Cell separation and the maintenance of cell integrity during cytokinesis in yeast: the assembly of a septum

Cell Wall Integrity Mediated by CfCHS1 Is Important for Growth, Stress Responses and Pathogenicity in Colletotrichum fructicola

Camellia oleifera, a woody plant that produces edible oil, is indigenous to China. The devastating disease of anthracnose inflicts significant financial losses on Ca. oleifera. The primary causative agent of anthracnose on Ca. oleifera is Colletotrichum fructicola. Chitin, a pivotal constituent of fungal cell walls, assumes a critical function in their proliferation and maturation. To study the biological functions of chitin synthase 1(Chs1) in C. fructicola, the CfCHS1 gene knockout mutants, ∆Cfchs1-1 and ∆Cfchs1-2, and their complementary strain, ∆Cfchs1/CfCHS1, of C. fructicola were generated. Our results showed that the colony diameters of wild-type and complement-strain ∆Cfchs1/CfCHS1, mutant ∆Cfchs1-1 and ∆Cfchs1-2 cultured on the CM and MM medium were 5.2, 5.0, 2.2 and 2.4 cm and 4.0, 4.0, 2.1 and 2.6 cm, respectively, which were significantly smaller for the mutant than for the wild type and complement strain; the inhibition rates on the CM medium supplemented with H₂O₂, DTT, SDS and CR were 87.0% and 88.5%, 29.6% and 27.1%, 88.0% and 89.4%, and 41.7% and 28.7%, respectively, for the mutant strains, ∆Cfchs1-1 and ∆Cfchs1-2, which were significantly higher than those for the other two strains; the rate of hyphal tips with CFW fluorescence in ∆Cfchs1-1 and ∆Cfchs1-2 was 13.3% and 15.0%, which was significantly lower than those for the other two strains; the mutant strains, ∆Cfchs1-1 and ∆Cfchs1-2, lost the ability to produce conidia; the mutant strains showed weaker pathogenicity on wounded and unwounded Ca. oleifera leaves than the wild type and complement strain. The findings of this study suggest that CfChs1 plays a crucial role in the growth and development, stress responses, and pathogenicity of C. fructicola. Thus, this gene could be a potential target for developing novel fungicide.

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.

In vitro and in vivo efficacies of Dectin-1-Fc(IgG)(s) fusion proteins against invasive fungal infections

Fungal infections have increased in the last years, particularly associated to an increment in the number of immunocompromised individuals and the emergence of known or new resistant species, despite the difficulties in the often time-consuming diagnosis. The controversial efficacy of the currently available strategies for their clinical management, apart from their high toxicity and severe side effects, have renewed the interest in the research and development of new broad antifungal alternatives. These encompass vaccines and passive immunization strategies with monoclonal antibodies (mAbs), recognizing ubiquitous fungal targets, such as fungal cell wall β-1,3-glucan polysaccharides, which could be used in early therapeutic intervention without the need for the diagnosis at species-level. As additional alternatives, based on the Dectin-1 great affinity to β-1,3-glucan, our group developed broad antibody-like Dectin1-Fc(IgG)(s) from distinct subclasses (IgG2a and IgG2b) and compared their antifungal in vitro and passive immunizations in vivo performances. Dectin1-Fc(IgG2a) and Dectin1-Fc(IgG2b) demonstrated high affinity to laminarin and the fungal cell wall by ELISA, flow cytometry and microscopy. Both Dectin-1-Fc(IgG)(s) inhibited H. capsulatum and C. neoformans growth in a dose-dependent fashion. For C. albicans, such inhibitory effect was observed with concentrations as low as 0.098 and 0.049 µg/mL, respectively, which correlated with the impairment of the kinetics and lengths of germ tubes in comparison to controls. Previous opsonization with Dectin-1-Fc(IgG)(s) enhanced considerably the macrophage antifungal effector functions, increasing the fungi macrophages-interactions and significantly reducing the intraphagosome fungal survival, as lower CFUs were observed. The administration of both Dectin1-Fc(IgG)(s) reduced the fungal burden and mortality in murine histoplasmosis and candidiasis models, in accordance with previous evaluations in aspergillosis model. These results altogether strongly suggested that therapeutic interventions with Dectin-1-Fc(IgG)(s) fusion proteins could directly impact the innate immunity and disease outcome in favor of the host, by direct neutralization, opsonization, phagocytosis, and fungal elimination, providing interesting information on the potential of these new strategies for the control of invasive fungal infections.

Nanochitin: Chemistry, Structure, Assembly, and Applications

Chitin, a fascinating biopolymer found in living organisms, fulfills current demands of availability, sustainability, biocompatibility, biodegradability, functionality, and renewability. A feature of chitin is its ability to structure into hierarchical assemblies, spanning the nano- and macroscales, imparting toughness and resistance (chemical, biological, among others) to multicomponent materials as well as adding adaptability, tunability, and versatility. Retaining the inherent structural characteristics of chitin and its colloidal features in dispersed media has been central to its use, considering it as a building block for the construction of emerging materials. Top-down chitin designs have been reported and differentiate from the traditional molecular-level, bottom-up synthesis and assembly for material development. Such topics are the focus of this Review, which also covers the origins and biological characteristics of chitin and their influence on the morphological and physical-chemical properties. We discuss recent achievements in the isolation, deconstruction, and fractionation of chitin nanostructures of varying axial aspects (nanofibrils and nanorods) along with methods for their modification and assembly into functional materials. We highlight the role of nanochitin in its native architecture and as a component of materials subjected to multiscale interactions, leading to highly dynamic and functional structures. We introduce the most recent advances in the applications of nanochitin-derived materials and industrialization efforts, following green manufacturing principles. Finally, we offer a critical perspective about the adoption of nanochitin in the context of advanced, sustainable materials.

Gene by Environment Interactions reveal new regulatory aspects of signaling network plasticity

Phenotypes can change during exposure to different environments through the regulation of signaling pathways that operate in integrated networks. How signaling networks produce different phenotypes in different settings is not fully understood. Here, Gene by Environment Interactions (GEIs) were used to explore the regulatory network that controls filamentous/invasive growth in the yeast Saccharomyces cerevisiae. GEI analysis revealed that the regulation of invasive growth is decentralized and varies extensively across environments. Different regulatory pathways were critical or dispensable depending on the environment, microenvironment, or time point tested, and the pathway that made the strongest contribution changed depending on the environment. Some regulators even showed conditional role reversals. Ranking pathways' roles across environments revealed an under-appreciated pathway (OPI1) as the single strongest regulator among the major pathways tested (RAS, RIM101, and MAPK). One mechanism that may explain the high degree of regulatory plasticity observed was conditional pathway interactions, such as conditional redundancy and conditional cross-pathway regulation. Another mechanism was that different pathways conditionally and differentially regulated gene expression, such as target genes that control separate cell adhesion mechanisms (FLO11 and SFG1). An exception to decentralized regulation of invasive growth was that morphogenetic changes (cell elongation and budding pattern) were primarily regulated by one pathway (MAPK). GEI analysis also uncovered a round-cell invasion phenotype. Our work suggests that GEI analysis is a simple and powerful approach to define the regulatory basis of complex phenotypes and may be applicable to many systems.

Echinocandin Drugs Induce Differential Effects in Cytokinesis Progression and Cell Integrity

Fission yeast contains three essential β(1,3)-D-glucan synthases (GSs), Bgs1, Bgs3, and Bgs4, with non-overlapping roles in cell integrity and morphogenesis. Only the bgs4⁺ mutants pbr1-8 and pbr1-6 exhibit resistance to GS inhibitors, even in the presence of the wild-type (WT) sequences of bgs1⁺ and bgs3⁺. Thus, Bgs1 and Bgs3 functions seem to be unaffected by those GS inhibitors. To learn more about echinocandins' mechanism of action and resistance, cytokinesis progression and cell death were examined by time-lapse fluorescence microscopy in WT and pbr1-8 cells at the start of treatment with sublethal and lethal concentrations of anidulafungin, caspofungin, and micafungin. In WT, sublethal concentrations of the three drugs caused abundant cell death that was either suppressed (anidulafungin and micafungin) or greatly reduced (caspofungin) in pbr1-8 cells. Interestingly, the lethal concentrations induced differential phenotypes depending on the echinocandin used. Anidulafungin and caspofungin were mostly fungistatic, heavily impairing cytokinesis progression in both WT and pbr1-8. As with sublethal concentrations, lethal concentrations of micafungin were primarily fungicidal in WT cells, causing cell lysis without impairing cytokinesis. The lytic phenotype was suppressed again in pbr1-8 cells. Our results suggest that micafungin always exerts its fungicidal effect by solely inhibiting Bgs4. In contrast, lethal concentrations of anidulafungin and caspofungin cause an early cytokinesis arrest, probably by the combined inhibition of several GSs.

Rapid screening method of Saccharomyces cerevisiae mutants using calcofluor white and aniline blue

Fungal cell walls are composed of polysaccharide scaffold that changes in response to environment. The structure and biosynthesis of the wall are unique to fungi, with plant and mammalian immune systems evolved to recognize wall components. Additionally, the enzymes that assemble fungal cell wall components are excellent targets for antifungal chemotherapies and fungicides. Understanding changes in the cell wall are important for fundamental understanding of cell wall dynamics and for drug development. Here we describe a screening technique to monitor the gross morphological changes of two key cell wall polysaccharides of chitin and β-1,3-glucan combined with polymerase chain reaction (PCR) genotyping. Changes in chitin and β-1,3-glucan were detected microscopically by using the dyes calcofluor white and aniline blue. Combining PCR and fluorescence microscopy, as a quick and easy screening technique, confirmed both the phenotype and genotype of the wild-type, h chitin synthase mutants (chs1Δ and chs3Δ) and one β-1,3-glucan synthase mutant fks2Δ from Saccharomyces cerevisiae knockout library. This combined screening method highlighted that the fks1Δ strain obtained commercially was in fact not FKS1 deletion strain, and instead had both wild-type genotype and phenotype. A new β-1,3-glucan synthase knockout fks1::URA3 strain was created. Fluorescence microscopy confirmed its phenotype revealing that the chitin and the new β-1,3-glucan profiles were elevated in the mother cells and in the emerging buds respectively in the fks1Δ cell walls. This combination of PCR with fluorescence microscopy is a quick and easy screening method to determine and verify morphological changes in the S. cerevisiae cell wall.

Synergistic role of nucleotides and lipids for the self-assembly of Shs1 septin oligomers

Budding yeast septins are essential for cell division and polarity. Septins assemble as palindromic linear octameric complexes. The function and ultra-structural organization of septins are finely governed by their molecular polymorphism. In particular, in budding yeast, the end subunit can stand either as Shs1 or Cdc11. We have dissected, here, for the first time, the behavior of the Shs1 protomer bound to membranes at nanometer resolution, in complex with the other septins. Using electron microscopy, we have shown that on membranes, Shs1 protomers self-assemble into rings, bundles, filaments or two-dimensional gauzes. Using a set of specific mutants we have demonstrated a synergistic role of both nucleotides and lipids for the organization and oligomerization of budding yeast septins. Besides, cryo-electron tomography assays show that vesicles are deformed by the interaction between Shs1 oligomers and lipids. The Shs1-Shs1 interface is stabilized by the presence of phosphoinositides, allowing the visualization of micrometric long filaments formed by Shs1 protomers. In addition, molecular modeling experiments have revealed a potential molecular mechanism regarding the selectivity of septin subunits for phosphoinositide lipids.

A novel checkpoint pathway controls actomyosin ring constriction trigger in fission yeast

In fission yeast, the septation initiation network (SIN) ensures temporal coordination between actomyosin ring (CAR) constriction with membrane ingression and septum synthesis. However, questions remain about CAR regulation under stress conditions. We show that Rgf1p (Rho1p GEF), participates in a delay of cytokinesis under cell wall stress (blankophor, BP). BP did not interfere with CAR assembly or the rate of CAR constriction, but did delay the onset of constriction in the wild type cells but not in the rgf1Δ cells. This delay was also abolished in the absence of Pmk1p, the MAPK of the cell integrity pathway (CIP), leading to premature abscission and a multi-septated phenotype. Moreover, cytokinesis delay correlates with maintained SIN signaling and depends on the SIN to be achieved. Thus, we propose that the CIP participates in a checkpoint, capable of triggering a CAR constriction delay through the SIN pathway to ensure that cytokinesis terminates successfully.

Rsr1 Palmitoylation and GTPase Activity Status Differentially Coordinate Nuclear, Septin, and Vacuole Dynamics in Candida albicans

Directional growth and tissue invasion by hyphae of the pathogenic fungus, Candida albicans, are disrupted by deletion of the small GTPase, Rsr1, which localizes Cdc42 and its kinase, Cla4, to the site of polarized growth. We investigated additional abnormalities observed in rsr1Δ hyphae, including vacuole development, cytoplasm inheritance, mitochondrial morphology, septin ring organization, nuclear division and migration, and branching frequency, which together demonstrate a fundamental role for Rsr1 in cellular organization. Rsr1 contains a C-terminal CCAAX box, which putatively undergoes both reversible palmitoylation and farnesylation for entry into the secretory pathway. We expressed variants of Rsr1 with mutated C244 or C245, or which lacked GTPase activity (Rsr1^K16N and Rsr1^G12V), in the rsr1Δ background and compared the resulting phenotypes with those of mutants lacking Bud5 (Rsr1 GEF), Bud2 (Rsr1 GAP), or Cla4. Bud5 was required only for cell size and bud site selection in yeast, suggesting there are alternative activators for Rsr1 in hyphae. Septin ring and vacuole dynamics were restored by expression of unpalmitoylated Rsr1^C244S, which localized to endomembranes, but not by cytoplasmic Rsr1^C245A or GTP/GDP-locked Rsr1, suggesting Rsr1 functions at intracellular membranes in addition to the plasma membrane. Rsr1^K16N or cytoplasmic Rsr1^C245A restored normal nuclear division but not septin ring or vacuole dynamics. Rsr1-GDP therefore plays a specific role in suppressing START, which can be signaled from the cytosol. Via differential palmitoylation and activity states, Rsr1 operates at diverse cell sites to orchestrate proper nuclear division and inheritance during constitutive polarized growth. As cla4Δ phenocopied rsr1Δ, it is likely these functions involve Cdc42-Cla4 activity.IMPORTANCE Understanding how single eukaryotic cells self-organize to replicate and migrate is relevant to health and disease. In the fungal pathogen, Candida albicans, the small GTPase, Rsr1, guides the directional growth of hyphae that invade human tissue during life-threatening infections. Rsr1 is a Ras-like GTPase and a homolog of the conserved Rap1 subfamily, which directs migration in mammalian cells. Research into how this single GTPase delivers complex intracellular patterning is challenging established views of GTPase regulation, trafficking, and interaction. Here, we show that Rsr1 directly and indirectly coordinates the spatial and temporal development of key intracellular macrostructures, including septum formation and closure, vacuole dynamics, and nuclear division and segregation, as well as whole-cell morphology by determining branching patterns. Furthermore, we categorize these functions by differential Rsr1 localization and activity state and provide evidence to support the emerging view that the cytosolic pool of Ras-like GTPases is functionally active.

Effects of 5'-3' Exonuclease Xrn1 on Cell Size, Proliferation and Division, and mRNA Levels of Periodic Genes in Cryptococcus neoformans

Cell size affects almost all biosynthetic processes by controlling the size of organelles and disrupting the nutrient uptake process. Yeast cells must reach a critical size to be able to enter a new cell cycle stage. Abnormal changes in cell size are often observed under pathological conditions such as cancer disease. Thus, cell size must be strictly controlled during cell cycle progression. Here, we reported that the highly conserved 5′-3′ exonuclease Xrn1 could regulate the gene expression involved in the cell cycle pathway of Cryptococcus neoformans. Chromosomal deletion of XRN1 caused an increase in cell size, defects in cell growth and altered DNA content at 37 °C. RNA-sequencing results showed that the difference was significantly enriched in genes involved in membrane components, DNA metabolism, integration and recombination, DNA polymerase activity, meiotic cell cycle, nuclear division, organelle fission, microtubule-based process and reproduction. In addition, the proportion of the differentially expressed periodic genes was up to 19.8% when XRN1 was deleted, including cell cycle-related genes, chitin synthase genes and transcription factors, indicating the important role of Xrn1 in the control of cell cycle. This work provides insights into the roles of RNA decay factor Xrn1 in maintaining appropriate cell size, DNA content and cell cycle progression.

The Glucan-Remodeling Enzyme Phr1p and the Chitin Synthase Chs1p Cooperate to Maintain Proper Nuclear Segregation and Cell Integrity in Candida albicans

GH72 family of β-(1,3)-glucanosyltransferases is unique to fungi and is required for cell wall biogenesis, morphogenesis, virulence, and in some species is essential for life. Candida albicans PHR1 and PHR2 are pH-regulated genes that encode GH72 enzymes highly similar to Gas1p of Saccharomyces cerevisiae. PHR1 is expressed at pH ≥ 5.5 while PHR2 is transcribed at pH ≤ 5.5. Both are essential for C. albicans morphogenesis and virulence. During growth at neutral-alkaline pH, Phr1p-GFP preferentially localizes to sites of active cell wall formation as the incipient bud, the mother-daughter neck, the bud periphery, and concentrates in the septum at cytokinesis. We further investigated this latter localization. In chs3Δ cells, lacking the chitin of the chitin ring and lateral cell wall, Phr1p-GFP still concentrated along the thin line of the primary septum formed by chitin deposited by chitin synthase I (whose catalytic subunit is Chs1p) suggesting that it plays a role during formation of the secondary septa. RO-09-3143, a highly specific inhibitor of Chs1p activity, inhibits septum formation and blocks cell division. However, alternative septa are produced and are crucial for cell survival. Phr1p-GFP is excluded from such aberrant septa. Finally, we determined the effects of RO-09-3143 in cells lacking Phr1p. PHR1 null mutant was more susceptible to the drug than the wild type. The phr1Δ cells were larger, devoid of septa, and underwent endomitosis and cell death. Phr1p and Chs1p cooperate in maintaining cell integrity and in coupling morphogenesis with nuclear division in C. albicans.

Coordinating septum formation and the actomyosin ring during cytokinesis in Schizosaccharomyces pombe

During cytokinesis, animal and fungal cells form a membrane furrow via actomyosin ring constriction. Our understanding of actomyosin ring-driven cytokinesis stems extensively from the fission yeast model system. However, unlike animal cells, actomyosin ring constriction occurs simultaneously with septum formation in fungi. While the formation of an actomyosin ring is essential for cytokinesis in fission yeast, proper furrow formation also requires septum deposition. The molecular mechanisms of spatiotemporal coordination of septum deposition with actomyosin ring constriction are poorly understood. Although the role of the actomyosin ring as a mechanical structure driving furrow formation is better understood, its role as a spatiotemporal landmark for septum deposition is not widely discussed. Here we review and discuss the recent advances describing how the actomyosin ring spatiotemporally regulates membrane traffic to promote septum-driven cytokinesis in fission yeast. Finally, we explore emerging questions in cytokinesis, and discuss the role of extracellular matrix during cytokinesis in other organisms.

Members of chitin synthase family in Metarhizium acridum differentially affect fungal growth, stress tolerances, cell wall integrity and virulence

Chitin is an important component of the fungal cell wall with a family of chitin synthases mediating its synthesis. Here, we report on the genetic characterization of the full suite of seven chitin synthases (MaChsI-VII) identified in the insect pathogenic fungus, Metarhizium acridum. Aberrant distribution of chitin was most evident in targeted gene knockouts of MaChsV and MaChsVII. Mutants of MaChsI, MaChsIII, MaChsIV showed delayed conidial germination, whereas ΔMaChsII and ΔMaChsV mutants germinated more rapidly when compared to the wild-type parent. All MaChs genes impacted conidial yield, but differentially affected stress tolerances. Inactivation of MaChsIII, MaChsV, MaChsVII resulted in cell wall fragility, and ΔMaChsV and ΔMaChsVII mutants showed high sensitivity to Congo red and calcofluor white, suggesting that the three genes are required for cell wall integrity. In addition, ΔMaChsIII and ΔMaChsVII mutants showed the highest sensitivities to heat and UV-B stress. Three of seven chitin synthase genes, MaChsIII, MaChsV, MaChsVII, were found to contribute to fungal virulence. Compared with the wild-type strain, ΔMaChsIII and ΔMaChsV mutants were reduced in virulence by topical inoculation, while the ΔMaChsVII mutant showed more severe virulence defects. Inactivation of MaChsIII, MaChsV, or MaChsVII impaired appressorium formation, affected growth of in insecta produced hyphal bodies, and altered the surface properties of conidia and hyphal bodies, resulting in defects in the ability of the mutant strains to evade insect immune responses. These data provide important links between the physiology of the cell wall and the ability of the fungus to parasitize insects and reveal differential functional consequences of the chitin synthase family in M. acridum growth, stress tolerances, cell wall integrity and virulence.

The fungal cell wall is a dynamic and flexible organelle that modulates the interaction of the pathogen with its host and acts as a critical recognition and evasion interface with host defenses. Chitin is a hallmark constituent of the fungal cell wall and all fungi contain multiple chitin synthase (Chs) genes. However, systematic characterization of chitin synthase genes has not yet been reported in entomopathogenic fungi. By using the insect pathogen Metarhizium acridum as a model, we performed a systematic genetic analysis of the seven member Chs family (ChsI-VII) in the insect pathogenic fungus. Construction of strains bearing targeted single gene mutations revealed differential contributions of specific Chs genes to growth, cell wall integrity, and stress responses. In addition, we revealed that three chitin synthase genes MaChsIII, MaChsV and MaChsVII were shown to be important for fungal appressorium formation and evasion of insect cellular and/or humoral defenses, promoting the fungal dimorphic transition to the production of hyphal bodies that occurs within hosts, and ultimately to virulence. These data provide new insights into the roles of Chs genes and chitin as critical components affecting fungal membrane structure and function.

Chitin synthase is involved in vegetative growth, asexual reproduction and pathogenesis of Phytophthora capsici and Phytophthora sojae

Chitin is a structural and functional component of the fungal cell wall and also serves as a pathogen-associated molecular pattern (PAMP) that triggers the innate immune responses of host plants. However, no or very little chitin is found in the fungus-like oomycetes. In Phytophthora spp., the presence of chitin has not been demonstrated so far, although putative chitin synthase (CHS) genes, which encode the enzymes that synthesize chitin, are present in their genomes. Here, we revealed that chitin is present in the zoospores and released sporangia of Phytophthora, and this is most consistent with the transcriptional pattern of PcCHS in Phytophthora capsici and PsCHS1 in Phytophthora sojae. Disruption of the CHS genes indicated that PcCHS and PsCHS1, but not PsCHS2 (which exhibited very weak transcription), have similar functions involved in mycelial growth, sporangial production, zoospore release and the pathogenesis of P. capsici and P. sojae. We also suggest that chitin in the zoospores of P. capsici can act as a PAMP that is recognized by the chitin receptors AtLYK5 or AtCERK1 of Arabidopsis. These results provide new insights into the biological significance of chitin and CHSs in Phytophthora and help with the identification of potential targets for disease control.

The fungal cell wall as a target for the development of new antifungal therapies

In the past three decades invasive mycoses have globally emerged as a persistent source of healthcare-associated infections. The cell wall surrounding the fungal cell opposes the turgor pressure that otherwise could produce cell lysis. Thus, the cell wall is essential for maintaining fungal cell shape and integrity. Given that this structure is absent in host mammalian cells, it stands as an important target when developing selective compounds for the treatment of fungal infections. Consequently, treatment with echinocandins, a family of antifungal agents that specifically inhibits the biosynthesis of cell wall (1-3)β-D-glucan, has been established as an alternative and effective antifungal therapy. However, the existence of many pathogenic fungi resistant to single or multiple antifungal families, together with the limited arsenal of available antifungal compounds, critically affects the effectiveness of treatments against these life-threatening infections. Thus, new antifungal therapies are required. Here we review the fungal cell wall and its relevance in biotechnology as a target for the development of new antifungal compounds, disclosing the most promising cell wall inhibitors that are currently in experimental or clinical development for the treatment of some invasive mycoses.

Glucanases and Chitinases

In many yeast and fungi, β-(1,3)-glucan and chitin are essential components of the cell wall, an important structure that surrounds cells and which is responsible for their mechanical protection and necessary for maintaining the cellular shape. In addition, the cell wall is a dynamic structure that needs to be remodelled along with the different phases of the fungal life cycle or in response to extracellular stimuli. Since β-(1,3)-glucan and chitin perform a central structural role in the assembly of the cell wall, it has been postulated that β-(1,3)-glucanases and chitinases should perform an important function in cell wall softening and remodelling. This review focusses on fungal glucanases and chitinases and their role during fungal morphogenesis.

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.

Gene overexpression screen for chromosome instability in yeast primarily identifies cell cycle progression genes

Loss of heterozygosity (LOH) in a vegetatively growing diploid cell signals irregularity of mitosis. Therefore, assays of LOH serve to discover pathways critical for proper replication and segregation of chromosomes. We screened for enhanced LOH in a whole-genome collection of diploid yeast strains in which a single gene was strongly overexpressed. We found 39 overexpression strains with substantially increased LOH caused either by recombination or by chromosome instability. Most of them, 32 in total, belonged to the category of "cell division", a broadly defined biological process. Of those, only one, TOP3, coded for an enzyme that uses DNA as a substrate. The rest related to establishment and maintenance of cell polarity, chromosome segregation, and cell cycle checkpoints. Former studies, in which gene deletions were used, showed that an absence of a protein participating in the DNA processing machinery is a potent stimulator of genome instability. As our results suggest, overexpression of such proteins is not comparably damaging as the absence of them. It may mean that the harmful effect of overexpression is more likely to occur in more complex and multistage processes, such as chromosome segregation. We also report a side finding, resulting from the fact that we worked with the yeast strains bearing a 2-micron plasmid. We noted that intense transcription from such a plasmid led to an enhanced rate of an entire chromosome loss (as opposed to LOH produced by recombination). This observation may support models linking segregation of 2-micron plasmids to segregation of chromosomes.

Impact of Fungal MAPK Pathway Targets on the Cell Wall

The fungal cell wall is an extracellular organelle that provides structure and protection to cells. The cell wall also influences the interactions of cells with each other and surfaces. The cell wall can be reorganized in response to changing environmental conditions and different types of stress. Signaling pathways control the remodeling of the cell wall through target proteins that are in many cases not well defined. The Mitogen Activated Protein Kinase pathway that controls filamentous growth in yeast (fMAPK) was required for normal growth in media containing the cell wall perturbing agent Calcofluor White (CFW). A mass spectrometry (MASS-SPEC) approach and analysis of expression profiling data identified cell wall proteins and modifying enzymes whose levels were influenced by the fMAPK pathway. These include Flo11p, Flo10p, Tip1p, Pry2p and the mannosyltransferase, Och1p. Cells lacking Flo11p or Och1p were sensitive to CFW. The identification of cell wall proteins controlled by a MAPK pathway may provide insights into how signaling pathways regulate the cell wall.

Cell wall biosynthesis impairment affects the budding lifespan of the Saccharomyces cerevisiae yeast

The Saccharomyces cerevisiae yeast is one of the most widely used model in studies of cellular and organismal biology, including as aging and proliferation. Although several constraints of aging and budding lifespan have been identified, these processes have not yet been fully understood. Previous studies of aging in yeast have focused mostly on the molecular basics of the underlying mechanisms, while physical aspects, particularly those related to the cell wall, were rather neglected. In this paper, we examine for the first time, to our knowledge, the impact of cell wall biosynthesis disturbances on the lifespan in the budding yeast. We have used a set of cell wall mutants, including knr4Δ, cts1Δ, chs3Δ, fks1Δ and mnn9Δ, which affect biosynthesis of all major cell wall compounds. Our results indicated that impairment of chitin biosynthesis and cell wall protein mannosylation reduced the budding lifespan, while disruption in the 1,3-β-glucan synthase activity had no adverse effect on that parameter. The impact varied in the severity and the most notable effect was observed for the mnn9Δ mutant. What was interesting, in the case of the dysfunction of the Knr4 protein playing the role of the transcriptional regulator of cell wall chitin and glucan synthesis, the lifespan increased significantly. We also report the phenotypic characteristics of cell wall-associated mutants as revealed by imaging of the cell wall using transmission electron microscopy, scanning electron microscopy and atomic force microscopy. In addition, our findings support the conviction that achievement of the state of hypertrophy may not be the only factor that determines the budding lifespan.

Studying the Role of the Mitotic Exit Network in Cytokinesis

In budding yeast cells, cytokinesis is achieved by the successful division of the cytoplasm into two daughter cells, but the precise mechanisms of cell division and its regulation are still rather poorly understood. The Mitotic Exit Network (MEN) is the signaling cascade that is responsible for the release of Cdc14 phosphatase leading to the inactivation of the kinase activity associated to cyclin-dependent kinases (CDK), which drives exit from mitosis and a rapid and efficient cytokinesis. Mitotic CDK impairs the activation of MEN before anaphase, and activation of MEN in anaphase leads to the inactivation of CDK, which presents a challenge to determine the contribution that each pathway makes to the successful onset of cytokinesis. To determine CDK and MEN contribution to cytokinesis irrespectively of each other, here we present methods to induce cytokinesis after the inactivation of CDK activity in temperature sensitive mutants of the MEN pathway. An array of methods to monitor the cellular events associated with the successful cytokinesis is included.

The chitin synthase FgChs2 and other FgChss co-regulate vegetative development and virulence in F. graminearum

Fusarium graminearum contains eight chitin synthase (Chs) genes belonging to seven classes. Previous studies have found that deletion of FgChs3b is lethal to F. graminearum, and deletion of FgChs1, FgChs2, FgChs7 and FgChs5 caused diverse defects in chitin content, mycelial growth, conidiation, virulence or stress responses. However, little is known about the functional relationships among these FgChss. In this study, FgChs2 deletion mutant ΔFgChs2 exhibited reduced mycelial growth and virulence as reported previously. In addition, we found that the mutant produced thickened and “wavy” septa. Quantitative real-time PCR (qRT-PCR) assays showed that the expression levels of FgChs1, FgChs3a, FgChs4, FgChs7, FgChs5 and FgChs6 in ΔFgChs2 were significantly higher than those in the wild type. Therefore, we generated six double deletion mutants of FgChs2 and each of the above six FgChss, and found that FgChs2 shares a function with FgChs1 in regulating mycelial growth, and co-regulates conidiation with FgChs1, FgChs4, FgChs7 and FgChs5. Furthermore, FgChs2 and other six FgChss have overlapped functions in virulence, DON production and septum formation. Taken together, these results indicate that although each chitin synthase of F. graminearum plays certain roles, FgChss may co-regualte various cellular processes in F. graminearum.

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.

Cellular localization of Sun4p and its interaction with proteins in the yeast birth scar

Yeast harbor several proteins with predicted glucanase activity that are potentially involved in cell wall remodeling during different processes, including mitosis. Here, we showed that 2 of these putative glucanases, Sun4p and Dse2p, co-localize to the yeast birth scar, dependently on presence of the third glucanase, Egt2p. The absence of these glucanases results in inefficient mother-daughter cell separation. The Sun4p, Dse2p and Egt2p localize to the daughter side of the bud neck, possibly forming a complex, and are involved in the separation of the virgin daughter from the mother cell during mitosis. The formation of properly assembled birth scars that delimitate cell wall area restricted in the next budding is dependent on the presence of Aim44p and its transcriptional regulator, Swi5p. AIM44 or SWI5 deletion caused the "budding within the birth scar" phenotype, together with altered localization of the birth scar proteins Sun4p and Dse2p, indicating the impairment of birth scar protein complexes.

Investigation of the role of four mitotic septins and chitin synthase 2 for cytokinesis in Kluyveromyces lactis

Septins are key components of the cell division machinery from yeast to humans. The model yeast Saccharomyces cerevisiae has five mitotic septins, Cdc3, Cdc10, Cdc11, Cdc12, and Shs1. Here we characterized the five orthologs from the genetically less-redundant milk yeast Kluyveromyces lactis. We found that except for KlSHS1 all septin genes are essential. Klshs1 deletions displayed temperature-sensitive growth and morphological defects. Heterologous complementation analyses revealed that all five K. lactis genes encode functional orthologs of their S. cerevisiae counterparts. Fluorophore-tagged versions of the K. lactis septins localized to a ring at the incipient bud site and split into two separate rings at the bud neck later in cytokinesis. One of the key proteins recruited to the bud neck by septins in S. cerevisiae is the chitin synthase Chs2, which synthesizes the primary septum. KlCHS2 was found to be essential and deletions showed cytokinetic defects upon spore germination. KlChs2-GFP also localized to the bud neck and to punctate structures in K. lactis. We conclude that cytokinesis in K. lactis is similar to S. cerevisiae and chimeric septin complexes are fully functional in both yeasts. In contrast to some S. cerevisiae strains, KlChs2 and KlCdc10 were found to be essential.

Roles of the novel coiled-coil protein Rng10 in septum formation during fission yeast cytokinesis

The regulation of Rho-GAP localization is not well understood. A novel coiled-coil protein Rng10 is characterized that localizes the Rho-GAP Rga7 in fission yeast. Rng10 and Rga7 physically interact and work together to regulate the accumulation and dynamics of glucan synthases for successful septum formation during cytokinesis.

Rho GAPs are important regulators of Rho GTPases, which are involved in various steps of cytokinesis and other processes. However, regulation of Rho-GAP cellular localization and function is not fully understood. Here we report the characterization of a novel coiled-coil protein Rng10 and its relationship with the Rho-GAP Rga7 in fission yeast. Both rng10Δ and rga7Δ result in defective septum and cell lysis during cytokinesis. Rng10 and Rga7 colocalize on the plasma membrane at the cell tips during interphase and at the division site during cell division. Rng10 physically interacts with Rga7 in affinity purification and coimmunoprecipitation. Of interest, Rga7 localization is nearly abolished without Rng10. Moreover, Rng10 and Rga7 work together to regulate the accumulation and dynamics of glucan synthases for successful septum formation in cytokinesis. Our results show that cellular localization and function of the Rho-GAP Rga7 are regulated by a novel protein, Rng10, during cytokinesis in fission yeast.

Overview of fission yeast septation

Cytokinesis is the final process of the vegetative cycle, which divides a cell into two independent daughter cells once mitosis is completed. In fungi, as in animal cells, cytokinesis requires the formation of a cleavage furrow originated by constriction of an actomyosin ring which is connected to the plasma membrane and causes its invagination. Additionally, because fungal cells have a polysaccharide cell wall outside the plasma membrane, cytokinesis requires the formation of a septum coincident with the membrane ingression. Fission yeast Schizosaccharomyces pombe is a unicellular, rod-shaped fungus that has become a popular model organism for the study of actomyosin ring formation and constriction during cell division. Here we review the current knowledge of the septation and separation processes in this fungus, as well as recent advances in understanding the functional interaction between the transmembrane enzymes that build the septum and the actomyosin ring proteins.

The price of independence: cell separation in fission yeast

The ultimate goal of cell division is to give rise to two viable independent daughter cells. A tight spatial and temporal regulation between chromosome segregation and cytokinesis ensures the viability of the daughter cells. Schizosaccharomyces pombe, commonly known as fission yeast, has become a leading model organism for studying essential and conserved mechanisms of the eukaryotic cell division process. Like many other eukaryotic cells it divides by binary fission and the cleavage furrow undergoes ingression due to the contraction of an actomyosin ring. In contrast to mammalian cells, yeasts as cell-walled organisms, also need to form a division septum made of cell wall material to complete the process of cytokinesis. The division septum is deposited behind the constricting ring and it will constitute the new ends of the daughter cells. Cell separation also involves cell wall degradation and this process should be precisely regulated to avoid cell lysis. In this review, we will give a brief overview of the whole cytokinesis process in fission yeast, from the positioning and assembly of the contractile ring to the final step of cell separation, and the problems generated when these processes are not precise.

Ingression Progression Complexes Control Extracellular Matrix Remodelling during Cytokinesis in Budding Yeast

Eukaryotic cells must coordinate contraction of the actomyosin ring at the division site together with ingression of the plasma membrane and remodelling of the extracellular matrix (ECM) to support cytokinesis, but the underlying mechanisms are still poorly understood. In eukaryotes, glycosyltransferases that synthesise ECM polysaccharides are emerging as key factors during cytokinesis. The budding yeast chitin synthase Chs2 makes the primary septum, a special layer of the ECM, which is an essential process during cell division. Here we isolated a group of actomyosin ring components that form complexes together with Chs2 at the cleavage site at the end of the cell cycle, which we named ‘ingression progression complexes’ (IPCs). In addition to type II myosin, the IQGAP protein Iqg1 and Chs2, IPCs contain the F-BAR protein Hof1, and the cytokinesis regulators Inn1 and Cyk3. We describe the molecular mechanism by which chitin synthase is activated by direct association of the C2 domain of Inn1, and the transglutaminase-like domain of Cyk3, with the catalytic domain of Chs2. We used an experimental system to find a previously unanticipated role for the C-terminus of Inn1 in preventing the untimely activation of Chs2 at the cleavage site until Cyk3 releases the block on Chs2 activity during late mitosis. These findings support a model for the co-ordinated regulation of cell division in budding yeast, in which IPCs play a central role.

Cytokinesis is the process by which a cell divides in two and occurs once cells have replicated and segregated their chromosomes. Eukaryotic cells assemble a molecular machine called the actomyosin ring that drives cytokinesis. Contraction of the actomyosin ring is coupled to ingression of the plasma membrane and extracellular matrix remodelling. In eukaryotes, glycosyltransferases that synthesise polysaccharides of the extracellular matrix are emerging as essential factors during cytokinesis. Defects associated with the function of those glycosyltransferases induce the failure of cell division, which promotes the formation of genetically unstable tetraploid cells. Budding yeast cells contain a glycosyltransferase called Chs2 that makes a special layer of extracellular matrix and is essential during cell division. Our findings provide new insights into the molecular mechanism by which the cytokinesis regulators Inn1 and Cyk3 finely regulate the activity of glycosyltransferase Chs2 at the end of mitosis. In addition we isolated a group of actomyosin ring components that form complexes together with Chs2 and Inn1 at the cleavage site, which we have named ‘ingression progression complexes’. These complexes coordinate the contraction of the actomyosin ring, ingression of the plasma membrane and extracellular matrix remodelling in a precise manner. Chs2 is indeed a key factor for coordinating these events. It appears that similar principles could apply to other eukaryotic species, such as fission yeast even if the identity of the relevant glycosyltransferase has changed over the evolution. Taking into account the conservation of the basic cytokinetic mechanisms future studies should try to determine whether a glycosyltransferase similar to Chs2 plays a key role during cytokinesis in human cells.

Time-Lapse Fluorescence Microscopy of Budding Yeast Cells

The discovery of green fluorescent protein (GFP) allowed visualization of a wide variety of processes within living cells. Thanks to the development of differently colored fluorophores, it is now possible to simultaneously follow distinct subcellular events at the single cell level. Here, we describe a basic method to visualize multiple events during cytokinesis by time-lapse fluorescence microscopy in the budding yeast Saccharomyces cerevisiae. In this organism, contraction of an actomyosin-based ring drives ingression of the plasma membrane at the mother-bud division site to partition the cytoplasm of the dividing cell. Simultaneous visualization of distinct cytokinesis steps in living cells, such as ring contraction and membrane ingression, will facilitate a complete understanding of the mechanisms of eukaryotic cell division.

Monitoring Chitin Deposition During Septum Assembly in Budding Yeast

The synthesis of the septum is a critical step during cytokinesis in the fungal cell. Moreover, in Saccharomyces cerevisiae septum assembly depends mostly on the proper synthesis and deposition of chitin and, accordingly, on the timely regulation of chitin synthases. In this chapter, we will see how to follow chitin synthesis by two complementary approaches: monitoring chitin deposition in vivo at the septum by calcofluor staining and fluorescence microscopy, and measuring the chitin synthase activities responsible for this synthesis.

Regulation of contractile ring formation and septation in Schizosaccharomyces pombe

The fission yeast Schizosaccharomyces pombe has become a powerful model organism for cytokinesis studies, propelled by pioneering genetic screens in the 1980s and 1990s. S. pombe cells are rod-shaped and divide similarly to mammalian cells, utilizing a medially-placed actin-and myosin-based contractile ring. A cell wall division septum is deposited behind the constricting ring, forming the new ends of each daughter cell. Here we discuss recent advances in our understanding of the regulation of contractile ring formation through formin proteins and the role of the division septum in S. pombe cell division.

Regulation of Ace2-dependent genes requires components of the PBF complex in Schizosaccharomyces pombe

The division cycle of unicellular yeasts is completed with the activation of a cell separation program that results in the dissolution of the septum assembled during cytokinesis between the 2 daughter cells, allowing them to become independent entities. Expression of the eng1(+) and agn1(+) genes, encoding the hydrolytic enzymes responsible for septum degradation, is activated at the end of each cell cycle by the transcription factor Ace2. Periodic ace2(+) expression is regulated by the transcriptional complex PBF (PCB Binding Factor), composed of the forkhead-like proteins Sep1 and Fkh2 and the MADS box-like protein Mbx1. In this report, we show that Ace2-dependent genes contain several combinations of motifs for Ace2 and PBF binding in their promoters. Thus, Ace2, Fkh2 and Sep1 were found to bind in vivo to the eng1(+) promoter. Ace2 binding was coincident with maximum level of eng1(+) expression, whereas Fkh2 binding was maximal when mRNA levels were low, supporting the notion that they play opposing roles. In addition, we found that the expression of eng1(+) and agn1(+) was differentially affected by mutations in PBF components. Interestingly, agn1(+) was a major target of Mbx1, since its ectopic expression resulted in the suppression of Mbx1 deletion phenotypes. Our results reveal a complex regulation system through which the transcription factors Ace2, Fkh2, Sep1 and Mbx1 in combination control the expression of the genes involved in separation at the end of the cell division cycle.

The Carboxy-Terminal Tails of Septins Cdc11 and Shs1 Recruit Myosin-II Binding Factor Bni5 to the Bud Neck in Saccharomyces cerevisiae

Septins are a conserved family of GTP-binding proteins that form heterooctameric complexes that assemble into higher-order structures. In yeast, septin superstructure at the bud neck serves as a barrier to separate a daughter cell from its mother and as a scaffold to recruit the proteins that execute cytokinesis. However, how septins recruit specific factors has not been well characterized. In the accompanying article in this issue, (Finnigan et al. 2015), we demonstrated that the C-terminal extensions (CTEs) of the alternative terminal subunits of septin heterooctamers, Cdc11 and Shs1, share a role required for optimal septin function in vivo. Here we describe our use of unbiased genetic approaches (both selection of dosage suppressors and analysis of synthetic interactions) that pinpointed Bni5 as a protein that interacts with the CTEs of Cdc11 and Shs1. Furthermore, we used three independent methods-construction of chimeric proteins, noncovalent tethering mediated by a GFP-targeted nanobody, and imaging by fluorescence microscopy-to confirm that a physiologically important function of the CTEs of Cdc11 and Shs1 is optimizing recruitment of Bni5 and thereby ensuring efficient localization at the bud neck of Myo1, the type II myosin of the actomyosin contractile ring.Related article in

GENETICS

Finnigan, G. C. et al., 2015 Comprehensive Genetic Analysis of Paralogous Terminal Septin Subunits Shs1 and Cdc11 in Saccharomyces cerevisiae. Genetics 200: 841-861.

Rho4 interaction with exocyst and septins regulates cell separation in fission yeast

Rho GTPases are small proteins present in all eukaryotic cells, from yeast to mammals, with a function in actin organization and morphogenetic processes. Schizosaccharomyces pombe Rho4 is not essential but it displays a role during cell separation at high temperature. In fact, Rho4 is involved in the secretion of the hydrolytic enzymes that are required for cell septum degradation during this process. In rho4Δ cells, vesicles accumulate in the septum area and the glucanases Eng1 and Agn1 are not secreted to the culture medium. The localization of Eng1 and Agn1 depends on the exocyst and the septins. The exocyst is a conserved multiprotein complex important for the targeting and fusion of Golgi-derived vesicles with the plasma membrane. Septins are a family of GTP-binding proteins conserved in eukaryotes that function during cytokinesis. Here we show that Rho4 is required for the proper localization of the exocyst and septins at high temperature. Moreover, pull-down experiments demonstrate that Rho4 can interact with exocyst subunits, such as Sec8 and Exo70, and septin proteins, such as Spn3. We observe that Sec8 preferentially binds to activated GTP-Rho4, suggesting that Sec8 could be an effector of this GTPase. We propose that the interaction of Rho4 with the exocyst and septins confers a precise regulation for the secretion of glucanases at the appropriate place and time during the cell cycle.

The contractile ring coordinates curvature-dependent septum assembly during fission yeast cytokinesis

Cytokinesis in fission yeast is accomplished by inward growth of the cell wall septum guided by the contractile ring. The ring promotes local septum growth in a curvature-dependent manner, allowing even a misshapen septum to grow into a more regular shape. This suggests that the ring regulates cell wall assembly through a mechanosensitive mechanism.

The functions of the actin-myosin–based contractile ring in cytokinesis remain to be elucidated. Recent findings show that in the fission yeast Schizosaccharomyces pombe, cleavage furrow ingression is driven by polymerization of cell wall fibers outside the plasma membrane, not by the contractile ring. Here we show that one function of the ring is to spatially coordinate septum cell wall assembly. We develop an improved method for live-cell imaging of the division apparatus by orienting the rod-shaped cells vertically using microfabricated wells. We observe that the septum hole and ring are circular and centered in wild-type cells and that in the absence of a functional ring, the septum continues to ingress but in a disorganized and asymmetric manner. By manipulating the cleavage furrow into different shapes, we show that the ring promotes local septum growth in a curvature-dependent manner, allowing even a misshapen septum to grow into a more regular shape. This curvature-dependent growth suggests a model in which contractile forces of the ring shape the septum cell wall by stimulating the cell wall machinery in a mechanosensitive manner. Mechanical regulation of the cell wall assembly may have general relevance to the morphogenesis of walled cells.

The putative exchange factor Gef3p interacts with Rho3p GTPase and the septin ring during cytokinesis in fission yeast

The small GTP-binding proteins of the Rho family and its regulatory proteins play a central role in cytokinetic actomyosin ring assembly and cytokinesis. Here we show that the fission yeast guanine nucleotide exchange factor Gef3p interacts with Rho3p at the division site. Gef3p contains a putative DH homology domain and a BAR/IMD-like domain. The protein localized to the division site late in mitosis, where it formed a ring that did not constrict with actomyosin ring (cytokinetic actomyosin ring) invagination; instead, it split into a double ring that resembled the septin ring. Gef3p co-localized with septins and Mid2p and required septins and Mid2p for its localization. Gef3p interacts physically with the GTP-bound form of Rho3p. Although Gef3p is not essential for cell separation, the simultaneous disruption of gef3(+) and Rho3p-interacting proteins, such as Sec8p, an exocyst component, Apm1p, a subunit of the clathrin adaptor complex or For3p, an actin-polymerizing protein, yielded cells with strong defects in septation and polarity respectively. Our results suggest that interactions between septins and Rho-GEFs provide a new targeting mechanism for GTPases in cytokinesis, in this case probably contributing to Rho3p function in vesicle tethering and vesicle trafficking in the later steps of cell separation.

Proteins involved in building, maintaining and remodeling of yeast cell walls

The cell wall defines the shape and provides osmotic stability to the yeast cell. It also serves to anchor proteins required for communication of the yeast cell with surrounding molecules and other cells. It is synthesized as a complex structure with β-1,3-glucan chains forming the basic network to which β-1,6-glucan, chitin and a number of mannoproteins are attached. Synthesis, maintaining and remodeling of this complex structure require a set of different synthases, hydrolases and transglycosidases whose concerted activities provide necessary firmness but at the same time flexibility of the wall moiety. The present state of comprehension of the interplay of these proteins in the yeast cell wall is the subject of this article.

Extracellular cell wall β(1,3)glucan is required to couple septation to actomyosin ring contraction

β(1,3)glucan is critical for contractile ring positioning and for coupling septum synthesis to constriction of the contractile ring and plasma membrane extension during cytokinesis.

Cytokinesis has been extensively studied in different models, but the role of the extracellular cell wall is less understood. Here we studied this process in fission yeast. The essential protein Bgs4 synthesizes the main cell wall β(1,3)glucan. We show that Bgs4-derived β(1,3)glucan is required for correct and stable actomyosin ring positioning in the cell middle, before the start of septum formation and anchorage to the cell wall. Consequently, β(1,3)glucan loss generated ring sliding, oblique positioned rings and septa, misdirected septum synthesis indicative of relaxed rings, and uncoupling between a fast ring and membrane ingression and slow septum synthesis, suggesting that cytokinesis can progress with defective septum pushing and/or ring pulling forces. Moreover, Bgs4-derived β(1,3)glucan is essential for secondary septum formation and correct primary septum completion. Therefore, our results show that extracellular β(1,3)glucan is required for cytokinesis to connect the cell wall with the plasma membrane and for contractile ring function, as proposed for the equivalent extracellular matrix in animal cells.

Mitotic exit and separation of mother and daughter cells

Productive cell proliferation involves efficient and accurate splitting of the dividing cell into two separate entities. This orderly process reflects coordination of diverse cytological events by regulatory systems that drive the cell from mitosis into G1. In the budding yeast Saccharomyces cerevisiae, separation of mother and daughter cells involves coordinated actomyosin ring contraction and septum synthesis, followed by septum destruction. These events occur in precise and rapid sequence once chromosomes are segregated and are linked with spindle organization and mitotic progress by intricate cell cycle control machinery. Additionally, critical paarts of the mother/daughter separation process are asymmetric, reflecting a form of fate specification that occurs in every cell division. This chapter describes central events of budding yeast cell separation, as well as the control pathways that integrate them and link them with the cell cycle.

SUN proteins belong to a novel family of β-(1,3)-glucan-modifying enzymes involved in fungal morphogenesis

BACKGROUND

SUN proteins are involved in yeast morphogenesis, but their function is unknown.

RESULTS

SUN protein plays a role in the Aspergillus fumigatus morphogenesis. Biochemical properties of recombinant SUN proteins were elucidated.

CONCLUSION

Both Candida albicans and Aspergillus fumigatus sun proteins show a β-(1,3)-glucanase activity.

SIGNIFICANCE

The mode of action of SUN proteins on β-(1,3)-glucan is unique, new, and original. In yeasts, the family of SUN proteins has been involved in cell wall biogenesis. Here, we report the characterization of SUN proteins in a filamentous fungus, Aspergillus fumigatus. The function of the two A. fumigatus SUN genes was investigated by combining reverse genetics and biochemistry. During conidial swelling and mycelial growth, the expression of AfSUN1 was strongly induced, whereas the expression of AfSUN2 was not detectable. Deletion of AfSUN1 negatively affected hyphal growth and conidiation. A closer examination of the morphological defects revealed swollen hyphae, leaky tips, intrahyphal growth, and double cell wall, suggesting that, like in yeast, AfSun1p is associated with cell wall biogenesis. In contrast to AfSUN1, deletion of AfSUN2 either in the parental strain or in the AfSUN1 single mutant strain did not affect colony and hyphal morphology. Biochemical characterization of the recombinant AfSun1p and Candida albicans Sun41p showed that both proteins had a unique hydrolysis pattern: acting on β-(1,3)-oligomers from dimer up to insoluble β-(1,3)-glucan. Referring to the CAZy database, it is clear that fungal SUN proteins represent a new family of glucan hydrolases (GH132) and play an important morphogenetic role in fungal cell wall biogenesis and septation.

Hof1 and Rvs167 have redundant roles in actomyosin ring function during cytokinesis in budding yeast

The Hof1 protein (Homologue of Fifteen) regulates formation of the primary septum during cytokinesis in the budding yeast Saccharomyces cerevisiae, whereas the orthologous Cdc15 protein in fission yeast regulates the actomyosin ring by using its F-BAR domain to recruit actin nucleators to the cleavage site. Here we show that budding yeast Hof1 also contributes to actin ring assembly in parallel with the Rvs167 protein. Simultaneous deletion of the HOF1 and RVS167 genes is lethal, and cells fail to assemble the actomyosin ring as they progress through mitosis. Although Hof1 and Rvs167 are not orthologues, they both share an analogous structure, with an F-BAR or BAR domain at the amino terminus, capable of inducing membrane curvature, and SH3 domains at the carboxyl terminus that bind to specific proline-rich targets. The SH3 domain of Rvs167 becomes essential for assembly of the actomyosin ring in cells lacking Hof1, suggesting that it helps to recruit a regulator of the actin cytoskeleton. This new function of Rvs167 appears to be independent of its known role as a regulator of the Arp2/3 actin nucleator, as actin ring assembly is not abolished by the simultaneous inactivation of Hof1 and Arp2/3. Instead we find that recruitment to the bud-neck of the Iqg1 actin regulator is defective in cells lacking Hof1 and Rvs167, though future studies will be needed to determine if this reflects a direct interaction between these factors. The redundant role of Hof1 in actin ring assembly suggests that the mechanism of actin ring assembly has been conserved to a greater extent across evolution than anticipated previously.

The Kin1 kinase and the calcineurin phosphatase cooperate to link actin ring assembly and septum synthesis in fission yeast

BACKGROUND INFORMATION

The Kin1 protein kinase of fission yeast, which regulates cell surface cohesiveness during interphase cell growth, is also present at the cell division site during mitosis; however, its function in cell division has remained elusive.

RESULTS

In FK506-mediated calcineurin deficient cells, mitosis is extended and ring formation is transiently compromised but septation remains normal. Here, we show that Kin1 inhibition in these cells leads to polyseptation and defects in membrane closure. Actomyosin ring disassembly is prevented and ultimately the daughter cells fail to separate. We show that the Pmk1 MAP kinase pathway and the type V myosin Myo4 act downstream of the cytokinetic function of Kin1. Kin1 inhibition also promotes polyseptation in myo3Δ, a type II myosin heavy-chain mutant defective in ring assembly. In contrast, Kin1 inactivation rescues septation in a myosin light-chain cdc4-8 thermosensitive mutant. A structure/function analysis of the Kin1 protein sequence identified a novel motif outside the kinase domain that is important for its polarised localisation and its catalytic activity. This motif is remarkably conserved in all fungal Kin1 homologues but is absent in related kinases of metazoans.

CONCLUSIONS

We conclude that calcineurin and Kin1 activities must be tightly coordinated to link actomyosin ring assembly with septum synthesis and membrane closure and to ensure separation of the daughter cells.

Blastomyces dermatitidis septins CDC3, CDC10, and CDC12 impact the morphology of yeast and hyphae, but are not required for the phase transition

Blastomyces dermatitidis, the etiologic agent of blastomycosis, belongs to a group of thermally dimorphic fungi that change between mold (22°C) and yeast (37°C) in response to temperature. The contribution of structural proteins such as septins to this phase transition in these fungi remains poorly understood. Septins are GTPases that serve as a scaffold for proteins involved with cytokinesis, cell polarity, and cell morphology. In this study, we use a GFP sentinel RNA interference system to investigate the impact of CDC3, CDC10, CDC12, and ASPE on the morphology and phase transition of B. dermatitidis. Targeting CDC3, CDC10, and CDC12 by RNA interference resulted in yeast with aberrant morphology at 37°C with defects in cytokinesis. Downshifting the temperature to 22°C promoted the conversion to the mold phase, but did not abrogate the morphologic defects. CDC3, CDC10, and CDC12 knockdown strains grew as mold with curved, thickened hyphae. Knocking down ASPE transcript did not alter morphology of yeast at 37°C or mold at 22°C. Following an increase in temperature from 22°C to 37°C, all septin knockdown strains were able to revert to yeast. In conclusion, CDC3, CDC10, and CDC12 septin- encoding genes are required for proper morphology of yeast and hyphae, but are dispensable for the phase transition.

An S/T-Q cluster domain census unveils new putative targets under Tel1/Mec1 control

Background

The cellular response to DNA damage is immediate and highly coordinated in order to maintain genome integrity and proper cell division. During the DNA damage response (DDR), the sensor kinases Tel1 and Mec1 in Saccharomyces cerevisiae and ATM and ATR in human, phosphorylate multiple mediators which activate effector proteins to initiate cell cycle checkpoints and DNA repair. A subset of kinase substrates are recognized by the S/T-Q cluster domain (SCD), which contains motifs of serine (S) or threonine (T) followed by a glutamine (Q). However, the full repertoire of proteins and pathways controlled by Tel1 and Mec1 is unknown.

Results

To identify all putative SCD-containing proteins, we analyzed the distribution of S/T-Q motifs within verified Tel1/Mec1 targets and arrived at a unifying SCD definition of at least 3 S/T-Q within a stretch of 50 residues. This new SCD definition was used in a custom bioinformatics pipeline to generate a census of SCD-containing proteins in both yeast and human. In yeast, 436 proteins were identified, a significantly larger number of hits than were expected by chance. These SCD-containing proteins did not distribute equally across GO-ontology terms, but were significantly enriched for those involved in processes related to the DDR. We also found a significant enrichment of proteins involved in telophase and cytokinesis, protein transport and endocytosis suggesting possible novel Tel1/Mec1 targets in these pathways. In the human proteome, a wide range of similar proteins were identified, including homologs of some SCD-containing proteins found in yeast. This list also included high concentrations of proteins in the Mediator, spindle pole body/centrosome and actin cytoskeleton complexes.

Conclusions

Using a bioinformatic approach, we have generated a census of SCD-containing proteins that are involved not only in known DDR pathways but several other pathways under Tel1/Mec1 control suggesting new putative targets for these kinases.

Architecture and biosynthesis of the Saccharomyces cerevisiae cell wall

The wall gives a Saccharomyces cerevisiae cell its osmotic integrity; defines cell shape during budding growth, mating, sporulation, and pseudohypha formation; and presents adhesive glycoproteins to other yeast cells. The wall consists of β1,3- and β1,6-glucans, a small amount of chitin, and many different proteins that may bear N- and O-linked glycans and a glycolipid anchor. These components become cross-linked in various ways to form higher-order complexes. Wall composition and degree of cross-linking vary during growth and development and change in response to cell wall stress. This article reviews wall biogenesis in vegetative cells, covering the structure of wall components and how they are cross-linked; the biosynthesis of N- and O-linked glycans, glycosylphosphatidylinositol membrane anchors, β1,3- and β1,6-linked glucans, and chitin; the reactions that cross-link wall components; and the possible functions of enzymatic and nonenzymatic cell wall proteins.

Inn1 and Cyk3 regulate chitin synthase during cytokinesis in budding yeasts

The chitin synthase that makes the primary septum during cell division in budding yeasts is an important therapeutic target with an unknown activation mechanism. We previously found that the C2-domain of the Saccharomyces cerevisiae Inn1 protein plays an essential but uncharacterised role at the cleavage site during cytokinesis. By combining a novel degron allele of INN1 with a point mutation in the C2-domain, we screened for mutations in other genes that suppress the resulting defect in cell division. In this way, we identified 22 dominant mutations of CHS2 (chitin synthase II) that map to two neighbouring sites in the catalytic domain. Chs2 in isolated cell membranes is normally nearly inactive (unless protease treatment is used to bypass inhibition); however, the dominant suppressor allele Chs2-V377I has enhanced activity in vitro. We show that Inn1 associates with Chs2 in yeast cell extracts. It also interacts in a yeast two-hybrid assay with the N-terminal 65% of Chs2, which contains the catalytic domain. In addition to compensating for mutations in the Inn1 C2-domain, the dominant CHS2 alleles suppress cytokinesis defects produced by the lack of the Cyk3 protein. Our data support a model in which the C2-domain of Inn1 acts in conjunction with Cyk3 to regulate the catalytic domain of Chs2 during cytokinesis. These findings suggest novel approaches for developing future drugs against important fungal pathogens.