A Bub2p-dependent spindle checkpoint pathway regulates the Dbf2p kinase in budding yeast

Cell growth and nutrient availability control the mitotic exit signaling network in budding yeast

Cell growth is required for cell cycle progression. The amount of growth required for cell cycle progression is reduced in poor nutrients, which leads to a reduction in cell size. In budding yeast, nutrients can influence cell size by modulating the extent of bud growth, which occurs predominantly in mitosis. However, the mechanisms are unknown. Here, we used mass spectrometry to identify proteins that modulate bud growth in response to nutrient availability. This led to the discovery that nutrients regulate numerous components of the mitotic exit network (MEN), which controls exit from mitosis. A key component of the MEN undergoes gradual multisite phosphorylation during bud growth that is dependent upon bud growth and correlated with the extent of growth. Furthermore, activation of the MEN is sufficient to override a growth requirement for mitotic exit. The data suggest a model in which the MEN ensures that mitotic exit occurs only when an appropriate amount of bud growth has occurred.

SIN-like Pathway Kinases Regulate the End of Mitosis in the Methylotrophic Yeast Ogataea polymorpha

The mitotic exit network (MEN) is a conserved signalling pathway essential for the termination of mitosis in the budding yeast Saccharomyces cerevisiae. All MEN components are highly conserved in the methylotrophic budding yeast Ogataea polymorpha, except for Cdc15 kinase. Instead, we identified two essential kinases OpHcd1 and OpHcd2 (homologue candidate of ScCdc15) that are homologous to SpSid1 and SpCdc7, respectively, components of the septation initiation network (SIN) of the fission yeast Schizosaccharomyces pombe. Conditional mutants for OpHCD1 and OpHCD2 exhibited significant delay in late anaphase and defective cell separation, suggesting that both genes have roles in mitotic exit and cytokinesis. Unlike Cdc15 in S. cerevisiae, the association of OpHcd1 and OpHcd2 with the yeast centrosomes (named spindle pole bodies, SPBs) is restricted to the SPB in the mother cell body. SPB localisation of OpHcd2 is regulated by the status of OpTem1 GTPase, while OpHcd1 requires the polo-like kinase OpCdc5 as well as active Tem1 to ensure the coordination of mitotic exit (ME) signalling and cell cycle progression. Our study suggests that the divergence of molecular mechanisms to control the ME-signalling pathway as well as the loss of Sid1/Hcd1 kinase in the MEN occurred relatively recently during the evolution of budding yeast.

A Novel Hyperactive Nud1 Mitotic Exit Network Scaffold Causes Spindle Position Checkpoint Bypass in Budding Yeast

Mitotic exit is a critical cell cycle transition that requires the careful coordination of nuclear positioning and cyclin B destruction in budding yeast for the maintenance of genome integrity. The mitotic exit network (MEN) is a Ras-like signal transduction pathway that promotes this process during anaphase. A crucial step in MEN activation occurs when the Dbf2-Mob1 protein kinase complex associates with the Nud1 scaffold protein at the yeast spindle pole bodies (SPBs; centrosome equivalents) and thereby becomes activated. This requires prior priming phosphorylation of Nud1 by Cdc15 at SPBs. Cdc15 activation, in turn, requires both the Tem1 GTPase and the Polo kinase Cdc5, but how Cdc15 associates with SPBs is not well understood. We have identified a hyperactive allele of NUD1, nud1-A308T, that recruits Cdc15 to SPBs in all stages of the cell cycle in a CDC5-independent manner. This allele leads to early recruitment of Dbf2-Mob1 during metaphase and requires known Cdc15 phospho-sites on Nud1. The presence of nud1-A308T leads to loss of coupling between nuclear position and mitotic exit in cells with mispositioned spindles. Our findings highlight the importance of scaffold regulation in signaling pathways to prevent improper activation.

Budding Yeast BFA1 Has Multiple Positive Roles in Directing Late Mitotic Events

The proper regulation of cell cycle transitions is paramount to the maintenance of cellular genome integrity. In Saccharomyces cerevisiae, the mitotic exit network (MEN) is a Ras-like signaling cascade that effects the transition from M phase to G1 during the cell division cycle in budding yeast. MEN activation is tightly regulated. It occurs during anaphase and is coupled to mitotic spindle position by the spindle position checkpoint (SPoC). Bfa1 is a key component of the SPoC and functions as part of a two-component GAP complex along with Bub2 The GAP activity of Bfa1-Bub2 keeps the MEN GTPase Tem1 inactive in cells with mispositioned spindles, thereby preventing inappropriate mitotic exit and preserving genome integrity. Interestingly, a GAP-independent role for Bfa1 in mitotic exit regulation has been previously identified. However the nature of this Bub2-independent role and its biological significance are not understood. Here we show that Bfa1 also activates the MEN by promoting the localization of Tem1 primarily to the daughter spindle pole body (dSPB). We demonstrate that the overexpression of BFA1 is lethal due to defects in Tem1 localization, which is required for its activity. In addition, our studies demonstrate a Tem1-independent role for Bfa1 in promoting proper cytokinesis. Cells lacking TEM1, in which the essential mitotic exit function is bypassed, exhibit cytokinesis defects. These defects are suppressed by the overexpression of BFA1 We conclude that Bfa1 functions to both inhibit and activate late mitotic events.

Temporal and compartment-specific signals coordinate mitotic exit with spindle position

The spatiotemporal control of mitotic exit is crucial for faithful chromosome segregation during mitosis. In budding yeast, the mitotic exit network (MEN) drives cells out of mitosis, whereas the spindle position checkpoint (SPOC) blocks MEN activity when the anaphase spindle is mispositioned. How the SPOC operates at a molecular level remains unclear. Here, we report novel insights into how mitotic signalling pathways orchestrate chromosome segregation in time and space. We establish that the key function of the central SPOC kinase, Kin4, is to counterbalance MEN activation by the cdc fourteen early anaphase release (FEAR) network in the mother cell compartment. Remarkably, Kin4 becomes dispensable for SPOC function in the absence of FEAR. Cells lacking both FEAR and Kin4 show that FEAR contributes to mitotic exit through regulation of the SPOC component Bfa1 and the MEN kinase Cdc15. Furthermore, we uncover controls that specifically promote mitotic exit in the daughter cell compartment.

The mitotic exit network (MEN) triggers mitotic exit and can be blocked by the spindle position checkpoint (SPOC). Here the authors show that SPOC kinase Kin4 counterbalances MEN activation by the Cdc fourteen early anaphase release (FEAR) network in the mother cell and that in the absence of FEAR mitotic exit requires daughter cell-confined factors.

Nur1 dephosphorylation confers positive feedback to mitotic exit phosphatase activation in budding yeast

Substrate dephosphorylation by the cyclin-dependent kinase (Cdk)-opposing phosphatase, Cdc14, is vital for many events during budding yeast mitotic exit. Cdc14 is sequestered in the nucleolus through inhibitory binding to Net1, from which it is released in anaphase following Net1 phosphorylation. Initial Net1 phosphorylation depends on Cdk itself, in conjunction with proteins of the Cdc14 Early Anaphase Release (FEAR) network. Later on, the Mitotic Exit Network (MEN) signaling cascade maintains Cdc14 release. An important unresolved question is how Cdc14 activity can increase in early anaphase, while Cdk activity, that is required for Net1 phosphorylation, decreases and the MEN is not yet active. Here we show that the nuclear rim protein Nur1 interacts with Net1 and, in its Cdk phosphorylated form, inhibits Cdc14 release. Nur1 is dephosphorylated by Cdc14 in early anaphase, relieving the inhibition and promoting further Cdc14 release. Nur1 dephosphorylation thus describes a positive feedback loop in Cdc14 phosphatase activation during mitotic exit, required for faithful chromosome segregation and completion of the cell division cycle.

During the cell cycle, a specific sequence of events leads to the formation of two daughter cells from one mother cell. Progression through the cell cycle is tightly controlled, with events occurring in the right place at the right time. Exactly how this is achieved is still being elucidated. In budding yeast, the events occurring during the final cell cycle phase – “mitotic exit” – are controlled by the phosphatase Cdc14. It is kept sequestered and inactive until it is needed for mitotic exit, at which time it is rapidly released. In this study, we have identified a new regulator of Cdc14 activity, the protein Nur1. In a series of experiments, we saw that Nur1 acts both upstream and downstream of Cdc14 activation, thereby creating a positive feedback loop. On the one hand, Nur1 contributes to inhibiting Cdc14 until the start of mitotic exit. On the other hand, through the actions of Cdc14 itself, Nur1 is disabled as an opponent of the phosphatase. This creates a robust system, rapidly switching between two opposing states and thus driving forward the mitotic exit transition.

The composition, functions, and regulation of the budding yeast kinetochore

The propagation of all organisms depends on the accurate and orderly segregation of chromosomes in mitosis and meiosis. Budding yeast has long served as an outstanding model organism to identify the components and underlying mechanisms that regulate chromosome segregation. This review focuses on the kinetochore, the macromolecular protein complex that assembles on centromeric chromatin and maintains persistent load-bearing attachments to the dynamic tips of spindle microtubules. The kinetochore also serves as a regulatory hub for the spindle checkpoint, ensuring that cell cycle progression is coupled to the achievement of proper microtubule-kinetochore attachments. Progress in understanding the composition and overall architecture of the kinetochore, as well as its properties in making and regulating microtubule attachments and the spindle checkpoint, is discussed.

A dynamical model of the spindle position checkpoint

The spindle position checkpoint (SPOC) is an important surveillance mechanism in the budding yeast cell cycle. An integrated approach, combining quantitative experimental cell biology with mathematical modeling, reveals how the SPOC inhibits mitotic exit at the molecular level.

We used fluorescence microscopy to quantify the number of molecules of Bfa1, Bub2 and Tem1 at the spindle pole bodies, and the size of the GAP-dependent and -independent Tem1 pools that coexist during mitosis.

We constructed a dynamical model of Tem1 regulation by Bfa1–Bub2.

Based on in-silico evidence supported by in-vivo data, we propose that cytoplasmic regulation of Tem1 by the GAP complex is critical for robust spindle position checkpoint arrest.

Our model also indicates the necessity of additional mechanisms of GAP inhibition for checkpoint silencing after spindle realignment.

The orientation of the mitotic spindle with respect to the polarity axis is crucial for the accuracy of asymmetric cell division. In budding yeast, a surveillance mechanism called the spindle position checkpoint (SPOC) prevents exit from mitosis when the mitotic spindle fails to align along the mother-to-daughter polarity axis. SPOC arrest relies upon inhibition of the GTPase Tem1 by the GTPase-activating protein (GAP) complex Bfa1–Bub2. Importantly, reactions signaling mitotic exit take place at yeast centrosomes (named spindle pole bodies, SPBs) and the GAP complex also promotes SPB localization of Tem1. Yet, whether the regulation of Tem1 by Bfa1–Bub2 takes place only at the SPBs remains elusive. Here, we present a quantitative analysis of Bfa1–Bub2 and Tem1 localization at the SPBs. Based on the measured SPB-bound protein levels, we introduce a dynamical model of the SPOC that describes the regulation of Bfa1 and Tem1. Our model suggests that Bfa1 interacts with Tem1 in the cytoplasm as well as at the SPBs to provide efficient Tem1 inhibition.

Monitoring spindle orientation: Spindle position checkpoint in charge

Every cell division in budding yeast is inherently asymmetric and counts on the correct positioning of the mitotic spindle along the mother-daughter polarity axis for faithful chromosome segregation. A surveillance mechanism named the spindle position checkpoint (SPOC), monitors the orientation of the mitotic spindle and prevents cells from exiting mitosis when the spindle fails to align along the mother-daughter axis. SPOC is essential for maintenance of ploidy in budding yeast and similar mechanisms might exist in higher eukaryotes to ensure faithful asymmetric cell division. Here, we review the current model of SPOC activation and highlight the importance of protein localization and phosphorylation for SPOC function.

Periodic cyclin-Cdk activity entrains an autonomous Cdc14 release oscillator

One oscillation of Cyclin-dependent kinase (Cdk) activity, largely driven by periodic synthesis and destruction of cyclins, is tightly coupled to a single complete eukaryotic cell division cycle. Tight linkage of different steps in diverse cell-cycle processes to Cdk activity has been proposed to explain this coupling. Here, we demonstrate an intrinsically oscillatory module controlling nucleolar release and resequestration of the Cdc14 phosphatase, which is essential for mitotic exit in budding yeast. We find that this Cdc14 release oscillator functions at constant and physiological cyclin-Cdk levels, and is therefore independent of the Cdk oscillator. However, the frequency of the release oscillator is regulated by cyclin-Cdk activity. This observation together with its mechanism suggests that the intrinsically autonomous Cdc14 release cycles are locked at once-per-cell-cycle through entrainment by the Cdk oscillator in wild-type cells. This concept may have broad implications for the structure and evolution of eukaryotic cell-cycle control.

Integrating high-throughput genetic interaction mapping and high-content screening to explore yeast spindle morphogenesis

A combination of yeast genetics, synthetic genetic array analysis, and high-throughput screening reveals that sumoylation of Mcm21p promotes disassembly of the mitotic spindle.

We describe the application of a novel screening approach that combines automated yeast genetics, synthetic genetic array (SGA) analysis, and a high-content screening (HCS) system to examine mitotic spindle morphogenesis. We measured numerous spindle and cellular morphological parameters in thousands of single mutants and corresponding sensitized double mutants lacking genes known to be involved in spindle function. We focused on a subset of genes that appear to define a highly conserved mitotic spindle disassembly pathway, which is known to involve Ipl1p, the yeast aurora B kinase, as well as the cell cycle regulatory networks mitotic exit network (MEN) and fourteen early anaphase release (FEAR). We also dissected the function of the kinetochore protein Mcm21p, showing that sumoylation of Mcm21p regulates the enrichment of Ipl1p and other chromosomal passenger proteins to the spindle midzone to mediate spindle disassembly. Although we focused on spindle disassembly in a proof-of-principle study, our integrated HCS-SGA method can be applied to virtually any pathway, making it a powerful means for identifying specific cellular functions.

Lte1 contributes to Bfa1 localization rather than stimulating nucleotide exchange by Tem1

Lte1 is a mitotic regulator long envisaged as a guanosine nucleotide exchange factor (GEF) for Tem1, the small guanosine triphosphatase governing activity of the Saccharomyces cerevisiae mitotic exit network. We demonstrate that this model requires reevaluation. No GEF activity was detectable in vitro, and mutational analysis of Lte1's putative GEF domain indicated that Lte1 activity relies on interaction with Ras for localization at the bud cortex rather than providing nucleotide exchange. Instead, we found that Lte1 can determine the subcellular localization of Bfa1 at spindle pole bodies (SPBs). Under conditions in which Lte1 is essential, Lte1 promoted the loss of Bfa1 from the maternal SPB. Moreover, in cells with a misaligned spindle, mislocalization of Lte1 in the mother cell promoted loss of Bfa1 from one SPB and allowed bypass of the spindle position checkpoint. We observed that lte1 mutants display aberrant localization of the polarity cap, which is the organizer of the actin cytoskeleton. We propose that Lte1's role in cell polarization underlies its contribution to mitotic regulation.

The protein phosphatase 2A functions in the spindle position checkpoint by regulating the checkpoint kinase Kin4

In budding yeast, a surveillance mechanism known as the spindle position checkpoint (SPOC) ensures accurate genome partitioning. In the event of spindle misposition, the checkpoint delays exit from mitosis by restraining the activity of the mitotic exit network (MEN). To date, the only component of the checkpoint to be identified is the protein kinase Kin4. Furthermore, how the kinase is regulated by spindle position is not known. Here, we identify the protein phosphatase 2A (PP2A) in complex with the regulatory subunit Rts1 as a component of the SPOC. Loss of PP2A-Rts1 function abrogates the SPOC but not other mitotic checkpoints. We further show that the protein phosphatase functions upstream of Kin4, regulating the kinase's phosphorylation and localization during an unperturbed cell cycle and during SPOC activation, thus defining the phosphatase as a key regulator of SPOC function.

Bub2 regulation of cytokinesis and septation in budding yeast

Background

The mitotic exit network (MEN) is required for events at the end of mitosis such as degradation of mitotic cyclins and cytokinesis. Bub2 and its binding partner Bfa1 act as a GTPase activating protein (GAP) to negatively regulate the MEN GTPase Tem1. The Bub2/Bfa1 checkpoint pathway is required to delay the cell cycle in response to mispositioned spindles. In addition to its role in mitotic exit, Tem1 is required for actomyosin ring contraction.

Results

To test the hypothesis that the Bub2 pathway prevents premature actin ring assembly, we compared the timing of actin ring formation in wild type, bub2Δ, mad2Δ, and bub2Δmad2Δ cells both with and without microtubules. There was no difference in the timing of actin ring formation between wild type and mutant cells in a synchronized cell cycle. In the presence of nocodazole, both bub2Δ and mad2Δ cells formed rings after a delay of the same duration. Double mutant bub2Δmad2Δ and bfa1Δmad2Δ cells formed rings at the same time with and without nocodazole. To determine if Bub2 has an effect on actomyosin ring contraction through its regulation of Tem1, we used live cell imaging of Myo1-GFP in a bub2Δ strain. We found a significant decrease in the total time of contraction and an increase in rate of contraction compared to wild type cells. We also examined myosin contraction using Myo1-GFP in cells overexpressing an epitope tagged Bub2. Surprisingly, overexpression of Bub2 also led to a significant increase in the rate of contraction, as well as morphological defects. The chained cell phenotype caused by Bub2 overexpression could be rescued by co-overexpression of Tem1, and was not rescued by deletion of BFA1.

Conclusion

Our data indicate that the Bub2 checkpoint pathway does not have a specific role in delaying actin ring formation. The observed increase in the rate of myosin contraction in the bub2Δ strain provides evidence that the MEN regulates actomyosin ring contraction. Our data suggest that the overexpression of the Bub2 fusion protein acts as a dominant negative, leading to septation defects by a mechanism that is Tem1-dependent.

Cdc7p-Dbf4p regulates mitotic exit by inhibiting Polo kinase

Cdc7p-Dbf4p is a conserved protein kinase required for the initiation of DNA replication. The Dbf4p regulatory subunit binds Cdc7p and is essential for Cdc7p kinase activation, however, the N-terminal third of Dbf4p is dispensable for its essential replication activities. Here, we define a short N-terminal Dbf4p region that targets Cdc7p-Dbf4p kinase to Cdc5p, the single Polo kinase in budding yeast that regulates mitotic progression and cytokinesis. Dbf4p mediates an interaction with the Polo substrate-binding domain to inhibit its essential role during mitosis. Although Dbf4p does not inhibit Polo kinase activity, it nonetheless inhibits Polo-mediated activation of the mitotic exit network (MEN), presumably by altering Polo substrate targeting. In addition, although dbf4 mutants defective for interaction with Polo transit S-phase normally, they aberrantly segregate chromosomes following nuclear misorientation. Therefore, Cdc7p-Dbf4p prevents inappropriate exit from mitosis by inhibiting Polo kinase and functions in the spindle position checkpoint.

Cdc7p-Dbf4p is a two-subunit enzyme required to copy the genetic material present on every chromosome in a process termed DNA replication. Dbf4p is an essential regulatory subunit of this enzyme that likely directs the Cdc7p subunit to its targets within the cell. We found that Dbf4p physically interacts with another protein called Polo that acts during mitosis, a later step in the cell cycle when the newly copied chromosomes are equally divided to mother and daughter cells. Polo is a master regulator of mitosis and impacts many other proteins required for cell division. We determined that Cdc7p-Dbf4p is a Polo inhibitor and, further, that Cdc7p-Dbf4p delayed or prevented chromosome segregation when errors occurred during the cell division process. Interestingly, Dbf4p may bind the Polo substrate-binding domain using a type of interaction not previously described. Thus, we have uncovered a new activity for Cdc7p-Dbf4p in the cell cycle to inhibit chromosome segregation, and these findings impact multiple fields that investigate how cells accurately copy and segregate their chromosomes.

IQGAP and mitotic exit network (MEN) proteins are required for cytokinesis and re-polarization of the actin cytoskeleton in the budding yeast, Saccharomyces cerevisiae

In budding yeast the final stages of the cell division cycle, cytokinesis and cell separation, are distinct events that require to be coupled, both together and with mitotic exit. Here we demonstrate that mutations in genes of the mitotic exit network (MEN) prevent cell separation and are synthetically lethal in combination with both cytokinesis and septation defective mutations. Analysis of the synthetic lethal phenotypes reveals that Iqg1p functions in combination with the MEN components, Tem1p, Cdc15p Dbf20p and Dbf2p to govern the re-polarization of the actin cytoskeleton to either side of the bud neck. In addition phosphorylation of the conserved PCH protein, Hof1p, is dependent upon these activities and requires actin ring assembly. Recruitment of Dbf2p to the bud neck is dependent upon actin ring assembly and correlates with Hof1p phosphorylation. Failure to phosphorylate Hof1p results in the increased stability of the protein and its persistence at the bud neck. These data establish a mechanistic dependency of cell separation upon an intermediate step requiring actomyosin ring assembly.

Centrosomes in cellular regulation

Centrosomes, spindle pole bodies, and related structures in other organisms are a morphologically diverse group of organelles that share a common ability to nucleate and organize microtubules and are thus referred to as microtubule organizing centers or MTOCs. Features associated with MTOCs include organization of mitotic spindles, formation of primary cilia, progression through cytokinesis, and self-duplication once per cell cycle. Centrosomes bind more than 100 regulatory proteins, whose identities suggest roles in a multitude of cellular functions. In fact, recent work has shown that MTOCs are required for several regulatory functions including cell cycle transitions, cellular responses to stress, and organization of signal transduction pathways. These new liaisons between MTOCs and cellular regulation are the focus of this review. Elucidation of these and other previously unappreciated centrosome functions promises to yield exciting scientific discovery for some time to come.

Human LATS1 is a mitotic exit network kinase

The kinase LATS/WARTS is a tumor suppressor protein conserved in evolution, but its function at the molecular level is not well understood. We report here that human LATS1 interacts with MOB1A, a protein whose homologue in budding yeast associates with kinases involved in mitotic exit. This suggested that LATS1 may be a component of the previously uncharacterized mitotic exit network in higher eukaryotes. Indeed, moderate overexpression of human LATS1 in cells exposed to microtubule poisons facilitated mitotic exit, and this activity required MOB1A. Reciprocally, small interfering RNA-mediated suppression of LATS1 or MOB1A prolonged telophase, but had no effect on the length of the earlier phases of mitosis. A role of LATS1 in mitotic exit may explain its previously described abilities to induce G2 arrest and promote cytokinesis.

A small-molecule inhibitor of Mps1 blocks the spindle-checkpoint response to a lack of tension on mitotic chromosomes

The spindle checkpoint prevents chromosome loss by preventing chromosome segregation in cells with improperly attached chromosomes [1, 2 and 3]. The checkpoint senses defects in the attachment of chromosomes to the mitotic spindle [4] and the tension exerted on chromosomes by spindle forces in mitosis [5, 6 and 7]. Because many cancers have defects in chromosome segregation, this checkpoint may be required for survival of tumor cells and may be a target for chemotherapy. We performed a phenotype-based chemical-genetic screen in budding yeast and identified an inhibitor of the spindle checkpoint, called cincreasin. We used a genome-wide collection of yeast gene-deletion strains and traditional genetic and biochemical analysis to show that the target of cincreasin is Mps1, a protein kinase required for checkpoint function [8]. Despite the requirement for Mps1 for sensing both the lack of microtubule attachment and tension at kinetochores, we find concentrations of cincreasin that selectively inhibit the tension-sensitive branch of the spindle checkpoint. At these concentrations, cincreasin causes lethal chromosome missegregation in mutants that display chromosomal instability. Our results demonstrate that Mps1 can be exploited as a target and that inhibiting the tension-sensitive branch of the spindle checkpoint may be a way of selectively killing cancer cells that display chromosomal instability.

The mitotic exit network Mob1p-Dbf2p kinase complex localizes to the nucleus and regulates passenger protein localization

The Saccharomyces cerevisiae mitotic exit network (MEN) is a conserved signaling network that coordinates CDK inactivation, cytokinesis and G1 gene transcription. The MEN Cdc14p phosphatase is sequestered in the nucleolus and transiently released in early anaphase and telophase. Cdc14p mediates mitotic exit by dephosphorylating Cdk1p substrates and promoting Cdk1p inactivation. Cdc14p also regulates the localization of chromosomal passenger proteins, which redistribute from kinetochores to the mitotic spindle during anaphase. Here we present evidence that the MEN protein kinase complex Mob1p-Dbf2p localizes to mitotic nuclei and partially colocalizes with Cdc14p and kinetochore proteins. Chromatin immunoprecipitation (ChIP) experiments reveal that Mob1p, Dbf2p, and Cdc14p associate with centromere DNA and require the centromere binding protein Ndc10p for this association. We establish that Mob1p is essential for maintaining the localization of Aurora, INCENP, and Survivin chromosomal passenger proteins on anaphase spindles, whereas Cdc14p and the Mob1p-Dbf2p-activating kinase Cdc15p are required for establishing passenger protein localization on the spindle. Moreover, Mob1p, but not Cdc15p, is required for dissociating Aurora from the kinetochore region. These findings reveal kinetochores as sites for MEN signaling and implicate MEN in coordinating chromosome segregation and/or spindle integrity with mitotic exit and cytokinesis via regulation of chromosome passenger proteins.

Essential tension and constructive destruction: the spindle checkpoint and its regulatory links with mitotic exit

Replicated genetic material must be partitioned equally between daughter cells during cell division. The precision with which this is accomplished depends critically on the proper functioning of the mitotic spindle. The assembly, orientation and attachment of the spindle to the kinetochores are therefore constantly monitored by a surveillance mechanism termed the SCP (spindle checkpoint). In the event of malfunction, the SCP not only prevents chromosome segregation, but also inhibits subsequent mitotic events, such as cyclin destruction (mitotic exit) and cytokinesis. This concerted action helps to maintain temporal co-ordination among mitotic events. It appears that the SCP is primarily activated by either a lack of occupancy or the absence of tension at kinetochores. Once triggered, the inhibitory circuit bifurcates, where one branch restrains the sister chromatid separation by inhibiting the E3 ligase APC(Cdc20) (anaphase-promoting complex activated by Cdc20) and the other impinges on the MEN (mitotic exit network). A large body of investigations has now led to the identification of the control elements, their targets and the functional coupling among them. Here we review the emerging regulatory network and discuss the remaining gaps in our understanding of this effective mechanochemical control system.

Closing mitosis: the functions of the Cdc14 phosphatase and its regulation

Completion of the cell cycle requires the temporal and spatial coordination of chromosome segregation with mitotic spindle disassembly and cytokinesis. In budding yeast, the protein phosphatase Cdc14 is a key regulator of these late mitotic events. Here, we review the functions of Cdc14 and how this phosphatase is regulated to accomplish the coupling of mitotic processes. We also discuss the function and regulation of Cdc14 in other eukaryotes, emphasizing conserved features.

Mitotic checkpoint function in the formation of gross chromosomal rearrangements in Saccharomyces cerevisiae

The accumulation of gross chromosomal rearrangements (GCRs) is characteristic of cancer cells. Multiple pathways that prevent GCRs, including S-phase cell cycle checkpoints, homologous recombination, telomere maintenance, suppression of de novo telomere addition, chromatin assembly, and mismatch repair, have been identified in Saccharomyces cerevisiae. However, pathways that promote the formation of GCRs are not as well understood. Of these, the de novo telomere addition pathway and nonhomologous end-joining are the best characterized. Here, we demonstrate that defects in the mitotic checkpoint and the mitotic exit network can suppress GCRs in strains containing defects that increase the GCR rate. These data suggest that functional mitotic checkpoints can play a role in the formation of genome rearrangements.

Mathematical Modeling of Complex Regulatory Networks

Cellular regulation comprises overwhelmingly complex interactions between genes and proteins that ultimately will only be rendered understandable by employing formal approaches. Developing large-scale mathematical models of such systems in an efficient and reliable way, however, requires careful evaluation of structuring principles for the models, of the description of the system dynamics, and of the experimental data basis for adjusting the models to reality. We discuss these three aspects of model development using the example of cell cycle regulation in yeast and suggest that capturing complex dynamic networks is feasible despite incomplete (quantitative) biological knowledge.

Functional characterization of Dma1 and Dma2, the budding yeast homologues of Schizosaccharomyces pombe Dma1 and human Chfr

Proper transmission of genetic information requires correct assembly and positioning of the mitotic spindle, responsible for driving each set of sister chromatids to the two daughter cells, followed by cytokinesis. In case of altered spindle orientation, the spindle position checkpoint inhibits Tem1-dependent activation of the mitotic exit network (MEN), thus delaying mitotic exit and cytokinesis until errors are corrected. We report a functional analysis of two previously uncharacterized budding yeast proteins, Dma1 and Dma2, 58% identical to each other and homologous to human Chfr and Schizosaccharomyces pombe Dma1, both of which have been previously implicated in mitotic checkpoints. We show that Dma1 and Dma2 are involved in proper spindle positioning, likely regulating septin ring deposition at the bud neck. DMA2 overexpression causes defects in septin ring disassembly at the end of mitosis and in cytokinesis. The latter defects can be rescued by either eliminating the spindle position checkpoint protein Bub2 or overproducing its target, Tem1, both leading to MEN hyperactivation. In addition, dma1Delta dma2Delta cells fail to activate the spindle position checkpoint in response to the lack of dynein, whereas ectopic expression of DMA2 prevents unscheduled mitotic exit of spindle checkpoint mutants treated with microtubule-depolymerizing drugs. Although their primary functions remain to be defined, our data suggest that Dma1 and Dma2 might be required to ensure timely MEN activation in telophase.

The spindle assembly and spindle position checkpoints

The mitotic spindle segregates chromosomes to opposite ends of the cell in preparation for cell division. Chromosome attachment to the spindle is monitored by the spindle assembly checkpoint, and at least in yeast cells, penetration of one spindle pole into the bud is monitored by the spindle position checkpoint. We review the historical origins of these checkpoints and recent progress in understanding their surveillance pathways. We also highlight fascinating but as yet unresolved questions, and examine crosstalk between the checkpoints.

The differential roles of budding yeast Tem1p, Cdc15p, and Bub2p protein dynamics in mitotic exit

In the budding yeast Saccharomyces cerevisiae the mitotic spindle must be positioned along the mother-bud axis to activate the mitotic exit network (MEN) in anaphase. To examine MEN proteins during mitotic exit, we imaged the MEN activators Tem1p and Cdc15p and the MEN regulator Bub2p in vivo. Quantitative live cell fluorescence microscopy demonstrated the spindle pole body that segregated into the daughter cell (dSPB) signaled mitotic exit upon penetration into the bud. Activation of mitotic exit was associated with an increased abundance of Tem1p-GFP and the localization of Cdc15p-GFP on the dSPB. In contrast, Bub2p-GFP fluorescence intensity decreased in mid-to-late anaphase on the dSPB. Therefore, MEN protein localization fluctuates to switch from Bub2p inhibition of mitotic exit to Cdc15p activation of mitotic exit. The mechanism that elevates Tem1p-GFP abundance in anaphase is specific to dSPB penetration into the bud and Dhc1p and Lte1p promote Tem1p-GFP localization. Finally, fluorescence recovery after photobleaching (FRAP) measurements revealed Tem1p-GFP is dynamic at the dSPB in late anaphase. These data suggest spindle pole penetration into the bud activates mitotic exit, resulting in Tem1p and Cdc15p persistence at the dSPB to initiate the MEN signal cascade.

The role of the polo kinase Cdc5 in controlling Cdc14 localization

In budding yeast, the protein phosphatase Cdc14 controls exit from mitosis. Its activity is regulated by a competitive inhibitor Cfi1/Net1, which binds to and sequesters Cdc14 in the nucleolus. During anaphase, Cdc14 is released from its inhibitor by the action of two regulatory networks. The Cdc Fourteen Early Anaphase Release (FEAR) network initiates Cdc14 release from Cfi1/Net1 during early anaphase, and the Mitotic Exit Network (MEN) promotes Cdc14 release during late anaphase. Here, we investigate the relationship among FEAR network components and propose an order in which they function to promote Cdc14 release from the nucleolus. Furthermore, we examine the role of the protein kinase Cdc5, which is a component of both the FEAR network and the MEN, in Cdc14 release from the nucleolus. We find that overexpression of CDC5 led to Cdc14 release from the nucleolus in S phase-arrested cells, which correlated with the appearance of phosphorylated forms of Cdc14 and Cfi1/Net1. Cdc5 promotes Cdc14 phosphorylation and, by stimulating the MEN, Cfi1/Net1 phosphorylation. Furthermore, we suggest that Cdc14 release from the nucleolus only occurs when Cdc14 and Cfi1/Net1 are both phosphorylated.

Mitotic exit regulation through distinct domains within the protein kinase Cdc15

The mitotic exit network (MEN), a Ras-like signaling cascade, promotes the release of the protein phosphatase Cdc14 from the nucleolus and is essential for cells to exit from mitosis in Saccharomyces cerevisiae. We have characterized the functional domains of one of the MEN components, the protein kinase Cdc15, and investigated the role of these domains in mitotic exit. We show that a region adjacent to Cdc15's kinase domain is required for self-association and for binding to spindle pole bodies and that this domain is essential for CDC15 function. Furthermore, we find that overexpression of CDC15 lacking the C-terminal 224 amino acids results in hyperactivation of MEN and premature release of Cdc14 from the nucleolus, suggesting that this domain within Cdc15 functions to inhibit MEN signaling. Our findings indicate that multiple modes of MEN regulation occur through the protein kinase Cdc15.

In vitro regulation of budding yeast Bfa1/Bub2 GAP activity by Cdc5

The Cdc5 protein of budding yeast is a polo-like kinase that has multiple roles in mitosis including control of the mitotic exit network (MEN). MEN activity brings about loss of mitotic kinase activity so that the mitotic spindle is disassembled and cytokinesis can proceed. Activity of the MEN is regulated by a small GTPase, Tem1, which in turn is controlled by a two-component GTPase-activating protein (GAP) formed by Bfa1 and Bub2. Bfa1 has been identified as a regulatory target of Cdc5 but there are conflicting deductions from indirect in vivo assays as to whether phosphorylation inhibits or stimulates Bfa1 activity. To resolve this question, we have used direct in vitro assays to observe the effects of phosphorylation on Bfa1 activity. We show that when Bfa1 is phosphorylated by Cdc5, its GAP activity with Bub2 is inhibited although its ability to interact with Tem1 is unaffected. Thus, in vivo inactivation of Bfa1-Bub2 by Cdc5 would have a positive regulatory effect by increasing levels of Tem1-GTP so stimulating exit from mitosis.

Nuclear localization of the cell cycle regulator CDH1 and its regulation by phosphorylation

The anaphase-promoting complex activated by CDC20 and CDH1 is a major ubiquitination system that controls the destruction of cell cycle regulators. Exactly how ubiquitination is regulated in time and space is incompletely understood. Here we report on the cell cycle-dependent localization of CDH1 and its regulation by phosphorylation. CDH1 localizes dynamically to the nucleus during interphase and to the centrosome during metaphase and anaphase. The nuclear accumulation of CDH1 correlates with a reduction in the steady-state amount of cyclin A, but not of cyclin E. A nuclear localization signal conserved in various species was identified in CDH1, and it sufficiently targets green fluorescent protein to the nucleus. Interestingly, a CDH1-4D mutant mimicking the hyperphosphorylated form was constitutively found in the cytoplasm. In further support of the notion that phosphorylation inhibits nuclear import, the nuclear localization signal of CDH1 with two phospho-accepting serine/threonine residues changed into aspartates was unable to drive heterologous protein into the nucleus. On the other hand, abolition of the cyclin-binding ability of CDH1 has no influence on its nuclear localization. Taken together, our findings document the phosphorylation-dependent localization of CDH1 in vertebrate cells.

Human MPS1 kinase is required for mitotic arrest induced by the loss of CENP-E from kinetochores

We have determined that the previously identified dual-specificity protein kinase TTK is the human orthologue of the yeast MPS1 kinase. Yeast MPS1 (monopolar spindle) is required for spindle pole duplication and the spindle checkpoint. Consistent with the recently identified vertebrate MPS1 homologues, we found that hMPS1 is localized to centrosomes and kinetochores. In addition, hMPS1 is part of a growing list of kinetochore proteins that are localized to nuclear pores. hMPS1 is required by cells to arrest in mitosis in response to spindle defects and kinetochore defects resulting from the loss of the kinesin-like protein, CENP-E. The pattern of kinetochore localization of hMPS1 in CENP-E defective cells suggests that their interaction with the kinetochore is sensitive to microtubule occupancy rather than kinetochore tension. hMPS1 is required for MAD1, MAD2 but not hBUB1, hBUBR1 and hROD to bind to kinetochores. We localized the kinetochore targeting domain in hMPS1 and found that it can abrogate the mitotic checkpoint in a dominant negative manner. Last, hMPS1 was found to associate with the anaphase promoting complex, thus raising the possibility that its checkpoint functions extend beyond the kinetochore.

Budding yeast PAK kinases regulate mitotic exit by two different mechanisms

We report the characterization of the dominant-negative CLA4t allele of the budding yeast CLA4 gene, encoding a member of the p21-activated kinase (PAK) family of protein kinases, which, together with its homologue STE20, plays an essential role in promoting budding and cytokinesis. Overproduction of the Cla4t protein likely inhibits both endogenous Cla4 and Ste20 and causes a delay in the onset of anaphase that correlates with inactivation of Cdc20/anaphase-promoting complex (APC)-dependent proteolysis of both the cyclinB Clb2 and securin. Although the precise mechanism of APC inhibition by Cla4t remains to be elucidated, our results suggest that Cla4 and Ste20 may regulate the first wave of cyclinB proteolysis mediated by Cdc20/APC, which has been shown to be crucial for activation of the mitotic exit network (MEN). We show that the Cdk1-inhibitory kinase Swe1 is required for the Cla4t-dependent delay in cell cycle progression, suggesting that it might be required to prevent full Cdc20/APC and MEN activation. In addition, inhibition of PAK kinases by Cla4t prevents mitotic exit also by a Swe1-independent mechanism impinging directly on the MEN activator Tem1.

The spindle checkpoint: structural insights into dynamic signalling

Chromosome segregation is a complex and astonishingly accurate process whose inner working is beginning to be understood at the molecular level. The spindle checkpoint plays a key role in ensuring the fidelity of this process. It monitors the interactions between chromosomes and microtubules, and delays mitotic progression to allow extra time to correct defects. Here, we review and integrate findings on the dynamics of checkpoint proteins at kinetochores with structural information about signalling complexes.

Control of mitotic exit in budding yeast. In vitro regulation of Tem1 GTPase by Bub2 and Bfa1

The elimination of mitotic kinase activity at the end of mitosis is essential for progression to the next stage of the eukaryotic cell cycle. In budding yeast, this process is controlled by a regulatory cascade called the mitotic exit network. Extensive genetic data indicate that mitotic exit network activity is determined by a GTP-binding protein, Tem1, and its putative regulators, Bub2, Bfa1, and Lte1. Here we describe the purification and in vitro activities of Tem1, Bub2, and Bfa1. We describe the nucleotide binding properties of Tem1 and characterize its intrinsic GTPase activity. The combination of Bfa1 and Bub2 acts as a two-component GTPase-activating protein for Tem1. In the absence of Bub2, Bfa1 inhibits the GTPase and GTP exchange activities of Tem1. This inhibition is elicited by either the N- or C-terminal regions of Bfa1, which also retain some ability to co-activate GTPase activity in the presence of Bub2. Although the C-terminal region of Bfa1 binds to Bub2, no interaction of the N-terminal half of Bfa1 with Bub2 was detected despite their combined GAP activity. Therefore, we propose that Bfa1 acts both as an adaptor to connect Bub2 and Tem1 and as an allosteric effector that facilitates this interaction.

Mitotic exit: the Cdc14 double cross

The release of Cdc14 from the nucleolus occurs in two waves in early and late anaphase, controlled by the FEAR and MEN pathways, respectively. Two new papers report the localisation at the spindle pole body of the Cdc14 released in early anaphase and, surprisingly, show that the two pulses of released Cdc14 have opposite effects on MEN activation.

Mitotic exit network controls the localization of Cdc14 to the spindle pole body in Saccharomyces cerevisiae

Budding yeast Cdc14 phosphatase plays essential roles in mitotic exit. Cdc14 is sequestered in the nucleolus by its inhibitor Net1/Cfi1 and is only released from the nucleolus during anaphase to inactivate mitotic CDK. It is believed that the mitotic exit network (MEN) is required for the release of Cdc14 from the nucleolus because liberation of Cdc14 by net1/cfi1 mutations bypasses the essential role of the MEN. But how the MEN residing at the spindle pole body (SPB) controls the association of Cdc14 with Net1/Cfi1 in the nucleolus is not yet understood. We found that Cdc14-5GFP was released from the nucleolus in the MEN mutants (tem1, cdc15, dbf2, and nud1), but not in the cdc5 cells during early anaphase. The Cdc14 liberation from the nucleolus was inhibited by the Mad2 checkpoint and by the Bub2 checkpoint in a different manner when microtubule organization was disrupted. We observed Cdc14-5GFP at the SPB in addition to the nucleolus. The SPB localization of Cdc14 was significantly affected by the MEN mutations and the bub2 mutation. We conclude that Cdc14 is released from the nucleolus at the onset of anaphase in a CDC5-dependent manner and that MEN factors possibly regulate Cdc14 release from the SPB.

Cytokinesis in eukaryotes

Cytokinesis is the final event of the cell division cycle, and its completion results in irreversible partition of a mother cell into two daughter cells. Cytokinesis was one of the first cell cycle events observed by simple cell biological techniques; however, molecular characterization of cytokinesis has been slowed by its particular resistance to in vitro biochemical approaches. In recent years, the use of genetic model organisms has greatly advanced our molecular understanding of cytokinesis. While the outcome of cytokinesis is conserved in all dividing organisms, the mechanism of division varies across the major eukaryotic kingdoms. Yeasts and animals, for instance, use a contractile ring that ingresses to the cell middle in order to divide, while plant cells build new cell wall outward to the cortex. As would be expected, there is considerable conservation of molecules involved in cytokinesis between yeast and animal cells, while at first glance, plant cells seem quite different. However, in recent years, it has become clear that some aspects of division are conserved between plant, yeast, and animal cells. In this review we discuss the major recent advances in defining cytokinesis, focusing on deciding where to divide, building the division apparatus, and dividing. In addition, we discuss the complex problem of coordinating the division cycle with the nuclear cycle, which has recently become an area of intense research. In conclusion, we discuss how certain cells have utilized cytokinesis to direct development.

IBD2 encodes a novel component of the Bub2p-dependent spindle checkpoint in the budding yeast Saccharomyces cerevisiae

During mitosis, genomic integrity is maintained by the proper coordination of mitotic events through the spindle checkpoint. The bifurcated spindle checkpoint blocks cell cycle progression at metaphase by monitoring unattached kinetochores and inhibits mitotic exit in response to the incorrect orientation of the mitotic spindle. Bfa1p is a spindle checkpoint regulator of budding yeast in the Bub2p checkpoint pathway for proper mitotic exit. We have isolated a novel Bfa1p interacting protein named Ibd2p in the budding yeast Saccharomyces cerevisiae. We found that IBD2 (Inhibition of Bud Division 2) is not an essential gene but its deletion mutant proceeded through the cell cycle in the presence of microtubule-destabilizing drugs, thereby inducing a sharp decrease in viability. In addition, overexpression of Mps1p caused partial mitotic arrest in ibd2Delta as well as in bub2Delta, suggesting that IBD2 encodes a novel component of the spindle checkpoint downstream of MPS1. Overexpression of Ibd2p induced mitotic arrest with increased levels of Clb2p in wild type and mad2Delta, but not in deletion mutants of BUB2 and BFA1. Pds1p was also stabilized by the overexpression of Ibd2p in wild-type cells. The mitotic arrest defects observed in ibd2Delta in the presence of nocodazole were restored by additional copies of BUB2, BFA1, and CDC5, whereas an extra copy of IBD2 could not rescue the mitotic arrest defects of bub2Delta and bfa1Delta. The mitotic arrest defects of ibd2Delta were not recovered by MAD2, or vice versa. Analysis of the double mutant combinations ibd2Deltamad2Delta, ibd2Deltabub2Delta, and ibd2Deltadyn1Delta showed that IBD2 belongs to the BUB2 epistasis group. Taken together, these data demonstrate that IBD2 encodes a novel component of the BUB2-dependent spindle checkpoint pathway that functions upstream of BUB2 and BFA1.

Regulation of the Bfa1p-Bub2p complex at spindle pole bodies by the cell cycle phosphatase Cdc14p

The budding yeast mitotic exit network (MEN) is a GTPase-driven signal transduction cascade that controls the release of the phosphatase Cdc14p from the nucleolus in anaphase and thereby drives mitotic exit. We show that Cdc14p is partially released from the nucleolus in early anaphase independent of the action of the MEN components Cdc15p, Dbf2p, and Tem1p. Upon release, Cdc14p binds to the spindle pole body (SPB) via association with the Bfa1p-Bub2p GTPase activating protein complex, which is known to regulate the activity of the G protein Tem1p. Cdc14p also interacts with this GTPase. The association of the MEN component Mob1p with the SPB acts as a marker of MEN activation. The simultaneous binding of Cdc14p and Mob1p to the SPB in early anaphase suggests that Cdc14p initially activates the MEN. In a second, later step, which coincides with mitotic exit, Cdc14p reactivates the Bfa1p-Bub2p complex by dephosphorylating Bfa1p. This inactivates the MEN and displaces Mob1p from SPBs. These data indicate that Cdc14p activates the MEN in early anaphase but later inactivates it through Bfa1p dephosphorylation and so restricts MEN activity to a short period in anaphase.

Bfa1 can regulate Tem1 function independently of Bub2 in the mitotic exit network of Saccharomyces cerevisiae

In budding yeast, exit from mitosis is achieved by inactivation of Cdc28/Clb2 activity. Although it is not clear at present how mitotic exit is triggered, a growing body of evidence suggests that the Tem1 GTPase plays a critical role in mediating this pathway and that Bfa1 and Bub2 constitute a two-component GTPase-activating protein to negatively regulate Tem1. Here, we have demonstrated that introduction of bfa1 Delta suppresses the growth defects associated with the cdc5-1 mutation significantly better than that of bub2 Delta, suggesting that Bfa1 may have a previously uncharacterized role in this pathway. Overexpression of BFA1 efficiently arrested the cell cycle at postanaphase even in the absence of BUB2, whereas overexpression of BUB2 weakly induced mitotic arrest only in the presence of BFA1. Coimmunoprecipitation and in vitro binding studies indicate that Bfa1 binds strongly to Tem1 independently of Bub2. Provision of GDP+AlF(4)(-), which mimics the GTPase transition state, enhanced the Bub2-Tem1 interaction both in vitro and in vivo. Interestingly, introduction of bfa1 Delta, but not bub2 Delta, greatly increased the interaction between Tem1 and Cdc15, a step that is thought to be critical for activating the mitotic exit network. Our data suggest that, in addition to its role as a putative, two-component GTPase-activating protein with Bub2, Bfa1 also can play a role in the regulation of mitotic exit by directly inhibiting the interaction between Tem1 and Cdc15 even in the absence of Bub2.

Bub3 interaction with Mad2, Mad3 and Cdc20 is mediated by WD40 repeats and does not require intact kinetochores

The kinetochore checkpoint pathway, involving the Mad1, Mad2, Mad3, Bub1, Bub3 and Mps1 proteins, prevents anaphase entry and mitotic exit by inhibiting the anaphase promoting complex activator Cdc20 in response to monopolar attachment of sister kinetochores to spindle fibres. We show here that Cdc20, which had previously been shown to interact physically with Mad2 and Mad3, associates also with Bub3 and association is up-regulated upon checkpoint activation. Moreover, co-fractionation experiments suggest that Mad2, Mad3 and Bub3 may be concomitantly present in protein complexes with Cdc20. Formation of the Bub3-Cdc20 complex requires all kinetochore checkpoint proteins but, surprisingly, not intact kinetochores. Conversely, point mutations altering the conserved WD40 motifs of Bub3, which might be involved in the formation of a beta-propeller fold devoted to protein-protein interactions, disrupt its association with Mad2, Mad3 and Cdc20, as well as proper checkpoint response. We suggest that Bub3 could serve as a platform for interactions between kinetochore checkpoint proteins, and its association with Mad2, Mad3 and Cdc20 might be instrumental for checkpoint activation.

The role of the yeast spindle pole body and the mammalian centrosome in regulating late mitotic events

Centrosomes of vertebrate cells and spindle pole bodies (SPBs) of fungi were first recognized through their ability to organize microtubules. Recent studies suggest that centrosomes and SPBs also have a function in the regulation of cell cycle progression, in particular in controlling late mitotic events. Regulators of mitotic exit and cytokinesis are associated with the SPB of budding and fission yeast. Elucidation of the molecular roles played by these regulators is helping to clarify the function of the SPB in controlling progression though mitosis.

The budding yeast protein kinase Ipl1/Aurora allows the absence of tension to activate the spindle checkpoint

The spindle checkpoint prevents cell cycle progression in cells that have mitotic spindle defects. Although several spindle defects activate the spindle checkpoint, the exact nature of the primary signal is unknown. We have found that the budding yeast member of the Aurora protein kinase family, Ipl1p, is required to maintain a subset of spindle checkpoint arrests. Ipl1p is required to maintain the spindle checkpoint that is induced by overexpression of the protein kinase Mps1. Inactivating Ipl1p allows cells overexpressing Mps1p to escape from mitosis and segregate their chromosomes normally. Therefore, the requirement for Ipl1p in the spindle checkpoint is not a consequence of kinetochore and/or spindle defects. The requirement for Ipl1p distinguishes two different activators of the spindle checkpoint: Ipl1p function is required for the delay triggered by chromosomes whose kinetochores are not under tension, but is not required for arrest induced by spindle depolymerization. Ipl1p localizes at or near kinetochores during mitosis, and we propose that Ipl1p is required to monitor tension at the kinetochore.