Kin4 kinase delays mitotic exit in response to spindle alignment defects

A noncanonical GTPase signaling mechanism controls exit from mitosis in budding yeast

In the budding yeast Saccharomyces cerevisiae, exit from mitosis is coupled to spindle position to ensure successful genome partitioning between mother and daughter cell. This coupling occurs through a GTPase signaling cascade known as the mitotic exit network (MEN). The MEN senses spindle position via a Ras-like GTPase Tem1 which localizes to the spindle pole bodies (SPBs, yeast equivalent of centrosomes) during anaphase and signals to its effector protein kinase Cdc15. How Tem1 couples the status of spindle position to MEN activation is not fully understood. Here, we show that Cdc15 has a relatively weak preference for Tem 1 GTP and Tem1's nucleotide state does not change upon MEN activation. Instead, we find that Tem1's nucleotide cycle establishes a localization-based concentration difference in the cell where only Tem 1 GTP is recruited to the SPB, and spindle position regulates the MEN by controlling Tem1 localization. SPB localization of Tem1 primarily functions to promote Tem1-Cdc15 interaction for MEN activation by increasing the effective concentration of Tem1. Consistent with this model, we demonstrate that artificially tethering Tem1 to the SPB or concentrating Tem1 in the cytoplasm with genetically encoded multimeric nanoparticles could bypass the requirement of Tem 1 GTP and correct spindle position for MEN activation. This localization/concentration-based GTPase signaling mechanism for Tem1 differs from the canonical Ras-like GTPase signaling paradigm and is likely relevant to other localization-based signaling scenarios.

The Ser/Thr protein kinase FonKin4-poly(ADP-ribose) polymerase FonPARP1 phosphorylation cascade is required for the pathogenicity of watermelon fusarium wilt fungus Fusarium oxysporum f. sp. niveum

Poly(ADP-ribosyl)ation (PARylation), catalyzed by poly(ADP-ribose) polymerases (PARPs) and hydrolyzed by poly(ADP-ribose) glycohydrolase (PARG), is a kind of post-translational protein modification that is involved in various cellular processes in fungi, plants, and mammals. However, the function of PARPs in plant pathogenic fungi remains unknown. The present study investigated the roles and mechanisms of FonPARP1 in watermelon Fusarium wilt fungus Fusarium oxysporum f. sp. niveum (Fon). Fon has a single PARP FonPARP1 and one PARG FonPARG1. FonPARP1 is an active PARP and contributes to Fon pathogenicity through regulating its invasive growth within watermelon plants, while FonPARG1 is not required for Fon pathogenicity. A serine/threonine protein kinase, FonKin4, was identified as a FonPARP1-interacting partner by LC-MS/MS. FonKin4 is required for vegetative growth, conidiation, macroconidia morphology, abiotic stress response and pathogenicity of Fon. The S_TKc domain is sufficient for both enzyme activity and pathogenicity function of FonKin4 in Fon. FonKin4 phosphorylates FonPARP1 in vitro to enhance its poly(ADP-ribose) polymerase activity; however, FonPARP1 does not PARylate FonKin4. These results establish the FonKin4-FonPARP1 phosphorylation cascade that positively contributes to Fon pathogenicity. The present study highlights the importance of PARP-catalyzed protein PARylation in regulating the pathogenicity of Fon and other plant pathogenic fungi.

Meiotic Cytokinesis in Saccharomyces cerevisiae: Spores That Just Need Closure

In the budding yeast Saccharomyces cerevisiae, sporulation occurs during starvation of a diploid cell and results in the formation of four haploid spores forming within the mother cell ascus. Meiosis divides the genetic material that is encapsulated by the prospore membrane that grows to surround the haploid nuclei; this membrane will eventually become the plasma membrane of the haploid spore. Cellularization of the spores occurs when the prospore membrane closes to capture the haploid nucleus along with some cytoplasmic material from the mother cell, and thus, closure of the prospore membrane is the meiotic cytokinetic event. This cytokinetic event involves the removal of the leading-edge protein complex, a complex of proteins that localizes to the leading edge of the growing prospore membrane. The development and closure of the prospore membrane must be coordinated with other meiotic exit events such as spindle disassembly. Timing of the closure of the prospore membrane depends on the meiotic exit pathway, which utilizes Cdc15, a Hippo-like kinase, and Sps1, an STE20 family GCKIII kinase, acting in parallel to the E3 ligase Ama1-APC/C. This review describes the sporulation process and focuses on the development of the prospore membrane and the regulation of prospore membrane closure.

A Number of the N-terminal RASSF Family: RASSF7

The Ras association domain family 7 (RASSF7, also named HRC1), a potential tumor-related gene, located on human chromosome 11p15, has been identified as an important member of the N-terminal RASSF family. Whereas, the molecular biological mechanisms of RASSF7 in tumorigenesis remain to be further established. We perform a systematic review of the literature and assessment from PUBMED and MEDLINE databases in this article. RASSF7 plays a significant role in mitosis, microtubule growth, apoptosis, proliferation and differentiation. Many research literature shows that the RASSF7 could promote the occurrence and advance of human tumors by regulating Aurora B, MKK4, MKK7, JNK, YAP, MEK, and ERK, whereas, it might inhibit c-Myc and thus lead to the suppression of tumorigenesis. The pregulation of RASSF7 often occurs in various malignancies such as lung cancer, neuroblastoma, thyroid neoplasm, hepatocellular cancer, breast cancer and gastric cancer. The expression stage of RASSF7 is positively correlated with the tumor TNM stage. In this review, we primarily elaborate on the acknowledged structure and progress in the various biomechanisms and research advances of RASSF7, especially the potential relevant signaling pathways. We hope that RASSF7 , a prospective therapeutic target for human malignancies, could play an available role in future anti-cancer treatment.

Asymmetric Stem Cell Division and Germline Immortality

Germ cells are the only cell type that is capable of transmitting genetic information to the next generation, which has enabled the continuation of multicellular life for the last 1.5 billion years. Surprisingly little is known about the mechanisms supporting the germline's remarkable ability to continue in this eternal cycle, termed germline immortality. Even unicellular organisms age at a cellular level, demonstrating that cellular aging is inevitable. Extensive studies in yeast have established the framework of how asymmetric cell division and gametogenesis may contribute to the resetting of cellular age. This review examines the mechanisms of germline immortality-how germline cells reset the aging of cells-drawing a parallel between yeast and multicellular organisms.

The signalling lipid PI3,5P2 is essential for timely mitotic exit

Coordination of mitotic exit with chromosome segregation is key for successful mitosis. Mitotic exit in budding yeast is executed by the mitotic exit network (MEN), which is negatively regulated by the spindle position checkpoint (SPOC). SPOC kinase Kin4 is crucial for SPOC activation in response to spindle positioning defects. Here, we report that the lysosomal signalling lipid phosphatidylinositol-3,5-bisphosphate (PI3,5P2) has an unanticipated role in the timely execution of mitotic exit. We show that the lack of PI3,5P2 causes a delay in mitotic exit, whereas elevated levels of PI3,5P2 accelerates mitotic exit in mitotic exit defective cells. Our data indicate that PI3,5P2 promotes mitotic exit in part through impairment of Kin4. This process is largely dependent on the known PI3,5P2 effector protein Atg18. Our work thus uncovers a novel link between PI3,5P2 and mitotic exit.

Meiosis II spindle disassembly requires two distinct pathways

During exit from meiosis II, cells undergo several structural rearrangements, including disassembly of the meiosis II spindles and cytokinesis. Each of these changes is regulated to ensure that they occur at the proper time. Previous studies have demonstrated that both SPS1, which encodes a STE20-family GCKIII kinase, and AMA1, which encodes a meiosis-specific activator of the Anaphase Promoting Complex, are required for both meiosis II spindle disassembly and cytokinesis in the budding yeast Saccharomyces cerevisiae. We examine the relationship between meiosis II spindle disassembly and cytokinesis and find that the meiosis II spindle disassembly failure in sps1Δ and ama1∆ cells is not the cause of the cytokinesis defect. We also see that the spindle disassembly defects in sps1Δ and ama1∆ cells are phenotypically distinct. We examined known microtubule-associated proteins Ase1, Cin8, and Bim1, and found that AMA1 is required for the proper loss of Ase1 and Cin8 on meiosis II spindles while SPS1 is required for Bim1 loss in meiosis II. Taken together, these data indicate that SPS1 and AMA1 promote distinct aspects of meiosis II spindle disassembly, and that both pathways are required for the successful completion of meiosis.

Characterization of a novel interaction of the Nup159 nucleoporin with asymmetrically localized spindle pole body proteins and its link with autophagy

Both the spindle microtubule-organizing centers and the nuclear pore complexes (NPCs) are convoluted structures where many signaling pathways converge to coordinate key events during cell division. Interestingly, despite their distinct molecular conformation and overall functions, these structures share common components and collaborate in the regulation of essential processes. We have established a new link between microtubule-organizing centers and nuclear pores in budding yeast by unveiling an interaction between the Bfa1/Bub2 complex, a mitotic exit inhibitor that localizes on the spindle pole bodies, and the Nup159 nucleoporin. Bfa1/Bub2 association with Nup159 is reduced in metaphase to not interfere with proper spindle positioning. However, their interaction is stimulated in anaphase and assists the Nup159-dependent autophagy pathway. The asymmetric localization of Bfa1/Bub2 during mitosis raises the possibility that its interaction with Nup159 could differentially promote Nup159-mediated autophagic processes, which might be relevant for the maintenance of the replicative lifespan.

Spindle Position Checkpoint Kinase Kin4 Regulates Organelle Transport in Saccharomyces cerevisiae

Membrane-bound organelles play important, frequently essential, roles in cellular metabolism in eukaryotes. Hence, cells have evolved molecular mechanisms to closely monitor organelle dynamics and maintenance. The actin cytoskeleton plays a vital role in organelle transport and positioning across all eukaryotes. Studies in the budding yeast Saccharomyces cerevisiae (S. cerevisiae) revealed that a block in actomyosin-dependent transport affects organelle inheritance to daughter cells. Indeed, class V Myosins, Myo2, and Myo4, and many of their organelle receptors, have been identified as key factors in organelle inheritance. However, the spatiotemporal regulation of yeast organelle transport remains poorly understood. Using peroxisome inheritance as a proxy to study actomyosin-based organelle transport, we performed an automated genome-wide genetic screen in S. cerevisiae. We report that the spindle position checkpoint (SPOC) kinase Kin4 and, to a lesser extent, its paralog Frk1, regulates peroxisome transport, independent of their role in the SPOC. We show that Kin4 requires its kinase activity to function and that both Kin4 and Frk1 protect Inp2, the peroxisomal Myo2 receptor, from degradation in mother cells. In addition, vacuole inheritance is also affected in kin4/frk1-deficient cells, suggesting a common regulatory mechanism for actin-based transport for these two organelles in yeast. More broadly our findings have implications for understanding actomyosin-based transport in cells.

Functional genomic analysis of Candida albicans protein kinases reveals modulators of morphogenesis in diverse environments

Candida albicans is a leading cause of mycotic infection. The ability to transition between yeast and filamentous forms is critical to C. albicans virulence and complex signaling pathways regulate this process. Here, we screened a C. albicans protein kinase mutant library in six environmental conditions to identify regulators of morphogenesis. We identified the uncharacterized gene orf19.3751 as a negative regulator of filamentation and follow-up investigations implicated a role for orf19.3751 in cell cycle regulation. We also uncovered a dual role for the kinases Ire1 and protein kinase A (Tpk1 and Tpk2) in C. albicans morphogenesis, specifically as negative regulators of wrinkly colony formation on solid medium but positive regulators of filamentation in liquid medium. Further analyses suggested Ire1 modulates morphogenesis in both media states in part through the transcription factor Hac1 and in part through independent mechanisms. Overall, this work provides insights into the signaling governing morphogenesis in C. albicans.

SWR1 chromatin remodeling complex prevents mitotic slippage during spindle position checkpoint arrest

Faithful chromosome segregation in budding yeast requires correct positioning of the mitotic spindle along the mother to daughter cell polarity axis. When the anaphase spindle is not correctly positioned, a surveillance mechanism, named as the spindle position checkpoint (SPOC), prevents the progression out of mitosis until correct spindle positioning is achieved. How SPOC works on a molecular level is not well understood. Here we performed a genome-wide genetic screen to search for components required for SPOC. We identified the SWR1 chromatin-remodeling complex (SWR1-C) among several novel factors that are essential for SPOC integrity. Cells lacking SWR1-C were able to activate SPOC upon spindle misorientation but underwent mitotic slippage upon prolonged SPOC arrest. This mitotic slippage required the Cdc14-early anaphase release pathway and other factors including the SAGA (Spt-Ada-Gcn5 acetyltransferase) histone acetyltransferase complex, proteasome components and the mitotic cyclin-dependent kinase inhibitor Sic1. Together, our data establish a novel link between SWR1-C chromatin remodeling and robust checkpoint arrest in late anaphase.

The budding yeast GSK-3 homologue Mck1 is an essential component of the spindle position checkpoint

The spindle position checkpoint (SPOC) is a mitotic surveillance mechanism in Saccharomyces cerevisiae that prevents cells from completing mitosis in response to spindle misalignment, thereby contributing to genomic integrity. The kinase Kin4, one of the most downstream SPOC components, is essential to stop the mitotic exit network (MEN), a signalling pathway that promotes the exit from mitosis and cell division. Previous work, however, suggested that a Kin4-independent pathway contributes to SPOC, yet the underlying mechanisms remain elusive. Here, we established the glycogen-synthase-kinase-3 (GSK-3) homologue Mck1, as a novel component that works independently of Kin4 to engage SPOC. Our data indicate that both Kin4 and Mck1 work in parallel to counteract MEN activation by the Cdc14 early anaphase release (FEAR) network. We show that Mck1's function in SPOC is mediated by the pre-replication complex protein and mitotic cyclin-dependent kinase (M-Cdk) inhibitor, Cdc6, which is degraded in a Mck1-dependent manner prior to mitosis. Moderate overproduction of Cdc6 phenocopies MCK1 deletion and causes SPOC deficiency via its N-terminal, M-Cdk inhibitory domain. Our data uncover an unprecedented role of GSK-3 kinases in coordinating spindle orientation with cell cycle progression.

The budding yeast GSK-3 homologue Mck1 is an essential component of the spindle position checkpoint

The spindle position checkpoint (SPOC) is a mitotic surveillance mechanism in Saccharomyces cerevisiae that prevents cells from completing mitosis in response to spindle misalignment, thereby contributing to genomic integrity. The kinase Kin4, one of the most downstream SPOC components, is essential to stop the mitotic exit network (MEN), a signalling pathway that promotes the exit from mitosis and cell division. Previous work, however, suggested that a Kin4-independent pathway contributes to SPOC, yet the underlying mechanisms remain elusive. Here, we established the glycogen-synthase-kinase-3 (GSK-3) homologue Mck1, as a novel component that works independently of Kin4 to engage SPOC. Our data indicate that both Kin4 and Mck1 work in parallel to counteract MEN activation by the Cdc14 early anaphase release (FEAR) network. We show that Mck1's function in SPOC is mediated by the pre-replication complex protein and mitotic cyclin-dependent kinase (M-Cdk) inhibitor, Cdc6, which is degraded in a Mck1-dependent manner prior to mitosis. Moderate overproduction of Cdc6 phenocopies MCK1 deletion and causes SPOC deficiency via its N-terminal, M-Cdk inhibitory domain. Our data uncover an unprecedented role of GSK-3 kinases in coordinating spindle orientation with cell cycle progression.

The N-Terminal Domain of Bfa1 Coordinates Mitotic Exit Independent of GAP Activity in Saccharomyces cerevisiae

The spindle position checkpoint (SPOC) of budding yeast delays mitotic exit in response to misaligned spindles to ensure cell survival and the maintenance of genomic stability. The GTPase-activating protein (GAP) complex Bfa1–Bub2, a key SPOC component, inhibits the GTPase Tem1 to induce mitotic arrest in response to DNA and spindle damage, as well as spindle misorientation. However, previous results strongly suggest that Bfa1 exerts a GAP-independent function in blocking mitotic exit in response to misaligned spindles. Thus, the molecular mechanism by which Bfa1 controls mitotic exit in response to misaligned spindles remains unclear. Here, we observed that overexpression of the N-terminal domain of Bfa1 (Bfa1-D16), which lacks GAP activity and cannot localize to the spindle pole body (SPB), induced cell cycle arrest along with hyper-elongation of astral microtubules (aMTs) as Bfa1 overexpression in Δbub2. We found that Δbub2 cells overexpressing Bfa1 or Bfa1-D16 inhibited activation of Mob1, which is responsible for mitotic exit. In anaphase-arrested cells, Bfa1-D16 overexpression inhibited Tem1 binding to the SPB as well as Bfa1 overexpression. Additionally, endogenous levels of Bfa1-D16 showed minor SPOC activity that was not regulated by Kin4. These results suggested that Bfa1-D16 may block mitotic exit through inhibiting Tem1 activity outside of SPBs. Alternatively, Bfa1-D16 dispersed out of SPBs may block Tem1 binding to SPBs by physically interacting with Tem1 as previously reported. Moreover, we observed hyper-elongated aMTs in tem1-3, cdc15-2, and dbf2-2 mutants that induce anaphase arrest and cannot undergo mitotic exit at restrictive temperatures, suggesting that aMT dynamics are closely related to the regulation of mitotic exit. Altogether, these observations suggest that Bfa1 can control the SPOC independent of its GAP activity and SPB localization.

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.

Protein phosphatase 1 in association with Bud14 inhibits mitotic exit in Saccharomyces cerevisiae

Mitotic exit in budding yeast is dependent on correct orientation of the mitotic spindle along the cell polarity axis. When accurate positioning of the spindle fails, a surveillance mechanism named the spindle position checkpoint (SPOC) prevents cells from exiting mitosis. Mutants with a defective SPOC become multinucleated and lose their genomic integrity. Yet, a comprehensive understanding of the SPOC mechanism is missing. In this study, we identified the type 1 protein phosphatase, Glc7, in association with its regulatory protein Bud14 as a novel checkpoint component. We further showed that Glc7-Bud14 promotes dephosphorylation of the SPOC effector protein Bfa1. Our results suggest a model in which two mechanisms act in parallel for a robust checkpoint response: first, the SPOC kinase Kin4 isolates Bfa1 away from the inhibitory kinase Cdc5, and second, Glc7-Bud14 dephosphorylates Bfa1 to fully activate the checkpoint effector.

Exact Maximal Reduction Of Stochastic Reaction Networks By Species Lumping

MOTIVATION

Stochastic reaction networks are a widespread model to describe biological systems where the presence of noise is relevant, such as in cell regulatory processes. Unfortunately, in all but simplest models the resulting discrete state-space representation hinders analytical tractability and makes numerical simulations expensive. Reduction methods can lower complexity by computing model projections that preserve dynamics of interest to the user.

RESULTS

We present an exact lumping method for stochastic reaction networks with mass-action kinetics. It hinges on an equivalence relation between the species, resulting in a reduced network where the dynamics of each macro-species is stochastically equivalent to the sum of the original species in each equivalence class, for any choice of the initial state of the system. Furthermore, by an appropriate encoding of kinetic parameters as additional species, the method can establish equivalences that do not depend on specific values of the parameters. The method is supported by an efficient algorithm to compute the largest species equivalence, thus the maximal lumping. The effectiveness and scalability of our lumping technique, as well as the physical interpretability of resulting reductions, is demonstrated in several models of signaling pathways and epidemic processes on complex networks.

AVAILABILITY

The algorithms for species equivalence have been implemented in the software tool ERODE, freely available for download from https://www.erode.eu.

Unifying the mechanism of mitotic exit control in a spatiotemporal logical model

The transition from mitosis into the first gap phase of the cell cycle in budding yeast is controlled by the Mitotic Exit Network (MEN). The network interprets spatiotemporal cues about the progression of mitosis and ensures that release of Cdc14 phosphatase occurs only after completion of key mitotic events. The MEN has been studied intensively; however, a unified understanding of how localisation and protein activity function together as a system is lacking. In this paper, we present a compartmental, logical model of the MEN that is capable of representing spatial aspects of regulation in parallel to control of enzymatic activity. We show that our model is capable of correctly predicting the phenotype of the majority of mutants we tested, including mutants that cause proteins to mislocalise. We use a continuous time implementation of the model to demonstrate that Cdc14 Early Anaphase Release (FEAR) ensures robust timing of anaphase, and we verify our findings in living cells. Furthermore, we show that our model can represent measured cell-cell variation in Spindle Position Checkpoint (SPoC) mutants. This work suggests a general approach to incorporate spatial effects into logical models. We anticipate that the model itself will be an important resource to experimental researchers, providing a rigorous platform to test hypotheses about regulation of mitotic exit.

A Noncanonical Hippo Pathway Regulates Spindle Disassembly and Cytokinesis During Meiosis in Saccharomyces cerevisiae

Meiosis in the budding yeast Saccharomyces cerevisiae is used to create haploid yeast spores from a diploid mother cell. During meiosis II, cytokinesis occurs by closure of the prospore membrane, a membrane that initiates at the spindle pole body and grows to surround each of the haploid meiotic products. Timely prospore membrane closure requires SPS1, which encodes an STE20 family GCKIII kinase. To identify genes that may activate SPS1, we utilized a histone phosphorylation defect of sps1 mutants to screen for genes with a similar phenotype and found that cdc15 shared this phenotype. CDC15 encodes a Hippo-like kinase that is part of the mitotic exit network. We find that Sps1 complexes with Cdc15, that Sps1 phosphorylation requires Cdc15, and that CDC15 is also required for timely prospore membrane closure. We also find that SPS1, like CDC15, is required for meiosis II spindle disassembly and sustained anaphase II release of Cdc14 in meiosis. However, the NDR-kinase complex encoded by DBF2/DBF20MOB1 which functions downstream of CDC15 in mitotic cells, does not appear to play a role in spindle disassembly, timely prospore membrane closure, or sustained anaphase II Cdc14 release. Taken together, our results suggest that the mitotic exit network is rewired for exit from meiosis II, such that SPS1 replaces the NDR-kinase complex downstream of CDC15.

The regulation of Net1/Cdc14 by the Hog1 MAPK upon osmostress unravels a new mechanism regulating mitosis

During evolution, cells have developed a plethora of mechanisms to optimize survival in a changing and unpredictable environment. In this regard, they have evolved networks that include environmental sensors, signaling transduction molecules and response mechanisms. Hog1 (yeast) and p38 (mammals) stress-activated protein kinases (SAPKs) are activated upon stress and they drive a full collection of cell adaptive responses aimed to maximize survival. SAPKs are extensively used to learn about the mechanisms through which cells adapt to changing environments. In addition to regulating gene expression and metabolism, SAPKs control cell cycle progression. In this review, we will discuss the latest findings related to the SAPK-driven regulation of mitosis upon osmostress in yeast.

A guiding torch at the poles: the multiple roles of spindle microtubule-organizing centers during cell division

The spindle constitutes the cellular machinery that enables the segregation of the chromosomes during eukaryotic cell division. The microtubules that form this fascinating and complex genome distribution system emanate from specialized structures located at both its poles and known as microtubule-organizing centers (MTOCs). Beyond their structural function, the spindle MTOCs play fundamental roles in cell cycle control, the activation and functionality of the mitotic checkpoints and during cellular aging. This review highlights the pivotal importance of spindle-associated MTOCs in multiple cellular processes and their central role as key regulatory hubs where diverse intracellular signals are integrated and coordinated to ensure the successful completion of cell division and the maintenance of the replicative lifespan.

Spindle pole bodies function as signal amplifiers in the Mitotic Exit Network

The Mitotic Exit Network (MEN), a budding yeast Ras-like signal transduction cascade, translates nuclear position into a signal to exit from mitosis. Here we describe how scaffolding the MEN onto spindle pole bodies (SPB—centrosome equivalent) allows the MEN to couple the final stages of mitosis to spindle position. Through the quantitative analysis of the localization of MEN components, we determined the relative importance of MEN signaling from the SPB that is delivered into the daughter cell (dSPB) during anaphase and the SPB that remains in the mother cell. Movement of half of the nucleus into the bud during anaphase causes the active form of the MEN GTPase Tem1 to accumulate at the dSPB. In response to Tem1’s activity at the dSPB, the MEN kinase cascade, which functions downstream of Tem1, accumulates at both SPBs. This localization to both SPBs serves an important role in promoting efficient exit from mitosis. Cells that harbor only one SPB delay exit from mitosis. We propose that MEN signaling is initiated by Tem1 at the dSPB and that association of the downstream MEN kinases with both SPBs serves to amplify MEN signaling, enabling the timely exit from mitosis.

Distinct surfaces on Cdc5/PLK Polo-box domain orchestrate combinatorial substrate recognition during cell division

Polo-like kinases (Plks) are key cell cycle regulators. They contain a kinase domain followed by a polo-box domain that recognizes phosphorylated substrates and enhances their phosphorylation. The regulatory subunit of the Dbf4-dependent kinase complex interacts with the polo-box domain of Cdc5 (the sole Plk in Saccharomyces cerevisiae) in a phosphorylation-independent manner. We have solved the crystal structures of the polo-box domain of Cdc5 on its own and in the presence of peptides derived from Dbf4 and a canonical phosphorylated substrate. The structure bound to the Dbf4-peptide reveals an additional density on the surface opposite to the phospho-peptide binding site that allowed us to propose a model for the interaction. We found that the two peptides can bind simultaneously and non-competitively to the polo-box domain in solution. Furthermore, point mutations on the surface opposite to the phosphopeptide binding site of the polo-box domain disrupt the interaction with the Dbf4 peptide in solution and cause an early anaphase arrest phenotype distinct from the mitotic exit defect typically observed in cdc5 mutants. Collectively, our data illustrates the importance of non-canonical interactions mediated by the polo-box domain and provide key mechanistic insights into the combinatorial recognition of substrates by Polo-like kinases.

Regulation of Mitotic Exit by Cell Cycle Checkpoints: Lessons From Saccharomyces cerevisiae

In order to preserve genome integrity and their ploidy, cells must ensure that the duplicated genome has been faithfully replicated and evenly distributed before they complete their division by mitosis. To this end, cells have developed highly elaborated checkpoints that halt mitotic progression when problems in DNA integrity or chromosome segregation arise, providing them with time to fix these issues before advancing further into the cell cycle. Remarkably, exit from mitosis constitutes a key cell cycle transition that is targeted by the main mitotic checkpoints, despite these surveillance mechanisms being activated by specific intracellular signals and acting at different stages of cell division. Focusing primarily on research carried out using Saccharomyces cerevisiae as a model organism, the aim of this review is to provide a general overview of the molecular mechanisms by which the major cell cycle checkpoints control mitotic exit and to highlight the importance of the proper regulation of this process for the maintenance of genome stability during the distribution of the duplicated chromosomes between the dividing cells.

The Multiple Roles of the Cdc14 Phosphatase in Cell Cycle Control

The Cdc14 phosphatase is a key regulator of mitosis in the budding yeast Saccharomyces cerevisiae. Cdc14 was initially described as playing an essential role in the control of cell cycle progression by promoting mitotic exit on the basis of its capacity to counteract the activity of the cyclin-dependent kinase Cdc28/Cdk1. A compiling body of evidence, however, has later demonstrated that this phosphatase plays other multiple roles in the regulation of mitosis at different cell cycle stages. Here, we summarize our current knowledge about the pivotal role of Cdc14 in cell cycle control, with a special focus in the most recently uncovered functions of the phosphatase.

PP2A Functions during Mitosis and Cytokinesis in Yeasts

Protein phosphorylation is a common mechanism for the regulation of cell cycle progression. The opposing functions of cell cycle kinases and phosphatases are crucial for accurate chromosome segregation and exit from mitosis. Protein phosphatases 2A are heterotrimeric complexes that play essential roles in cell growth, proliferation, and regulation of the cell cycle. Here, we review the function of the protein phosphatase 2A family as the counteracting force for the mitotic kinases. We focus on recent findings in the regulation of mitotic exit and cytokinesis by PP2A phosphatases in S. cerevisiae and other fungal species.

The DNA Damage Checkpoint and the Spindle Position Checkpoint Maintain Meiotic Commitment in Saccharomyces cerevisiae

During meiosis, diploid progenitor cells undergo one round of DNA replication followed by two rounds of chromosome segregation to form haploid gametes. Once cells initiate the meiotic divisions, it is imperative that they finish meiosis. A failure to maintain meiosis can result in highly aberrant polyploid cells, which could lead to oncogenesis in the germline. How cells stay committed to finishing meiosis, even in the presence of a mitosis-inducing signal, is poorly understood. We addressed this question in budding yeast, in which cells enter meiosis when starved. If nutrient-rich medium is added before a defined commitment point in mid-prometaphase I, they can return to mitosis. Cells in stages beyond the commitment point will finish meiosis, even with nutrient addition. Because checkpoints are signaling pathways known to couple cell-cycle processes with one another, we asked if checkpoints could ensure meiotic commitment. We find that two checkpoints with well-defined functions in mitosis, the DNA damage checkpoint and the spindle position checkpoint, have crucial roles in meiotic commitment. With nutrient-rich medium addition at stages beyond the commitment point, cells that are deficient in both checkpoints because they lack Rad53 and either Bub2, Bfa1, or Kin4 can return to mitotic growth and go on to form polyploid cells. The results demonstrate that the two checkpoints prevent cells from exiting meiosis in the presence of a mitosis-inducing signal. This study reveals a previously unknown function for the DNA damage checkpoint and the spindle position checkpoint in maintaining meiotic commitment.

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.

Candida albicans Cdc15 is essential for mitotic exit and cytokinesis

Candida albicans displays a variety of morphological forms, and the ability to switch forms must be linked with cell cycle control. In budding yeast the Mitotic Exit Network (MEN) acts to drive mitotic exit and signal for cytokinesis and cell separation. However, previous reports on the MEN in C. albicans have raised questions on its role in this organism, with the components analysed to date demonstrating differing levels of importance in the processes of mitotic exit, cytokinesis and cell separation. This work focuses on the role of the Cdc15 kinase in C. albicans and demonstrates that, similar to Saccharomyces cerevisiae, it plays an essential role in signalling for mitotic exit and cytokinesis. Cells depleted of Cdc15 developed into elongated filaments, a common response to cell cycle arrest in C. albicans. These filaments emerged exclusively from large budded cells, contained two nuclear bodies and exhibited a hyper-extended spindle, all characteristic of these cells failing to exit mitosis. Furthermore these filaments displayed a clear cytokinesis defect, and CDC15 over-expression led to aberrant cell separation following hyphal morphogenesis. Together, these results are consistent with Cdc15 playing an essential role in signalling for mitotic exit, cytokinesis and cell separation in C. albicans.

Coupling spindle position with mitotic exit in budding yeast: The multifaceted role of the small GTPase Tem1

The budding yeast S. cerevisiae divides asymmetrically and is an excellent model system for asymmetric cell division. As for other asymmetrically dividing cells, proper spindle positioning along the mother-daughter polarity axis is crucial for balanced chromosome segregation. Thus, a surveillance mechanism named Spindle Position Checkpoint (SPOC) inhibits mitotic exit and cytokinesis until the mitotic spindle is properly oriented, thereby preventing the generation of cells with aberrant ploidies. The small GTPase Tem1 is required to trigger a Hippo-like protein kinase cascade, named Mitotic Exit Network (MEN), that is essential for mitotic exit and cytokinesis but also contributes to correct spindle alignment in metaphase. Importantly, Tem1 is the target of the SPOC, which relies on the activity of the GTPase-activating complex (GAP) Bub2-Bfa1 to keep Tem1 in the GDP-bound inactive form. Tem1 forms a hetero-trimeric complex with Bub2-Bfa1 at spindle poles (SPBs) that accumulates asymmetrically on the bud-directed spindle pole during mitosis when the spindle is properly positioned. In contrast, the complex remains symmetrically localized on both poles of misaligned spindles. We have recently shown that Tem1 residence at SPBs depends on its nucleotide state and, importantly, asymmetry of the Bub2-Bfa1-Tem1 complex does not promote mitotic exit but rather controls spindle positioning.

Interrogation of γ-tubulin alleles using high-resolution fitness measurements reveals a distinct cytoplasmic function in spindle alignment

γ-Tubulin has a well-established role in nucleating the assembly of microtubules, yet how phosphorylation regulates its activity remains unclear. Here, we use a time-resolved, fitness-based SGA approach to compare two γ-tubulin alleles, and find that the genetic interaction profile of γtub-Y362E is enriched in spindle positioning and cell polarity genes relative to that of γtub-Y445D, which is enriched in genes involved in spindle assembly and stability. In γtub-Y362E cells, we find a defect in spindle alignment and an increase in the number of astral microtubules at both spindle poles. Our results suggest that the γtub-Y362E allele is a separation-of-function mutation that reveals a role for γ-tubulin phospho-regulation in spindle alignment. We propose that phosphorylation of the evolutionarily conserved Y362 residue of budding yeast γ-tubulin contributes to regulating the number of astral microtubules associated with spindle poles, and promoting efficient pre-anaphase spindle alignment.

Intrinsic and Extrinsic Determinants Linking Spindle Pole Fate, Spindle Polarity, and Asymmetric Cell Division in the Budding Yeast S. cerevisiae

The budding yeast S. cerevisiae is a powerful model to understand the multiple layers of control driving an asymmetric cell division. In budding yeast, asymmetric targeting of the spindle poles to the mother and bud cell compartments respectively orients the mitotic spindle along the mother-bud axis. This program exploits an intrinsic functional asymmetry arising from the age distinction between the spindle poles-one inherited from the preceding division and the other newly assembled. Extrinsic mechanisms convert this age distinction into differential fate. Execution of this program couples spindle orientation with the segregation of the older spindle pole to the bud. Remarkably, similar stereotyped patterns of inheritance occur in self-renewing stem cell divisions underscoring the general importance of studying spindle polarity and differential fate in yeast. Here, we review the mechanisms accounting for this pivotal interplay between intrinsic and extrinsic asymmetries that translate spindle pole age into differential fate.

The Mitotic Exit Network Regulates Spindle Pole Body Selection During Sporulation of Saccharomyces cerevisiae

Age-based inheritance of centrosomes in eukaryotic cells is associated with faithful chromosome distribution in asymmetric cell divisions. During Saccharomyces cerevisiae ascospore formation, such an inheritance mechanism targets the yeast centrosome equivalents, the spindle pole bodies (SPBs) at meiosis II onset. Decreased nutrient availability causes initiation of spore formation at only the younger SPBs and their associated genomes. This mechanism ensures encapsulation of nonsister genomes, which preserves genetic diversity and provides a fitness advantage at the population level. Here, by usage of an enhanced system for sporulation-induced protein depletion, we demonstrate that the core mitotic exit network (MEN) is involved in age-based SPB selection. Moreover, efficient genome inheritance requires Dbf2/20-Mob1 during a late step in spore maturation. We provide evidence that the meiotic functions of the MEN are more complex than previously thought. In contrast to mitosis, completion of the meiotic divisions does not strictly rely on the MEN whereas its activity is required at different time points during spore development. This is reminiscent of vegetative MEN functions in spindle polarity establishment, mitotic exit, and cytokinesis. In summary, our investigation contributes to the understanding of age-based SPB inheritance during sporulation of S. cerevisiae and provides general insights on network plasticity in the context of a specialized developmental program. Moreover, the improved system for a developmental-specific tool to induce protein depletion will be useful in other biological contexts.

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.

Evaluation of the Dynamicity of Mitotic Exit Network and Spindle Position Checkpoint Components on Spindle Pole Bodies by Fluorescence Recovery After Photobleaching (FRAP)

Fluorescence recovery after photobleaching (FRAP) is a powerful technique to study in vivo binding and diffusion dynamics of fluorescently labeled proteins. In this chapter, we describe how to determine spindle pole body (SPB) binding dynamics of mitotic exit network (MEN) and spindle position checkpoint (SPOC) proteins using FRAP microscopy. Procedures presented here include the growth of the yeast cultures, sample preparation, image acquisition and analysis.

Asymmetric Localization of Components and Regulators of the Mitotic Exit Network at Spindle Pole Bodies

Most proteins of the Mitotic Exit Network (MEN) and their upstream regulators localize at spindle pole bodies (SPBs) at least in some stages of the cell cycle. Studying the SPB localization of MEN factors has been extremely useful to elucidate their biological roles, organize them in a hierarchical pathway, and define their dynamics under different conditions.Recruitment to SPBs of the small GTPase Tem1 and the downstream kinases Cdc15 and Mob1/Dbf2 is thought to be essential for Cdc14 activation and mitotic exit, while that of the upstream Tem1 regulators (the Kin4 kinase and the GTPase activating protein Bub2-Bfa1) is important for MEN inhibition upon spindle mispositioning. Here, we describe the detailed fluorescence microscopy procedures that we use in our lab to analyze the localization at SPBs of Mitotic Exit Network (MEN) components tagged with GFP or HA epitopes.

Regulation of Mitotic Exit in Saccharomyces cerevisiae

The Mitotic Exit Network (MEN) is an essential signaling pathway, closely related to the Hippo pathway in mammals, which promotes mitotic exit and initiates cytokinesis in the budding yeast Saccharomyces cerevisiae. Here, we summarize the current knowledge about the MEN components and their regulation.

Septin-Associated Protein Kinases in the Yeast Saccharomyces cerevisiae

Septins are a family of eukaryotic GTP-binding proteins that associate into linear rods, which, in turn, polymerize end-on-end into filaments, and further assemble into other, more elaborate super-structures at discrete subcellular locations. Hence, septin-based ensembles are considered elements of the cytoskeleton. One function of these structures that has been well-documented in studies conducted in budding yeast Saccharomyces cerevisiae is to serve as a scaffold that recruits regulatory proteins, which dictate the spatial and temporal control of certain aspects of the cell division cycle. In particular, septin-associated protein kinases couple cell cycle progression with cellular morphogenesis. Thus, septin-containing structures serve as signaling platforms that integrate a multitude of signals and coordinate key downstream networks required for cell cycle passage. This review summarizes what we currently understand about how the action of septin-associated protein kinases and their substrates control information flow to drive the cell cycle into and out of mitosis, to regulate bud growth, and especially to direct timely and efficient execution of cytokinesis and cell abscission. Thus, septin structures represent a regulatory node at the intersection of many signaling pathways. In addition, and importantly, the activities of certain septin-associated protein kinases also regulate the state of organization of the septins themselves, creating a complex feedback loop.

LTE1 promotes exit from mitosis by multiple mechanisms

In budding yeast, the spindle position checkpoint ensures that cells exit from mitosis only when their spindle is properly aligned along the mother–bud axis. Exit from mitosis is controlled by both negative signals in the mother cell compartment and positive signals in the bud.

In budding yeast, alignment of the anaphase spindle along the mother–bud axis is crucial for maintaining genome integrity. If the anaphase spindle becomes misaligned in the mother cell compartment, cells arrest in anaphase because the mitotic exit network (MEN), an essential Ras-like GTPase signaling cascade, is inhibited by the spindle position checkpoint (SPoC). Distinct localization patterns of MEN and SPoC components mediate MEN inhibition. Most components of the MEN localize to spindle pole bodies. If the spindle becomes mispositioned in the mother cell compartment, cells arrest in anaphase due to inhibition of the MEN by the mother cell–restricted SPoC kinase Kin4. Here we show that a bud-localized activating signal is necessary for full MEN activation. We identify Lte1 as this signal and show that Lte1 activates the MEN in at least two ways. It inhibits small amounts of Kin4 that are present in the bud via its central domain. An additional MEN-activating function of Lte1 is mediated by its N- and C-terminal GEF domains, which, we propose, directly activate the MEN GTPase Tem1. We conclude that control of the MEN by spindle position is exerted by both negative and positive regulatory elements that control the pathway’s GTPase activity.

Spatial signals link exit from mitosis to spindle position

In budding yeast, if the spindle becomes mispositioned, cells prevent exit from mitosis by inhibiting the mitotic exit network (MEN). The MEN is a signaling cascade that localizes to spindle pole bodies (SPBs) and activates the phosphatase Cdc14. There are two competing models that explain MEN regulation by spindle position. In the 'zone model', exit from mitosis occurs when a MEN-bearing SPB enters the bud. The 'cMT-bud neck model' posits that cytoplasmic microtubule (cMT)-bud neck interactions prevent MEN activity. Here we find that 1) eliminating cMT– bud neck interactions does not trigger exit from mitosis and 2) loss of these interactions does not precede Cdc14 activation. Furthermore, using binucleate cells, we show that exit from mitosis occurs when one SPB enters the bud despite the presence of a mispositioned spindle. We conclude that exit from mitosis is triggered by a correctly positioned spindle rather than inhibited by improper spindle position.

DOI:http://dx.doi.org/10.7554/eLife.14036.001

Most cells duplicate their genetic material and then separate the two copies before they divide. This is true for budding yeast cells, which divide in an unusual manner. New daughter cells grow as a bud on the side of a larger mother cell and are eventually pinched off. To make healthy daughter cells, yeast must share their chromosomes between the mother cell and the bud. This involves threading the chromosomes through a small opening called the bud neck, which connects the mother cell and the bud.

A surveillance mechanism in budding yeast monitors the placement of the molecular machine (called the spindle) that separates the chromosomes before a cell divides. This mechanism stops the cell from dividing if the spindle is not positioned correctly. Two models could explain how an incorrectly positioned spindle prevents budding yeast from dividing. The first model proposes that yeast cells do not divide if protein filaments (called microtubules) touch the bud neck. This only occurs if the spindle is not properly threaded into the bud through the small opening of the bud neck. The second model proposes that specific proteins required for cell division (which are found at the ends of the spindle) are inhibited while they are inside the mother cell. This means that the cell cannot divide until one end of its spindle moves out of the mother cell and into the bud.

Now, Falk et al. report the results of experiments that aimed to distinguish between these two models. First, a laser was used to cut the spindle filaments in live yeast cells. This stopped the filaments from touching the neck between the mother cell and the bud, but did not cause the cell to divide. Therefore, these results refute the first model. Next, Falk et al. generated yeast cells that had essentially been tricked into forming two separate spindles before they started to divide. As would be predicted by the second model, these cells could divide as long as an end from at least one of the spindles entered the bud.

These findings strongly suggest that the second model provides the best explanation for how yeast cells sense spindle position to control cell division. The findings also lend further support to previous work that showed that activators of cell division are found in the bud, while inhibitors of cell division are found in the mother cell. Finally, in a related study, Gryaznova, Caydasi et al. identify a protein at the ends of the spindle that acts like a regulatory hub to coordinate cell division with spindle position. Their findings also suggest that the surveillance mechanism is switched off in the bud to allow the cell to divide.

DOI:http://dx.doi.org/10.7554/eLife.14036.002

A FRET-based study reveals site-specific regulation of spindle position checkpoint proteins at yeast centrosomes

The spindle position checkpoint (SPOC) is a spindle pole body (SPB, equivalent of mammalian centrosome) associated surveillance mechanism that halts mitotic exit upon spindle mis-orientation. Here, we monitored the interaction between SPB proteins and the SPOC component Bfa1 by FRET microscopy. We show that Bfa1 binds to the scaffold-protein Nud1 and the γ-tubulin receptor Spc72. Spindle misalignment specifically disrupts Bfa1-Spc72 interaction by a mechanism that requires the 14-3-3-family protein Bmh1 and the MARK/PAR-kinase Kin4. Dissociation of Bfa1 from Spc72 prevents the inhibitory phosphorylation of Bfa1 by the polo-like kinase Cdc5. We propose Spc72 as a regulatory hub that coordinates the activity of Kin4 and Cdc5 towards Bfa1. In addition, analysis of spc72∆ cells shows that a mitotic-exit-promoting dominant signal, which is triggered upon elongation of the spindle into the bud, overrides the SPOC. Our data reinforce the importance of daughter-cell-associated factors and centrosome-based regulations in mitotic exit and SPOC control.

Asymmetry of the budding yeast Tem1 GTPase at spindle poles is required for spindle positioning but not for mitotic exit

The asymmetrically dividing yeast S. cerevisiae assembles a bipolar spindle well after establishing the future site of cell division (i.e., the bud neck) and the division axis (i.e., the mother-bud axis). A surveillance mechanism called spindle position checkpoint (SPOC) delays mitotic exit and cytokinesis until the spindle is properly positioned relative to the mother-bud axis, thereby ensuring the correct ploidy of the progeny. SPOC relies on the heterodimeric GTPase-activating protein Bub2/Bfa1 that inhibits the small GTPase Tem1, in turn essential for activating the mitotic exit network (MEN) kinase cascade and cytokinesis. The Bub2/Bfa1 GAP and the Tem1 GTPase form a complex at spindle poles that undergoes a remarkable asymmetry during mitosis when the spindle is properly positioned, with the complex accumulating on the bud-directed old spindle pole. In contrast, the complex remains symmetrically localized on both poles of misaligned spindles. The mechanism driving asymmetry of Bub2/Bfa1/Tem1 in mitosis is unclear. Furthermore, whether asymmetry is involved in timely mitotic exit is controversial. We investigated the mechanism by which the GAP Bub2/Bfa1 controls GTP hydrolysis on Tem1 and generated a series of mutants leading to constitutive Tem1 activation. These mutants are SPOC-defective and invariably lead to symmetrical localization of Bub2/Bfa1/Tem1 at spindle poles, indicating that GTP hydrolysis is essential for asymmetry. Constitutive tethering of Bub2 or Bfa1 to both spindle poles impairs SPOC response but does not impair mitotic exit. Rather, it facilitates mitotic exit of MEN mutants, likely by increasing the residence time of Tem1 at spindle poles where it gets active. Surprisingly, all mutant or chimeric proteins leading to symmetrical localization of Bub2/Bfa1/Tem1 lead to increased symmetry at spindle poles of the Kar9 protein that mediates spindle positioning and cause spindle misalignment. Thus, asymmetry of the Bub2/Bfa1/Tem1 complex is crucial to control Kar9 distribution and spindle positioning during mitosis.

The centrosome and its duplication cycle

The centrosome was discovered in the late 19th century when mitosis was first described. Long recognized as a key organelle of the spindle pole, its core component, the centriole, was realized more than 50 or so years later also to comprise the basal body of the cilium. Here, we chart the more recent acquisition of a molecular understanding of centrosome structure and function. The strategies for gaining such knowledge were quickly developed in the yeasts to decipher the structure and function of their distinctive spindle pole bodies. Only within the past decade have studies with model eukaryotes and cultured cells brought a similar degree of sophistication to our understanding of the centrosome duplication cycle and the multiple roles of this organelle and its component parts in cell division and signaling. Now as we begin to understand these functions in the context of development, the way is being opened up for studies of the roles of centrosomes in human disease.

A lysine deacetylase Hos3 is targeted to the bud neck and involved in the spindle position checkpoint

The Saccharomyces cerevisiae lysine deacetylase Hos3 is asymmetrically targeted to the daughter side of the neck, dependent on the morphogenesis checkpoint member Hsl7, and to the daughter spindle pole body (SPB). In the presence of spindle misalignment, Hos3 at the SPBs functions as a brake component to inhibit mitotic exit.

An increasing number of cellular activities can be regulated by reversible lysine acetylation. Targeting the enzymes responsible for such posttranslational modifications is instrumental in defining their substrates and functions in vivo. Here we show that a Saccharomyces cerevisiae lysine deacetylase, Hos3, is asymmetrically targeted to the daughter side of the bud neck and to the daughter spindle pole body (SPB). The morphogenesis checkpoint member Hsl7 recruits Hos3 to the neck region. Cells with a defect in spindle orientation trigger Hos3 to load onto both SPBs. When associated symmetrically with both SPBs, Hos3 functions as a spindle position checkpoint (SPOC) component to inhibit mitotic exit. Neck localization of Hos3 is essential for its symmetric association with SPBs in cells with misaligned spindles. Our data suggest that Hos3 facilitates cross-talk between the morphogenesis checkpoint and the SPOC as a component of the intricate monitoring of spindle orientation after mitotic entry and before commitment to mitotic exit.

The 14-3-3 protein Bmh1 functions in the spindle position checkpoint by breaking Bfa1 asymmetry at yeast centrosomes

Phosphorylation of Bfa1 by Kin4 creates a docking site on Bfa1 for the 14-3-3 family protein Bmh1, which in turn weakens Bfa1–centrosome association and promotes symmetric Bfa1 localization to engage the spindle position checkpoint.

In addition to their well-known role in microtubule organization, centrosomes function as signaling platforms and regulate cell cycle events. An important example of such a function is the spindle position checkpoint (SPOC) of budding yeast. SPOC is a surveillance mechanism that ensures alignment of the mitotic spindle along the cell polarity axis. Upon spindle misalignment, phosphorylation of the SPOC component Bfa1 by Kin4 kinase engages the SPOC by changing the centrosome localization of Bfa1 from asymmetric (one centrosome) to symmetric (both centrosomes). Here we show that, unexpectedly, Kin4 alone is unable to break Bfa1 asymmetry at yeast centrosomes. Instead, phosphorylation of Bfa1 by Kin4 creates a docking site on Bfa1 for the 14-3-3 family protein Bmh1, which in turn weakens Bfa1–centrosome association and promotes symmetric Bfa1 localization. Consistently, BMH1-null cells are SPOC deficient. Our work thus identifies Bmh1 as a new SPOC component and refines the molecular mechanism that breaks Bfa1 centrosome asymmetry upon SPOC activation.

Dual Regulation of the mitotic exit network (MEN) by PP2A-Cdc55 phosphatase

Exit from mitosis in budding yeast is triggered by activation of the key mitotic phosphatase Cdc14. At anaphase onset, the protease separase and Zds1 promote the downregulation of PP2A^Cdc55 phosphatase, which facilitates Cdk1-dependent phosphorylation of Net1 and provides the first wave of Cdc14 activity. Once Cdk1 activity starts to decline, the mitotic exit network (MEN) is activated to achieve full Cdc14 activation. Here we describe how the PP2A^Cdc55 phosphatase could act as a functional link between FEAR and MEN due to its action on Bfa1 and Mob1. We demonstrate that PP2A^Cdc55 regulates MEN activation by facilitating Cdc5- and Cdk1-dependent phosphorylation of Bfa1 and Mob1, respectively. Downregulation of PP2A^Cdc55 initiates MEN activity up to Cdc15 by Bfa1 inactivation. Surprisingly, the premature Bfa1 inactivation observed does not entail premature MEN activation, since an additional Cdk1-Clb2 inhibitory signal acting towards Dbf2-Mob1 activity restrains MEN activity until anaphase. In conclusion, we propose a clear picture of how PP2A^Cdc55 functions affect the regulation of various MEN components, contributing to mitotic exit.

Cell cycle studies over the years have tried to elucidate the molecular mechanisms behind cell division, one of the most highly regulated of all cell processes, which ensures life in all organisms. Protein phosphorylation emerged as a key regulatory mechanism in the cell cycle. The highly conserved family of cyclin-dependent kinases, the Cdks, are considered the main component of the cell cycle control system. However, it has become clear that opposing phosphatases also play a key role in determining the phosphorylation state of the proteins. Cells enter mitosis when mitotic Cdk activity increases, having its pick of activity during metaphase. To exit mitosis, cells must coordinate chromosome segregation with Cdk inactivation processes involving the activation of protein phosphatases. Here we show that the phosphatase PP2A regulates the mitotic exit network (MEN) by counteracting the phosphorylation of Bfa1 and Mob1. Our findings provide new insights into the mechanism by which PP2A-Cdc55 functions affect the regulation of various MEN components that contribute to mitotic exit. The core signalling elements of the MEN, SIN and Hippo pathways are highly conserved. Therefore, studies of MEN regulation will contribute to our understanding of MEN-related pathways in other organisms.

The Mitotic Exit Network: new turns on old pathways

In budding yeast, the Mitotic Exit Network (MEN) is a signaling pathway known to drive cells out of mitosis and promote the faithful division of cells. The MEN triggers inactivation of cyclin-dependent kinase (Cdk1), the master regulator of mitosis, and the onset of cytokinesis after segregation of the daughter nuclei. The current model of the MEN suggests that MEN activity is restricted to late anaphase and coordinated with proper alignment of the spindle pole bodies (SPBs) with the division axis. However, recent evidence suggests that MEN activity may function earlier in mitosis, prompting re-evaluation of the current model. Here we attempt to integrate this recent progress into the current view of mitotic exit.

Inhibition of the mitotic exit network in response to damaged telomeres

When chromosomal DNA is damaged, progression through the cell cycle is halted to provide the cells with time to repair the genetic material before it is distributed between the mother and daughter cells. In Saccharomyces cerevisiae, this cell cycle arrest occurs at the G2/M transition. However, it is also necessary to restrain exit from mitosis by maintaining Bfa1-Bub2, the inhibitor of the Mitotic Exit Network (MEN), in an active state. While the role of Bfa1 and Bub2 in the inhibition of mitotic exit when the spindle is not properly aligned and the spindle position checkpoint is activated has been extensively studied, the mechanism by which these proteins prevent MEN function after DNA damage is still unclear. Here, we propose that the inhibition of the MEN is specifically required when telomeres are damaged but it is not necessary to face all types of chromosomal DNA damage, which is in agreement with previous data in mammals suggesting the existence of a putative telomere-specific DNA damage response that inhibits mitotic exit. Furthermore, we demonstrate that the mechanism of MEN inhibition when telomeres are damaged relies on the Rad53-dependent inhibition of Bfa1 phosphorylation by the Polo-like kinase Cdc5, establishing a new key role of this kinase in regulating cell cycle progression.