CHS5, a gene involved in chitin synthesis and mating in Saccharomyces cerevisiae

Antifungal effects of carvacrol, the main volatile compound in Origanum vulgare L. essential oil, against Aspergillus flavus in postharvest wheat

Plant volatile organic compounds (VOCs) with antimicrobial activity could potentially be extremely useful fumigants to prevent and control the fungal decay of agricultural products postharvest. In this study, antifungal effects of volatile compounds in essential oils extracted from Origanum vulgare L. against Aspergillus flavus growth were investigated using transcriptomic and biochemical analyses. Carvacrol was identified as the major volatile constituent of the Origanum vulgare L. essential oil, accounting for 66.01 % of the total content. The minimum inhibitory concentrations of carvacrol were 0.071 and 0.18 μL/mL in gas-phase fumigation and liquid contact, respectively. Fumigation with 0.60 μL/mL of carvacrol could completely inhibit A. flavus proliferation in wheat grains with 20 % moisture, showing its potential as a biofumigant. Scanning electron microscopy revealed that carvacrol treatment caused morphological deformation of A. flavus mycelia, and the resulting increased electrolyte leakage indicates damage to the plasma membrane. Confocal laser scanning microscopy confirmed that the carvacrol treatment caused a decrease in mitochondrial membrane potential, reactive oxygen species accumulation, and DNA damage. Transcriptome analysis revealed that differentially expressed genes were mainly associated with fatty acid degradation, autophagy, peroxisomes, the tricarboxylic acid cycle, oxidative phosphorylation, and DNA replication in A. flavus mycelia exposed to carvacrol. Biochemical analyses of hydrogen peroxide and superoxide anion content, and catalase, superoxide dismutase, and glutathione S-transferase activities showed that carvacrol induced oxidative stress in A. flavus, which agreed with the transcriptome results. In summary, this study provides an experimental basis for the use of carvacrol as a promising biofumigant for the prevention of A. flavus contamination during postharvest grain storage.

Recent Advances in Chitin Biosynthesis Associated with the Morphology and Secondary Metabolite Synthesis of Filamentous Fungi in Submerged Fermentation

Metabolites produced by filamentous fungi are used extensively in the food and drug industries. With the development of the morphological engineering of filamentous fungi, numerous biotechnologies have been applied to alter the morphology of fungal mycelia and enhance the yields and productivity of target metabolites during submerged fermentation. Disruption of chitin biosynthesis can modify the cell growth and mycelial morphology of filamentous fungi and regulate the biosynthesis of metabolites during submerged fermentation. In this review, we present a comprehensive coverage of the categories and structures of the enzyme chitin synthase, chitin biosynthetic pathways, and the association between chitin biosynthesis and cell growth and metabolism in filamentous fungi. Through this review, we hope to increase awareness of the metabolic engineering of filamentous fungal morphology, provide insights into the molecular mechanisms of morphological control via chitin biosynthesis, and describe strategies for the application of morphological engineering to enhance the production of target metabolites in filamentous fungi during submerged fermentation.

SERCA interacts with chitin synthase and participates in cuticular chitin biogenesis in Drosophila

The biogenesis of chitin, a major structural polysaccharide found in the cuticle and peritrophic matrix, is crucial for insect growth and development. Chitin synthase, a membrane-integral β-glycosyltransferase, has been identified as the core of the chitin biogenesis machinery. However, a yet unknown number of auxiliary proteins appear to assist in chitin biosynthesis, whose precise function remains elusive. Here, we identified a sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA), in the fruit fly Drosophila melanogaster, as a chitin biogenesis-associated protein. The physical interaction between DmSERCA and epidermal chitin synthase (Krotzkopf verkehrt, Kkv) was demonstrated and analyzed using split-ubiquitin membrane yeast two-hybrid, bimolecular fluorescent complementation, pull-down, and immunoprecipitation assays. The interaction involves N-terminal regions (aa 48-81 and aa 247-33) and C-terminal regions (aa 743-783 and aa 824-859) of DmSERCA and two N-terminal regions (aa 121-179 and aa 369-539) of Kkv, all of which are predicted be transmembrane helices. While tissue-specific knock-down of DmSERCA in the epidermis caused larval and pupal lethality, the knock-down of DmSERCA in wings resulted in smaller and crinkled wings, a significant decrease in chitin deposition, and the loss of chitin lamellar structure. Although DmSERCA is well-known for its role in muscular contraction, this study reveals a novel role in chitin synthesis, contributing to our knowledge on the machinery of chitin biogenesis.

Exomer Is Part of a Hub Where Polarized Secretion and Ionic Stress Connect

Plasma membrane and membranous organelles contribute to the physiology of the Eukaryotic cell by participating in vesicle trafficking and the maintenance of ion homeostasis. Exomer is a protein complex that facilitates vesicle transport from the trans-Golgi network to the plasma membrane, and its absence leads to the retention of a set of selected cargoes in this organelle. However, this retention does not explain all phenotypes observed in exomer mutants. The Schizosaccharomyces pombe exomer is composed of Cfr1 and Bch1, and cfr1Δ and bch1Δ were sensitive to high concentrations of potassium salts but not sorbitol, which showed sensitivity to ionic but not osmotic stress. Additionally, the activity of the plasma membrane ATPase was higher in exomer mutants than in the wild-type, pointing to membrane hyperpolarization, which caused an increase in intracellular K⁺ content and mild sensitivity to Na⁺, Ca²⁺, and the aminoglycoside antibiotic hygromycin B. Moreover, in response to K⁺ shock, the intracellular Ca²⁺ level of cfr1Δ cells increased significantly more than in the wild-type, likely due to the larger Ca²⁺ spikes in the mutant. Microscopy analyses showed a defective endosomal morphology in the mutants. This was accompanied by an increase in the intracellular pools of the K⁺ exporting P-type ATPase Cta3 and the plasma membrane Transient Receptor Potential (TRP)-like Ca²⁺ channel Pkd2, which were partially diverted from the trans-Golgi network to the prevacuolar endosome. Despite this, most Cta3 and Pkd2 were delivered to the plasma membrane at the cell growing sites, showing that their transport from the trans-Golgi network to the cell surface occurred in the absence of exomer. Nevertheless, shortly after gene expression in the presence of KCl, the polarized distribution of Cta3 and Pkd2 in the plasma membrane was disturbed in the mutants. Finally, the use of fluorescent probes suggested that the distribution and dynamics of association of some lipids to the plasma membrane in the presence of KCl were altered in the mutants. Thus, exomer participation in the response to K⁺ stress was multifaceted. These results supported the notion that exomer plays a general role in protein sorting at the trans-Golgi network and in polarized secretion, which is not always related to a function as a selective cargo adaptor.

RNA-seq transcriptomic analysis of green tea polyphenols regulation of differently expressed genes in Saccharomyces cerevisiae under ethanol stress

Saccharomyces cerevisiae has been widely used to produce alcoholic beverages and bio-fuels; however, its performance is remarkably compromised by the increased ethanol concentration during the fermentation process. In this study, RNA-sequence analysis was used to investigate the protective effect of green tea polyphenols (GTP) on S. cerevisiae cells from ethanol-induced damage. GO and KEGG analysis showed that to deal with the stress of ethanol, large amounts of genes related to cell wall, cell membrane, basic metabolism and redox regulation were significantly differentially expressed (P < 0.05), while these undesired changes could be partly relieved by administration of GTP, suggesting its potential to enhance the ethanol tolerance of S. cerevisiae. The present study provided a global view of the transcriptomic changes of S. cerevisiae in response to the accumulation of ethanol and the treatment of GTP, which might deepen our understanding about S. cerevisiae and the fermentation process, and thus benefit the development of the bioethanol production industry.

Cinnamaldehyde Exerts Its Antifungal Activity by Disrupting the Cell Wall Integrity of Geotrichum citri-aurantii

Our previous study showed that cinnamaldehyde (CA) significantly inhibited the mycelial growth of Geotrichum citri-aurantii, one of the main postharvest pathogens in citrus fruits. This study investigated the antifungal mechanism of CA against G. citri-aurantii. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images showed that CA treatment led to clear morphological changes in the cell walls and membranes of G. citri-aurantii. However, the membrane integrity, total lipids and ergosterol contents were not apparently affected by CA treatment. Notably, the extracellular alkaline phosphatase (AKP) activity was increased after CA treatment, suggesting impairment in cell wall permeability. A weakened fluorescence in the cell wall, a decrease in the chitin contents, and changes of ten genes involved in cell wall integrity were also observed. These results suggested that CA may exhibit its antifungal activity against G. citri-aurantii by interfering the build of cell wall and therefore lead to the damage of cell wall permeability and integrity.

Imaging the secretory compartments involved in the intracellular traffic of CHS-4, a class IV chitin synthase, in Neurospora crassa

In Neurospora crassa hyphae the localization of all seven chitin synthases (CHSs) at the Spitzenkörper (SPK) and at developing septa has been well analyzed. Hitherto, the mechanisms of CHSs traffic and sorting from synthesis to delivery sites remain largely unexplored. In Saccharomyces cerevisiae exit of Chs3p from the endoplasmic reticulum (ER) requires chaperone Chs7p. Here, we analyzed the role of CSE-7, N. crassa Chs7p orthologue, in the biogenesis of CHS-4 (orthologue of Chs3p). In a N. crassa Δcse-7 mutant, CHS-4-GFP no longer accumulated at the SPK and septa. Instead, fluorescence was retained in hyphal subapical regions in an extensive network of elongated cisternae (NEC) referred to previously as tubular vacuoles. In a complemented strain expressing a copy of cse-7 the localization of CHS-4-GFP at the SPK and septa was restored, providing evidence that CSE-7 is necessary for the localization of CHS-4 at hyphal tips and septa. CSE-7 was revealed at delimited regions of the ER at the immediacies of nuclei, at the NEC, and remarkably also at septa and the SPK. The organization of the NEC was dependent on the cytoskeleton. SEC-63, an extensively used ER marker, and NCA-1, a SERCA-type ATPase previously localized at the nuclear envelope, were used as markers to discern the nature of the membranes containing CSE-7. Both SEC-63 and NCA-1 were found at the nuclear envelope, but also at regions of the NEC. However, at the NEC only NCA-1 co-localized extensively with CSE-7. Observations by transmission electron microscopy revealed abundant rough ER sheets and distinct electron translucent smooth flattened cisternae, which could correspond collectively to the NEC, thorough the subapical cytoplasm. This study identifies CSE-7 as the putative ER receptor for its cognate cargo, the polytopic membrane protein CHS-4, and elucidates the complexity of the ER system in filamentous fungi.

Traffic Through the Trans-Golgi Network and the Endosomal System Requires Collaboration Between Exomer and Clathrin Adaptors in Fission Yeast

Despite its biological and medical relevance, traffic from the Golgi to the plasma membrane (PM) is one of the least understood steps of secretion. Exomer is a protein complex that mediates the trafficking of certain cargoes from the trans-Golgi network/early endosomes to the PM in budding yeast. Here, we show that in Schizosaccharomyces pombe the Cfr1 and Bch1 proteins constitute the simplest form of an exomer. Cfr1 co-immunoprecipitates with Assembly Polypeptide adaptor 1 (AP-1), AP-2, and Golgi-localized, gamma-adaptin ear domain homology, ARF-binding (GGA) subunits, and cfr1⁺ interacts genetically with AP-1 and GGA genes. Exomer-defective cells exhibit multiple mild defects, including alterations in the morphology of Golgi stacks and the distribution of the synaptobrevin-like Syb1 protein, carboxypeptidase missorting, and stress sensitivity. S. pombe apm1Δ cells exhibit a defect in trafficking through the early endosomes that is severely aggravated in the absence of exomer. apm1Δ cfr1Δ cells exhibit a dramatic disorganization of intracellular compartments, including massive accumulation of electron-dense tubulovesicular structures. While the trans-Golgi network/early endosomes are severely disorganized in the apm1Δ cfr1Δ strain, gga21Δ gga22Δ cfr1Δ cells exhibit a significant disturbance of the prevacuolar/vacuolar compartments. Our findings show that exomer collaborates with clathrin adaptors in trafficking through diverse cellular compartments, and that this collaboration is important to maintain their integrity. These results indicate that the effect of eliminating exomer is more pervasive than that described to date, and suggest that exomer complexes might participate in diverse steps of vesicle transport in other organisms.

The Pch2 AAA+ ATPase promotes phosphorylation of the Hop1 meiotic checkpoint adaptor in response to synaptonemal complex defects

Meiotic cells possess surveillance mechanisms that monitor critical events such as recombination and chromosome synapsis. Meiotic defects resulting from the absence of the synaptonemal complex component Zip1 activate a meiosis-specific checkpoint network resulting in delayed or arrested meiotic progression. Pch2 is an evolutionarily conserved AAA+ ATPase required for the checkpoint-induced meiotic block in the zip1 mutant, where Pch2 is only detectable at the ribosomal DNA array (nucleolus). We describe here that high levels of the Hop1 protein, a checkpoint adaptor that localizes to chromosome axes, suppress the checkpoint defect of a zip1 pch2 mutant restoring Mek1 activity and meiotic cell cycle delay. We demonstrate that the critical role of Pch2 in this synapsis checkpoint is to sustain Mec1-dependent phosphorylation of Hop1 at threonine 318. We also show that the ATPase activity of Pch2 is essential for its checkpoint function and that ATP binding to Pch2 is required for its localization. Previous work has shown that Pch2 negatively regulates Hop1 chromosome abundance during unchallenged meiosis. Based on our results, we propose that, under checkpoint-inducing conditions, Pch2 also possesses a positive action on Hop1 promoting its phosphorylation and its proper distribution on unsynapsed chromosome axes.

The yeast Arf-GAP Glo3p is required for the endocytic recycling of cell surface proteins

Small GTP-binding proteins of the Ras superfamily play diverse roles in intracellular trafficking. Among them, the Rab, Arf, and Rho families function in successive steps of vesicle transport, in forming vesicles from donor membranes, directing vesicle trafficking toward target membranes and docking vesicles onto target membranes. These proteins act as molecular switches that are controlled by a cycle of GTP binding and hydrolysis regulated by guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). In this study we explored the role of GAPs in the regulation of the endocytic pathway using fluorescently labeled yeast mating pheromone α-factor. Among 25 non-essential GAP mutants, we found that deletion of the GLO3 gene, encoding Arf-GAP protein, caused defective internalization of fluorescently labeled α-factor. Quantitative analysis revealed that glo3Δ cells show defective α-factor binding to the cell surface. Interestingly, Ste2p, the α-factor receptor, was mis-localized from the plasma membrane to the vacuole in glo3Δ cells. Domain deletion mutants of Glo3p revealed that a GAP-independent function, as well as the GAP activity, of Glo3p is important for both α-factor binding and Ste2p localization at the cell surface. Additionally, we found that deletion of the GLO3 gene affects the size and number of Arf1p-residing Golgi compartments and causes a defect in transport from the TGN to the plasma membrane. Furthermore, we demonstrated that glo3Δ cells were defective in the late endosome-to-TGN transport pathway, but not in the early endosome-to-TGN transport pathway. These findings suggest novel roles for Arf-GAP Glo3p in endocytic recycling of cell surface proteins.

Proteins involved in flor yeast carbon metabolism under biofilm formation conditions

A lack of sugars during the production of biologically aged wines after fermentation of grape must causes flor yeasts to metabolize other carbon molecules formed during fermentation (ethanol and glycerol, mainly). In this work, a proteome analysis involving OFFGEL fractionation prior to LC/MS detection was used to elucidate the carbon metabolism of a flor yeast strain under biofilm formation conditions (BFC). The results were compared with those obtained under non-biofilm formation conditions (NBFC). Proteins associated to processes such as non-fermentable carbon uptake, the glyoxylate and TCA cycles, cellular respiration and inositol metabolism were detected at higher concentrations under BFC than under the reference conditions (NBFC). This study constitutes the first attempt at identifying the flor yeast proteins responsible for the peculiar sensory profile of biologically aged wines. A better metabolic knowledge of flor yeasts might facilitate the development of effective strategies for improved production of these special wines.

Comparative pathobiology of Heterobasidion annosum during challenge on Pinus sylvestris and Arabidopsis roots: an analysis of defensin gene expression in two pathosystems

Heterobasidion annosum is widely known as a major root and butt rot pathogen of conifer trees, but little information is available on its interaction with the roots of herbaceous angiosperm plants. We investigated the infection biology of H. annosum during challenge with the angiosperm model Arabidopsis and monitored the host response after exposure to different hormone elicitors, chemicals (chitin, glucan and chitosan) and fungal species that represent diverse basidiomycete life strategies [e.g., pathogen (H. annosum), saprotroph (Stereum sanguinolentum) and mutualist (Lactarius rufus)]. The results revealed that the tree pathogen (H. annosum) and the saprotroph (S. sanguinolentum) could infect the Col-8 (Columbia) ecotype of Arabidopsis in laboratory inoculation experiments. Germinated H. annosum spores had appressorium-like penetration structures attached to the surface of the Arabidopsis roots. Subsequent invasive fungal growth led to the disintegration of the vascular region of the root tissues. Progression of root rot symptoms in Arabidopsis was similar to the infection development that was previously documented in Scots pine seedlings. Scots pine PsDef1 and Arabidopsis DEFLs (AT5G44973.1) and PDF1.2 were induced at the initial stage of the infection. However, differences in the expression patterns of the defensin gene homologs from the two plant groups were observed under various conditions, suggesting functional differences in their regulation. The potential use of the H. annosum-Arabidopsis pathosystem as a model for studying forest tree diseases is discussed.

Distinct roles of cell wall biogenesis in yeast morphogenesis as revealed by multivariate analysis of high-dimensional morphometric data

To better define how cell wall structure affects morphogenesis, the morphology of yeast cells was analyzed quantitatively after treatment with the three drugs that inhibit different aspects of cell wall synthesis. These drugs induced both similar effects, including broader necks and increased morphological variation, and distinct effects.

The cell wall of budding yeast is a rigid structure composed of multiple components. To thoroughly understand its involvement in morphogenesis, we used the image analysis software CalMorph to quantitatively analyze cell morphology after treatment with drugs that inhibit different processes during cell wall synthesis. Cells treated with cell wall–affecting drugs exhibited broader necks and increased morphological variation. Tunicamycin, which inhibits the initial step of N-glycosylation of cell wall mannoproteins, induced morphologies similar to those of strains defective in α-mannosylation. The chitin synthase inhibitor nikkomycin Z induced morphological changes similar to those of mutants defective in chitin transglycosylase, possibly due to the critical role of chitin in anchoring the β-glucan network. To define the mode of action of echinocandin B, a 1,3-β-glucan synthase inhibitor, we compared the morphology it induced with mutants of Fks1 that contains the catalytic domain for 1,3-β-glucan synthesis. Echinocandin B exerted morphological effects similar to those observed in some fks1 mutants, with defects in cell polarity and reduced glucan synthesis activity, suggesting that echinocandin B affects not only 1,3-β-glucan synthesis, but also another functional domain. Thus our multivariate analyses reveal discrete functions of cell wall components and increase our understanding of the pharmacology of antifungal drugs.

The exomer cargo adaptor features a flexible hinge domain

Exomer is a cargo adaptor that mediates the sorting of specific plasma membrane proteins into vesicles at the trans-Golgi network. Cargo adaptors must bind to multiple partners, including their cargo, regulatory proteins, and the membrane surface. During biogenesis of a vesicle, the membrane makes a transition from a relatively flat surface to one of high curvature, requiring cargo adaptors to somehow maintain protein-protein and protein-membrane interactions on a changing membrane environment. Here, we present the crystal structure of a tetrameric Chs5/Bch1 exomer complex and use small-angle X-ray scattering to demonstrate its flexibility in solution. The structural data suggest that the complex flexes primarily around the dimeric N-terminal domain of the Chs5 subunits, which adopts a noncanonical β sandwich fold. We propose that this flexible hinge domain enables exomer to maintain interactions in the context of a dynamic membrane environment.

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.

Sorting signals that mediate traffic of chitin synthase III between the TGN/endosomes and to the plasma membrane in yeast

Traffic of the integral yeast membrane protein chitin synthase III (Chs3p) from the trans-Golgi network (TGN) to the cell surface and to and from the early endosomes (EE) requires active protein sorting decoded by a number of protein coats. Here we define overlapping signals on Chs3p responsible for sorting in both exocytic and intracellular pathways by the coats exomer and AP-1, respectively. Residues 19DEESLL24, near the N-terminal cytoplasmically-exposed domain, comprise both an exocytic di-acidic signal and an intracellular di-leucine signal. Additionally we show that the AP-3 complex is required for the intracellular retention of Chs3p. Finally, residues R374 and W391, comprise another signal responsible for an exomer-independent alternative pathway that conveys Chs3p to the cell surface. These results establish a role for active protein sorting at the trans-Golgi en route to the plasma membrane (PM) and suggest a possible mechanism to regulate protein trafficking.

The exomer cargo adaptor structure reveals a novel GTPase-binding domain

Cargo adaptors control intracellular trafficking of transmembrane proteins by sorting them into membrane transport carriers. The COPI, COPII, and clathrin cargo adaptors are structurally well characterized, but other cargo adaptors remain poorly understood. Exomer is a specialized cargo adaptor that sorts specific proteins into trans-Golgi network (TGN)-derived vesicles in response to cellular signals. Exomer is recruited to the TGN by the Arf1 GTPase, a universally conserved trafficking regulator. Here, we report the crystal structure of a tetrameric exomer complex composed of two copies each of the Chs5 and Chs6 subunits. The structure reveals the FN3 and BRCT domains of Chs5, which together we refer to as the FBE domain (FN3-BRCT of exomer), project from the exomer core complex. The overall architecture of the FBE domain is reminiscent of the appendage domains of other cargo adaptors, although it exhibits a distinct topology. In contrast to appendage domains, which bind accessory factors, we show that the primary role of the FBE domain is to bind Arf1 for recruitment of exomer to membranes.

Chitinases in Pneumocystis carinii pneumonia

Pneumocystis pneumonia remains an important complication of immune suppression. The cell wall of Pneumocystis has been demonstrated to potently stimulate host inflammatory responses, with most studies focusing on β-glucan components of the Pneumocystis cell wall. In the current study, we have elaborated the potential role of chitins and chitinases in Pneumocystis pneumonia. We demonstrated differential host mammalian chitinase expression during Pneumocystis pneumonia. We further characterized a chitin synthase gene in Pneumocystis carinii termed Pcchs5, a gene with considerable homolog to the fungal chitin biosynthesis protein Chs5. We also observed the impact of chitinase digestion on Pneumocystis-induced host inflammatory responses by measuring TNFα release and mammalian chitinase expression by cultured lung epithelial and macrophage cells stimulated with Pneumocystis cell wall isolates in the presence and absence of exogenous chitinase digestion. These findings provide evidence supporting a chitin biosynthetic pathway in Pneumocystis organisms and that chitinases modulate inflammatory responses in lung cells. We further demonstrate lung expression of chitinase molecules during Pneumocystis pneumonia.

Regulation of expression, activity and localization of fungal chitin synthases

The fungal cell wall represents an attractive target for pharmacologic inhibition, as many of the components are fungal-specific. Though targeted inhibition of β-glucan synthesis is effective treatment for certain fungal infections, the ability of the cell wall to dynamically compensate via the cell wall integrity pathway may limit overall efficacy. To date, chitin synthesis inhibitors have not been successfully deployed in the clinical setting. Fungal chitin synthesis is a complex and highly regulated process. Regulation of chitin synthesis occurs on multiple levels, thus targeting of these regulatory pathways may represent an exciting alternative approach. A variety of signaling pathways have been implicated in chitin synthase regulation, at both transcriptional and post-transcriptional levels. Recent research suggests that localization of chitin synthases likely represents a major regulatory mechanism. However, much of the regulatory machinery is not necessarily shared among different chitin synthases. Thus, an in-depth understanding of the precise roles of each protein in cell wall maintenance and repair will be essential to identifying the most likely therapeutic targets.

Expression and β-glucan binding properties of Scots pine (Pinus sylvestris L.) antimicrobial protein (Sp-AMP)

Scots pine (Pinus sylvestris) secretes a number of small, highly-related, disulfide-rich proteins (Sp-AMPs) in response to challenges with fungal pathogens such as Heterobasidion annosum, although their biological role has been unknown. Here, we examined the expression patterns of these genes, as well as the structure and function of the encoded proteins. Northern blots and quantitative real time PCR showed increased levels of expression that are sustained during the interactions of host trees with pathogens, but not non-pathogens, consistent with a function in conifer tree defenses. Furthermore, the genes were up-regulated after treatment with salicylic acid and an ethylene precursor, 1-aminocyclopropane-1-carboxylic-acid, but neither methyl jasmonate nor H(2)O(2) induced expression, indicating that Sp-AMP gene expression is independent of the jasmonic acid signaling pathways. The cDNA encoding one of the proteins was cloned and expressed in Pichia pastoris. The purified protein had antifungal activity against H. annosum, and caused morphological changes in its hyphae and spores. It was directly shown to bind soluble and insoluble β-(1,3)-glucans, specifically and with high affinity. Furthermore, addition of exogenous glucan is linked to higher levels of Sp-AMP expression in the conifer. Homology modeling and sequence comparisons suggest that a conserved patch on the surface of the globular Sp-AMP is a carbohydrate-binding site that can accommodate approximately four sugar units. We conclude that these proteins belong to a new family of antimicrobial proteins (PR-19) that are likely to act by binding the glucans that are a major component of fungal cell walls.

The FN3 and BRCT motifs in the exomer component Chs5p define a conserved module that is necessary and sufficient for its function

Chs5p is a component of the exomer, a coat complex required to transport the chitin synthase Chs3p from the trans-Golgi network to the plasma membrane. The Chs5p N-terminal region exhibits fibronectin type III (FN3) and BRCT domains. FN3 domains are present in proteins that mediate adhesion processes, whereas BRCT domains are involved in DNA repair. Several fungi--including Schizosaccharomyces pombe, which has no detectable amounts of chitin--have proteins similar to Chs5p. Here we show that the FN3 and BRCT motifs in Chs5p behave as a module that is necessary and sufficient for Chs5p localization and for cargo delivery. The N-terminal regions of S. cerevisiae Chs5p and S. pombe Cfr1p are interchangeable in terms of Golgi localization, but not in terms of exomer assembly, showing that the conserved function of this module is protein retention in this organelle and that the interaction between the exomer components is organism-specific.

Identification of yeast genes involved in k homeostasis: loss of membrane traffic genes affects k uptake

Using the homozygous diploid Saccharomyces deletion collection, we searched for strains with defects in K(+) homeostasis. We identified 156 (of 4653 total) strains unable to grow in the presence of hygromycin B, a phenotype previously shown to be indicative of ion defects. The most abundant group was that with deletions of genes known to encode membrane traffic regulators. Nearly 80% of these membrane traffic defective strains showed defects in uptake of the K(+) homolog, (86)Rb(+). Since Trk1, a plasma membrane protein localized to lipid microdomains, is the major K(+) influx transporter, we examined the subcellular localization and Triton-X 100 insolubility of Trk1 in 29 of the traffic mutants. However, few of these showed defects in the steady state levels of Trk1, the localization of Trk1 to the plasma membrane, or the localization of Trk1 to lipid microdomains, and most defects were mild compared to wild-type. Three inositol kinase mutants were also identified, and in contrast, loss of these genes negatively affected Trk1 protein levels. In summary, this work reveals a nexus between K(+) homeostasis and membrane traffic, which does not involve traffic of the major influx transporter, Trk1.

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.

Yeast mating: a model system for studying cell and nuclear fusion

Haploid yeast cells mate to form a zygote, whose progeny are diploid cells. A fundamentally sexual event, related to fertilization, yeast mating nevertheless exhibits cytological properties that appear similar to somatic cell fusion. A large collection of mutations that lead to defects in various stages of mating, including cell fusion, has allowed a detailed dissection of the overall pathway. Recent advances in imaging methods, together with powerful methods of genetic analysis, make yeast mating a superb platform for investigation of cell fusion. An understanding of yeast cell fusion will provide insight into fundamental mechanisms of cell signaling, cell polarization, and membrane fusion.

The PKC, HOG and Ca2+ signalling pathways co-ordinately regulate chitin synthesis in Candida albicans

Chitin is an essential component of the fungal cell wall and its synthesis is under tight spatial and temporal regulation. The fungal human pathogen Candida albicans has a four member chitin synthase gene family comprising of CHS1 (class II), CHS2 (class I), CHS3 (class IV) and CHS8 (class I). LacZ reporters were fused to each CHS promoter to examine the transcriptional regulation of chitin synthesis. Each CHS promoter had a unique regulatory profile and responded to the addition of cell wall damaging agents, to mutations in specific CHS genes and exogenous Ca2+. The regulation of both CHS gene expression and chitin synthesis was co-ordinated by the PKC, HOG MAP kinase and Ca2+/calcineurin signalling pathways. Activation of these pathways also resulted in increased chitin synthase activity in vitro and elevated cell wall chitin content. Combinations of treatments that activated multiple pathways resulted in synergistic increases in CHS expression and in cell wall chitin content. Therefore, at least three pathways co-ordinately regulate chitin synthesis and activation of chitin synthesis operates at both transcriptional and post-transcriptional levels.

Chitin synthase III requires Chs4p-dependent translocation of Chs3p into the plasma membrane

In Saccharomyces cerevisiae, Chs4p is required for chitin synthase III (CSIII) activity and hence for chitin synthesis. This protein is transported in vesicles in a polarized fashion independently of the other Chs proteins. Its association with membranes depends not only on prenylation, but also on its interaction with other proteins, mainly Chs3p, which is the catalytic subunit of CSIII and is able to properly direct Chs4p to the bud neck in the absence of prenylation. Chs4p is present in functionally limiting amounts and its overexpression increases Chs3p accumulation at the plasma membrane with a concomitant increase in chitin synthesis. In the absence of Chs4p, Chs3p is delivered to the plasma membrane but fails to accumulate there because it is rapidly endocytosed and accumulates in intracellular vesicles. A blockade of endocytosis stops Chs3p internalization, triggering a significant increase in chitin synthesis. This blockade is independent of Chs4p function, allowing the accumulation of Chs3p at the plasma membrane even in the chs4Δ mutant. However, the absence of Chs4p renders CSIII functionally inactive, independently of Chs3p accumulation at the plasma membrane. Chs4p thus promotes Chs3p translocation into the plasma membrane in a stable and active form. Proper CSIII turnover is maintained through the endocytic internalization of Chs3p.

Central roles of small GTPases in the development of cell polarity in yeast and beyond

SUMMARY

The establishment of cell polarity is critical for the development of many organisms and for the function of many cell types. A large number of studies of diverse organisms from yeast to humans indicate that the conserved, small-molecular-weight GTPases function as key signaling proteins involved in cell polarization. The budding yeast Saccharomyces cerevisiae is a particularly attractive model because it displays pronounced cell polarity in response to intracellular and extracellular cues. Cells of S. cerevisiae undergo polarized growth during various phases of their life cycle, such as during vegetative growth, mating between haploid cells of opposite mating types, and filamentous growth upon deprivation of nutrition such as nitrogen. Substantial progress has been made in deciphering the molecular basis of cell polarity in budding yeast. In particular, it becomes increasingly clear how small GTPases regulate polarized cytoskeletal organization, cell wall assembly, and exocytosis at the molecular level and how these GTPases are regulated. In this review, we discuss the key signaling pathways that regulate cell polarization during the mitotic cell cycle and during mating.

The Hsp40 molecular chaperone Ydj1p, along with the protein kinase C pathway, affects cell-wall integrity in the yeast Saccharomyces cerevisiae

Molecular chaperones, such as Hsp40, regulate cellular processes by aiding in the folding, localization, and activation of multi-protein machines. To identify new targets of chaperone action, we performed a multi-copy suppressor screen for genes that improved the slow-growth defect of yeast lacking the YDJ1 chromosomal locus and expressing a defective Hsp40 chimera. Among the genes identified were MID2, which regulates cell-wall integrity, and PKC1, which encodes protein kinase C and is linked to cell-wall biogenesis. We found that ydj1delta yeast exhibit phenotypes consistent with cell-wall defects and that these phenotypes were improved by Mid2p or Pkc1p overexpression or by overexpression of activated downstream components in the PKC pathway. Yeast containing a thermosensitive allele in the gene encoding Hsp90 also exhibited cell-wall defects, and Mid2p or Pkc1p overexpression improved the growth of these cells at elevated temperatures. To determine the physiological basis for suppression of the ydj1delta growth defect, wild-type and ydj1delta yeast were examined by electron microscopy and we found that Mid2p overexpression thickened the mutant's cell wall. Together, these data provide the first direct link between cytoplasmic chaperone function and cell-wall integrity and suggest that chaperones orchestrate the complex biogenesis of this structure.

Exomer: A coat complex for transport of select membrane proteins from the trans-Golgi network to the plasma membrane in yeast

Ayeast plasma membrane protein, Chs3p, transits to the mother–bud neck from a reservoir comprising the trans-Golgi network (TGN) and endosomal system. Two TGN/endosomal peripheral proteins, Chs5p and Chs6p, and three Chs6p paralogues form a complex that is required for the TGN to cell surface transport of Chs3p. The role of these peripheral proteins has not been clear, and we now provide evidence that they create a coat complex required for the capture of membrane proteins en route to the cell surface. Sec7p, a Golgi protein required for general membrane traffic and functioning as a nucleotide exchange factor for the guanosine triphosphate (GTP)–binding protein Arf1p, is required to recruit Chs5p to the TGN surface in vivo. Recombinant forms of Chs5p, Chs6p, and the Chs6p paralogues expressed in baculovirus form a complex of approximately 1 MD that binds synthetic liposomes in a reaction requiring acidic phospholipids, Arf1p, and the nonhydrolyzable GTPγS. The complex remains bound to liposomes centrifuged on a sucrose density gradient. Thin section electron microscopy reveals a spiky coat structure on liposomes incubated with the full complex, Arf1p, and GTPγS. We termed the novel coat exomer for its role in exocytosis from the TGN to the cell surface. Unlike other coats (e.g., coat protein complex I, II, and clathrin/adaptor protein complex), the exomer does not form buds or vesicles on liposomes.

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.

Chs5/6 complex: a multiprotein complex that interacts with and conveys chitin synthase III from the trans-Golgi network to the cell surface

In Saccharomyces cerevisiae, the polysaccharide chitin is deposited at the mother bud junction by an integral membrane enzyme, chitin synthase 3 (Chs3p). The traffic of Chs3p to the cell surface from the trans-Golgi network (TGN) depends on two proteins, Chs5p and Chs6p, which sort selected cargo proteins into secretory vesicles. We have found that Chs5p forms a large higher-order complex of around 1 MDa with Chs6p and three Chs6 paralogs: Bch1p, Bud7p, and Bch2p. The Chs5/6 complex transiently interacts with its cargo, Chs3p, and the presence of Chs3p in the complex is dependent on every subunit. Chs5p and Chs6p have unique and crucial roles in Chs3p transport because either a chs5delta or chs6delta mutant drastically reduces the level of Chs3p bound to the remaining subunits of the complex. Bch1p and Bud7p appear to have a redundant function in Chs3p transport because deletion of both is necessary to displace Chs3p from the complex. The role of Bch2p in Chs3p binding is the least important. Chs5p is essential for structural integrity of the Chs5/6 complex and may act as a scaffold through which the other subunits assemble. Our results suggest a model of protein sorting at the TGN that involves a peripheral, possibly coat, complex that includes multiple related copies of a specificity determining subunit.

The Schizosaccharomyces pombe cfr1+ gene participates in mating through a new pathway that is independent of fus1+

Conjugation is a complex event directed to ensure the transfer of genetic material, which is achieved by the union of two cells. In fungi, success of this relevant process requires digestion of the cell wall at the point where both cells have agglutinated and, later, the union of the plasma membranes and nuclei from the mating partners. In order to gain information about cell fusion, we have cloned and disrupted the cfr1+ gene from the fission yeast Schizosaccharomyces pombe. cfr1+ gene is slightly induced at the beginning of mating but Cfr1p protein is degraded soon after the cells are transferred to nitrogen-lacking medium. cfr1Delta mutants present a defect in cell fusion owing to a failure in the digestion of the cell walls between the two parental cells. Finally, cytological and genetic analyses show that cfr1+ acts in a new pathway involved in conjugation that is independent of fus1+, the only gene that has been found to be specifically required for cell fusion during mating in the fission yeast.

Cdc42p GDP/GTP cycling is necessary for efficient cell fusion during yeast mating

The highly conserved small Rho G-protein, Cdc42p plays a critical role in cell polarity and cytoskeleton organization in all eukaryotes. In the yeast Saccharomyces cerevisiae, Cdc42p is important for cell polarity establishment, septin ring assembly, and pheromone-dependent MAP-kinase signaling during the yeast mating process. In this study, we further investigated the role of Cdc42p in the mating process by screening for specific mating defective cdc42 alleles. We have identified and characterized novel mating defective cdc42 alleles that are unaffected in vegetative cell polarity. Replacement of the Cdc42p Val36 residue with Met resulted in a specific cell fusion defect. This cdc42[V36M] mutant responded to mating pheromone but was defective in cell fusion and in localization of the cell fusion protein Fus1p, similar to a previously isolated cdc24 (cdc24-m6) mutant. Overexpression of a fast cycling Cdc42p mutant suppressed the cdc24-m6 fusion defect and conversely, overexpression of Cdc24p suppressed the cdc42[V36M] fusion defect. Taken together, our results indicate that Cdc42p GDP-GTP cycling is critical for efficient cell fusion.

Arf1p, Chs5p and the ChAPs are required for export of specialized cargo from the Golgi

In Saccharomyces cerevisiae, the synthesis of chitin is temporally and spatially regulated through the transport of Chs3p (chitin synthase III) to the plasma membrane in the bud neck region. Traffic of Chs3p from the trans-Golgi network (TGN)/early endosome to the plasma membrane requires the function of Chs5p and Chs6p. Chs6p belongs to a family of four proteins that we have named ChAPs for Chs5p-Arf1p-binding Proteins. Here, we show that all ChAPs physically interact not only with Chs5p but also with the small GTPase Arf1p. A short sequence at the C-terminus of the ChAPs is required for protein function and the ability to bind to Chs5p. Simultaneous disruption of two members, Deltabud7 and Deltabch1, phenocopies a Deltachs6 or Deltachs5 deletion with respect to Chs3p transport. Moreover, the ChAPs interact with each other and can form complexes. In addition, they are all at least partially localized to the TGN in a Chs5p-dependent manner. Most importantly, several ChAPs can interact physically with Chs3p. We propose that the ChAPs facilitate export of cargo out of the Golgi.

Ascospore formation in the yeast Saccharomyces cerevisiae

Sporulation of the baker's yeast Saccharomyces cerevisiae is a response to nutrient depletion that allows a single diploid cell to give rise to four stress-resistant haploid spores. The formation of these spores requires a coordinated reorganization of cellular architecture. The construction of the spores can be broadly divided into two phases. The first is the generation of new membrane compartments within the cell cytoplasm that ultimately give rise to the spore plasma membranes. Proper assembly and growth of these membranes require modification of aspects of the constitutive secretory pathway and cytoskeleton by sporulation-specific functions. In the second phase, each immature spore becomes surrounded by a multilaminar spore wall that provides resistance to environmental stresses. This review focuses on our current understanding of the cellular rearrangements and the genes required in each of these phases to give rise to a wild-type spore.

Saccharomyces cerevisiae Sps1p regulates trafficking of enzymes required for spore wall synthesis

SPS1 encodes a sporulation-specific protein with homology to the Ste20/p21-activated kinase family. Deletion of SPS1 impinges on the formation of the spore wall, which surrounds each of the haploid nuclei generated by the meiotic divisions. Here, we demonstrate that the new internal membranes that surround the meiotic nuclei appear normal in the absence of Sps1p. Analyses of spore wall layers by immunohistochemistry suggest that the inner layers are not efficiently deposited. The defect in spore wall morphogenesis is most likely a consequence of mislocalization of enzymes required for the synthesis of the spore wall layers as both Chs3p, the major chitin synthase in yeast, and Gsc2/Fks2p, a glucan synthase transcriptionally upregulated during sporulation, fail to reach the prospore membrane in the sps1 mutant. Furthermore, localization of Chs3p to the prospore membrane is not dependent on Shc1p, a sporulation-specific homolog of Chs4p, which is required for recruitment of Chs3p to the bud neck in vegetative cells. Sps1p colocalized with Chs3p to peripheral and internal punctate structures and prospore membranes. We propose that Sps1p promotes sporulation, in part, by regulating the intracellular movement of proteins required for spore wall formation.

The exchange factor Cdc24 is required for cell fusion during yeast mating

During Saccharomyces cerevisiae mating, chemotropic growth and cell fusion are critical for zygote formation. Cdc24p, the guanine nucleotide exchange factor for the Cdc42 G protein, is necessary for oriented growth along a pheromone gradient during mating. To understand the functions of this critical Cdc42p activator, we identified additional cdc24 mating mutants. Two mating-specific mutants, the cdc24-m5 and cdc24-m6 mutants, each were isolated with a mutated residue in the conserved catalytic domain. The cdc24-m6 mutant responds normally to pheromone and orients its growth towards a mating partner yet accumulates prezygotes during mating. cdc24-m6 prezygotes have two apposed intact cell walls and do not correctly localize proteins required for cell fusion, despite normal exocytosis. Our results indicate that the exchange factor Cdc24p is necessary for maintaining or restricting specific proteins required for cell fusion to the cell contact region during mating.

Lrg1p Is a Rho1 GTPase-activating protein required for efficient cell fusion in yeast

To identify additional cell fusion genes in Saccharomyces cerevisiae, we performed a high-copy suppressor screen of fus2Delta. Higher dosage of three genes, BEM1, LRG1, and FUS1, partially suppressed the fus2Delta cell fusion defect. BEM1 and FUS1 were high-copy suppressors of many cell-fusion-defective mutations, whereas LRG1 suppressed only fus2Delta and rvs161Delta. Lrg1p contains a Rho-GAP homologous region. Complete deletion of LRG1, as well as deletion of the Rho-GAP coding region, caused decreased rates of cell fusion and diploid formation comparable to that of fus2Delta. Furthermore, lrg1Delta caused a more severe mating defect in combination with other cell fusion mutations. Consistent with an involvement in cell fusion, Lrg1p localized to the tip of the mating projection. Lrg1p-GAP domain strongly and specifically stimulated the GTPase activity of Rho1p, a regulator of beta(1-3)-glucan synthase in vitro. beta(1-3)-glucan deposition was increased in lrg1Delta strains and mislocalized to the tip of the mating projection in fus2Delta strains. High-copy LRG1 suppressed the mislocalization of beta(1-3) glucan in fus2Delta strains. We conclude that Lrg1p is a Rho1p-GAP involved in cell fusion and speculate that it acts to locally inhibit cell wall synthesis to aid in the close apposition of the plasma membranes of mating cells.

Fus1p interacts with components of the Hog1p mitogen-activated protein kinase and Cdc42p morphogenesis signaling pathways to control cell fusion during yeast mating

Cell fusion in the budding yeast Saccharomyces cerevisiae is a temporally and spatially regulated process that involves degradation of the septum, which is composed of cell wall material, and occurs between conjugating cells within a prezygote, followed by plasma membrane fusion. The plasma membrane protein Fus1p is known to be required for septum degradation during cell fusion, yet its role at the molecular level is not understood. We identified Sho1p, an osmosensor for the HOG MAPK pathway, as a binding partner for Fus1 in a two-hybrid screen. The Sho1p-Fus1p interaction occurs directly and is mediated through the Sho1p-SH3 domain and a proline-rich peptide ligand on the Fus1p COOH-terminal cytoplasmic region. The cell fusion defect associated with fus1Delta mutants is suppressed by a sho1Delta deletion allele, suggesting that Fus1p negatively regulates Sho1p signaling to ensure efficient cell fusion. A two-hybrid matrix containing fusion proteins and pheromone response pathway signaling molecules reveals that Fus1p may participate in a complex network of interactions. In particular, the Fus1p cytoplasmic domain interacts with Chs5p, a protein required for secretion of specialized Chs3p-containing vesicles during bud development, and chs5Delta mutants were defective in cell surface localization of Fus1p. The Fus1p cytoplasmic domain also interacts with the activated GTP-bound form of Cdc42p and the Fus1p-SH3 domain interacts with Bni1p, a yeast formin that participates in cell fusion and controls the assembly of actin cables to polarize secretion in response to Cdc42p signaling. Taken together, our results suggest that Fus1p acts as a scaffold for the assembly of a cell surface complex involved in polarized secretion of septum-degrading enzymes and inhibition of HOG pathway signaling to promote cell fusion.

Differential expression of the chitin synthase genes of Aspergillus nidulans, chsA, chsB, and chsC, in response to developmental status and environmental factors

To understand the role of the chitin synthase genes of Aspergillus nidulans, we analyzed the expression of chsA, chsB, and chsC both by Northern blotting and by a vital reporter system with sgfp encoding a modified version of green fluorescent protein, sGFP. chsA was expressed specifically during asexual differentiation, but not during either vegetative growth or sexual differentiation. The expression of chsB was ubiquitous throughout the fungal body and relatively independent of the change in developmental status of the cells. chsC was expressed moderately during sexual development as well as during the early phase of vegetative growth, but was expressed weakly in old vegetative mycelia and in asexual structures. Furthermore its expression was spatially differentiated, i.e., relatively strong in young cleistothecia and in mature ascospores, but negligible in Hülle cells. Osmostress caused by high concentrations (up to 1.2M) of KCl or NaCl stimulated the expression of chsA and chsC, but not that of chsB. Sodium acetate, especially at high concentration (3%), strongly enhanced the expression of all the three genes. Neither heat shock nor the sugar carbon sources tested (glucose, sucrose, or lactose) affected the expression of any of the three chitin synthase genes.

CHS8-a fourth chitin synthase gene of Candida albicans contributes to in vitro chitin synthase activity, but is dispensable for growth

In silico analysis of the genome sequence of the human pathogenic fungus Candida albicans identified an open reading frame encoding a putative fourth member of the chitin synthase gene family. This gene, named CaCHS8, encodes an 1105 amino acid open reading frame with the conserved motifs characteristic of class I zymogenic chitin synthases with closest sequence similarity to the non-essential C. albicans class I CHS2 gene. Although the CaCHS8 gene was expressed in both yeast and hyphal cells, homozygous chs8 Delta null mutants had normal growth rates, cellular morphologies and chitin contents. The null mutant strains had a 25% reduction in chitin synthase activity and were hypersensitive to Calcofluor White. A chs2 Delta chs8 Delta double mutant had less than 3% of normal chitin synthase activity and had increased wall glucan and decreased mannan but was unaffected in growth or cell morphology. The C. albicans class I double mutant did not exhibit a bud-lysis phenotype as found in the class I chs1 Delta mutant of Saccharomyces cerevisiae. Therefore, C. albicans has four chitin synthases with two non-essential class I Chs isoenzymes that contribute collectively to more than 97% of the in vitro chitin synthase activity.

Specific protein targeting during cell differentiation: polarized localization of Fus1p during mating depends on Chs5p in Saccharomyces cerevisiae

In budding yeast, chs5 mutants are defective in chitin synthesis and cell fusion during mating. Chs5p is a late-Golgi protein required for the polarized transport of the chitin synthase Chs3p to the membrane. Here we show that Chs5p is also essential for the polarized targeting of Fus1p, but not of other cell fusion proteins, to the membrane during mating.

Characterization of Grp1p, a novel cis-Golgi matrix protein

A high copy suppressor screen with sec34-2, a temperature-sensitive mutant defective in the late stages of ER to Golgi transport, has resulted in the identification of a novel gene called GRP1 (also called RUD3). GRP1 encodes a hydrophilic yeast protein related to the mammalian Golgi matrix protein golgin-160. A large portion of the protein is predicted to form a coiled-coil structure. Although GRP1 is not essential for growth, the loss of Grp1p results in a growth defect at high temperature. GRP1 genetically interacts with several genes involved in vesicle targeting/fusion stages of ER to Golgi transport. Despite these interactions, pulse chase analysis using Grp1p-depleted cells did not reveal a significant delay in the transit of the vacuolar protease carboxypeptidase Y. Grp1p-depleted cells efficiently secreted invertase which was underglycosylated, suggesting some disturbance of Golgi function. Grp1p-GFP predominantly colocalizes with the cis-Golgi marker Och1p. Despite lacking a signal peptide and a significant stretch of hydrophobic amino acids, Grp1p pellets with membranes. It is extracted with 1M NaCl or 0.1M Na(2)CO(3) (pH 11.0), but is surprisingly insoluble in 1% Triton X-100. Grp1p does not recycle to the ER when forward transport is blocked and a cis-Golgi marker (Och1p-HA), but not a trans-Golgi marker (Chs5p-HA), became dispersed in grp1 Delta cells after 1.5h incubation at 38.5 degrees C. Together, these data suggest that Grp1p is a novel matrix protein that is involved in the structural organization of the cis-Golgi.

Maintenance of cell integrity in the gas1 mutant of Saccharomyces cerevisiae requires the Chs3p-targeting and activation pathway and involves an unusual Chs3p localization

Chitin synthase III is essential for the increase in chitin level and for cell integrity in cells lacking Gas1p, a beta(1,3)-glucanosyltransferase. In order to discover whether the upregulation of chitin synthesis proceeds through the canonical transport and activation pathway of Chs3p or through an alternative one, here we studied the effects of the inactivation of the GAS1 and CHS4-5-6-7 genes. All the double-null mutants showed a temperature-sensitive cell lysis phenotype that could be suppressed by the presence of an osmotic stabilizer. In liquid YEPD at 30 degrees C, chs4 delta gas1 delta, chs5 delta gas1 delta, chs6 deltagas1 delta and chs7 delta gas1 delta mutants were unable to grow, whereas they grew very slowly in minimal medium and showed low viability. High osmolarity suppressed the defective phenotype and restored growth. In chs4 gas1, chs5 gas1 and chs7 gas1, chitin levels did not increase and were reduced to only 10%, while in chs6 gas1 the value of gas1 was reduced to 20-40%. To investigate at which level the upregulation of chitin synthesis could occur, mRNA levels were monitored. The expression of CHS4-5-6-7 did not change significantly in gas1 delta. In strains expressing HA-tagged forms, the localization of Chs3p and Chs5p was examined. In the gas1 mutant the fluorescence pattern was affected and the proteins appeared abnormally present in the bud. The results indicate that: (a) the function of the CHS4-7 genes is required for chitin hyperaccumulation in gas1 mutant and for cell integrity; (b) homologous genes do not replace their function; (c) the regulation of CHS4-7 genes does not occur at transcriptional level. Control of the position of chitin synthesis could be important in protecting the bud from lysis.

The zygomycetous fungus, Benjaminiella poitrasii contains a large family of differentially regulated chitin synthase genes

Benjaminiella poitrasii is a zygomycetous, non-pathogenic dimorphic fungus. Chitin synthases are the membrane bound enzymes involved in the synthesis of chitin and are key enzymes in the cell wall metabolism. Multiplicity of these enzymes is a common occurrence. Here, we identify eight distinct CHS genes in B. poitrasii as confirmed through DNA sequence and Southern analysis. These genes are related to other fungal CHS genes. BpCHS1-4 are class I-III chitin synthases while BpCHS5-8 are class IV-V chitin synthases. These eight genes are differentially expressed during morphogenesis and under different growth conditions. Two of these genes viz. BpCHS2 and BpCHS3 appear to be specific to the mycelial growth form. These are the first B. poitrasii sequences to be reported. Based on CHS gene sequences, B. poitrasii chitin synthase genes place it with other zygomycetes on a fungal phylogenetic tree.

Mechanisms for targeting of theSaccharomyces cerevisiaeGPI-anchored cell wall protein Crh2p to polarised growth sites

The cell wall is an essential structure that preserves the osmotic integrity of fungal cells and determines cellular morphology during developmental programs. The high number of different wall components demands a variety of processes to deliver precursors and synthetic proteins to the proper location at the right time for wall development and modification. Here,the specificity of the mechanisms that regulate the temporal and spatial localisation of cell wall proteins to sites of polarised growth in Saccharomyces cerevisiae is investigated. For this purpose, the localisation of Crh2p, a cell wall glycosylphosphatidylinositol (GPI)-anchored mannoprotein that we have recently described as involved in cell wall construction and localised to polarised growth sites, was followed using a Crh2p-GFP fusion protein. Crh2p distribution was studied in several genetic backgrounds affected in different steps of the cell polarity establishment machinery or/and bud morphogenesis. Crh2p is localised at the mother-bud neck in bud1 cells following the random budding pattern characteristic of this mutant. The Crh2p distribution was greatly altered in a cdc42-1mutant, indicating complete dependence on an organised actin cytoskeleton for polarised Crh2p distribution. The usual deposition of Crh2p in a ring at the base of growing buds was lacking in cdc10-11 cells growing under restrictive temperature conditions, whereas Crh2p deposition at the septum region was absent in both cdc10-11 and cdc15-lyt1 cells. These results point to the dependence of Crh2p localisation at the bud-neck on both septins and septum integrity. Furthermore, in the absence of Bni4p, a scaffold protein involved in the targeting of the chitin synthase III complex to the bud neck, Crh2p was not longer found at the neck in large-budded cells undergoing cytokinesis. Finally, Crh2p was not properly localised in cells deleted in CHS5, which encodes a protein involved in the transport of Chs3p, and was completely mislocalised in sbe2/sbe22 mutants,suggesting that the transport systems for Chs3p and Crh2p are to a certain extent coincident. The transport of other GPI-cell wall proteins, such as Cwp1p, however, does not depend on these systems as the localisation of the latter protein was not affected in either of these mutants.

Control of chitin synthesis through Shc1p, a functional homologue of Chs4p specifically induced during sporulation

The Saccharomyces cerevisiae SHC1 gene encodes a protein with a high homology to Chs4p, a positive regulator of chitin synthase III (CSIII) during vegetative growth. SHC1 is not expressed during vegetative growth but is strongly induced during sporulation as a mid-late gene. shc1/shc1 mutants do not show any defect in the total rate of sporulation and meiosis occurs normally. However, shc1/shc1 ascospores be-come highly permeable to DAPI, much more sensitive to glusulase treatment, and have very low levels of chitosan in their cell walls. All these observations indicate that Shc1p is required for proper maturation of the ascospore through its participation in the synthesis of the chitosan layer. Lack of SHC1 during sporulation can be partially compensated by over-expression of the CHS4 gene. During vegetative growth, SHC1 has no apparent function but, when ectopically overexpressed, it can substitute Chs4p as an activator of the CSIII activity; however, Shc1p fails to localize it properly, as Chs4p does. In conclusion, S. cerevisiae contains two functionally redundant genes in the control of CSIII activity: CHS4, whose function is restricted to vegetative growth because Chs4p is rapidly degraded during sporulation, and SHC1, whose function in cell wall ascospore assembly is transcriptionally restricted to the sporulation process.