Feedback control of mitosis in budding yeast

Three-dimensional analysis and ultrastructural design of mitotic spindles from the cdc20 mutant of Saccharomyces cerevisiae

The three-dimensional organization of mitotic microtubules in a mutant strain of Saccharomyces cerevisiae has been studied by computer-assisted serial reconstruction. At the nonpermissive temperature, cdc20 cells arrested with a spindle length of approximately 2.5 microns. These spindles contained a mean of 81 microtubules (range, 56-100) compared with 23 in wild-type spindles of comparable length. This increase in spindle microtubule number resulted in a total polymer length up to four times that of wild-type spindles. The spindle pole bodies in the cdc20 cells were approximately 2.3 times the size of wild-type, thereby accommodating the abnormally large number of spindle microtubules. The cdc20 spindles contained a large number of interpolar microtubules organized in a "core bundle." A neighbor density analysis of this bundle at the spindle midzone showed a preferred spacing of approximately 35 nm center-to-center between microtubules of opposite polarity. Although this is evidence of specific interaction between antiparallel microtubules, mutant spindles were less ordered than the spindle of wild-type cells. The number of noncore microtubules was significantly higher than that reported for wild-type, and these microtubules did not display a characteristic metaphase configuration. cdc20 spindles showed significantly more cross-bridges between spindle microtubules than were seen in the wild type. The cross-bridge density was highest between antiparallel microtubules. These data suggest that spindle microtubules are stabilized in cdc20 cells and that the CDC20 gene product may be involved in cell cycle processes that promote spindle microtubule disassembly.

The genetic defect in ataxia-telangiectasia

The autosomal recessive human disorder ataxia-telangiectasia (A-T) was first described as a separate disease entity 40 years ago. It is a multisystem disease characterized by progressive cerebellar ataxia, oculocutaneous telangiectasia, radiosensitivity, predisposition to lymphoid malignancies and immunodeficiency, with defects in both cellular and humoral immunity. The pleiotropic nature of the clinical and cellular phenotype suggests that the gene product involved is important in maintaining stability of the genome but also plays a more general role in signal transduction. The chromosomal instability and radiosensitivity so characteristic of this disease appear to be related to defective activation of cell cycle checkpoints. Greater insight into the nature of the defect in A-T has been provided by the recent identification, by positional cloning, of the responsible gene, ATM. The ATM gene is related to a family of genes involved in cellular responses to DNA damage and/or cell cycle control. These genes encode large proteins containing a phosphatidylinositol 3-kinase domain, some of which have protein kinase activity. The mutations causing A-T completely inactivate or eliminate the ATM protein. This protein has been detected and localized to different subcellular compartments.

Cell cycle checkpoints: preventing an identity crisis

Cell cycle checkpoints are regulatory pathways that control the order and timing of cell cycle transitions and ensure that critical events such as DNA replication and chromosome segregation are completed with high fidelity. In addition, checkpoints respond to damage by arresting the cell cycle to provide time for repair and by inducing transcription of genes that facilitate repair. Checkpoint loss results in genomic instability and has been implicated in the evolution of normal cells into cancer cells. Recent advances have revealed signal transduction pathways that transmit checkpoint signals in response to DNA damage, replication blocks, and spindle damage. Checkpoint pathways have components shared among all eukaryotes, underscoring the conservation of cell cycle regulatory machinery.

Mitosis: don't get mad, get even

The 'mitotic spindle checkpoint' ensures that, before a cell exits from mitosis, all of its chromosomes are aligned on the spindle to form the metaphase plate. Mad2 is an essential component of this checkpoint system and it binds specifically to unattached kinetochores.

Protein phosphatase 2A regulates MPF activity and sister chromatid cohesion in budding yeast

BACKGROUND

Mitosis is regulated by MPF (maturation promoting factor), the active form of Cdc2/28-cyclin B complexes. Increasing levels of cyclin B abundance and the loss of inhibitory phosphates from Cdc2/28 drives cells into mitosis, whereas cyclin B destruction inactivates MPF and drives cells out of mitosis. Cells with defective spindles are arrested in mitosis by the spindle-assembly checkpoint, which prevents the destruction of mitotic cyclins and the inactivation of MPF. We have investigated the relationship between the spindle-assembly checkpoint, cyclin destruction, inhibitory phosphorylation of Cdc2/28, and exit from mitosis.

RESULTS

The previously characterized budding yeast mad mutants lack the spindle-assembly checkpoint. Spindle depolymerization does not arrest them in mitosis because they cannot stabilize cyclin B. In contrast, a newly isolated mutant in the budding yeast CDC55 gene, which encodes a protein phosphatase 2A (PP2A) regulatory subunit, shows a different checkpoint defect. In the presence of a defective spindle, these cells separate their sister chromatids and leave mitosis without inducing cyclin B destruction. Despite the persistence of B-type cyclins, cdc55 mutant cells inactivate MPF. Two experiments show that this inactivation is due to inhibitory phosphorylation on Cdc28: phosphotyrosine accumulates on Cdc28 in cdc55 delta cells whose spindles have been depolymerized, and a cdc28 mutant that lacks inhibitory phosphorylation sites on Cdc28 allows spindle defects to arrest cdc55 mutants in mitosis with active MPF and unseparated sister chromatids.

CONCLUSIONS

We conclude that perturbations of protein phosphatase activity allow MPF to be inactivated by inhibitory phosphorylation instead of by cyclin destruction. Under these conditions, sister chromatid separation appears to be regulated by MPF activity rather than by protein degradation. We discuss the role of PP2A and Cdc28 phosphorylation in cell-cycle control, and the possibility that the novel mitotic exit pathway plays a role in adaptation to prolonged activation of the spindle-assembly checkpoint.

GFP tagging of budding yeast chromosomes reveals that protein-protein interactions can mediate sister chromatid cohesion

BACKGROUND

Precise control of sister chromatid separation is essential for the accurate transmission of genetic information. Sister chromatids must remain linked to each other from the time of DNA replication until the onset of chromosome segregation, when the linkage must be promptly dissolved. Recent studies suggest that the machinery that is responsible for the destruction of mitotic cyclins also degrades proteins that play a role in maintaining sister chromatid linkage, and that this machinery is regulated by the spindle-assembly checkpoint. Studies on these problems in budding yeast are hampered by the inability to resolve its chromosomes by light or electron microscopy.

RESULTS

We have developed a novel method for visualizing specific DNA sequences in fixed and living budding yeast cells. A tandem array of 256 copies of the Lac operator is integrated at the desired site in the genome and detected by the binding of a green fluorescent protein (GFP)-Lac repressor fusion expressed from the HIS3 promoter. Using this method, we show that sister chromatid segregation precedes the destruction of cyclin B. In mad or bub cells, which lack the spindle-assembly checkpoint, sister chromatid separation can occur in the absence of microtubules. The expression of a tetramerizing form of the GFP-Lac repressor, which can bind Lac operators on two different DNA molecules, can hold sister chromatids together under conditions in which they would normally separate.

CONCLUSIONS

We conclude that sister chromatid separation in budding yeast can occur in the absence of microtubule-dependent forces, and that protein complexes that can bind two different DNA molecules are capable of holding sister chromatids together.

The spindle assembly checkpoint

The spindle assembly checkpoint monitors proper chromosome attachment to spindle microtubules and is conserved from yeast to humans. Checkpoint components reside on kinetochores of chromosomes and show changes in phosphorylation and localization as cells proceed through mitosis. Adaptation to prolonged checkpoint arrest can occur by inhibitory phosphorylation of Cdc2.

Inhibition of mouse egg chromosome decondensation due to meiotic apparatus derangement induced by the protein phosphatase inhibitor, okadaic acid

The transition from metaphase to interphase involves protein dephosphorylation. Genetic and immunologic evidence suggest that protein phosphatase (PP) 1 and PP 2A may be required for this transition. Okadaic acid, a specific inhibitor of PP 1 and 2A, prevents the exit from metaphase in mammalian cells, but also disrupts the mitotic apparatus. Since disruption of the spindle itself causes cell cycle arrest, the present study was carried out to determine whether okadaic acid-treated cells fail to exit from metaphase because PP 1 and/or 2A activity is required, or because of spindle disruption. It was possible to distinguish between these two alternatives by including the protein kinase inhibitor, 6-dimethylaminopurine (6-DMAP), in the culture medium, since cells treated with 6-DMAP exit from metaphase despite disruption of the spindle. Mouse eggs, physiologically arrested at metaphase of the second meiotic division, complete meiosis and enter interphase when exposed to the calcium ionophore A23187. When eggs were exposed to 80 or 100 nM okadaic acid for 8 h, the meiotic spindle disappeared and the chromosomes disjoined. Nuclei did not form in eggs treated with okadaic acid and A23187, but did form in eggs treated with okadaic acid, A23187, and 6-DMAP. Therefore, eggs treated with okadaic acid have the capacity to exit from metaphase and enter interphase.

Fission yeast mal2+ is required for chromosome segregation

By a screen designed to isolate new fission yeast genes required for chromosome segregation, we have identified mal2+. The conditionally lethal mal2-1 allele gives rise to increased loss of a nonessential minichromosome at the permissive temperature and leads to severe missegregation of the chromosomes at the nonpermissive temperature. Cloning by complementation and subsequent sequence analysis revealed that mal2 is a novel protein with a mass of 34 kDa. Cells containing a mal2 null allele were inviable, indicating that mal2+ is an essential gene. Fusion of mal2 protein to the green fluorescent protein (GFP) showed that mal2 was predominantly localized in the nucleus. Sensitivity to microtubule-destabilizing drugs and strong genetic interactions with alpha1-tubulin suggest an interaction of the mal2 protein with the microtubule system. Spindle formation and elongation were not detectably affected in the mal2-1 mutant as determined by indirect immunofluorescence. However, anomalous chromosome movement on the spindle leading to aberrant distribution of the chromosomal material was observed.

Expression of Bcl-xL and loss of p53 can cooperate to overcome a cell cycle checkpoint induced by mitotic spindle damage

During somatic cell division, faithful chromosomal segregation must follow DNA replication to prevent aneuploidy or polyploidy. Damage to the mitotic spindle is one potential mechanism that interferes with chromosomal segregation. The accumulation of aneuploid or polyploid cells resulting from a disrupted mitotic spindle is presumably prevented by cell cycle checkpoint controls. In the course of studying cells that overexpress the apoptosis-inhibiting protein Bcl-xL, we found that these cells have an increased rate of spontaneous tetraploidization, suggesting that apoptosis may play an important role in eliminating cells that fail to complete mitosis properly. When cells expressing Bcl-xL are treated with mitotic spindle inhibitors, a significant percentage reinitiate DNA replication and become polyploid. Nevertheless, the majority of cells expressing Bcl-xL undergo a prolonged p53-dependent cell cycle arrest following mitotic spindle damage. Unexpectedly, p53 expression is not induced in mitosis, nor does it influence M-phase arrest. Instead, cells with mitotic spindle damage only transiently arrest in M phase, and despite failing to complete mitosis, appear to proceed to G1. During this subsequent growth factor-dependent phase, p53 is induced and mediates cell cycle arrest. In cells that do not overexpress Bcl-xL, elimination of the p53-dependent growth arrest with a dominant negative mutant also results in polyploidy after mitotic spindle damage, but under these conditions most cells die by apoptosis. Expression of Bcl-xL and abrogation of p53 cooperate to allow rapid and progressive polyploidization following mitotic spindle damage. Our results suggest that suppression of apoptosis by bcl-2-related genes and loss of p53 function can act cooperatively to contribute to genetic instability.

Mitotic chromosome condensation in the rDNA requires TRF4 and DNA topoisomerase I in Saccharomyces cerevisiae

DNA topoisomerase I (topo I) is known to participate in the process of DNA replication, but is not essential in Saccharomyces cerevisiae. The TRF4 gene is also nonessential and was identified in a screen for mutations that are inviable in combination with a top1 null mutation. Here we report the surprising finding that a top1 trf4-ts double mutant is defective in the mitotic events of chromosome condensation, spindle elongation, and nuclear segregation, but not in DNA replication. Direct examination of rDNA-containing mitotic chromosomes demonstrates that a top1 trf4-ts mutant fails both to establish and to maintain chromosome condensation in the rDNA at mitosis. We show that the Trf4p associates physically with both Smclp and Smc2p, the S. cerevisiae homologs of Xenopus proteins that are required for mitotic chromosome condensation in vitro. The defect in the top1 trf4-ts mutant is sensed by the MAD1-dependent spindle assembly checkpoint but not by the RAD9-dependent DNA damage checkpoint, further supporting the notion that chromosome structure influences spindle assembly. These data indicate that TOP1 (encoding topo I) and TRF4 participate in overlapping or dependent steps in mitotic chromosome condensation and serve to define a previously unrecognized biological function of topo I.

Association of spindle assembly checkpoint component XMAD2 with unattached kinetochores

The spindle assembly checkpoint delays anaphase until all chromosomes are attached to a mitotic spindle. The mad (mitotic arrest-deficient) and bub (budding uninhibited by benzimidazole) mutants of budding yeast lack this checkpoint and fail to arrest the cell cycle when microtubules are depolymerized. A frog homolog of MAD2 (XMAD2) was isolated and found to play an essential role in the spindle assembly checkpoint in frog egg extracts. XMAD2 protein associated with unattached kinetochores in prometaphase and in nocodazole-treated cells and disappeared from kinetochores at metaphase in untreated cells, suggesting that XMAD2 plays a role in the activation of the checkpoint by unattached kinetochores. This study furthers understanding of the mechanism of cell cycle checkpoints in metazoa and provides a marker for studying the role of the spindle assembly checkpoint in the genetic instability of tumors.

Identification of a human mitotic checkpoint gene: hsMAD2

In Saccharomyces cerevisiae, MAD2 is required for mitotic arrest if the spindle assembly is perturbed. The human homolog of MAD2 was isolated and shown to be a necessary component of the mitotic checkpoint in HeLa cells by antibody electroporation experiments. Human, or Homo sapiens, MAD2 (hsMAD2) was localized at the kinetochore after chromosome condensation but was no longer observed at the kinetochore in metaphase, suggesting that MAD2 might monitor the completeness of the spindle-kinetochore attachment. Finally, T47D, a human breast tumor cell line that is sensitive to taxol and nocodazole, had reduced MAD2 expression and failed to arrest in mitosis after nocodazole treatment. Thus, defects in the mitotic checkpoint may contribute to the sensitivity of certain tumors to mitotic spindle inhibitors.

An actin point-mutation neighboring the 'hydrophobic plug' causes defects in the maintenance of cell polarity and septum organization in the fission yeast Schizosaccharomyces pombe

The fission yeast cps8 mutation gives rise to abnormally enlarged and dispolarized cells, each of which contains several nuclei with aberrant multisepta. Molecular cloning and sequence analysis of the cps8 gene indicated that it encodes an actin with an amino acid substitution of aspartic acid for glycine at residue 273 in the hydrophobic loop that is located between actin subdomains 3 and 4. Fluorescence microscopy using phalloidin and anti-actin antibody revealed changes in the F-actin structure and distribution in the mutant cells. These results indicate that the hydrophobic loop plays an essential role for creating normal F-actin structure, only by which cell polarity and the late mitotic events can be maintained properly.

Activation of the budding yeast spindle assembly checkpoint without mitotic spindle disruption

The spindle assembly checkpoint keeps cells with defective spindles from initiating chromosome segregation. The protein kinase Mps1 phosphorylates the yeast protein Mad1p when this checkpoint is activated, and the overexpression of Mps1p induces modification of Mad1p and arrests wild-type yeast cells in mitosis with morphologically normal spindles. Spindle assembly checkpoint mutants overexpressing Mps1p pass through mitosis without delay and can produce viable progeny, which demonstrates that the arrest of wild-type cells results from inappropriate activation of the checkpoint in cells whose spindle is fully functional. Ectopic activation of cell-cycle checkpoints might be used to exploit the differences in checkpoint status between normal and tumor cells and thus improve the selectivity of chemotherapy.

Abnormal kinetochore structure activates the spindle assembly checkpoint in budding yeast

Saccharomyces cerevisiae cells containing one or more abnormal kinetochores delay anaphase entry. The delay can be produced by using centromere DNA mutations present in single-copy or kinetochore protein mutations. This observation is strikingly similar to the preanaphase delay or arrest exhibited in animal cells that experience spontaneous or induced failures in bipolar attachment of one or more chromosomes and may reveal the existence of a conserved surveillance pathway that monitors the state of chromosome attachment to the spindle before anaphase. We find that three genes (MAD2, BUB1, and BUB2) that are required for the spindle assembly checkpoint in budding yeast (defined by antimicrotubule drug-induced arrest or delay) are also required in the establishment and/or maintenance of kinetochore-induced delays. This was tested in strains in which the delays were generated by limited function of a mutant kinetochore protein (ctf13-30) or by the presence of a single-copy centromere DNA mutation (CDEII delta 31). Whereas the MAD2 and BUB1 genes were absolutely required for delay, loss of BUB2 function resulted in a partial delay defect, and we suggest that BUB2 is required for delay maintenance. The inability of mad2-1 and bub1 delta mutants to execute kinetochore-induced delay is correlated with striking increases in chromosome missegregation, indicating that the delay does indeed have a role in chromosome transmission fidelity. Our results also indicated that the yeast RAD9 gene, necessary for DNA damage-induced arrest, had no role in the kinetochore-induced delays. We conclude that abnormal kinetochore structures induce preanaphase delay by activating the same functions that have defined the spindle assembly checkpoint in budding yeast.

Budding yeast SKP1 encodes an evolutionarily conserved kinetochore protein required for cell cycle progression

The budding yeast SKP1 gene, identified as a dosage suppressor of a known kinetochore protein mutant, encodes an intrinsic 22.3 kDa subunit of CBF3, a multiprotein complex that binds centromere DNA in vitro. Temperature-sensitive mutations in SKP1 define two distinct phenotypic classes. skp1-4 mutants arrest predominantly as large budded cells with a G2 DNA content and short mitotic spindle, consistent with a role in kinetochore function. skp1-3 mutants, however, arrest predominantly as multiply budded cells with a G1 DNA content, suggesting an additional role during the G1/S phase. Identification of Skp1p homologs from C. elegans, A. thaliana, and H. sapiens indicates that SKP1 is evolutionarily highly conserved. Skp1p therefore represents an intrinsic kinetochore protein conserved throughout eukaryotic evolution and may be directly involved in linking kinetochore function with the cell cycle-regulatory machinery.

gamma-Tubulin-like Tub4p of Saccharomyces cerevisiae is associated with the spindle pole body substructures that organize microtubules and is required for mitotic spindle formation

Tub4p is a novel tubulin in Saccharomyces cerevisiae that most closely resembles gamma-tubulin. We report in this manuscript that the essential Tub4p is associated with the inner and outer plaques of the yeast microtubule organizing center, the spindle pole body (SPB). These SPB substructures are involved in the attachment of the nuclear and cytoplasmic microtubules, respectively (Byers, B., and L. Goetsch. 1975. J. Bacteriol. 124:511-523). Study of a temperature sensitive tub4-1 allele revealed that TUB4 has essential functions in microtubule organization. Remarkably, SPB duplication and separation are not impaired in tub4-1 cells incubated at the nonpermissive temperature. However, SPBs from such cells contain less or misdirected nuclear microtubules. Further analysis revealed that tub4-1 cells are able to assemble a short bipolar spindle, suggesting that the defect in microtubule organization occurs after spindle formation. A role of Tub4p in microtubule organization is further suggested by an increase in chromosome loss in tub4-1 cells. In addition, cell cycle arrest and survival of tub4-1 cells is dependent on the mitotic checkpoint control gene BUB2 (Hoyt, M.A., L. Totis, B.T. Roberts. 1991. Cell. 66:507-517), one of the cell's monitors of spindle integrity.

The spindle-assembly checkpoint: aiming for a perfect mitosis, every time

Checkpoints reduce the frequency of errors in cell division by delaying the progress of the cell cycle until certain processes are complete. The spindle-assembly checkpoint prevents the onset of anaphase until a bipolar spindle is present and all chromosomes are attached to the spindle. Evidence from yeast and mammalian cells suggests that kinetochores are at least one source of the signal that stops the cell cycle. Recent studies in budding yeast have begun to define the signal-transduction pathway involved in the spindle-assembly checkpoint, but details of the endpoint of the pathway, where these signals interact with the cell-cycle machinery, remain to be characterized.

Mutations which block the binding of calmodulin to Spc110p cause multiple mitotic defects

We have generated three temperature-sensitive alleles of SPC110, which encodes the 110 kDa component of the yeast spindle pole body (SPB). Each of these alleles carries point mutations within the calmodulin (CaM) binding site of Spc110p which affect CaM binding in vitro; two of the mutant proteins fail to bind CaM detectably (spc110-111, spc110-118) while binding to the third (spc110-124) is temperature-sensitive. All three alleles are suppressed to a greater or lesser extent by elevated dosage of the CaM gene (CMD1), suggesting that disruption of CaM binding is the primary defect in each instance. To determine the consequences on Spc110p function of loss of effective CaM binding, we have therefore examined in detail the progression of synchronous cultures through the cell division cycle at the restrictive temperature. In each case, cells replicate their DNA but then lose viability. In spc110-124, most cells duplicate and partially separate the SPBs but fail to generate a functional mitotic spindle, a phenotype which we term ‘abnormal metaphase’. Conversely, spc110-111 cells initially produce nuclear microtubules which appear well-organised but on entry into mitosis accumulate cells with ‘broken spindles’, where one SPB has become completely detached from the nuclear DNA. In both cases, the bulk of the cells suffer a lethal failure to segregate the DNA.

Dominant mutant alleles of yeast protein kinase gene CDC15 suppress the lte1 defect in termination of M phase and genetically interact with CDC14

LTE1 encodes a homolog of GDP-GTP exchange factors for the Ras superfamily and is required at low temperatures for cell cycle progression at the stage of the termination of M phase in Saccharomyces cerevisiae. We isolated extragenic suppressors which suppress the cold sensitivity of lte1 cells and confer a temperature-sensitive phenotype on cells. Cells mutant for the suppressor alone were arrested at telophase at non-permissive temperatures and the terminal phenotype was almost identical to that of lte1 cells at non-permissive temperatures. Genetic analysis revealed that the suppressor is allelic to CDC15, which encodes a protein kinase. The cdc15 mutations thus isolated were recessive with regard to the temperature-sensitive phenotype and were dominant with respect to suppression of lte1. We isolated CDC14 as a low-copy-number suppressor of cdc15-rlt1. CDC14 encodes a phosphotyrosine phosphatase (PTPase) and is essential for termination of M phase. An extra copy of CDC14 suppressed the temperature sensitivity of cdc15-rlt1 cells, but not that of cdc15-1 cells. In addition, some residues that are essential for the CDC14 PTPase activity were found to be non-essential for the suppression. These results strongly indicate that Cdc14 possesses dual functions; PTPase activity is needed for one function but not for the other. We postulate that the cooperative action of Cdc14 and Cdc15 plays an essential role in the termination of M phase.

TPR proteins required for anaphase progression mediate ubiquitination of mitotic B-type cyclins in yeast

The abundance of B-type cyclin-CDK complexes is determined by regulated synthesis and degradation of cyclin subunits. Cyclin proteolysis is required for the final exit from mitosis and for the initiation of a new cell cycle. In extracts from frog or clam eggs, degradation is accompanied by ubiquitination of cyclin. Three genes, CDC16, CDC23, and CSE1 have recently been shown to be required specifically for cyclin B proteolysis in yeast. To test whether these genes are required for cyclin ubiquitination, we prepared extracts from G1-arrested yeast cells capable of conjugating ubiquitin to the B-type cyclin Clb2. The ubiquitination activity was cell cycle regulated, required Clb2's destruction box, and was low if not absent in cdc16, cdc23, cdc27, and cse1 mutants. Furthermore all these mutants were also defective in ubiquitination of another mitotic B-type cyclin, Clb3. The Cdc16, Cdc23, and Cdc27 proteins all contain several copies of the tetratricopeptide repeat and are subunits of a complex that is required for the onset of anaphase. The finding that gene products that are required for ubiquitination of Clb2 and Clb3 are also required for cyclin proteolysis in vivo provides the best evidence so far that cyclin B is degraded via the ubiquitin pathway in living cells. Xenopus homologues of Cdc16 and Cdc27 have meanwhile been shown to be associated with a 20S particle that appears to function as a cell cycle-regulated ubiquitin-protein ligase.

Distinct roles of yeast MEC and RAD checkpoint genes in transcriptional induction after DNA damage and implications for function

In eukaryotic cells, checkpoint genes cause arrest of cell division when DNA is damaged or when DNA replication is blocked. In this study of budding yeast checkpoint genes, we identify and characterize another role for these checkpoint genes after DNA damage-transcriptional induction of genes. We found that three checkpoint genes (of six genes tested) have strong and distinct roles in transcriptional induction in four distinct pathways of regulation (each defined by induction of specific genes). MEC1 mediates the response in three transcriptional pathways, RAD53 mediates two of these pathways, and RAD17 mediates but a single pathway. The three other checkpoint genes (including RAD9) have small (twofold) but significant roles in transcriptional induction in all pathways. One of the pathways that we identify here leads to induction of MEC1 and RAD53 checkpoint genes themselves. This suggests a positive feedback circuit that may increase the cell's ability to respond to DNA damage. We make two primary conclusions from these studies. First, MEC1 appears to be the key regulator because it is required for all responses (both transcriptional and cell cycle arrest), while other genes serve only a subset of these responses. Second, the two types of responses, transcriptional induction and cell cycle arrest, appear distinct because both require MEC1 yet only cell cycle arrest requires RAD9. These and other results were used to formulate a working model of checkpoint gene function that accounts for roles of different checkpoint genes in different responses and after different types of damage. The conclusion that the yeast MEC1 gene is a key regulator also has implications for the role of a putative human homologue, the ATM gene.

Role of calmodulin and Spc110p interaction in the proper assembly of spindle pole body compenents

Previously we demonstrated that calmodulin binds to the carboxy terminus of Spc110p, an essential component of the Saccharomyces cerevisiae spindle pole body (SPB), and that this interaction is required for chromosome segregation. Immunoelectron microscopy presented here shows that calmodulin and thus the carboxy terminus of Spc110p localize to the central plaque. We created temperature-sensitive SPC110 mutations by combining PCR mutagenesis with a plasmid shuffle strategy. The temperature-sensitive allele spc110-220 differs from wild type at two sites. The cysteine 911 to arginine mutation resides in the calmodulin-binding site and alone confers a temperature-sensitive phenotype. Calmodulin overproduction suppresses the temperature sensitivity of spc110-220. Furthermore, calmodulin levels at the SPB decrease in the mutant cells at the restrictive temperature. Thus, calmodulin binding to Spc110-220p is defective at the nonpermissive temperature. Synchronized mutant cells incubated at the nonpermissive temperature arrest as large budded cells with a G2 content of DNA and suffer considerable lethality. Immunofluorescent staining demonstrates failure of nuclear DNA segregation and breakage of many spindles. Electron microscopy reveals an aberrant nuclear structure, the intranuclear microtubule organizer (IMO), that differs from a SPB but serves as a center of microtubule organization. The IMO appears during nascent SPB formation and disappears after SPB separation. The IMO contains both the 90-kD and the mutant 110-kD SPB components. Our results suggest that disruption of the calmodulin Spc110p interaction leads to the aberrant assembly of SPB components into the IMO, which in turn perturbs spindle formation.

Sequence of ptb1, a gene for the beta subunit of the type-II geranylgeranyltransferase from the fission yeast Schizosaccharomyces pombe

We have isolated and sequenced the ptb1 gene from the fission yeast Schizosaccharomyces pombe. Sequence analysis suggests that Ptb1 is the beta subunit of the type-II geranylgeranyltransferase that is responsible for geranylgeranylation of the Rab-like YPT proteins in this yeast.

Pds1p, an inhibitor of anaphase in budding yeast, plays a critical role in the APC and checkpoint pathway(s)

We report the isolation and characterization of pds1 mutants in Saccharomyces cerevisiae. The initial pds1-1 allele was identified by its inviability after transient exposure to microtubule inhibitors and its precocious dissociation of sister chromatids in the presence of these microtubule inhibitors. These findings suggest that pds1 mutants might be defective in anaphase arrest that normally is imposed by a spindle-damage checkpoint. To further examine a role for Pds1p in anaphase arrest, we compared the cell cycle arrest of pds1 mutants and PDS1 cells after: (a) the inactivation of Cdc16p or Cdc23p, two proteins that are required for the degradation of mitotic cyclins and are putative components of the yeast anaphase promoting complex (APC); (b) the inactivation of Cdc20p, another protein implicated in the degradation of mitotic cyclins; and (c) the inactivation of Cdc13 protein or gamma irradiation, two circumstances that induce a DNA-damage checkpoint. Under all these conditions, anaphase is inhibited in PDS1 cells but not in pds1 mutants. From these results we suggest that Pds1 protein is an anaphase inhibitor in PDS1 cells but not in pds1 mutants. From these results we suggest that Pds1 protein is an anaphase inhibitor that plays a critical role in the control of anaphase by both APC and checkpoints. We also show that pds1 mutants exit mitosis and initiate new rounds of cell division after gamma irradiation and Cdc13p inactivation but no after nocodazole-treatment or inactivation of Cdc16p, Cdc20p or Cdc23p function. Therefore, in the DNA-damage checkpoint, Pds1p is required for the inhibition of cytokinesis and DNA replication as well as anaphase. The role of Pds1 protein in anaphase inhibition and general cell cycle regulation is discussed.

Aberrantly segregating centromeres activate the spindle assembly checkpoint in budding yeast

The spindle assembly checkpoint is the mechanism or set of mechanisms that prevents cells with defects in chromosome alignment or spindle assembly from passing through mitosis. We have investigated the effects of mini-chromosomes on this checkpoint in budding yeast by performing pedigree analysis. This method allowed us to observe the frequency and duration of cell cycle delays in individual cells. Short, centromeric linear mini-chromosomes, which have a low fidelity of segregation, cause frequent delays in mitosis. Their circular counterparts and longer linear mini-chromosomes, which segregate more efficiently, show a much lower frequency of mitotic delays, but these delays occur much more frequently in divisions where the mini-chromosome segregates to only one of the two daughter cells. Using a conditional centromere to increase the copy number of a circular mini-chromosome greatly increases the frequency of delayed divisions. In all cases the division delays are completely abolished by the mad mutants that inactivate the spindle assembly checkpoint, demonstrating that the Mad gene products are required to detect the subtle defects in chromosome behavior that have been observed to arrest higher eukaryotic cells in mitosis.

Induction of polyploidy in adenovirus E1-transformed cells by the mitotic inhibitor colcemid

Adenovirus-transformed cells were tested for their ability to synthesize DNA in the presence of cell cycle inhibitory drugs. We show that transformed cells are completely resistant to the mitotic inhibitor colcemid, partly resistant to lovastatin, mimosine, aphidicolin and genistein but not to hydroxyurea or thymidine. When treated with colcemid, AdE1-transformed cells continue to synthesize DNA but do not divide and, therefore, become highly polyploid. This effect is dependent on the presence of both E1A and E1B.

The Saccharomyces cerevisiae spindle pole body duplication gene MPS1 is part of a mitotic checkpoint

M-phase checkpoints inhibit cell division when mitotic spindle function is perturbed. Here we show that the Saccharomyces cerevisiae MPS1 gene product, an essential protein kinase required for spindle pole body (SPB) duplication (Winey et al., 1991; Lauze et al., 1995), is also required for M-phase check-point function. In cdc31-2 and mps2-1 mutants, conditional failure of SPB duplication results in cell cycle arrest with high p34CDC28 kinase activity that depends on the presence of the wild-type MAD1 checkpoint gene, consistent with checkpoint arrest of mitosis. In contrast, mps1 mutant cells fail to duplicate their SPBs and do not arrest division at 37 degrees C, exhibiting a normal cycle of p34CDC28 kinase activity despite the presence of a monopolar spindle. Double mutant cdc31-2, mps1-1 cells also fail to arrest mitosis at 37 degrees C, despite having SPB structures similar to cdc31-2 single mutants as determined by EM analysis. Arrest of mitosis upon microtubule depolymerization by nocodazole is also conditionally absent in mps1 strains. This is observed in mps1 cells synchronized in S phase with hydroxyurea before exposure to nocodazole, indicating that failure of checkpoint function in mps1 cells is independent of SPB duplication failure. In contrast, hydroxyurea arrest and a number of other cdc mutant arrest phenotypes are unaffected by mps1 alleles. We propose that the essential MPS1 protein kinase functions both in SPB duplication and in a mitotic checkpoint monitoring spindle integrity.

Two genes required for the binding of an essential Saccharomyces cerevisiae kinetochore complex to DNA

Kinetochores are DNA-protein structures that assemble on centromeric DNA and attach chromosomes to spindle microtubules. Because of their simplicity, the 125-bp centromeres of Saccharomyces cerevisiae are particularly amenable to molecular analysis. Budding yeast centromeres contain three sequence elements of which centromere DNA sequence element III (CDEIII) appears to be particularly important. cis-acting mutations in CDEIII and trans-acting mutations in genes encoding subunits of the CDEIII-binding complex (CBF3) prevent correct chromosome transmission. Using temperature-sensitive mutations in CBF3 subunits, we show a strong correlation between DNA-binding activity measured in vitro and kinetochore activity in vivo. We extend previous findings by Goh and Kilmartin [Goh, P.-Y. & Kilmartin, J.V. (1993) J. Cell Biol. 121, 503-512] to argue that DNA-bound CBF3 may be involved in the operation of a mitotic checkpoint but that functional CBF3 is not required for the assembly of a bipolar spindle.

Fission yeast sta mutations that stabilize an unstable minichromosome are novel cdc2-interacting suppressors and are involved in regulation of spindle dynamics

Cytological observations have shown that the presence of unstable minichromosomes can delay progression through the early stages of mitosis in fission yeast (Schizosaccharomyces pombe), suggesting that such minichromosomes may provide a useful tool for examining the system that regulates the coordinated segregation of chromosomes. One such unstable minichromosome is a large circular minichromosome. We previously showed that the mitotic instability of this minichromosome is probably due to the frequent occurrence of catenated forms of DNA after replication. To identify genes involved in the regulation of chromosome behavior in mitosis, we isolated mutants which stabilized this minichromosome. Three loci (sta1, sta2, and sta3) were identified. Two of them were found to be suppressors of temperature-sensitive mutations in cdc2, which encodes the catalytic subunit of muturation promoting factor (MPF). They show no linkage to, and are thus different from, suc1, and cdc13, previously identified as genes that interact with cdc2. The other mutation mapped to a gene previously identified as being required for the correct formation of the mitotic spindle. Data provided in this study suggest that the sta genes are involved in the regulation of spindle dynamics to ensure proper chromosome segregation during mitosis.

Checkpoint genes required to delay cell division in response to nocodazole respond to impaired kinetochore function in the yeast Saccharomyces cerevisiae

Inhibition of mitosis by antimitotic drugs is thought to occur by destruction of microtubules, causing cells to arrest through the action of one or more mitotic checkpoints. We have patterned experiments in the yeast Saccharomyces cerevisiae after recent studies in mammalian cells that demonstrate the effectiveness of antimitotic drugs at concentrations that maintain spindle structure. We show that low concentrations of nocodazole delay cell division under the control of the previously identified mitotic checkpoint genes BUB1, BUB3, MAD1, and MAD2 and independently of BUB2. The same genes mediate the cell cycle delay induced in ctf13 mutants, limited for an essential kinetochore component. Our data suggest that a low concentration of nocodazole induces a cell cycle delay through checkpoint control that is sensitive to impaired kinetochore function. The BUB2 gene may be part of a separate checkpoint that responds to abnormal spindle structure.

Mad1p, a phosphoprotein component of the spindle assembly checkpoint in budding yeast

The spindle assembly checkpoint prevents cells from initiating anaphase until the spindle has been fully assembled. We previously isolated mitotic arrest deficient (mad) mutants that inactivate this checkpoint and thus increase the sensitivity of cells to benomyl, a drug that interferes with mitotic spindle assembly by depolymerizing microtubules. We have cloned the MAD1 gene and show that when it is disrupted yeast cells have the same phenotype as the previously isolated mad1 mutants: they fail to delay the metaphase to anaphase transition in response to microtubule depolymerization. MAD1 is predicted to encode a 90-kD coiled-coil protein. Anti-Mad1p antibodies give a novel punctate nuclear staining pattern and cell fractionation reveals that the bulk of Mad1p is soluble. Mad1p becomes hyperphosphorylated when wild-type cells are arrested in mitosis by benomyl treatment, or by placing a cold sensitive tubulin mutant at the restrictive temperature. This modification does not occur in G1-arrested cells treated with benomyl or in cells arrested in mitosis by defects in the mitotic cyclin proteolysis machinery, suggesting that Mad1p hyperphosphorylation is a step in the activation of the spindle assembly checkpoint. Analysis of Mad1p phosphorylation in other spindle assembly checkpoint mutants reveals that this response to microtubule-disrupting agents is defective in some (mad2, bub1, and bub3) but not all (mad3, bub2) mutant strains. We discuss the possible functions of Mad1p at this cell cycle checkpoint.

The Aspergillus nidulans bimE (blocked-in-mitosis) gene encodes multiple cell cycle functions involved in mitotic checkpoint control and mitosis

The bimE (blocked-in-mitosis) gene appears to function as a negative mitotic regulator because the recessive bimE7 mutation can override certain interphase-arresting treatments and mutations, causing abnormal induction of mitosis. We have further investigated the role of bimE in cell cycle checkpoint control by: (1) coordinately measuring mitotic induction and DNA content of bimE7 mutant cells; and (2) analyzing epistasis relationships between bimE7 and 16 different nim mutations. A combination of cytological and flow cytometric techniques was used to show that bimE7 cells at restrictive temperature (44 degrees C) undergo a normal, although somewhat slower cell cycle prior to mitotic arrest. Most bimE7 cells were fully reversible from restrictive temperature arrest, indicating that they are able to enter mitosis normally, and therefore require bimE function in order to finish mitosis. Furthermore, epistasis studies between bimE7 and mutations in cdc2 pathway components revealed that the induction of mitosis caused by inactivation of bimE requires functional p34cdc2 kinase, and that mitotic induction by bimE7 depends upon several other nim genes whose functions are not yet known. The involvement of bimE in S phase function and mitotic checkpoint control was suggested by three lines of evidence. First, at restrictive temperature the bimE7 mutation slowed the cell cycle by delaying the onset or execution of S phase. Second, at permissive temperature (30 degrees C) the bimE7 mutation conferred enhanced sensitivity to the DNA synthesis inhibitor hydroxyurea. Finally, the checkpoint linking M phase to the completion of S phase was abolished when bimE7 was combined with two nim mutations that cause arrest in G1 or S phase. A model for bimE function based on these findings is presented.

The chk1 pathway is required to prevent mitosis following cell-cycle arrest at 'start'

BACKGROUND

The G2-M-phase transition is controlled by cell-cycle checkpoint pathways which inhibit mitosis if previous events are incomplete or if the DNA is damaged. Genetic analyses in yeast have defined two related, but distinct, pathways which prevent mitosis--one which acts when S phase is inhibited, and one which acts when the DNA is damaged. In the fission yeast Schizosaccharomyces pombe, many of the gene products involved have been identified. Six 'radiation checkpoint' (rad) gene products are required for both the S-M and DNA-damage checkpoints, whereas Chk1, a putative protein kinase, is required only for the DNA-damage checkpoint and not for the S-M checkpoint following the inhibition of DNA synthesis.

RESULTS

We have genetically defined a third mitotic control checkpoint pathway in fission yeast which prevents mitosis when passage through 'start' (the commitment point in G1) is compromized. In cycling cells arrested at start, mitosis is prevented by a Chk1-dependent pathway. In the absence of Chk1, G1 cells attempt an abortive mitosis with a 1C DNA content without entering S phase. Similar results are seen in the absence of Rad17, a typical example of a rad gene product.

CONCLUSIONS

Genetic dissection of checkpoints in logarithmically growing fission yeast has identified a pathway that couples mitosis to correct passage through start. This pathway is related to the DNA-structure check-points which ensure that mitosis is dependent on the completion of replication and the integrity of the DNA. We propose that all three mitotic control checkpoints monitor distinct DNA or protein structures at different stages in the cell cycle.

Segregation of unreplicated chromosomes in Saccharomyces cerevisiae reveals a novel G1/M-phase checkpoint

Saccharomyces cerevisiae dbf4 and cdc7 cell cycle mutants block initiation of DNA synthesis (i.e., are iDS mutants) at 37 degrees C and arrest the cell cycle with a 1C DNA content. Surprisingly, certain dbf4 and cdc7 strains divide their chromatin at 37 degrees C. We found that the activation of the Cdc28 mitotic protein kinase and the Dbf2 kinase occurred with the correct relative timing with respect to each other and the observed division of the unreplicated chromatin. Furthermore, the division of unreplicated chromatin depended on a functional spindle. Therefore, the observed nuclear division resembled a normal mitosis, suggesting that S. cerevisiae commits to M phase in late G1 independently of S phase. Genetic analysis of dbf4 and cdc7 strains showed that the ability to restrain mitosis during a late G1 block depended on the genetic background of the strain concerned, since the dbf4 and cdc7 alleles examined showed the expected mitotic restraint in other backgrounds. This restraint was genetically dominant to lack of restraint, indicating that an active arrest mechanism, or checkpoint, was involved. However, none of the previously described mitotic checkpoint pathways were defective in the iDS strains that carry out mitosis without replicated DNA, therefore indicating that the checkpoint pathway that arrests mitosis in iDS mutants is novel. Thus, spontaneous strain differences have revealed that S. cerevisiae commits itself to mitosis in late G1 independently of entry into S phase and that a novel checkpoint mechanism can restrain mitosis if cells are blocked in late G1. We refer to this as the G1/M-phase checkpoint since it acts in G1 to restrain mitosis.

Evolution of the cell cycle

Cell proliferation involves duplication of all cell constituents and their more-or-less equal segregation to daughter cells. It seems probable that the performance of primitive cell-like structures would have been dogged by poor duplication and segregation fidelity, and by parasitism. This favoured evolution of the genome and with it the distinction between 'genomic' components like chromosomes whose synthesis is periodic and most other 'functional' components whose synthesis is continuous. Eukaryotic cells evolved from bacterial ancestors whose fused genome was replicated from a single origin and whose means of segregating sister chromatids depended on fixing their identity at replication. Evolution of an endo- or cytoskeleton, initially as means of consuming other bacteria, eventually enabled evolution of the mitotic spindle and a new means of segregating sister chromatids whose replication could be initiated from multiple origins. In this primitive eukaryotic cell, S and M phases might have been triggered by activation of a single cyclin-dependent kinase whose destruction along with that of other proteins would have triggered anaphase. Mitotic non-disjunction would have greatly facilitated genomic expansion, now possible due to multiple origins, and thereby accelerated the tempo of evolution when permitted by environmental conditions.

Microtubule stability in budding yeast: characterization and dosage suppression of a benomyl-dependent tubulin mutant

To better understand the dynamic regulation of microtubule structures in yeast, we studied a conditional-lethal beta-tubulin mutation tub2-150. This mutation is unique among the hundreds of tubulin mutations isolated in Saccharomyces cerevisiae in that it appears to cause an increase in the stability of microtubules. We report here that this allele is a mutation of threonine 238 to alanine, and that tub2-150 prevents the spindle from elongating during anaphase, suggesting a nuclear microtubule defect. To identify regulators of microtubule stability and/or anaphase, yeast genes were selected that, when overexpressed, could suppress the tub2-150 temperature-sensitive phenotype. One of these genes, JSN1, encodes a protein of 125 kDa that has limited similarity to a number of proteins of unknown function. Overexpression of the JSN1 gene in a TUB2 strain causes that strain to become more sensitive to benomyl, a microtubule-destabilizing drug. Of a representative group of microtubule mutants, only one other mutation, tub2-404, could be suppressed by JSN1 overexpression, showing that JSN1 is an allele-specific suppressor. As tub2-404 mutants are also defective for spindle elongation, this provides additional support for a role for JSN1 during anaphase.

Microtubule dynamics: taking aim at a moving target

Antitumor drugs of the vinca alkaloid and taxane classes function by suppressing the dynamics of microtubules in spindles, blocking cell division at metaphase. The drugs bind to various sites on the tubulin dimer and at different positions within the microtubule, suggesting that there are many unexplored targets for the design of novel drugs of this type.

Kinetochore chemistry is sensitive to tension and may link mitotic forces to a cell cycle checkpoint

Some cells have a quality control checkpoint that can detect a single misattached chromosome and delay the onset of anaphase, thus allowing time for error correction. The mechanical error in attachment must somehow be linked to the chemical regulation of cell cycle progression. The 3F3 antibody detects phosphorylated kinetochore proteins that might serve as the required link (Gorbsky, G. J., and W. A. Ricketts. 1993. J. Cell Biol. 122:1311-1321). We show by direct micromanipulation experiments that tension alters the phosphorylation of kinetochore proteins. Tension, whether from a micromanipulation needle or from normal mitotic forces, causes dephosphorylation of the kinetochore proteins recognized by 3F3. If tension is absent, either naturally or as a result of chromosome detachment by micromanipulation, the proteins are phosphorylated. Equally direct experiments identify tension as the checkpoint signal: tension from a microneedle on a misattached chromosome leads to anaphase (Li, X., and R. B. Nicklas. 1995. Nature (Lond.). 373:630-632), and we show here that the absence of tension caused by detaching chromosomes from the spindle delays anaphase indefinitely. Thus, the absence of tension is linked to both kinetochore phosphorylation and delayed anaphase onset. We propose that the kinetochore protein dephosphorylation caused by tension is the all clear signal to the checkpoint. The evidence is circumstantial but rich. In any event, tension alters kinetochore chemistry. Very likely, tension affects chemistry directly, by altering the conformation of a tension-sensitive protein, which leads directly to dephosphorylation.

NAP1 acts with Clb1 to perform mitotic functions and to suppress polar bud growth in budding yeast

NAP1 is a 60-kD protein that interacts specifically with mitotic cyclins in budding yeast and frogs. We have examined the ability of the yeast mitotic cyclin Clb2 to function in cells that lack NAP1. Our results demonstrate that Clb2 is unable to carry out its full range of functions without NAP1, even though Clb2/p34CDC28-associated kinase activity rises to normal levels. In the absence of NAP1, Clb2 is unable to efficiently induce mitotic events, and cells undergo a prolonged delay at the short spindle stage with normal levels of Clb2/p34CDC28 kinase activity. NAP1 is also required for the ability of Clb2 to induce the switch from polar to isotropic bud growth. As a result, polar bud growth continues during mitosis, giving rise to highly elongated cells. Our experiments also suggest that NAP1 is required for the ability of the Clb2/p34CDC28 kinase complex to amplify its own production, and that NAP1 plays a role in regulation of microtubule dynamics during mitosis. Together, these results demonstrate that NAP1 is required for the normal function of the activated Clb2/p34CDC28 kinase complex, and provide a step towards understanding how cyclin-dependent kinase complexes induce specific events during the cell cycle.

Histone H4 and the maintenance of genome integrity

The normal progression of Saccharomyces cerevisiae through nuclear division requires the function of the amino-terminal domain of histone H4. Mutations that delete the domain, or alter 4 conserved lysine residues within the domain, cause a marked delay during the G2+M phases of the cell cycle. Site-directed mutagenesis of single and multiple lysine residues failed to map this phenotype to any particular site; the defect was only observed when all four lysines were mutated. Starting with a quadruple lysine-to-glutamine substitution allele, the insertion of a tripeptide containing a single extra lysine residue suppressed the G2+M cell cycle defect. Thus, the amino-terminal domain of histone H4 has novel genetic functions that depend on the presence of lysine per se, and not a specific primary peptide sequence. To determine the nature of this function, we examined H4 mutants that were also defective for G2/M checkpoint pathways. Disruption of the mitotic spindle checkpoint pathway had no effect on the phenotype of the histone amino-terminal domain mutant. However, disruption of RAD9, which is part of the pathway that monitors DNA integrity, caused precocious progression of the H4 mutant through nuclear division and increased cell death. These results indicate that the lysine-dependent function of histone H4 is required for the maintenance of genome integrity, and that DNA damage resulting from the loss of this function activates the RAD9-dependent G2/M checkpoint pathway.

p93dis1, which is required for sister chromatid separation, is a novel microtubule and spindle pole body-associating protein phosphorylated at the Cdc2 target sites

Fission yeast cold-sensitive (cs) dis1 mutants are defective in sister chromatid separation. The dis1+ gene was isolated by chromosome walking. The null mutant showed the same phenotype as that of cs mutants. The dis1+ gene product was identified as a novel 93-kD protein, and its localization was determined by use of anti-dis1 antibodies and green fluorescent protein (GFP) tagged to the carboxyl end of p93dis1. The tagged p93dis1 in living cells localizes along cytoplasmic microtubule arrays in interphase and the elongating anaphase spindle in mitosis, but association with the short metaphase spindle microtubules is strikingly reduced. In the spindle, the tagged p93dis1 is enriched at the spindle pole bodies (SPBs). Time-lapse video images of single cells support the localization shift of p93dis1 to the SPBs in metaphase and spindle microtubules in anaphase. The carboxy-terminal fragment, which is essential for Dis1 function, accumulates around the mitotic SPB. We propose that these localization shifts of p93dis1 in mitosis facilitates sister chromatid separation by affecting SPB and anaphase spindle function.

Kinetochore motility after severing between sister centromeres using laser microsurgery: evidence that kinetochore directional instability and position is regulated by tension

During mitosis in vertebrate somatic cells, the single attached kinetochore on a mono-oriented chromosome exhibits directional instability: abruptly and independently switching between constant velocity poleward and away from the pole motility states. When the non-attached sister becomes attached to the spindle (chromosome bi-orientation), the motility of the sister kinetochores becomes highly coordinated, one moving poleward while the other moves away from the pole, allowing chromosomes to congress to the spindle equator. In our kinetochore-tensiometer model, we hypothesized that this coordinated behavior is regulated by tension across the centromere produced by kinetochore movement relative to the sister kinetochore and bulk of the chromosome arms. To test this model, we severed or severely weakened the centromeric chromatin between sister kinetochores on bi-oriented newt lung cell chromosomes with a laser microbeam. This procedure converted a pair of tightly linked sister kinetochores into two mono-oriented single kinetochore-chromatin fragments that were tethered to their chromosome arms by thin compliant chromatin strands. These single kinetochore-chromatin fragments moved substantial distances off the metaphase plate, stretching their chromatin strands, before the durations of poleward and away from the pole movement again became similar. In contrast, the severed arms remained at or moved closer to the spindle equator. The poleward and away from the pole velocities of single kinetochore-chromatin fragments in prometaphase were typical of velocities exhibited by sister kinetochores on intact chromosomes from prometaphase through midanaphase A. However, severing the chromatin between sister kinetochores uncoupled the normally coordinated motility of sister kinetochores. Laser ablation also uncoupled the motilities of the single kinetochore fragments from the bulk of the arms. These results reveal that kinetochore directional instability is a fundamental property of the kinetochore and that the motilities of sister kinetochores are coordinated during congression by a stiff centromere linkage. We conclude that kinetochores act as tensiometers that sense centromere tension generated by differential movement of sister kinetochores and their chromosome arms to control switching between constant velocity P and AP motility states.

Gene disruption of a G4-DNA-dependent nuclease in yeast leads to cellular senescence and telomere shortening

The yeast gene KEM1 (also named SEP1/DST2/XRN1/RAR5) produces a G4-DNA-dependent nuclease that binds to G4 tetraplex DNA structure and cuts in a single-stranded region 5' to the G4 structure. G4-DNA generated from yeast telomeric oligonucleotides competitively inhibits the cleavage reaction, suggesting that this enzyme may interact with yeast telomeres in vivo. Homozygous deletions of the KEM1 gene in yeast block meiosis at the pachytene stage, which is consistent with the hypothesis that G4 tetraplex DNA may be involved in homologous chromosome pairing during meiosis. We conjectured that the mitotic defects of kem1/sep1 mutant cells, such as a higher chromosome loss rate, are also due to failure in processing G4-DNA, especially at telomeres. Here we report two phenotypes associated with a kem1-null allele, cellular senescence and telomere shortening, that provide genetic evidence that G4 tetraplex DNA may play a role in telomere functioning. In addition, our results reveal that chromosome ends in the same cells behave differently in a fashion dependent on the KEM1 gene product.

Frontier questions about sister chromatid separation in anaphase

Sister chromatid separation in anaphase is an important event in the cell's transmission of genetic information to a descendent. It has been investigated from different aspects: cell cycle regulation, spindle and chromosome dynamics within the three-dimensional cell architecture, transmission fidelity control and cellular signaling. Integrated studies directed toward unified understanding are possible using multidisciplinary methods with model organisms. Ubiquitin-dependent proteolysis, protein dephosphorylation, an unknown function by the TPR repeat proteins, chromosome transport by microtubule-based motors and DNA topological change by DNA topoisomerase II are all necessary for progression from metaphase to anaphase. Chromosome condensation, mitotic kinetochore function and spindle formation require a larger number of proteins, which are prerequisites for successful sister chromatid separation. Factors that help to retain sister chromatid connection after replication and prevent premature separation remain to be determined. Although sister chromatid separation occurs in anaphase, gene functions in other cell cycle stages also ensure the progression of correct chromatid separation.

Microinjection of mitotic cells with the 3F3/2 anti-phosphoepitope antibody delays the onset of anaphase

The transition from metaphase to anaphase is regulated by a checkpoint system that prevents chromosome segregation in anaphase until all the chromosomes have aligned at the metaphase plate. We provide evidence indicating that a kinetochore phosphoepitope plays a role in this checkpoint pathway. The 3F3/2 monoclonal antibody recognizes a kinetochore phosphoepitope in mammalian cells that is expressed on chromosomes before their congression to the metaphase plate. Once chromosomes are aligned, expression is lost and cells enter anaphase shortly thereafter. When microinjected into prophase cells, the 3F3/2 antibody caused a concentration-dependent delay in the onset of anaphase. Injected antibody inhibited the normal dephosphorylation of the 3F3/2 phosphoepitope at kinetochores. Microinjection of the antibody eliminated the asymmetric expression of the phosphoepitope normally seen on sister kinetochores of chromosomes during their movement to the metaphase plate. Chromosome movement to the metaphase plate appeared unaffected in cells injected with the antibody suggesting that asymmetric expression of the phosphoepitope on sister kinetochores is not required for chromosome congression to the metaphase plate. In antibody-injected cells, the epitope remained expressed at kinetochores throughout the prolonged metaphase, but had disappeared by the onset of anaphase. When normal cells in metaphase, lacking the epitope at kinetochores, were treated with agents that perturb microtubules, the 3F3/2 phosphoepitope quickly reappeared at kinetochores. Immunoelectron microscopy revealed that the 3F3/2 epitope is concentrated in the middle electronlucent layer of the trilaminar kinetochore structure. We propose that the 3F3/2 kinetochore phosphoepitope is involved in detecting stable kinetochore-microtubule attachment or is a signaling component of the checkpoint pathway regulating the metaphase to anaphase transition.