A novel mutant allele of Schizosaccharomyces pombe rad26 defective in monitoring S-phase progression to prevent premature mitosis

Extracellular signal-regulated kinases modulate DNA damage response - a contributing factor to using MEK inhibitors in cancer therapy

The Raf-MEK-ERK pathway is commonly activated in human cancers, largely attributable to the extracellular signal-regulated kinases (ERKs) being a common downstream target of growth factor receptors, Ras, and Raf. Elevation of these up-stream signals occurs frequently in a variety of malignancies and ERK kinases play critical roles in promoting cell proliferation. Therefore, inhibition of MEK-mediated ERK activation is very appealing in cancer therapy. Consequently, numerous MEK inhibitors have been developed over the years. However, clinical trials have yet to produce overwhelming support for using MEK inhibitors in cancer therapy. Although complex reasons may have contributed to this outcome, an alternative possibility is that the MEK-ERK pathway may not solely provide proliferation signals to malignancies, the central scientific rationale in developing MEK inhibitors for cancer therapy. Recent developments may support this alternative possibility. Accumulating evidence now demonstrated that the MEK-ERK pathway contributes to the proper execution of cellular DNA damage response (DDR), a major pathway of tumor suppression. During DDR, the MEK-ERK pathway is commonly activated, which facilitates the proper activation of DDR checkpoints to prevent cell division. Inhibition of MEK-mediated ERK activation, therefore, compromises checkpoint activation. As a result, cells may continue to proliferate in the presence of DNA lesions, leading to the accumulation of mutations and thereby promoting tumorigenesis. Alternatively, reduction in checkpoint activation may prevent efficient repair of DNA damages, which may cause apoptosis or cell catastrophe, thereby enhancing chemotherapy’s efficacy. This review summarizes our current understanding of the participation of the ERK kinases in DDR.

Phosphorylation of Chk1 by ATM- and Rad3-related (ATR) in Xenopus egg extracts requires binding of ATRIP to ATR but not the stable DNA-binding or coiled-coil domains of ATRIP

ATR, a critical regulator of DNA replication and damage checkpoint responses, possesses a binding partner called ATRIP. We have studied the functional properties of Xenopus ATR and ATRIP in incubations with purified components and in frog egg extracts. In purified systems, ATRIP associates with DNA in both RPA-dependent and RPA-independent manners, depending on the composition of the template. However, in egg extracts, only the RPA-dependent mode of binding to DNA can be detected. ATRIP adopts an oligomeric state in egg extracts that depends upon binding to ATR. In addition, ATR and ATRIP are mutually dependent on one another for stable binding to DNA in egg extracts. The ATR-dependent oligomerization of ATRIP does not require an intact coiled-coil domain in ATRIP and does not change in the presence of checkpoint-inducing DNA templates. Egg extracts containing a mutant of ATRIP that cannot bind to ATR are defective in the phosphorylation of Chk1. However, extracts containing mutants of ATRIP lacking stable DNA-binding and coiled-coil domains show no reduction in the phosphorylation of Chk1 in response to defined DNA templates. Furthermore, activation of Chk1 does not depend upon RPA under these conditions. These results suggest that ATRIP must associate with ATR in order for ATR to carry out the phosphorylation of Chk1 effectively. However, this function of ATRIP does not involve its ability to mediate the stable binding of ATR to defined checkpoint-inducing DNA templates in egg extracts, does not require an intact coiled-coil domain, and does not depend on RPA.

ATRIP binding to replication protein A-single-stranded DNA promotes ATR-ATRIP localization but is dispensable for Chk1 phosphorylation

ATR associates with the regulatory protein ATRIP that has been proposed to localize ATR to sites of DNA damage through an interaction with single-stranded DNA (ssDNA) coated with replication protein A (RPA). We tested this hypothesis and found that ATRIP is required for ATR accumulation at intranuclear foci induced by DNA damage. A domain at the N terminus of ATRIP is necessary and sufficient for interaction with RPA-ssDNA. Deletion of the ssDNA-RPA interaction domain of ATRIP greatly diminished accumulation of ATRIP into foci. However, the ATRIP-RPA-ssDNA interaction is not sufficient for ATRIP recognition of DNA damage. A splice variant of ATRIP that cannot bind to ATR revealed that ATR association is also essential for proper ATRIP localization. Furthermore, the ATRIP-RPA-ssDNA interaction is not absolutely essential for ATR activation because ATR phosphorylates Chk1 in cells expressing only a mutant of ATRIP that does not bind to RPA-ssDNA. These data suggest that binding to RPA-ssDNA is not the essential function of ATRIP in ATR-dependent checkpoint signaling and ATR has an important function in properly localizing the ATR-ATRIP complex.

Fission yeast Rad17 associates with chromatin in response to aberrant genomic structures

Fission yeast checkpoint protein Rad17 is required for the DNA integrity checkpoint responses. A fraction of Rad17 is chromatin bound independent of the other checkpoint proteins throughout the cell cycle. Here we show that in response to DNA damage induced by either methyl methanesulfonate treatment or ionizing radiation, increased levels of Rad17 bind to chromatin. Following S-phase stall induced by hydroxyurea or a cdc22 mutation, the chromatin-bound Rad17 progressively dissociates from the chromatin. After S-phase arrest by hydroxyurea in cds1Delta or rad3Delta cells or by replication mutants, Rad17 remains chromatin bound. Rad17 is able to complex in vivo with an Rfc small subunit, Rfc2, but not with Rfc1. Furthermore, cells with rfc1Delta are checkpoint proficient, suggesting that Rfc1 does not have a role in checkpoint function. A checkpoint-defective mutant protein, Rad17(K118E), which has similar nuclear localization to that of the wild type, is unable to bind ATP and has reduced ability in chromatin binding. Mutant Rad17(K118E) protein also has reduced ability to complex with Rfc2, suggesting that Lys(118) of Rad17 plays a role in Rad17-Rfc small-subunit complex formation and chromatin association. However, in the rad17.K118E mutant cells, Cds1 can be activated by hydroxyurea. Together, these results suggest that Rad17 binds to chromatin in response to an aberrant genomic structure generated from DNA damage, replication mutant arrest, or hydroxyurea arrest in the absence of Cds1. Rad17 is not required to bind chromatin when genomic structures are protected by hydroxyurea-activated Cds1. The possible checkpoint events induced by chromatin-bound Rad17 are discussed.

Pie1, a protein interacting with Mec1, controls cell growth and checkpoint responses in Saccharomyces cerevisiae

In eukaryotes, the ATM and ATR family proteins play a critical role in the DNA damage and replication checkpoint controls. These proteins are characterized by a kinase domain related to the phosphatidylinositol 3-kinase, but they have the ability to phosphorylate proteins. In budding yeast, the ATR family protein Mec1/Esr1 is essential for checkpoint responses and cell growth. We have isolated the PIE1 gene in a two-hybrid screen for proteins that interact with Mec1, and we show that Pie1 interacts physically with Mec1 in vivo. Like MEC1, PIE1 is essential for cell growth, and deletion of the PIE1 gene causes defects in the DNA damage and replication block checkpoints similar to those observed in mec1Delta mutants. Rad53 hyperphosphorylation following DNA damage and replication block is also decreased in pie1Delta cells, as in mec1Delta cells. Pie1 has a limited homology to fission yeast Rad26, which forms a complex with the ATR family protein Rad3. Mutation of the region in Pie1 homologous to Rad26 results in a phenotype similar to that of the pie1Delta mutation. Mec1 protein kinase activity appears to be essential for checkpoint responses and cell growth. However, Mec1 kinase activity is unaffected by the pie1Delta mutation, suggesting that Pie1 regulates some essential function other than Mec1 kinase activity. Thus, Pie1 is structurally and functionally related to Rad26 and interacts with Mec1 to control checkpoints and cell proliferation.

Role of fission yeast primase catalytic subunit in the replication checkpoint

To investigate the cell cycle checkpoint response to aberrant S phase-initiation, we analyzed mutations of the two DNA primase subunit genes of Schizosaccharomyces pombe, spp1(+) and spp2(+) (S. pombe primase 1 and 2). spp1(+) encodes the catalytic subunit that synthesizes the RNA primer, which is then utilized by Polalpha to synthesize the initiation DNA. Here, we reported the isolation of the fission yeast spp1(+) gene and cDNA and the characterization of Spp1 protein and its cellular localization during the cell cycle. Spp1 is essential for cell viability, and thermosensitive mutants of spp1(+) exhibit an allele-specific abnormal mitotic phenotype. Mutations of spp1(+) reduce the steady-state cellular levels of Spp1 protein and compromised the formation of Polalpha-primase complex. The spp1 mutant displaying an aberrant mitotic phenotype also fails to properly activate the Chk1 checkpoint kinase, but not the Cds1 checkpoint kinase. Mutational analysis of Polalpha has previously shown that activation of the replication checkpoint requires the initiation of DNA synthesis by Polalpha. Together, these have led us to propose that suboptimal cellular levels of polalpha-primase complex due to the allele-specific mutations of Spp1 might not allow Polalpha to synthesize initiation DNA efficiently, resulting in failure to activate a checkpoint response. Thus, a functional Spp1 is required for the Chk1-mediated, but not the Cds1-mediated, checkpoint response after an aberrant initiation of DNA synthesis.

Analysis of fission yeast primase defines the checkpoint responses to aberrant S phase initiation

To investigate the checkpoint response to aberrant initiation, we analyzed the cell cycle checkpoint response induced by mutations of Schizosaccharomyces pombe DNA primase. DNA primase has two subunits, Spp1 and Spp2 (S. pombe primases 1 and 2). Spp1 is the catalytic subunit that synthesizes the RNA primer, which is then extended by DNA polymerase alpha (Polalpha) to synthesize an initiation DNA structure, and this catalytic function of Polalpha is a prerequisite for generating the S-M phase checkpoint. Here we show that Spp2 is required for coupling the function of Spp1 to Polalpha. Thermosensitive mutations of spp2(+) destabilize the Polalpha-primase complex, resulting in an allele-specific S phase checkpoint defect. The mutant exhibiting a more severe checkpoint defect also has a higher extent of Polalpha-primase complex instability and deficiency in the hydroxyurea-induced Cds1-mediated intra-S phase checkpoint response. However, this mutant is able to activate the Cds1 response to S phase arrest induced by temperature. These findings suggest that the Cds1 response to the S-phase arrest signal(s) induced by a initiation mutant is different from that induced by hydroxyurea. Interestingly, a polalphats mutant with a defective S-M phase checkpoint and an spp2 mutant with an intact checkpoint have a similar Polalpha-primase complex stability, and the Cds1 response induced by hydroxyurea or by the mutant arrests at the restrictive temperature. Thus, the Cds1-mediated intra-S phase checkpoint response induced by hydroxyurea can also be distinguished from the S-M phase checkpoint response that requires the initiation DNA synthesis by Polalpha.

A novel mutant allele of the chromatin-bound fission yeast checkpoint protein Rad17 separates the DNA structure checkpoints

To further dissect the genetic differences between the checkpoint pathway following S-phase cdc arrest versus DNA damage, a genetic screen was performed for checkpoint mutants that were unable to arrest mitosis following cell-cycle arrest with a temperature-sensitive DNA polymerase delta mutant, cdc20-M10. One such checkpoint mutant, rad17-d14, was found to display the cut phenotype following S-phase arrest by cdc20-M10, but not by the DNA synthesis inhibitor hydroxyurea, reminiscent of the chk1 mutant. Unlike chk1, rad17-d14 was not sensitive to UV irradiation. Interestingly, the ionising radiation sensitivity of rad17-d14 was only at higher doses, and cells were found to be defective in properly arresting cell division following irradiation in S phase, but not G(2) phase. Biochemical analysis attributes the checkpoint defects of rad17-d14 to the failure to phosphorylate the checkpoint effector Chk1p. To investigate if Rad17p monitors the genome for abnormal DNA structures specifically during DNA synthesis, chromatin association of Rad17p was analysed. Rad17p was found to be chromatin associated throughout the cell cycle, not just during S phase. This interaction occurred irrespective of the arrest with cdc20-M10 and, surprisingly, was also independent of the other checkpoint Rad proteins, and the cell-cycle effectors Chk1p and Cds1p.

Replication factor C3 of Schizosaccharomyces pombe, a small subunit of replication factor C complex, plays a role in both replication and damage checkpoints

We report here the isolation and functional analysis of the rfc3(+) gene of Schizosaccharomyces pombe, which encodes the third subunit of replication factor C (RFC3). Because the rfc3(+) gene was essential for growth, we isolated temperature-sensitive mutants. One of the mutants, rfc3-1, showed aberrant mitosis with fragmented or unevenly separated chromosomes at the restrictive temperature. In this mutant protein, arginine 216 was replaced by tryptophan. Pulsed-field gel electrophoresis suggested that rfc3-1 cells had defects in DNA replication. rfc3-1 cells were sensitive to hydroxyurea, methanesulfonate (MMS), and gamma and UV irradiation even at the permissive temperature, and the viabilities after these treatments were decreased. Using cells synchronized in early G2 by centrifugal elutriation, we found that the replication checkpoint triggered by hydroxyurea and the DNA damage checkpoint caused by MMS and gamma irradiation were impaired in rfc3-1 cells. Association of Rfc3 and Rad17 in vivo and a significant reduction of the phosphorylated form of Chk1 in rfc3-1 cells after treatments with MMS and gamma or UV irradiation suggested that the checkpoint signal emitted by Rfc3 is linked to the downstream checkpoint machinery via Rad17 and Chk1. From these results, we conclude that rfc3(+) is required not only for DNA replication but also for replication and damage checkpoint controls, probably functioning as a checkpoint sensor.

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.

Genetic control of telomere integrity in Schizosaccharomyces pombe: rad3(+) and tel1(+) are parts of two regulatory networks independent of the downstream protein kinases chk1(+) and cds1(+)

The Schizosaccharomyces pombe checkpoint gene named rad3(+) encodes an ATM-homologous protein kinase that shares a highly conserved motif with proteins involved in DNA metabolism. Previous studies have shown that Rad3 fulfills its function via the regulation of the Chk1 and Cds1 protein kinases. Here we describe a novel role for Rad3 in the control of telomere integrity. Mutations in the rad3(+) gene alleviated telomeric silencing and produced shortened lengths in the telomere repeat tracts. Genetic analysis revealed that the other checkpoint rad mutations rad1, rad17, and rad26 belong to the same phenotypic class with rad3 with regard to control of the telomere length. Of these mutations, rad3 and rad26 have a drastic effect on telomere shortening. tel1(+), another ATM homologue in S. pombe, carries out its telomere maintenance function in parallel with the checkpoint rad genes. Furthermore, either a single or double disruption of cds1(+) and chk1(+) caused no obvious changes in the telomeric DNA structure. Our results demonstrate a novel role of the S. pombe ATM homologues that is independent of chk1(+) and cds1(+).

Schizosaccharomyces pombe replication and repair proteins: proliferating cell nuclear antigen (PCNA)

Schizosaccharomyces pombe has a cell cycle progression with distinctive phases that serves as a perfect model system for investigating DNA replication and repair of eukaryotic cells. Here, we use proliferating cell nuclear antigen (PCNA) of S. pombe to demonstrate how the function of this protein in both DNA replication and repair can be assessed by genetic and biochemical approaches. We describe a method of introducing site-specific mutations into the fission yeast PCNA gene pcn1(+). The in vivo effects of these pcn1 mutants in a strain with a null pcn1 background are described and their in vitro biochemical properties are characterized. Mutants described here are those that are defective in enhancing processivity of DNA polymerase delta, show temperature-sensitive growth, and have increased sensitivity to hydroxyurea (HU), UV and gamma irradiation, and methyl methanesulfonate (MMS). Three mutants that show reduced growth rate in vivo and decreased capacity to enhance polymerase delta DNA synthetic activity and processivity in vitro-pcn1-1, pcn1-5, and pcn1-26-are described as examples of using a genetic approach to identify the biochemical function of replication proteins. One cold-sensitive growth allele, pcn1-3, that has a recessive cold-sensitive cdc phenotype and shows sensitivity to HU and UV and gamma irradiation is used as an example of using the genetic approach to reveal the function of replication proteins in repair. The power of combining both biochemical and genetic disciplines is emphasized. Methods for site-directed mutagenesis, in vitro analysis of mutant proteins, and in vivo characterization of mutants in response to UV or gamma irradiation, MMS, HU, and temperature, as well as genetic epistasis are described. Locations of functionally significant residues on the PCNA tertiary structure are summarized.

DNA damage and replication checkpoints in the fission yeast, Schizosaccharomyces pombe

Eukaryotic organisms have developed an array of mechanisms for minimizing the consequences of damage to their DNA molecules and the consequences of interference with their DNA replication. Among these mechanisms are the DNA damage and replication checkpoints, which inhibit passage from one cell cycle stage to the next when DNA is damaged or replication is incomplete. Studies of these checkpoints in the fission yeast, Schizosaccharomyces pombe, complement studies in other organisms and provide valuable insight into the nature of the proteins responsible for these checkpoints and how such proteins may function.

DNA structure checkpoint pathways in Schizosaccharomyces pombe

The response to DNA damage includes a delay to progression through the cell cycle to aid DNA repair. Incorrectly replicated chromosomes (replication checkpoint) or DNA damage (DNA damage checkpoint) delay the onset of mitosis. These checkpoint pathways detect DNA perturbations and generate a signal. The signal is amplified and transmitted to the cell cycle machinery. Since the checkpoint pathways are essential for genome stability, the related proteins which are found in all eukaryotes (from yeast to mammals) are expected to have similar functions to the yeast progenitors. This review article focuses on the function of checkpoint proteins in the model system Schizosaccharomyces pombe. Checkpoint controls in Saccharomyces cerevisiae and mammalian cells are mentioned briefly to underscore common or diverse features.

Suppressors of cdc25p overexpression identify two pathways that influence the G2/M checkpoint in fission yeast

Checkpoints maintain the order of cell-cycle events. At G2/M, a checkpoint blocks mitosis in response to damaged or unreplicated DNA. There are significant differences in the checkpoint responses to damaged DNA and unreplicated DNA, although many of the same genes are involved in both responses. To identify new genes that function specifically in the DNA replication checkpoint pathway, we searched for high-copy suppressors of overproducer of Cdc25p (OPcdc25(+)), which lacks a DNA replication checkpoint. Two classes of suppressors were isolated. One class includes a new gene encoding a putative DEAD box helicase, suppressor of uncontrolled mitosis (sum3(+)). This gene negatively regulates the cell-cycle response to stress when overexpressed and restores the checkpoint response by a mechanism that is independent of Cdc2p tyrosine phosphorylation. The second class includes chk1(+) and the two Schizosaccharomyces pombe 14-3-3 genes, rad24(+) and rad25(+), which appear to suppress the checkpoint defect by inhibiting Cdc25p. We show that rad24Delta mutants are defective in the checkpoint response to the DNA replication inhibitor hydroxyurea at 37 degrees and that cds1Delta rad24Delta mutants, like cds1Delta chk1Delta mutants, are entirely checkpoint deficient at 29 degrees. These results suggest that chk1(+) and rad24(+) may function redundantly with cds1(+) in the checkpoint response to unreplicated DNA.

Mitotic DNA damage and replication checkpoints in yeast

Studies of the genetics of G2/M checkpoints in budding and fission yeasts have produced many of the defining concepts of checkpoint biology. Recent progress in the biochemistry of the checkpoint gene products is adding a mechanistic understanding to our models and identifying the components of the normal cell cycle machinery that are targeted by checkpoints.

Replication checkpoint requires phosphorylation of the phosphatase Cdc25 by Cds1 or Chk1

Checkpoints maintain the order and fidelity of events of the cell cycle by blocking mitosis in response to unreplicated or damaged DNA. In most species this is accomplished by preventing activation of the cell-division kinase Cdc2, which regulates entry into mitosis. The Chk1 kinase, an effector of the DNA-damage checkpoint, phosphorylates Cdc25, an activator of Cdc2. Phosphorylation of Cdc25 promotes its binding to 14-3-3 proteins, preventing it from activating Cdc2. Here we propose that a similar pathway is required for mitotic arrest in the presence of unreplicated DNA (that is, in the replication checkpoint) in fission yeast. We show by mutagenesis that Chk1 functions redundantly with the kinase Cds1 at the replication checkpoint and that both kinases phosphorylate Cdc25 on the same sites, which include serine residues at positions 99, 192 and 359. Mutation of these residues reduces binding of 14-3-3 proteins to Cdc25 in vitro and disrupts the replication checkpoint in vivo. We conclude that both Cds1 and Chk1 regulate the binding of Cdc25 to 14-3-3 proteins as part of the checkpoint response to unreplicated DNA.

Mutational effect of fission yeast polalpha on cell cycle events

Polalpha is the principal DNA polymerase for initiation of DNA replication and also functions in postinitiation DNA synthesis. In this study, we investigated the cell cycle responses induced by mutations in polalpha+. Germinating spores carrying either a deletion of polalpha+ (polalphaDelta) or a structurally intact but catalytically dead polalpha mutation proceed to inappropriate mitosis with no DNA synthesis. This suggests that the catalytic function, and not the physical presence of Polalpha, is required to generate the signal that prevents the cells from entering mitosis prematurely. Cells with a polalphats allele arrest the cell cycle near the hydroxyurea arrest point, but, surprisingly, polalphats in cdc20 (polepsilon mutant) background arrested with a cdc phenoytpe, not a polalphats-like phenotype. At 25 degrees C, replication perturbation caused by polalphats alleles induces Cds1 kinase activity and requires the checkpoint Rads, Cds1, and Rqh1, but not Chk1, to maintain cell viability. At 36 degrees C, replication disruption caused by polalphats alleles induces the phosphorylation of Chk1; however, mutant cells arrest with heterogeneous cell sizes with a population of the cells entering aberrant mitosis. Together, our results indicate that the initiation DNA structure synthesized by Polalpha is required to bring about the S phase to mitosis checkpoint, whereas replication defects of different severity caused by polalphats mutations induce differential downstream kinase responses.

Hus1p, a conserved fission yeast checkpoint protein, interacts with Rad1p and is phosphorylated in response to DNA damage

The hus1+ gene is one of six fission yeast genes, termed the checkpoint rad genes, which are essential for both the S-M and DNA damage checkpoints. Classical genetics suggests that these genes are required for activation of the PI-3 kinase-related (PIK-R) protein, Rad3p. Using a dominant negative allele of hus1+, we have demonstrated a genetic interaction between hus1+ and another checkpoint rad gene, rad1+. Hus1p and Rad1p form a stable complex in wild-type fission yeast, and the formation of this complex is dependent on a third checkpoint rad gene, rad9+, suggesting that these three proteins may exist in a discrete complex in the absence of checkpoint activation. Hus1p is phosphorylated in response to DNA damage, and this requires rad3+ and each of the other checkpoint rad genes. Although there is no gene related to hus1+ in the Saccharomyces cerevisiae genome, we have identified closely related mouse and human genes, suggesting that aspects of the checkpoint control mechanism are conserved between fission yeast and higher eukaryotes.

S-phase-specific activation of Cds1 kinase defines a subpathway of the checkpoint response in Schizosaccharomyces pombe

Checkpoints that respond to DNA structure changes were originally defined by the inability of yeast mutants to prevent mitosis following DNA damage or S-phase arrest. Genetic analysis has subsequently identified subpathways of the DNA structure checkpoints, including the reversible arrest of DNA synthesis. Here, we show that the Cds1 kinase is required to slow S phase in the presence of DNA-damaging agents. Cds1 is phosphorylated and activated by S-phase arrest and activated by DNA damage during S phase, but not during G1 or G2. Activation of Cds1 during S phase is dependent on all six checkpoint Rad proteins, and Cds1 interacts both genetically and physically with Rad26. Unlike its Saccharomyces cerevisiae counterpart Rad53, Cds1 is not required for the mitotic arrest checkpoints and, thus, defines an S-phase specific subpathway of the checkpoint response. We propose a model for the DNA structure checkpoints that offers a new perspective on the function of the DNA structure checkpoint proteins. This model suggests that an intrinsic mechanism linking S phase and mitosis may function independently of the known checkpoint proteins.

The anaphase inhibitor of Saccharomyces cerevisiae Pds1p is a target of the DNA damage checkpoint pathway

Inhibition of DNA replication and physical DNA damage induce checkpoint responses that arrest cell cycle progression at two different stages. In Saccharomyces cerevisiae, the execution of both checkpoint responses requires the Mec1 and Rad53 proteins. This observation led to the suggestion that these checkpoint responses are mediated through a common signal transduction pathway. However, because the checkpoint-induced arrests occur at different cell cycle stages, the downstream effectors mediating these arrests are likely to be distinct. We have previously shown that the S. cerevisiae protein Pds1p is an anaphase inhibitor and is essential for cell cycle arrest in mitosis in the presence DNA damage. Herein we show that DNA damage, but not inhibition of DNA replication, induces the phosphorylation of Pds1p. Analyses of Pds1p phosphorylation in different checkpoint mutants reveal that in the presence of DNA damage, Pds1p is phosphorylated in a Mec1p- and Rad9p-dependent but Rad53p-independent manner. Our data place Pds1p and Rad53p on parallel branches of the DNA damage checkpoint pathway. We suggest that Pds1p is a downstream target of the DNA damage checkpoint pathway and that it is involved in implementing the DNA damage checkpoint arrest specifically in mitosis.

DNA polymerase delta isolated from Schizosaccharomyces pombe contains five subunits

DNA polymerase delta (pol delta) plays an essential role in DNA replication, repair, and recombination. We have purified pol delta from Schizosaccharomyces pombe more than 10(3)-fold and demonstrated that the polymerase activity of purified S. pombe pol delta is completely dependent on proliferating cell nuclear antigen and replication factor C. SDS/PAGE analysis of the purified fraction indicated that the pol delta complex consists of five subunits that migrate with apparent molecular masses of 125, 55, 54, 42, and 22 kDa. Western blot analysis indicated that the 125, 55, and 54 kDa proteins are the large catalytic subunit (Pol3), Cdc1, and Cdc27, respectively. The identity of the other two subunits, p42 and p22, was determined following proteolytic digestion and sequence analysis of the resulting peptides. The peptide sequences derived from the p22 subunit indicated that this subunit is identical to Cdm1, previously identified as a multicopy suppressor of the temperature-sensitive cdc1-P13 mutant, whereas peptide sequences derived from the p42 subunit were identical to a previously uncharacterized ORF located on S. pombe chromosome 1.

DNA replication and order of cell cycle events: a role for protein isoprenylation?

When the aya1+ gene is mutated, Schizosaccharomyces pombe cells become unable to react appropriately to a delay in DNA replication. Instead of stalling the cell cycle to allow completion of DNA synthesis, they proceed unperturbed towards mitosis and attempt to segregate the still unreplicated chromosomes. As a result, the genetic material segregates unevenly and the nuclei assume a mitotic catastrophe phenotype, characterized by torn chromosomes (cut), anucleated cells and scattered chromosomes. Interestingly, the aya1 phenotype can be suppressed by overexpression of either the catalytic subunit of S. pombe DNA polymerase alpha or of a novel protein called hur1 +p. The latter bears significant homology to the core of the human Rab escort protein, which belongs to a family of factors necessary to the post-translational isoprenylation of proteins like Ras, Rab and lamin B. When isoprenylation is chemically inhibited with R-limonene (a monoterpene derived from orange rind), wild type S. pombe cells become insensitive to an S phase delay, in a manner strongly reminiscent of aya1 mutants. Moreover, overexpression of hur1 +p in wild type cells rescues the failing checkpoint function. We propose that there is a strong correlation between the aya1 phenotype, S-M phase checkpoint function, and isoprenylation events in fission yeast.