheless inhibits Polo-mediated activation of the mitotic exit network (MEN), presumably by altering Polo substrate targeting. In addition, although dbf4 mutants defective for interaction with Polo transit S-phase normally, they aberrantly segregate chromosomes following nuclear misorientation. Therefore, Cdc7p-Dbf4p prevents inappropriate exit from mitosis by inhibiting Polo kinase and functions in the spindle position checkpoint.

Cdc7p-Dbf4p is a two-subunit enzyme required to copy the genetic material present on every chromosome in a process termed DNA replication. Dbf4p is an essential regulatory subunit of this enzyme that likely directs the Cdc7p subunit to its targets within the cell. We found that Dbf4p physically interacts with another protein called Polo that acts during mitosis, a later step in the cell cycle when the newly copied chromosomes are equally divided to mother and daughter cells. Polo is a master regulator of mitosis and impacts many other proteins required for cell division. We determined that Cdc7p-Dbf4p is a Polo inhibitor and, further, that Cdc7p-Dbf4p delayed or prevented chromosome segregation when errors occurred during the cell division process. Interestingly, Dbf4p may bind the Polo substrate-binding domain using a type of interaction not previously described. Thus, we have uncovered a new activity for Cdc7p-Dbf4p in the cell cycle to inhibit chromosome segregation, and these findings impact multiple fields that investigate how cells accurately copy and segregate their chromosomes.

The role of the Saccharomyces cerevisiae Cdc7-Dbf4 complex in the replication checkpoint

The Cdc7-Dbf4 complex is a conserved serine/threonine protein kinase essential for the initiation of eukaryotic DNA replication. Although an mcm5-bob1 mutation bypasses lethality conferred by mutations in CDC7 or DBF4, the Deltacdc7 mcm5-bob1 mutant is sensitive to hydroxyurea (HU), which induces replication stress. To elucidate the reasons for HU sensitivity conferred by deletion of CDC7, we examined the role of Cdc7-Dbf4 in the replication checkpoint. We found that in Cdc7-Dbf4-deficient cells exposed to replication stress, Rad53 remains in a hypophosphorylated form, anaphase spindle is elongated, and checkpoint-specific transcription is not induced. The hypophosphorylated Rad53 exhibits a low autophosphorylation activity, and recombinant Cdc7-Dbf4 phosphorylates Rad53 in vitro. These results suggest that Cdc7-Dbf4 is required for full activation of Rad53 in response to replication stress.

The Xenopus cell cycle: an overview

Oocytes, eggs and embryos from the frog Xenopus laevis have been an important model system for studying cell-cycle regulation for several decades. First, progression through meiosis in the oocyte has been extensively investigated. Oocyte maturation has been shown to involve complex networks of signal transduction pathways, culminating in the cyclic activation and inactivation of Maturation Promoting Factor (MPF), composed of cyclin B and cdc2. After fertilisation, the early embryo undergoes rapid simplified cell cycles which have been recapitulated in cell-free extracts of Xenopus eggs. Experimental manipulation of these extracts has given a wealth of biochemical information about the cell cycle, particularly concerning DNA replication and mitosis. Finally, cells of older embryos adopt a more somatic-type cell cycle and have been used to study the balance between cell cycle and differentiation during development.

Identification and functional characterization of ASK/Dbf4, a novel cell survival gene in cutaneous melanoma with prognostic relevance

Malignant melanoma is one of the most aggressive and invasive metastatic tumors derived from melanocytes that have undergone malignant transformation by acquisition of genetic and epigenetic alterations. Oligonucleotide microarray-based screening of distinct stages in the tumor progression model of cutaneous melanoma identified ASK/Dbf4, as a novel determinant for melanoma development. Quantitative real-time polymerase chain reaction-based confirmation of ASK/Dbf4 on a series of benign nevi, dysplastic nevi, primary cutaneous melanomas and cutaneous melanoma metastases; and a number of other controls using normal human melanocytes as calibrator not only revealed a melanoma-specific over-expression but also revealed that higher ASK/Dbf4-expressing melanomas were associated with lower relapse-free survival. Additionally, we also confirmed the observed over-expression of ASK/Dbf4 in melanoma using western blot analysis and immunohistochemistry. As ASK/Dbf4 is known to be a cyclin-like regulatory subunit of mammalian Cdc7 from the studies in yeast, the present study investigated its role in melanoma cells. In keeping with its expected role, our data suggest that up-regulated ASK/Dbf4 is localized in the nucleus and binds to human Cdc7 to form Cdc7-ASK/Dbf4 complexes in several analyzed melanoma cell lines. Further, we demonstrate that small interfering RNA-mediated depletion of ASK/Dbf4 retarded melanoma cell survival and proliferation. In summary, we report the differential regulation of a novel gene, namely ASK/Dbf4, in melanoma and suggest that up-regulation of ASK/Dbf4 is a novel molecular determinant with prognostic relevance that confers a proliferative advantage in cutaneous melanoma.