Ultrastructural changes of the fission yeast (Schizosaccharomyces pombe) during ascospore formation

The Multiple Functions of Rho GTPases in Fission Yeasts

The Rho family of GTPases represents highly conserved molecular switches involved in a plethora of physiological processes. Fission yeast Schizosaccharomyces pombe has become a fundamental model organism to study the functions of Rho GTPases over the past few decades. In recent years, another fission yeast species, Schizosaccharomyces japonicus, has come into focus offering insight into evolutionary changes within the genus. Both fission yeasts contain only six Rho-type GTPases that are spatiotemporally controlled by multiple guanine-nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs), and whose intricate regulation in response to external cues is starting to be uncovered. In the present review, we will outline and discuss the current knowledge and recent advances on how the fission yeasts Rho family GTPases regulate essential physiological processes such as morphogenesis and polarity, cellular integrity, cytokinesis and cellular differentiation.

Wake-up alarm: virtual time-lapse gene expression landscape illuminates mechanisms underlying dormancy breaking of germinating spores

Dormancy breaking is a common physiological phenomenon that is shared by eukaryotes. Germination of spores in fungi is one of the most representative cases of dormancy breaking. Understanding the mechanisms of spore germination is therefore fundamental to basic studies on the control of cell proliferation and differentiation, as well as agricultural applications and medical investigation of fungal pathogenesis. In fission yeast, spores are generated as a consequence of sexual differentiation under nutrient starvation, remaining dormant until further nourishment, but little is known about how dormant spores germinate in response to environmental change. In a breakthrough, methods for single-cell-based gene expression profiling have recently been introduced. Several mRNA expression profiles were assembled from single spore cells during dormancy or germination. Single-cell RNA-seq profiles were aligned sequentially according to their similarities. The alignment of transcriptomes visualised how gene expression varies over time upon dormancy breaking. In this review, we revisit knowledge from previous studies on germination, select candidate genes that may be involved in germination, and query their expression from the temporal transcriptomic dataset so that studies on S. pombe germination can be extended further.

Quick-Freeze, Deep-Etch Electron Microscopy Reveals the Characteristic Architecture of the Fission Yeast Spore

The spore of the fission yeast Schizosaccharomyces pombe is a dormant cell that is resistant to a variety of environmental stresses. The S. pombe spore is coated by a proteinaceous surface layer, termed the Isp3 layer because it comprises mainly Isp3 protein. Although thin-section electron microscopy and scanning electron microscopy have revealed the fundamental structure of the spore, its architecture remains unclear. Here we visualized S. pombe spores by using a quick-freeze replica electron microscopy (QFDE-EM) at nanometer resolution, which revealed novel characteristic structures. QFDE-EM revealed that the Isp3 layer exists as an interwoven fibrillar layer. On the spore cell membrane, many deep invaginations, which are longer than those on the vegetative cell membrane, are aligned in parallel. We also observed that during spore germination, the cell surface changes from a smooth to a dendritic filamentous structure, the latter being characteristic of vegetative cells. These findings provide significant insight into not only the structural composition of the spore, but also the mechanism underlying the stress response of the cell.

The Fission Yeast RNA-Binding Protein Meu5 Is Involved in Outer Forespore Membrane Breakdown during Spore Formation

In Schizosaccharomyces pombe, the spore wall confers strong resistance against external stress. During meiosis II, the double-layered intracellular forespore membrane (FSM) forms de novo and encapsulates the nucleus. Eventually, the inner FSM layer becomes the plasma membrane of the spore, while the outer layer breaks down. However, the molecular mechanism and biological significance of this membrane breakdown remain unknown. Here, by genetic investigation of an S. pombe mutant (E22) with normal prespore formation but abnormal spores, we showed that Meu5, an RNA-binding protein known to bind to and stabilize more than 80 transcripts, is involved in this process. We confirmed that the E22 mutant does not produce Meu5 protein, while overexpression of meu5⁺ in E22 restores the sporulation defect. Furthermore, electron microscopy revealed that the outer membrane of the FSM persisted in meu5∆ spores. Investigation of the target genes of meu5⁺ showed that a mutant of cyc1⁺ encoding cytochrome c also showed a severe defect in outer FSM breakdown. Lastly, we determined that outer FSM breakdown occurs coincident with or after formation of the outermost Isp3 layer of the spore wall. Collectively, our data provide novel insights into the molecular mechanism of spore formation.

Metabolism of Storage Lipids and the Role of Lipid Droplets in the Yeast Schizosaccharomyces pombe

Storage lipids, triacylglycerols (TAG), and steryl esters (SE), are predominant constituents of lipid droplets (LD) in fungi. In several yeast species, metabolism of TAG and SE is linked to various cellular processes, including cell division, sporulation, apoptosis, response to stress, and lipotoxicity. In addition, TAG are an important source for the generation of value-added lipids for industrial and biomedical applications. The fission yeast Schizosaccharomyces pombe is a widely used unicellular eukaryotic model organism. It is a powerful tractable system used to study various aspects of eukaryotic cellular and molecular biology. However, the knowledge of S. pombe neutral lipids metabolism is quite limited. In this review, we summarize and discuss the current knowledge of the homeostasis of storage lipids and of the role of LD in the fission yeast S. pombe with the aim to stimulate research of lipid metabolism and its connection with other essential cellular processes. We also discuss the advantages and disadvantages of fission yeast in lipid biotechnology and recent achievements in the use of S. pombe in the biotechnological production of valuable lipid compounds.

The fission yeast SPB component Dms1 is required to initiate forespore membrane formation and maintain meiotic SPB components

The spindle pole body (SPB) plays a central role in spore plasma membrane formation in addition to its recognized role in microtubule organization. During meiosis, a biomembrane called the forespore membrane (FSM) is newly formed at the SPB. Although several SPB proteins essential for the initiation of FSM formation (meiotic SPB components) have been identified, the molecular mechanism is still unknown. Here, we report the isolation and functional characterization of Dms1 as a component of the SPB. We show that FSM formation does not initiate in dms1Δ cells. Dms1 protein is constitutively expressed throughout the life cycle and localizes to the SPB and the nuclear envelope. The predicted Dms1 protein has a transmembrane domain, which is required for correct localization at the SPB. Dms1 is essential for the proper localization of three meiotic SPB components, Spo15, Spo2, and Spo13, but these components do not affect localization of Dms1. Collectively, these results suggest that Dms1 anchors these meiotic SPB components to the SPB, thereby facilitating the initiation of FSM formation.

Meiosis-specific localization of the exocytic Rab Ypt2 in fission yeast

Fission yeast Ypt2, an orthologue of the mammalian small GTPase Rab8, is responsible for post-Golgi membrane trafficking. During meiosis, Ypt2 localizes at the spindle pole body (SPB), where it regulates de novo biogenesis of the spore plasma membrane. Recruitment of Ypt2 to the SPB is dependent on its meiosis-specific GDP/GTP exchange factor (GEF), the SPB-resident protein Spo13. Here we have examined the SPB recruitment of Ypt2 by Spo13. The GEF activity of Spo13 was required, but not essential for recruitment. Furthermore, Ypt2 recruitment was regulated in a meiosis-specific manner and partially regulated by the nuclear Dbf2-related (NDR) kinase Sid2, indicating the existence of a novel regulatory mechanism for localization of Rab GTPases during meiosis.

A large gene family in fission yeast encodes spore killers that subvert Mendel's law

Spore killers in fungi are selfish genetic elements that distort Mendelian segregation in their favor. It remains unclear how many species harbor them and how diverse their mechanisms are. Here, we discover two spore killers from a natural isolate of the fission yeast Schizosaccharomyces pombe. Both killers belong to the previously uncharacterized wtf gene family with 25 members in the reference genome. These two killers act in strain-background-independent and genome-location-independent manners to perturb the maturation of spores not inheriting them. Spores carrying one killer are protected from its killing effect but not that of the other killer. The killing and protecting activities can be uncoupled by mutation. The numbers and sequences of wtf genes vary considerably between S. pombe isolates, indicating rapid divergence. We propose that wtf genes contribute to the extensive intraspecific reproductive isolation in S. pombe, and represent ideal models for understanding how segregation-distorting elements act and evolve.

The exocytic Rabs Ypt3 and Ypt2 regulate the early step of biogenesis of the spore plasma membrane in fission yeast

Two Rabs, Ypt3 and Ypt2, regulating the trafficking of Golgi-derived secretory vesicles have key roles in biogenesis of the spore plasma membrane in fission yeast. During sporulation, the Rabs and secretory vesicles relocalize at the meiotic spindle pole body, where spore plasma membrane formation subsequently initiates.

During fission yeast sporulation, a membrane compartment called the forespore membrane (FSM) is newly formed on the spindle pole body (SPB). The FSM expands by membrane vesicle fusion, encapsulates the daughter nucleus resulting from meiosis, and eventually matures into the plasma membrane of the spore. Although many of the genes involved in FSM formation have been identified, its molecular mechanism is not fully understood. Here a genetic screen for sporulation-deficient mutations identified Ypt3, a Rab-family small GTPase known to function in the exocytic pathway. The ypt3-ki8 mutant showed defects in both the initiation of FSM biogenesis and FSM expansion. We also show that a mutation in Ypt2, another Rab protein that may function in the same pathway as Ypt3, compromises the initiation of FSM formation. As meiosis proceeds, both GFP-Ypt3 and GFP-Ypt2 are observed at the SPB and then relocalize to the FSM. Their localizations at the SPB precede FSM formation and depend on the meiotic SPB component Spo13, a putative GDP/GTP exchange factor for Ypt2. Given that Spo13 is essential for initiating FSM formation, these results suggest that two exocytic Rabs, Ypt3 and Ypt2, regulate the initiation of FSM formation on the SPB in concert with Spo13.

Label-free Chemical Imaging of Fungal Spore Walls by Raman Microscopy and Multivariate Curve Resolution Analysis

Fungal cell walls are medically important since they represent a drug target site for antifungal medication. So far there is no method to directly visualize structurally similar cell wall components such as α-glucan, β-glucan and mannan with high specificity, especially in a label-free manner. In this study, we have developed a Raman spectroscopy based molecular imaging method and combined multivariate curve resolution analysis to enable detection and visualization of multiple polysaccharide components simultaneously at the single cell level. Our results show that vegetative cell and ascus walls are made up of both α- and β-glucans while spore wall is exclusively made of α-glucan. Co-localization studies reveal the absence of mannans in ascus wall but are distributed primarily in spores. Such detailed picture is believed to further enhance our understanding of the dynamic spore wall architecture, eventually leading to advancements in drug discovery and development in the near future.

A genome-wide screen for sporulation-defective mutants in Schizosaccharomyces pombe

Yeast sporulation is a highly regulated developmental program by which diploid cells generate haploid gametes, termed spores. To better define the genetic pathways regulating sporulation, a systematic screen of the set of ~3300 nonessential Schizosaccharomyces pombe gene deletion mutants was performed to identify genes required for spore formation. A high-throughput genetic method was used to introduce each mutant into an h(90) background, and iodine staining was used to identify sporulation-defective mutants. The screen identified 34 genes whose deletion reduces sporulation, including 15 that are defective in forespore membrane morphogenesis. In S. pombe, the total number of sporulation-defective mutants is a significantly smaller fraction of coding genes than in S. cerevisiae, which reflects the different evolutionary histories and biology of the two yeasts.

The fission yeast spore is coated by a proteinaceous surface layer comprising mainly Isp3

The spore is a dormant cell that is resistant to various environmental stresses. As compared with the vegetative cell wall, the spore wall has a more extensive structure that confers resistance on spores. In the fission yeast Schizosaccharomyces pombe, the polysaccharides glucan and chitosan are major components of the spore wall; however, the structure of the spore surface remains unknown. We identify the spore coat protein Isp3/Meu4. The isp3 disruptant is viable and executes meiotic nuclear divisions as efficiently as the wild type, but isp3∆ spores show decreased tolerance to heat, digestive enzymes, and ethanol. Electron microscopy shows that an electron-dense layer is formed at the outermost region of the wild-type spore wall. This layer is not observed in isp3∆ spores. Furthermore, Isp3 is abundantly detected in this layer by immunoelectron microscopy. Thus Isp3 constitutes the spore coat, thereby conferring resistance to various environmental stresses.

Dma1-dependent degradation of Septation Initiation Network proteins during meiosis in Schizosaccharomyces pombe

The Schizosaccharomyces pombe septation initiation network (SIN) is required for cytokinesis during vegetative growth and spore formation during meiosis. Regulation of the SIN during mitosis has been studied extensively, but less is known about its meiotic regulation. Here, we show that several aspects of the SIN regulation differ between mitosis and meiosis. First, the presence of GTP-bound spg1p is not the main determinant of the timing of cdc7p and sid1p association with the SPB during meiosis. Second, the localisation dependencies of SIN proteins differ from those in mitotic cells, suggesting a modified functional organisation of the SIN during meiosis. Third, there is stage-specific degradation of SIN components in meiosis; byr4p is degraded after meiosis I, while the degradation of cdc7p, cdc11p and sid4p occurs after the second meiotic division and depends upon the ubiquitin ligase dma1p. Finally, dma1p-dependent degradation is not restricted to the SIN, for we show that dma1p is needed for the degradation of mcp6p/hrs1p in meiosis I. Together, these data suggest that stage-specific targetted proteolysis will play an important role in regulating meiotic progression.

The fission yeast synaptobrevin ortholog Syb1 plays an important role in forespore membrane formation and spore maturation

Synaptobrevin, also called vesicle-associated membrane protein (VAMP), is a component of the plasma membrane N-methylmaleimide-sensitive factor attachment protein receptor (SNARE) complex, which plays a key role in intracellular membrane fusion. Previous studies have revealed that, similar to synaptobrevin in other organisms, the fission yeast synaptobrevin ortholog Syb1 associates with post-Golgi secretory vesicles and is essential for cytokinesis and cell elongation. Here, we report that Syb1 has a role in sporulation. After nitrogen starvation, green fluorescent protein (GFP)-Syb1 is found in intracellular dots. As meiosis proceeds, GFP-Syb1 accumulates around the nucleus and then localizes at the forespore membrane (FSM). We isolated a syb-S1 mutant, which exhibits a defect in sporulation. In syb1-S1 mutants, the FSM begins to form but fails to develop a normal morphology. Electron microscopy shows that an abnormal spore wall is often formed in syb1-S1 mutant spores. Although most syb1-S1 mutant spores are germinated, they are less tolerant to ethanol than wild-type spores. The syb1-S1 allele carries a missense mutation, resulting in replacement of a conserved cysteine residue adjacent to the transmembrane domain, which reduces the stability and abundance of the Syb1 protein. Taken together, these results indicate that Syb1 plays an important role in both FSM assembly and spore wall formation.

The fission yeast pleckstrin homology domain protein Spo7 is essential for initiation of forespore membrane assembly and spore morphogenesis

Sporulation in fission yeast represents a unique mode of cell division in which a new cell is formed within the cytoplasm of a mother cell. This event is accompanied by formation of the forespore membrane (FSM), which becomes the plasma membrane of spores. At prophase II, the spindle pole body (SPB) forms an outer plaque, from which formation of the FSM is initiated. Several components of the SPB play an indispensable role in SPB modification, and therefore in sporulation. In this paper, we report the identification of a novel SPB component, Spo7, which has a pleckstrin homology (PH) domain. We found that Spo7 was essential for initiation of FSM assembly, but not for SPB modification. Spo7 directly bound to Meu14, a component of the leading edge of the FSM, and was essential for proper localization of Meu14. The PH domain of Spo7 had affinity for phosphatidylinositol 3-phosphate (PI3P). spo7 mutants lacking the PH domain showed aberrant spore morphology, similar to that of meu14 and phosphatidylinositol 3-kinase (pik3) mutants. Our study suggests that Spo7 coordinates formation of the leading edge and initiation of FSM assembly, thereby accomplishing accurate formation of the FSM.

Endocytosis is essential for dynamic translocation of a syntaxin 1 orthologue during fission yeast meiosis

Fission yeast sporulation seems to accompany a dynamic alteration of membrane traffic pathways in which the destination of secretory vesicles changes from the plasma membrane to the developing spore membrane. Evidence shows that endocytosis is responsible for this alteration in traffic pathways via the relocalization of syntaxin 1.

Syntaxin is a component of the target soluble N-ethylmaleimide–sensitive factor attachment protein receptor complex, which is responsible for fusion of membrane vesicles at the target membrane. The fission yeast syntaxin 1 orthologue Psy1 is essential for both vegetative growth and spore formation. During meiosis, Psy1 disappears from the plasma membrane (PM) and dramatically relocalizes on the nascent forespore membrane, which becomes the PM of the spore. Here we report the molecular details and biological significance of Psy1 relocalization. We find that, immediately after meiosis I, Psy1 is selectively internalized by endocytosis. In addition, a meiosis-specific signal induced by the transcription factor Mei4 seems to trigger this internalization. The internalization of many PM proteins is facilitated coincident with the initiation of meiosis, whereas Pma1, a P-type ATPase, persists on the PM even during the progression of meiosis II. Ergosterol on the PM is also important for the internalization of PM proteins in general during meiosis. We consider that during meiosis in Schizosaccharomyces pombe cells, the characteristics of endocytosis change, thereby facilitating internalization of Psy1 and accomplishing sporulation.

Schizosaccharomyces pombe calmodulin, Cam1, plays a crucial role in sporulation by recruiting and stabilizing the spindle pole body components responsible for assembly of the forespore membrane

Calmodulin in Schizosaccharomyces pombe is encoded by the cam1(+) gene, which is indispensable for both vegetative growth and sporulation. Here, we report how Cam1 functions in spore formation. We found that Cam1 preferentially localized to the spindle pole body (SPB) during meiosis and sporulation. Formation of the forespore membrane, a precursor of the plasma membrane in spores, was blocked in a missense cam1 mutant, which was viable but unable to sporulate. Three SPB proteins necessary for the onset of forespore membrane formation, Spo2, Spo13, and Spo15, were unable to localize to the SPB in the cam1 mutant although five core SPB components that were tested were present. Recruitment of Spo2 and Spo13 is known to require the presence of Spo15 in the SPB. Notably, Spo15 was unstable in the cam1 mutant, and as a result, SPB localization of Spo2 and Spo13 was lost. Overexpression of Spo15 partially alleviated the sporulation defect in the cam1 mutant. These results indicate that calmodulin plays an essential role in forespore membrane formation by stably maintaining Spo15, and thus Spo2 and Spo13, at the SPB in meiotic cells.

The role of Schizosaccharomyces pombe dma1 in spore formation during meiosis

Meiosis is a specialised form of the cell cycle that gives rise to haploid gametes. In Schizosaccharomyces pombe, the products of meiosis are four spores, which are formed by encapsulation of the four meiosis II nuclei within the cytoplasm of the zygote produced by fusion of the mating cells. The S. pombe spindle pole body is remodelled during meiosis II and membrane vesicles are then recruited there to form the forespore membrane, which encapsulates the haploid nucleus to form a prespore. Spore wall material is then deposited, giving rise to the mature spore. The septation initiation network is required to coordinate cytokinesis and mitosis in the vegetative cycle and for spore formation in the meiotic cycle. We have investigated the role of the SIN regulator dma1p in meiosis; we find that although both meiotic divisions occur in the absence of dma1p, asci frequently contain fewer than four spores, which are larger than in wild-type meiosis. Our data indicate that dma1p acts in parallel to the leading-edge proteins and septins to assure proper formation for the forespore membrane. Dma1p also contributes to the temporal regulation of the abundance of the meiosis-specific SIN component mug27p.

Ectopic overproduction of a sporulation-specific transcription factor induces assembly of prespore-like membranous compartments in vegetative cells of fission yeast

Mei4 is a key sporulation-specific transcription factor in fission yeast. Ectopic expression of Mei4 in vegetative cells caused formation of nucleated membranous compartments, which shared common features with normal forespore membranes, thereby perturbing nuclear division. These results suggest why expression of development-specific transcription factors must be strictly controlled.

Live observation of forespore membrane formation in fission yeast

Sporulation in the fission yeast Schizosaccharomyces pombe is a unique biological process in that the plasma membrane of daughter cells is assembled de novo within the mother cell cytoplasm. A double unit membrane called the forespore membrane (FSM) is constructed dynamically during meiosis. To obtain a dynamic view of FSM formation, we visualized FSM in living cells by using green fluorescent protein fused with Psy1, an FSM-resident protein, together with the nucleus or microtubules. The assembly of FSM initiates in prophase II, and four FSMs in a cell expand in a synchronous manner at the same rate throughout meiosis II. After the meiosis II completes, FSMs continue to expand until closure to form the prespore, a spore precursor. Prespores are initially ellipsoidal, and eventually become spheres. FSM formation was also observed in the sporulation-deficient mutants spo3, spo14, and spo15. In the spo15 mutant, the initiation of FSM formation was completely blocked. In the spo3 mutant, the FSM expanded normally during early meiosis II, but it was severely inhibited during late and postmeiosis, whereas in the spo14 mutant, membrane expansion was more severely inhibited throughout meiosis II. These observations suggest that FSM expansion is composed of two steps, early meiotic FSM expansion and late and post meiotic FSM expansion. Possible regulatory mechanisms of FSM formation in fission yeast are discussed.

Meiotic spindle pole bodies acquire the ability to assemble the spore plasma membrane by sequential recruitment of sporulation-specific components in fission yeast

The spindle pole body (SPB) of Schizosaccharomyces pombe is required for assembly of the forespore membrane (FSM) during meiosis. Before de novo biogenesis of the FSM, the meiotic SPB forms outer plaques, an event referred to as SPB modification. A constitutive SPB component, Spo15, plays an indispensable role in SPB modification and sporulation. Here, we analyzed two sporulation-specific genes, spo13(+) and spo2(+), which are not required for progression of meiotic nuclear divisions, but are essential for sporulation. Spo13 is a 16-kDa coiled-coil protein, and Spo2 is a 15-kDa nonconserved protein. Both Spo13 and Spo2 specifically associated with the meiotic SPB. The respective deletion mutants are viable, but defective in SPB modification and in the onset of FSM formation. Spo13 and Spo2 localized on the cytoplasmic side of the SPB in close contact with the nascent FSM. Localization of Spo13 to the SPB was dependent on Spo15 and Spo2; that of Spo2 depended only on Spo15, suggesting that their recruitment to the SPB is strictly controlled. Spo2 physically associated with both Spo15 and Spo13, but Spo13 and Spo15 did not interact directly. Taken together, these observations indicate that Spo2 is recruited to the SPB during meiosis and then assists in the localization of Spo13 to the outer surface of the SPB.

Geranylgeranyl diphosphate synthase in fission yeast is a heteromer of farnesyl diphosphate synthase (FPS), Fps1, and an FPS-like protein, Spo9, essential for sporulation

Both farnesyl diphosphate synthase (FPS) and geranylgeranyl diphosphate synthase (GGPS) are key enzymes in the synthesis of various isoprenoid-containing compounds and proteins. Here, we describe two novel Schizosaccharomyces pombe genes, fps1(+) and spo9(+), whose products are similar to FPS in primary structure, but whose functions differ from one another. Fps1 is essential for vegetative growth, whereas, a spo9 null mutant exhibits temperature-sensitive growth. Expression of fps1(+), but not spo9(+), suppresses the lethality of a Saccharomyces cerevisiae FPS-deficient mutant and also restores ubiquinone synthesis in an Escherichia coli ispA mutant, which lacks FPS activity, indicating that S. pombe Fps1 in fact functions as an FPS. In contrast to a typical FPS gene, no apparent GGPS homologues have been found in the S. pombe genome. Interestingly, although neither fps1(+) nor spo9(+) expression alone in E. coli confers clear GGPS activity, coexpression of both genes induces such activity. Moreover, the GGPS activity is significantly reduced in the spo9 mutant. In addition, the spo9 mutation perturbs the membrane association of a geranylgeranylated protein, but not that of a farnesylated protein. Yeast two-hybrid and coimmunoprecipitation analyses indicate that Fps1 and Spo9 physically interact. Thus, neither Fps1 nor Spo9 alone functions as a GGPS, but the two proteins together form a complex with GGPS activity. Because spo9 was originally identified as a sporulation-deficient mutant, we show here that expansion of the forespore membrane is severely inhibited in spo9Delta cells. Electron microscopy revealed significant accumulation membrane vesicles in spo9Delta cells. We suggest that lack of GGPS activity in a spo9 mutant results in impaired protein prenylation in certain proteins responsible for secretory function, thereby inhibiting forespore membrane formation.

Role of the alpha-glucanase Agn2p in ascus-wall endolysis following sporulation in fission yeast

During sporulation in the ascomyceteous fungus Schizosaccharomyces pombe, diploid cells undergo differentiation into asci containing four haploid ascospores, which are highly resistant to environmental stresses. Although the morphogenetic processes involved in ascospore formation have been studied extensively, little is known about the molecular mechanism that ensures the release of mature ascospores from the ascus, allowing their dispersal into the environment. Recently, we identified Agn2p as the paralogue of the characterized endo-(1,3)-alpha-glucanase Agn1p, and observed that asci deleted for agn2 are defective in ascospore dispersal. Here, we focus on the cellular and biochemical functions of Agn2p. By placing agn2 under the control of an inducible promoter, we show that expression of agn2 is required for the efficient release of ascospores from their asci. Furthermore, we characterize the enzyme activity of purified recombinant Agn2p and show that Agn2p, like Agn1p, is an endo-(1,3)-alpha-glucanase that produces predominantly (1,3)-alpha-glucan pentasaccharides. Finally, we demonstrate that exogenous addition of purified Agn2p liberated the ascospores from asci deleted for agn2. We propose that Agn2p participates in the endolysis of the ascus wall by hydrolysing its (1,3)-alpha-glucan, thereby assisting in the release of ascospores.

The cation-transporting P-type ATPase Cta4 is required for assembly of the forespore membrane in fission yeast

A novel sporulation-deficient mutant, sev4-L5, was isolated in a genetic screen of a collection of temperature-sensitive mutants of Schizosaccharomyces pombe. The wild-type sev4 gene was identified as cta4+, which encodes a putative cation-transporting P-type ATPase. The sev4-L5 allele harbored a single missense mutation that caused replacement of Gly615 with a glutamate at the putative ATP-binding site. Similar to cta4-null mutants, sev4-L5 exhibited defects in growth at high and low temperatures, and sensitivity to high and extremely low concentrations of Ca2+. The cta4+ mRNA level was considerably enhanced during meiosis. When sev4-L5 cells were incubated in sporulation medium at the permissive temperature, meiotic nuclear divisions proceeded with normal kinetics, but spores were not formed. Structural alteration of the spindle pole body, which is prerequisite to construction of the forespore membrane in wild type, was incomplete. Consequently, formation of the forespore membrane was severely impaired. These observations show that perturbation of Ca2+ homeostasis by mutation of cta4/sev4 blocks sporulation mainly by interfering with forespore membrane assembly.

A role for fission yeast Rab GTPase Ypt7p in sporulation

Ypt7p, a fission yeast (Schizosaccharomyces pombe) homologue of Rab7 GTPase, mediates fusion of endosomes to vacuoles and homotypic vacuole fusion. Here, we report that Ypt7p plays important roles in sporulation. Most ypt7Delta asci produced less than four spores, which were apparently immature and germinated at low frequency. Furthermore, ypt7Delta cells were defective in development of the forespore membranes. Vacuoles in sporulating cells were found to undergo extensive homotypic vacuole fusion to form a few large compartments occupying the entire cytoplasm of asci. This extensive vacuole fusion depended on Ypt7p.

Localization of Type I Myosin and F-actin to the Leading Edge Region of the Forespore Membrane in Schizosaccharomyces pombe

Myo1, a heavy chain of type I myosin of the fission yeast Schizosaccharomyces pombe, is essential for sporulation. Here we have analyzed the expression, localization and cellular function of the type I myosin light chain calmodulin, Cam2, encoded by cam2(+). Transcription of cam2(+) was constitutive and markedly enhanced in meiosis. The cam2 null mutant was viable and completed sporulation normally at 28 degrees C, but formed four-spored asci poorly at 34 degrees C. In those sporulation-defective cells, the forespore membrane was formed abnormally. A Cam2-GFP fusion protein accumulated at the cell poles in interphase cells and at the medial septation site in postmitotic cells, colocalizing with Myo1 and F-actin patches. During the mating process, a single Cam2-GFP dot was detected at the tip of the mating projection. During meiosis-I, the Cam2-GFP dots dispersed into the cell periphery and the cytoplasm. At metaphase-II, intense Cam2-GFP signals appeared near Meu14 rings which were formed at the leading edge of expanding forespore membranes. This localization of Cam2 was dependent upon Myo1; and sporulation defect of cam2Delta at 34 degrees C was alleviated by overexpressing Myo1DeltaIQ. These results suggest a close relationship between Cam2 and Myo1. In addition, both F-actin and Myo1 localized with Cam2 in the leading edge region. In summary, type I myosin and F-actin accumulate at the leading edge area of the forespore membrane and may play a pivotal role in its assembly.

Role of the alpha-glucanase Agn1p in fission-yeast cell separation

Cell division in the fission yeast Schizosaccharomyces pombe yields two equal-sized daughter cells. Medial fission is achieved by deposition of a primary septum flanked by two secondary septa within the dividing cell. During the final step of cell division, cell separation, the primary septum is hydrolyzed by an endo-(1,3)-beta-glucanase, Eng1p. We reasoned that the cell wall material surrounding the septum, referred to here as the septum edging, also must be hydrolyzed before full separation of the daughter cells can occur. Because the septum edging contains (1,3)-alpha-glucan, we investigated the cellular functions of the putative (1,3)-alpha-glucanases Agn1p and Agn2p. Whereas agn2 deletion results in a defect in endolysis of the ascus wall, deletion of agn1 leads to clumped cells that remained attached to each other by septum-edging material. Purified Agn1p hydrolyzes (1,3)-alpha-glucan predominantly into pentasaccharides, indicating an endo-catalytic mode of hydrolysis. Furthermore, we show that the transcription factors Sep1p and Ace2p regulate both eng1 and agn1 expression in a cell cycle-dependent manner. We propose that Agn1p acts in concert with Eng1p to achieve efficient cell separation, thereby exposing the secondary septa as the new ends of the daughter cells.

Forespore membrane assembly in yeast: coordinating SPBs and membrane trafficking

In the yeasts Schizosaccharomyces pombe and Saccharomyces cerevisiae, sporulation involves de novo synthesis of forespore membrane (FSM) within the cytoplasm of mother cells. The FSM ultimately becomes the plasma membrane of the developing ascospores. Several protein components of the FSM have been identified. Visualization of these proteins has demonstrated the dynamic nature of the genesis and development of the FSM. It begins to develop at the differentiated outer plaque of the spindle pole bodies (SPBs) and extends outwards, encapsulating each of the haploid nuclei produced by meiosis. Several coiled-coil proteins are specifically recruited to the SPBs and play indispensable roles in FSM assembly. Temporal and spatial coordination of meiotic nuclear divisions and membrane assembly is of special importance. Comparison of the processes of FSM assembly in these yeasts shows that the basic mechanism has been conserved, even though the individual proteins involved are often different. Understanding these dynamic aspects of yeast sporulation will help to elucidate a general mechanism for the cellularization of cytoplasm containing multiple nuclei.

Fission yeast meu14+ is required for proper nuclear division and accurate forespore membrane formation during meiosis II

Using a meiosis-specific subtracted cDNA library of Schizosaccharomyces pombe, we identified meu14+ as a gene whose expression is upregulated during meiosis. Transcription of meu14+ is induced abruptly after the cell enters meiosis. Its transcription is dependent on the meiosis-specific transcription factor Mei4. In meu14Delta cells, the segregation and modification of the SPBs (spindle pole bodies) and microtubule elongation during meiosis II were aberrant. Meiotic meu14Delta cells consequently produced a high frequency of abnormal tetranucleate cells harboring aberrant forespore membranes and failed to produce asci. In wild-type cells harboring the integrated meu14+-gfp fusion gene, Meu14-GFP first appeared inside the nuclear region at prophase II, after which it accumulated beside the two SPBs at metaphase II. Thereafter, it formed two ring-shaped structures that surrounded the nucleus at early anaphase II. At post-anaphase II, it disappeared. Meu14-GFP appears to localize at the border of the forespore membrane that later develops into spore walls at the end of sporulation. This was confirmed by coexpressing Spo3-HA, a component of the forespore membrane, with Meu14-GFP. Taken together, we conclude that meu14+ is crucial in meiosis in that it participates in both the nuclear division during meiosis II and the accurate formation of the forespore membrane.

The fission yeast spo14+ gene encoding a functional homologue of budding yeast Sec12 is required for the development of forespore membranes

The Schizosaccharomyces pombe spo14-B221 mutant was originally isolated as a sporulation-deficient mutant. However, the spo14(+) gene is essential for cell viability and growth. spo14(+) is identical to the previously characterized stl1(+) gene encoding a putative homologue of Saccharomyces cerevisiae Sec12, which is essential for protein transport from the endoplasmic reticulum (ER) to the Golgi apparatus. In the spo14 mutant cells, ER-like membranes were accumulated beneath the plasma membrane and the ER/Golgi shuttling protein Rer1 remained in the ER. Sec12 is a guanine nucleotide exchange factor for the Sar1 GTPase. Overproduction of psr1(+) coding for an S. pombe Sar1 homologue suppressed both the sporulation defect of spo14-B221 and cold-sensitive growth of newly isolated spo14-6 and spo14-7 mutants. These results indicate that Spo14 is involved in early steps of the protein secretory pathway. The spo14-B221 allele carries a single nucleotide change in the branch point consensus of the fifth intron, which reduces the abundance of the spo14 mRNA. During meiosis II, the forespore membrane was initiated near spindle pole bodies; however, subsequent extension of the membrane was arrested before its closure into a sac. We conclude that Spo14 is responsible for the assembly of the forespore membrane by supplying membrane vesicles.

The Schizosaccharomyces pombe spo3+ gene is required for assembly of the forespore membrane and genetically interacts with psy1(+)-encoding syntaxin-like protein

Formation of the forespore membrane, which becomes the plasma membrane of spores, is an intriguing step in the sporulation of the fission yeast Schizosaccharomyces pombe. Here we report two novel proteins that localize to the forespore membrane. spo3(+) encodes a potential membrane protein, which was expressed only during sporulation. Green fluorescent protein (GFP) fusion revealed that Spo3 localized to the forespore membrane. The spo3 disruptant was viable and executed meiotic nuclear divisions as efficiently as the wild type but did not form spores. One of the spo3 alleles, spo3-KC51, was dose-dependently suppressed by psy1(+), which encodes a protein similar to mammalian syntaxin-1A, a component of the plasma membrane docking/fusion complex. psy1(+) was essential for vegetative growth, and its transcription was enhanced during sporulation. As expected, Psy1 localized to the plasma membrane during vegetative growth. Interestingly, Psy1 on the plasma membrane disappeared immediately after first meiotic division and relocalized to the forespore membrane as the second division initiated. In the spo3 null mutant, the forespore membrane was initiated but failed to develop a normal morphology. Electron microscopy revealed that membrane vesicles were accumulated in the cytoplasm of immature spo3Delta asci. These results suggest that Spo3 is a key component of the forespore membrane and is essential for its assembly acting in collaboration with the syntaxin-like protein.

The Schizosaccharomyces pombe spo20(+) gene encoding a homologue of Saccharomyces cerevisiae Sec14 plays an important role in forespore membrane formation

The Schizosaccharomyces pombe spo20-KC104 mutation was originally isolated in a screen for sporulation-deficient mutants, and the spo20-KC104 mutant exhibits temperature-sensitive growth. Herein, we report that S. pombe, spo20(+) is essential for fission yeast cell viability and is constitutively expressed throughout the life cycle. We also demonstrate that the spo20(+) gene product is structurally homologous to Saccharomyces cerevisiae Sec14, the major phosphatidylinositol transfer protein of budding yeast. This structural homology translates to a significant degree of functional relatedness because reciprocal complementation experiments demonstrate that each protein is able to fulfill the essential function of the other. Moreover, biochemical experiments show that, like Sec14, Spo20 is a phosphatidylinositol/phosphatidylcholine-transfer protein. That Spo20 is required for Golgi secretory function in vegetative cells is indicated by our demonstration that the spo20-KC104 mutant accumulates aberrant Golgi cisternae at restrictive temperatures. However, a second phenotype observed in Spo20-deficient fission yeast is arrest of cell division before completion of cell separation. Consistent with a direct role for Spo20 in controlling cell septation in vegetatively growing cells, localization experiments reveal that Spo20 preferentially localizes to the cell poles and to sites of septation of fission yeast cells. We also report that, when fission yeasts are challenged with nitrogen starvation, Spo20 translocates to the nucleus. This nuclear localization persists during conjugation and meiosis. On completion of meiosis, Spo20 translocates to forespore membranes, and it is the assembly of forespore membranes that is abnormal in spo20-KC104 cells. In such mutants, a considerable fraction of forming prespores fail to encapsulate the haploid nucleus. Our results indicate that Spo20 regulates the formation of specialized membrane structures in addition to its recognized role in regulating Golgi secretory function.

bgs2+, a sporulation-specific glucan synthase homologue is required for proper ascospore wall maturation in fission yeast

The formation of the ascospore cell wall of Schizosaccharomyces pombe requires the co-ordinated activity of enzymes involved in the biosynthesis of its components, such as glucans. We have cloned the bgs2+ gene. bgs2+ belongs to the glucan synthase family of S. pombe and is homologous to the Saccharomyces cerevisiae FKS1 and FKS2 genes. Deletion or overexpression of this gene does not lead to any apparent defect during vegetative growth, but homozygous bgs2Delta diploids do show a sporulation defect. Although meiosis takes place normally, ascospores are unable to mature, and their wall differs from that of wild-type ascospores. Moreover, bgs2Delta zygotes were not able to release ascospores spontaneously, and the ascospores were unable to germinate. We show that expression of bgs2+ is restricted to sporulation and that a bgs2-green fluorescent protein (GFP) fusion protein localizes to the ascospore envelope. The glucan synthase activity in sporulating diploids bearing a bgs2 deletion was diminished in comparison with that of the wild-type diploids, a fact that underscores the importance of the bgs2+ gene and glucan synthesis for the proper formation and maturation of the ascospore wall.

Smashed fission yeast walls. Structural discontinuities related to wall growth

Twenty-three samples of fission yeast cells (Schizosaccharomyces pombe) were smashed by shaking them with glass beads. The samples represented all phases of the culture cycle, with the lag and log phases emphasized. Ruptured walls of the smashed cells were observed by phase-contrast and electron microscopy. Ruptures were tabulated with respect to their magnitudes and locations. Ruptures occurred not at random, nor at sites directed by geometry, but predominated in certain definable wall regions. These discontinuities were correlated with morphogenetic activities of the cell. Thus, the extensile end was found to be most fragile through most of the culture cycle. Also fragile was the nonextensile end, its edge more than its middle. Further, the data were applied to the testing of predictions from extant models (Johnson endohydrolytic softening model and Wessels presoftened-posthardened and crosslinking model) for hyphal tip extension. The frequency of rupture at the extensile (old) end of the cell was qualitatively predicted by both models; the frequency at the nonextensile (new) end was not predictable by either. Rupture frequencies and characteristics at other regions conformed to predictions by one or the other model, but rarely by both.

Structural modification of spindle pole bodies during meiosis II is essential for the normal formation of ascospores in Schizosaccharomyces pombe: ultrastructural analysis of spo mutants

In order to characterize the morphological steps defined by sporulation (spo) genes during the formation of ascospores in the fission yeast Schizosaccharomyces pombe, we performed an electron microscopic study of the ultrastructure of the spindle pole body (SPB) and of the development of the forespore membrane during the second meiotic division (meiosis II) in sporulation-deficient (spo) mutants (spo4, spo5, spo14 and spo18). No difference was found in terms of the function and the structure of the SPB during the first meiotic division (meiosis I) between the four mutants and wild-type cells. However, during meiosis II, the spo4 and spo18 mutants underwent nuclear division but in neither case were the SPBs modified nor were forespore membranes formed. The SPBs of the spo18 mutant diminished in size after meiosis II and eventually disappeared after 18 h in sporulation medium. By contrast, the SPBs of the spo4 mutant remained unchanged even after an 18-h incubation. The outer plaques of SPBs of spo5 and spo14 mutants were sufficiently modified to allow them to initiate development of the forespore membrane, but the membrane had an abnormally expanded lumen and did not enclose the nuclei during meiosis II. The spo5 mutant produced anucleate spore-like bodies while the spo14 mutant formed unorganized structures with irregular peripheries which, presumably, contained spore-wall precursors, instead of anucleate spore-like bodies. We conclude that the modification of the SPB is essential for the formation of ascospores and at least two genes (spo5 and spo14) participate in the development of the forespore membrane. The defective phenotypes define discrete steps in the development of ascospores, which proceeds via steps defined by the mutant spo4, spo18, spo14 and spo5 genes respectively. Our observations provide further substantial evidence that the SPB plays a pivotal role in the normal development of ascospores in yeasts.

A meiotic mutant of the fission yeast Schizosaccharomyces pombe that produces mature asci containing two diploid spores

A mutant of the fission yeast Schizosaccharomyces pombe grew normally in the mitotic cycle but produced two-spored asci in the meiosis cycle. These spores were diploid, and the segregation of centromere-linked markers in the dyads was mostly reductional. Only the first meiotic division appears to occur in this tws1 mutant, resulting in enclosure of diploid nuclei into spores.

Estimation of meiosis and sporulation efficiencies in the fission yeast by ascus analysis

Populations of linear asci are classified according to the number of spores in an ascus. The resultant five numerical classes are further classified into ten spatial classes according to the arrangement of the spores in an ascus and, by inference, into ten historical classes according to the number and origins of failures during the developmental process. An analysis of the observed frequencies of numerical classes allows derivation of the efficiencies of the first meiotic division, the second meiotic division and sporulation in a fission yeast. The analytical method may be useful in locating the site of action of sporulation inhibitors and in identifying meiosis mutants from sporulation mutants.

Temperature sensitivity of flocculation induction, conjugation and sporulation in fission yeast

Homothallic cultures of Schizosaccharomyces pombe, anaerobically grown to stationary phase in broth at 32 degrees C, were induced by aeration to flocculate. Flocculation was followed by copulation, conjugation, zygote formation, meiosis and sporulation. Cultures grown to stationary phase at 32 degrees C and then aerated at 37 degrees C did not sporulate. Grown to stationary phase at 37 degrees C, cultures were not immediately inducible when aerated at 32 degrees C. To identify which events in the developmental sequence were thermosensitive, we grew and induced cultures at 32 degrees C and then shifted them at various times to 37 degrees C. We observed the following events to be thermosensitive: development of respiratory sufficiency, readiness (inducibility of a culture within 1 h), flocculation induction, copulation, conjugation and early sporulation (including meiosis). Respiration, flocculation and spore maturation were thermoresistant. Conjugation-induced lysis and postdevelopmental deflocculation were enhanced at 37 degrees C.

Conjugation-induced lysis of Schizosaccharomyces pombe

About 15% of the conjugating cells of Schizosaccharomyces pombe were observed to lyse spontaneously during the conjugation process. Lysis occurred at the site of union.

Meiosis in protists. Some structural and physiological aspects of meiosis in algae, fungi, and protozoa

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