Defects in Saccharomyces cerevisiae protein phosphatase type I activate the spindle/kinetochore checkpoint

Analog-sensitive Cdk1 as a tool to study mitotic exit: protein phosphatase 1 is required downstream from Cdk1 inactivation in budding yeast

We show that specific inactivation of the protein kinase Cdk1/cyclin B (Cdc28/Clb2) triggers exit from mitosis in the budding yeast Saccharomyces cerevisiae. Cells carrying the allele cdc28-as1, which makes Cdk1 (Cdc28) uniquely sensitive to the ATP analog 1NM-PP1, were arrested with spindle poisons and then treated with 1NM-PP1 to inhibit Cdk1. This caused the cells to leave mitosis and enter G1-phase as shown by initiation of rebudding (without cytokinesis), induction of mating projections ("shmoos") by α-factor, stabilization of Sic1, and degradation of Clb2. It is known that Cdk1 must be inactivated for cells to exit mitosis, but our results show that inactivation of Cdk1 is not only necessary but also sufficient to initiate the transition from mitosis to G1-phase. This result suggests a system in which to test requirements for particular gene products downstream from Cdk1 inactivation, for example, by combining cdc28-as1 with conditional mutations in the genes of interest. Using this approach, we demonstrate that protein phosphatase 1 (PPase1; Glc7 in S. cerevisiae) is required for mitotic exit and reestablishment of interphase following Cdk1 inactivation. This system could be used to test the need for other protein phosphatases downstream from Cdk1 inactivation, such as PPase 2A and Cdc14, and it could be combined with phosphoproteomics to gain information about the substrates that the various phosphatases act upon during mitotic exit.

Exit from Mitosis in Budding Yeast: Protein Phosphatase 1 is Required Downstream from Cdk1 Inactivation

We show that inactivation of the protein kinase Cdk1/Cyclin B (Cdc28/Clb 2 in the budding yeast Saccharomyces cerevisiae ) is not only necessary for cells to leave mitosis, as is well known, but also sufficient to trigger mitotic exit. Cells carrying the mutation cdc28-as1 , which makes Cdc28 (Cdk1) uniquely sensitive to the ATP analog 1NM-PP1, were arrested with spindle poisons and then treated with 1NM-PP1 to inhibit Cdk1. This treatment caused the cells to exit mitosis and enter G1-phase as shown by initiation of rebudding (without cytokinesis), production of "shmoos" (when α-factor was present), stabilization of Sic1, and degradation of Clb2. This result provides a system in which to test whether particular gene products are required downstream from Cdk1 inactivation in exit from mitosis. In this system, the mutation cdc28-as1 is combined with a conditional mutation in the gene of interest. Using this approach, we demonstrate that Protein Phosphatase 1 (PPase1; Glc7 in S. cerevisiae ) is required for reestablishment of G1-phase following Cdk1 inactivation. This system could be used to test whether other protein phosphatases are also needed downstream from Cdk1 inactivation, and it could be combined with phosphoproteomics to gain information about the substrates those phosphatases act on during mitotic exit.

D-Xylose Sensing in Saccharomyces cerevisiae: Insights from D-Glucose Signaling and Native D-Xylose Utilizers

Extension of the substrate range is among one of the metabolic engineering goals for microorganisms used in biotechnological processes because it enables the use of a wide range of raw materials as substrates. One of the most prominent examples is the engineering of baker's yeast Saccharomyces cerevisiae for the utilization of d-xylose, a five-carbon sugar found in high abundance in lignocellulosic biomass and a key substrate to achieve good process economy in chemical production from renewable and non-edible plant feedstocks. Despite many excellent engineering strategies that have allowed recombinant S. cerevisiae to ferment d-xylose to ethanol at high yields, the consumption rate of d-xylose is still significantly lower than that of its preferred sugar d-glucose. In mixed d-glucose/d-xylose cultivations, d-xylose is only utilized after d-glucose depletion, which leads to prolonged process times and added costs. Due to this limitation, the response on d-xylose in the native sugar signaling pathways has emerged as a promising next-level engineering target. Here we review the current status of the knowledge of the response of S. cerevisiae signaling pathways to d-xylose. To do this, we first summarize the response of the native sensing and signaling pathways in S. cerevisiae to d-glucose (the preferred sugar of the yeast). Using the d-glucose case as a point of reference, we then proceed to discuss the known signaling response to d-xylose in S. cerevisiae and current attempts of improving the response by signaling engineering using native targets and synthetic (non-native) regulatory circuits.

The Opposing Functions of Protein Kinases and Phosphatases in Chromosome Bipolar Attachment

Accurate chromosome segregation during cell division is essential to maintain genome integrity in all eukaryotic cells, and chromosome missegregation leads to aneuploidy and therefore represents a hallmark of many cancers. Accurate segregation requires sister kinetochores to attach to microtubules emanating from opposite spindle poles, known as bipolar attachment or biorientation. Recent studies have uncovered several mechanisms critical to chromosome bipolar attachment. First, a mechanism exists to ensure that the conformation of sister centromeres is biased toward bipolar attachment. Second, the phosphorylation of some kinetochore proteins destabilizes kinetochore attachment to facilitate error correction, but a protein phosphatase reverses this phosphorylation. Moreover, the activity of the spindle assembly checkpoint is regulated by kinases and phosphatases at the kinetochore, and this checkpoint prevents anaphase entry in response to faulty kinetochore attachment. The fine-tuned kinase/phosphatase balance at kinetochores is crucial for faithful chromosome segregation during both mitosis and meiosis. Here, we discuss the function and regulation of protein phosphatases in the establishment of chromosome bipolar attachment with a focus on the model organism budding yeast.

A Kinase-Phosphatase Network that Regulates Kinetochore-Microtubule Attachments and the SAC

The KMN network (for KNL1, MIS12 and NDC80 complexes) is a hub for signalling at the outer kinetochore. It integrates the activities of two kinases (MPS1 and Aurora B) and two phosphatases (PP1 and PP2A-B56) to regulate kinetochore-microtubule attachments and the spindle assembly checkpoint (SAC). We will first discuss each of these enzymes separately, to describe how they are regulated at kinetochores and why this is important for their primary function in controlling either microtubule attachments or the SAC. We will then discuss why inhibiting any one of them individually produces secondary effects on all the others. This cross-talk may help to explain why all enzymes have been linked to both processes, even though the direct evidence suggests they each control only one. This chapter therefore describes how a network of kinases and phosphatases work together to regulate two key mitotic processes.

Phosphatases in Mitosis: Roles and Regulation

Mitosis requires extensive rearrangement of cellular architecture and of subcellular structures so that replicated chromosomes can bind correctly to spindle microtubules and segregate towards opposite poles. This process originates two new daughter nuclei with equal genetic content and relies on highly-dynamic and tightly regulated phosphorylation of numerous cell cycle proteins. A burst in protein phosphorylation orchestrated by several conserved kinases occurs as cells go into and progress through mitosis. The opposing dephosphorylation events are catalyzed by a small set of protein phosphatases, whose importance for the accuracy of mitosis is becoming increasingly appreciated. This review will focus on the established and emerging roles of mitotic phosphatases, describe their structural and biochemical properties, and discuss recent advances in understanding the regulation of phosphatase activity and function.

PP1-Dependent Formin Bnr1 Dephosphorylation and Delocalization from a Cell Division Site

Cell cycle ends with cytokinesis that is the physical separation of a cell into two daughter cells. For faithful cytokinesis, cells integrate multiple processes, such as actomyosin ring formation, contraction and plasma membrane closure, into coherent responses. Linear actin assembly by formins is essential for formation and maintenance of actomyosin ring. Although budding yeast’s two formins, Bni1 and Bnr1, are known to switch their subcellular localization at the division site prior to cytokinesis, the underlying mechanisms were not completely understood. Here, we provide evidence showing that Bnr1 is dephosphorylated concomitant with its release from the division site. Impaired PP1/Glc7 activity delayed Bnr1 release and dephosphorylation, Bni1 recruitment and actomyosin ring formation at the division site. These results suggest the involvement of Glc7 in this regulation. Further, we identified Ref2 as the PP1 regulatory subunit responsible for this regulation. Taken together, Glc7 and Ref2 may have a role in actomyosin ring formation by modulating the localization of formins during cytokinesis.

The budding yeast Cdc48(Shp1) complex promotes cell cycle progression by positive regulation of protein phosphatase 1 (Glc7)

The conserved, ubiquitin-selective AAA ATPase Cdc48 regulates numerous cellular processes including protein quality control, DNA repair and the cell cycle. Cdc48 function is tightly controlled by a multitude of cofactors mediating substrate specificity and processing. The UBX domain protein Shp1 is a bona fide substrate-recruiting cofactor of Cdc48 in the budding yeast S. cerevisiae. Even though Shp1 has been proposed to be a positive regulator of Glc7, the catalytic subunit of protein phosphatase 1 in S. cerevisiae, its cellular functions in complex with Cdc48 remain largely unknown. Here we show that deletion of the SHP1 gene results in severe growth defects and a cell cycle delay at the metaphase to anaphase transition caused by reduced Glc7 activity. Using an engineered Cdc48 binding-deficient variant of Shp1, we establish the Cdc48^Shp1 complex as a critical regulator of mitotic Glc7 activity. We demonstrate that shp1 mutants possess a perturbed balance of Glc7 phosphatase and Ipl1 (Aurora B) kinase activities and show that hyper-phosphorylation of the kinetochore protein Dam1, a key mitotic substrate of Glc7 and Ipl1, is a critical defect in shp1. We also show for the first time a physical interaction between Glc7 and Shp1 in vivo. Whereas loss of Shp1 does not significantly affect Glc7 protein levels or localization, it causes reduced binding of the activator protein Glc8 to Glc7. Our data suggest that the Cdc48^Shp1 complex controls Glc7 activity by regulating its interaction with Glc8 and possibly further regulatory subunits.

Suppressors of ipl1-2 in components of a Glc7 phosphatase complex, Cdc48 AAA ATPase, TORC1, and the kinetochore

Ipl1/Aurora B is the catalytic subunit of a protein kinase complex required for chromosome segregation and nuclear division. Before anaphase, Ipl1 is required to establish proper kinetochore-microtubule associations and to regulate the spindle assembly checkpoint (SAC). The phosphatase Glc7/PP1 opposes Ipl1 for these activities. To investigate Ipl1 and Glc7 regulation in more detail, we isolated and characterized mutations in the yeast Saccharomyces cerevisiae that raise the restrictive temperature of the ipl-2 mutant. These suppressors include three intragenic, second-site revertants in IPL1; 17 mutations in Glc7 phosphatase components (GLC7, SDS22, YPI1); two mutations in SHP1, encoding a regulator of the AAA ATPase Cdc48; and a mutation in TCO89, encoding a subunit of the TOR Complex 1. Two revertants contain missense mutations in microtubule binding components of the kinetochore. rev76 contains the missense mutation duo1-S115F, which alters an essential component of the DAM1/DASH complex. The mutant is cold sensitive and arrests in G2/M due to activation of the SAC. rev8 contains the missense mutation ndc80-K204E. K204 of Ndc80 corresponds to K166 of human Ndc80 and the human Ndc80 K166E variant was previously shown to be defective for microtubule binding in vitro. In a wild-type IPL1 background, ndc80-K204E cells grow slowly and the SAC is activated. The slow growth and cell cycle delay of ndc80-K204E cells are partially alleviated by the ipl1-2 mutation. These data provide biological confirmation of a biochemically based model for the effect of phosphorylation on Ndc80 function.

Shp1, a regulator of protein phosphatase 1 Glc7, has important roles in cell morphogenesis, cell cycle progression and DNA damage response in Candida albicans

In yeast, the type 1 protein phosphatase (PP1) catalytic subunit Glc7 is involved in the regulation of multiple cellular processes and thought to achieve specificity through association with different regulatory subunits. Here, we report that the Glc7 regulator Shp1 plays important roles in cell morphogenesis, cell cycle progression and DNA damage response in Candida albicans. SHP1 deletion caused the formation of rod-shaped yeast cells with slow growth. Flow cytometry analysis revealed that shp1Δ cells showed a prolonged G(2)/M phase, which was rescued by deleting the spindle-checkpoint gene MAD2. Furthermore, shp1Δ cells were hypersensitive to heat and genotoxic stresses. Interestingly, depletion of Glc7 caused defects similar to the shp1Δ mutant such as arrest at G(2)/M transition; and the GLC7/glc7Δ heterozygous mutant exhibited increased sensitivity to genotoxic stresses, consistent with the recent finding that Saccharomyces cerevisiae Glc7 has a role in DNA damage response. We also show that Shp1 is required for the nuclear accumulation of Glc7, suggesting that Shp1 executes its cellular function partly by regulating Glc7 localization.

Temperature-sensitive ipl1-2/Aurora B mutation is suppressed by mutations in TOR complex 1 via the Glc7/PP1 phosphatase

Ipl1/Aurora B is the catalytic subunit of a complex that is required for chromosome segregation and nuclear division. Before anaphase, Ipl1 localizes to kinetochores, where it is required to establish proper kinetochore-microtubule associations and regulate the spindle assembly checkpoint. The protein phosphatase Glc7/PP1 opposes Ipl1 for some of these activities. To more thoroughly characterize the Glc7 phosphatase that opposes Ipl1, we have identified mutations that suppress the thermosensitivity of an ipl1-2 mutant. In addition to mutations in genes previously associated with ipl1 suppression, we recovered a null mutant in TCO89, which encodes a subunit of the TOR complex 1 (TORC1), the conserved rapamycin-sensitive kinase activity that regulates cell growth in response to nutritional status. The temperature sensitivity of ipl1-2 can also be suppressed by null mutation of TOR1 or by administration of pharmacological TORC1 inhibitors, indicating that reduced TORC1 activity is responsible for the suppression. Suppression of the ipl1-2 growth defect is accompanied by increased fidelity of chromosome segregation and increased phosphorylation of the Ipl1 substrates histone H3 and Dam1. Nuclear Glc7 levels are reduced in a tco89 mutant, suggesting that TORC1 activity is required for the nuclear accumulation of Glc7. In addition, several mutant GLC7 alleles that suppress the temperature sensitivity of ipl1-2 exhibit negative synthetic genetic interactions with TORC1 mutants. Together, our results suggest that TORC1 positively regulates the Glc7 activity that opposes Ipl1 and provide a mechanism to tie nutritional status with mitotic regulation.

Kinetochore-microtubule interactions: steps towards bi-orientation

Eukaryotic cells segregate their chromosomes accurately to opposite poles during mitosis, which is necessary for maintenance of their genetic integrity. This process mainly relies on the forces generated by kinetochore-microtubule (KT-MT) attachment. During prometaphase, the KT initially interacts with a single MT extending from a spindle pole and then moves towards a spindle pole. Subsequently, MTs from the other spindle pole also interact with the KT. Eventually, one sister KT becomes attached to MTs from one pole while the other sister to those from the other pole (sister KT bi-orientation). If sister KTs interact with MTs with aberrant orientation, this must be corrected to attain proper bi-orientation (error correction) before the anaphase is initiated. Here, I discuss how KTs initially interact with MTs and how this interaction develops into bi-orientation; both processes are fundamentally crucial for proper chromosome segregation in the subsequent anaphase.

Function of protein phosphatase-1, Glc7, in Saccharomyces cerevisiae

Budding yeast, Saccharomyces cerevisiae, and its close relatives are unique among eukaryotes in having a single gene, GLC7, encoding protein phosphatase-1 (PP1). This enzyme with a highly conserved amino acid sequence controls many processes in all eukaryotic cells. Therefore, the study of Glc7 function offers a unique opportunity to gain a comprehensive understanding of this critical regulatory enzyme. This review summarizes our current knowledge of how Glc7 function modulates processes in the cytoplasm and nucleus. Additionally, global Glc7 regulation is described.

The AAA-ATPase Cdc48 and cofactor Shp1 promote chromosome bi-orientation by balancing Aurora B activity

The assembly, disassembly and dynamic movement of macromolecules are integral to cell physiology. The ubiquitin-selective chaperone Cdc48 (p97 in Metazoa), an AAA-ATPase, might facilitate such processes in the cell cycle. Cdc48 in budding yeast was initially isolated from a mitotic mutant. However, its function in mitosis remained elusive. Here we show that the temperature-sensitive cdc48-3 mutant and depletion of cofactor Shp1 (p47 in Metazoa) cause cell-cycle arrest at metaphase. The arrest is due to a defect in bipolar attachment of the kinetochore that activates the spindle checkpoint. Furthermore, Cdc48-Shp1 positively regulates Glc7/protein phosphatase 1 by facilitating nuclear localization of Glc7, whereas it opposes Ipl1/Aurora B kinase activity. Thus, we propose that Cdc48-Shp1 promotes nuclear accumulation of Glc7 to counteract Ipl1 activity. Our results identify Cdc48 and Shp1 as critical components that balance the kinase and phosphatase activities at the kinetochore in order to achieve stable bipolar attachment.

Changes in Bni4 localization induced by cell stress in Saccharomyces cerevisiae

Septin complexes at the bud neck in Saccharomyces cerevisiae serve as a scaffold for proteins involved in signaling, cell cycle control, and cell wall synthesis. Many of these bind asymmetrically, associating with either the mother- or daughter-side of the neck. Septin structures are inherently apolar so the basis for the asymmetric binding remains unknown. Bni4, a regulatory subunit of yeast protein phosphatase type 1, Glc7, binds to the outside of the septin ring prior to bud formation and remains restricted to the mother-side of the bud neck after bud emergence. Bni4 is responsible for targeting Glc7 to the mother-side of the bud neck for proper deposition of the chitin ring. We show here that Bni4 localizes symmetrically, as two distinct rings on both sides of the bud neck following energy depletion or activation of cell cycle checkpoints. Our data indicate that loss of Bni4 asymmetry can occur via at least two different mechanisms. Furthermore, we show that Bni4 has a Swe1-dependent role in regulating the cell morphogenesis checkpoint in response to hydroxyurea, which suggests that the change in localization of Bni4 following checkpoint activation may help stabilize the cell cycle regulator Swe1 during cell cycle arrest.

Cell division: righting the check

Studies in fission and budding yeast have continuously led the way for analyzing pathways of cell division. Two elegant studies, one from each yeast species, are opening the gates to study one of the final steps of mitosis--silencing the spindle checkpoint.

Protein phosphatase 1 regulates exit from the spindle checkpoint in budding yeast

Accurate chromosome segregation depends on sister kinetochores coming under tension when they make bioriented attachments to microtubules from opposite poles. The spindle checkpoint halts the cell cycle in response to defects in generating proper attachments or tension on kinetochores, although the precise signal that triggers the checkpoint is unclear because tension and attachment are coupled. The target of the checkpoint is the Cdc20 protein, which initiates the anaphase-promoting complex (APC)-dependent degradation of the anaphase inhibitor Pds1/securin. Although the molecular details of spindle checkpoint activation are still being elucidated, phosphorylation by at least four kinases is a crucial requirement. However, less is known about the mechanisms that silence the checkpoint after kinetochores biorient. Here, we show that the catalytic subunit of the budding yeast protein phosphatase 1 (PP1) homolog, Glc7, regulates exit from the checkpoint. Glc7 overexpression prevents spindle checkpoint activation in response to both tension and attachment defects. Although glc7 mutant cells are able to efficiently release from a non-checkpoint-mediated metaphase arrest, they are uniquely sensitive to transient spindle checkpoint activation as a result of a failure in spindle checkpoint exit. We therefore propose that PP1 activity silences the checkpoint by reversing key phosphorylation events.

Genetic interaction network of the Saccharomyces cerevisiae type 1 phosphatase Glc7

Background

Protein kinases and phosphatases regulate protein phosphorylation, a critical means of modulating protein function, stability and localization. The identification of functional networks for protein phosphatases has been slow due to their redundant nature and the lack of large-scale analyses. We hypothesized that a genome-scale analysis of genetic interactions using the Synthetic Genetic Array could reveal protein phosphatase functional networks. We apply this approach to the conserved type 1 protein phosphatase Glc7, which regulates numerous cellular processes in budding yeast.

Results

We created a novel glc7 catalytic mutant (glc7-E101Q). Phenotypic analysis indicates that this novel allele exhibits slow growth and defects in glucose metabolism but normal cell cycle progression and chromosome segregation. This suggests that glc7-E101Q is a hypomorphic glc7 mutant. Synthetic Genetic Array analysis of glc7-E101Q revealed a broad network of 245 synthetic sick/lethal interactions reflecting that many processes are required when Glc7 function is compromised such as histone modification, chromosome segregation and cytokinesis, nutrient sensing and DNA damage. In addition, mitochondrial activity and inheritance and lipid metabolism were identified as new processes involved in buffering Glc7 function. An interaction network among 95 genes genetically interacting with GLC7 was constructed by integration of genetic and physical interaction data. The obtained network has a modular architecture, and the interconnection among the modules reflects the cooperation of the processes buffering Glc7 function.

Conclusion

We found 245 genes required for the normal growth of the glc7-E101Q mutant. Functional grouping of these genes and analysis of their physical and genetic interaction patterns bring new information on Glc7-regulated processes.

An integrated platform of genomic assays reveals small-molecule bioactivities

Bioactive compounds are widely used to modulate protein function and can serve as important leads for drug development. Identifying the in vivo targets of these compounds remains a challenge. Using yeast, we integrated three genome-wide gene-dosage assays to measure the effect of small molecules in vivo. A single TAG microarray was used to resolve the fitness of strains derived from pools of (i) homozygous deletion mutants, (ii) heterozygous deletion mutants and (iii) genomic library transformants. We demonstrated, with eight diverse reference compounds, that integration of these three chemogenomic profiles improves the sensitivity and specificity of small-molecule target identification. We further dissected the mechanism of action of two protein phosphatase inhibitors and in the process developed a framework for the rational design of multidrug combinations to sensitize cells with specific genotypes more effectively. Finally, we applied this platform to 188 novel synthetic chemical compounds and identified both potential targets and structure-activity relationships.

Myosin phosphatase-targeting subunit 1 regulates mitosis by antagonizing polo-like kinase 1

Myosin phosphatase-targeting subunit 1 (MYPT1) binds to the catalytic subunit of protein phosphatase 1 (PP1C). This binding is believed to target PP1C to specific substrates including myosin II, thus controlling cellular contractility. Surprisingly, we found that during mitosis, mammalian MYPT1 binds to polo-like kinase 1 (PLK1). MYPT1 is phosphorylated during mitosis by proline-directed kinases including cdc2, which generates the binding motif for the polo box domain of PLK1. Depletion of PLK1 by small interfering RNAs is known to result in loss of gamma-tubulin recruitment to the centrosomes, blocking centrosome maturation and leading to mitotic arrest. We found that codepletion of MYPT1 and PLK1 reinstates gamma-tubulin at the centrosomes, rescuing the mitotic arrest. MYPT1 depletion increases phosphorylation of PLK1 at its activating site (Thr210) in vivo, explaining, at least in part, the rescue phenotype by codepletion. Taken together, our results identify a previously unrecognized role for MYPT1 in regulating mitosis by antagonizing PLK1.

Ypi1, a positive regulator of nuclear protein phosphatase type 1 activity in Saccharomyces cerevisiae

The catalytic subunit of protein phosphatase type 1 (PP1) has an essential role in mitosis, acting in opposition to the Ipl1/Aurora B protein kinase to ensure proper kinetochore-microtubule interactions. However, the regulatory subunit(s) that completes the PP1 holoenzyme that functions in this capacity is not known. We show here that the budding yeast Ypi1 protein is a nuclear protein that functions with PP1 (Glc7) in this mitotic role. Depletion of cellular Ypi1 induces mitotic arrest due to activation of the spindle checkpoint. Ypi1 depletion is accompanied by a reduction of nuclear PP1 and by loss of nuclear Sds22, a Glc7 binding partner that is found in a ternary complex with Ypi1 and Glc7. Expression of a Ypi1 variant that binds weakly to PP1 also activates the spindle checkpoint and suppresses the temperature sensitivity of an ipl1-2 mutant. These results, together with genetic interactions among YPI1, GLC7, and SDS22 mutants, indicate that Ypi1 and Sds22 are positive regulators of the nuclear Glc7 activity that is required for mitosis.

Kinetochore capture and bi-orientation on the mitotic spindle

Kinetochores are large protein complexes that are formed on chromosome regions known as centromeres. For high-fidelity chromosome segregation, kinetochores must be correctly captured on the mitotic spindle before anaphase onset. During prometaphase, kinetochores are initially captured by a single microtubule that extends from a spindle pole and are then transported poleward along the microtubule. Subsequently, microtubules that extend from the other spindle pole also interact with kinetochores and, eventually, each sister kinetochore attaches to microtubules that extend from opposite poles - this is known as bi-orientation. Here we discuss the molecular mechanisms of these processes, by focusing on budding yeast and drawing comparisons with other organisms.

Glc7/protein phosphatase 1 regulatory subunits can oppose the Ipl1/aurora protein kinase by redistributing Glc7

Faithful chromosome segregation depends on the opposing activities of the budding yeast Glc7/PP1 protein phosphatase and Ipl1/Aurora protein kinase. We explored the relationship between Glc7 and Ipl1 and found that the phosphorylation of the Ipl1 substrate, Dam1, was altered by decreased Glc7 activity, whereas Ipl1 levels, localization, and kinase activity were not. These data strongly suggest that Glc7 ensures accurate chromosome segregation by dephosphorylating Ipl1 targets rather than regulating the Ipl1 kinase. To identify potential Glc7 and Ipl1 substrates, we isolated ipl1-321 dosage suppressors. Seven genes (SDS22, BUD14, GIP3, GIP4, SOL1, SOL2, and PEX31) encode newly identified ipl1 dosage suppressors, and all 10 suppressors encode proteins that physically interact with Glc7. The overexpression of the Gip3 and Gip4 suppressors altered Glc7 localization, indicating they are previously unidentified Glc7 regulatory subunits. In addition, the overexpression of Gip3 and Gip4 from the galactose promoter restored Dam1 phosphorylation in ipl1-321 mutant cells and caused wild-type cells to arrest in metaphase with unsegregated chromosomes, suggesting that Gip3 and Gip4 overexpression impairs Glc7's mitotic functions. We therefore propose that the overexpression of Glc7 regulatory subunits can titrate Glc7 away from relevant Ipl1 targets and thereby suppress ipl1-321 cells by restoring the balance of phosphatase/kinase activity.

Characterization of Schistosoma mansoni Sds homologue, a leucine-rich repeat protein that interacts with protein phosphatase type 1 and interrupts a G2/M cell-cycle checkpoint

The suppressor of the dis2 mutant (sds22+) has been shown to be an essential regulator in cell division of fission and budding yeast where its deletion causes mitotic arrest. Its role seems to take place through the activation of PP1 (protein phosphatase type 1) in Schizosaccharomyces pombe. In the trematode Schistosoma mansoni, we have identified the Sds22 homologue (SmSds), and the PP1 (SmPP1). We showed by using a GST (glutathione S-transferase) pull-down assay that the SmSds gene product interacts with SmPP1 and that the SmSds-SmPP1 complex is present in parasite extracts. Furthermore, we observed that SmSds inhibited PP1 activity. Functional studies showed that the microinjection of SmSds into Xenopus oocytes interacted with the Xenopus PP1 and disrupted the G2/M cell-cycle checkpoint by promoting progression to GVBD (germinal vesicle breakdown). Similar results showing the appearance of GVBD were observed when oocytes were treated with anti-PP1 antibodies. Taken together, these observations suggest that SmSds can regulate the cell cycle by binding to PP1.

Glucose signaling in Saccharomyces cerevisiae

Eukaryotic cells possess an exquisitely interwoven and fine-tuned series of signal transduction mechanisms with which to sense and respond to the ubiquitous fermentable carbon source glucose. The budding yeast Saccharomyces cerevisiae has proven to be a fertile model system with which to identify glucose signaling factors, determine the relevant functional and physical interrelationships, and characterize the corresponding metabolic, transcriptomic, and proteomic readouts. The early events in glucose signaling appear to require both extracellular sensing by transmembrane proteins and intracellular sensing by G proteins. Intermediate steps involve cAMP-dependent stimulation of protein kinase A (PKA) as well as one or more redundant PKA-independent pathways. The final steps are mediated by a relatively small collection of transcriptional regulators that collaborate closely to maximize the cellular rates of energy generation and growth. Understanding the nuclear events in this process may necessitate the further elaboration of a new model for eukaryotic gene regulation, called "reverse recruitment." An essential feature of this idea is that fine-structure mapping of nuclear architecture will be required to understand the reception of regulatory signals that emanate from the plasma membrane and cytoplasm. Completion of this task should result in a much improved understanding of eukaryotic growth, differentiation, and carcinogenesis.

The Set1 methyltransferase opposes Ipl1 aurora kinase functions in chromosome segregation

A balance in the activities of the Ipl Aurora kinase and the Glc7 phosphatase is essential for normal chromosome segregation in yeast. We report here that this balance is modulated by the Set1 methyltransferase. Deletion of SET1 suppresses chromosome loss in ipl1-2 cells. Conversely, combination of SET1 and GLC7 mutations is lethal. Strikingly, these effects are independent of previously defined functions for Set1 in transcription initiation and histone H3 methylation. We find that Set1 is required for methylation of conserved lysines in a kinetochore protein, Dam1. Biochemical and genetic experiments indicate that Dam1 methylation inhibits Ipl1-mediated phosphorylation of flanking serines. Our studies demonstrate that Set1 has important, unexpected functions in mitosis. Moreover, our findings suggest that antagonism between lysine methylation and serine phosphorylation is a fundamental mechanism for controlling protein function.

Changes in the localization of the Saccharomyces cerevisiae anaphase-promoting complex upon microtubule depolymerization and spindle checkpoint activation

The anaphase-promoting complex/cyclosome (APC/C) is an E3 ubiquitin ligase in the ubiquitin-mediated proteolysis pathway (UMP). To understand how the APC/C was targeted to its substrates, we performed a detailed analysis of one of the APC/C components, Cdc23p. In live cells, Cdc23-GFP localized to punctate nuclear spots surrounded by homogenous nuclear signal throughout the cell cycle. These punctate spots colocalized with two outer kinetochore proteins, Slk19p and Okp1p, but not with the spindle pole body protein, Spc42p. In late anaphase, the Cdc23-GFP was also visualized along the length of the mitotic spindle. We hypothesized that spindle checkpoint activation may affect the APC/C nuclear spot localization. Localization of Cdc23-GFP was disrupted upon nocodazole treatment in the kinetochore mutant okp1-5 and in the cdc20-1 mutant. Cdc23-GFP nuclear spot localization was not affected in the ndc10-1 mutant, which is defective in spindle checkpoint function. Additional studies using a mad2Delta strain revealed a microtubule dependency of Cdc23-GFP spot localization, whether or not the checkpoint response was activated. On the basis of these data, we conclude that Cdc23p localization was dependent on microtubules and was affected by specific types of kinetochore disruption.

Retinoblastoma tumor suppressor and genome stability

Retinoblastoma gene (Rb) is the prototype of tumor suppressors. Germline mutation in the retinoblastoma gene is susceptible to cancer and reintroduction of wild-type Rb is able to suppress neoplastic phenotypes. The fundamental cellular functions of Rb in the control of cell growth and differentiation are important for its tumor suppression. In general, cancer susceptibility caused by inactivation of a tumor suppressor gene results from genome instability. Accordingly, Rb may function in the maintenance of chromosome stability by influencing mitotic progression, faithful chromosome segregation, and structural remodeling of mitotic chromosomes. Rb is also implicated in the regulation of replication machinery and in the control of cell cycle checkpoints in response to DNA damage, further supporting such a role for Rb. Moreover, the mechanistic basis for Rb-mediated transcriptional repression has revealed its connection to global chromatin remodeling. It is likely that Rb suppresses tumor formation by virtue of its multiple biological activities, and a theme throughout its multiple cellular functions is its central role in controlling activities that involve chromatin remodeling. A model in which Rb controls global genome fluidity is thus proposed. Finally, a recent study provides direct evidence indicating that loss of Rb function leads to genome instability. Therefore, tumor suppressors have a common role in the maintenance of genome stability, and such a role may be pivotal for their functions in tumor suppression.

Pho85 phosphorylates the Glc7 protein phosphatase regulator Glc8 in vivo

The budding yeast Glc7 serine/threonine protein phosphatase-1 is regulated by Glc8, the yeast ortholog of mammalian phosphatase inhibitor-2. In this work, we demonstrated that similarly to inhibitor-2, Glc8 function is regulated by phosphorylation. The cyclin-dependent protein kinase, Pho85, in conjunction with the related cyclins Pcl6 and Pcl7 comprise the major Glc8 kinase in vivo and in vitro. Several glc7 mutations are dependent on the presence of Glc8 for viability. For example, glc7 alleles R121K, R142H, and R198D are lethal in combination with a glc8 deletion. We found that glc7-R121K is lethal in combination with a pho85 deletion. This finding indicates that Pho85 is the sole Glc8 kinase in vivo. Furthermore, glc7-R121K is also lethal when combined with deletions of pcl6, plc7, pcl8, and pcl10, indicating that these related cyclins redundantly activate Pho85 for Glc8 phosphorylation in vivo. In vitro kinase assays and genetic results indicate that Pho85 cyclins Pcl6 and Pcl7 comprise the predominant Glc8 kinase.

A Bni4-Glc7 phosphatase complex that recruits chitin synthase to the site of bud emergence

Bni4 is a scaffold protein in the yeast Saccharomyces cerevisiae that tethers chitin synthase III to the bud neck by interacting with septin neck filaments and with Chs4, a regulatory subunit of chitin synthase III. We show herein that Bni4 is also a limiting determinant for the targeting of the type 1 serine/threonine phosphatase (Glc7) to the bud neck. Yeast cells containing a Bni4 variant that fails to associate with Glc7 fail to tether Chs4 to the neck, due in part to the failure of Bni4(V831A/F833A) to localize properly. Conversely, the Glc7-129 mutant protein fails to bind Bni4 properly and glc7-129 mutants exhibit reduced levels of Bni4 at the bud neck. Bni4 is phosphorylated in a cell cycle-dependent manner and Bni4(V831A/F833A) is both hyperphosphorylated and mislocalized in vivo. Yeast cells lacking the protein kinase Hsl1 exhibit increased levels of Bni4-GFP at the bud neck. GFP-Chs4 does not accumulate at the incipient bud site in either a bni4::TRP1 or a bni4(V831A/F833A) mutant but does mobilize to the neck at cytokinesis. Together, these results indicate that the formation of the Bni4-Glc7 complex is required for localization to the site of bud emergence and for subsequent targeting of chitin synthase.

The Glc7p-interacting protein Bud14p attenuates polarized growth, pheromone response, and filamentous growth in Saccharomyces cerevisiae

A genetic selection in Saccharomyces cerevisiae for mutants that stimulate the mating pathway uncovered a mutant that had a hyperactive pheromone response pathway and also had hyperpolarized growth. Cloning and segregation analysis demonstrated that BUD14 was the affected gene. Disruption of BUD14 in wild-type cells caused mild stimulation of pheromone response pathway reporters, an increase in sensitivity to mating factor, and a hyperelongated shmoo morphology. The bud14 mutant also had hyperfilamentous growth. Consistent with a role in the control of cell polarity, a Bud14p-green fluorescent protein fusion was localized to sites of polarized growth in the cell. Bud14p shared morphogenetic functions with the Ste20p and Bni1p proteins as well as with the type 1 phosphatase Glc7p. The genetic interactions between BUD14 and GLC7 suggested a role for Glc7p in filamentous growth, and Glc7p was found to have a positive function in filamentous growth in yeast.

Dynamic distribution of BIMG(PP1) in living hyphae of Aspergillus indicates a novel role in septum formation

Mutation of bimG, the major protein phosphatase 1 gene in Aspergillus nidulans, causes multiple cell cycle and hyphal growth defects that are associated with overphosphorylation of subcellular components. We have used functional translational fusions with the green fluorescent protein (GFP) to show that BIMG has at least four discrete locations within growing hyphae. Three of these locations, the hyphal tip, the spindle pole body and the nucleus, correlate with previously known requirements for bimG(PP1) in mitosis and hyphal growth and are highly dynamic. BIMG-GFP in the hyphal tip seemed to be associated with the plasma membrane and formed a collar of fluorescence within the apical dome. The distribution of nuclear BIMG-GFP varied depending on nutritional conditions; on poor medium, it concentrated more in the nucleolus than in the nucleoplasm, whereas on rich medium, it was more evenly distributed between the two nuclear regions. The association of BIMG-GFP with developing septa was transient, and we present evidence that BIMG phosphatase plays a direct role in septum formation, distinct from its role in mitosis. We conclude that, by being physically present at several sites, the BIMG phosphatase has roles in multiple cellular processes.

Glc8 is a glucose-repressible activator of Glc7 protein phosphatase-1

Regulation of Glc7 type 1 protein phosphatase stability and activity was studied in budding yeast. We found that the Glc7 protein has a half-life of over 180min, which is sufficient for several generations. Glc7 protein stability was constant during the cell cycle and in batch culture growth. Furthermore, deletion of regulatory subunit Gac1, Reg1, Reg2, Sds22, or Glc8 had no influence on Glc7 protein half-life. The activity of Glc7 assayed as okadaic acid-resistant phosphorylase phosphatase activity was constant during the cell cycle. Deletion of the aforementioned regulatory subunits revealed that only Glc8 deletion had a significant effect in reducing Glc7 activity. Glc7 activity was induced during stationary phase in a Glc8-dependent manner. In addition, extracellular glucose repressed the induction of Glc7 activity. These results are consistent with glucose repression of Glc8 expression and favor the role of Glc8 as a major Glc7 activator.

Plasmodium falciparum protein phosphatase type 1 functionally complements a glc7 mutant in Saccharomyces cerevisiae

We have identified a new homologue of protein phosphatase type 1 from Plasmodium falciparum, designated PfPP1, which shows 83-87% sequence identity with yeast and mammalian PP1s at the amino acid level. The PfPP1 sequence is strikingly different from all other P. falciparum Ser/Thr phosphatases cloned so far. The deduced 304 amino acid sequence revealed the signature sequence of Ser/Thr phosphatase LRGNHE, and two putative protein kinase C and five putative casein kinase II phosphorylation sites. Calyculin A, a potent inhibitor of Ser/Thr phosphatase 1 and 2A showed hyperphosphorylation of a 51kDa protein among other parasite proteins. Okadaic acid on the other hand, was without any effect suggesting that PP1 activity might predominate over PP2A activity in intra-erythrocytic P. falciparum. Complementation studies showed that PfPP1 could rescue low glycogen phenotype of Saccharomyces cerevisiae glc7 (PP1) mutant, strongly suggesting functional interaction of PfPP1 and yeast proteins involved in glycogen metabolism.

Protein phosphatase type 1 regulates ion homeostasis in Saccharomyces cerevisiae

Protein phosphatase type 1 (PP1) is encoded by the essential gene GLC7 in Saccharomyces cerevisiae. glc7-109 (K259A, R260A) has a dominant, hyperglycogen defect and a recessive, ion and drug sensitivity. Surprisingly, the hyperglycogen phenotype is partially retained in null mutants of GAC1, GIP2, and PIG1, which encode potential glycogen-targeting subunits of Glc7. The R260A substitution in GLC7 is responsible for the dominant and recessive traits of glc7-109. Another mutation at this residue, glc7-R260P, confers only salt sensitivity, indicating that the glycogen and salt traits of glc7-109 are due to defects in distinct physiological pathways. The glc7-109 mutant is sensitive to cations, aminoglycosides, and alkaline pH and exhibits increased rates of l-leucine and 3,3'-dihexyloxacarbocyanine iodide uptake, but it is resistant to molar concentrations of sorbitol or KCl, indicating that it has normal osmoregulation. KCl suppresses the ion and drug sensitivities of the glc7-109 mutant. The CsCl sensitivity of this mutant is suppressed by recessive mutations in PMA1, which encodes the essential plasma membrane H(+)ATPase. Together, these results indicate that Glc7 regulates ion homeostasis by controlling ion transport and/or plasma membrane potential, a new role for Glc7 in budding yeast.

Evidence that the Ipl1-Sli15 (Aurora kinase-INCENP) complex promotes chromosome bi-orientation by altering kinetochore-spindle pole connections

How sister kinetochores attach to microtubules from opposite spindle poles during mitosis (bi-orientation) remains poorly understood. In yeast, the ortholog of the Aurora B-INCENP protein kinase complex (Ipl1-Sli15) may have a role in this crucial process, because it is necessary to prevent attachment of sister kinetochores to microtubules from the same spindle pole. We investigated IPL1 function in cells that cannot replicate their chromosomes but nevertheless duplicate their spindle pole bodies (SPBs). Kinetochores detach from old SPBs and reattach to old and new SPBs with equal frequency in IPL1+ cells, but remain attached to old SPBs in ipl1 mutants. This raises the possibility that Ipl1-Sli15 facilitates bi-orientation by promoting turnover of kinetochore-SPB connections until traction of sister kinetochores toward opposite spindle poles creates tension in the surrounding chromatin.

Essential functions of Sds22p in chromosome stability and nuclear localization of PP1

Sds22p is a conserved, leucine-rich repeat protein that interacts with the catalytic subunit of protein phosphatase 1 (PP1C) and which has been proposed to regulate one or more functions of PP1C during mitosis. Here we show that Saccharomyces cerevisiae Sds22p is a largely nuclear protein, most of which is present as a sTable 1:1 complex with yeast PP1C (Glc7p). Temperature-sensitive (Ts–) S. cerevisiae sds22 mutants show profound chromosome instability at elevated growth temperatures but do not confer a cell cycle stage-specific arrest. In the sds22-6 Ts– mutant, nuclear Glc7p is both reduced in level and aberrantly localized at 37°C and the interaction between Glc7p and Sds22p in vitro is reduced at higher temperatures, consistent with the in vivo Ts– growth defect. Like some glc7 mutations, sds22-6 can suppress the Ts– growth defect associated with ipl1-2, a loss of function mutation in a protein kinase that is known to work in opposition to PP1 on at least two nuclear substrates. This, together with reciprocal genetic interactions between GLC7 and SDS22, suggests that Sds22p functions positively with Glc7p to promote dephosphorylation of nuclear substrates required for faithful transmission of chromosomes during mitosis, and this role is at least partly mediated by effects of Sds22p on the nuclear distribution of Glc7p

Reactive oxygen species-induced phosphorylation of p53 on serine 20 is mediated in part by polo-like kinase-3

Upon exposure of cells to hydrogen peroxide (H(2)O(2)) phosphorylation of p53 was rapidly induced in human fibroblast GM00637, and this phosphorylation occurred on serine 9, serine 15, serine 20, but not on serine 392. In addition, H(2)O(2)-induced phosphorylation of p53 was followed by induction of p21, suggesting functional activation of p53. Induction of phosphorylation of p53 on multiple serine residues by H(2)O(2) was caffeine-sensitive and blocked in ATM(-/-) cells. Polo-like kinase-3 (Plk3) activity was also activated upon H(2)O(2) treatment, and this activation was ATM-dependent. Recombinant His(6)-Plk3 phosphorylated glutathione S-transferase (GST)-p53 fusion protein but not GST alone. When phoshorylated in vitro by His(6)-Plk3, but not by the kinase-defective mutant His6-Plk3(K52R), GST-p53 was recognized by an antibody specifically to serine 20-phosphorylated p53, indicating that serine 20 is an in vitro target of Plk3. Also serine 20-phosphorylated p53 was coimmunoprecipitated with Plk3 in cells treated with H(2)O(2). Furthermore, although H(2)O(2) strongly induced serine 15 phosphorylation of p53, it failed to induce serine 20 phosphorylation in Plk3-dificient Daudi cells. Ectopic expression of a Plk3 dominant negative mutant, Plk3(K52R), in GM00637 cells suppressed H(2)O(2)-induced serine 20 phosphorylation. Taken together, our studies strongly suggest that the oxidative stress-induced activation of p53 is at least in part mediated by Plk3.

The retinoblastoma gene: a prototypic and multifunctional tumor suppressor

Genome instability has been implicated in the generation of multiple somatic mutations that underlie cancer. Germline mutation in the retinoblastoma (RB) gene leads to tumor formation in both human and experimental animal models, and reintroduction of wild-type RB is able to suppress neoplastic phenotypes. Rb governs the passage of cells through the G1 phase-restriction point and this control is lost in most cancer cells. Rb has also been shown to promote terminal differentiation and prevent cell cycle reentry. Recent studies implicate Rb in mitotic progression, faithful chromosome segregation, checkpoint control, and chromatin remodeling, suggesting that Rb may function in the maintenance of genome integrity. It is likely that Rb suppresses tumor formation by virtue of its multiple biological activities. A single protein capable of performing multiple antioncogenic functions may be a common characteristic of other tumor suppressors including p53 and BRCA1/2.

The mitotic serine/threonine kinase Aurora2/AIK is regulated by phosphorylation and degradation

Aurora2 is a cell cycle regulated serine/threonine protein kinase which is overexpressed in many tumor cell lines. We demonstrate that Aurora2 is regulated by phosphorylation in a cell cycle dependent manner. This phosphorylation occurs on a conserved residue, Threonine 288, within the activation loop of the catalytic domain of the kinase and results in a significant increase in the enzymatic activity. Threonine 288 resides within a consensus motif for the cAMP dependent kinase and can be phosphorylated by PKA in vitro. The protein phosphatase 1 is shown to dephosphorylate this site in vitro, and in vivo the phosphorylation of T288 is induced by okadaic acid treatment. Furthermore, we show that the Aurora2 kinase is regulated by proteasome dependent degradation and that Aurora2 phosphorylated on T288 may be targeted for degradation during mitosis. Our experiments suggest that phosphorylation of T288 is important for regulation of the Aurora2 kinase both for its activity and its stability.

Mitotic phosphorylation of histone H3 is governed by Ipl1/aurora kinase and Glc7/PP1 phosphatase in budding yeast and nematodes

Phosphorylation of histone H3 at serine 10 occurs during mitosis and meiosis in a wide range of eukaryotes and has been shown to be required for proper chromosome transmission in Tetrahymena. Here we report that Ipl1/aurora kinase and its genetically interacting phosphatase, Glc7/PP1, are responsible for the balance of H3 phosphorylation during mitosis in Saccharomyces cerevisiae and Caenorhabditis elegans. In these models, both enzymes are required for H3 phosphorylation and chromosome segregation, although a causal link between the two processes has not been demonstrated. Deregulation of human aurora kinases has been implicated in oncogenesis as a consequence of chromosome missegregation. Our findings reveal an enzyme system that regulates chromosome dynamics and controls histone phosphorylation that is conserved among diverse eukaryotes.

Centromeres: getting a grip of chromosomes

On monocentric chromosomes the centromere is the chromosomal site at which the kinetochore complex is assembled. This complex mediates the attachment and movement of chromosomes along spindle microtubules. The centromere is usually the last site to retain cohesion between sister centromeres. The location of the main sensor for defective spindle assembly at the kinetochore allows the release of this cohesion, and thus progression through mitosis, to be held in check until key events have been completed. The intricate nature of the centromere-kinetochore complexes and the events they co-ordinate and react to is presently being dissected by studies in several organisms. In particular, several new kinetochore proteins have been identified in many organisms over the last year.

Retinoblastoma protein enhances the fidelity of chromosome segregation mediated by hsHec1p

Retinoblastoma protein (Rb) plays important roles in cell cycle progression and cellular differentiation. It may also participate in M phase events, although heretofore only circumstantial evidence has suggested such involvement. Here we show that Rb interacts, through an IxCxE motif and specifically during G(2)/M phase, with hsHec1p, a protein essential for proper chromosome segregation. The interaction between Rb and hsHec1p was reconstituted in a yeast strain in which human hsHEC1 rescues the null mutation of scHEC1. Expression of Rb reduced chromosome segregation errors fivefold in yeast cells sustained by a temperature-sensitive (ts) hshec1-113 allele and enhanced the ability of wild-type hsHec1p to suppress lethality caused by a ts smc1 mutation. The interaction between Hec1p and Smc1p was important for the specific DNA-binding activity of Smc1p. Expression of Rb restored part of the inactivated function of hshec1-113p and thereby increased the DNA-binding activity of Smc1p. Rb thus increased the fidelity of chromosome segregation mediated by hsHec1p in a heterologous yeast system.

Genetic interactions between GLC7, PPZ1 and PPZ2 in saccharomyces cerevisiae

GLC7 encodes an essential serine/threonine protein type I phosphatase in Saccharomyces cerevisiae. Three other phosphatases (Ppz1p, Ppz2p, and Sal6p) share >59% identity in their catalytic region with Glc7p. ppz1 ppz2 null mutants have no apparent growth defect on rich media. However, null alleles of PPZ1 and PPZ2, in combination with mutant alleles of GLC7, confer a range of growth defects varying from slow growth to lethality. These results indicate that Glc7p, Ppz1p, and Ppz2p may have overlapping functions. To determine if this overlap extends to interaction with targeting subunits, Glc7p-binding proteins were tested for interaction in the two-hybrid system with the functional catalytic domain of Ppz1p. Ppz1p interacts strongly with a number of Glc7p regulatory subunits, including Glc8p, a protein that shares homology with mammalian PP1 inhibitor I2. Genetic data suggest that Glc8p positively affects both Glc7p and Ppz1p functions. Together our data suggest that Ppz1p and Ppz2p may have overlapping functions with Glc7p and that all three phosphatases may act through common regulatory proteins.

Pachytene exit controlled by reversal of Mek1-dependent phosphorylation

During yeast meiosis, a checkpoint prevents exit from pachytene in response to defects in meiotic recombination and chromosome synapsis. This pachytene checkpoint requires two meiotic chromosomal proteins, Red1 and Mek1; Mek1 is a kinase that phosphorylates Red1. In mutants that undergo checkpoint-mediated pachytene arrest, Mek1 is active and Red1 remains phosphorylated. Activation of Mek1 requires the initiation of meiotic recombination and certain DNA damage checkpoint proteins. Mek1 kinase activity and checkpoint-induced pachytene arrest are counteracted by protein phosphatase type 1 (Glc7). Glc7 coimmunoprecipitates with Red1, colocalizes with Red1 on chromosomes, and dephosphorylates Red1 in vitro. We speculate that phosphorylated Red1 prevents exit from pachytene and that completion of meiotic recombination triggers Glc7-dependent dephosphorylation of Red1.

Dynamic localization of protein phosphatase type 1 in the mitotic cell cycle of Saccharomyces cerevisiae

Protein phosphatase type I (PP1), encoded by the single essential gene GLC7 in Saccharomyces cerevisiae, functions in diverse cellular processes. To identify in vivo subcellular location(s) where these processes take place, we used a functional green fluorescent protein (GFP)-Glc7p fusion protein. Time-lapse fluorescence microscopy revealed GFP-Glc7p localizes predominantly in the nucleus throughout the mitotic cell cycle, with the highest concentrations in the nucleolus. GFP-Glc7p was also observed in a ring at the bud neck, which was dependent upon functional septins. Supporting a role for Glc7p in bud site selection, a glc7-129 mutant displayed a random budding pattern. In alpha-factor treated cells, GFP-Glc7p was located at the base of mating projections, again in a septin-dependent manner. At the start of anaphase, GFP-Glc7p accumulated at the spindle pole bodies and remained there until cytokinesis. After anaphase, GFP-Glc7p became concentrated in a ring that colocalized with the actomyosin ring. A GFP-Glc7-129 fusion was defective in localizing to the bud neck and SPBs. Together, these results identify sites of Glc7p function and suggest Glc7p activity is regulated through dynamic changes in its location.

Type 1 protein phosphatase is required for maintenance of cell wall integrity, morphogenesis and cell cycle progression in Saccharomyces cerevisiae

GLC7 encodes the catalytic subunit of type 1 protein serine/threonine phosphatase (PP1) in the yeast Saccharomyces cerevisiae. Here we have characterized the temperature-sensitive glc7-10 allele, which displays aberrant bud morphology and an abnormal actin cytoskeleton at the restrictive temperature. At 37 degrees C glc7-10 strains accumulated a high proportion of budded cells with an unmigrated nucleus, duplicated spindle pole bodies, a short spindle, delocalized cortical actin and 2C DNA content, indicating a cell cycle block prior to the metaphase to anaphase transition. glc7-10 was suppressed by growth on high osmolarity medium and exhibited temperature-sensitive cell lysis upon hypo-osmotic stress. Pkc1p, the yeast protein kinase C homolog which is thought to regulate the Mpk1p MAP kinase pathway involved in cell wall remodelling and polarized cell growth, was found to act as a dosage suppressor of glc7-10. Although neither activation of BCK1 (MEKK) by the dominant BCK1-20 mutation nor increased dosage of MKK1 (MEK) or MPK1 (MAP kinase) mimicked PKC1 as a glc7-10 dosage suppressor, extra copies of genes encoding upstream components of the Pkc1p pathway such as ROM2, RHO2, HCS77/WSC1/SLG1 and MID2 also suppressed glc7-10 effectively. Conversely, mpk1delta glc7-10 and bck1delta glc7-10 double mutants displayed a synthetic cell lysis defect compared with each single mutant and glc7-10 was hypersensitive to reduced PKC1 function, displaying highly aberrant morphologies and inviability even at the normally permissive temperature of 26 degrees C. Dephosphorylation by PP1 therefore functions positively to promote cell integrity, bud morphology and polarization of the actin cytoskeleton and glc7-10 cells require higher levels of Pkc1p activity to sustain these functions.