Triap1 upregulation promotes escape from mitotic-slippage-induced G1 arrest

Drugs targeting microtubules rely on the mitotic checkpoint to arrest cell proliferation. The prolonged mitotic arrest induced by such drugs is followed by a G1 arrest. Here, we follow for several weeks the fate of G1-arrested human cells after treatment with nocodazole. We find that a small fraction of cells escapes from the arrest and resumes proliferation. These escaping cells experience reduced DNA damage and p21 activation. Cells surviving treatment are enriched for anti-apoptotic proteins, including Triap1. Increasing Triap1 levels allows cells to survive the first treatment with reduced DNA damage and lower levels of p21; accordingly, decreasing Triap1 re-sensitizes cells to nocodazole. We show that Triap1 upregulation leads to the retention of cytochrome c in the mitochondria, opposing the partial activation of caspases caused by nocodazole. In summary, our results point to a potential role of Triap1 upregulation in the emergence of resistance to drugs that induce prolonged mitotic arrest.

Epistasis, aneuploidy, and functional mutations underlie evolution of resistance to induced microtubule depolymerization

Cells with blocked microtubule polymerization are delayed in mitosis, but eventually manage to proliferate despite substantial chromosome missegregation. While several studies have analyzed the first cell division after microtubule depolymerization, we have asked how cells cope long-term with microtubule impairment. We allowed 24 clonal populations of yeast cells with beta-tubulin mutations preventing proper microtubule polymerization, to evolve for ˜150 generations. At the end of the laboratory evolution experiment, cells had regained the ability to form microtubules and were less sensitive to microtubule-depolymerizing drugs. Whole-genome sequencing identified recurrently mutated genes, in particular for tubulins and kinesins, as well as pervasive duplication of chromosome VIII. Recreating these mutations and chromosome VIII disomy prior to evolution confirmed that they allow cells to compensate for the original mutation in beta-tubulin. Most of the identified mutations did not abolish function, but rather restored microtubule functionality. Analysis of the temporal order of resistance development in independent populations repeatedly revealed the same series of events: disomy of chromosome VIII followed by a single additional adaptive mutation in either tubulins or kinesins. Since tubulins are highly conserved among eukaryotes, our results have implications for understanding resistance to microtubule-targeting drugs widely used in cancer therapy.

Cellular response upon proliferation in the presence of an active mitotic checkpoint

Cells that replicate with an active mitotic checkpoint remain capable to mount multiple times an efficient arrest, are bigger than unperturbed cells, rely more heavily on Cdh1, and have an altered protein expression profile.

Eukaryotic cells treated with microtubule-targeting agents activate the spindle assembly checkpoint to arrest in mitosis and prevent chromosome mis-segregation. A fraction of mitotically arrested cells overcomes the block and proliferates even under persistent checkpoint-activating conditions. Here, we asked what allows proliferation in such unfavourable conditions. We report that yeast cells are delayed in mitosis at each division, implying that their spindle assembly checkpoint remains responsive. The arrest causes their cell cycle to be elongated and results in a size increase. Growth saturates at mitosis and correlates with the repression of various factors involved in translation. Contrary to unperturbed cells, growth of cells with an active checkpoint requires Cdh1. This peculiar cell cycle correlates with global changes in protein expression whose signatures partly overlap with the environmental stress response. Hence, cells dividing with an active checkpoint develop recognisable specific traits that allow them to successfully complete cell division notwithstanding a constant mitotic checkpoint arrest. These properties distinguish them from unperturbed cells. Our observation may have implications for the identification of new therapeutic windows and targets in tumors.

Cells Escape an Operational Mitotic Checkpoint through a Stochastic Process

Improperly attached chromosomes activate the mitotic checkpoint that arrests cell division before anaphase. Cells can maintain an arrest for several hours but eventually will resume proliferation, a process we refer to as adaptation. Whether adapting cells bypass an active block or whether the block has to be removed to resume proliferation is not clear. Likewise, it is not known whether all cells of a genetically homogeneous population are equally capable to adapt. Here, we show that the mitotic checkpoint is operational when yeast cells adapt and that each cell has the same propensity to adapt. Our results are consistent with a model of the mitotic checkpoint where adaptation is driven by random fluctuations of APC/C^Cdc20, the molecular species inhibited by the checkpoint. Our data provide a quantitative framework for understanding how cells overcome a constant stimulus that halts cell cycle progression.

Budding yeast Swe1 is involved in the control of mitotic spindle elongation and is regulated by Cdc14 phosphatase during mitosis

Cyclin-dependent kinase (Cdk1) activity is required for mitotic entry, and this event is restrained by an inhibitory phosphorylation of the catalytic subunit Cdc28 on a conserved tyrosine (Tyr(19)). This modification is brought about by the protein kinase Swe1 that inhibits Cdk1 activation thus blocking mitotic entry. Swe1 levels are regulated during the cell cycle, and they decrease during G2/M concomitantly to Cdk1 activation, which drives entry into mitosis. However, after mitotic entry, a pool of Swe1 persists, and we collected evidence that it is involved in controlling mitotic spindle elongation. We also describe that the protein phosphatase Cdc14 is implicated in Swe1 regulation; in fact, we observed that Swe1 dephosphorylation in vivo depends on Cdc14 that, in turn, is able to control its subcellular localization. In addition we show that the lack of Swe1 causes premature mitotic spindle elongation and that high levels of Swe1 block mitotic spindle elongation, indicating that Swe1 inhibits this process. Importantly, these effects are not dependent upon the role of in Cdk1 inhibition. These data fit into a model in which Cdc14 binds and inhibits Swe1 to allow timely mitotic spindle elongation.

Vhs2 is a novel regulator of septin dynamics in budding yeast

In budding yeast, septins are assembled into structures that undergo dramatic changes during the cell cycle. The molecular mechanisms that drive these remodelings are not fully uncovered. In this study, we describe a characterization of Vhs2, a nonessential protein that revealed to be a new player in septin dynamics. In particular, we report that Vhs2 is important to maintain the stability of the double septin ring structure until telophase. In addition, we show that Vhs2 undergoes multiple phosphorylations during the cell cycle, being phosphorylated during S phase until nuclear division and dephosphorylated just before cell division. Importantly we report that cyclin-dependent protein kinase Cdk1 and protein phosphatase Cdc14 control these Vhs2 post-translational modifications. These results reveal that Vhs2 is a novel Cdc14 substrate that is involved in the control of septin organization.

Adaptation to the spindle checkpoint is regulated by the interplay between Cdc28/Clbs and PP2ACdc55

PP2A^Cdc55 dephosphorylates APC/C^Cdc20 to prevent anaphase, an effect that is counteracted by Cdc28/Clbs to allow for spindle checkpoint adaptation.

The spindle checkpoint arrests cells in metaphase until all chromosomes are properly attached to the chromosome segregation machinery. Thereafter, the anaphase promoting complex (APC/C) is activated and chromosome segregation can take place. Cells remain arrested in mitosis for hours in response to checkpoint activation, but not indefinitely. Eventually, they adapt to the checkpoint and proceed along the cell cycle. In yeast, adaptation requires the phosphorylation of APC/C. Here, we show that the protein phosphatase PP2A^Cdc55 dephosphorylates APC/C, thereby counteracting the activity of the mitotic kinase Cdc28. We also observe that the key regulator of Cdc28, the mitotic cyclin Clb2, increases before cells adapt and is then abruptly degraded at adaptation. Adaptation is highly asynchronous and takes place over a range of several hours. Our data suggest the presence of a double negative loop between PP2A^Cdc55 and APC/C^Cdc20 (i.e., a positive feedback loop) that controls APC/C^Cdc20 activity. The circuit could guarantee sustained APC/C^Cdc20 activity after Clb2 starts to be degraded.

Bub3 reads phosphorylated MELT repeats to promote spindle assembly checkpoint signaling

Regulation of macromolecular interactions by phosphorylation is crucial in signaling networks. In the spindle assembly checkpoint (SAC), which enables errorless chromosome segregation, phosphorylation promotes recruitment of SAC proteins to tensionless kinetochores. The SAC kinase Mps1 phosphorylates multiple Met-Glu-Leu-Thr (MELT) motifs on the kinetochore subunit Spc105/Knl1. The phosphorylated MELT motifs (MELT^P) then promote recruitment of downstream signaling components. How MELT^P motifs are recognized is unclear. In this study, we report that Bub3, a 7-bladed β-propeller, is the MELT^P reader. It contains an exceptionally well-conserved interface that docks the MELT^P sequence on the side of the β-propeller in a previously unknown binding mode. Mutations targeting the Bub3 interface prevent kinetochore recruitment of the SAC kinase Bub1. Crucially, they also cause a checkpoint defect, showing that recognition of phosphorylated targets by Bub3 is required for checkpoint signaling. Our data provide the first detailed mechanistic insight into how phosphorylation promotes recruitment of checkpoint proteins to kinetochores.

DOI:http://dx.doi.org/10.7554/eLife.01030.001

The cell cycle is the process by which a cell divides to produce two near-identical daughter cells. Two crucial parts of the cell cycle are the duplication of the chromosomes in the original cell, and the segregation of these chromosomes between the two daughter cells. These and other parts of the cell cycle are strictly regulated to prevent errors, which can lead to cancer and other diseases.

After chromosome duplication has taken place, the pairs of identical chromosomes, known as sister chromatids, remain tightly bound to each other. These sister chromatids line up in the middle of the cell, with protein filaments called microtubules connecting them to a bipolar structure called the spindle. For the cell to divide correctly, the sister chromatids in each pair must be connected to opposite poles of the spindle. A signalling network known as the spindle assembly checkpoint (SAC) ensures that the sister chromatids have enough time to line up correctly and to correct possible problems. Once everything is in place, the SAC releases its ‘break’, and the microtubules then pull the sister chromatids away from each other. This way, each daughter cell receives the same complement of chromosomes that was present in the mother cell.

The microtubules are not directly attached to the sister chromatids but to protein complexes called kinetochores that assemble on each sister chromatid. In particular, each microtubule binds to a very large protein complex called the KMN network. Knl1, which is part of this network, recruits two SAC proteins–Bub1 and Bub3–to the kinetochore. It is known that a phosphate group is added to Knl1 when the SAC is active, and that Knl1 can only recruit Bub1 and Bub3 after it has been phosphorylated. However, the details of the interactions between Knl1, Bub1 and Bub3 are not understood, and it is not clear whether these interactions are essential for the SAC.

Now Primorac et al. have shown that Bub3 binds directly to Knl1 through a region that contains multiple MELT motifs (where M, E, L and T are all amino acids), and that this interaction only happens if these ‘MELT repeats’ have been phosphorylated. Moreover, once bound to the Knl1, Bub3 then recruits Bub1 to the kinetochore. By showing that the recognition of phosphorylated Knl1 by the Bub1-Bub3 complex has a central role in the spindle assembly checkpoint, these results highlight the importance of phosphorylation as a way of regulating the timing of events during the cell cycle.

DOI:http://dx.doi.org/10.7554/eLife.01030.002

Saccharomyces cerevisiae Dma proteins participate in cytokinesis by controlling two different pathways

Cytokinesis completion in the budding yeast S. cerevisiae is driven by tightly regulated pathways, leading to actomyosin ring contraction coupled to plasma membrane constriction and to centripetal growth of the primary septum, respectively. These pathways can partially substitute for each other, but their concomitant inactivation leads to cytokinesis block and cell death. Here we show that both the lack of the functionally redundant FHA-RING ubiquitin ligases Dma1 and Dma2 and moderate Dma2 overproduction affect actomyosin ring contraction as well as primary septum deposition, although they do not apparently alter cell cycle progression of otherwise wild-type cells. In addition, overproduction of Dma2 impairs the interaction between Tem1 and Iqg1, which is thought to be required for AMR contraction, and causes asymmetric primary septum deposition as well as mislocalization of the Cyk3-positive regulator of this process. In agreement with these multiple inhibitory effects, a Dma2 excess that does not cause any apparent defect in wild-type cells leads to lethal cytokinesis block in cells lacking the Hof1 protein, which is essential for primary septum formation in the absence of Cyk3. Altogether, these findings suggest that the Dma proteins act as negative regulators of cytokinesis.

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.

The budding yeast PP2ACdc55 protein phosphatase prevents the onset of anaphase in response to morphogenetic defects

Faithful chromosome transmission requires establishment of sister chromatid cohesion during S phase, followed by its removal at anaphase onset. Sister chromatids are tethered together by cohesin, which is displaced from chromosomes through cleavage of its Mcd1 subunit by the separase protease. Separase is in turn inhibited, up to this moment, by securin. Budding yeast cells respond to morphogenetic defects by a transient arrest in G2 with high securin levels and unseparated chromatids. We show that neither securin elimination nor forced cohesin cleavage is sufficient for anaphase in these conditions, suggesting that other factors contribute to cohesion maintainance in G2. We find that the protein phosphatase PP2A bound to its regulatory subunit Cdc55 plays a key role in this process, uncovering a new function for PP2A(Cdc55) in controlling a noncanonical pathway of chromatid cohesion removal.

The spindle position checkpoint in budding yeast: the motherly care of MEN

Mitotic exit and cytokinesis must be tightly coupled to nuclear division both in time and space in order to preserve genome stability and to ensure that daughter cells inherit the right set of chromosomes after cell division. This is achieved in budding yeast through control over a signal transduction cascade, the mitotic exit network (MEN), which is required for mitotic CDK inactivation in telophase and for cytokinesis. Current models of MEN activation emphasize on the bud as the place where most control is exerted. This review focuses on recent data that instead point to the mother cell as being the residence of key regulators of late mitotic events.

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.