Feedback control of mitosis in budding yeast

CDC44: a putative nucleotide-binding protein required for cell cycle progression that has homology to subunits of replication factor C

To investigate the means by which a cell regulates the progression of the mitotic cell cycle, we characterized cdc44, a mutation that causes Saccharomyces cerevisiae cells to arrest before mitosis. CDC44 encodes a 96-kDa basic protein with significant homology to a human protein that binds DNA (PO-GA) and to three subunits of human replication factor C (also called activator 1). The hypothesis that Cdc44p is involved in DNA metabolism is supported by the observations that (i) levels of mitotic recombination suggest elevated rates of DNA damage in cdc44 mutants and (ii) the cell cycle arrest observed in cdc44 mutants is alleviated by the DNA damage checkpoint mutations rad9, mec1, and mec2. The predicted amino acid sequence of Cdc44p contains GTPase consensus sites, and mutations in these regions cause a conditional cell cycle arrest. Taken together, these observations suggest that the essential CDC44 gene may encode the large subunit of yeast replication factor C.

The mitotic feedback control gene MAD2 encodes the alpha-subunit of a prenyltransferase

The mad2-1 mutation inactivates the cell-cycle feedback control that prevents budding yeast cells from leaving mitosis until spindle assembly is complete. The gene product of MAD2 shows significant sequence similarity to the alpha-subunit of prenyltransferases. Here we isolate a new temperature-sensitive mad2 mutant, mad2-2ts, and find that Mad2p is required for the membrane association of Ypt1p and Sec4p, two prenylated small GTP-binding proteins involved in protein trafficking. Extracts from mad2-2ts mutant cells fail to geranylgeranylate a number of substrates at the non-permissive temperature. mad2-2ts is synthetically lethal with bet2-1, a mutation in the gene that encodes for the beta-subunit of the Ypt1p and Sec4p geranylgeranyl transferase. Therefore MAD2 and BET2 gene products may physically interact to form a geranylgeranyl transferase complex. In addition, the difference between the phenotypes of mad2-1 and mad2-2ts suggests that MAD2 has distinct roles in protein transport and the mitotic feedback control.

Bet2p and Mad2p are components of a prenyltransferase that adds geranylgeranyl onto Ypt1p and Sec4p

Three different prenyltransferases have been identified in yeast and higher cells, the farnesyltransferase and the type I and type II geranylgeranyltransferases (GGTase). The farnesyltransferase and GGTase-I modify peptides in vitro with the CAAX (C, Cys; A, aliphatic residue; X, terminal amino acid) consensus motif. These enzymes are heterodimers that have different beta-subunits and a shared alpha-subunit. In yeast, the RAM2 gene encodes this alpha-subunit. RAM2 is also homologous to MAD2, a yeast gene whose product has been implicated in the feedback control of mitosis. We have shown that Bet2p is a component of the yeast GGTase-II (refs 6, 12) that geranylgeranylates Ypt1p, a small GTP-binding protein that mediates transport from the endoplasmic reticulum to the Golgi complex. Here we report that Mad2p is a component of this enzyme. Bet2p forms a complex with Mad2p that appears to bind geranylgeranyl pyrophosphate, but not farnesyl pyrophosphate. The efficient transfer of geranylgeranyl onto small GTP-binding proteins requires the presence of an additional activity.

The calcium-binding protein cell division cycle 31 of Saccharomyces cerevisiae is a component of the half bridge of the spindle pole body

Cdc31 mutants of Saccharomyces cerevisiae arrest at the nonpermissive temperature with large buds, G2 DNA content and, a single, abnormally large spindle pole body (SPB) (Byers, B. 1981. Molecular Genetics in Yeast. Alfred Benzon Symposium. 16:119-133). In this report, we show that the CDC31 gene product is essential for cell viability. We demonstrate that purified CDC31 protein binds Ca2+ and that this binding is highly specific. Taken together, three lines of evidence indicate that CDC31 is a component of the SPB. First, CDC31 cofractionates with enriched preparations of SPBs. Second, immunofluorescence staining indicates that CDC31 colocalizes with a known SPB component. Third, immunoelectron microscopy with whole cells and with isolated SPBs reveals that CDC31 is localized to the half bridge of the SPB, which lies immediately adjacent to the SPB plaques. CDC31 was detected mainly at the cytoplasmic side of the half bridge and, therefore, defines a further substructure of the SPB. We suggest that CDC31 is a member of a family of calcium-binding, centrosome-associated proteins from a phylogenetically diverse group of organisms.

Suppression of yeast geranylgeranyl transferase I defect by alternative prenylation of two target GTPases, Rho1p and Cdc42p

Geranylgeranyl transferase I (GGTase I), which modifies proteins containing the sequence Cys-Ali-Ali-Leu (Ali: aliphatic) at their C-termini, is indispensable for growth in the budding yeast Saccharomyces cerevisiae. We report here that GGTase I is no longer essential when Rho1p and Cdc42p are simultaneously overproduced. The lethality of a GGTase I deletion is most efficiently suppressed by provision of both Rho1p and Cdc42p with altered C-terminal sequences (Cys-Ali-Ali-Met) corresponding to the C-termini of substrates of farnesyl transferase (FTase). Under these circumstances, the FTase, normally not essential for growth of yeast, becomes essential.

Differential expression of a phosphoepitope at the kinetochores of moving chromosomes

A phosphorylated epitope is differentially expressed at the kinetochores of chromosomes in mitotic cells and may be involved in regulating chromosome movement and cell cycle progression. During prophase and early prometaphase, the phosphoepitope is expressed equally among all the kinetochores. In mid-prometaphase, some chromosomes show strong labeling on both kinetochores; others exhibit weak or no labeling; while in other chromosomes, one kinetochore is intensely labeled while its sister kinetochore is unlabeled. Chromosomes moving toward the metaphase plate express the phosphoepitope strongly on the leading kinetochore but weakly on the trailing kinetochore. This is the first demonstration of a biochemical difference between the two kinetochores of a single chromosome. During metaphase and anaphase, the kinetochores are unlabeled. At metaphase, a single misaligned chromosome can inhibit further progression into anaphase. Misaligned chromosomes express the phosphoepitope strongly on both kinetochores, even when all the other chromosomes of a cell are assembled at the metaphase plate and lack expression. This phosphoepitope may be involved in regulating chromosome movement to the metaphase plate during prometaphase and may be part of a cell cycle checkpoint by which the onset of anaphase is inhibited until complete metaphase alignment is achieved.

Delays in anaphase initiation occur in individual nuclei of the syncytial Drosophila embryo

The syncytial divisions of the Drosophila melanogaster embryo lack some of the well established cell-cycle checkpoints. It has been suggested that without these checkpoints the divisions would display a reduced fidelity. To test this idea, we examined division error frequencies in individuals bearing an abnormally long and rearranged second chromosome, designated C(2)EN. Relative to a normal chromosome, this chromosome imposes additional structural demands on the mitotic apparatus in both the early syncytial embryonic divisions and the later somatic divisions. We demonstrate that the C(2)EN chromosome does not increase the error frequency of the late larva neuroblast divisions. However, in the syncytial embryonic nuclear divisions, the C(2)EN chromosome produces a 10-fold increase in division errors relative to embryos with a normal karyotype. During late anaphase of the neuroblast divisions, the sister C(2)EN chromosomes cleanly separate from one another. In contrast, during late anaphase of the syncytial divisions in C(2)EN-bearing nuclei, large amounts of chromatin often lag on the metaphase plate. Live analysis of C(2)EN-bearing embryos demonstrates that individual nuclei in the syncytial population of dividing nuclei often delay in their initiation of anaphase. These delays frequently lead to division errors. Eventually the products of the nuclei delayed in anaphase sink inward and are removed from the dividing population of syncytial nuclei. These results suggest that the Drosophila embryo may be equipped with mechanisms that monitor the fidelity of the syncytial nuclear divisions. Unlike checkpoints that rely on cell cycle delays to identify and correct division errors, these embryonic mechanisms rely on cell cycle delays to identify and discard the products of division errors.

Mutations in the Drosophila melanogaster gene three rows permit aspects of mitosis to continue in the absence of chromatid segregation

We have cloned the three rows (thr) gene, by a combination of chromosome microdissection and P element tagging. We describe phenotypes of embryos homozygous for mutations at the thr locus. Maternal mRNA and protein appear to be sufficient to allow 14 rounds of mitosis in embryos homozygous for thr mutations. However, a small percentage of cells in syncytial blastoderm stage thr embryos sink into the interior of the embryo as if they have failed to divide properly. Following cellularisation all cells complete mitosis 14 normally. All cells become delayed at mitosis 15 with their chromosomes remaining aligned on the spindle in a metaphase-like configuration, even though both cyclins A and B have both been degraded. As cyclin B degradation occurs at the metaphase-anaphase transition, subsequent to the microtubule integrity checkpoint, the delay induced by mutations at the thr locus defines a later point in mitotic progression. Chromosomes in the cells of thr embryos do not undertake anaphase separation, but remain at the metaphase plate. Subsequently they decondense. A subset of nuclei go on to replicate their DNA but there is no further mitotic division.

Assembly and functions of the spindle pole body in budding yeast

The spindle pole body (SPB) serves as the centrosome in yeasts and in a variety of other lower eukaryotes. In Saccharomyces cerevisiae, this organelle controls the assembly of all microtubules in the cell, acting not only as a pole of the mitotic or meiotic spindle but also as the site from which cytoplasmic microtubules emanate. The distinctive structure of the SPB has permitted definition of discrete stages in its duplication and behavior at all stages of the yeast life cycle. In association with genetic analyses, studies of the yeast SPB are providing insights into the mechanisms that control centrosomal behavior in this model eukaryote.

Expression of p60v-src in Saccharomyces cerevisiae results in elevation of p34CDC28 kinase activity and release of the dependence of DNA replication on mitosis

Expression of the oncogenic protein tyrosine kinase p60v-src in the yeast Saccharomyces cerevisiae has been shown to result in rapid cell death (J. S. Brugge, G. Jarosik, J. Andersen, A. Queral-Lustig, M. Fedor-Chaiken, and J. R. Broach, Mol. Cell. Biol. 7:2180-2187, 1987). Work described here demonstrates that v-Src expression results in accumulation of large-budded cells and a nuclear division block without blocking cytokinesis. Flow-cytometric analysis indicates that the DNA content of these cells is elevated beyond the G2 DNA content, and genetic studies indicate that v-Src expression causes aneuploidy. The activity of Cdc28 kinase, which controls the G1/S and G2/M transitions in S. cerevisiae, increases during galactose induction in a Src+ strain but not in an isogenic Src- strain. These observations indicate that v-Src expression disrupts p34CDC28 kinase regulation, allowing DNA replication to proceed in the absence of a prior mitotic event.

Expression of p60v-src in Saccharomyces cerevisiae results in elevation of p34CDC28 kinase activity and release of the dependence of DNA replication on mitosis

Expression of the oncogenic protein tyrosine kinase p60v-src in the yeast Saccharomyces cerevisiae has been shown to result in rapid cell death (J. S. Brugge, G. Jarosik, J. Andersen, A. Queral-Lustig, M. Fedor-Chaiken, and J. R. Broach, Mol. Cell. Biol. 7:2180-2187, 1987). Work described here demonstrates that v-Src expression results in accumulation of large-budded cells and a nuclear division block without blocking cytokinesis. Flow-cytometric analysis indicates that the DNA content of these cells is elevated beyond the G2 DNA content, and genetic studies indicate that v-Src expression causes aneuploidy. The activity of Cdc28 kinase, which controls the G1/S and G2/M transitions in S. cerevisiae, increases during galactose induction in a Src+ strain but not in an isogenic Src- strain. These observations indicate that v-Src expression disrupts p34CDC28 kinase regulation, allowing DNA replication to proceed in the absence of a prior mitotic event.

A mutation in PLC1, a candidate phosphoinositide-specific phospholipase C gene from Saccharomyces cerevisiae, causes aberrant mitotic chromosome segregation

We identified a putative Saccharomyces cerevisiae homolog of a phosphoinositide-specific phospholipase C (PI-PLC) gene, PLC1, which encodes a protein most similar to the delta class of PI-PLC enzymes. The PLC1 gene was isolated during a study of yeast strains that exhibit defects in chromosome segregation. plc1-1 cells showed a 10-fold increase in aberrant chromosome segregation compared with the wild type. Molecular analysis revealed that PLC1 encodes a predicted protein of 101 kDa with approximately 50 and 26% identity to the highly conserved X and Y domains of PI-PLC isozymes from humans, bovines, rats, and Drosophila melanogaster. The putative yeast protein also contains a consensus EF-hand domain that is predicted to bind calcium. Interestingly, the temperature-sensitive and chromosome missegregation phenotypes exhibited by plc1-1 cells were partially suppressed by exogenous calcium.

A mutation in PLC1, a candidate phosphoinositide-specific phospholipase C gene from Saccharomyces cerevisiae, causes aberrant mitotic chromosome segregation

We identified a putative Saccharomyces cerevisiae homolog of a phosphoinositide-specific phospholipase C (PI-PLC) gene, PLC1, which encodes a protein most similar to the delta class of PI-PLC enzymes. The PLC1 gene was isolated during a study of yeast strains that exhibit defects in chromosome segregation. plc1-1 cells showed a 10-fold increase in aberrant chromosome segregation compared with the wild type. Molecular analysis revealed that PLC1 encodes a predicted protein of 101 kDa with approximately 50 and 26% identity to the highly conserved X and Y domains of PI-PLC isozymes from humans, bovines, rats, and Drosophila melanogaster. The putative yeast protein also contains a consensus EF-hand domain that is predicted to bind calcium. Interestingly, the temperature-sensitive and chromosome missegregation phenotypes exhibited by plc1-1 cells were partially suppressed by exogenous calcium.

The lethality of p60v-src in Saccharomyces cerevisiae and the activation of p34CDC28 kinase are dependent on the integrity of the SH2 domain

The lethal effects of the expression of the oncogenic protein tyrosine kinase p60v-src in Saccharomyces cerevisiae are associated with a loss of cell cycle control at the G1/S and G2/M checkpoints. Results described here indicate that the ability of v-Src to kill yeast is dependent on the integrity of the SH2 domain, a region of the Src protein involved in recognition of proteins phosphorylated on tyrosine. Catalytically active v-Src proteins with deletions in the SH2 domain have little effect on yeast growth, unlike wild-type v-Src protein, which causes accumulation of large-budded cells, perturbation of spindle microtubules and increased DNA content when expressed. The proteins phosphorylated on tyrosine in cells expressing v-Src differ from those in cells expressing a Src protein with a deletion in the SH2 domain. Also, unlike the wild-type v-Src protein, which drastically increases histone H1-associated Cdc28 kinase activity, c-Src and an altered v-Src protein have no effect on Cdc28 kinase activity. These results indicate that the SH2 domain is functionally important in the disruption of the yeast cell cycle by v-Src.

In vivo inhibition of cyclin B degradation and induction of cell-cycle arrest in mammalian cells by the neutral cysteine protease inhibitor N-acetylleucylleucylnorleucinal

The cytotoxic neutral cysteine protease inhibitor N-acetylleucylleucylnorleucinal (ALLN) inhibits cell-cycle progression in CHO cells, affecting the G1/S and metaphase-anaphase transition points, as well as S phase. Mitotic arrest induced by ALLN is associated with the inhibition of cyclin B degradation. At mitosis-inhibiting concentrations of ALLN, cells undergo nuclear-envelope breakdown, spindle formation, chromosome condensation, and congression to the metaphase plate. However, normal anaphase events do not occur, and cells arrest in a metaphase configuration for a prolonged period. Steady-state levels of cyclin B increase to greater than normal mitotic levels, and cyclin B is not degraded for an extended period. Histone H1 kinase activity remains elevated during mitotic arrest. Duration of mitotic arrest depends on ALLN concentration; high concentrations (> 50 micrograms/ml) produce a prolonged mitotic arrest, whereas at lower concentrations, cells are transiently delayed through mitosis (up to 4-12 hr), after which they undergo aberrant cell division resulting in randomly sized daughter cells containing variable amounts of DNA. Cyclin B degradation fails to occur, and histone H1 kinase remains activated for the duration of mitotic arrest at all ALLN concentrations.

Control of the yeast cell cycle by the Cdc28 protein kinase

It is becoming increasingly apparent that the diverse functions of Cdc28 during the yeast cell cycle are performed by forms of the kinase that are distinguished by their cyclin subunits. Entry into the cell cycle at START involves the Cln cyclins. S phase needs Clb5 or Clb6 B-type cyclins. Bipolar mitotic spindle formation involves Clb1-4 B-type cyclins. Much of the order and timing of the cell cycle events may involve the progressive activation of Cdc28 kinase activities associated with different cyclins, whose periodicity during the cycle is determined by both transcriptional and post-transcriptional controls.

Unravelling the tangled web at the microtubule-organizing center

The last year has seen dramatic progress in the use of genetic and biochemical approaches to identify microtubule-organizing center components. The use of vertebrate and invertebrate egg extracts has allowed the development of novel assays for centrosome duplication and activation. A variety of mutations in fungi are being used to sort out the pathway of spindle pole body duplication.

Mos and the cell cycle: the molecular basis of the transformed phenotype

The product of the mos proto-oncogene is a serine/threonine kinase that is expressed at high levels in germ cells. Mos is a regulator of meiotic maturation, and is required for the initiation and progression of oocyte meiotic maturation that leads to the production of unfertilized eggs. Mos is also a component of cytostatic factor, an activity that is believed to arrest oocyte maturation at meiotic metaphase II. There is evidence showing that the Mos protein is associated with tubulin in unfertilized eggs and transformed cells, raising the possibility that it is involved in the microtubular reorganization that occurs during M-phase. Inappropriate expression of its M-phase activity during interphase of the cell cycle may be responsible for its transforming activity.

Suppression of the bimC4 mitotic spindle defect by deletion of klpA, a gene encoding a KAR3-related kinesin-like protein in Aspergillus nidulans

To investigate the relationship between structure and function of kinesin-like proteins, we have identified by polymerase chain reaction (PCR) a new kinesin-like protein in the filamentous fungus Aspergillus nidulans, which we have designated KLPA. DNA sequence analysis showed that the predicted KLPA protein contains a COOH terminal kinesin-like motor domain. Despite the structural similarity of KLPA to the KAR3 and NCD kinesin-like proteins of Saccharomyces cerevisiae and Drosophila melanogaster, which also posses COOH-terminal kinesin-like motor domains, there are no significant sequence similarities between the nonmotor or tail portions of these proteins. Nevertheless, expression studies in S. cerevisiae showed that klpA can complement a null mutation in KAR3, indicating that primary amino acid sequence conservation between the tail domains of kinesin-like proteins is not necessarily required for conserved function. Chromosomal deletion of the klpA gene exerted no observable mutant phenotype, suggesting that in A. nidulans there are likely to be other proteins functionally redundant with KLPA. Interestingly, the temperature sensitive phenotype of a mutation in another gene, bimC, which encodes a kinesin-like protein involved in mitotic spindle function in A. nidulans, was suppressed by deletion of klpA. We hypothesize that the loss of KLPA function redresses unbalanced forces within the spindle caused by mutation in bimC, and that the KLPA and BIMC kinesin-like proteins may play opposing roles in spindle function.

Mutations of the fizzy locus cause metaphase arrest in Drosophila melanogaster embryos

We describe the effects of mutations in the fizzy gene of Drosophila melanogaster and show that fizzy mutations cause cells in mitosis to arrest at metaphase. We show that maternally supplied fizzy activity is required for normal nuclear division in the preblastoderm embryo and, during later embryogenesis, that zygotic fizzy activity is required for the development of the ventrally derived epidermis and the central and peripheral nervous systems. In fizzy embryos, dividing cells in these tissues arrest at metaphase, fail to differentiate and ultimately die. In the ventral epidermis, if cells are prevented from entering mitosis by using a string mutation, cell death is prevented and the ability to differentiate ventral epidermis is restored in fizzy; string double mutant embryos. These results demonstrate that fizzy is a cell cycle mutation and that the normal function of the fizzy gene is required for dividing cells to exit metaphase and complete mitosis.

Requirement for ESP1 in the nuclear division of Saccharomyces cerevisiae

Mutations in the ESP1 gene of Saccharomyces cerevisiae disrupt normal cell-cycle control and cause many cells in a mutant population to accumulate extra spindle pole bodies. To determine the stage at which the esp1 gene product becomes essential for normal cell-cycle progression, synchronous cultures of ESP1 mutant cells were exposed to the nonpermissive temperature for various periods of time. The mutant cells retained viability until the onset of mitosis, when their viability dropped markedly. Examination of these cells by fluorescence and electron microscopy showed the first detectable defect to be a structural failure in the spindle. Additionally, flow cytometric analysis of DNA content demonstrated that massive chromosome missegregation accompanied this failure of spindle function. Cytokinesis occurred despite the aberrant nuclear division, which often resulted in segregation of both spindle poles to the same cell. At later times, the missegregated spindle pole bodies entered a new cycle of duplication, thereby leading to the accumulation of extra spindle pole bodies within a single nucleus. The DNA sequence predicts a protein product similar to those of two other genes that are also required for nuclear division: the cut1 gene of Schizosaccharomyces pombe and the bimB gene of Aspergillus nidulans.

Chromatin remodeling in mammalian zygotes

With sperm-egg fusion at the time of fertilization the gamete nuclei are remodeled from genetically quiescent structures into pronuclei capable of DNA synthesis. Features of this process that are critical to insure the genetic integrity of the zygote and the success of subsequent embryonic development include: oocyte responses that prevent polyspermy; completion of the 2nd meiotic division by the oocyte; exchange of proteins in the sperm nucleus; and, remodelling of the oocyte chromosomes and sperm nucleus into functional pronuclei. Elucidation of the biological and molecular mechanisms underlying zygote formation and chromatin remodeling should enhance our understanding of the potential vulnerability of the zygote to toxicant-induced damage.

Cell division in Aspergillus

Amenable to sophisticated genetic and molecular analysis, the simple filamentous fungus Aspergillus nidulans has provided some novel insights into the mechanisms and regulation of cell division. Mutational analysis has identified over fifty genes necessary for nuclear division, nuclear movement and cytokinesis. Molecular and cellular analysis of these mutants has led to the discovery of novel components of the cytoskeleton as well as to clarifying the role of established cytoskeletal proteins. Mutations leading to defects in the kinases (i.e. p34cdc2) and phosphatases (i.e. cdc25 and PP1), which are known to regulate mitosis in other eukaryotes, have been identified in Aspergillus. Additional, as yet novel, mitotic regulatory molecules, encoded by the nimA and bimE genes, have also been discovered in Aspergillus.

Fission yeast genes involved in coupling mitosis to completion of DNA replication

We have isolated fission yeast mutants that enter mitosis when DNA replication is blocked with hydroxyurea. The mutants define eight linkage groups, three of which consist of alleles of the rad1, rad3, and rad17 genes. Recently, these fission yeast genes have been shown to be required for radiation-induced cell cycle arrest, as is the budding yeast RAD9 gene. The other five genes are called hus (hydroxyurea sensitive) 1-5. We propose that these genes participate in an intracellular signal transduction pathway that monitors the completion of DNA replication and transmits information to the mitotic control protein cdc2. Mutations that bypass the requirement for cdc25 (an activator of the mitotic regulator cdc2) also uncouple mitosis from DNA replication. However, mitosis is blocked by inhibitors of DNA replication in strains in which the cdc25 gene has been deleted, indicating that although cdc25 influences the coupling of mitosis to the completion of DNA replication, it is not essential for this control.

Poleward kinetochore fiber movement occurs during both metaphase and anaphase-A in newt lung cell mitosis

Microtubules in the mitotic spindles of newt lung cells were marked using local photoactivation of fluorescence. The movement of marked segments on kinetochore fibers was tracked by digital fluorescence microscopy in metaphase and anaphase and compared to the rate of chromosome movement. In metaphase, kinetochore oscillations toward and away from the poles were coupled to kinetochore fiber shortening and growth. Marked zones on the kinetochore microtubules, meanwhile, moved slowly polewards at a rate of approximately 0.5 micron/min, which identifies a slow polewards movement, or "flux," of kinetochore microtubules accompanied by depolymerization at the pole, as previously found in PtK2 cells (Mitchison, 1989b). Marks were never seen moving away from the pole, indicating that growth of the kinetochore microtubules occurs only at their kinetochore ends. In anaphase, marked zones on kinetochore microtubules also moved polewards, though at a rate slower than overall kinetochore-to-pole movement. Early in anaphase-A, microtubule depolymerization at kinetochores accounted on average for 75% of the rate of chromosome-to-pole movement, and depolymerization at the pole accounted for 25%. When chromosome-to-pole movement slowed in late anaphase, the contribution of depolymerization at the kinetochores lessened, and flux became the dominant component in some cells. Over the whole course of anaphase-A, depolymerization at kinetochores accounted on average for 63% of kinetochore fiber shortening, and flux for 37%. In some anaphase cells up to 45% of shortening resulted from the action of flux. We conclude that kinetochore microtubules change length predominantly through polymerization and depolymerization at the kinetochores during both metaphase and anaphase as the kinetochores move away from and towards the poles. Depolymerization, though not polymerization, also occurs at the pole during metaphase and anaphase, so that flux contributes to polewards chromosome movements throughout mitosis. Poleward force production for chromosome movements is thus likely to be generated by at least two distinct molecular mechanisms.

Creative blocks: cell-cycle checkpoints and feedback controls

Before division, cells must ensure that they finish DNA replication, DNA repair and chromosome segregation. They do so by using feedback controls which can detect the failure to complete replication, repair or spindle assembly to arrest the progress of the cell cycle at one of three checkpoints. Failures in feedback controls can contribute to the generation of cancer.

Astral microtubules are not required for anaphase B in Saccharomyces cerevisiae

tub2-401 is a cold-sensitive allele of TUB2, the sole gene encoding beta-tubulin in the yeast, Saccharomyces cerevisiae. At 18 degrees C, tub2-401 cells are able to assemble spindle microtubules but lack astral microtubules. Under these conditions, movement of the spindle to the bud neck is blocked. However, spindle elongation and chromosome separation are unimpeded and occur entirely within the mother cell. Subsequent cytokinesis produces one cell with two nuclei and one cell without a nucleus. The anucleate daughter can not bud. The binucleate daughter proceeds through another cell cycle to produce a cell with four nuclei and another anucleate cell. With additional time in the cold, the number of nuclei in the nucleated cells continues to increase and the percentage of anucleate cells in the population rises. The results indicate that astral microtubules are needed to position the spindle in the bud neck but are not required for spindle elongation at anaphase B. In addition, cell cycle progression does not depend on the location or orientation of the spindle.

Centromere DNA mutations induce a mitotic delay in Saccharomyces cerevisiae

Cytological observations of animal cell mitoses have shown that the onset of anaphase is delayed when chromosome attachment to the spindle is spontaneously retarded or experimentally interrupted. This report demonstrates that a centromere DNA (CEN) mutation carried on a single chromosome can induce a cell cycle delay observed as retarded mitosis in the yeast Saccharomyces cerevisiae. A 31-base-pair deletion within centromere DNA element II (CDEII delta 31) that causes chromosome missegregation in only 1% of cell division elicited a dramatic mitotic delay phenotype. Other CEN DNA mutations, including mutations in centromere DNA elements I and III, similarly delayed mitosis. Single division pedigree analysis of strains containing the CDEII delta 31 CEN mutation indicated that most (and possibly all) cells experienced delay in each cell cycle and that the delay was not due to increased chromosome copy number. Furthermore, a synchronous population of cells containing the CDEII delta 31 mutation underwent DNA synthesis on schedule with wild-type kinetics, but subsequently exhibited late chromosomal separation and concomitant late cell separation. We speculate that this delay in cell cycle progression before the onset of anaphase provides a mechanism for the stabilization of chromosomes with defective kinetochore structure. Further, we suggest that the delay may be mediated by surveillance at a cell cycle checkpoint that monitors the completion of chromosomal attachment to the spindle.

A delay in the Saccharomyces cerevisiae cell cycle that is induced by a dicentric chromosome and dependent upon mitotic checkpoints

Dicentric chromosomes are genetically unstable and depress the rate of cell division in Saccharomyces cerevisiae. We have characterized the effects of a conditionally dicentric chromosome on the cell division cycle by using microscopy, flow cytometry, and an assay for histone H1 kinase activity. Activating the dicentric chromosome induced a delay in the cell cycle after DNA replication and before anaphase. The delay occurred in the absence of RAD9, a gene required to arrest cell division in response to DNA damage. The rate of dicentric chromosome loss, however, was elevated in the rad9 mutant. A mutation in BUB2, a gene required for arrest of cell division in response to loss of microtubule function, diminished the delay. Both RAD9 and BUB2 appear to be involved in the cellular response to a dicentric chromosome, since the conditionally dicentric rad9 bub2 double mutant was highly inviable. We conclude that a dicentric chromosome results in chromosome breakage and spindle aberrations prior to nuclear division that normally activate mitotic checkpoints, thereby delaying the onset of anaphase.

A delay in the Saccharomyces cerevisiae cell cycle that is induced by a dicentric chromosome and dependent upon mitotic checkpoints

Dicentric chromosomes are genetically unstable and depress the rate of cell division in Saccharomyces cerevisiae. We have characterized the effects of a conditionally dicentric chromosome on the cell division cycle by using microscopy, flow cytometry, and an assay for histone H1 kinase activity. Activating the dicentric chromosome induced a delay in the cell cycle after DNA replication and before anaphase. The delay occurred in the absence of RAD9, a gene required to arrest cell division in response to DNA damage. The rate of dicentric chromosome loss, however, was elevated in the rad9 mutant. A mutation in BUB2, a gene required for arrest of cell division in response to loss of microtubule function, diminished the delay. Both RAD9 and BUB2 appear to be involved in the cellular response to a dicentric chromosome, since the conditionally dicentric rad9 bub2 double mutant was highly inviable. We conclude that a dicentric chromosome results in chromosome breakage and spindle aberrations prior to nuclear division that normally activate mitotic checkpoints, thereby delaying the onset of anaphase.

pp39mos is associated with p34cdc2 kinase in c-mosxe-transformed NIH 3T3 cells

We investigated the possible interactions between pp39mos and p34cdc2 kinase in NIH 3T3 cells transformed by c-mosxe. pp39mos is coprecipitated with p34cdc2 when using either anti-PSTAIR antibody or p13suc1-Sepharose beads. Likewise, p34cdc2 is coprecipitated with pp39mos when using anti-mos antibody. However, pp39mos was not present in histone H1 kinase-active p34cdc2 complexes precipitated with anti-p34cdc2 C-terminal peptide antibody even during metaphase of the cell cycle. The molar ratio of p34 to pp39mos in the p13suc1 complex is approximately 2:1. Consistent with the tight association between pp39mos and tubulin, tubulin was also present in equivalent amounts with pp39mos and p34 in the p13suc1 complex. This pp39mos-p34cdc2-tubulin complex may be important in transformation by the mos oncogene.

The Drosophila l(1)zw10 gene product, required for accurate mitotic chromosome segregation, is redistributed at anaphase onset

Mutations in the gene l(1)zw10 disrupt the accuracy of chromosome segregation in a variety of cell types during the course of Drosophila development. Cytological analysis of mutant larval brain neuroblasts shows very high levels of aneuploid cells. Many anaphase figures are aberrant, the most frequent abnormality being the presence of lagging chromosomes that remain in the vicinity of the metaphase plate when the other chromosomes have migrated toward the spindle poles. Finally, the centromeric connection between sister chromatids in mutant neuroblasts treated with colchicine often appears to be broken, in contrast with similarly treated control neuroblasts. The 85-kD protein encoded by the l(1)zw10 locus displays a dynamic pattern of localization in the course of the embryonic cell cycle. It is excluded from the nuclei during interphase, but migrates into the nuclear zone during prometaphase. At metaphase, the zw10 antigen is found in a novel filamentous structure that may be specifically associated with kinetochore microtubules. Upon anaphase onset, there is an extremely rapid redistribution of the zw10 protein to a location at or near the kinetochores of the separating chromosomes.

CIK1: a developmentally regulated spindle pole body-associated protein important for microtubule functions in Saccharomyces cerevisiae

A genetic screen was devised to identify genes important for spindle pole body (SPB) and/or microtubule functions. Four mutants defective in both nuclear fusion (karyogamy) and chromosome maintenance were isolated; these mutants termed cik (for chromosome instability and karyogamy) define three complementation groups. The CIK1 gene was cloned and characterized. Sequence analysis of the CIK1 gene predicts that the CIK1 protein is 594 amino acids in length and possesses a central 300-amino-acid coiled-coil domain. Two different CIK1-beta-galactosidase fusions localize to the SPB region in vegetative cells, and antibodies against the authentic protein detect CIK1 in the SPB region of alpha-factor-treated cells. Evaluation of cells deleted for CIK1 (cik1-delta) indicates that CIK1 is important for the formation or maintenance of a spindle apparatus. Longer and slightly more microtubule bundles are visible in cik1-delta strains than in wild type. Thus, CIK1 encodes a SPB-associated component that is important for proper organization of microtubule arrays and the establishment of a spindle during vegetative growth. Furthermore, the CIK1 gene is essential for karyogamy, and the level of the CIK1 protein at the SPB appears to be dramatically induced by alpha-factor treatment. These results indicate that molecular changes occur at the microtubule-organizing center (MTOC) as the yeast cell prepares for karyogamy and imply that specialization of the MTOC or its associated microtubules occurs in preparation for particular microtubule functions in the yeast life cycle.

pp39mos is associated with p34cdc2 kinase in c-mosxe-transformed NIH 3T3 cells

We investigated the possible interactions between pp39mos and p34cdc2 kinase in NIH 3T3 cells transformed by c-mosxe. pp39mos is coprecipitated with p34cdc2 when using either anti-PSTAIR antibody or p13suc1-Sepharose beads. Likewise, p34cdc2 is coprecipitated with pp39mos when using anti-mos antibody. However, pp39mos was not present in histone H1 kinase-active p34cdc2 complexes precipitated with anti-p34cdc2 C-terminal peptide antibody even during metaphase of the cell cycle. The molar ratio of p34 to pp39mos in the p13suc1 complex is approximately 2:1. Consistent with the tight association between pp39mos and tubulin, tubulin was also present in equivalent amounts with pp39mos and p34 in the p13suc1 complex. This pp39mos-p34cdc2-tubulin complex may be important in transformation by the mos oncogene.

The cell cycle then and now

In the last few years a general model of cell cycle control has been established for all eukaryotic cells. Experiments from a variety of organisms and from a variety of experimental approaches have identified a protein kinase and its unstable regulatory subunit as the activator of mitosis; related molecules seem to be involved in the activation of chromosome replication. The identification of the biochemical components of these important regulatory pathways is providing several new insights into homeostatic and developmental control mechanisms in higher organisms.

Kinesin-related proteins required for assembly of the mitotic spindle

We identified two new Saccharomyces cerevisiae kinesin-related genes, KIP1 and KIP2, using polymerase chain reaction primers corresponding to highly conserved regions of the kinesin motor domain. Both KIP proteins are expressed in vivo, but deletion mutations conferred no phenotype. Moreover, kip1 kip2 double mutants and a triple mutant with kinesin-related kar3 had no synthetic phenotype. Using a genetic screen for mutations that make KIP1 essential, we identified another gene, KSL2, which proved to be another kinesin-related gene, CIN8. KIP1 and CIN8 are functionally redundant: double mutants arrested in mitosis whereas the single mutants did not. The microtubule organizing centers of arrested cells were duplicated but unseparated, indicating that KIP1 or CIN8 is required for mitotic spindle assembly. Consistent with this role, KIP1 protein was found to colocalize with the mitotic spindle.

Yeast calmodulin: structural and functional elements essential for the cell cycle

The budding yeast Saccharomyces cerevisiae is a suitable organism for studying calmodulin function in cell proliferation. Genetic studies in yeast demonstrate that vertebrate calmodulin can functionally replace yeast calmodulin. In addition, expression of half of the yeast calmodulin molecule is found to be sufficient for cell growth. Characterization of conditional-lethal mutants of yeast calmodulin as well as the intracellular distribution of calmodulin have suggested that at least two cell cycle steps require calmodulin function. One is nuclear division and the other is the maintenance of cell polarity. A current focus is to understand which kinds of target proteins are involved in mediating the essential functions of yeast calmodulin in these processes. Thus far, three yeast enzymes whose activity is regulated by calmodulin have been identified.

The rad3+ gene of Schizosaccharomyces pombe is involved in multiple checkpoint functions and in DNA repair

A number of important molecular checkpoints are believed to control the orderly progression of cell cycle events. We have found that the radiation-sensitive Schizosaccharomyces pombe mutant rad3-136 is deficient in two molecular checkpoint functions. Unlike wild-type cells, the mutant cells are unable to arrest in the G2 phase of the cell cycle after DNA damage by gamma-irradiation and are also incapable of maintaining the dependence of mitosis upon the completion of DNA synthesis. An S. pombe genomic clone that complements the UV sensitivity of the rad3-136 mutant completely restores the missing checkpoint functions. The rad3+ gene is also likely to play a role in DNA repair.

Drugs affecting microtubule dynamics increase alpha-tubulin mRNA accumulation via transcription in Tetrahymena thermophila

In cultured mammalian cells, an increase in the amount of tubulin monomer due to treatment with a microtubule-depolymerizing agent results in a rapid decline in tubulin synthesis. This autoregulatory response is mediated through a posttranscriptional mechanism which decreases the stability of tubulin message with no change in transcriptional activity of tubulin genes. Conversely, treatment with a microtubule-polymerizing drug, such as taxol, results in a slight increase in the synthesis of tubulin. Surprisingly, we find that two microtubule-depolymerizing agents, colchicine and oryzalin, actually cause an increase in alpha-tubulin synthesis and alpha-tubulin message in starved Tetrahymena thermophila. This increase is paralleled by an increase in transcription of alpha-tubulin sequences measured by run-on transcription, while the half-life of tubulin message measured by decay in the presence of actinomycin D does not change appreciably. Treatment of starved cells with taxol also produces an increase in alpha-tubulin synthesis via an increase in message abundance due to an increase in transcription of the alpha-tubulin gene. These results indicate that tubulin synthesis in T. thermophila is regulated very differently than in cultured mammalian cells.

Drugs affecting microtubule dynamics increase alpha-tubulin mRNA accumulation via transcription in Tetrahymena thermophila

In cultured mammalian cells, an increase in the amount of tubulin monomer due to treatment with a microtubule-depolymerizing agent results in a rapid decline in tubulin synthesis. This autoregulatory response is mediated through a posttranscriptional mechanism which decreases the stability of tubulin message with no change in transcriptional activity of tubulin genes. Conversely, treatment with a microtubule-polymerizing drug, such as taxol, results in a slight increase in the synthesis of tubulin. Surprisingly, we find that two microtubule-depolymerizing agents, colchicine and oryzalin, actually cause an increase in alpha-tubulin synthesis and alpha-tubulin message in starved Tetrahymena thermophila. This increase is paralleled by an increase in transcription of alpha-tubulin sequences measured by run-on transcription, while the half-life of tubulin message measured by decay in the presence of actinomycin D does not change appreciably. Treatment of starved cells with taxol also produces an increase in alpha-tubulin synthesis via an increase in message abundance due to an increase in transcription of the alpha-tubulin gene. These results indicate that tubulin synthesis in T. thermophila is regulated very differently than in cultured mammalian cells.

Mitotic checkpoints

Entry into mitosis is triggered by activation of maturation promoting factor and a complex of p34cdc2 kinase and cyclin B. Activation induces nuclear lamina breakdown, chromosome condensation and mitotic spindle assembly. Exit from mitosis is initiated by the degradation of cyclin B and the subsequent inactivation of maturation-promoting factor. A more thorough understanding of the checkpoints for initiation of and exit from mitosis has evolved during the past few years.

2-Aminopurine overrides multiple cell cycle checkpoints in BHK cells

BHK cells blocked at any of several points in the cell cycle override their drug-induced arrest and proceed in the cycle when exposed concurrently to the protein kinase inhibitor 2-aminopurine (2-AP). For cells arrested at various points in interphase, 2-AP-induced cell cycle progression is made evident by arrival of the drug-treated cell population in mitosis. Cells that have escaped from mimosine G1 arrest, from hydroxyurea or aphidicolin S-phase arrest, or from VM-26-induced G2 arrest subsequently have all the hallmarks of mitosis--such as a mitotic microtubule array, nuclear envelope breakdown, and chromatin condensation. In a synchronous population, the time course of arrival in mitosis and its duration in 2-AP-treated cells that have escaped drug-induced cell cycle blocks is indistinguishable from control cells. Cells arrested in mitosis by nocodazole or taxol quickly escape mitotic arrest and enter interphase when exposed to 2-AP. 2-AP by itself does not influence the timing of cell cycle progression. We conclude that 2-AP acts to override checkpoints in every phase of the cell cycle, perhaps by inhibiting a protein kinase responsible for control of multiple cell cycle checkpoints.