Cdc18p can block mitosis by two independent mechanisms

Role of Cdc6 During Oogenesis and Early Embryo Development in Mouse and Xenopus laevis

Cdc6 is an important player in cell cycle regulation. It is involved in the regulation of both S-phase and M-phase. Its role during oogenesis is crucial for repression of the S-phase between the first and the second meiotic M-phases, and it also regulates, via CDK1 inhibition, the M-phase entry and exit. This is of special importance for the reactivation of the major M-phase-regulating kinase CDK1 (Cyclin-Dependent Kinase 1) in oocytes entering metaphase II of meiosis and in embryo cleavage divisions, in which precise timing allows coordination between cell cycle events and developmental program of the embryo. In this chapter, we discuss the role of Cdc6 protein in oocytes and early embryos.

Regulation of the final stage of mitosis by components of the pre-replicative complex and a polo kinase

The accurate division of duplicated DNA is essential for maintenance of genomic stability in proliferating eukaryotic cells. Errors in DNA replication and chromosomal segregation may lead to cell death or genomic mutations that lead to oncogenic properties. Thus, tight regulation of DNA replication and mitosis is essential for maintaining genomic integrity. Cell division cycle 6 (Cdc6) is an essential factor for initiating DNA replication. Recent work shows that phosphorylation of Cdc6 by polo-like kinase 1 (Plk1), one of the essential mitotic kinases, regulates mitotic exit mediated by Cdk1 and separase. Here we discuss how pre-replicative complex factors are connected with Plk1 and affect mitotic exit.

Cell division cycle 6, a mitotic substrate of polo-like kinase 1, regulates chromosomal segregation mediated by cyclin-dependent kinase 1 and separase

Defining the links between cell division and DNA replication is essential for understanding normal cell cycle progression and tumorigenesis. In this report we explore the effect of phosphorylation of cell division cycle 6 (Cdc6), a DNA replication initiation factor, by polo-like kinase 1 (Plk1) on the regulation of chromosomal segregation. In mitosis, the phosphorylation of Cdc6 was highly increased, in correlation with the level of Plk1, and conversely, Cdc6 is hypophosphorylated in Plk1-depleted cells, although cyclin A- and cyclin B1-dependent kinases are active. Binding between Cdc6 and Plk1 occurs through the polo-box domain of Plk1, and Cdc6 is phosphorylated by Plk1 on T37. Immunohistochemistry studies reveal that Cdc6 and Plk1 colocalize to the central spindle in anaphase. Expression of T37V mutant of Cdc6 (Cdc6-TV) induces binucleated cells and incompletely separated nuclei. Wild-type Cdc6 but not Cdc6-TV binds cyclin-dependent kinase 1 (Cdk1). Expression of wild-type Plk1 but not kinase-defective mutant promotes the binding of Cdc6 to Cdk1. Cells expressing wild-type Cdc6 display lower Cdk1 activity and higher separase activity than cells expressing Cdc6-TV. These results suggest that Plk1-mediated phosphorylation of Cdc6 promotes the interaction of Cdc6 and Cdk1, leading to the attenuation of Cdk1 activity, release of separase, and subsequent anaphase progression.

Genome-wide mapping of nuclear mitochondrial DNA sequences links DNA replication origins to chromosomal double-strand break formation in Schizosaccharomyces pombe

Chromosomal double-strand breaks (DSBs) threaten genome integrity and repair of these lesions is often mutagenic. How and where DSBs are formed is a major question conveniently addressed in simple model organisms like yeast. NUMTs, nuclear DNA sequences of mitochondrial origin, are present in most eukaryotic genomes and probably result from the capture of mitochondrial DNA (mtDNA) fragments into chromosomal breaks. NUMT formation is ongoing and was reported to cause de novo human genetic diseases. Study of NUMTs is likely to contribute to the understanding of naturally occurring chromosomal breaks. We show that Schizosaccharomyces pombe NUMTs are exclusively located in noncoding regions with no preference for gene promoters and, when located into promoters, do not affect gene transcription level. Strikingly, most noncoding regions comprising NUMTs are also associated with a DNA replication origin (ORI). Chromatin immunoprecipitation experiments revealed that chromosomal NUMTs are probably not acting as ORI on their own but that mtDNA insertions occurred directly next to ORIs, suggesting that these loci may be prone to DSB formation. Accordingly, induction of excessive DNA replication origin firing, a phenomenon often associated with human tumor formation, resulted in frequent nucleotide deletion events within ORI3001 subtelomeric chromosomal locus, illustrating a novel aspect of DNA replication-driven genomic instability. How mtDNA is fragmented is another important issue that we addressed by sequencing experimentally induced NUMTs. This highlighted regions of S. pombe mtDNA prone to breaking. Together with an analysis of human NUMTs, we propose that these fragile sites in mtDNA may correspond to replication pause sites.

Checkpoint effects and telomere amplification during DNA re-replication in fission yeast

Background

Although much is known about molecular mechanisms that prevent re-initiation of DNA replication on newly replicated DNA during a single cell cycle, knowledge is sparse regarding the regions that are most susceptible to re-replication when those mechanisms are bypassed and regarding the extents to which checkpoint pathways modulate re-replication. We used microarrays to learn more about these issues in wild-type and checkpoint-mutant cells of the fission yeast, Schizosaccharomyces pombe.

Results

We found that over-expressing a non-phosphorylatable form of the replication-initiation protein, Cdc18 (known as Cdc6 in other eukaryotes), drove re-replication of DNA sequences genome-wide, rather than forcing high level amplification of just a few sequences. Moderate variations in extents of re-replication generated regions spanning hundreds of kilobases that were amplified (or not) ~2-fold more (or less) than average. However, these regions showed little correlation with replication origins used during S phase. The extents and locations of amplified regions in cells deleted for the checkpoint genes encoding Rad3 (ortholog of human ATR and budding yeast Mec1) and Cds1 (ortholog of human Chk2 and budding yeast Rad53) were similar to those in wild-type cells. Relatively minor but distinct effects, including increased re-replication of heterochromatic regions, were found specifically in cells lacking Rad3. These might be due to Cds1-independent roles for Rad3 in regulating re-replication and/or due to the fact that cells lacking Rad3 continued to divide during re-replication, unlike wild-type cells or cells lacking Cds1. In both wild-type and checkpoint-mutant cells, regions near telomeres were particularly susceptible to re-replication. Highly re-replicated telomere-proximal regions (50–100 kb) were, in each case, followed by some of the least re-replicated DNA in the genome.

Conclusion

The origins used, and the extent of replication fork progression, during re-replication are largely independent of the replication and DNA-damage checkpoint pathways mediated by Cds1 and Rad3. The fission yeast pattern of telomere-proximal amplification adjacent to a region of under-replication has also been seen in the distantly-related budding yeast, which suggests that subtelomeric sequences may be a promising place to look for DNA re-replication in other organisms.

Mitotic Cdc6 stabilizes anaphase-promoting complex substrates by a partially Cdc28-independent mechanism, and this stabilization is suppressed by deletion of Cdc55

Ectopic expression of Cdc6p results in mitotic delay, and this has been attributed to Cdc6p-mediated inhibition of Cdc28 protein kinase and failure to activate the anaphase-promoting complex (APC). Here we show that endogenous Cdc6p delays a specific subset of mitotic events and that Cdc28 inhibition is not sufficient to account for it. The depletion of Cdc6p in G(2)/M cells reveals that Cdc6p is rate limiting for the degradation of the APC/Cdc20 substrates Pds1p and Clb2p. Conversely, the premature expression of Cdc6p delays the degradation of APC/Cdc20 substrates. Abolishing Cdc6p/Cdc28p interaction does not eliminate the Cdc6-dependent delay of these anaphase events. To identify additional Cdc6-mediated, APC-inhibitory mechanisms, we looked for mutants that reversed the mitotic delay. The deletion of SWE1, RAD24, MAD2, or BUB2 had no effect. However, disrupting CDC55, a PP2A regulatory subunit, suppressed the Cdc6p-dependent delay of Pds1 and Clb2 destruction. A specific role for CDC55 was supported by demonstrating that the lethality of Cdc6 ectopic expression in a cdc16-264 mutant is suppressed by the deletion of CDC55, that endogenous Cdc6p coimmunoprecipitates with the Cdc55 and Tpd3 subunits of PP2A, that Cdc6p/Cdc55p/Tpd3 interaction occurs only during mitosis, and that Cdc6 affects PP2A-Cdc55 activity during anaphase. This demonstrates that the levels and timing of accumulation of Cdc6p in mitosis are appropriate for mediating the modulation of APC/Cdc20.

Integrative analysis of cell cycle control in budding yeast

The adaptive responses of a living cell to internal and external signals are controlled by networks of proteins whose interactions are so complex that the functional integration of the network cannot be comprehended by intuitive reasoning alone. Mathematical modeling, based on biochemical rate equations, provides a rigorous and reliable tool for unraveling the complexities of molecular regulatory networks. The budding yeast cell cycle is a challenging test case for this approach, because the control system is known in exquisite detail and its function is constrained by the phenotypic properties of >100 genetically engineered strains. We show that a mathematical model built on a consensus picture of this control system is largely successful in explaining the phenotypes of mutants described so far. A few inconsistencies between the model and experiments indicate aspects of the mechanism that require revision. In addition, the model allows one to frame and critique hypotheses about how the division cycle is regulated in wild-type and mutant cells, to predict the phenotypes of new mutant combinations, and to estimate the effective values of biochemical rate constants that are difficult to measure directly in vivo.

A novel chk1-dependent G1/M checkpoint in fission yeast

Fission yeast cells with a temperature-sensitive Orp1 protein, a component of the origin recognition complex, cannot perform DNA replication at the restrictive temperature. Seventy percent of orp1-4 cells arrest with a 1C DNA content, whereas 30% proceed to mitosis ('cut'). The arrest depends upon the checkpoint Rad proteins and, surprisingly, the Chk1 protein, which is thought to act only from late S phase. The arrested cells maintain a 1C DNA content, as judged by flow cytometry, and the early origin ars3001 has not been initiated, as judged by 2D gel analysis. We show that in G1-arrested orp1-4 cells, Wee1 phosphorylates and inactivates Cdc2. Activation of Chk1 occurs earlier than Cdc2 phosphorylation, indicating a novel role for Chk1, namely to induce and/or maintain Cdc2 phosphorylation upon checkpoint activation in G1. We also show that commitment to cutting occurs already in early G1 phase.

CDK phosphorylation of Drc1 regulates DNA replication in fission yeast

Cyclin-dependent kinases (CDKs) are absolutely required for DNA replication in eukaryotic cells. CDKs are thought to activate one or more replication factors, but the identities of these proteins are unknown. Here we describe fission yeast Drc1, a protein required for DNA replication that is phosphorylated by Cdc2. Drc1 depletion leads to catastrophic mitotic divisions with incompletely replicated DNA, indicating that Drc1 is required for DNA synthesis and S-M replication checkpoint control. Drc1 associates with Cdc2 and is phosphorylated at the onset of S phase when Cdc2 is activated. Mutant Drc1 that lacks CDK phosphorylation sites is nonfunctional and fails to interact with Cut5 replication factor. These data suggest that Cdc2 promotes DNA replication by phosphorylating Drc1 and regulating its association with Cut5.

Redundant control of rereplication in fission yeast

The initiation of DNA replication at replication origins in eukaryotic cells is tightly controlled to ensure that the genome is duplicated only once each cell cycle. We present evidence that in fission yeast, independent regulation of two essential components of the initiation complex, Cdc18 and Cdt1, contributes to the prevention of reinitiation of DNA replication. Cdc18 is negatively controlled by cyclin-dependent kinase (CDK) phosphorylation, but low level expression of a mutant form of Cdc18 lacking CDK phosphorylation sites (Cdc18(CDK)) is not sufficient to induce rereplication. Similar to Cdc18, Cdt1 is expressed periodically in the cell cycle, accumulating in the nucleus in G(1) and declining in G(2). When Cdt1 is expressed constitutively from an ectopic promoter, it accumulates in the nucleus throughout the cell cycle but does not promote reinitiation. However, constitutive expression of Cdt1, together with Cdc18(CDK), is sufficient to induce extra rounds of DNA replication in the absence of mitosis. Significantly greater levels of rereplication can be induced by coexpression of Cdc18(CDK) and a Cdt1 mutant lacking a conserved C-terminal motif. In contrast, uncontrolled DNA replication does not occur when either mutant protein is expressed in the absence of the other. Constitutive expression of wild-type or mutant Cdt1 also leads to an increase in the levels of Cdc18(CDK), possibly as a result of increased protein stability. Our data are consistent with the hypothesis that control of rereplication depends on a redundant mechanism in which negative regulation of Cdt1 functions in parallel with the negative regulation of Cdc18.

mik1(+) G1-S transcription regulates mitotic entry in fission yeast

In the fission yeast Schizosaccharomyces pombe Mik1p, in combination with Wee1p, is an important inhibitor of mitosis through direct phosphorylation of Cdc2p. Here we present the observation that mik1(+) is transcribed during G1- and S-phase in normally dividing cells. mik1(+) transcription is regulated by the MCB-DSC1 system, which controls expression of other genes at the G1-S interval. mik1(+) is shown to be an important target of MCB-DSC1 as it is epistatic for the mitotic delay phenotype displayed in cdc10-C4 cells, which are mutated in a component of DSC1. The mitotic delay in cdc10-C4 cells is bypassed by cdc2-1w, suggesting that mik1(+) acts directly on cdc2(+), with no checkpoint function involved. Thus, mik1(+) represents a new type of MCB-DSC1 regulated gene in fission yeast, whose gene product is exclusively expressed during G1- and S-phase to prevent premature mitosis during this cell cycle stage.

Regulation of chromosome replication

The initiation of DNA replication in eukaryotic cells is tightly controlled to ensure that the genome is faithfully duplicated once each cell cycle. Genetic and biochemical studies in several model systems indicate that initiation is mediated by a common set of proteins, present in all eukaryotic species, and that the activities of these proteins are regulated during the cell cycle by specific protein kinases. Here we review the properties of the initiation proteins, their interactions with each other, and with origins of DNA replication. We also describe recent advances in understanding how the regulatory protein kinases control the progress of the initiation reaction. Finally, we describe the checkpoint mechanisms that function to preserve the integrity of the genome when the normal course of genome duplication is perturbed by factors that damage the DNA or inhibit DNA synthesis.

A role for the START gene-specific transcription factor complex in the inactivation of cyclin B and Cut2 destruction

Hyperactivation of Cdc2 in fission yeast causes cells to undergo a lethal premature mitosis called mitotic catastrophe. This phenotype is observed in cdc2-3w wee1-50 cells at high temperature. Eleven of 17 mutants that suppress this phenotype define a single complementation group, mcs1. The mcs1-77 mutant also suppresses lethal inactivation of the Wee1 and Mik1 tyrosine kinases and thus delays mitosis independently of Cdc2 tyrosine phosphorylation. We have cloned mcs1 by isolating suppressors of the cell cycle arrest phenotype of mcs1-77 cdc25-22 cells and found that it encodes Res2, a component of the START gene-specific transcription factor complex MBF (also known as DSC-1). The mcs1-77 mutant bears a single point mutation in the DNA-binding domain of Res2 that causes glycine 68 to be replaced by a serine residue. Importantly, two substrates of the anaphase-promoting complex (APC), the major B-type cyclin, Cdc13, and the anaphase inhibitor, Cut2, are unstable in G2-phase mcs1-77 cells. Consistent with this, we observe abnormal sister chromatid separation in mcs1-77 cdc25-22 cells at the restrictive temperature. Mutation of either Cdc10 or Res1 also deregulates MBF-dependent transcription and causes a G2 delay. We find that this cell cycle delay is abolished in the absence of the APC regulator Ste9/Srw1 and that the periodic expression of Ste9/Srw1 is controlled by the MBF complex. These data suggest that in fission yeast the MBF complex plays a key role in the inactivation of cyclin B and Cut2 destruction by controlling the periodic production of APC regulators.

The G(2) DNA damage checkpoint targets both Wee1 and Cdc25

The onset of mitosis is controlled by the cyclin dependent kinase Cdc2p. Cdc2p activity is controlled through the balance of phosphorylation and dephosphorylation of tyrosine-15 (Y15) by the Wee1p kinase and Cdc25p phosphatase. In the fission yeast Schizosaccharomyces pombe, detection of DNA damage in G(2) activates a checkpoint that prevents entry into mitosis through the maintenance of Y15 phosphorylation of Cdc2p, thus ensuring DNA repair precedes chromosome segregation. The protein kinase Chk1p is the endpoint of this checkpoint pathway. We have previously reported that overexpression of Chk1p causes a wee1(+)-dependent G(2) arrest, and this or irradiation leads to hyperphosphorylation of Wee1p. Moreover, Chk1p directly phosphorylates Wee1p in vitro. These data suggested that Wee1p is a key target of Chk1p action in checkpoint control. However, cells lacking wee1(+) are checkpoint proficient and sustained Chk1p overexpression arrests cell cycle progression independently of Wee1p. Therefore, up-regulation of Wee1p alone cannot enforce a checkpoint arrest. Chk1p can also phosphorylate Cdc25p in vitro. These phosphorylation events are thought to promote the interaction with 14–3-3 proteins the cytoplasmic retention of the 14–3-3/Cdc25p complexes. However, we show here that the G(2) DNA damage checkpoint is intact in cells that regulate mitotic entry independently of Cdc25p. Further, these cells are still sensitive to Chk1p-mediated arrest, and so down-regulation of Cdc25p is also insufficient to regulate checkpoint arrest. Conversely, inactivation of both wee1(+) and cdc25(+)abolishes checkpoint control. We also show that activation of the G(2) DNA damage checkpoint induces a transient increase in Wee1p levels. We conclude that the G(2) DNA damage checkpoint simultaneously signals via both up-regulation of Wee1p and down-regulation of Cdc25p, thus providing a double-lock mechanism to ensure cell cycle arrest and genomic stability.

The Cdt1 protein is required to license DNA for replication in fission yeast

To maintain genome stability in eukaryotic cells, DNA is licensed for replication only after the cell has completed mitosis, ensuring that DNA synthesis (S phase) occurs once every cell cycle. This licensing control is thought to require the protein Cdc6 (Cdc18 in fission yeast) as a mediator for association of minichromosome maintenance (MCM) proteins with chromatin. The control is overridden in fission yeast by overexpressing Cdc18 (ref. 11) which leads to continued DNA synthesis in the absence of mitosis. Other factors acting in this control have been postulated and we have used a re-replication assay to identify Cdt1 (ref. 14) as one such factor. Cdt1 cooperates with Cdc18 to promote DNA replication, interacts with Cdc18, is located in the nucleus, and its concentration peaks as cells finish mitosis and proceed to S phase. Both Cdc18 and Cdt1 are required to load the MCM protein Cdc21 onto chromatin at the end of mitosis and this is necessary to initiate DNA replication. Genes related to Cdt1 have been found in Metazoa and plants (A. Whitaker, I. Roysman and T. Orr-Weaver, personal communication), suggesting that the cooperation of Cdc6/Cdc18 with Cdt1 to load MCM proteins onto chromatin may be a generally conserved feature of DNA licensing in eukaryotes.

The cyclin-dependent kinase Cdc28p regulates distinct modes of Cdc6p proteolysis during the budding yeast cell cycle

BACKGROUND

Cdc28p, the major cyclin-dependent kinase in budding yeast, prevents re-replication within each cell cycle by preventing the reassembly of Cdc6p-dependent pre-replicative complexes (pre-RCs) once origins have fired. Cdc6p is a rapidly degraded protein that must be synthesised in each cell cycle and is present only during the G1 phase.

RESULTS

We found that, at different times in the cell cycle, there are distinct modes of Cdc6p proteolysis. Before Start, Cdc6p proteolysis did not require either the anaphase-promoting complex (APC/C) or the SCF complex, which mediate the major cell cycle regulated ubiquitination pathways, nor did it require Cdc28p activity or any of the potential Cdc28p phosphorylation sites in Cdc6p. In fact, the activation of B cyclin (Clb)-Cdc28p kinase inactivated this pathway of Cdc6p degradation later in the cell cycle. Activation of the G1 cyclins (Clns) caused Cdc6p degradation to become extremely rapid. This degradation required the SCF(CDC4) and Cdc28p consensus sites in Cdc6p, but did not require Clb5 and Clb6. Later in the cell cycle, SCF(CDC4)-dependent Cdc6p proteolysis remained active but became less rapid.

CONCLUSIONS

Levels of Cdc6p are regulated in several ways by the Cdc28p cyclin-dependent kinase. The Cln-dependent elimination of Cdc6p, which does not require the S-phase-promoting cyclins Clb5 and Clb6, suggests that the ability to assemble pre-RCs is lost before, not concomitant with, origin firing.

Checkpoint defects leading to premature mitosis also cause endoreplication of DNA in Aspergillus nidulans

The G2 DNA damage and slowing of S-phase checkpoints over mitosis function through tyrosine phosphorylation of NIMX(cdc2) in Aspergillus nidulans. We demonstrate that breaking these checkpoints leads to a defective premature mitosis followed by dramatic rereplication of genomic DNA. Two additional checkpoint functions, uvsB and uvsD, also cause the rereplication phenotype after their mutation allows premature mitosis in the presence of low concentrations of hydroxyurea. uvsB is shown to encode a rad3/ATR homologue, whereas uvsD displays homology to rad26, which has only previously been identified in Schizosaccharomyces pombe. uvsB(rad3) and uvsD(rad26) have G2 checkpoint functions over mitosis and another function essential for surviving DNA damage. The rereplication phenotype is accompanied by lack of NIME(cyclinB), but ectopic expression of active nondegradable NIME(cyclinB) does not arrest DNA rereplication. DNA rereplication can also be induced in cells that enter mitosis prematurely because of lack of tyrosine phosphorylation of NIMX(cdc2) and impaired anaphase-promoting complex function. The data demonstrate that lack of checkpoint control over mitosis can secondarily cause defects in the checkpoint system that prevents DNA rereplication in the absence of mitosis. This defines a new mechanism by which endoreplication of DNA can be triggered and maintained in eukaryotic cells.