The dynamics of chromosome movement in the budding yeast Saccharomyces cerevisiae

A Yeast Mitotic Tale for the Nucleus and the Vacuoles to Embrace

The morphology of the nucleus is roughly spherical in most eukaryotic cells. However, this organelle shape needs to change as the cell travels through narrow intercellular spaces during cell migration and during cell division in organisms that undergo closed mitosis, i.e., without dismantling the nuclear envelope, such as yeast. In addition, the nuclear morphology is often modified under stress and in pathological conditions, being a hallmark of cancer and senescent cells. Thus, understanding nuclear morphological dynamics is of uttermost importance, as pathways and proteins involved in nuclear shaping can be targeted in anticancer, antiaging, and antifungal therapies. Here, we review how and why the nuclear shape changes during mitotic blocks in yeast, introducing novel data that associate these changes with both the nucleolus and the vacuole. Altogether, these findings suggest a close relationship between the nucleolar domain of the nucleus and the autophagic organelle, which we also discuss here. Encouragingly, recent evidence in tumor cell lines has linked aberrant nuclear morphology to defects in lysosomal function.

CDC20: a novel therapeutic target in cancer

Cell division cycle protein 20 (Cdc20) is a member of the cell cyclin family. In the early stage of mitosis, it activates the anaphase-promoting complex (APC) and forms the E3 ubiquitin ligase complex APC^Cdc20, which destroys key regulators of the cell cycle and promotes mitosis. Cdc20 serves as a target for the spindle checkpoint, ensuring proper chromosome segregation. As an oncoprotein, Cdc20 is highly expressed in a variety of malignant tumors, and Cdc20 overexpression is associated with poor prognosis of these tumors. This review aims to dissect the tumorigenic role of Cdc20 in human malignancies and its targeting strategies.

The vacuole shapes the nucleus and the ribosomal DNA loop during mitotic delays

Chromosome structuring and condensation is one of the main features of mitosis. Here, Matos-Perdomo et al show how the nuclear envelope reshapes around the vacuole to give rise to the outstanding ribosomal DNA loop in budding yeast.

The ribosomal DNA (rDNA) array of Saccharomyces cerevisiae has served as a model to address chromosome organization. In cells arrested before anaphase (mid-M), the rDNA acquires a highly structured chromosomal organization referred to as the rDNA loop, whose length can double the cell diameter. Previous works established that complexes such as condensin and cohesin are essential to attain this structure. Here, we report that the rDNA loop adopts distinct presentations that arise as spatial adaptations to changes in the nuclear morphology triggered during mid-M arrests. Interestingly, the formation of the rDNA loop results in the appearance of a space under the loop (SUL) which is devoid of nuclear components yet colocalizes with the vacuole. We show that the rDNA-associated nuclear envelope (NE) often reshapes into a ladle to accommodate the vacuole in the SUL, with the nucleus becoming bilobed and doughnut-shaped. Finally, we demonstrate that the formation of the rDNA loop and the SUL require TORC1, membrane synthesis and functional vacuoles, yet is independent of nucleus–vacuole junctions and rDNA-NE tethering.

Production of nascent ribosome precursors within the nucleolar microenvironment of Saccharomyces cerevisiae

35S rRNA transcripts include a 5'-external transcribed spacer followed by rRNAs of the small and large ribosomal subunits. Their processing yields massive precursors that include dozens of assembly factor proteins. In Saccharomyces cerevisiae, nucleolar assembly factors form 2 coaxial layers/volumes around ribosomal DNA. Most of these factors are cyclically recruited from a latent state to an operative state, and are extensively conserved. The layers match, at least approximately, known subcompartments found in higher eukaryotic cells. ∼80% of assembly factors are essential. The number of copies of these assembly factors is comparable to the number of nascent transcripts. Moreover, they exhibit "isoelectric balance," with RNA-binding candidate "nucleator" assembly factors being notably basic. The physical properties of pre-small subunit and pre-large subunit assembly factors are similar, as are their 19 motif signatures detected by hierarchical clustering, unlike motif signatures of the 5'-external transcribed spacer rRNP. Additionally, many assembly factors lack shared motifs. Taken together with the progression of rRNP composition during subunit maturation, and the realization that the ribosomal DNA cable is initially bathed in a subunit-nonspecific assembly factor reservoir/microenvironment, we propose a "3-step subdomain assembly model": Step (1): predominantly basic assembly factors sequentially nucleate sites along nascent rRNA; Step (2): the resulting rRNPs recruit numerous less basic assembly factors along with notably basic ribosomal proteins; Step (3): rRNPs in nearby subdomains consolidate. Cleavages of rRNA then promote release of rRNPs to the nucleoplasm, likely facilitated by the persistence of assembly factors that were already associated with nucleolar precursors.

Cell cycle arrest of S. cerevisiae in conjunction with labeling of the cell wall

S. cerevisiae can be arrested in metaphase by depleting Cdc20. We describe (1) how to achieve this arrest and verify it, (2) how to label cell surface glycans covalently to distinguish mother from bud, and (3) how to detect the nucleolus and learn that it remains in the mother domain upon arrest. For complete details on the use and execution of this protocol, please refer to Tartakoff et al. (2021), Rai et al. (2017), and Zapanta Rinonos et al. (2014).

The nucleolus as a polarized coaxial cable in which the rDNA axis is surrounded by dynamic subunit-specific phases

In ribosomal DNA (rDNA) repeats, sequences encoding small-subunit (SSU) rRNA precede those encoding large-subunit (LSU) rRNAs. Processing the composite transcript and subunit assembly requires >100 subunit-specific nucleolar assembly factors (AFs). To investigate the functional organization of the nucleolus, we localized AFs in S. cerevisiae in which the rDNA axis was "linearized" to reduce its dimensionality, thereby revealing its coaxial organization. In this situation, rRNA synthesis and processing continue. The axis is embedded in an inner layer/phase of SSU AFs that is surrounded by an outer layer/phase of LSU AFs. When subunit production is inhibited, subsets of AFs differentially relocate between the inner and outer layers, as expected if there is a cycle of repeated relocation whereby "latent" AFs become "operative" when recruited to nascent subunits. Recognition of AF cycling and localization of segments of rRNA make it possible to infer the existence of assembly intermediates that span between the inner and outer layers and to chart the cotranscriptional assembly of each subunit. AF cycling also can explain how having more than one protein phase in the nucleolus makes possible "vectorial 2-phase partitioning" as a driving force for relocation of nascent rRNPs. Because nucleoplasmic AFs are also present in the outer layer, we propose that critical surface remodeling occurs at this site, thereby partitioning subunit precursors into the nucleoplasm for post-transcriptional maturation. Comparison to observations on higher eukaryotes shows that the coaxial paradigm is likely to be applicable for the many other organisms that have rDNA repeats.

DNA damage checkpoint activation impairs chromatin homeostasis and promotes mitotic catastrophe during aging

Genome instability is a hallmark of aging and contributes to age-related disorders such as cancer and Alzheimer's disease. The accumulation of DNA damage during aging has been linked to altered cell cycle dynamics and the failure of cell cycle checkpoints. Here, we use single cell imaging to study the consequences of increased genomic instability during aging in budding yeast and identify striking age-associated genome missegregation events. This breakdown in mitotic fidelity results from the age-related activation of the DNA damage checkpoint and the resulting degradation of histone proteins. Disrupting the ability of cells to degrade histones in response to DNA damage increases replicative lifespan and reduces genomic missegregations. We present several lines of evidence supporting a model of antagonistic pleiotropy in the DNA damage response where histone degradation, and limited histone transcription are beneficial to respond rapidly to damage but reduce lifespan and genomic stability in the long term.

The mode of mitosis is dramatically modified by deletion of a single nuclear pore complex gene in Aspergillus nidulans

Nuclear pore complex (NPC) proteins (Nups) play multiple roles during mitosis. In this study we expand these roles and reveal that in Aspergillus nidulans, compromising the core Nup84-120 subcomplex of the NPC modifies the mitotic behavior of the nuclear envelope (NE). In wildtype cells, the NE undergoes simultaneous double pinching events to separate daughter nuclei during mitotic exit, whereas in Nup84-120 complex mutants, only one restriction of the NE is observed. Investigating the basis for this modified behavior of the NE in Nup deleted cells uncovered previously unrealized roles for core Nups in mitotic exit. During wildtype anaphase, the NE surrounds the two separating daughter DNA masses which typically flank the central nucleolus, to form three distinct nuclear compartments. In contrast, deletion of core Nups frequently results in early nucleolar eviction from the mitotic nucleus, in turn causing an uncharacteristic dumbbell-shaped NE morphology of anaphase nuclei with a nuclear membrane bridge connecting the two forming G1 nuclei. Importantly, the absence of the nucleolus between the separating daughter nuclei during anaphase delays chromosome segregation and progression into G1 as nuclei remain connected by chromatin bridges. Proteins localizing to late segregating chromosome arms are observed between forming daughter nuclei, and the mitotic spindle fails to resolve in a timely manner. These chromatin bridges are occupied by the Aurora kinase until nuclei have fully separated, suggesting involvement of Aurora in monitoring mitotic spindle and nuclear membrane resolution during mitotic exit. Our findings thus reveal a novel requirement for core Nups in mediating nucleolar positioning during mitosis, which dictates the pattern of NE fissions during karyokinesis and facilitates normal chromosome segregation. The findings additionally demonstrate that the mode of mitosis can be dramatically modified by deletion of a single NPC gene and reveals surprising fluidity in mitotic mechanisms.

Developmental regulation of an organelle tether coordinates mitochondrial remodeling in meiosis

Cellular differentiation involves remodeling cellular architecture to transform one cell type to another. By investigating mitochondrial dynamics during meiotic differentiation in budding yeast, we sought to understand how organelle morphogenesis is developmentally controlled in a system where regulators of differentiation and organelle architecture are known, but the interface between them remains unexplored. We analyzed the regulation of mitochondrial detachment from the cell cortex, a known meiotic alteration to mitochondrial morphology. We found that mitochondrial detachment is enabled by the programmed destruction of the mitochondria-endoplasmic reticulum-cortex anchor (MECA), an organelle tether that bridges mitochondria and the plasma membrane. MECA regulation is governed by a meiotic transcription factor, Ndt80, which promotes the activation of a conserved kinase, Ime2. We further present evidence for Ime2-dependent phosphorylation and degradation of MECA in a temporally controlled manner. Our study defines a key mechanism that coordinates mitochondrial morphogenesis with the landmark events of meiosis and demonstrates that cells can developmentally regulate tethering to induce organelle remodeling.

Nucleolar asymmetry and the importance of septin integrity upon cell cycle arrest

Cell cycle arrest can be imposed by inactivating the anaphase promoting complex (APC). In S. cerevisiae this arrest has been reported to stabilize a metaphase-like intermediate in which the nuclear envelope spans the bud neck, while chromatin repeatedly translocates between the mother and bud domains. The present investigation was undertaken to learn how other features of nuclear organization are affected upon depletion of the APC activator, Cdc20. We observe that the spindle pole bodies and the spindle repeatedly translocate across the narrow orifice at the level of the neck. Nevertheless, we find that the nucleolus (organized around rDNA repeats on the long right arm of chromosome XII) remains in the mother domain, marking the polarity of the nucleus. Accordingly, chromosome XII is polarized: TelXIIR remains in the mother domain and its centromere is predominantly located in the bud domain. In order to learn why the nucleolus remains in the mother domain, we studied the impact of inhibiting rRNA synthesis in arrested cells. We observed that this fragments the nucleolus and that these fragments entered the bud domain. Taken together with earlier observations, the restriction of the nucleolus to the mother domain therefore can be attributed to its massive structure. We also observed that inactivation of septins allowed arrested cells to complete the cell cycle, that the alternative APC activator, Cdh1, was required for completion of the cell cycle and that induction of Cdh1 itself caused arrested cells to progress to the end of the cell cycle.

Daucosterol promotes the proliferation of neural stem cells

Neural stem cells (NSCs) are self-regenerating cells, but their regenerative capacity is limited. The present study was conducted to investigate the effect of daucosterol (a sterolin) on the promotion of NSC proliferation and determine the corresponding molecular mechanism. Results of cell counting kit-8 (CCK-8) assay showed that daucosterol significantly increased the quantity of viable cells and the effectiveness of daucosterol was similar to that of basic fibroblast growth factor (bFGF) and epidermal growth factor (EGF). Flow cytometry detection of CFSE-labeled (CFSE, carboxyfluorescein diacetate succinimidyl ester) NSCs showed that Div Index (or the average number of cell divisions) and % Divided (or the percentage of cells that divided at least once) of the cells were increased, indicating that daucosterol increased the percentage of NSCs re-entering the cell cycle. mRNA microarray analysis showed that 333 genes that are mostly involved in the mitotic cell cycle were up-regulated. By contrast, 627 genes that are mostly involved in differentiation were down-regulated. In particular, insulin-like growth factor I (IGF1) was considered as an important regulatory gene that functionally promoted NSC proliferation, and the increased expression of IGF1 protein was validated by ELISA. In addition, the phosphorylation of AKT was increased, indicating that the proliferation-enhancing activity of daucosterol may be involved in IGF1-AKT pathway. Our study provided information about daucosterol as an efficient and inexpensive growth factor alternative that could be used in clinical medicine and research applications.

The molecular chaperone Hsp90 is required for cell cycle exit in Drosophila melanogaster

The coordination of cell proliferation and differentiation is crucial for proper development. In particular, robust mechanisms exist to ensure that cells permanently exit the cell cycle upon terminal differentiation, and these include restraining the activities of both the E2F/DP transcription factor and Cyclin/Cdk kinases. However, the full complement of mechanisms necessary to restrain E2F/DP and Cyclin/Cdk activities in differentiating cells are not known. Here, we have performed a genetic screen in Drosophila melanogaster, designed to identify genes required for cell cycle exit. This screen utilized a PCNA-miniwhite⁺ reporter that is highly E2F-responsive and results in a darker red eye color when crossed into genetic backgrounds that delay cell cycle exit. Mutation of Hsp83, the Drosophila homolog of mammalian Hsp90, results in increased E2F-dependent transcription and ectopic cell proliferation in pupal tissues at a time when neighboring wild-type cells are postmitotic. Further, these Hsp83 mutant cells have increased Cyclin/Cdk activity and accumulate proteins normally targeted for proteolysis by the anaphase-promoting complex/cyclosome (APC/C), suggesting that APC/C function is inhibited. Indeed, reducing the gene dosage of an inhibitor of Cdh1/Fzr, an activating subunit of the APC/C that is required for timely cell cycle exit, can genetically suppress the Hsp83 cell cycle exit phenotype. Based on these data, we propose that Cdh1/Fzr is a client protein of Hsp83. Our results reveal that Hsp83 plays a heretofore unappreciated role in promoting APC/C function during cell cycle exit and suggest a mechanism by which Hsp90 inhibition could promote genomic instability and carcinogenesis.

Cells must permanently stop dividing when they terminally differentiate for development to occur normally. Maintenance of this postmitotic state is also important, as unscheduled proliferation of differentiated cells can result in cancer. To identify genes important for restraining cell proliferation during terminal differentiation, we performed a genetic screen in Drosophila and found that mutation of Hsp90 caused ectopic cell proliferation in differentiating tissues. Hsp90 is a molecular chaperone that is essential for viability in all eukaryotes and has been shown to facilitate the activity of hundreds of “client” proteins. Indeed, several inhibitors of Hsp90 are currently being tested in clinical trials for use as anti-cancer therapeutics due to their ability to silence multiple client oncoproteins simultaneously. Our data suggest that Hsp90 is necessary to halt cell proliferation during differentiation because the protein Cdh1, which is required for normal cell cycle exit, may be a client of Hsp90. As reduced Cdh1 function results in genomic instability and tumorigenesis, our work highlights the need to design more precisely targeted Hsp90 inhibitors for use as cancer treatments.

An auxiliary, membrane-based mechanism for nuclear migration in budding yeast

The preanaphase nucleus of budding yeast deforms and builds protrusions into the bud. Formation of the nuclear protrusions requires membrane growth and DNA replication. Nuclear protrusions anchor the nuclear envelope to the cortical ER in an actin- and exocyst-dependent manner, facilitating spindle positioning relative to the cleavage apparatus.

How nuclear shape correlates with nuclear movements during the cell cycle is poorly understood. We investigated changes in nuclear morphology during nuclear migration in budding yeast. In preanaphase cells, nuclear protrusions (nucleopodia [NP]) extend into the bud, preceding insertion of chromosomes into the bud neck. Surprisingly, formation of nucleopodia did not depend on the established nuclear migration pathways. We show that generation and maintenance of NP requires nuclear membrane expansion, actin, and the exocyst complex. Exocyst mutations cause nuclear positioning defects and display genetic interactions with mutations that deactivate astral microtubule-dependent nuclear migration. Cells that cannot perform DNA replication also fail to form nucleopodia. We propose that nuclear membrane expansion, DNA replication, and exocyst-dependent anchoring of the nuclear envelope to the bud affect nuclear morphology and facilitate correct positioning of nucleus and chromosomes relative to the cleavage apparatus.

Mitotic spindle form and function

The Saccharomyces cerevisiae mitotic spindle in budding yeast is exemplified by its simplicity and elegance. Microtubules are nucleated from a crystalline array of proteins organized in the nuclear envelope, known as the spindle pole body in yeast (analogous to the centrosome in larger eukaryotes). The spindle has two classes of nuclear microtubules: kinetochore microtubules and interpolar microtubules. One kinetochore microtubule attaches to a single centromere on each chromosome, while approximately four interpolar microtubules emanate from each pole and interdigitate with interpolar microtubules from the opposite spindle to provide stability to the bipolar spindle. On the cytoplasmic face, two to three microtubules extend from the spindle pole toward the cell cortex. Processes requiring microtubule function are limited to spindles in mitosis and to spindle orientation and nuclear positioning in the cytoplasm. Microtubule function is regulated in large part via products of the 6 kinesin gene family and the 1 cytoplasmic dynein gene. A single bipolar kinesin (Cin8, class Kin-5), together with a depolymerase (Kip3, class Kin-8) or minus-end-directed kinesin (Kar3, class Kin-14), can support spindle function and cell viability. The remarkable feature of yeast cells is that they can survive with microtubules and genes for just two motor proteins, thus providing an unparalleled system to dissect microtubule and motor function within the spindle machine.

Mitotic exit in the absence of separase activity

In budding yeast, three interdigitated pathways regulate mitotic exit (ME): mitotic cyclin-cyclin-dependent kinase (Cdk) inactivation; the Cdc14 early anaphase release (FEAR) network, including a nonproteolytic function of separase (Esp1); and the mitotic exit network (MEN) driven by interaction between the spindle pole body and the bud cortex. Here, we evaluate the contributions of these pathways to ME kinetics. Reducing Cdk activity is critical for ME, and the MEN contributes strongly to ME efficiency. Esp1 contributes to ME kinetics mainly through cohesin cleavage: the Esp1 requirement can be largely bypassed if cells are provided Esp1-independent means of separating sister chromatids. In the absence of Esp1 activity, we observed only a minor ME delay consistent with a FEAR defect. Esp1 overexpression drives ME in Cdc20-depleted cells arrested in metaphase. We have found that this activity of overexpressed Esp1 depended on spindle integrity and the MEN. We defined the first quantitative measure for Cdc14 release based on colocalization with the Net1 nucleolar anchor. This measure indicates efficient Cdc14 release upon MEN activation; release driven by Esp1 in the absence of microtubules was inefficient and incapable of driving ME. We also found a novel role for the MEN: activating Cdc14 nuclear export, even in the absence of Net1.

The yeast DNA damage checkpoint proteins control a cytoplasmic response to DNA damage

A single HO endonuclease-induced double-strand break (DSB) is sufficient to activate the DNA damage checkpoint and cause Saccharomyces cells to arrest at G(2)/M for 12-14 h, after which cells adapt to the presence of the DSB and resume cell cycle progression. The checkpoint signal leading to G(2)/M arrest was previously shown to be nuclear-limited. Cells lacking ATR-like Mec1 exhibit no DSB-induced cell cycle delay; however, cells lacking Mec1's downstream protein kinase targets, Rad53 or Chk1, still have substantial G(2)/M delay, as do cells lacking securin, Pds1. This delay is eliminated only in the triple mutant chk1Delta rad53Delta pds1Delta, suggesting that Rad53 and Chk1 control targets other than the stability of securin in enforcing checkpoint-mediated cell cycle arrest. The G(2)/M arrest in rad53Delta and chk1Delta revealed a unique cytoplasmic phenotype in which there are frequent dynein-dependent excursions of the nucleus through the bud neck, without entering anaphase. Such excursions are infrequent in wild-type arrested cells, but have been observed in cells defective in mitotic exit, including the semidominant cdc5-ad mutation. We suggest that Mec1-dependent checkpoint signaling through Rad53 and Chk1 includes the repression of nuclear movements that are normally associated with the execution of anaphase.

Deficiency of centromere-associated protein Slk19 causes premature nuclear migration and loss of centromeric elasticity

The cohesin complex prevents premature segregation of duplicated chromosomes by providing resistance to the pole-ward pull by spindle microtubules. The centromeric region (or sister kinetochores) bears the majority of this force and undergoes transient separation prior to anaphase, indicative of its elastic nature. A cysteine protease, separase, cleaves the cohesin subunit Scc1 and dissolves cohesion between sister chromatids, initiating their separation. Separase also cleaves the kinetochore protein Slk19 during anaphase. Slk19 has been implicated in stabilization of the mitotic spindle and regulation of mitotic exit, but it is not known what role it plays at the kinetochores. We show that during pre-anaphase arrest, the spindle in slk19Delta cells is excessively dynamic and the nuclei move into mother-daughter junction prematurely. As a result, the chromatin mass undergoes partial division that requires neither anaphase promoting complex (APC) activity nor Scc1 cleavage. Partial division of the chromatin mass is accompanied by the loss of the centromeric region's ability to resist pole-ward pull by the spindle. Slk19 physically associates with Scc1 and this association appears necessary for efficient cleavage of Slk19 by separase. Our results suggest that Slk19 participates in regulating nuclear migration and, in conjunction with cohesin complex, may be involved in the maintenance of centromeric tensile strength to resist the pole-ward pull.

Model of chromosome motility in Drosophila embryos: adaptation of a general mechanism for rapid mitosis

During mitosis, ensembles of dynamic MTs and motors exert forces that coordinate chromosome segregation. Typically, chromosomes align at the metaphase spindle equator where they oscillate along the pole-pole axis before disjoining and moving poleward during anaphase A, but spindles in different cell types display differences in MT dynamicity, in the amplitude of chromosome oscillations and in rates of chromatid-to-pole motion. Drosophila embryonic mitotic spindles, for example, display remarkably dynamic MTs, barely detectable metaphase chromosome oscillations, and a rapid rate of "flux-pacman-dependent" anaphase chromatid-to-pole motility. Here we develop a force-balance model that describes Drosophila embryo chromosome motility in terms of a balance of forces acting on kinetochores and kMTs that is generated by multiple polymer ratchets and mitotic motors coupled to tension-dependent kMT dynamics. The model shows that i), multiple MTs displaying high dynamic instability can drive steady and rapid chromosome motion; ii), chromosome motility during metaphase and anaphase A can be described by a single mechanism; iii), high kinetochore dynein activity is deployed to dampen metaphase oscillations, to augment the basic flux-pacman mechanism, and to drive rapid anaphase A; iv), modulation of the MT rescue frequency by the kinetochore-associated kinesin-13 depolymerase promotes metaphase chromosome oscillations; and v), this basic mechanism can be adapted to a broad range of spindles.

Yeast nuclear envelope subdomains with distinct abilities to resist membrane expansion

Little is known about what dictates the round shape of the yeast Saccharomyces cerevisiae nucleus. In spo7Delta mutants, the nucleus is misshapen, exhibiting a single protrusion. The Spo7 protein is part of a phosphatase complex that represses phospholipid biosynthesis. Here, we report that the nuclear protrusion of spo7Delta mutants colocalizes with the nucleolus, whereas the nuclear compartment containing the bulk of the DNA is unaffected. Using strains in which the nucleolus is not intimately associated with the nuclear envelope, we show that the single nuclear protrusion of spo7Delta mutants is not a result of nucleolar expansion, but rather a property of the nuclear membrane. We found that in spo7Delta mutants the peripheral endoplasmic reticulum (ER) membrane was also expanded. Because the nuclear membrane and the ER are contiguous, this finding indicates that in spo7Delta mutants all ER membranes, with the exception of the membrane surrounding the bulk of the DNA, undergo expansion. Our results suggest that the nuclear envelope has distinct domains that differ in their ability to resist membrane expansion in response to increased phospholipid biosynthesis. We further propose that in budding yeast there is a mechanism, or structure, that restricts nuclear membrane expansion around the bulk of the DNA.

The DNA damage checkpoint and PKA pathways converge on APC substrates and Cdc20 to regulate mitotic progression

The conserved checkpoint kinases Chk1 and Rad53-Dun1 block the metaphase to anaphase transition by the phosphorylation and stabilization of securin, and block the mitotic exit network regulated by the Bfa1-Bub2 complex. However, both chk1 and rad53 mutants are able to exit from mitosis and initiate a new cell cycle, suggesting that both pathways have supporting functions in restraining anaphase and in blocking the inactivation of mitotic cyclin-Cdk1 complexes. Here we find that the cyclic-AMP-dependent protein kinase (PKA) pathway supports Chk1 in the regulation of mitosis by targeting the mitotic inducer Cdc20. Cdc20 is phosphorylated on PKA consensus sites after DNA damage, and this phosphorylation requires the Atr orthologue Mec1 and the PKA catalytic subunits Tpk1 and Tpk2. We show that the inactivation of PKA or expression of phosphorylation-defective Cdc20 proteins accelerates securin and Clb2 destruction in chk1 mutants and is sufficient to remove most of the DNA damage-induced delay. Mutation of the Cdc20 phosphorylation sites permitted the interaction of Cdc20 with Clb2 under conditions that should halt cell cycle progression. These data show that PKA pathways regulate mitotic progression through Cdc20 and support the DNA damage checkpoint pathways in regulating the destruction of Clb2 and securin.

Subcellular localization of yeast ribonucleotide reductase regulated by the DNA replication and damage checkpoint pathways

The fidelity of DNA replication and repair processes is critical for maintenance of genomic stability. Ribonucleotide reductase (RNR) catalyzes the rate-limiting step in dNTP production and thus plays an essential role in DNA synthesis. The level and activity of RNR are highly regulated by the cell cycle and DNA damage checkpoints, which maintain optimal dNTP pools required for genetic fidelity. RNRs are composed of a large subunit that binds the nucleoside diphosphate substrates and allosteric effectors and a small subunit that houses the di-iron tyrosyl radical cofactor essential for the reduction process. In Saccharomyces cerevisiae, there are two large subunits (Rnr1 and Rnr3) and two small subunits (Rnr2 and Rnr4). Here we report the subcellular localization of Rnr1-4 during normal cell growth and the redistribution of Rnr2 and Rnr4 in response to DNA damage and replicational stress. During the normal cell cycle, Rnr1 and Rnr3 are predominantly localized to the cytoplasm and Rnr2 and Rnr4 are predominantly present in the nucleus. Under genotoxic stress, Rnr2 and Rnr4 become redistributed to the cytoplasm in a checkpoint-dependent manner. Subcellular redistribution of Rnr2 and Rnr4 can occur in the absence of the transcriptional induction of the RNR genes after DNA damage and likely represents a posttranslational event. These results suggest a mechanism by which DNA damage checkpoint modulates RNR activity through the temporal and spatial regulation of its subunits.

Nuclear oscillations and nuclear filament formation accompany single-strand annealing repair of a dicentric chromosome in Saccharomyces cerevisiae

Dicentric chromosomes undergo breakage during mitosis as a result of the attachment of two centromeres on one sister chromatid to opposite spindle poles. Studies utilizing a conditional dicentric chromosome III in Saccharomyces cerevisiae have shown that dicentric chromosome repair occurs primarily by deletion of one centromere via a RAD52-dependent recombination pathway. We report that dicentric chromosome resolution requires RAD1, a gene involved in the single-strand annealing DNA repair pathway. We additionally show that single-strand annealing repair of a dicentric chromosome can occur in the absence of RAD52. RAD52-independent repair requires the adaptation-defective cdc5-ad allele of the yeast polo kinase and the DNA damage checkpoint gene RAD9. Dicentric chromosome breakage in cdc5-ad rad52 mutant cells is associated with a prolonged mitotic arrest, during which nuclei undergo microtubule-dependent oscillations, accompanied by dynamic changes in nuclear morphology. We further demonstrate that the frequency of spontaneous direct repeat recombination is suppressed in yeast cells treated with benomyl, a drug that perturbs microtubules. Our findings indicate that microtubule-dependent processes facilitate recombination.

Mutation in the beta-tubulin signature motif suppresses microtubule GTPase activity and dynamics, and slows mitosis

We introduced a threonine-to-glycine point mutation at position 143 in the "tubulin signature motif" 140Gly-Gly-Gly-Thr-Gly-Ser-Gly146 of Saccharomyces cerevisiae beta-tubulin. In an electron diffraction model of the tubulin dimer, this sequence comes close to the phosphates of a guanine nucleotide bound in the beta-tubulin exchangeable E site. Both the GTP-binding affinity and the microtubule (MT)-dependent GTPase activity of tubulin isolated from haploid tub2-T143G mutant cells were reduced by at least 15-fold, compared to tubulin isolated from control wild-type cells. The growing and shortening dynamics of MTs assembled from alphabeta:Thr143Gly-mutated dimers were also strongly suppressed, compared to control MTs. The in vitro properties of the mutated MTs (slower growing and more stable) are consistent with the effects of the tub2-T143G mutation in haploid cells. The average length of MT spindles in large-budded mutant cells was only 3.7 +/- 0.2 microm, approximately half of the size of MT arrays in large-budded wild-type cells (average length = 7.1 +/- 0.4 microm), suggesting that there is a delay in mitosis in the mutant cells. There was also a higher proportion of large-budded cells with unsegregated nuclei in mutant cultures (30% versus 12% for wild-type cells), again suggesting such a delay. The results show that beta:Thr143 of the tubulin signature motif plays an important role in GTP binding and hydrolysis by the beta-tubulin E site and support the idea that tubulins belong to a family of proteins within the GTPase superfamily that are structurally distinct from the classic GTPases, such as EF-Tu and p21(ras). The data also suggest that MT dynamics are critical for MT function in yeast cells and that spindle MT assembly and disassembly could be coordinated with other cell-cycle events by regulating beta-tubulin GTPase activity.

Pds1p of budding yeast has dual roles: inhibition of anaphase initiation and regulation of mitotic exit

Progression through mitosis is controlled by protein degradation that is mediated by the anaphase-promoting complex/cyclosome (APC/C) and its associated specificity factors. In budding yeast, APC/C(Cdc20) promotes the degradation of the Pds1p anaphase inhibitor at the metaphase-to-anaphase transition, whereas APC/C(Cdh1) promotes the degradation of the mitotic cyclins at the exit from mitosis. Here we show that Pds1p has a novel activity as an inhibitor of mitotic cyclin destruction, apparently by preventing the activation of APC/C(Cdh1). This activity of Pds1p is independent of its activity as an anaphase inhibitor. We propose that the dual role of Pds1p as an inhibitor of anaphase and of cyclin degradation allows the cell to couple the exit from mitosis to the prior completion of anaphase. Finally, these observations provide a novel regulatory paradigm in which the sequential degradation of two substrates is determined by the substrates themselves, such that an early substrate inhibits the degradation of a later one.

Cdc20 associates with the kinase aurora2/Aik

Cdc20/fizzy family proteins are involved in activation of the anaphase-promoting complex/cyclosome, which catalyzes the ubiquitin-dependent proteolysis of cell cycle regulatory proteins such as anaphase inhibitors and mitotic cyclins, leading to chromosome segregation and exit from mitosis. Previous work has shown that human Cdc20 (hCdc20/p55CDC) associates with one or more kinases. We report here that Cdc20-associated myelin basic protein kinase activity peaks sharply in early M phase (embryonic cells) or in G2 phase (somatic cells). In HeLa cells, Cdc20 is associated with the kinase aurora2/Aik. Aurora2/Aik is a member of the aurora/Ipl1 family of kinases that, like Cdc20, previously has been shown to be localized at mitotic spindle poles and is involved in regulating chromosome segregation and maintaining genomic stability. The demonstration that Cdc20 is associated with aurora2/Aik suggests that some function of Cdc20 is carried out or regulated through its association with aurora2/Aik.

Esa1p is an essential histone acetyltransferase required for cell cycle progression

Histones are dynamically modified during chromatin assembly, as specific transcriptional patterns are established, and during mitosis and development. Modifications include acetylation, phosphorylation, ubiquitination, methylation, and ADP-ribosylation, but the biological significance of each of these is not well understood. For example, distinct acetylation patterns correlate with nucleosome formation and with transcriptionally activated or silenced chromatin, yet mutations in genes encoding several yeast histone acetyltransferase (HAT) activities result in either no cellular phenotype or only modest growth defects. Here we report characterization of ESA1, an essential gene that is a member of the MYST family that includes two yeast silencing genes, human genes associated with leukemia and with the human immunodeficiency virus type 1 Tat protein, and Drosophila mof, a gene essential for male dosage compensation. Esa1p acetylates histones in a pattern distinct from those of other yeast enzymes, and temperature-sensitive mutant alleles abolish enzymatic activity in vitro and result in partial loss of an acetylated isoform of histone H4 in vivo. Strains carrying these mutations are also blocked in the cell cycle such that at restrictive temperatures, esa1 mutants succeed in replicating their DNA but fail to proceed normally through mitosis and cytokinesis. Recent studies show that Esa1p enhances transcription in vitro and thus may modulate expression of genes important for cell cycle control. These observations therefore link an essential HAT activity to cell cycle progression, potentially through discrete transcriptional regulatory events.

Control of mitotic spindle position by the Saccharomyces cerevisiae formin Bni1p

Alignment of the mitotic spindle with the axis of cell division is an essential process in Saccharomyces cerevisiae that is mediated by interactions between cytoplasmic microtubules and the cell cortex. We found that a cortical protein, the yeast formin Bni1p, was required for spindle orientation. Two striking abnormalities were observed in bni1Delta cells. First, the initial movement of the spindle pole body (SPB) toward the emerging bud was defective. This phenotype is similar to that previously observed in cells lacking the kinesin Kip3p and, in fact, BNI1 and KIP3 were found to be in the same genetic pathway. Second, abnormal pulling interactions between microtubules and the cortex appeared to cause preanaphase spindles in bni1Delta cells to transit back and forth between the mother and the bud. We therefore propose that Bni1p may localize or alter the function of cortical microtubule-binding sites in the bud. Additionally, we present evidence that other bipolar bud site determinants together with cortical actin are also required for spindle orientation.

The leptotene-zygotene transition of meiosis

The leptotene/zygotene transition of meiosis, as defined by classical cytological studies, is the period when homologous chromosomes, already being discernible individualized entities, begin to be close together or touching over portions of their lengths. This period also includes the bouquet stage: Chromosome ends, which have already become integral components of the inner nuclear membrane, move into a polarized configuration, along with other nuclear envelope components. Chromosome movements, active or passive, also occur. The detailed nature of interhomologue interactions during this period, with special emphasis on the involvement of chromosome ends, and the overall role for meiosis and recombination of chromosome movement and, especially, the bouquet stage are discussed.

Coronin promotes the rapid assembly and cross-linking of actin filaments and may link the actin and microtubule cytoskeletons in yeast

Coronin is a highly conserved actin-associated protein that until now has had unknown biochemical activities. Using microtubule affinity chromatography, we coisolated actin and a homologue of coronin, Crn1p, from Saccharomyces cerevisiae cell extracts. Crn1p is an abundant component of the cortical actin cytoskeleton and binds to F-actin with high affinity (Kd 6 x 10(-9) M). Crn1p promotes the rapid barbed-end assembly of actin filaments and cross-links filaments into bundles and more complex networks, but does not stabilize them. Genetic analyses with a crn1Delta deletion mutation also are consistent with Crn1p regulating filament assembly rather than stability. Filament cross-linking depends on the coiled coil domain of Crn1p, suggesting a requirement for Crn1p dimerization. Assembly-promoting activity is independent of cross-linking and could be due to nucleation and/or accelerated polymerization. Crn1p also binds to microtubules in vitro, and microtubule binding is enhanced by the presence of actin filaments. Microtubule binding is mediated by a region of Crn1p that contains sequences (not found in other coronins) homologous to the microtubule binding region of MAP1B. These activities, considered with microtubule defects observed in crn1Delta cells and in cells overexpressing Crn1p, suggest that Crn1p may provide a functional link between the actin and microtubule cytoskeletons in yeast.

Regulation of Cdc28 cyclin-dependent protein kinase activity during the cell cycle of the yeast Saccharomyces cerevisiae

The cyclin-dependent protein kinase (CDK) encoded by CDC28 is the master regulator of cell division in the budding yeast Saccharomyces cerevisiae. By mechanisms that, for the most part, remain to be delineated, Cdc28 activity controls the timing of mitotic commitment, bud initiation, DNA replication, spindle formation, and chromosome separation. Environmental stimuli and progress through the cell cycle are monitored through checkpoint mechanisms that influence Cdc28 activity at key cell cycle stages. A vast body of information concerning how Cdc28 activity is timed and coordinated with various mitotic events has accrued. This article reviews that literature. Following an introduction to the properties of CDKs common to many eukaryotic species, the key influences on Cdc28 activity-cyclin-CKI binding and phosphorylation-dephosphorylation events-are examined. The processes controlling the abundance and activity of key Cdc28 regulators, especially transcriptional and proteolytic mechanisms, are then discussed in detail. Finally, the mechanisms by which environmental stimuli influence Cdc28 activity are summarized.

Meiotic behaviours of chromosomes and microtubules in budding yeast: relocalization of centromeres and telomeres during meiotic prophase

BACKGROUND

Meiosis is a process of universal importance in eukaryotic organisms, generating variation in the heritable haploid genome by recombination and re-assortment of chromosomes. The intranuclear movement of chromosomes is expected to achieve pairing and recombination of homologous chromosomes during meiosis. Meiosis in the budding yeast Saccharomyces cerevisiae has been extensively studied, both genetically and by molecular biology; here we report cytological observations of meiotic chromosomal events in this organism.

RESULTS

Using fluorescence microscopy, we have examined the behaviour of chromosomes and microtubules during meiosis in S. cerevisiae. We first observed the dynamic behaviour of nuclei in living cells using jellyfish green fluorescent protein (GFP) fused with nucleoplasmin, a Xenopus oocyte nuclear protein. The characterization of nuclear movement in living cells was extended by an analysis of chromosomes and microtubules in fixed specimens. In addition, the nuclear localization of centromeres and telomeres was determined by indirect immunofluorescence microscopy in synchronous populations of meiotic cells. While telomeres remain in clusters of 5-8 throughout meiosis, centromeres change their nuclear localization dramatically during the progression of meiosis: centromeres are first clustered at a single site near the spindle-pole body before the induction of meiosis, and become scattered during the meiotic prophase.

CONCLUSIONS

Our observations have demonstrated that nuclear and cytoskeletal reorganization take place with meiosis in S. cerevisiae. In particular, the distinct relocalization of centromeres during meiosis indicates a considerable movement of chromosomes within the meiotic prophase nucleus.

The yeast dynactin complex is involved in partitioning the mitotic spindle between mother and daughter cells during anaphase B

Although vertebrate cytoplasmic dynein can move to the minus ends of microtubules in vitro, its ability to translocate purified vesicles on microtubules depends on the presence of an accessory complex known as dynactin. We have cloned and characterized a novel gene, NIP100, which encodes the yeast homologue of the vertebrate dynactin complex protein p150(glued). Like strains lacking the cytoplasmic dynein heavy chain Dyn1p or the centractin homologue Act5p, nip100Delta strains are viable but undergo a significant number of failed mitoses in which the mitotic spindle does not properly partition into the daughter cell. Analysis of spindle dynamics by time-lapse digital microscopy indicates that the precise role of Nip100p during anaphase is to promote the translocation of the partially elongated mitotic spindle through the bud neck. Consistent with the presence of a true dynactin complex in yeast, Nip100p exists in a stable complex with Act5p as well as Jnm1p, another protein required for proper spindle partitioning during anaphase. Moreover, genetic depletion experiments indicate that the binding of Nip100p to Act5p is dependent on the presence of Jnm1p. Finally, we find that a fusion of Nip100p to the green fluorescent protein localizes to the spindle poles throughout the cell cycle. Taken together, these results suggest that the yeast dynactin complex and cytoplasmic dynein together define a physiological pathway that is responsible for spindle translocation late in anaphase.

Cell cycle arrest in cdc20 mutants of Saccharomyces cerevisiae is independent of Ndc10p and kinetochore function but requires a subset of spindle checkpoint genes

The spindle checkpoint ensures accurate chromosome segregation by inhibiting anaphase onset in response to altered microtubule function and impaired kinetochore function. In this study, we report that the ability of the anti-microtubule drug nocodazole to inhibit cell cycle progression in Saccharomyces cerevisiae depends on the function of the kinetochore protein encoded by NDC10. We examined the role of the spindle checkpoint in the arrest in cdc20 mutants that arrest prior to anaphase with an aberrant spindle. The arrest in cdc20 defective cells is dependent on the BUB2 checkpoint and independent of the BUB1, BUB3, and MAD spindle checkpoint genes. We show that the lesion recognized by Bub2p is not excess microtubules, and the cdc20 arrest is independent of kinetochore function. We show that Cdc20p is not required for cyclin proteolysis at two points in the cell cycle, suggesting that CDC20 is distinct from genes encoding integral proteins of the anaphase promoting complex.

Kar9p is a novel cortical protein required for cytoplasmic microtubule orientation in yeast

kar9 was originally identified as a bilateral karyogamy mutant, in which the two zygotic nuclei remained widely separated and the cytoplasmic microtubules were misoriented (Kurihara, L.J., C.T. Beh, M. Latterich, R. Schekman, and M.D. Rose. 1994. J. Cell Biol. 126:911-923.). We now report a general defect in nuclear migration and microtubule orientation in kar9 mutants. KAR9 encodes a novel 74-kD protein that is not essential for life. The kar9 mitotic defect was similar to mutations in dhc1/dyn1 (dynein heavy chain gene), jnm1, and act5. kar9Delta dhc1Delta, kar9Delta jnm1Delta, and kar9Delta act5Delta double mutants were synthetically lethal, suggesting that these genes function in partially redundant pathways to carry out nuclear migration. A functional GFP-Kar9p fusion protein localized to a single dot at the tip of the shmoo projection. In mitotic cells, GFP-Kar9p localized to a cortical dot with both mother-daughter asymmetry and cell cycle dependence. In small-budded cells through anaphase, GFP-Kar9p was found at the tip of the growing bud. In telophase and G1 unbudded cells, no localization was observed. By indirect immunofluorescence, cytoplasmic microtubules intersected the GFP-Kar9p dot. Nocodazole experiments demonstrated that Kar9p's cortical localization was microtubule independent. We propose that Kar9p is a component of a cortical adaptor complex that orients cytoplasmic microtubules.

A novel yeast screen for mitotic arrest mutants identifies DOC1, a new gene involved in cyclin proteolysis

B-type cyclins are rapidly degraded at the transition between metaphase and anaphase and their ubiquitin-mediated proteolysis is required for cells to exit mitosis. We used a novel enrichment to isolate new budding mutants that arrest the cell cycle in mitosis. Most of these mutants lie in the CDC16, CDC23, and CDC27 genes, which have already been shown to play a role in cyclin proteolysis and encode components of a 20S complex (called the cyclosome or anaphase promoting complex) that ubiquitinates mitotic cyclins. We show that mutations in CDC26 and a novel gene, DOC1, also prevent mitotic cyclin proteolysis. Mutants in either gene arrest as large budded cells with high levels of the major mitotic cyclin (Clb2) protein at 37 degrees C and cannot degrade Clb2 in G1-arrested cells. Cdc26 associates in vivo with Doc1, Cdc16, Cdc23, and Cdc27. In addition, the majority of Doc1 cosediments at 20S with Cdc27 in a sucrose gradient, indicating that Cdc26 and Doc1 are components of the anaphase promoting complex.

Kinesin-related KIP3 of Saccharomyces cerevisiae is required for a distinct step in nuclear migration

Spindle orientation and nuclear migration are crucial events in cell growth and differentiation of many eukaryotes. Here we show that KIP3, the sixth and final kinesin-related gene in Saccharomyces cerevisiae, is required for migration of the nucleus to the bud site in preparation for mitosis. The position of the nucleus in the cell and the orientation of the mitotic spindle was examined by microscopy of fixed cells and by time-lapse microscopy of individual live cells. Mutations in KIP3 and in the dynein heavy chain gene defined two distinct phases of nuclear migration: a KIP3-dependent movement of the nucleus toward the incipient bud site and a dynein-dependent translocation of the nucleus through the bud neck during anaphase. Loss of KIP3 function disrupts the unidirectional movement of the nucleus toward the bud and mitotic spindle orientation, causing large oscillations in nuclear position. The oscillatory motions sometimes brought the nucleus in close proximity to the bud neck, possibly accounting for the viability of a kip3 null mutant. The kip3 null mutant exhibits normal translocation of the nucleus through the neck and normal spindle pole separation kinetics during anaphase. Simultaneous loss of KIP3 and kinesin-related KAR3 function, or of KIP3 and dynein function, is lethal but does not block any additional detectable movement. This suggests that the lethality is due to the combination of sequential and possibly overlapping defects. Epitope-tagged Kip3p localizes to astral and central spindle microtubules and is also present throughout the cytoplasm and nucleus.

Microtubules orient the mitotic spindle in yeast through dynein-dependent interactions with the cell cortex

Proper orientation of the mitotic spindle is critical for successful cell division in budding yeast. To investigate the mechanism of spindle orientation, we used a green fluorescent protein (GFP)-tubulin fusion protein to observe microtubules in living yeast cells. GFP-tubulin is incorporated into microtubules, allowing visualization of both cytoplasmic and spindle microtubules, and does not interfere with normal microtubule function. Microtubules in yeast cells exhibit dynamic instability, although they grow and shrink more slowly than microtubules in animal cells. The dynamic properties of yeast microtubules are modulated during the cell cycle. The behavior of cytoplasmic microtubules revealed distinct interactions with the cell cortex that result in associated spindle movement and orientation. Dynein-mutant cells had defects in these cortical interactions, resulting in misoriented spindles. In addition, microtubule dynamics were altered in the absence of dynein. These results indicate that microtubules and dynein interact to produce dynamic cortical interactions, and that these interactions result in the force driving spindle orientation.

A proteasome cap subunit required for spindle pole body duplication in yeast

Proteasome-mediated protein degradation is a key regulatory mechanism in a diversity of complex processes, including the control of cell cycle progression. The selection of substrates for degradation clearly depends on the specificity of ubiquitination mechanisms, but further regulation may occur within the proteasomal 19S cap complexes, which attach to the ends of the 20S proteolytic core and are thought to control entry of substrates into the core. We have characterized a gene from Saccharomyces cerevisiae that displays extensive sequence similarity to members of a family of ATPases that are components of the 19S complex, including human subunit p42 and S. cerevisiae SUG1/CIM3 and CIM5 products. This gene, termed PCS1 (for proteasomal cap subunit), is identical to the recently described SUG2 gene (Russell, S.J., U.G. Sathyanarayana, and S.A. Johnston. 1996. J. Biol. Chem. 271:32810-32817). We have shown that PCS1 function is essential for viability. A temperature-sensitive pcs1 strain arrests principally in the second cycle after transfer to the restrictive temperature, blocking as large-budded cells with a G2 content of unsegregated DNA. EM reveals that each arrested pcs1 cell has failed to duplicate its spindle pole body (SPB), which becomes enlarged as in other monopolar mutants. Additionally, we have shown localization of a functional Pcs1-green fluorescent protein fusion to the nucleus throughout the cell cycle. We hypothesize that Pcs1p plays a role in the degradation of certain potentially nuclear component(s) in a manner that specifically is required for SPB duplication.

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.

Analysis of the cell cycle in the budding yeast Candida albicans by positioning of chromosomes by fluorescence in situ hybridization (FISH) with repetitive sequences

BACKGROUND

In the budding yeasts, including Saccharomyces cerevisiae, in which individual chromosomes cannot be visualized by microscopy, the mitotic phases in the cell cycle have not been correlated with the chromosome behaviour. We used various repetitive sequences, namely, rDNA, telomeric sequences and RPSs, which are localized in limited regions in almost all chromosomes, as probes for fluorescence in situ hybridization (FISH) to analyse the cell cycle phases in a pathogenic yeast Candida albicans. The positioning of the FISH signals was analysed quantitatively in relation to the length of spindle microtubules in the nuclear domain.

RESULTS

RPSs were randomly distributed in the interphase nucleus, and they formed aggregates with the development of the spindle. DNA synthesis was complete before RPSs came closest to the spindle. As the spindle elongated, they were scattered along the spindle and then separated into two clusters at the spindle poles at the end of anaphase. rDNA was localized in the nucleolar domain, and telomere signals were randomly distributed throughout mitosis.

CONCLUSION

By estimating quantitatively the proportions of mitotic cells with particular configurations of both microtubules and chromosomes in a population of rapidly proliferating cells, we were able to define various stages in the progression of mitosis. The S phase and pro-to-prometaphase were overlapping and the G2 phase was lacking. Unexpectedly, the pole-to-pole elongation of the spindle (anaphase B) was predominating and was followed by movement of chromosomes to the poles (anaphase A).

Forces on chromosomal DNA during anaphase

In the course of anaphase, the chromosomal DNA is submitted to the traction of the spindle. Several physical problems are associated with this action. In particular, the sister chromatids are generally topologically intertwined at the onset of anaphase, and the removal of the intertwinings results from a coupling between the enzymatic action of type II DNA topoisomerases and the force exerted by the spindle. We propose a physical analysis of some of these problems: 1) We compare the maximum force the spindle can produce with the force required to break a DNA molecule, and define the conditions compatible with biological safety during anaphase. 2) We show that the behavior of the sister chromatids in the absence of type II DNA topoisomerases can be described by two distinct models: a chain pullout model accounts for the experimental observations made in the budding yeast, and a model of the mechanical rupture of rubbers accounts for the nondisjunction in standard cases. 3) Using the fluctuation-dissipation theorem, we introduce an effective protein friction associated with the strand-passing activity of type II DNA topoisomerases. We show that this friction can be used to describe the situation in which one chromosome passes entirely through another one. Possible experiments that could test these theoretical analyses are discussed.

Kinetics of spindle pole body separation in budding yeast

In the budding yeast Saccharomyces cerevisiae, the spindle pole body (SPB) serves as the microtubule-organizing center and is the functional analog of the centrosome of higher organisms. By expressing a fusion of a yeast SPB-associated protein to the Aequorea victoria green fluorescent protein, the movement of the SPBs in living yeast cells undergoing mitosis was observed by fluorescence microscopy. The ability to visualize SPBs in vivo has revealed previously unidentified mitotic events. During anaphase, the mitotic spindle has four sequential activities: alignment at the mother-daughter junction, fast elongation, translocation into the bud, and slow elongation. These results indicate that distinct forces act upon the spindle at different times during anaphase.

Two microtubule-associated proteins required for anaphase spindle movement in Saccharomyces cerevisiae

In many eucaryotic cells, the midzone of the mitotic spindle forms a distinct structure containing a specific set of proteins. We have isolated ASE1, a gene encoding a component of the Saccharomyces cerevisiae spindle midzone. Strains lacking both ASE1 and BIK1, which encodes an S. cerevisiae microtubule-associated protein, are inviable. The analysis of the phenotype of a bik1 ase1 conditional double mutant suggests that BIK1 and ASE1 are not required for the assembly of a bipolar spindle, but are essential for anaphase spindle elongation. The steady-state levels of Ase1p are regulated in a manner that is consistent with a function during anaphase: they are low in G1, accumulate to maximal levels after S phase and then drop as cells exit mitosis. Components of the spindle midzone may therefore be required in vivo for anaphase spindle movement. Additionally, anaphase spindle movement may depend on a dedicated set of genes whose expression is induced at G2/M.

Spindle dynamics and cell cycle regulation of dynein in the budding yeast, Saccharomyces cerevisiae

We have used time-lapse digital- and video-enhanced differential interference contrast (DE-DIC, VE-DIC) microscopy to study the role of dynein in spindle and nuclear dynamics in the yeast Saccharomyces cerevisiae. The real-time analysis reveals six stages in the spindle cycle. Anaphase B onset appears marked by a rapid phase of spindle elongation, simultaneous with nuclear migration into the daughter cell. The onset and kinetics of rapid spindle elongation are identical in wild type and dynein mutants. In the absence of dynein the nucleus does not migrate as close to the neck as in wild-type cells and initial spindle elongation is confined primarily to the mother cell. Rapid oscillations of the elongating spindle between the mother and bud are observed in wild-type cells, followed by a slower growth phase until the spindle reaches its maximal length. This stage is protracted in the dynein mutants and devoid of oscillatory motion. Thus dynein is required for rapid penetration of the nucleus into the bud and anaphase B spindle dynamics. Genetic analysis reveals that in the absence of a functional central spindle (ndcl), dynein is essential for chromosome movement into the bud. Immunofluorescent localization of dynein-beta-galactosidase fusion proteins reveals that dynein is associated with spindle pole bodies and the cell cortex: with spindle pole body localization dependent on intact microtubules. A kinetic analysis of nuclear movement also revealed that cytokinesis is delayed until nuclear translocation is completed, indicative of a surveillance pathway monitoring nuclear transit into the bud.

The Drosophila cell cycle gene fizzy is required for normal degradation of cyclins A and B during mitosis and has homology to the CDC20 gene of Saccharomyces cerevisiae

The Drosophila cell cycle gene fizzy (fzy) is required for normal execution of the metaphase-anaphase transition. We have cloned fzy, and confirmed this by P-element mediated germline transformation rescue. Sequence analysis predicts that fzy encodes a protein of 526 amino acids, the carboxy half of which has significant homology to the Saccharomyces cerevisiae cell cycle gene CDC20. A monoclonal antibody against fzy detects a single protein of the expected size, 59 kD, in embryonic extracts. In early embryos fzy is expressed in all proliferating tissues; in late embryos fzy expression declines in a tissue-specific manner correlated with cessation of cell division. During interphase fzy protein is present in the cytoplasm; while in mitosis fzy becomes ubiquitously distributed throughout the cell except for the area occupied by the chromosomes. The metaphase arrest phenotype caused by fzy mutations is associated with failure to degrade both mitotic cyclins A and B, and an enrichment of spindle microtubules at the expense of astral microtubules. Our data suggest that fzy function is required for normal cell cycle-regulated proteolysis that is necessary for successful progress through mitosis.

CEP3 encodes a centromere protein of Saccharomyces cerevisiae

We have designed a screen to identify mutants specifically affecting kinetochore function in the yeast Saccharomyces cerevisiae. The selection procedure was based on the generation of "synthetic acentric" minichromosomes. "Synthetic acentric" minichromosomes contain a centromere locus, but lack centromere activity due to combination of mutations in centromere DNA and in a chromosomal gene (CEP) encoding a putative centromere protein. Ten conditional lethal cep mutants were isolated, seven were found to be alleles of NDC10 (CEP2) encoding the 110-kD protein of yeast kinetochore. Three mutants defined a novel essential gene CEP3. The CEP3 product (Cep3p) is a 71-kD protein with a potential DNA-binding domain (binuclear Zn-cluster). At nonpermissive temperature the cep3 cells arrest with an undivided nucleus and a short mitotic spindle. At permissive temperature the cep3 cells are unable to support segregation of minichromosomes with mutations in the central part of element III of yeast centromere DNA. These minichromosomes, when isolated from cep3 cultures, fail to bind bovine microtubules in vitro. The sum of genetic, cytological and biochemical data lead us to suggest that the Cep3 protein is a DNA-binding component of yeast centromere. Molecular mass and sequence comparison confirm that Cep3p is the p64 component of centromere DNA binding complex Cbf3 (Lechner, 1994).

Saccharomyces cerevisiae kinesin- and dynein-related proteins required for anaphase chromosome segregation

The Saccharomyces cerevisiae kinesin-related gene products Cin8p and Kip1p function to assemble the bipolar mitotic spindle. The cytoplasmic dynein heavy chain homologue Dyn1p (also known as Dhc1p) participates in proper cellular positioning of the spindle. In this study, the roles of these motor proteins in anaphase chromosome segregation were examined. While no single motor was essential, loss of function of all three completely halted anaphase chromatin separation. As combined motor activity was diminished by mutation, both the velocity and extent of chromatin movement were reduced, suggesting a direct role for all three motors in generating a chromosome-separating force. Redundancy for function between different types of microtubule-based motor proteins was also indicated by the observation that cin8 dyn1 double-deletion mutants are inviable. Our findings indicate that the bulk of anaphase chromosome segregation in S. cerevisiae is accomplished by the combined actions of these three motors.

The spindle pole body of yeast

Microtubule organizing centers play an essential cellular role in nucleating microtubule assembly and establishing the microtubule array. The microtubule organizing center of yeast, the spindle pole body (SPB), shares many functions and properties with those other organisms. In recent years considerable new information has been generated concerning components associated with the SPB, and the mechanism by which it duplicates. This article reviews our current view of the cytology and molecular composition of the SPB of the budding yeast, Saccharomyces cerevisiae, and the fission yeast, Schizosaccharomyces pombe. Genetic studies in these organisms has revealed information about how the SPB duplicates and separates, and its roles during vegetative growth, mating and meiosis.