Correct spindle elongation at the metaphase/anaphase transition is an APC-dependent event in budding yeast

TORC1 inactivation promotes APC/C-dependent mitotic slippage in yeast and human cells

Unsatisfied kinetochore-microtubule attachment activates the spindle assembly checkpoint to inhibit the metaphase-anaphase transition. However, some cells eventually override mitotic arrest by mitotic slippage. Here, we show that inactivation of TORC1 kinase elicits mitotic slippage in budding yeast and human cells. Yeast mitotic slippage was accompanied with aberrant aspects, such as degradation of the nucleolar protein Net1, release of phosphatase Cdc14, and anaphase-promoting complex/cyclosome (APC/C)-Cdh1-dependent degradation of securin and cyclin B in metaphase. This mitotic slippage caused chromosome instability. In human cells, mammalian TORC1 (mTORC1) inactivation also invoked mitotic slippage, indicating that TORC1 inactivation-induced mitotic slippage is conserved from yeast to mammalian cells. However, the invoked mitotic slippage in human cells was not dependent on APC/C-Cdh1. This study revealed an unexpected involvement of TORC1 in mitosis and provides information on undesirable side effects of the use of TORC1 inhibitors as immunosuppressants and anti-tumor drugs.

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Yeast TORC1 inhibition promotes Net1 degradation and Cdc14 release

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Yeast TORC1 inhibition invokes mitotic slippage in an APC/C-Cdh1-dependent manner

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Human mTORC1 inhibition also elicits mitotic slippage

Coupling DNA Replication and Spindle Function in Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae DNA replication and spindle assembly can overlap. Therefore, signaling mechanisms modulate spindle dynamics in order to ensure correct timing of chromosome segregation relative to genome duplication, especially when replication is incomplete or the DNA becomes damaged. This review focuses on the molecular mechanisms that coordinate DNA replication and spindle dynamics, as well as on the role of spindle-dependent forces in DNA repair. Understanding the coupling between genome duplication and spindle function in yeast cells can provide important insights into similar processes operating in other eukaryotic organisms, including humans.

A CLK1-KKT2 Signaling Pathway Regulating Kinetochore Assembly in Trypanosoma brucei

During mitosis, eukaryotic cells must duplicate and separate their chromosomes in a precise and timely manner. The apparatus responsible for this is the kinetochore, which is a large protein structure that links chromosomal DNA and spindle microtubules to facilitate chromosome alignment and segregation. The proteins that comprise the kinetochore in the protozoan parasite Trypanosoma brucei are divergent from yeast and mammals and comprise an inner kinetochore complex composed of 24 distinct proteins (KKT1 to KKT23, KKT25) that include four protein kinases, CLK1 (KKT10), CLK2 (KKT19), KKT2, and KKT3. We recently reported the identification of a specific trypanocidal inhibitor of T. brucei CLK1, an amidobenzimidazole, AB1. We now show that chemical inhibition of CLK1 with AB1 impairs inner kinetochore recruitment and compromises cell cycle progression, leading to cell death. Here, we show that KKT2 is a substrate for CLK1 and identify phosphorylation of S508 by CLK1 to be essential for KKT2 function and for kinetochore assembly. Additionally, KKT2 protein kinase activity is required for parasite proliferation but not for assembly of the inner kinetochore complex. We also show that chemical inhibition of the aurora kinase AUK1 does not affect CLK1 phosphorylation of KKT2, indicating that AUK1 and CLK1 are in separate regulatory pathways. We propose that CLK1 is part of a divergent signaling cascade that controls kinetochore function via phosphorylation of the inner kinetochore protein kinase KKT2. IMPORTANCE In eukaryotic cells, kinetochores are large protein complexes that link chromosomes to dynamic microtubule tips, ensuring proper segregation and genomic stability during cell division. Several proteins tightly coordinate kinetochore functions, including the protein kinase aurora kinase B. The kinetochore has diverse evolutionary roots. For example, trypanosomatids, single-cell parasitic protozoa that cause several neglected tropical diseases, possess a unique repertoire of kinetochore components whose regulation during the cell cycle remains unclear. Here, we shed light on trypanosomatid kinetochore biology by showing that the protein kinase CLK1 coordinates the assembly of the inner kinetochore by phosphorylating one of its components, KKT2, allowing the timely spatial recruitment of the rest of the kinetochore proteins and posterior attachment to microtubules in a process that is aurora kinase B independent.

Inhibition of spindle extension through the yeast S phase checkpoint is coupled to replication fork stability and the integrity of centromeric DNA

Budding yeast treated with hydroxyurea (HU) activate the S phase checkpoint kinase Rad53, which prevents DNA replication forks from undergoing aberrant structural transitions and nuclease processing. Rad53 is also required to prevent premature extension of the mitotic spindle that assembles during a HU-extended S phase. Here we present evidence that checkpoint restraint of spindle extension is directly coupled to Rad53 control of replication fork stability. In budding yeast, centromeres are flanked by replication origins that fire in early S phase. Mutations affecting the Zn²⁺-finger of Dbf4, an origin activator, preferentially reduce centromere-proximal origin firing in HU, corresponding with suppression of rad53 spindle extension. Inactivating Exo1 nuclease or displacing centromeres from origins provides a similar suppression. Conversely, short-circuiting Rad53 targeting of Dbf4, Sld3, and Dun1, substrates contributing to fork stability, induces spindle extension. These results reveal spindle extension in HU-treated rad53 mutants is a consequence of replication fork catastrophes at centromeres. When such catastrophes occur, centromeres become susceptible to nucleases, disrupting kinetochore function and spindle force balancing mechanisms. At the same time, our data indicate centromere duplication is not required to stabilize S phase spindle structure, leading us to propose a model for how monopolar kinetochore-spindle attachments may contribute to spindle force balance in HU.

The Mitotic Exit Network integrates temporal and spatial signals by distributing regulation across multiple components

GTPase signal transduction pathways control cellular decision making by integrating multiple cellular events into a single signal. The Mitotic Exit Network (MEN), a Ras-like GTPase signaling pathway, integrates spatial and temporal cues to ensure that cytokinesis only occurs after the genome has partitioned between mother and daughter cells during anaphase. Here we show that signal integration does not occur at a single step of the pathway. Rather, sequential components of the pathway are controlled in series by different signals. The spatial signal, nuclear position, regulates the MEN GTPase Tem1. The temporal signal, commencement of anaphase, is mediated by mitotic cyclin-dependent kinase (CDK) phosphorylation of the GTPase's downstream kinases. We propose that integrating multiple signals through sequential steps in the GTPase pathway represents a generalizable principle in GTPase signaling and explains why intracellular signal transmission is a multi-step process. Serial signal integration rather than signal amplification makes multi-step signal transduction necessary.

Replication-dependent and independent mechanisms for the chromosome-coupled persistence of a selfish genome

The yeast 2-micron plasmid epitomizes the evolutionary optimization of selfish extra-chromosomal genomes for stable persistence without jeopardizing their hosts' fitness. Analyses of fluorescence-tagged single-copy reporter plasmids and/or the plasmid partitioning proteins in native and non-native hosts reveal chromosome-hitchhiking as the likely means for plasmid segregation. The contribution of the partitioning system to equal segregation is bipartite- replication-independent and replication-dependent. The former nearly eliminates 'mother bias' (preferential plasmid retention in the mother cell) according to binomial distribution, thus limiting equal segregation of a plasmid pair to 50%. The latter enhances equal segregation of plasmid sisters beyond this level, elevating the plasmid close to chromosome status. Host factors involved in plasmid partitioning can be functionally separated by their participation in the replication-independent and/or replication-dependent steps. In the hitchhiking model, random tethering of a pair of plasmids to chromosomes signifies the replication-independent component of segregation; the symmetric tethering of plasmid sisters to sister chromatids embodies the replication-dependent component. The 2-micron circle broadly resembles the episomes of certain mammalian viruses in its chromosome-associated propagation. This unifying feature among otherwise widely differing selfish genomes suggests their evolutionary convergence to the common logic of exploiting, albeit via distinct molecular mechanisms, host chromosome segregation machineries for self-preservation.

Protease-dead separase is dominant negative in the C. elegans embryo

Separase is a protease that promotes chromosome segregation at anaphase by cleaving cohesin. Several non-proteolytic functions of separase have been identified in other organisms. We created a transgenic C. elegans line that expresses protease-dead separase in embryos to further characterize separase function. We find that expression of protease-dead separase is dominant-negative in C. elegans embryos, not previously reported in other systems. The C. elegans embryo is an ideal system to study developmental processes in a genetically tractable system. However, a major limitation is the lack of an inducible gene expression system for the embryo. We have developed two methods that allow for the propagation of lines carrying dominant-negative transgenes and have applied them to characterize expression of protease-dead separase in embryos. Using these methods, we show that protease-dead separase causes embryo lethality, and that protease-dead separase cannot rescue separase mutants. These data suggest that protease-dead separase interferes with endogenous separase function, possibly by binding substrates and protecting them from cleavage.

Increased Aurora B activity causes continuous disruption of kinetochore-microtubule attachments and spindle instability

Aurora B kinase regulates the proper biorientation of sister chromatids during mitosis. Lack of Aurora B kinase function results in the inability to correct erroneous kinetochore-microtubule attachments and gives rise to aneuploidy. Interestingly, increased Aurora B activity also leads to problems with chromosome segregation, and overexpression of this kinase has been observed in various types of cancer. However, little is known about the mechanisms by which an increase in Aurora B kinase activity can impair mitotic progression and cell viability. Here, using a yeast model, we demonstrate that increased Aurora B activity as a result of the overexpression of the Aurora B and inner centromere protein homologs triggers defects in chromosome segregation by promoting the continuous disruption of chromosome-microtubule attachments even when sister chromatids are correctly bioriented. This disruption leads to a constitutive activation of the spindle-assembly checkpoint, which therefore causes a lack of cytokinesis even though spindle elongation and chromosome segregation take place. Finally, we demonstrate that this increase in Aurora B activity causes premature collapse of the mitotic spindle by promoting instability of the spindle midzone.

Coordination of chromatid separation and spindle elongation by antagonistic activities of mitotic and S-phase CDKs

Because cohesion prevents sister-chromatid separation and spindle elongation, cohesion dissolution may trigger these two events simultaneously. However, the relatively normal spindle elongation kinetics in yeast cohesin mutants indicates an additional mechanism for the temporal control of spindle elongation. Here we show evidence indicating that S-phase CDK (cyclin dependent kinase) negatively regulates spindle elongation. In contrast, mitotic CDK promotes spindle elongation by activating Cdc14 phosphatase, which reverses the protein phosphorylation imposed by S-phase CDK. Our data suggest that S-phase CDK negatively regulates spindle elongation partly through its phosphorylation of a spindle pole body (SPB) protein Spc110. We also show that hyperactive S-phase CDK compromises the microtubule localization of Stu2, a processive microtubule polymerase essential for spindle elongation. Strikingly, we found that hyperactive mitotic CDK induces uncoupled spindle elongation and sister-chromatid separation in securin mutants (pds1Δ), and we speculate that asynchronous chromosome segregation in pds1Δ cells contributes to this phenotype. Therefore, the tight temporal control of spindle elongation and cohesin cleavage assure orchestrated chromosome separation and spindle elongation.

Temporal sequence and cell cycle cues in the assembly of host factors at the yeast 2 micron plasmid partitioning locus

The Saccharomyces cerevisiae 2 micron plasmid exemplifies a benign but selfish genome, whose stability approaches that of the chromosomes of its host. The plasmid partitioning locus STB (stability locus) displays certain functional analogies with centromeres along with critical distinctions, a significant one being the absence of the kinetochore complex at STB. The remodels the structure of chromatin (RSC) chromatin remodeling complex, the nuclear motor Kip1, the histone H3 variant Cse4 and the cohesin complex associate with both loci. These factors appear to contribute to plasmid segregation either directly or indirectly through their roles in chromosome segregation. Assembly and disassembly of the plasmid-coded partitioning proteins Rep1 and Rep2 and host factors at STB follow a temporal hierarchy during the cell cycle. Assembly is initiated by STB association of [Rsc8-Rsc58], followed by [Rep1-Rep2-Kip1] and [Cse4-Rsc2-Sth1] recruitment, and culminates in cohesin assembly. Disassembly starts with dissociation of RSC components, is followed by cohesin disassembly and Cse4 exit during anaphase and late telophase, respectively. [Rep1-Rep2-Kip1] persists through G1 of the ensuing cell cycle. The de novo assembly of the 'partitioning complex' is cued by the innate cell cycle clock and is dependent on DNA replication. Shared functional attributes of STB and centromere (CEN) are consistent with a potential evolutionary link between them.

Unrestrained spindle elongation during recovery from spindle checkpoint activation in cdc15-2 cells results in mis-segregation of chromosomes

During normal metaphase in Saccharomyces cerevisiae, chromosomes are captured at the kinetochores by microtubules emanating from the spindle pole bodies at opposite poles of the dividing cell. The balance of forces between the cohesins holding the replicated chromosomes together and the pulling force from the microtubules at the kinetochores result in the biorientation of the sister chromatids before chromosome segregation. The absence of kinetochore-microtubule interactions or loss of cohesion between the sister chromatids triggers the spindle checkpoint which arrests cells in metaphase. We report here that an MEN mutant, cdc15-2, though competent in activating the spindle assembly checkpoint when exposed to Noc, mis-segregated chromosomes during recovery from spindle checkpoint activation. cdc15-2 cells arrested in Noc, although their Pds1p levels did not accumulate as well as in wild-type cells. Genetic analysis indicated that Pds1p levels are lower in a mad2Delta cdc15-2 and bub2Delta cdc15-2 double mutants compared with the single mutants. Chromosome mis-segregation in the mutant was due to premature spindle elongation in the presence of unattached chromosomes, likely through loss of proper control on spindle midzone protein Slk19p and kinesin protein, Cin8p. Our data indicate that a slower rate of transition through the cell division cycle can result in an inadequate level of Pds1p accumulation that can compromise recovery from spindle assembly checkpoint activation.

Phosphorylation-dependent protein interactions at the spindle midzone mediate cell cycle regulation of spindle elongation

The metaphase-to-anaphase transition is one of the most dramatic and highly regulated steps in cell division. At anaphase onset the protease separase dissolves sister chromatid cohesion. Simultaneously, the mitotic spindle elongates as interpolar microtubules (iMTs) slide apart at the spindle midzone, ensuring chromosome segregation. However, it remains unclear how spindle elongation is coordinated with cell cycle progression. Here we demonstrate that phosphorylation of the midzone organizer Ase1 controls localization and function of Cin8, a kinesin-5 that slides iMTs relative to each other. Phosphorylation of Ase1 by Cdk1 (cyclin-dependent kinase) inhibits Cin8 binding to iMTs, preventing bending and collapse of the metaphase spindle. In anaphase Ase1 dephosphorylation by the separase-activated phosphatase Cdc14 is necessary and sufficient for Cin8 recruitment to the midzone, where it drives spindle elongation. Our results reveal that sliding forces at the midzone are activated by separase and explain how spindle elongation is triggered with anaphase entry.

Structure and function of the PP2A-shugoshin interaction

Accurate chromosome segregation during mitosis and meiosis depends on shugoshin proteins that prevent precocious dissociation of cohesin from centromeres. Shugoshins associate with PP2A, which is thought to dephosphorylate cohesin and thereby prevent cleavage by separase during meiosis I. A crystal structure of a complex between a fragment of human Sgo1 and an AB'C PP2A holoenzyme reveals that Sgo1 forms a homodimeric parallel coiled coil that docks simultaneously onto PP2A's C and B' subunits. Sgo1 homodimerization is a prerequisite for PP2A binding. While hSgo1 interacts only with the AB'C holoenzymes, its relative, Sgo2, interacts with all PP2A forms and may thus lead to dephosphorylation of distinct substrates. Mutant shugoshin proteins defective in the binding of PP2A cannot protect centromeric cohesin from separase during meiosis I or support the spindle assembly checkpoint in yeast. Finally, we provide evidence that PP2A's recruitment to chromosomes may be sufficient to protect cohesin from separase in mammalian oocytes.

Cdc14 inhibition by the spindle assembly checkpoint prevents unscheduled centrosome separation in budding yeast

The spindle assembly checkpoint (SAC) is an evolutionarily conserved surveillance mechanism that delays anaphase onset and mitotic exit in response to the lack of kinetochore attachment. The target of the SAC is the E3 ubiquitin ligase anaphase-promoting complex (APC) bound to its Cdc20 activator. The Cdc20/APC complex is in turn required for sister chromatid separation and mitotic exit through ubiquitin-mediated proteolysis of securin, thus relieving inhibition of separase that unties sister chromatids. Separase is also involved in the Cdc-fourteen early anaphase release (FEAR) pathway of nucleolar release and activation of the Cdc14 phosphatase, which regulates several microtubule-linked processes at the metaphase/anaphase transition and also drives mitotic exit. Here, we report that the SAC prevents separation of microtubule-organizing centers (spindle pole bodies [SPBs]) when spindle assembly is defective. Under these circumstances, failure of SAC activation causes unscheduled SPB separation, which requires Cdc20/APC, the FEAR pathway, cytoplasmic dynein, and the actin cytoskeleton. We propose that, besides inhibiting sister chromatid separation, the SAC preserves the accurate transmission of chromosomes also by preventing SPBs to migrate far apart until the conditions to assemble a bipolar spindle are satisfied.

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.

Mitotic CDKs control the metaphase-anaphase transition and trigger spindle elongation

Mitotic cyclin-dependent kinases (CDKs) control entry into mitosis, but their role during mitotic progression is less well understood. Here we characterize the functions of CDK activity associated with the mitotic cyclins Clb1, Clb2, and Clb3. We show that Clb-CDKs are important for the activation of the ubiquitin ligase Anaphase-Promoting Complex/Cyclosome (APC/C)-Cdc20 that triggers the metaphase-anaphase transition. Furthermore, we define an essential role for Clb-CDK activity in anaphase spindle elongation. Thus, mitotic CDKs serve not only to initiate M phase, but are also needed continuously throughout mitosis to trigger key mitotic events such as APC/C activation and anaphase spindle elongation.

Spindle checkpoint activation at meiosis I advances anaphase II onset via meiosis-specific APC/C regulation

During mitosis, the spindle assembly checkpoint (SAC) inhibits the Cdc20-activated anaphase-promoting complex/cyclosome (APC/C(Cdc20)), which promotes protein degradation, and delays anaphase onset to ensure accurate chromosome segregation. However, the SAC function in meiotic anaphase regulation is poorly understood. Here, we examined the SAC function in fission yeast meiosis. As in mitosis, a SAC factor, Mad2, delayed anaphase onset via Slp1 (fission yeast Cdc20) when chromosomes attach to the spindle improperly. However, when the SAC delayed anaphase I, the interval between meiosis I and II shortened. Furthermore, anaphase onset was advanced and the SAC effect was reduced at meiosis II. The advancement of anaphase onset depended on a meiosis-specific, Cdc20-related factor, Fzr1/Mfr1, which contributed to anaphase cyclin decline and anaphase onset and was inefficiently inhibited by the SAC. Our findings show that impacts of SAC activation are not confined to a single division at meiosis due to meiosis-specific APC/C regulation, which has probably been evolved for execution of two meiotic divisions.

The protease activity of yeast separase (esp1) is required for anaphase spindle elongation independently of its role in cleavage of cohesin

Separase is a caspase-family protease required for the metaphase-anaphase transition in eukaryotes. In budding yeast, the separase ortholog, Esp1, has been shown to cleave a subunit of cohesin, Mcd1 (Scc1), thereby releasing sister chromatids from cohesion and allowing anaphase. However, whether Esp1 has other substrates required for anaphase has been controversial. Whereas it has been reported that cleavage of Mcd1 is sufficient to trigger anaphase in the absence of Esp1 activation, another study using a temperature-sensitive esp1 mutant concluded that depletion of Mcd1 was not sufficient for anaphase in the absence of Esp1 function. Here we revisit the issue and demonstrate that neither depletion of Mcd1 nor ectopic cleavage of Mcd1 by Tev1 protease is sufficient to support anaphase in an esp1 temperature-sensitive mutant. Furthermore, we demonstrate that the catalytic activity of the Esp1 protease is required for this Mcd1-independent anaphase function. These data suggest that another protein, possibly a spindle-associated protein, is cleaved by Esp1 to allow anaphase. Such a function is consistent with the previous observation that Esp1 localizes to the mitotic spindle during anaphase.

Kinetochore microtubule interaction during S phase in Saccharomyces cerevisiae

In the budding yeast Saccharomyces cerevisiae, microtubule-organizing centers called spindle pole bodies (SPBs) are embedded in the nuclear envelope, which remains intact throughout the cell cycle (closed mitosis). Kinetochores are tethered to SPBs by microtubules during most of the cell cycle, including G1 and M phases; however, it has been a topic of debate whether microtubule interaction is constantly maintained or transiently disrupted during chromosome duplication. Here, we show that centromeres are detached from microtubules for 1-2 min and displaced away from a spindle pole in early S phase. These detachment and displacement events are caused by centromere DNA replication, which results in disassembly of kinetochores. Soon afterward, kinetochores are reassembled, leading to their recapture by microtubules. We also show how kinetochores are subsequently transported poleward by microtubules. Our study gives new insights into kinetochore-microtubule interaction and kinetochore duplication during S phase in a closed mitosis.

Pericentric chromatin is an elastic component of the mitotic spindle

BACKGROUND

Prior to chromosome segregation, the mitotic spindle bi-orients and aligns sister chromatids along the metaphase plate. During metaphase, spindle length remains constant, which suggests that spindle forces (inward and outward) are balanced. The contribution of microtubule motors, regulators of microtubule dynamics, and cohesin to spindle stability has been previously studied. In this study, we examine the contribution of chromatin structure on kinetochore positioning and spindle-length control. After nucleosome depletion, by either histone H3 or H4 repression, spindle organization was examined by live-cell fluorescence microscopy.

RESULTS

Histone repression led to a 2-fold increase in sister-centromere separation and an equal increase in metaphase spindle length. Histone H3 repression does not impair kinetochores, whereas H4 repression disrupts proper kinetochore function. Deletion of outward force generators, kinesins Cin8p and Kip1p, shortens the long spindles observed in histone-repressed cells. Oscillatory movements of individual sister chromatid pairs are not altered after histone repression.

CONCLUSIONS

The increase in spindle length upon histone repression and restoration of wild-type spindle length by the loss of plus-end-directed motors suggests that during metaphase, centromere separation and spindle length are governed in part by the stretching of pericentric chromatin. Chromatin is an elastic molecule that is stretched in direct opposition to the outward force generators Cin8p and Kip1p. Thus, we assign a new role to chromatin packaging as an integral biophysical component of the mitotic apparatus.

A Tel1/MRX-dependent checkpoint inhibits the metaphase-to-anaphase transition after UV irradiation in the absence of Mec1

In Saccharomyces cerevisiae, Mec1/ATR plays a primary role in sensing and transducing checkpoint signals in response to different types of DNA lesions, while the role of the Tel1/ATM kinase in DNA damage checkpoints is not as well defined. We found that UV irradiation in G(1) in the absence of Mec1 activates a Tel1/MRX-dependent checkpoint, which specifically inhibits the metaphase-to-anaphase transition. Activation of this checkpoint leads to phosphorylation of the downstream checkpoint kinases Rad53 and Chk1, which are required for Tel1-dependent cell cycle arrest, and their adaptor Rad9. The spindle assembly checkpoint protein Mad2 also partially contributes to the G(2)/M arrest of UV-irradiated mec1Delta cells independently of Rad53 phosphorylation and activation. The inability of UV-irradiated mec1Delta cells to undergo anaphase can be relieved by eliminating the anaphase inhibitor Pds1, whose phosphorylation and stabilization in these cells depend on Tel1, suggesting that Pds1 persistence may be responsible for the inability to undergo anaphase. Moreover, while UV irradiation can trigger Mec1-dependent Rad53 phosphorylation and activation in G(1)- and G(2)-arrested cells, Tel1-dependent checkpoint activation requires entry into S phase independently of the cell cycle phase at which cells are UV irradiated, and it is decreased when single-stranded DNA signaling is affected by the rfa1-t11 allele. This indicates that UV-damaged DNA molecules need to undergo structural changes in order to activate the Tel1-dependent checkpoint. Active Clb-cyclin-dependent kinase 1 (CDK1) complexes also participate in triggering this checkpoint and are required to maintain both Mec1- and Tel1-dependent Rad53 phosphorylation, suggesting that they may provide critical phosphorylation events in the DNA damage checkpoint cascade.

Maintenance of cohesin at centromeres after meiosis I in budding yeast requires a kinetochore-associated protein related to MEI-S332

BACKGROUND

The halving of chromosome number that occurs during meiosis depends on three factors. First, homologs must pair and recombine. Second, sister centromeres must attach to microtubules that emanate from the same spindle pole, which ensures that homologous maternal and paternal pairs can be pulled in opposite directions (called homolog biorientation). Third, cohesion between sister centromeres must persist after the first meiotic division to enable their biorientation at the second.

RESULTS

A screen performed in fission yeast to identify meiotic chromosome missegregation mutants has identified a conserved protein called Sgo1 that is required to maintain sister chromatid cohesion after the first meiotic division. We describe here an orthologous protein in the budding yeast S. cerevisiae (Sc), which has not only meiotic but also mitotic chromosome segregation functions. Deletion of Sc SGO1 not only causes frequent homolog nondisjunction at meiosis I but also random segregation of sister centromeres at meiosis II. Meiotic cohesion fails to persist at centromeres after the first meiotic division, and sister centromeres frequently separate precociously. Sgo1 is a kinetochore-associated protein whose abundance declines at anaphase I but, nevertheless, persists on chromatin until anaphase II.

CONCLUSIONS

The finding that Sgo1 is localized to the centromere at the time of the first division suggests that it may play a direct role in preventing the removal of centromeric cohesin. The similarity in sequence composition, chromosomal location, and mutant phenotypes of sgo1 mutants in two distant yeasts with that of MEI-S332 in Drosophila suggests that these proteins define an orthologous family conserved in most eukaryotic lineages.

Spindle checkpoint proteins and chromosome-microtubule attachment in budding yeast

Accurate chromosome segregation depends on precise regulation of mitosis by the spindle checkpoint. This checkpoint monitors the status of kinetochore–microtubule attachment and delays the metaphase to anaphase transition until all kinetochores have formed stable bipolar connections to the mitotic spindle. Components of the spindle checkpoint include the mitotic arrest defective (MAD) genes MAD1–3, and the budding uninhibited by benzimidazole (BUB) genes BUB1 and BUB3. In animal cells, all known spindle checkpoint proteins are recruited to kinetochores during normal mitoses. In contrast, we show that whereas Saccharomyces cerevisiae Bub1p and Bub3p are bound to kinetochores early in mitosis as part of the normal cell cycle, Mad1p and Mad2p are kinetochore bound only in the presence of spindle damage or kinetochore lesions that interfere with chromosome–microtubule attachment. Moreover, although Mad1p and Mad2p perform essential mitotic functions during every division cycle in mammalian cells, they are required in budding yeast only when mitosis goes awry. We propose that differences in the behavior of spindle checkpoint proteins in animal cells and budding yeast result primarily from evolutionary divergence in spindle assembly pathways.

Chromosomal cohesin forms a ring

The cohesin complex is essential for sister chromatid cohesion during mitosis. Its Smc1 and Smc3 subunits are rod-shaped molecules with globular ABC-like ATPases at one end and dimerization domains at the other connected by long coiled coils. Smc1 and Smc3 associate to form V-shaped heterodimers. Their ATPase heads are thought to be bridged by a third subunit, Scc1, creating a huge triangular ring that could trap sister DNA molecules. We address here whether cohesin forms such rings in vivo. Proteolytic cleavage of Scc1 by separase at the onset of anaphase triggers its dissociation from chromosomes. We show that N- and C-terminal Scc1 cleavage fragments remain connected due to their association with different heads of a single Smc1/Smc3 heterodimer. Cleavage of the Smc3 coiled coil is sufficient to trigger cohesin release from chromosomes and loss of sister cohesion, consistent with a topological association with chromatin.

The molecular function of Ase1p: evidence for a MAP-dependent midzone-specific spindle matrix. Microtubule-associated proteins

The midzone is the domain of the mitotic spindle that maintains spindle bipolarity during anaphase and generates forces required for spindle elongation (anaphase B). Although there is a clear role for microtubule (MT) motor proteins at the spindle midzone, less is known about how microtubule-associated proteins (MAPs) contribute to midzone organization and function. Here, we report that budding yeast Ase1p is a member of a conserved family of midzone-specific MAPs. By size exclusion chromatography and velocity sedimentation, both Ase1p in extracts and purified Ase1p behaved as a homodimer. Ase1p bound and bundled MTs in vitro. By live cell microscopy, loss of Ase1p resulted in a specific defect: premature spindle disassembly in mid-anaphase. Furthermore, when overexpressed, Ase1p was sufficient to trigger spindle elongation in S phase-arrested cells. FRAP revealed that Ase1p has both a very slow rate of turnover within the midzone and limited lateral diffusion along spindle MTs. We propose that Ase1p functions as an MT cross-bridge that imparts matrix-like characteristics to the midzone. MT-dependent networks of spindle midzone MAPs may be one molecular basis for the postulated spindle matrix.

The budding yeast Ipl1/Aurora protein kinase regulates mitotic spindle disassembly

Ipl1p is the budding yeast member of the Aurora family of protein kinases, critical regulators of genomic stability that are required for chromosome segregation, the spindle checkpoint, and cytokinesis. Using time-lapse microscopy, we found that Ipl1p also has a function in mitotic spindle disassembly that is separable from its previously identified roles. Ipl1-GFP localizes to kinetochores from G1 to metaphase, transfers to the spindle after metaphase, and accumulates at the spindle midzone late in anaphase. Ipl1p kinase activity increases at anaphase, and ipl1 mutants can stabilize fragile spindles. As the spindle disassembles, Ipl1p follows the plus ends of the depolymerizing spindle microtubules. Many Ipl1p substrates colocalize with Ipl1p to the spindle midzone, identifying additional proteins that may regulate spindle disassembly. We propose that Ipl1p regulates both the kinetochore and interpolar microtubule plus ends to regulate its various mitotic functions.

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.

Spo13 regulates cohesin cleavage

A key aspect of meiotic chromosome segregation is that cohesin, the protein complex that holds sister chromatids together, dissociates from chromosome arms during meiosis I and from centromeric regions during meiosis II. The budding yeast protein Spo13 plays a key role in preventing centromeric cohesin from being lost during meiosis I. We have determined the molecular basis for the metaphase arrest obtained when SPO13 is overexpressed during the mitotic cell cycle. Overexpression of SPO13 inhibits anaphase onset by at least two mechanisms. First, Spo13 causes a transient delay in degradation of the anaphase inhibitor Pds1. Second, Spo13 inhibits cleavage of the cohesin subunit Scc1/Mcd1 or its meiosis-specific homolog, Rec8, by the separase Esp1. The finding that Spo13 did not prevent cleavage of another Esp1 substrate, Slk19, suggests that overexpression of SPO13 is sufficient to prevent cohesin cleavage by protecting specific substrates from separase activity.

Separase, polo kinase, the kinetochore protein Slk19, and Spo12 function in a network that controls Cdc14 localization during early anaphase

In budding yeast, the phosphatase Cdc14, a key regulator of exit from mitosis, is released from its inhibitor Cfi1/Net1 in the nucleolus during anaphase. A signaling cascade, known as the mitotic exit network (MEN), controls this release. We have identified a regulatory network, the FEAR (Cdc fourteen early anaphase release) network that promotes Cdc14 release from the nucleolus during early anaphase. The FEAR network is comprised of the polo kinase Cdc5, the separase Esp1, the kinetochore-associated protein Slk19, and Spo12. We also show that the FEAR network initiates Cdc14 release from Cfi1/Net1 during early anaphase, and MEN maintains Cdc14 in the released state during late anaphase. We propose that one function of Cdc14 released by the FEAR network is to stimulate MEN activity.