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

Regulation of sexual differentiation initiation in Schizosaccharomyces pombe

The fission yeast Schizosaccharomyces pombe is an excellent model organism to explore cellular events owing to rich tools in genetics, molecular biology, cellular biology, and biochemistry. Schizosaccharomyces pombe proliferates continuously when nutrients are abundant but arrests in G1 phase upon depletion of nutrients such as nitrogen and glucose. When cells of opposite mating types are present, cells conjugate, fuse, undergo meiosis, and finally form 4 spores. This sexual differentiation process in S. pombe has been studied extensively. To execute sexual differentiation, the glucose-sensing cAMP-PKA (cyclic adenosine monophosphate-protein kinase A) pathway, nitrogen-sensing TOR (target of rapamycin) pathway, and SAPK (stress-activating protein kinase) pathway are crucial, and the MAPK (mitogen-activating protein kinase) cascade is essential for pheromone sensing. These signals regulate ste11 at the transcriptional and translational levels, and Ste11 is modified in multiple ways. This review summarizes the initiation of sexual differentiation in S. pombe based on results I have helped to obtain, including the work of many excellent researchers.

Analysis of the Localization of Schizosaccharomyces pombe Glucan Synthases in the Presence of the Antifungal Agent Caspofungin

In recent years, invasive fungal infections have emerged as a common source of infections in immunosuppressed patients. All fungal cells are surrounded by a cell wall that is essential for cell integrity and survival. It prevents cell death and lysis resulting from high internal turgor pressure. Since the cell wall is not present in animal cells, it is an ideal target for selective invasive fungal infection treatments. The antifungal family known as echinocandins, which specifically inhibit the synthesis of the cell wall β(13)glucan, has been established as an alternative treatment for mycoses. To explore the mechanism of action of these antifungals, we analyzed the cell morphology and glucan synthases localization in Schizosaccharomyces pombe cells during the initial times of growth in the presence of the echinocandin drug caspofungin. S. pombe are rod-shaped cells that grow at the poles and divide by a central division septum. The cell wall and septum are formed by different glucans, which are synthesized by four essential glucan synthases: Bgs1, Bgs3, Bgs4, and Ags1. Thus, S. pombe is not only a perfect model for studying the synthesis of the fungal β(1-3)glucan, but also it is ideal for examining the mechanisms of action and resistance of cell wall antifungals. Herein, we examined the cells in a drug susceptibility test in the presence of either lethal or sublethal concentrations of caspofungin, finding that exposure to the drug for long periods at high concentrations (>10 µg/mL) induced cell growth arrest and the formation of rounded, swollen, and dead cells, whereas low concentrations (<10 µg/mL) permitted cell growth with a mild effect on cell morphology. Interestingly, short-term treatments with either high or low concentrations of the drug induced effects contrary to those observed in the susceptibility tests. Thus, low drug concentrations induced a cell death phenotype that was not observed at high drug concentrations, which caused transient fungistatic cell growth arrest. After 3 h, high concentrations of the drug caused the following: (i) a decrease in the GFP-Bgs1 fluorescence level; (ii) altered locations of Bgs3, Bgs4, and Ags1; and (iii) a simultaneous accumulation of cells with calcofluor-stained incomplete septa, which at longer times resulted in septation uncoupling from plasma membrane ingression. The incomplete septa revealed with calcofluor were found to be complete when observed via the membrane-associated GFP-Bgs or Ags1-GFP. Finally, we found that the accumulation of incomplete septa depended on Pmk1, the last kinase of the cell wall integrity pathway.

Fission Yeast Rho1p-GEFs: From Polarity and Cell Wall Synthesis to Genome Stability

Rho1p is a membrane-associated protein that belongs to the Rho family of small GTPases. These proteins coordinate processes such as actin remodelling and polarised secretion to maintain the shape and homeostasis of yeast cells. In response to extracellular stimuli, Rho1p undergoes conformational switching between a guanosine triphosphate (GTP)-bound active state and a guanosine diphosphate (GDP)-bound inactive state. Cycling is improved with guanine nucleotide exchange factor (GEF) activity necessary to activate signalling and GTPase activating protein (GAP) activity required for subsequent signal depletion. This review focuses on fission yeast Rho1p GEFs, Rgf1p, Rgf2p, and Rgf3p that belong to the family of DH-PH domain-containing Dbl-related GEFs. They are multi-domain proteins that detect biological signals that induce or inhibit their catalytic activity over Rho1p. Each of them activates Rho1p in different places and times. Rgf1p acts preferentially during polarised growth. Rgf2p is required for sporulation, and Rgf3p plays an essential function in septum synthesis. In addition, we outline the noncanonical roles of Rho1p-GEFs in genomic instability.

Sporulation: A response to starvation in the fission yeast Schizosaccharomyces pombe

The fission yeast Schizosaccharomyces pombe employs two main strategies to adapt to the environment and survive when starved for nutrients. The strategies employ sporulation via sexual differentiation and extension of the chronological lifespan. When a cell is exposed to nutrient starvation in the presence of a cell of the opposite sex, the cells undergo fusion through conjugation and sporulation through meiosis. S. pombe spores are highly resistant to diverse stresses and may survive for a very long time. In this minireview, among the various sexual differentiation processes induced by starvation, we focused on and summarized the findings of the molecular mechanisms of spore formation in fission yeast. Furthermore, comparative measurements of the chronological lifespan of stationary phase cells and G₀ cells and the survival period of spore cells revealed that the spore cells survived for a long period, indicating the presence of an effective mechanism for survival. Currently, many molecules involved in sporulation and their functions are being discovered; however, our understanding of these is not complete. Further understanding of spores may not only deepen our comprehension of sexual differentiation but may also provide hints for sustaining life.

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.

Cell wall glucans of fungi. A review

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Glucans are the most abundant compounds in the fungal cell walls.

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The most common type of glucose bonding is 1 → 3, both alpha and beta.

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Microfibrillar glucans with chitin provide rigidity to the fungal wall.

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Fungal beta glucans act as PAMPS during infection of animals and plants.

Glucans are the most abundant polysaccharides in the cell walls of fungi, and their structures are highly variable. Accordingly, their glucose moieties may be joined through either or both alpha (α) or beta (β) linkages, they are either lineal or branched, and amorphous or microfibrillar. Alpha 1,3 glucans sensu strictu (pseudonigerans) are the most abundant alpha glucans present in the cell walls of fungi, being restricted to dikarya. They exist in the form of structural microfibrils that provide resistance to the cell wall. The structure of beta glucans is more complex. They are linear or branched, and contain mostly β 1,3 and β 1,6 linkages, existing in the form of microfibrils. Together with chitin they constitute the most important structural components of fungal cell walls. They are the most abundant components of the cell walls in members of all fungal phyla, with the exception of Microsporidia, where they are absent.

Taking into consideration the importance of glucans in the structure and physiology of the fungi, in the present review we describe the following aspects of these polysaccharides: i) types and distribution of fungal glucans, ii) their structure, iii) their roles, iv) the mechanism of synthesis of the most important ones, and v) the phylogentic relationships of the enzymes involved in their synthesis.

Fission yeast cell wall biosynthesis and cell integrity signalling

The cell wall is a structure external to the plasma membrane that is essential for the survival of the fungi. This polysaccharidic structure confers resistance to the cell internal turgor pressure and protection against mechanical injury. The fungal wall is also responsible for the shape of these organisms due to different structural polysaccharides, such as β-(1,3)-glucan, which form fibers and confer rigidity to the cell wall. These polysaccharides are not present in animal cells and therefore they constitute excellent targets for antifungal chemotherapies. Cell wall damage leads to the activation of MAPK signaling pathways, which respond to the damage by activating the repair of the wall and the maintenance of the cell integrity. Fission yeast Schizosaccharomyces pombe is a model organism for the study morphogenesis, cell wall, and how different inputs might regulate this structure. We present here a short overview of the fission yeast wall composition and provide information about the main biosynthetic activities that assemble this cell wall. Additionally, we comment the recent advances in the knowledge of the cell wall functions and discuss the role of the cell integrity MAPK signaling pathway in the regulation of fission yeast wall.

Fission Yeast Cell Wall Analysis

The Schizosaccharomyces pombe cell wall is a rigid exoskeletal structure mainly composed of interlinked glucose polysaccharides and galactomannoproteins. It is essential for survival of the fission yeast, as it prevents cells from bursting from internal turgor pressure and protects them from mechanical injuries. Additionally, the cell wall determines the cell shape and, therefore, a better knowledge of cell wall structure and composition could provide valuable data in S. pombe morphogenetic studies. Here, we provide information about this structure and the current reliable methods for rapid analysis of the cell wall polymers by specific enzymatic and chemical degradations of purified cell walls.

The Cell Biology of Fission Yeast Septation

In animal cells, cytokinesis requires the formation of a cleavage furrow that divides the cell into two daughter cells. Furrow formation is achieved by constriction of an actomyosin ring that invaginates the plasma membrane. However, fungal cells contain a rigid extracellular cell wall surrounding the plasma membrane; thus, fungal cytokinesis also requires the formation of a special septum wall structure between the dividing cells. The septum biosynthesis must be strictly coordinated with the deposition of new plasma membrane material and actomyosin ring closure and must occur in such a way that no breach in the cell wall occurs at any time. Because of the high turgor pressure in the fungal cell, even a minor local defect might lead to cell lysis and death. Here we review our knowledge of the septum structure in the fission yeast Schizosaccharomyces pombe and of the recent advances in our understanding of the relationship between septum biosynthesis and actomyosin ring constriction and how the two collaborate to build a cross-walled septum able to support the high turgor pressure of the cell. In addition, we discuss the importance of the septum biosynthesis for the steady ingression of the cleavage furrow.

Fission yeast septation

In animal cells cytokinesis relies on the contraction of an actomyosin ring that pulls the plasma membrane to create a cleavage furrow, whose ingression finally divides the mother cell into two daughter cells. Fungal cells are surrounded by a tough and flexible structure called cell wall, which is considered to be the functional equivalent of the extracellular matrix in animal cells. Therefore, in addition to cleavage furrow ingression, fungal cytokinesis also requires the centripetal formation of a septum wall structure that develops between the dividing cells, whose genesis must be strictly coordinated with both the actomyosin ring closure and plasma membrane ingression. Here we briefly review what is known about the septum structure and composition in the fission yeast Schizosaccharomyces pombe, the recent progress about the relationship between septum biosynthesis and actomyosin ring constriction, and the importance of the septum and ring in the steady progression of the cleavage furrow.

Mechanics and morphogenesis of fission yeast cells

The integration of biochemical and biomechanical elements is at the heart of morphogenesis. While animal cells are relatively soft objects which shape and mechanics is mostly regulated by cytoskeletal networks, walled cells including those of plants, fungi and bacteria are encased in a rigid cell wall which resist high internal turgor pressure. How these particular mechanical properties may influence basic cellular processes, such as growth, shape and division remains poorly understood. Recent work using the model fungal cell fission yeast, Schizosaccharomyces pombe, highlights important contribution of cell mechanics to various morphogenesis processes. We envision this genetically tractable system to serve as a novel standard for the mechanobiology of walled cell.

A role for F-BAR protein Rga7p during cytokinesis in S. pombe

F-BAR proteins are known to participate in cytokinesis, but their mechanisms are not well understood. Here we investigated Rga7p, an Schizosaccharomyces pombe F-BAR protein with a RhoGAP domain. Localization of Rga7p to the cytokinetic cleavage furrow depends on its F-BAR domain, actin filaments, the formins Cdc12p and For3p, and the presence of a contractile ring. Rga7p is not required for the constriction of the contractile ring but does participate in the transport of a β-glucan synthetase (Bgs4p) from the late Golgi compartments to plasma membrane that is adjacent to the contractile ring. Cells without Rga7p moved Bgs4p normally from the poles to the Golgi complex near to the cell center, but Bgs4p then moved slowly from the late Golgi compartments to the cleavage site. The late arrival and lower than normal numbers of Bgs4p result in septal defects late in cytokinesis, and in the lysis of separating cells, similar to that in cells with mutations in the cwg1(+) gene (which encodes Bgs4p).

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.

Repression of meiotic genes by antisense transcription and by Fkh2 transcription factor in Schizosaccharomyces pombe

In S. pombe, about 5% of genes are meiosis-specific and accumulate little or no mRNA during vegetative growth. Here we use Affymetrix tiling arrays to characterize transcripts in vegetative and meiotic cells. In vegetative cells, many meiotic genes, especially those induced in mid-meiosis, have abundant antisense transcripts. Disruption of the antisense transcription of three of these mid-meiotic genes allowed vegetative sense transcription. These results suggest that antisense transcription represses sense transcription of meiotic genes in vegetative cells. Although the mechanism(s) of antisense mediated transcription repression need to be further explored, our data indicates that RNAi machinery is not required for repression. Previously, we and others used non-strand specific methods to study splicing regulation of meiotic genes and concluded that 28 mid-meiotic genes are spliced only in meiosis. We now demonstrate that the "unspliced" signal in vegetative cells comes from the antisense RNA, not from unspliced sense RNA, and we argue against the idea that splicing regulates these mid-meiotic genes. Most of these mid-meiotic genes are induced in mid-meiosis by the forkhead transcription factor Mei4. Interestingly, deletion of a different forkhead transcription factor, Fkh2, allows low levels of sense expression of some mid-meiotic genes in vegetative cells. We propose that vegetative expression of mid-meiotic genes is repressed at least two independent ways: antisense transcription and Fkh2 repression.

Kin1 is a plasma membrane-associated kinase that regulates the cell surface in fission yeast

Cell morphogenesis is a complex process that depends on cytoskeleton and membrane organization, intracellular signalling and vesicular trafficking. The rod shape of the fission yeast Schizosaccharomyces pombe and the availability of powerful genetic tools make this species an excellent model to study cell morphology. Here we have investigated the function of the conserved Kin1 kinase. Kin1-GFP associates dynamically with the plasma membrane at sites of active cell surface remodelling and is present in the membrane fraction. Kin1Δ null cells show severe defects in cell wall structure and are unable to maintain a rod shape. To explore Kin1 primary function, we constructed an ATP analogue-sensitive allele kin1-as1. Kin1 inhibition primarily promotes delocalization of plasma membrane-associated markers of actively growing cell surface regions. Kin1 itself is depolarized and its mobility is strongly reduced. Subsequently, amorphous cell wall material accumulates at the cell surface, a phenotype that is dependent on vesicular trafficking, and the cell wall integrity mitogen-activated protein kinase pathway is activated. Deletion of cell wall integrity mitogen-activated protein kinase components reduces kin1Δ hypersensitivity to stresses such as those induced by Calcofluor white and SDS. We propose that Kin1 is required for a tight link between the plasma membrane and the cell wall.

Differential activities of three families of specific beta(1,3)glucan synthase inhibitors in wild-type and resistant strains of fission yeast

Three specific β(1,3)glucan synthase (GS) inhibitor families, papulacandins, acidic terpenoids, and echinocandins, have been analyzed in Schizosaccharomyces pombe wild-type and papulacandin-resistant cells and GS activities. Papulacandin and enfumafungin produced similar in vivo effects, different from that of echinocandins. Also, papulacandin was the strongest in vitro GS inhibitor (IC(50) 10(3)-10(4)-fold lower than with enfumafungin or pneumocandin), but caspofungin was by far the most efficient antifungal because of the following. 1) It was the only drug that affected resistant cells (minimal inhibitory concentration close to that of the wild type). 2) It was a strong inhibitor of wild-type GS (IC(50) close to that of papulacandin). 3) It was the best inhibitor of mutant GS. Moreover, caspofungin showed a special effect for two GS inhibition activities, of high and low affinity, separated by 2 log orders, with no increase in inhibition. pbr1-8 and pbr1-6 resistances are due to single substitutions in the essential Bgs4 GS, located close to the resistance hot spot 1 region described in Saccharomyces and Candida Fks mutants. Bgs4(pbr)(1-8) contains the E700V change, four residues N-terminal from hot spot 1 defining a larger resistance hot spot 1-1 of 13 amino acids. Bgs4(pbr)(1-6) contains the W760S substitution, defining a new resistance hot spot 1-2. We observed spontaneous revertants of the spherical pbr1-6 phenotype and found that an additional A914V change is involved in the recovery of the wild-type cell shape, but it maintains the resistance phenotype. A better understanding of the mechanism of action of the antifungals available should help to improve their activity and to identify new antifungal targets.

β(1,3)-glucanosyl-transferase activity is essential for cell wall integrity and viability of Schizosaccharomyces pombe

Background

The formation of the cell wall in Schizosaccharomyces pombe requires the coordinated activity of enzymes involved in the biosynthesis and modification of β-glucans. The β(1,3)-glucan synthase complex synthesizes linear β(1,3)-glucans, which remain unorganized until they are cross-linked to other β(1,3)-glucans and other cell wall components. Transferases of the GH72 family play important roles in cell wall assembly and its rearrangement in Saccharomyces cerevisiae and Aspergillus fumigatus. Four genes encoding β(1,3)-glucanosyl-transferases -gas1⁺, gas2⁺, gas4⁺ and gas5⁺- are present in S. pombe, although their function has not been analyzed.

Methodology/Principal Findings

Here, we report the characterization of the catalytic activity of gas1p, gas2p and gas5p together with studies directed to understand their function during vegetative growth. From the functional point of view, gas1p is essential for cell integrity and viability during vegetative growth, since gas1Δ mutants can only grow in osmotically supported media, while gas2p and gas5p play a minor role in cell wall construction. From the biochemical point of view, all of them display β(1,3)-glucanosyl-transferase activity, although they differ in their specificity for substrate length, cleavage point and product size. In light of all the above, together with the differences in expression profiles during the life cycle, the S. pombe GH72 proteins may accomplish complementary, non-overlapping functions in fission yeast.

Conclusions/Significance

We conclude that β(1,3)-glucanosyl-transferase activity is essential for viability in fission yeast, being required to maintain cell integrity during vegetative growth.

A novel role of Dma1 in regulating forespore membrane assembly and sporulation in fission yeast

By characterizing the fission yeast Dma1's function during meiosis, we revealed that Dma1 is required for spore formation, while it is dispensable for fidelity of nuclear divisions. We also found that Dma1 is functionally related to SIN pathway and meiosis-specific kinase Slk1 during sporulation.

In fission yeast Schizosaccharomyces pombe, a diploid mother cell differentiates into an ascus containing four haploid ascospores following meiotic nuclear divisions, through a process called sporulation. Several meiosis-specific proteins of fission yeast have been identified to play essential roles in meiotic progression and sporulation. We report here an unexpected function of mitotic spindle checkpoint protein Dma1 in proper spore formation. Consistent with its function in sporulation, expression of dma1⁺ is up-regulated during meiosis I and II. We showed that Dma1 localizes to the SPB during meiosis and the maintenance of this localization at meiosis II depends on septation initiation network (SIN) scaffold proteins Sid4 and Cdc11. Cells lacking Dma1 display defects associated with sporulation but not nuclear division, leading frequently to formation of asci with fewer spores. Our genetic analyses support the notion that Dma1 functions in parallel with the meiosis-specific Sid2-related protein kinase Slk1/Mug27 and the SIN signaling during sporulation, possibly through regulating proper forespore membrane assembly. Our studies therefore revealed a novel function of Dma1 in regulating sporulation in fission yeast.

Characterization of glycoside hydrolase family 5 proteins in Schizosaccharomyces pombe

In yeast, enzymes with β-glucanase activity are thought to be necessary in morphogenetic events that require controlled hydrolysis of the cell wall. Comparison of the sequence of the Saccharomyces cerevisiae exo-β(1,3)-glucanase Exg1 with the Schizosaccharomyces pombe genome allowed the identification of three genes that were named exg1(+) (locus SPBC1105.05), exg2(+) (SPAC12B10.11), and exg3(+) (SPBC2D10.05). The three proteins have different localizations: Exg1 is secreted to the periplasmic space, Exg2 is a membrane protein, and Exg3 is a cytoplasmic protein. Characterization of the biochemical activity of the proteins indicated that Exg1 and Exg3 are active only against β(1,6)-glucans while no activity was detected for Exg2. Interestingly, Exg1 cleaves the glucans with an endohydrolytic mode of action. exg1(+) showed periodic expression during the cell cycle, with a maximum coinciding with the septation process, and its expression was dependent on the transcription factor Sep1. The Exg1 protein localizes to the septum region in a pattern that was different from that of the endo-β(1,3)-glucanase Eng1. Overexpression of Exg2 resulted in an increase in cell wall material at the poles and in the septum, but the putative catalytic activity of the protein was not required for this effect.

Mug28, a meiosis-specific protein of Schizosaccharomyces pombe, regulates spore wall formation

The meiosis-specific mug28⁺ gene of Schizosaccharomyces pombe encodes a putative RNA-binding protein. mug28Δ cells generated spores with low viability, due to the aberrant FSM formation. Meu14-GFP in mug28Δ cells showed that the FSM formed extra membranes with buds. We conclude that Mug28 is essential for the proper maturation of the FSM and the spore wall.

The meiosis-specific mug28⁺ gene of Schizosaccharomyces pombe encodes a putative RNA-binding protein with three RNA recognition motifs (RRMs). Live observations of meiotic cells that express Mug28 tagged with green fluorescent protein (GFP) revealed that Mug28 is localized in the cytoplasm, and accumulates around the nucleus from metaphase I to anaphase II. Disruption of mug28⁺ generated spores with low viability, due to the aberrant formation of the forespore membrane (FSM). Visualization of the FSM in living cells expressing GFP-tagged Psy1, an FSM protein, indicated that mug28Δ cells harbored abnormal FSMs that contained buds, and had a delayed disappearance of Meu14, a leading edge protein. Electron microscopic observation revealed that FSM formation was abnormal in mug28Δ cells, showing bifurcated spore walls that were thicker than the nonbifurcated spore walls of the wild type. Analysis of Mug28 mutants revealed that RRM3, in particular phenylalanin-466, is of primary importance for the proper localization of Mug28, spore viability, and FSM formation. Together, we conclude that Mug28 is essential for the proper maturation of the FSM and the spore wall.

Different steps of sexual development are differentially regulated by the Sec8p and Exo70p exocyst subunits

In this paper we show that in Schizosaccharomyces pombe, mating-specific cell adhesion is dependent on the exocyst subunit Sec8p, but independent of the exocyst subunit Exo70p. In the absence of Exo70p, the forespore membrane does not develop properly and the leading edge protein Meu14p is abnormally distributed. Additionally, the spindle pole body is aberrant in a significant number of exo70Delta asci. In both the sec8-1 and the exo70Delta mutants, the development of the spore cell wall is impaired. These results show that different steps of sexual development are differentially regulated by the exocyst and suggest the existence of exocyst subcomplexes with distinct roles in mating.

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.

{beta}-glucanase Eng2 is required for ascus wall endolysis after sporulation in the fission yeast Schizosaccharomyces pombe

Meiosis is the developmental program by which sexually reproducing diploid organisms generate haploid gametes. In yeast, meiosis is followed by spore morphogenesis. When Schizosaccharomyces pombe diploid cells undergo meiosis, they differentiate into asci containing four haploid ascospores that are highly resistant to environmental stress. The formation of the ascospore wall requires the activity of several enzymes involved in the biosynthesis and modification of its components, such as alpha- and beta-glucan synthases. Once the spores are completely mature, the wall of the ascus undergoes an endolytic process that results in the release of ascospores from the ascus, allowing their dispersal into the environment. This process requires the activity of the endo-alpha-1,3-glucanase Agn2. Here, we focus on the characterization of the endo-beta-1,3-glucanase Eng2, which is also required for ascospore release from the ascus. Although Eng2 is present during the mitotic cycle, the protein accumulates after meiosis II. The expression of eng2(+) is required for the efficient release of ascospores, as shown by placing eng2(+) under the control of a repressible promoter. Furthermore, a point mutation that destroys the catalytic activity of the protein results in a phenotype similar to that of the mutant strain. Finally, we demonstrate that exogenous addition of purified Eng2 releases the ascospores from asci generated by an eng2Delta mutant. We propose that Eng2 would act together with Agn2 to completely hydrolyze the ascus wall, thereby assisting in the release of ascospores in S. pombe.

The fission yeast SEL1 domain protein Cfh3p: a novel regulator of the glucan synthase Bgs1p whose function is more relevant under stress conditions

In Schizosaccharomyces pombe, Bgs1/Cps1p is a beta(1,3)-glucan synthase required for linear beta(1,3)-glucan synthesis and primary septum formation. Here, we have studied the regulation of Bgs1p by Cfh3/Chr4p, a member of a family of conserved adaptor proteins, which resembles the chitin synthase regulator Chs4p from Saccharomyces cerevisiae and Candida albicans. cfh3Delta cells showed a genetic interaction with cps1-191, and Cfh3p co-immunoprecipitated with Bgs1/Cps1p. In the absence of cfh3(+), cells were more sensitive to digestion by glucanases, and both Calcofluor staining and glucan synthesis were reduced. We found that in a wild-type strain, beta(1,3)-glucan synthesis was reduced under stress conditions. In the cfh3Delta, cps1-191, and cfh3Delta cps1-191 strains, beta(1,3)-glucan synthesis was further reduced, and growth was impaired under stress conditions, suggesting that Cfh3p and Bgs1p might play a role in ensuring growth in unfavorable environments. In a cfh3Delta mutant, Bgs1p was delocalized when the cells were distressed, but a blockade in endocytosis prevented this delocalization. Finally, we found that the SEL1 repeats are required for Cfh3p function. These results show that Cfh3p is a regulatory protein for Bgs1p and that its function is particularly necessary when the cells are undergoing stress.

Fission yeast rgf2p is a rho1p guanine nucleotide exchange factor required for spore wall maturation and for the maintenance of cell integrity in the absence of rgf1p

Schizosaccharomyces pombe Rho1p is essential, directly activates beta-1,3-glucan synthase, and participates in the regulation of morphogenesis. In S. pombe, Rho1p is activated by at least three guanine nucleotide exchange factors (GEFs): Rgf1p, Rgf2p, and Rgf3p. In this study we show that Rgf2p is a Rho1p GEF required for sporulation. The rgf2+ deletion did not affect forespore membrane formation and the nuclei were encapsulated properly. However, the mutant ascospores appeared dark and immature. The rgf2Delta zygotes were not able to release the ascospores spontaneously, and the germination efficiency was greatly reduced compared to wild-type (wt) spores. This phenotype resembles that of the mutants in bgs2+, which encodes a sporulation-specific glucan synthase subunit. In fact, glucan synthase activity was diminished in sporulating rgf2Delta diploids. Rgf2p also plays a role in beta-glucan biosynthesis during vegetative growth. Overexpression of rgf2+ specifically increased GTP-bound Rho1p, caused changes in cell morphology, and elicited an increase in beta-1,3-glucan synthase activity. Moreover, the simultaneous disruption of rgf1+ and rgf2+ was lethal and both Rgf1p and Rgf2p were able to partially substitute for each other. Our results suggest that Rgf1p and Rgf2p are alternative GEFs with an essential overlapping function in Rho1p activation during vegetative growth.

The meiosis-specific Sid2p-related protein Slk1p regulates forespore membrane assembly in fission yeast

Cytokinesis in all organisms involves the creation of membranous barriers that demarcate individual daughter cells. In fission yeast, a signaling module termed the septation initiation network (SIN) plays an essential role in the assembly of new membranes and cell wall during cytokinesis. In this study, we have characterized Slk1p, a protein-kinase related to the SIN component Sid2p. Slk1p is expressed specifically during meiosis and localizes to the spindle pole bodies (SPBs) during meiosis I and II in a SIN-dependent manner. Slk1p also localizes to the forespore membrane during sporulation. Cells lacking Slk1p display defects associated with sporulation, leading frequently to the formation of asci with smaller and/or fewer spores. The ability of slk1 Delta cells to sporulate, albeit inefficiently, is fully abolished upon compromise of function of Sid2p, suggesting that Slk1p and Sid2p play overlapping roles in sporulation. Interestingly, increased expression of the syntaxin Psy1p rescues the sporulation defect of sid2-250 slk1 Delta. Thus, it is likely that Slk1p and Sid2p play a role in forespore membrane assembly by facilitating recruitment of components of the secretory apparatus, such as Psy1p, to allow membrane expansion. These studies thereby provide a novel link between the SIN and vesicle trafficking during cytokinesis.

Schizosaccharomyces pombe Sst4p, a conserved Vps27/Hrs homolog, functions downstream of phosphatidylinositol 3-kinase Pik3p to mediate proper spore formation

Sporulation of the fission yeast Schizosaccharomyces pombe is a developmental process that generates gametes and that includes the formation of spore envelope precursors called the forespore membranes. Assembly and development of forespore membranes require vesicular trafficking from other intracellular membrane compartments. We have shown that phosphatidylinositol 3-kinase (PtdIns 3-kinase) is required for efficient and proper development of forespore membranes. The role of a FYVE domain protein, Sst4p, a homolog of Vps27p/Hrs, as a downstream factor for PtdIns 3-kinase in sporulation was investigated. sst4Delta asci formed spores with oval-shaped morphology and with reduced viability compared to that of the wild-type spores. The extension of forespore membranes was inefficient, and bubble-like structures emerged from the leading edges of the forespore membranes. Sst4p localization was examined using fluorescent protein fusions and was found to be adjacent to the forespore membranes during sporulation. The localization and function of Sst4p were dependent on its FYVE domain and on PtdIns 3-kinase. Sst4p colocalized and interacted with Hse1p, a homolog of Saccharomyces cerevisiae Hse1p and of mammalian STAM. Mutations in all three UIM domains of the Sst4p/Hse1p complex resulted in formation of spores with abnormal morphology. These results suggest that Sst4p is a downstream factor of PtdIns 3-kinase and functions in forespore membrane formation.

The (1,3)β-d-glucan synthase subunit Bgs1p is responsible for the fission yeast primary septum formation

Cytokinesis is a crucial event in the cell cycle of all living cells. In fungal cells, it requires co-ordinated contraction of an actomyosin ring and synthesis of both plasmatic membrane and a septum structure that will constitute the new cell wall end. Schizosaccharomyces pombe contains four essential putative (1,3)beta-d-glucan synthase catalytic subunits, Bgs1p to Bgs4p. Here we examined the function of Bgs1p in septation by studying the lethal phenotypes of bgs1(+) shut-off and bgs1Delta cells and demonstrated that Bgs1p is responsible and essential for linear (1,3)beta-d-glucan and primary septum formation. bgs1(+) shut-off generates a more than 300-fold Bgs1p reduction, but the septa still present large amounts of disorganized linear (1,3)beta-d-glucan and partial primary septa. Conversely, both structures are absent in bgs1Delta cells, where there is no Bgs1p. The septum analysis of bgs1(+)-repressed cells indicates that linear (1,3)beta-d-glucan is necessary but not sufficient for primary septum formation. Linear (1,3)beta-d-glucan is the polysaccharide that specifically interacts with the fluorochrome Calcofluor white in fission yeast. We also show that in the absence of Bgs1p abnormal septa are formed, but the cells cannot separate and eventually die.

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.

Role of Rho GTPases and Rho-GEFs in the regulation of cell shape and integrity in fission yeast

The Rho family of GTPases are highly conserved molecular switches that control some of the most fundamental processes of cell biology, including morphogenesis, vesicular transport, cell division and motility. Guanine nucleotide-exchange factors (GEFs) are directly responsible for the activation of Rho-family GTPases in response to extracellular stimuli. In fission yeast, there are seven Dbl-related GEFs and they activate six Rho-type GTPases within a particular spatio-temporal context. The failure to do so might have consequences reflected in aberrant phenotypes and in some cases lead to cell death. In this review, we briefly summarize the role of Rho GTPases and Rho-GEFs in the establishment and maintenance of cell polarity and cell integrity in Schizosaccharomyces pombe.

Homologous subunits of 1,3-beta-glucan synthase are important for spore wall assembly in Saccharomyces cerevisiae

During sporulation in Saccharomyces cerevisiae, the four haploid nuclei are encapsulated within multilayered spore walls. Glucan, the major constituent of the spore wall, is synthesized by 1,3-beta-glucan synthase, which is composed of a putative catalytic subunit encoded by FKS1 and FKS2. Although another homolog, encoded by FKS3, was identified by homology searching, its function is unknown. In this report, we show that FKS2 and FKS3 are required for spore wall assembly. The ascospores of fks2 and fks3 mutants were enveloped by an abnormal spore wall with reduced resistance to diethyl ether, elevated temperatures, and ethanol. However, deletion of the FKS1 gene did not result in a defective spore wall. The construction of fusion genes that expressed Fks1p and Fks2p under the control of the FKS2 promoter revealed that asci transformed with FKS2p-driven Fks1p and Fks2p were resistant to elevated temperatures, which suggests that the expression of FKS2 plays an important role in spore wall assembly. The expression of FKS1p-driven Fks3p during vegetative growth did not affect 1,3-beta-glucan synthase activity in vitro but effectively suppressed the growth defect of the temperature-sensitive fks1 mutant by stabilizing Rho1p, which is a regulatory subunit of glucan synthase. Based on these results, we propose that FKS2 encodes the primary 1,3-beta-glucan synthase in sporulation and that FKS3 is required for normal spore wall formation because it affects the upstream regulation of 1,3-beta-glucan synthase.

EST analysis of cDNA libraries from the entomopathogenic fungus Beauveria (Cordyceps) bassiana. I. Evidence for stage-specific gene expression in aerial conidia, in vitro blastospores and submerged conidia

The entomopathogenic fungus Beauveria (Cordyceps) bassiana holds much promise as a pest biological control agent. B. bassiana produces at least three in vitro single cell infectious propagules, including aerial conidia, vegetative cells termed blastospores and submerged conidia, that display different morphological, biochemical and virulence properties. Populations of aerial conidia, blastospores and submerged conidia were produced on agar plates, rich liquid broth cultures and under conditions of nutrient limitation in submerged cultures, respectively. cDNA libraries were generated from mRNA isolated from each B. bassiana cell type and approximately 2,500 5' end sequences were determined from each library. Sequences derived from aerial conidia clustered into 284 contigs and 963 singlets, with those derived from blastospores and submerged conidia forming 327 contigs with 788 singlets, and 303 contigs and 1,079 contigs, respectively. Almost half (40-45 %) of the sequences in each library displayed either no significant similarity (e value >10(-4)) or similarity to hypothetical proteins found in the NCBI database. The expressed sequence tag dataset also included sequences representing a significant portion of proteins in cellular metabolism, information storage and processing, transport and cell processes, including cell division and posttranslational modifications. Transcripts encoding a diverse array of pathogenicity-related genes, including proteases, lipases, esterases, phosphatases and enzymes producing toxic secondary metabolites, were also identified. Comparative analysis between the libraries identified 2,416 unique sequences, of which 20-30 % were unique to each library, and only approximately 6 % of the sequences were shared between all three libraries. The unique and divergent representation of the B. bassiana transcriptome in the cDNA libraries from each cell type suggests robust differential gene expression profiles in response to environmental conditions.

Cell wall assembly in Saccharomyces cerevisiae

An extracellular matrix composed of a layered meshwork of beta-glucans, chitin, and mannoproteins encapsulates cells of the yeast Saccharomyces cerevisiae. This organelle determines cellular morphology and plays a critical role in maintaining cell integrity during cell growth and division, under stress conditions, upon cell fusion in mating, and in the durable ascospore cell wall. Here we assess recent progress in understanding the molecular biology and biochemistry of cell wall synthesis and its remodeling in S. cerevisiae. We then review the regulatory dynamics of cell wall assembly, an area where functional genomics offers new insights into the integration of cell wall growth and morphogenesis with a polarized secretory system that is under cell cycle and cell type program controls.

Fission yeast Rho5p GTPase is a functional paralogue of Rho1p that plays a role in survival of spores and stationary-phase cells

The Rho GTPase family and their effectors are key regulators involved in many eukaryotic cell functions related to actin organization and polarity establishment. Schizosaccharomyces pombe Rho1p is essential, directly activates the (1,3)-beta-d-glucan synthase, and participates in regulation of cell wall growth and morphogenesis. Here we describe the characterization of the fission yeast Rho5p GTPase, highly homologous to Rho1p, sharing 86% identity and 95% similarity. Overexpression of the hyperactive allele rho5-G15V causes a morphological effect similar to that of rho1-G15V, but the penetrance is significantly lower, and overexpression of the dominant-negative allele rho5-T20N causes lysis like that of rho1-T20N. Importantly, overexpression of rho5(+) but no other rho genes is able to rescue the lethality of rho1Delta cells. Shutoff experiments indicated that Rho5p can replace Rho1p, but it is not as effective in maintaining cell wall integrity or actin organization. rho5(+) expression is hardly detected during log-phase growth but is induced under nutritional starvation conditions. rho5Delta cells are viable and do not display any defects during logarithmic growth. However, when rho1(+) expression is repressed during stationary phase, rho5Delta cells display reduced viability. Ascospores lacking Rho5p are less resistant to heat or lytic enzymes than wild-type spores. Moreover, h(90) mutant strains carrying the hyperactive rho5-G15V or the dominant-negative rho5-T20N alleles display severe ascospore formation defects. These results suggest that Rho5p functions in a way similar to, but less efficient than, Rho1p, plays a nonessential role during stationary phase, and participates in the spore wall formation.

Synthesis of alpha-glucans in fission yeast spores is carried out by three alpha-glucan synthase paralogues, Mok12p, Mok13p and Mok14p

Fission yeast possesses a family of (1,3)-alpha-glucan synthase-related genes; one of them, mok1+/ags1+, plays an essential function in morphogenesis during vegetative growth. Here we show that three mok1+ paralogues -mok12+, mok13+ and mok14+- are required for sporulation to succeed, acting at different stages of the spore wall maturation process. Mutation of mok12+ affected the efficiency of spore formation and spore viability. Deletion of mok13+ does not affect spore viability but the spores showed reduced resistance to stress conditions. mok14Delta mutant spores failed to accumulate the amylose-like spore wall-specific polymer. mok12+, mok13+ and mok14+ expression was restricted to sporulating cells and the proteins localized to the spore envelope but with different timing. mok11+ was also induced during the sporulation process although its deletion did not show apparently a sporulation defect. In vegetative cells, beta-glucans are more abundant than alpha-glucans (55% versus 28%). In spores, the situation was the opposite, alpha-glucans accounted for 46% while beta-glucans were approximately 38% of the total polysaccharides. We found at least two types of alpha-glucan polymers, Mok12p and Mok13p, were involved in the synthesis of the greater part of alpha-glucan in the spores envelope, a polymer that is mainly digested with alpha-1,3 glucanase, while Mok14p, homologous to starch synthases, was required for the synthesis of the iodine-reactive polymer that is made of alpha-1,4 glucose residues.

Rgf1p is a specific Rho1-GEF that coordinates cell polarization with cell wall biogenesis in fission yeast

Rho1p regulates cell integrity by controlling the actin cytoskeleton and cell wall synthesis. We have identified a new GEF, designated Rgf1p, which specifically regulates Rho1p during polarized growth. The phenotype of rgf1 null cells was very similar to that seen after depletion of Rho1p, 30% of cells being lysed. In addition, rgf1(+) deletion caused hypersensitivity to the antifungal drug Caspofungin and defects in the establishment of bipolar growth. rho1(+), but none of the other GTPases of the Rho-family, suppressed the rgf1Delta phenotypes. Moreover, deletion of rgf1(+) suppressed the severe growth defect in rga1(+) null mutants (a Rho1-GAP, negative regulator). Rgf1p and Rho1p coimmunoprecipitated and overexpression of rgf1(+) specifically increased the GTP-bound Rho1p; it caused changes in cell morphology, and a large increase in beta(1,3)-glucan synthase activity. These effects were similar to those elicited when the hyperactive rho1-G15V allele was expressed. A genetic relationship was observed between Rgf1p, Bgs4p (beta[1,3]-glucan synthase), and Pck1p (protein kinase C [PKC] homologue); Bgs4p and Pck1p suppressed the hypersensitivity to Caspofungin in rgf1Delta mutants. Rgf1p localized to the growing ends and the septum, where Rho1, Pck1p, and Bgs4p are known to function. Our results suggest that Rgf1p probably activates the Rho functions necessary for coordinating actin deposition with cell wall biosynthesis during bipolar growth, allowing the cells to remodel their wall without risk of rupture.

Morphogenetic pathway of spore wall assembly in Saccharomyces cerevisiae

The Saccharomyces cerevisiae spore is protected from environmental damage by a multilaminar extracellular matrix, the spore wall, which is assembled de novo during spore formation. A set of mutants defective in spore wall assembly were identified in a screen for mutations causing sensitivity of spores to ether vapor. The spore wall defects in 10 of these mutants have been characterized in a variety of cytological and biochemical assays. Many of the individual mutants are defective in the assembly of specific layers within the spore wall, leading to arrests at discrete stages of assembly. The localization of several of these gene products has been determined and distinguishes between proteins that likely are involved directly in spore wall assembly and probable regulatory proteins. The results demonstrate that spore wall construction involves a series of dependent steps and provide the outline of a morphogenetic pathway for assembly of a complex extracellular structure.

The novel fission yeast (1,3)beta-D-glucan synthase catalytic subunit Bgs4p is essential during both cytokinesis and polarized growth

Schizosaccharomyces pombe contains four putative (1,3)beta-D-glucan synthase (GS) catalytic subunits, Bgs1p-4p. In this work, we cloned bgs4+ and show that Bgs4p is the only subunit found to be a part of the GS enzyme and essential for maintaining cell integrity during cytokinesis and polarized growth. Here we show that bgs4+, cwg1+ (cwg1-1 shows reduced cell-wall beta-glucan and GS catalytic activity) and orb11+ (orb11-59 is defective in cell morphogenesis) are the same gene. bgs4+ is essential for spore germination and bgs4+ shut-off produces cell lysis at growing poles and mainly at the septum prior to cytokinesis, suggesting that Bgs4p is essential for cell wall growth and to compensate for an excess of cell wall degradation during cytokinesis. Shut-off and overexpression analysis suggest that Bgs4p forms part of a GS catalytic multiprotein complex and that Bgs4p-promoted cell-wall beta-glucan alterations induce compensatory mechanisms from other Bgs subunits and (1,3)alpha-D-glucan synthase. Physiological localization studies showed that Bgs4p localizes to the growing ends, the medial ring and septum, and at each stage of wall synthesis or remodeling that occurs during sexual differentiation: mating, zygote and spore formation, and spore germination. Bgs4p timing and requirements for proper positioning during cytokinesis and its localization pattern during spore maturation differ from those of Bgs1p. Bgs4p localizes overlapping the contractile ring once Bgs1p is present and a Calcofluor white-stained septum material is detected, suggesting that Bgs4p is involved in a late process of secondary or general septum synthesis. Unlike Bgs1p, Bgs4p needs the medial ring but not the septation initiation network proteins to localize with the other septation components. Furthermore, Bgs4p localization depends on the polarity establishment proteins. Finally, F-actin is necessary for Bgs4p delocalization from and relocalization to the growing regions, but it is not needed for the stable maintenance of Bgs4p at the growing sites, poles and septum. All these data show for the first time an essential role for a Bgs subunit in the synthesis of a (1,3)beta-D-glucan necessary to preserve cell integrity when cell wall synthesis or repair are needed.

A glucanase-driven fractionation allows redefinition of Schizosaccharomyces pombe cell wall composition and structure: assignment of diglucan

Purified endoglucanases have been used to determine the composition of Schizosaccharomyces pombe cell wall. This structure has been traditionally studied after isolating its components (mannoproteins, alpha1,3-glucan, beta1,3-glucan, and a branched beta-glucan) with hot alkali. Instead, we sequentially removed the polysaccharides by digesting with endo-beta1,3-glucanase and with a novel endo-alpha1,3-glucanase (mutanase). After this gentle isolation we observed that a branched beta1,3-beta1,6-glucan is much more abundant than previously described. By scaling-up the new protocol we prepared large amounts of the highly branched glucan and determined its structural features. We have named this highly branched beta-glucan diglucan, reflecting its two types of beta linkages. We have also identified an insoluble endoglucanase-resistant type of 1,3-linked glucan present in S. pombe cell walls. We redefined the wall composition of S. pombe vegetative cells by this new method. Finally, to demonstrate its application, we determined the cell wall composition of known mutant strains.

Schizosaccharomyces pombe Rgf3p is a specific Rho1 GEF that regulates cell wall beta-glucan biosynthesis through the GTPase Rho1p

Rho1p regulates cell integrity by controlling the actin cytoskeleton and cell-wall synthesis. Here, we describe the cloning and characterization of rgf3+, a member of the Rho family of guanine nucleotide exchange factors (Rho GEFs). The rgf3+ gene was cloned by complementation of a mutant (ehs2-1) hypersensitive to drugs that interfere with cell-wall biosynthesis. The rgf3+ gene was found to be essential for cell viability and depletion of Rgf3p afforded phenotypes similar to those obtained following depletion of Rho1p. However, the cell death caused by Rgf3p depletion could be rescued by the presence of 1.2 M sorbitol, whereas depletion of Rho1 was lethal under the same conditions. We show that Rgf3p is a specific Rho1-GEF. The hypersensitivity to drugs affecting the cell wall of the ehs2-1 mutant was suppressed by overexpression of rho1+ but not by any of the other GTPases of the Rho family. Rgf3p interacted with the GDP-bound form of Rho1p and promoted the GDP-GTP exchange. In addition, we show that overexpression of Rgf3p produces multiseptated cells and increases beta-1,3-glucan synthase activity and the amount of cell wall beta-1,3-glucan. Rgf3p localized to the septum and the mRNA level was regulated in a cell-cycle-dependent manner peaking during septation. Our results suggest that Rgf3p acts as a positive activator of Rho1p, probably activating the Rho functions that coordinate cell-wall biosynthesis to maintain cell integrity during septation.

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.

Overlapping omt1+ and omt2+ genes are required for spore wall maturation in Schizosaccharomyces pombe

BACKGROUND

Overlapping genes that are transcribed from the same genomic regions are rare in eukaryotes and to date few detailed functional analyses have been reported.

RESULTS

We report here three novel overlapping transcripts that are specifically expressed during meiosis of Schizosaccharomyces pombe. They are denoted as omt1+, omt2+ and omt3+ after overlapping meiotic transcripts. omt1+ encodes a 12-kDa protein and omt2+ encodes a 11-kDa protein with homology to the bifunctional mammalian protein DCoH/PCBD. omt3+ does not have a significant open reading frame. The omt2+ transcript overlaps with both the omt1+ and omt3+ transcripts but the latter two transcripts do not overlap. omt1Delta and omt2Delta but not omt3Delta failed to form mature spore walls. The Omt1-GFP and Omt2-GFP fusion proteins localized to the outside and the inside of the spore walls, respectively. The sporulation-specific protein Meu10 and the spore wall components were abnormally localized in the spore walls of omt1Delta and omt2Delta.

CONCLUSION

The overlapping genes omt1+ and omt2+ express functional proteins that participate in spore wall maturation, indicating that gene overlap does not affect the physiological functions of the proteins encoded by these genes. Generation of overlapped RNA may be due to loose regulation of transcription termination during meiosis of S. pombe.

Sterol-rich plasma membrane domains in the fission yeast Schizosaccharomyces pombe

Sterol-rich membrane domains exist in unicellular and multicellular eukaryotes. They are thought to provide a structural framework for interactions among a subset of proteins by selectively incorporating some proteins while excluding others. Although most studies have focused on the biophysical and biochemical properties of sterol-rich membrane domains and incorporated proteins, relatively little is known about their intracellular distribution. Using a cytological approach we show here that in the fission yeast Schizosaccharomyces pombe, sterols are enriched in the plasma membrane at the growing cell tips and at the site of cytokinesis. The distribution of sterols is regulated in a cell-cycle-dependent manner and requires a functional secretory pathway. By manipulating the integrity of sterol-rich membrane domains using sterol sequestering agents and genetic means, we show that these domains are important for multiple processes regulating cytokinesis. In these cells, defects in proper maintenance of the actomyosin ring and/or its attachment to the overlying plasma membrane were observed. Furthermore, the stability of a plasma membrane protein that colocalises with sterol-rich membrane domains was compromised. Taken together, our studies establish S. pombe as a genetically tractable model organism in which to study the role(s) of sterol-rich membrane domains in cell polarity and cytokinesis.

Bgs3p, a putative 1,3-beta-glucan synthase subunit, is required for cell wall assembly in Schizosaccharomyces pombe

beta-Glucans are the main components of the fungal cell wall. Fission yeast possesses a family of beta-glucan synthase-related genes. We describe here the cloning and characterization of bgs3(+), a new member of this family. bgs3(+) was cloned as a suppressor of a mutant hypersensitive to Echinocandin and Calcofluor White, drugs that interfere with cell wall biosynthesis. Disruption of the gene is lethal, and a decrease in Bgs3p levels leads to rounded cells with thicker walls, slightly reduces the amount of the beta-glucan, and raises the amount of alpha-glucan polymer. These cells finally died. bgs3(+) is expressed in vegetative cells grown in different conditions and during mating and germination and is not enhanced by stress situations. Consistent with the observed expression pattern, Bgs3-green fluorescence protein (GFP-Bgs3p) was found at the growing tips during interphase and at the septum prior to cytokinesis, always localized to growth areas. We also found GFP-Bgs3p in mating projections, during the early stages of zygote formation, and at the growing pole during ascospore germination. We conclude that Bgs3p localization is restricted to growth areas and that Bgs3p is a glucan synthase homologue required for cell wall biosynthesis and cell elongation in the fission yeast life cycle.

Localization of the (1,3)beta-D-glucan synthase catalytic subunit homologue Bgs1p/Cps1p from fission yeast suggests that it is involved in septation, polarized growth, mating, spore wall formation and spore germination

Schizosaccharomyces pombe Bgs1p/Cps1p has been identified as a putative (1,3)beta-D-glucan synthase (GS) catalytic subunit with a possible function during cytokinesis and polarized growth. To study this possibility, double mutants of cps1-12 and cdc septation mutants were made. The double mutants displayed several hypersensitive phenotypes and altered actin distribution. Epistasis analysis showed mutations prior to septum synthesis were dominant over cps1-12, while cps1-12 was dominant over the end of septation mutant cdc16-116, suggesting Bgs1p is involved in septum cell-wall (1,3)beta-D-glucan synthesis at cytokinesis. We have studied the in vivo physiological localization of Bgs1p in a bgs1delta strain containing a functional GFP-bgs1(+) gene (integrated single copy and expressed under its own promoter). During vegetative growth, Bgs1p always localizes to the growing zones: one or both ends during cell growth and contractile ring and septum during cytokinesis. Bgs1p localization in cdc septation mutants indicates that Bgs1p needs the medial ring and septation initiation network (SIN) proteins to localize properly with the rest of septation components. Bgs1p localization in the actin mutant cps8-188 shows it depends on actin localization. In addition, Bgs1p remains polarized in the mislocalized growing poles and septa of tea1-1 and tea2-1 mutants. During the meiotic process of the life cycle, Bgs1p localizes to the mating projection, to the cell-to-cell contact zone during cell fusion and to the neck area during zygote formation. Also, Bgs1p localization suggests that it collaborates in forespore and spore wall synthesis. During spore germination, Bgs1p localizes first around the spore during isotropic growth, then to the zone of polarized growth and finally, to the medial ring and septum. At the end of spore-cell division, the Bgs1p displacement to the old end occurs only in the new cell. All these data show that Bgs1p is localized to the areas of polarized cell wall growth and so we propose that it might be involved in synthesizing the lineal (1,3)beta-D-glucan of the primary septum, as well as a similar lineal (1,3)beta-D-glucan when other processes of cell wall growth or repair are needed.

A single FKS homologue in Yarrowia lipolytica is essential for viability

The synthesis of beta-1,3-glucan, the structural component of the yeast cell wall which gives shape to the cell, occurs at the plasma membrane and is the result of the activity of at least a two-component complex. Fks1p is the catalytic subunit directly responsible for the synthesis of beta-1,3-glucan, whilst the second subunit, Rho1p, has a GTP-dependent regulatory role. FKS1 has been characterized in Saccharomyces cerevisiae, where its function is at least partially redundant with that of FKS2/GSC2. FKS homologues have also been identified in several other fungal species, including Candida albicans, Schizosaccharomyces pombe, Aspergillus nidulans, Cryptococcus neoformans and Paracoccidiodes brasiliensis. In this work, we have used degenerate oligonucleotides derived from the conserved regions of Fks1ps to isolate the possible FKS homologue genes of the strictly aerobic non-conventional yeast Yarrowia lipolytica. Using this approach, we have isolated a single FKS homologue which we have named YlFKS1; this codes a 1961 amino acid protein that shows a high degree of homology with other Fksps. Expression analysis of YlFKS1 under different conditions affecting the cell wall did not reveal significant differences. Finally, attempts to obtain a Y. lipolytica strain containing a disrupted YlFKS1 allele failed, despite having used two different techniques. Taken together, these results suggest that, unlike S. cerevisiae, YlFKS1 is the only FKS1 homologue in Y. lipolytica and is essential for growth.

Meu10 is required for spore wall maturation in Schizosaccharomyces pombe

BACKGROUND

Many genes are meiosis and/or sporulation-specifically transcribed during this process. Isolation and analysis of these genes might help us to understand how meiosis and sporulation are regulated. For this purpose, we have isolated a large number of cDNA clones from Schizosaccharomyces pombe whose expression is up-regulated during meiosis.

RESULTS

We have isolated meu10+ gene, which encodes 416 amino acids and bears homology to SPS2 of Saccharomyces cerevisiae. A strain whose meu10+ gene has been deleted forms no viable spores. Thin-section electron micrographs showed that the meu10Delta strain has abnormally formed spore walls, and then they disrupt, allowing cytoplasmic material to escape. The Meu10-GFP fusion protein is localized to the spore periphery, thereafter returned to the cytoplasm after sporulation. Meu10-GFP localization to the spore wall was almost normal in the bgs2Delta or chs1Delta mutants that lack 1,3-beta-glucan or chitin, respectively. In contrast, 1,3-beta-glucan is abnormally localized in meu10Delta cells. Meu10 has an N-terminal domain with homology to the mammalian insulin receptor and a C-terminal domain with a transmembrane motif. Mutants whose N-terminal or C-terminal domain was truncated were severely defective for sporulation.

CONCLUSIONS

Meu10 is a spore wall component and plays a pivotal role in the formation of the mature spore wall structure.