ATP analog-sensitive Pat1 protein kinase for synchronous fission yeast meiosis at physiological temperature

An improved tetracycline-inducible expression system for fission yeast

The ability to manipulate gene expression is valuable for elucidating gene function. In the fission yeast Schizosaccharomyces pombe, the most widely used regulatable expression system is the nmt1 promoter and its two attenuated variants. However, these promoters have limitations, including a long lag, incompatibility with rich media and unsuitability for non-dividing cells. Here, we present a tetracycline-inducible system free of these shortcomings. Our system features the enotetS promoter, which achieves a similar induced level and a higher induction ratio compared to the nmt1 promoter, without exhibiting a lag. Additionally, our system includes four weakened enotetS variants, offering an expression range similar to that of the nmt1 series promoters but with more intermediate levels. To enhance usability, each promoter is combined with a Tet-repressor-expressing cassette in an integration plasmid. Importantly, our system can be used in non-dividing cells, enabling the development of a synchronous meiosis induction method with high spore viability. Moreover, our system allows for the shutdown of gene expression and the generation of conditional loss-of-function mutants. This system provides a versatile and powerful tool for manipulating gene expression in fission yeast.

Quantitative proteomics and phosphoproteomics profiling of meiotic divisions in the fission yeast Schizosaccharomyces pombe

In eukaryotes, chromosomal DNA is equally distributed to daughter cells during mitosis, whereas the number of chromosomes is halved during meiosis. Despite considerable progress in understanding the molecular mechanisms that regulate mitosis, there is currently a lack of complete understanding of the molecular mechanisms regulating meiosis. Here, we took advantage of the fission yeast Schizosaccharomyces pombe, for which highly synchronous meiosis can be induced, and performed quantitative proteomics and phosphoproteomics analyses to track changes in protein expression and phosphorylation during meiotic divisions. We compared the proteomes and phosphoproteomes of exponentially growing mitotic cells with cells harvested around meiosis I, or meiosis II in strains bearing either the temperature-sensitive pat1-114 allele or conditional ATP analog-sensitive pat1-as2 allele of the Pat1 kinase. Comparing pat1-114 with pat1-as2 also allowed us to investigate the impact of elevated temperature (25 °C versus 34 °C) on meiosis, an issue that sexually reproducing organisms face due to climate change. Using TMTpro 18plex labeling and phosphopeptide enrichment strategies, we performed quantification of a total of 4673 proteins and 7172 phosphosites in S. pombe. We found that the protein level of 2680 proteins and the rate of phosphorylation of 4005 phosphosites significantly changed during progression of S. pombe cells through meiosis. The proteins exhibiting changes in expression and phosphorylation during meiotic divisions were represented mainly by those involved in the meiotic cell cycle, meiotic recombination, meiotic nuclear division, meiosis I, centromere clustering, microtubule cytoskeleton organization, ascospore formation, organonitrogen compound biosynthetic process, carboxylic acid metabolic process, gene expression, and ncRNA processing, among others. In summary, our findings provide global overview of changes in the levels and phosphorylation of proteins during progression of S. pombe cells through meiosis at normal and elevated temperatures, laying the groundwork for further elucidation of the functions and importance of specific proteins and their phosphorylation in regulating meiotic divisions in this yeast.

CDK phosphorylation of Sfr1 downregulates Rad51 function in late-meiotic homolog invasions

Meiosis is the developmental program that generates gametes. To produce healthy gametes, meiotic recombination creates reciprocal exchanges between each pair of homologous chromosomes that facilitate faithful chromosome segregation. Using fission yeast and biochemical, genetic, and cytological approaches, we have studied the role of CDK (cyclin-dependent kinase) in the control of Swi5-Sfr1, a Rad51-recombinase auxiliary factor involved in homolog invasion during recombination. We show that Sfr1 is a CDK target, and its phosphorylation downregulates Swi5-Sfr1 function in the meiotic prophase. Expression of a phospho-mimetic sfr1-7D mutant inhibits Rad51 binding, its robust chromosome loading, and subsequently decreases interhomolog recombination. On the other hand, the non-phosphorylatable sfr1-7A mutant alters Rad51 dynamics at late prophase, and exacerbates chromatin segregation defects and Rad51 retention observed in dbl2 deletion mutants when combined with them. We propose Sfr1 phospho-inhibition as a novel cell-cycle-dependent mechanism, which ensures timely resolution of recombination intermediates and successful chromosome distribution into the gametes. Furthermore, the N-terminal disordered part of Sfr1, an evolutionarily conserved feature, serves as a regulatory platform coordinating this phospho-regulation, protein localization and stability, with several CDK sites and regulatory sequences being conserved.

AoRan1 Is Involved in Regulating Conidiation, Stress Resistance, Secondary Metabolism, and Pathogenicity in Arthrobotrys oligospora

Arthrobotrys oligospora is a representative nematode-trapping (NT) fungus that is able to capture, kill, and digest nematodes by producing specialized three-dimensional networks (traps) under nutrient-deprived conditions. Ran1 is a serine/threonine protein kinase that can act as a negative regulator of sexual conjugation and meiosis. However, the specific role of Ran1 remains largely unknown in NT fungi. Here, we identified AoRan1 (AOL_s00004g277) via gene disruption, phenotypic analysis, and metabolomic analysis. Our findings reveal that Aoran1 knockout caused a remarkable increase in conidial production, traps, and nematode feeding efficiency. In addition, the absence of Aoran1 resulted in the accumulation of lipid droplets and increased autophagic levels as well as increased tolerance to cell wall synthesis-disturbing reagents and oxidants. Metabolomic analyses also suggested that AoRan1 is involved in multiple metabolic processes, such as fatty acid biosynthesis. In summary, our results suggest that AoRan1 is crucial in conidiation, pathogenicity, and secondary metabolism. This study's results further our understanding of the molecular mechanisms by which AoRan1 regulates conidiation and trap formation in A. oligospora.

Sororin is an evolutionary conserved antagonist of WAPL

Cohesin mediates sister chromatid cohesion to enable chromosome segregation and DNA damage repair. To perform these functions, cohesin needs to be protected from WAPL, which otherwise releases cohesin from DNA. It has been proposed that cohesin is protected from WAPL by SORORIN. However, in vivo evidence for this antagonism is missing and SORORIN is only known to exist in vertebrates and insects. It is therefore unknown how important and widespread SORORIN's functions are. Here we report the identification of SORORIN orthologs in Schizosaccharomyces pombe (Sor1) and Arabidopsis thaliana (AtSORORIN). sor1Δ mutants display cohesion defects, which are partially alleviated by wpl1Δ. Atsororin mutant plants display dwarfism, tissue specific cohesion defects and chromosome mis-segregation. Furthermore, Atsororin mutant plants are sterile and separate sister chromatids prematurely at anaphase I. The somatic, but not the meiotic deficiencies can be alleviated by loss of WAPL. These results provide in vivo evidence for SORORIN antagonizing WAPL, reveal that SORORIN is present in organisms beyond the animal kingdom and indicate that it has acquired tissue specific functions in plants.

Metabolic stress-induced long ncRNA transcription governs the formation of meiotic DNA breaks in the fission yeast fbp1 gene

Meiotic recombination is a pivotal process that ensures faithful chromosome segregation and contributes to the generation of genetic diversity in offspring, which is initiated by the formation of double-strand breaks (DSBs). The distribution of meiotic DSBs is not uniform and is clustered at hotspots, which can be affected by environmental conditions. Here, we show that non-coding RNA (ncRNA) transcription creates meiotic DSBs through local chromatin remodeling in the fission yeast fbp1 gene. The fbp1 gene is activated upon glucose starvation stress, in which a cascade of ncRNA-transcription in the fbp1 upstream region converts the chromatin configuration into an open structure, leading to the subsequent binding of transcription factors. We examined the distribution of meiotic DSBs around the fbp1 upstream region in the presence and absence of glucose and observed several new DSBs after chromatin conversion under glucose starvation conditions. Moreover, these DSBs disappeared when cis-elements required for ncRNA transcription were mutated. These results indicate that ncRNA transcription creates meiotic DSBs in response to stress conditions in the fbp1 upstream region. This study addressed part of a long-standing unresolved mechanism underlying meiotic recombination plasticity in response to environmental fluctuation.

SILAC-Based Proteomic Analysis of Meiosis in the Fission Yeast Schizosaccharomyces pombe

Stable isotope labeling by amino acids in cell culture (SILAC) provides a powerful tool to quantify proteins and posttranslational modifications. Here we describe how to apply SILAC for protein identification and quantification in synchronous meiotic cultures induced by inactivation of the Pat1 kinase in the fission yeast Schizosaccharomyces pombe.

Rec8 Cohesin-mediated Axis-loop chromatin architecture is required for meiotic recombination

During meiotic prophase, cohesin-dependent axial structures are formed in the synaptonemal complex (SC). However, the functional correlation between these structures and cohesion remains elusive. Here, we examined the formation of cohesin-dependent axial structures in the fission yeast Schizosaccharomyces pombe. This organism forms atypical SCs composed of linear elements (LinEs) resembling the lateral elements of SC but lacking the transverse filaments. Hi-C analysis using a highly synchronous population of meiotic S. pombe cells revealed that the axis-loop chromatin structure formed in meiotic prophase was dependent on the Rec8 cohesin complex. In contrast, the Rec8-mediated formation of the axis-loop structure occurred in cells lacking components of LinEs. To dissect the functions of Rec8, we identified a rec8-F204S mutant that lost the ability to assemble the axis-loop structure without losing cohesion of sister chromatids. This mutant showed defects in the formation of the axis-loop structure and LinE assembly and thus exhibited reduced meiotic recombination. Collectively, our results demonstrate that the Rec8-dependent axis-loop structure provides a structural platform essential for LinE assembly, facilitating meiotic recombination of homologous chromosomes, independently of its role in sister chromatid cohesion.

Direct evaluation of cohesin-mediated sister kinetochore associations at meiosis I in fission yeast

Kinetochores drive chromosome segregation by mediating chromosome interactions with the spindle. In higher eukaryotes, sister kinetochores are separately positioned on opposite sides of sister centromeres during mitosis, but associate with each other during meiosis I. Kinetochore association facilitates the attachment of sister chromatids to the same pole, enabling the segregation of homologous chromosomes toward opposite poles. In the fission yeast, Schizosaccharomyces pombe, Rec8-containing meiotic cohesin is suggested to establish kinetochore associations by mediating cohesion of the centromere cores. However, cohesin-mediated kinetochore associations on intact chromosomes have never been demonstrated directly. In the present study, we describe a novel method for the direct evaluation of kinetochore associations on intact chromosomes in live S. pombe cells, and demonstrate that sister kinetochores and the centromere cores are positioned separately on mitotic chromosomes but associate with each other on meiosis I chromosomes. Furthermore, we demonstrate that kinetochore association depends on meiotic cohesin and the cohesin regulators Moa1 and Mrc1, and requires mating-pheromone signaling for its establishment. These results confirm cohesin-mediated kinetochore association and its regulatory mechanisms, along with the usefulness of the developed method for its analysis. This article has an associated First Person interview with the first author of the paper.

Mapping and Analysis of Swi5 and Sfr1 Phosphorylation Sites

The evolutionarily conserved Swi5-Sfr1 complex plays an important role in homologous recombination, a process crucial for the maintenance of genomic integrity. Here, we purified Schizosaccharomyces pombe Swi5-Sfr1 complex from meiotic cells and analyzed it by mass spectrometry. Our analysis revealed new phosphorylation sites on Swi5 and Sfr1. We found that mutations that prevent phosphorylation of Swi5 and Sfr1 do not impair their function but swi5 and sfr1 mutants encoding phosphomimetic aspartate at the identified phosphorylation sites are only partially functional. We concluded that during meiosis, Swi5 associates with Sfr1 and both Swi5 and Sfr1 proteins are phosphorylated. However, the functional relevance of Swi5 and Sfr1 phosphorylation remains to be determined.

Proteomic analysis of meiosis and characterization of novel short open reading frames in the fission yeast Schizosaccharomyces pombe

Meiosis is the process by which haploid gametes are produced from diploid precursor cells. We used stable isotope labeling by amino acids in cell culture (SILAC) to characterize the meiotic proteome in the fission yeast Schizosaccharomyces pombe. We compared relative levels of proteins extracted from cells harvested around meiosis I with those of meiosis II, and proteins from premeiotic S phase with the interval between meiotic divisions, when S phase is absent. Our proteome datasets revealed peptides corresponding to short open reading frames (sORFs) that have been previously identified by ribosome profiling as new translated regions. We verified expression of selected sORFs by Western blotting and analyzed the phenotype of deletion mutants. Our data provide a resource for studying meiosis that may help understand differences between meiosis I and meiosis II and how S phase is suppressed between the two meiotic divisions.

Chromosome-associated RNA-protein complexes promote pairing of homologous chromosomes during meiosis in Schizosaccharomyces pombe

Pairing of homologous chromosomes in meiosis is essential for sexual reproduction. We have previously demonstrated that the fission yeast sme2 RNA, a meiosis-specific long noncoding RNA (lncRNA), accumulates at the sme2 chromosomal loci and mediates their robust pairing in meiosis. However, the mechanisms underlying lncRNA-mediated homologous pairing have remained elusive. In this study, we identify conserved RNA-binding proteins that are required for robust pairing of homologous chromosomes. These proteins accumulate mainly at the sme2 and two other chromosomal loci together with meiosis-specific lncRNAs transcribed from these loci. Remarkably, the chromosomal accumulation of these lncRNA–protein complexes is required for robust pairing. Moreover, the lncRNA–protein complexes exhibit phase separation properties, since 1,6-hexanediol treatment reversibly disassembled these complexes and disrupted the pairing of associated loci. We propose that lncRNA–protein complexes assembled at specific chromosomal loci mediate recognition and subsequent pairing of homologous chromosomes.

During meiosis, pairing of homologous chromosomes is critical for sexual reproduction. Here the authors reveal in S. pombe the role of lncRNA–protein complexes during the pairing of homologues chromosomes that assemble at specific chromosomal loci to mediate recognition of the pairs.

Sexual Reproduction: Preventing Re-fertilization in Fission Yeast

During sexual reproduction, two haploid cells fuse to produce a diploid cell called a zygote. A new study describes how fission yeast prevents a zygote from being formed by the fusion of more than two cells.

Multiple Duties for Spindle Assembly Checkpoint Kinases in Meiosis

Cell division in mitosis and meiosis is governed by evolutionary highly conserved protein kinases and phosphatases, controlling the timely execution of key events such as nuclear envelope breakdown, spindle assembly, chromosome attachment to the spindle and chromosome segregation, and cell cycle exit. In mitosis, the spindle assembly checkpoint (SAC) controls the proper attachment to and alignment of chromosomes on the spindle. The SAC detects errors and induces a cell cycle arrest in metaphase, preventing chromatid separation. Once all chromosomes are properly attached, the SAC-dependent arrest is relieved and chromatids separate evenly into daughter cells. The signaling cascade leading to checkpoint arrest depends on several protein kinases that are conserved from yeast to man. In meiosis, haploid cells containing new genetic combinations are generated from a diploid cell through two specialized cell divisions. Though apparently less robust, SAC control also exists in meiosis. Recently, it has emerged that SAC kinases have additional roles in executing accurate chromosome segregation during the meiotic divisions. Here, we summarize the main differences between mitotic and meiotic cell divisions, and explain why meiotic divisions pose special challenges for correct chromosome segregation. The less-known meiotic roles of the SAC kinases are described, with a focus on two model systems: yeast and mouse oocytes. The meiotic roles of the canonical checkpoint kinases Bub1, Mps1, the pseudokinase BubR1 (Mad3), and Aurora B and C (Ipl1) will be discussed. Insights into the molecular signaling pathways that bring about the special chromosome segregation pattern during meiosis will help us understand why human oocytes are so frequently aneuploid.

Analysis of Schizosaccharomyces pombe Meiosis

Meiosis is a specialized cell cycle that generates haploid gametes from diploid cells. The fission yeast Schizosaccharomyces pombe is one of the best model organisms for studying the regulatory mechanisms of meiosis. S. pombe cells, which normally grow in the haploid state, diploidize by conjugation and initiate meiosis when starved for nutrients, especially nitrogen. Following two rounds of chromosome segregation, spore formation takes place. The switch from mitosis to meiosis is controlled by a kinase, Pat1, and an RNA-binding protein, Mei2. Mei2 is also a key factor for meiosis-specific gene expression. Studies on S. pombe have offered insights into cell cycle regulation and chromosome segregation during meiosis. Here we outline the current understanding of the molecular mechanisms regulating the initiation and progression of meiosis, and introduce methods for the study of meiosis in fission yeast.

Synchronous Induction of Meiosis in the Fission Yeast Schizosaccharomyces pombe

In fission yeast Schizosaccharomyces pombe, initiation of meiosis is repressed by Pat1 kinase. This protocol describes how ectopic inactivation of the temperature-sensitive Pat1-114 kinase in G₁-arrested h^- /h^- diploid cells carrying mat1-Pc induces a highly synchronized commitment to and execution of meiosis. Haploid or diploid pat1-114 mutants without mat1-Pc can also be used for convenience, although less synchrony may be attained compared with induction using true diploids. An essentially identical protocol can be used for induction via inhibition of genetically sensitized Pat1 kinase by ATP analogs.

Destabilization of the replication fork protection complex disrupts meiotic chromosome segregation

The replication fork protection complex (FPC) coordinates multiple processes that are crucial for unimpeded passage of the replisome through various barriers and difficult to replicate areas of the genome. We examine the function of Swi1 and Swi3, fission yeast's primary FPC components, to elucidate how replication fork stability contributes to DNA integrity in meiosis. We report that destabilization of the FPC results in reduced spore viability, delayed replication, changes in recombination, and chromosome missegregation in meiosis I and meiosis II. These phenotypes are linked to accumulation and persistence of DNA damage markers in meiosis and to problems with cohesion stability at the centromere. These findings reveal an important connection between meiotic replication fork stability and chromosome segregation, two processes with major implications to human reproductive health.

Selection of G1 Phase Yeast Cells for Synchronous Meiosis and Sporulation

Centrifugal elutriation is a procedure that allows the fractionation of cell populations based upon their size and shape. This allows cells in distinct cell cycle stages can be captured from an asynchronous population. The technique is particularly helpful when performing an experiment to monitor the progression of cells through the cell cycle or meiosis. Yeast sporulation like gametogenesis in other eukaryotes initiates from the G1 phase of the cell cycle. Conveniently, S. cerevisiae arrest in G1 phase when starved for nutrients and so withdrawal of nitrogen and glucose allows cells to abandon vegetative growth in G1 phase before initiating the sporulation program. This simple starvation protocol yields a partial synchronization that has been used extensively in studies of progression through meiosis and sporulation. By using centrifugal elutriation it is possible to isolate a homogeneous population of G1 phase cells and induce them to sporulate synchronously, which is beneficial for investigating progression through meiosis and sporulation. An additionally benefit of this protocol is that cell populations can be isolated based upon size and both large and small cell populations can be tested for progression through meiosis and sporulation. Here we present a protocol for purification of G1 phase diploid cells for examining synchronous progression through meiosis and sporulation.

Quantitative Genome-Wide Measurements of Meiotic DNA Double-Strand Breaks and Protein Binding in S. pombe

The fission yeast Schizosaccharomyces pombe is especially well suited for studying meiosis in molecular detail. Experiments with S. pombe strains that undergo a nearly synchronous meiosis-at variable temperatures-have elucidated the mechanisms of meiotic progression and the proteins that are involved. For example, studies focused on the initiation of meiotic recombination by programmed DNA double-strand breaks (DSBs) have proven exceptionally informative. In meiosis, some regions of DNA have more frequent DSBs than the surrounding regions. These DSB hotspots can be visualized by Southern blot hybridization of restriction fragments ranging from kilobases (kb) to megabases (Mb) in size. More recently, the benefits of genome-wide analysis to map the distribution and frequency of meiotic DSBs have been attained, with resolution down to the nucleotide level. Infrequent, non-hotspot DSBs previously not detectable have been observed, creating a better understanding of how recombination is regulated. Additional genome-wide analyses have shown proteins that bind specifically to DSB hotspots, providing insight into how the DSB initiation complex functions. We describe here detailed methods for achieving these results.

Modulation of plant growth in vivo and identification of kinase substrates using an analog-sensitive variant of CYCLIN-DEPENDENT KINASE A;1

BACKGROUND

Modulation of protein activity by phosphorylation through kinases and subsequent de-phosphorylation by phosphatases is one of the most prominent cellular control mechanisms. Thus, identification of kinase substrates is pivotal for the understanding of many - if not all - molecular biological processes. Equally, the possibility to deliberately tune kinase activity is of great value to analyze the biological process controlled by a particular kinase.

RESULTS

Here we have applied a chemical genetic approach and generated an analog-sensitive version of CDKA;1, the central cell-cycle regulator in Arabidopsis and homolog of the yeast Cdc2/CDC28 kinases. This variant could largely rescue a cdka;1 mutant and is biochemically active, albeit less than the wild type. Applying bulky kinase inhibitors allowed the reduction of kinase activity in an organismic context in vivo and the modulation of plant growth. To isolate CDK substrates, we have adopted a two-dimensional differential gel electrophoresis strategy, and searched for proteins that showed mobility changes in fluorescently labeled extracts from plants expressing the analog-sensitive version of CDKA;1 with and without adding a bulky ATP variant. A pilot set of five proteins involved in a range of different processes could be confirmed in independent kinase assays to be phosphorylated by CDKA;1 approving the applicability of the here-developed method to identify substrates.

CONCLUSION

The here presented generation of an analog-sensitive CDKA;1 version is functional and represent a novel tool to modulate kinase activity in vivo and identify kinase substrates. Our here performed pilot screen led to the identification of CDK targets that link cell proliferation control to sugar metabolism, proline proteolysis, and glucosinolate production providing a hint how cell proliferation and growth are integrated with plant development and physiology.

Dbl2 Regulates Rad51 and DNA Joint Molecule Metabolism to Ensure Proper Meiotic Chromosome Segregation

To identify new proteins required for faithful meiotic chromosome segregation, we screened a Schizosaccharomyces pombe deletion mutant library and found that deletion of the dbl2 gene led to missegregation of chromosomes during meiosis. Analyses of both live and fixed cells showed that dbl2Δ mutant cells frequently failed to segregate homologous chromosomes to opposite poles during meiosis I. Removing Rec12 (Spo11 homolog) to eliminate meiotic DNA double-strand breaks (DSBs) suppressed the segregation defect in dbl2Δ cells, indicating that Dbl2 acts after the initiation of meiotic recombination. Analyses of DSBs and Holliday junctions revealed no significant defect in their formation or processing in dbl2Δ mutant cells, although some Rec12-dependent DNA joint molecules persisted late in meiosis. Failure to segregate chromosomes in the absence of Dbl2 correlated with persistent Rad51 foci, and deletion of rad51 or genes encoding Rad51 mediators also suppressed the segregation defect of dbl2Δ. Formation of foci of Fbh1, an F-box helicase that efficiently dismantles Rad51-DNA filaments, was impaired in dbl2Δ cells. Our results suggest that Dbl2 is a novel regulator of Fbh1 and thereby Rad51-dependent DSB repair required for proper meiotic chromosome segregation and viable sex cell formation. The wide conservation of these proteins suggests that our results apply to many species.

Meiosis produces haploid gametes from diploid precursor cells. This reduction of chromosome number is achieved by two successive divisions after only a single round of DNA replication. To identify novel regulators of meiosis, we screened a library of fission yeast deletion mutants and found that deletion of the dbl2 gene led to missegregation of chromosomes during meiosis. Analysis of live dbl2Δ cells by fluorescence microscopy showed that chromosomes frequently failed to segregate during the first meiotic division. Further cytological and biochemical analyses revealed that this segregation defect is due to persistent intermediates of DNA double-strand break repair, also called DNA joint molecules. Our results indicate that Dbl2 is required for formation of Fbh1 DNA helicase foci at the sites of DNA double-strand break repair in order to process DNA joint molecules and allow segregation of chromosomes during meiotic divisions. Our bioinformatics searches revealed that Dbl2 is highly conserved in fungi, animals and plants, suggesting that Dbl2 plays a similar role in other organisms–the formation of viable sex cells and healthy progeny.

Spatiotemporal Control of Forkhead Binding to DNA Regulates the Meiotic Gene Expression Program

Meiosis is a differentiated program of the cell cycle that is characterized by high levels of recombination followed by two nuclear divisions. In fission yeast, the genetic program during meiosis is regulated at multiple levels, including transcription, mRNA stabilization, and splicing. Mei4 is a forkhead transcription factor that controls the expression of mid-meiotic genes. Here, we describe that Fkh2, another forkhead transcription factor that is essential for mitotic cell-cycle progression, also plays a pivotal role in the control of meiosis. Fkh2 binding preexists in most Mei4-dependent genes, inhibiting their expression. During meiosis, Fkh2 is phosphorylated in a CDK/Cig2-dependent manner, decreasing its affinity for DNA, which creates a window of opportunity for Mei4 binding to its target genes. We propose that Fkh2 serves as a placeholder until the later appearance of Mei4 with a higher affinity for DNA that induces the expression of a subset of meiotic genes.

A single cyclin-CDK complex is sufficient for both mitotic and meiotic progression in fission yeast

The dominant model for eukaryotic cell cycle control proposes that cell cycle progression is driven by a succession of CDK complexes with different substrate specificities. However, in fission yeast it has been shown that a single CDK complex generated by the fusion of the Cdc13 cyclin with the CDK protein Cdc2 can drive the mitotic cell cycle. Meiosis is a modified cell cycle programme in which a single S-phase is followed by two consecutive rounds of chromosome segregation. Here we systematically analyse the requirements of the different fission yeast cyclins for meiotic cell cycle progression. We also show that a single Cdc13-Cdc2 complex, in the absence of the other cyclins, can drive the meiotic cell cycle. We propose that qualitatively different CDK complexes are not absolutely required for cell cycle progression either during mitosis or meiosis, and that a single CDK complex can drive both cell cycle programmes.

Dissecting the first and the second meiotic divisions using a marker-less drug-hypersensitive fission yeast

Faithful chromosome segregation during meiosis is indispensable to prevent birth defects and infertility. Canonical genetic manipulations have not been very useful for studying meiosis II, since mutations of genes involved in cell cycle regulation or chromosome segregation may affect meiosis I, making interpretations of any defects observed in meiosis II complicated. Here we present a powerful strategy to dissect meiosis I and meiosis II, using chemical inhibitors in genetically tractable model organism fission yeast (Schizosaccharomyces pombe). As various chemical probes are not active in fission yeast, mainly due to an effective multidrug resistance (MDR) response, we have recently developed a drug-hypersensitive MDR-sup strain by suppression of the key genes responsible for MDR response. We further developed the MDR-supML (marker-less) strain by deleting 7 MDR genes without commonly used antibiotic markers. The new strain makes fluorescent tagging and gene deletion much simpler, which enables effective protein visualization in varied genetic backgrounds. Using the MDR-supML strain with chemical inhibitors and live cell fluorescence microscopy, we established cell cycle arrest at meiosis I and meiosis II and examined Aurora-dependent spindle assembly checkpoint (SAC) regulation during meiosis. We found that Aurora B/Ark1 kinase activity is required for recruitment of Bub1, an essential SAC kinase, to unattached kinetochore in prometaphase I and prometaphase II as in mitosis. Thus, Aurora's role in SAC activation is likely conserved in mitosis, meiosis I, and meiosis II. Together, our MDR-supML strain will be useful to dissect complex molecular mechanisms in mitosis and 2 successive meiotic divisions.

Synchronized fission yeast meiosis using an ATP analog-sensitive Pat1 protein kinase

Synchronous cultures are often indispensable for studying meiosis. Here we present an optimized protocol for induction of synchronous meiosis in the fission yeast Schizosaccharomyces pombe. Chemical inactivation of an ATP analog-sensitive form of the Pat1 kinase (pat1-as2) by adding the ATP analog 1-NM-PP1 in G1-arrested cells allows the induction of synchronous meiosis at optimal temperature (25°C). Importantly, this protocol eliminates detrimental effects of elevated temperature (34°C), which is required to inactivate the commonly used temperature-sensitive Pat1 kinase mutant (pat1-114). The addition of the mat-Pc gene to a mat1-M strain further improves chromosome segregation and spore viability. Thus, our protocol offers highly synchronous meiosis at optimal temperature, with most characteristics similar to those of wild-type meiosis. The synchronization protocol can be completed in 5 d (not including strain production, which may take as long as 2 or 3 months).

DNA intermediates of meiotic recombination in synchronous S. pombe at optimal temperature

Crossovers formed by recombination between homologous chromosomes are important for proper homolog segregation during meiosis and for generation of genetic diversity. Optimal molecular analysis of DNA intermediates of recombination requires synchronous cultures. We previously described a mutant, pat1-as2, of the fission yeast Schizosaccharomyces pombe that undergoes synchronous meiosis at 25°C when an ATP analog is added to the culture. Here, we compare recombination intermediates in pat1-as2 at 25°C with those in the widely used pat1-114 temperature-sensitive mutant at 34°C, a temperature higher than optimal. DNA double-strand breaks at most hotspots are similarly abundant in the two conditions but, remarkably, a few hotspots are distinctly deficient at 25°C. In both conditions, Holliday junctions at DNA break hotspots form more frequently between sister chromatids than between homologs, but a novel species, perhaps arising from invasion by only one end of broken DNA, is more readily observed at 25°C. Our results confirm the validity of previous assays of recombination intermediates in S. pombe and provide new information on the mechanism of meiotic recombination.

Mal3, the Schizosaccharomyces pombe homolog of EB1, is required for karyogamy and for promoting oscillatory nuclear movement during meiosis

Two successive rounds of chromosome segregation following a single round of DNA replication enable the production of haploid gametes during meiosis. In the fission yeast Schizosaccharomyces pombe, karyogamy is the process where the nuclei from 2 haploid cells fuse to create a diploid nucleus, which then undergoes meiosis to produce 4 haploid spores. By screening a collection of S. pombe deletion strains, we found that the deletion of 2 genes, mal3 and mto1, leads to the production of asci containing up to 8 spores. Here, we show that Mal3, the fission yeast member of the EB1 family of conserved microtubule plus-end tracking proteins, is required for karyogamy, oscillatory nuclear movement, and proper segregation of chromosomes during meiosis. In the absence of Mal3, meiosis frequently initiates before the completion of karyogamy, thus producing up to 8 nuclei in a single ascus. Our results provide new evidence that fission yeast can initiate meiosis prior to completing karyogamy.

Crosstalk between casein kinase II and Ste20-related kinase Nak1

Although the sterile 20 (Ste20) serine/threonine protein kinase was originally identified as a component of the S. cerevisiae mating pathway, it has homologs in higher eukaryotes and is part of a larger family of Ste20-like kinases. Ste20-like kinases are involved in multiple cellular processes, such as cell growth, morphogenesis, apoptosis and immune response. Carrying out such a diverse array of biological functions requires numerous regulatory inputs and outputs in the form of protein-protein interactions and post-translational modifications. Hence, a thorough knowledge of Ste20-like kinase binding partners and phosphorylation sites will be essential for understanding the various roles of these kinases. Our recent study revealed that Schizosaccharomyces pombe Nak1 (a conserved member of the GC-kinase sub-family of Ste20-like kinases) is in a complex with the leucine-rich repeat-containing protein Sog2. Here, we show a novel and unexpected interaction between the Nak1-Sog2 kinase complex and Casein kinase 2 (Cka1, Ckb1 and Ckb2) using tandem-affinity purification followed by mass spectrometric analysis. In addition, we identify unique phosphosites on Nak1, Sog2 and the catalytic subunit of casein kinase 2, Cka1. Given the conserved nature of these kinases, we expect this work will shed light on the functions of these proteins both in yeast and higher eukaryotes.

A knockout screen for protein kinases required for the proper meiotic segregation of chromosomes in the fission yeast Schizosaccharomyces pombe

The reduction of chromosome number during meiosis is achieved by two successive rounds of chromosome segregation after just single round of DNA replication. To identify novel proteins required for the proper segregation of chromosomes during meiosis, we analyzed the consequences of deleting Schizosaccharomyces pombe genes predicted to encode protein kinases that are not essential for cell viability. We show that Mph1, a member of the Mps1 family of spindle assembly checkpoint kinases, is required to prevent meiosis I homolog non-disjunction. We also provide evidence for a novel function of Spo4, the fission yeast ortholog of Dbf4-dependent Cdc7 kinase, in regulating the length of anaphase II spindles. In the absence of Spo4, abnormally elongated anaphase II spindles frequently overlap and thus destroy the linear order of nuclei in the ascus. Our observation that the spo4Δ mutant phenotype can be partially suppressed by inhibiting Cdc2-as suggests that dysregulation of the activity of this cyclin-dependent kinase may cause abnormal elongation of anaphase II spindles in spo4Δ mutant cells.

Global control of cell growth in fission yeast and its coordination with the cell cycle

Cell growth is a fundamental process for every cell but its pleiotropic complexity makes it difficult to comprehend. Global aspects of cellular growth, like the overall determinants of growth rate are not well understood. Here we examine the cell growth pattern of the fission yeast Schizosaccharomyces pombe during the mitotic and meiotic cell cycles. We also explore recent findings illuminating aspects of cell size homeostasis and cell growth regulation, and propose that there are global controls over growth acting at the level of the cell.