Characterization of the need for galactofuranose during the Neurospora crassa life cycle

Galactofuranose is a constituent of the cell walls of filamentous fungi. The galactofuranose can be found as a component of N-linked oligosaccharides, in O-linked oligosaccharides, in GPI-anchored galactomannan, and in free galactomannan. The Neurospora genome contains a single UDP-galactose mutase gene (ugm-1/NCU01824) and two UDP-galactofuranose translocases used to import UDP-galactofuranose into the lumen of the Golgi apparatus (ugt-1/NCU01826 and ugt-2/NCU01456). Our results demonstrate that loss of galactofuranose synthesis or its translocation into the lumen of the secretory pathway affects the morphology and growth rate of the vegetative hyphae, the production of conidia (asexual spores), and dramatically affects the sexual stages of the life cycle. In mutants that are unable to make galactofuranose or transport it into the lumen of the Golgi apparatus, ascospore development is aborted soon after fertilization and perithecium maturation is aborted prior to the formation of the neck and ostiole. The Neurospora genome contains three genes encoding possible galactofuranosyltransferases from the GT31 family of glycosyltransferases (gfs-1/NCU05878, gfs-2/NCU07762, and gfs-3/NCU02213) which might be involved in generating galactofuranose-containing oligosaccharide structures. Analysis of triple KO mutants in GT31 glycosyltransferases shows that these mutants have normal morphology, suggesting that these genes do not encode vital galactofuranosyltransferases.

Genetic Characterization of the Acidic and Neutral Glycosphingolipid Biosynthetic Pathways in Neurospora crassa

Fungal glycosphingolipids (GSLs) are important membrane components which play a key role in vesicle trafficking. To assess the importance of GSLs in the fungal life cycle, we performed a mutant phenotypic study of the acidic and neutral GSL biosynthetic pathways in Neurospora crassa. GSL biosynthesis begins with two reactions leading up to the formation of dihydrosphingosine. The first of these reactions is catalyzed by serine palmitoyltransferase and generates 3-keto dihydrosphinganine. In N. crassa, this reaction is catalyzed by GSL-1 and GSL-2 and is required for viability. The second reaction is carried out by GSL-3, a 3-keto dihydrosphinoganine reductase to generate dihydrosphingosine, which is used for the synthesis of neutral and acidic GSLs. We found that deletion mutations in the acidic GSL pathway leading up to the formation of mannosylinositol-phosphoceramide are lethal, indicating that acidic GSLs are essential for viability in N. crassa. Once mannosylinositol-phosphoceramide is made, it is further modified by GSL-5, an inositol-phosphoceramide-B C26 hydroxylase, which adds a hydroxyl group to the amide-linked fatty acid. GSL-5 is not required for viability but gives a clear mutant phenotype affecting all stages of the life cycle. Our results show that the synthesis of mannosylinositol-phosphoceramide is required for viability and that the modification of the amide-linked fatty acid is important for acidic GSL functionality. We also examined the neutral GSL biosynthetic pathway and identified the presence of glucosylceramide. The deletion of neutral GSL biosynthetic genes affected hyphal morphology, vegetative growth rate, conidiation, and female development. Our results indicate that the synthesis of neutral GSLs is essential for normal growth and development of N. crassa.

Characterization of Neurospora crassa GH16, GH17, and GH72 gene families of cell wall crosslinking enzymes

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Mutants lacking GH16 chitin transferases are sensitive to cell wall perturbation reagents.

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Mutants lacking GH17 β-1,3-glucan transferases are sensitive to cell wall perturbation reagents.

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In N. crassa, GH17 β-1,3-glucan transferases and GH72 β-1,3-glucan/lichenin transferases are not redundant activities.

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Neurospora GH72 enzymes form lichenin-enzyme intermediates.

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Neurospora GH72 enzymes are lichenin transferases.

GH16 chitin transferases, GH17 β-1,3-glucan transferases, and GH72 β-1,3-glucan/lichenin transferases are important fungal cell wall crosslinking enzymes. The Neurospora crassa genome encodes three genes from the GH17 gene family and five members in the GH16 subfamily 18 and 19 fungal chitin transferases. We created deletion mutants lacking all three GH17 genes and determined that they had wild type morphology and are more sensitive to cell wall perturbation reagents than the wild type. We also created deletion mutants lacking all five GH16 subfamily 18 and 19 genes and found that they had wild type morphology and are more sensitive to cell wall perturbation reagents than the wild type. We conclude that GH16 and GH17 enzymes play roles in cell wall biogenesis. In N. crassa, GH72 enzymes have been reported to be lichenin transferases, while in other fungi they have been shown to be the β-1,3-glucan transferases. Neurospora triple GH72 deletions give rise to a tight colonial morphology, sensitivity to cell wall perturbation reagents, and release of cell wall proteins into the medium. To ask if GH72 and GH17 enzymes might be redundant in N. crassa, we created sextuple mutants lacking the three GH72 genes and the three GH17 genes and found that they were indistinguishable from the GH72 triple mutant. We also found that a recombinant GH72 enzyme is able to form a lichenin-enzyme intermediate demonstrating that GH72 enzymes are lichenin transferases. The N. crassa GH72 enzymes are lichenin transferases and are not redundant with the GH17 β-1,3-glucan transferases.

The Genetics and Biochemistry of Cell Wall Structure and Synthesis in Neurospora crassa, a Model Filamentous Fungus

This review discusses the wealth of information available for the N. crassa cell wall. The basic organization and structure of the cell wall is presented and how the wall changes during the N. crassa life cycle is discussed. Over forty cell wall glycoproteins have been identified by proteomic analyses. Genetic and biochemical studies have identified many of the key enzymes needed for cell wall biogenesis, and the roles these enzymes play in cell wall biogenesis are discussed. The review includes a discussion of how the major cell wall components (chitin, β-1,3-glucan, mixed β-1,3-/ β-1,4- glucans, glycoproteins, and melanin) are synthesized and incorporated into the cell wall. We present a four-step model for how cell wall glycoproteins are covalently incorporated into the cell wall. In N. crassa, the covalent incorporation of cell wall glycoproteins into the wall occurs through a glycosidic linkage between lichenin (a mixed β-1,3-/β-1,4- glucan) and a "processed" galactomannan that has been attached to the glycoprotein N-linked oligosaccharides. The first step is the addition of the galactomannan to the N-linked oligosaccharide. Mutants affected in galactomannan formation are unable to incorporate glycoproteins into their cell walls. The second step is carried out by the enzymes from the GH76 family of α-1,6-mannanases, which cleave the galactomannan to generate a processed galactomannan. The model suggests that the third and fourth steps are carried out by members of the GH72 family of glucanosyltransferases. In the third step the glucanosyltransferases cleave lichenin and generate enzyme/substrate intermediates in which the lichenin is covalently attached to the active site of the glucanosyltransferases. In the final step, the glucanosyltransferases attach the lichenin onto the processed galactomannans, which creates new glycosidic bonds and effectively incorporates the glycoproteins into the cross-linked cell wall glucan/chitin matrix.

Trichophyton rubrum LysM proteins bind to fungal cell wall chitin and to the N-linked oligosaccharides present on human skin glycoproteins

The Trichophyton rubrum genome contains six proteins containing two or more lysin M (LysM) domains. We have characterized two of these proteins, LysM1 and LysM2, and demonstrated that these proteins have the capacity to bind two substrates, chitin and N-linked oligosaccharides associated with human skin glycoproteins. We have characterized the individual LysM domains in LysM1, and shown that the protein contains two functional LysM domains. Each of these domains can bind to chitin, to N-linked oligosaccharides in human skin glycoproteins, and to N-linked oligosaccharides on fungal glycoproteins. We hypothesize that LysM proteins could provide the pathogen with three important functions. First, the T. rubrum LysM proteins could shield host cell wall chitin from the human immune system. Second, the LysM proteins could shield the pathogen’s glycoproteins from host degradation and immune surveillance. Third, the LysM proteins could help facilitate pathogen adhesion to human skin.

Neurospora crassa family GH72 glucanosyltransferases function to crosslink cell wall glycoprotein N-linked galactomannan to cell wall lichenin

The formation of a glucan/chitin/glycoprotein cell wall matrix is vital for fungal survival, growth, and morphogenesis. The cell wall proteins are important cell wall components and function in adhesion, signal transduction, and as cell wall structural elements. In this report we demonstrate that Neurospora crassa GH72 glucan transferases function to crosslink cell wall glycoproteins into the cell wall. With an in vitro assay, we show that the glucan transferases are able to attach lichenin, a cell wall glucan with a repeating β-1,4-glucose-β-1,4-glucose-β-1,3-glucose structure, to cell wall glycoproteins. We propose that the pathway for attachment of lichenin to the glycoprotein has four steps. First, N-linked oligosaccharides present on the glycoproteins are modified by the addition of a galactomannan. As part of our report we have characterized the structure of the galactomannan, which consists of an α-1,6-mannose backbone with galactofuranose side chains. In the second step, the galactomannan is processed by members of the GH76 α-1,6-mannanases. In the third step, the glucan transferases cleave the lichenin and create substrate-enzyme intermediates. In the final step, the transferases transfer the lichenin to the processed galactomannan. We demonstrate that the N. crassa glucan transferases have demonstrate specificity for the processed galactomannan and for lichenin. The energy from the cleaved glycosidic bond in lichenin is retained in the substrate-enzyme intermediate and used to create a new glycosidic bond between the lichenin and the processed galactomannan. The pathway effectively crosslinks glycoproteins into the fungal cell wall.

Genetic and biochemical characterization of the GH72 family of cell wall transglycosylases in Neurospora crassa

The Neurospora crassa genome encodes five GH72 family transglycosylases, and four of these enzymes (GEL-1, GEL-2, GEL-3 and GEL-5) have been found to be present in the cell wall proteome. We carried out an extensive genetic analysis on the role of these four transglycosylases in cell wall biogenesis and demonstrated that the transglycosylases are required for the formation of a normal cell wall. As suggested by the proteomic analysis, we found that multiple transglycosylases were being expressed in N. crassa cells and that different combinations of the enzymes are required in different cell types. The combination of GEL-1, GEL-2 and GEL-5 is required for the growth of vegetative hyphae, while the GEL-1, GEL-2, GEL-3 combination is needed for the production of aerial hyphae and conidia. Our data demonstrates that the enzymes are redundant with partially overlapping enzymatic activities, which provides the fungus with a robust cell wall biosynthetic system. Characterization of the transglycosylase-deficient mutants demonstrated that the incorporation of cell wall proteins was severely compromised. Interestingly, we found that the transglycosylase-deficient mutant cell walls contained more β-1,3-glucan than the wild type cell wall. Our results demonstrate that the GH72 transglycosylases are not needed for the incorporation of β-1,3-glucan into the cell wall, but they are required for the incorporation of cell wall glycoprotein into the cell wall.

Characterization of the Sclerotinia sclerotiorum cell wall proteome

We used a proteomic analysis to identify cell wall proteins released from Sclerotinia sclerotiorum hyphal and sclerotial cell walls via a trifluoromethanesulfonic acid (TFMS) digestion. Cell walls from hyphae grown in Vogel's glucose medium (a synthetic medium lacking plant materials), from hyphae grown in potato dextrose broth and from sclerotia produced on potato dextrose agar were used in the analysis. Under the conditions used, TFMS digests the glycosidic linkages in the cell walls to release intact cell wall proteins. The analysis identified 24 glycosylphosphatidylinositol (GPI)-anchored cell wall proteins and 30 non-GPI-anchored cell wall proteins. We found that the cell walls contained an array of cell wall biosynthetic enzymes similar to those found in the cell walls of other fungi. When comparing the proteins in hyphal cell walls grown in potato dextrose broth with those in hyphal cell walls grown in the absence of plant material, it was found that a core group of cell wall biosynthetic proteins and some proteins associated with pathogenicity (secreted cellulases, pectin lyases, glucosidases and proteases) were expressed in both types of hyphae. The hyphae grown in potato dextrose broth contained a number of additional proteins (laccases, oxalate decarboxylase, peroxidase, polysaccharide deacetylase and several proteins unique to Sclerotinia and Botrytis) that might facilitate growth on a plant host. A comparison of the proteins in the sclerotial cell wall with the proteins in the hyphal cell wall demonstrated that sclerotia formation is not marked by a major shift in the composition of cell wall protein. We found that the S. sclerotiorum cell walls contained 11 cell wall proteins that were encoded only in Sclerotinia and Botrytis genomes.

A proteomic and genetic analysis of the Neurospora crassa conidia cell wall proteins identifies two glycosyl hydrolases involved in cell wall remodeling

A proteomic analysis of the conidial cell wall identified 35 cell wall proteins. A comparison with the proteome of the vegetative hyphae showed that 16 cell wall proteins were shared, and that these shared cell wall proteins were cell wall biosynthetic proteins or cell wall structural proteins. Deletion mutants for 34 of the genes were analyzed for phenotypes indicative of conidial cell wall defects. Mutants for two cell wall glycosyl hydrolases, the CGL-1 β-1,3-glucanase (NCU07523) and the NAG-1 exochitinase (NCU10852), were found to have a conidial separation phenotype. These two enzymes function in remodeling the cell wall between adjacent conidia to facilitate conidia formation and dissemination. Using promoter::RFP and promoter::GFP constructs, we demonstrated that the promoters for 15 of the conidia-specific cell wall genes, including cgl-1 and nag-1, provided for conidia-specific gene expression or for a significant increase in their expression during conidiation.

Neurospora crassa female development requires the PACC and other signal transduction pathways, transcription factors, chromatin remodeling, cell-to-cell fusion, and autophagy

Using a screening protocol we have identified 68 genes that are required for female development in the filamentous fungus Neurospora crassa. We find that we can divide these genes into five general groups: 1) Genes encoding components of the PACC signal transduction pathway, 2) Other signal transduction pathway genes, including genes from the three N. crassa MAP kinase pathways, 3) Transcriptional factor genes, 4) Autophagy genes, and 5) Other miscellaneous genes. Complementation and RIP studies verified that these genes are needed for the formation of the female mating structure, the protoperithecium, and for the maturation of a fertilized protoperithecium into a perithecium. Perithecia grafting experiments demonstrate that the autophagy genes and the cell-to-cell fusion genes (the MAK-1 and MAK-2 pathway genes) are needed for the mobilization and movement of nutrients from an established vegetative hyphal network into the developing protoperithecium. Deletion mutants for the PACC pathway genes palA, palB, palC, palF, palH, and pacC were found to be defective in two aspects of female development. First, they were unable to initiate female development on synthetic crossing medium. However, they could form protoperithecia when grown on cellophane, on corn meal agar, or in response to the presence of nearby perithecia. Second, fertilized perithecia from PACC pathway mutants were unable to produce asci and complete female development. Protein localization experiments with a GFP-tagged PALA construct showed that PALA was localized in a peripheral punctate pattern, consistent with a signaling center associated with the ESCRT complex. The N. crassa PACC signal transduction pathway appears to be similar to the PacC/Rim101 pathway previously characterized in Aspergillus nidulans and Saccharomyces cerevisiae. In N. crassa the pathway plays a key role in regulating female development.

Characterization of the Neurospora crassa cell fusion proteins, HAM-6, HAM-7, HAM-8, HAM-9, HAM-10, AMPH-1 and WHI-2

Intercellular communication of vegetative cells and their subsequent cell fusion is vital for different aspects of growth, fitness, and differentiation of filamentous fungi. Cell fusion between germinating spores is important for early colony establishment, while hyphal fusion in the mature colony facilitates the movement of resources and organelles throughout an established colony. Approximately 50 proteins have been shown to be important for somatic cell-cell communication and fusion in the model filamentous fungus Neurospora crassa. Genetic, biochemical, and microscopic techniques were used to characterize the functions of seven previously poorly characterized cell fusion proteins. HAM-6, HAM-7 and HAM-8 share functional characteristics and are proposed to function in the same signaling network. Our data suggest that these proteins may form a sensor complex at the cell wall/plasma membrane for the MAK-1 cell wall integrity mitogen-activated protein kinase (MAPK) pathway. We also demonstrate that HAM-9, HAM-10, AMPH-1 and WHI-2 have more general functions and are required for normal growth and development. The activation status of the MAK-1 and MAK-2 MAPK pathways are altered in mutants lacking these proteins. We propose that these proteins may function to coordinate the activities of the two MAPK modules with other signaling pathways during cell fusion.

Neurospora crassa 1,3-α-glucan synthase, AGS-1, is required for cell wall biosynthesis during macroconidia development

The Neurospora crassa genome encodes two 1,3-α-glucan synthases. One of these 1,3-α-glucan synthase genes, ags-1, was shown to be required for the synthesis of 1,3-α-glucan in the aerial hyphae and macroconidia cell walls. 1,3-α-Glucan was found in the conidia cell wall, but was absent from the vegetative hyphae cell wall. Deletion of ags-1 affected conidial development. Δags-1 produced only 5 % as many conidia as the WT and most of the conidia produced by Δags-1 were not viable. The ags-1 upstream regulatory elements were shown to direct cell-type-specific expression of red fluorescent protein in conidia and aerial hyphae. A haemagglutinin-tagged AGS-1 was found to be expressed in aerial hyphae and conidia. The research showed that 1,3-α-glucan is an aerial hyphae and conidia cell wall component, and is required for normal conidial differentiation.

Fungal cell wall organization and biosynthesis

The composition and organization of the cell walls from Saccharomyces cerevisiae, Candida albicans, Aspergillus fumigatus, Schizosaccharomyces pombe, Neurospora crassa, and Cryptococcus neoformans are compared and contrasted. These cell walls contain chitin, chitosan, β-1,3-glucan, β-1,6-glucan, mixed β-1,3-/β-1,4-glucan, α-1,3-glucan, melanin, and glycoproteins as major constituents. A comparison of these cell walls shows that there is a great deal of variability in fungal cell wall composition and organization. However, in all cases, the cell wall components are cross-linked together to generate a cell wall matrix. The biosynthesis and properties of each of the major cell wall components are discussed. The chitin and glucans are synthesized and extruded into the cell wall space by plasma membrane-associated chitin synthases and glucan synthases. The glycoproteins are synthesized by ER-associated ribosomes and pass through the canonical secretory pathway. Over half of the major cell wall proteins are modified by the addition of a glycosylphosphatidylinositol anchor. The cell wall glycoproteins are also modified by the addition of O-linked oligosaccharides, and their N-linked oligosaccharides are extensively modified during their passage through the secretory pathway. These cell wall glycoprotein posttranslational modifications are essential for cross-linking the proteins into the cell wall matrix. Cross-linking the cell wall components together is essential for cell wall integrity. The activities of four groups of cross-linking enzymes are discussed. Cell wall proteins function as cross-linking enzymes, structural elements, adhesins, and environmental stress sensors and protect the cell from environmental changes.

WSC-1 and HAM-7 are MAK-1 MAP kinase pathway sensors required for cell wall integrity and hyphal fusion in Neurospora crassa

A large number of cell wall proteins are encoded in the Neurospora crassa genome. Strains carrying gene deletions of 65 predicted cell wall proteins were characterized. Deletion mutations in two of these genes (wsc-1 and ham-7) have easily identified morphological and inhibitor-based defects. Their phenotypic characterization indicates that HAM-7 and WSC-1 function during cell-to-cell hyphal fusion and in cell wall integrity maintenance, respectively. wsc-1 encodes a transmembrane protein with extensive homology to the yeast Wsc family of sensor proteins. In N. crassa, WSC-1 (and its homolog WSC-2) activates the cell wall integrity MAK-1 MAP kinase pathway. The GPI-anchored cell wall protein HAM-7 is required for cell-to-cell fusion and the sexual stages of the N. crassa life cycle. Like WSC-1, HAM-7 is required for activating MAK-1. A Δwsc-1;Δham-7 double mutant fully phenocopies mutants lacking components of the MAK-1 MAP kinase cascade. The data identify WSC-1 and HAM-7 as the major cell wall sensors that regulate two distinct MAK-1-dependent cellular activities, cell wall integrity and hyphal anastomosis, respectively.

The Neurospora crassa dfg5 and dcw1 genes encode α-1,6-mannanases that function in the incorporation of glycoproteins into the cell wall

The covalent cross-linking of cell wall proteins into the cell wall glucan/chitin matrix is an important step in the biogenesis of the fungal cell wall. We demonstrate that the Neurospora crassa DFG5 (NCU03770) and DCW1 (NCU08127) enzymes function in vivo to cross-link glycoproteins into the cell wall. Mutants lacking DFG5 or DCW1 release slightly elevated levels of cell wall proteins into their growth medium. Mutants lacking both DFG5 and DCW1 have substantially reduced levels of cell wall proteins in their cell walls and release large amounts of known cell wall proteins into the medium. DFG5 and DCW1 are members of the GH76 family of glycosyl hydrolases, which have specificity to recognize and cleave α-1,6-mannans. A model for incorporation of glycoproteins into the cell wall through the α-1,6-mannan core of the N-linked galactomannan is presented. In this model, DFG5 and DCW1 recognize the N-linked galactomannan present on glycoproteins and cross-link it into the cell wall glucan/chitin matrix.

The ham-5, rcm-1 and rco-1 genes regulate hyphal fusion in Neurospora crassa

Mutants of Neurospora crassa unable to participate in vegetative hyphal fusion (anastomosis) were isolated and characterized. From this analysis, three genes, rcm-1, rco-1 and ham-5, were identified and shown to be required for hyphal fusion. The rcm-1 and rco-1 genes are homologues of the Saccharomyces cerevisiae SSN6 and TUP1 genes, which encode a dimeric transcription factor in yeast. We demonstrate that in N. crassa the rcm-1 and rco-1 genes are required for hyphal fusion and normal hyphal morphology, and influence both asexual and sexual development. The ham-5 gene encodes a 1686 amino acid protein with two putative WD40 domains, which might participate in protein–protein interactions. ham-5 deletion mutants had a reduced rate of hyphal extension and altered hyphal morphology, and were unable to produce the conidial anastomosis tubes that are required for hyphal fusion during colony initiation.

Characterization of GPIT-1 and GPIT-2, two auxiliary components of the Neurospora crassa GPI transamidase complex

The glycosylphosphatidylinositol (GPI) transamidase contains five known subunits and functions in the lumen of the ER to produce GPI-anchored proteins. The transamidase cleaves proteins containing a GPI anchor attachment signal at their C terminus and generates an amide bond between the newly generated carboxyl terminus of the protein and a GPI anchor. We have identified and characterized GPIT-1 and GPIT-2, two of the transamidase subunits from Neurospora crassa. GPIT-1 and GPIT-2 are homologs of the human PIG-T and PIG-U transamidase subunits respectively. We demonstrated that GPIT-2 is required for the addition of GPI anchors onto GPI-anchored proteins. We employed the Neurospora RIP (repeat-induced point mutation) phenomenon to generate 106 "noncritical" amino acid changes in GPIT-1 and 84 "noncritical" amino acid changes in GPIT-2. We used the data to evaluate three-dimensional models for the structures of GPIT-1 and GPIT-2. The mutational data for GPIT-1 is consistent with a multiple-blade propeller structure containing a central channel. The mutational analysis for GPIT-2 supports a structural model based on the karyopherin alpha subunit.

Phenotype of a mechanosensitive channel mutant, mid-1, in a filamentous fungus, Neurospora crassa

In the yeast Saccharomyces cerevisiae, the MID1 (mating-induced death) gene encodes a stretch-activated channel which is required for successful mating; the mutant phenotype is rescued by elevated extracellular calcium. Homologs of the MID1 gene are found in fungi that are morphologically complex compared to yeast, both Basidiomycetes and Ascomycetes. We explored the phenotype of a mid-1 knockout mutant in the filamentous ascomycete Neurospora crassa. The mutant exhibits lower growth vigor than the wild type (which is not rescued by replete calcium) and mates successfully. Thus, the role of the MID-1 protein differs from that of the homologous gene product in yeast. Hyphal cytology, growth on diverse carbon sources, turgor regulation, and circadian rhythms of the mid-1 mutant are all similar to those of the wild type. However, basal turgor is lower than wild type, as is the activity of the plasma membrane H(+)-ATPase (measured by cyanide [CN(-)]-induced depolarization of the energy-dependent component of the membrane potential). In addition, the mutant is unable to grow at low extracellular Ca(2+) levels or when cytoplasmic Ca(2+) is elevated with the Ca(2+) ionophore A23187. We conclude that the MID-1 protein plays a role in regulation of ion transport via Ca(2+) homeostasis and signaling. In the absence of normal ion transport activity, the mutant exhibits poorer growth.

The structure and synthesis of the fungal cell wall

The fungal cell wall is a dynamic structure that protects the cell from changes in osmotic pressure and other environmental stresses, while allowing the fungal cell to interact with its environment. The structure and biosynthesis of a fungal cell wall is unique to the fungi, and is therefore an excellent target for the development of anti-fungal drugs. The structure of the fungal cell wall and the drugs that target its biosynthesis are reviewed. Based on studies in a number of fungi, the cell wall has been shown to be primarily composed of chitin, glucans, mannans and glycoproteins. The biosynthesis of the various components of the fungal cell wall and the importance of the components in the formation of a functional cell wall, as revealed through mutational analyses, are discussed. There is strong evidence that the chitin, glucans and glycoproteins are covalently cross-linked together and that the cross-linking is a dynamic process that occurs extracellularly.

Mutational analysis of the glycosylphosphatidylinositol (GPI) anchor pathway demonstrates that GPI-anchored proteins are required for cell wall biogenesis and normal hyphal growth in Neurospora crassa

Using mutational and proteomic approaches, we have demonstrated the importance of the glycosylphosphatidylinositol (GPI) anchor pathway for cell wall synthesis and integrity and for the overall morphology of the filamentous fungus Neurospora crassa. Mutants affected in the gpig-1, gpip-1, gpip-2, gpip-3, and gpit-1 genes, which encode components of the N. crassa GPI anchor biosynthetic pathway, have been characterized. GPI anchor mutants exhibit colonial morphologies, significantly reduced rates of growth, altered hyphal growth patterns, considerable cellular lysis, and an abnormal "cell-within-a-cell" phenotype. The mutants are deficient in the production of GPI-anchored proteins, verifying the requirement of each altered gene for the process of GPI-anchoring. The mutant cell walls are abnormally weak, contain reduced amounts of protein, and have an altered carbohydrate composition. The mutant cell walls lack a number of GPI-anchored proteins, putatively involved in cell wall biogenesis and remodeling. From these studies, we conclude that the GPI anchor pathway is critical for proper cell wall structure and function in N. crassa.

Mannosyltransferase is required for cell wall biosynthesis, morphology and control of asexual development in Neurospora crassa

Two Neurospora mutants with a phenotype that includes a tight colonial growth pattern, an inability to form conidia and an inability to form protoperithecia have been isolated and characterized. The relevant mutations were mapped to the same locus on the sequenced Neurospora genome. The mutations responsible for the mutant phenotype then were identified by examining likely candidate genes from the mutant genomes at the mapped locus with PCR amplification and a sequencing assay. The results demonstrate that a map and sequence strategy is a feasible way to identify mutant genes in Neurospora. The gene responsible for the phenotype is a putative alpha-1,2-mannosyltransferase gene. The mutant cell wall has an altered composition demonstrating that the gene functions in cell wall biosynthesis. The results demonstrate that the mnt-1 gene is required for normal cell wall biosynthesis, morphology and for the regulation of asexual development.

Two divergent plasma membrane syntaxin-like SNAREs, nsyn1 and nsyn2, contribute to hyphal tip growth and other developmental processes in Neurospora crassa

Highly polarized exocytosis of vesicles at hyphal apices is an essential requirement of tip growth. This requirement may be met by the localization and/or activation of an apical SNARE-based machinery. We have cloned nsyn1 and nsyn2, SNAREs predicted to function at the plasma membrane in Neurospora crassa. Transformation of extra copies of nsyn1 into wild-type strains displayed effects consistent with quelling of nsyn1 expression, which was lethal in most transformants. All surviving transformants grew slowly, conidiated poorly, and were male sterile. In addition, antisense nsyn1 strains grew slowly, with abnormal hyphal diameters and polarity and defective conidiation. For nsyn2, several repeat induced point mutation (RIP) crosses produced no, or poorly germinating ascospores. Those that germinated produced slow-growing hyphae with abnormal branching. The defects in nsyn1 and nsyn2 mutants are consistent with differential impaired vesicle fusion in hyphal tips and other developmental stages.

Quantification of iNOS mRNA with reverse transcription polymerase chain reaction directly from cell lysates

Inducible nitric oxide synthase (iNOS) is a member of a family of primary inflammatory response genes. Quantitative measurement of iNOS mRNA levels is important for the study of gene expression of this enzyme during the process of inflammation. We report here a method for quantitative measurement of iNOS mRNA levels with rtPCR directly from cells lysed with a single step phenol/chloroform/ether extraction. Using a mouse macrophage cell line, J774.2, which expresses iNOS mRNA upon LPS + IFN-gamma treatment as the model, the effects of the extraction on iNOS mRNA recovery and cytosolic RNase removal have been studied. The cells are lysed and RNases denatured and removed by phenol/chloroform extraction. Trace amounts of the phenol partitioned in the samples are then removed by ether extraction. After the extraction, the samples can be used directly for reverse transcription and PCR without further purification of RNA. The recovery of specific mRNA is not affected by the extraction procedure and externally added iNOS cRNA shows no degradation by the extracted cell lysates. Measurement of iNOS mRNA with this procedure is linear using serially double-diluted cells in the range from 94 to 6000 cells. The efficiencies of rtPCR of iNOS wild-type and deletion cRNAs are also compared in our study. By controlling the molecular size of the deletion construct to within 10% of that of the wild type and maintaining PCR cycling below 25 cycles, the rtPCR efficiencies of iNOS wild type and deletion are identical. The detection of rtPCR products is enhanced by hybridization with specific probes. Under these conditions, iNOS mRNA concentration can directly be calculated from the internal standard in each tube without a standard curve. We conclude that our procedure provides an accurate method for quantitative measurement of iNOS mRNA from limited amount of cells without complete RNA isolation.

Mutational analysis of the Shab-encoded delayed rectifier K(+) channels in Drosophila

K(+) currents in Drosophila muscles have been resolved into two voltage-activated currents (I(A) and I(K)) and two Ca(2+)-activated currents (I(CF) and I(CS)). Mutations that affect I(A) (Shaker) and I(CF) (slowpoke) have helped greatly in the analysis of these currents and their role in membrane excitability. Lack of mutations that specifically affect channels for the delayed rectifier current (I(K)) has made their genetic and functional identity difficult to elucidate. With the help of mutations in the Shab K(+) channel gene, we show that this gene encodes the delayed rectifier K(+) channels in Drosophila. Three mutant alleles with a temperature-sensitive paralytic phenotype were analyzed. Analysis of the ionic currents from mutant larval body wall muscles showed a specific effect on delayed rectifier K(+) current (I(K)). Two of the mutant alleles contain missense mutations, one in the amino-terminal region of the channel protein and the other in the pore region of the channel. The third allele contains two deletions in the amino-terminal region and is a null allele. These observations identity the channels that carry the delayed rectifier current and provide an in vivo physiological role for the Shab-encoded K(+) channels in Drosophila. The availability of mutations that affect I(K) opens up possibilities for studying I(K) and its role in larval muscle excitability.

The isolation and characterization of nrc-1 and nrc-2, two genes encoding protein kinases that control growth and development in Neurospora crassa

Using an insertional mutagenesis approach, a series of Neurospora crassa mutants affected in the ability to control entry into the conidiation developmental program were isolated. One such mutant, GTH16-T4, was found to lack normal vegetative hyphae and to undergo constitutive conidiation. The affected gene has been named nrc-1, for nonrepressible conidiation gene #1. The nrc-1 gene was cloned from the mutant genomic DNA by plasmid rescue, and was found to encode a protein closely related to the protein products of the Saccharomyces cerevisiae STE11 and Schizosaccharomyces pombe byr2 genes. Both of these genes encode MAPKK kinases that are necessary for sexual development in these organisms. We conclude the nrc-1 gene encodes a MAPKK kinase that functions to repress the onset of conidiation in N. crassa. A second mutant, GTH16-T17, was found to lack normal vegetative hyphae and to constitutively enter, but not complete, the conidiation program. The affected locus is referred to as nrc-2 (nonrepressible conidiation gene #2). The nrc-2 gene was cloned and found to encode a serine-threonine protein kinase. The kinase is closely related to the predicted protein products of the S. pombe kad5, and the S. cerevisiae YNRO47w and KIN82 genes, three genes that have been identified in genome sequencing projects. The N. crassa nrc-2 gene is the first member of this group of kinases for which a phenotype has been defined. We conclude a functional nrc-2-encoded serine/threonine kinase is required to repress entry into the conidiation program.

Calcineurin subunit B is required for normal vegetative growth in Neurospora crassa

A series of Neurospora crassa mutants affected in the ability to regulate entry into conidiation (an asexual developmental program) were isolated by using an insertional mutagenesis procedure followed by a screening protocol. One of the mutants isolated by this approach consisted entirely of cells with an abnormal morphology. The mutant produces chains of swollen septated cells. The developmentally regulated ccg-1 gene is constitutively expressed in these cells, suggesting that they have entered the conidial developmental program. The insertionally disrupted gene cnb-1 was isolated by plasmid rescue and found to encode calcineurin B, the regulatory subunit of the Ca2+ and calmodulin-dependent protein phosphatase calcineurin. The data demonstrate that calcineurin B is required for normal vegetative growth in N. crassa and suggest that the cnb-1 mutant is unable to repress entry into the asexual developmental program. The results suggest that Ca2+ may play an important role in regulating fungal morphology.

The isolation and characterization of a Neurospora crassa gene (ubi::crp-6) encoding a ubiquitin-40S ribosomal protein fusion protein

We have isolated and sequenced a Neurospora crassa gene encoding a single copy of ubiquitin (UBI) fused to the S27a ribosomal (r) protein. We have opted to name this gene the ubiquitin/cytoplasmic r-protein gene 6 (ubi::crp-6). The ubi::crp-6 gene generates a 700-nucleotide (nt) transcript. It shares a 700-bp regulatory region with the cytoplasmic r-protein gene 5 (crp-5), a gene encoding the N. crassa S26 r-protein. The two genes are transcribed divergently from the common regulatory region and their mRNA levels are regulated in parallel during growth on a variety of carbon sources.

A cis-acting region required for the regulated expression of grg-1, a Neurospora glucose-repressible gene. Two regulatory sites (CRE and NRS) are required to repress grg-1 expression

Grg-1 is a Neurospora gene which was identified as being a highly expressed glucose-repressible gene. A cis-acting regulatory region required for the regulated expression of grg-1 has been characterized. The regulatory region is found between 440 and 500 nucleotides upstream of the first major grg-1 start of transcription site and contains two distinct cis-acting regulatory elements. The upstream element consists of the sequence GTGACGTCAC, which is identical to the previously identified CRE (Cyclic AMP-Responsive Element). The second element is a newly defined cis-acting regulatory site. The element has the sequence TTGCTAGCAA and has been named NRS (Neurospora Repressor Site). DNA binding proteins can be shown to bind to both of these cis-acting regulatory elements. Experiments in which these sites were deleted demonstrate that both cis-acting regulatory elements are required to turn off the in vitro expression of the grg-1 gene under conditions of glucose sufficiency.

Isolation, sequencing, and characterization of crp-5, a gene encoding a Neurospora ribosomal protein

A Neurospora crassa cytoplasmic ribosomal protein gene, crp-5, has been isolated and characterized. The cDNA was isolated by a differential screening of a cDNA library for glucose-inducible genes. The cDNA was subsequently used to identify and isolate crp-5 genomic sequences. Computer analysis of the DNA sequences showed that they contain an open reading frame which encodes a protein homologous to the rat ribosomal protein S26. The crp-5 mRNA levels are regulated in a carbon-source-dependent manner. The organization of the gene and the region upstream of the coding sequences are discussed.

Developing Dictyostelium cells contain the lysosomal enzyme alpha-mannosidase in a secretory granule

The prespore vesicle (PSV) is an organelle which secretes spore coat proteins and gal/galNAc polysaccharides from prespore cells of Dictyostelium. By combining the techniques of protein A-gold immunocytochemistry and ricin-gold affinity cytochemistry we have demonstrated colocalization of the lysosomal enzyme alpha-mannosidase with gal/galNAc polysaccharides in prespore vesicles and the spore coat. To determine the origin of prespore vesicles a series of pulse-chase experiments were performed. Cells were labeled with [35S]methionine or [35S]sulfate at different times during development and allowed to differentiate in the presence of unlabeled methionine or sulfate for various periods of time. The cells were homogenized and intracellular organelles were separated using Percoll density gradient centrifugation. The distribution of [35S]methionine-labeled alpha-mannosidase and [35S]sulfate-labeled glycoproteins in the Percoll gradients was determined. It was found that prespore vesicles contained protein which was previously found in lysosomes. Newly labeled protein also entered these vesicles. The data suggest that developing Dictyostelium cells either restructure preexisting lysosomes into prespore vesicles or transport protein between these two organelles. We propose that secretory granules and lysosomes may have a common biosynthetic origin and may be evolutionarily related.

Developing Dictyostelium discoideum cells contain two distinct acid hydrolase-containing vesicles

Two distinct populations of acid hydrolase-containing vesicles have been found in developing Dictyostelium discoideum cells. Percoll gradient centrifugation revealed these vesicle populations have densities of 1.07 and 1.13 g/ml. The 1.13 g/ml vesicle populations arose during the aggregation stage of differentiation. Although both vesicle populations contained an array of acid hydrolases, they could be shown to differ by several criteria. Electron micrographs of prespore cells showed they contained two types of vesicles with distinct acid phosphatase-staining patterns. One of these vesicle types appeared identical to the lysosomes found in vegetative cells. The second vesicle type had a morphology similar to that of a previously identified organelle, the prespore vesicle. The prespore vesicle is known to contain spore coat proteins which are exocytosed during the final stages of spore differentiation. The higher density acid hydrolase-containing vesicle population was found to contain spore coat proteins. Electron micrographs of the higher density vesicle population showed the presence of acid phosphatase-staining vesicles with a morphology similar to that of prespore vesicles. These data suggest that the higher density acid hydrolase-containing vesicles represent a subpopulation of lysosomes which appear during development and which may be identical to prespore vesicles.

Deoxyglucose-resistant mutants of Neurospora crassa: isolation, mapping, and biochemical characterization

Neurospora crassa mutants resistant to 2-deoxyglucose have been isolated, and their mutations have been mapped to four genetic loci. The mutants have the following characteristics: (i) they are resistant to sorbose as well as to 2-deoxyglucose; (ii) they are partially or completely constitutive for glucose transport system II, glucamylase, and invertase, which are usually repressed during growth on glucose; and (iii) they synthesize an invertase with abnormal thermostability and immunological properties, suggesting altered posttranslational modification. All of these characteristics could arise from defects in the regulation of carbon metabolism. In addition, mutants with mutations at three of the loci lack glucose transport system I, which is normally synthesized constitutively by wild-type N. crassa. Although the basis for this change is not yet clear, the mutants provide a way of studying the high-affinity system II uncomplicated by the presence of the low-affinity system I.

Isolation and characterization of a Neurospora glucose-repressible gene

Using differential hybridization, the cDNA copy of a Neurospora gene coding for an abundant glucose-repressible mRNA (grg-1) has been isolated. The cDNA was used to clone the genomic copy, and both were sequenced. The cDNA is nearly full length and contains putative translational start and termination codons. Conceptual translation indicates that grg-1 mRNA could direct the synthesis of a 7,000 molecular weight polypeptide. The genomic clone, contained in an 1,888 bp PvuII fragment, encompasses the entire cDNA as well as 838 bp of 5' and 369 bp of 3' flanking sequence. Comparison of the cDNA and genomic clones revealed the presence of two short introns in potential protein-coding sequences. grg-1 message levels were found to increase within minutes following the onset of glucose deprivation and rise 50 fold during the first 90 min of derepression.

A developmentally controlled change in the post-translational modifications on the lysosomal alpha-mannosidase of the cellular slime mould Dictyostelium discoideum

During the development of the cellular slime mould Dictyostelium discoideum, a second form of a number of lysosomal enzymes begins to accumulate. The second ('late') form of these enzymes differs from the pre-existing ('early') form in post-translational modification. Pulse-chase experiments using [35S]methionine show that the late form of alpha-mannosidase-1 is made by synthesis de novo starting 8 h after the onset of development. These experiments show there is no interconversion between early and late forms in vivo. A one-dimensional peptide map indicated that the early and late forms of alpha-mannosidase have similar amino acid sequences. The two forms have a similar half-life in vivo when measured during the same period of development. Double-labelling studies were performed with 35SO4 and [3H]leucine or 32PO4 and [3H]leucine. and these studies indicated that the oligosaccharides present on the early form of alpha-mannosidase contained more sulphate and phosphate than did those on the late form. The early enzyme had a 10-fold higher 35S/3H ratio and a 4-fold higher 32P/3H ratio. Endocytosis experiments using early and late alpha-mannoside showed that the early form was efficiently taken up by human fibroblasts, whereas the late form was poorly endocytosed. This suggests that the late form lacks the mannose 6-phosphate residue required for efficient uptake.

Neurospora trehalase and its structural gene

We have isolated Neurospora trehalaseless mutants and mapped the trehalase structural gene to linkage group I. The structural gene mutations not only affect thermostability and other characteristics of the enzyme but also affect the production of an inhibitor of the wild-type trehalase. The inhibitor appears to be the mutant trehalase. We suggest that the mutant subunits act as inhibitors by entering into the multimeric forms of the enzyme and altering the ability of the normal wild-type subunits to catalyze the cleavage of trehalose.--Wild type trehalase has been purified to near homogeneity, and its characteristics have been studied. It was purified as a tetramer, with each subunit having a molecular weight of 88,000.--We have studied the regulation of trehalase and found the production of trehalase to be glucose repressible. Cells begin to produce trehalase 60 min after being transferred to glucose-free medium.

Changes in the post-translational modification of lysosomal enzymes during development of Dictyostelium

The form of post-translational modification present on two lysosomal enzymes--acid phosphatase and alpha-mannosidase--changes as part of the developmental program of Dictyostelium discoideum. Prior to 8 h of development, all enzyme molecules are of a single modification type (early form enzyme). Starting at 8 h of development, enzyme molecules with a second type of modification (late-form enzymes) begin to appear in the cell. We separated the early and late forms of these enzymes from each other by chromatography on DEAE-cellulose. We found that the change in protein modification affects the enzymes' in vitro properties. The early and late forms of both of these enzymes differ in thermostability and susceptibility to proteolytic inactivation. We also found that the late form of alpha-mannosidase is preferentially secreted. We suggest that by synthesizing molecules with a second form of modification, the cell confers new characteristics to its lysosomal enzymes.

Neurospora glucamylase and a mutant affected in its regulation

Neurospora glucamylase is a glucose-repressible extracellular enzyme. The enzyme was purified to homogeneity and found to have a molecular weight of 82,000 and to release glucose from either maltose or amylose. The rate of glucamylase synthesis increases more than 100-fold when cells are transferred from a glucose-containing medium to a glucose-free medium. Increased from a glucose-containing medium to a glucose-free medium. Increased production of glucamylase begins within 30 min of the transfer. Glucamylase is rapidly secreted into the medium. A mutant affecting the ability of glucose to repress the synthesis of the glucose-repressible extracellular enzymes glucamylase and invertase has been isolated and studied. The mutant constitutively synthesizes and secretes a glucamylase which is indistinguishable from the wild-type enzyme.

Protein modification and its biological role

The modifications present on a polypeptide play an important role in determining its eventual fate. Modifications, particularly proteolysis, are important in the generation of biological activity. Modifications are used to "target" particular polypeptides to specific cellular locations. Protein modification also plays a role in determining the rate of polypeptide degradation. Cells have developed elaborate systems for the modification of their proteins because these modifications serve important biological functions.

Organization of the ribosomal ribonucleic acid genes in various wild-type strains and wild-collected strains of Neurospora

The organization of the ribosomal DNA (rDNA) repeat unit in the standard wild-type strain of Neurospora crassa, 74-OR23-1A, and in 30 other wild-type strains and wild-collected strains of N. crassa, . tetrasperma, N. sitophila, N. intermedia, and N. discreta isolated from nature, was investigated by restriction enzyme digestion of genomic DNA, and probing of the Southern-blotted DNA fragments with specific cloned pieces of the rDNA unit from 74-OR23-1A. The size of the rDNA unit in 74-OR23-1A was shown to be 9.20 kilobase pairs (kb) from blotting data, and the average for all strains was 9.11 + 0.21 kb; standard error = 0.038; coefficient of variation (C.V.) = 2.34%. These data indicate that the rDNA repeat unit size has been highly conserved among the Neurospora strains investigated. However, while all strains have a conserved HindIII site near the 5' end of the 25 S rDNA coding sequence, a polymorphism in the number and/or position of HindIII sites in the nontranscribed spacer region was found between strains. The 74-OR23-1A strain has two HindIII sites in the spacer, while others have from 0 to at least 3. This restriction site polymorphism is strain-specific and not species-specific. It was confirmed for some strains by restriction analysis of clones containing most of the rDNA repeat unit. The current restriction map of the 74-OR23-1A rDNA repeat unit is presented.

Isolation and characterization of Neurospora mutants affected in invertase synthesis

We have outlined a procedure that allows the large-scale screening of mutagenized Neurospora crassa populations for invertaseless mutants. We have isolated and characterized three mutations, inv(DBL1), inv(DBL9) and inv(DBL14), which have been mapped at or near the invertase structural gene. One of these, inv(DBL1), is particularly interesting. Our experiments indicate that the reduced level of invertase activity in the inv(DBL1)-containing cell can be explained as the result of a reduced number of normal enzyme molecules. We also show that wild-type Neurospora is able to respond rapidly to a change of medium and can dramatically increase its production of invertase within 20 min after a transfer to a carbon-free medium.

The synthesis of alkaline phosphatase in Neurospora crassa

Mutations which affect the regulation of Neurospora repressible alkaline phosphatase do so by altering the rate of de novo alkaline phosphatase synthesis. In regulatory mutants the rate of alkaline phosphatase polypeptide synthesis can vary over a 1000-fold range. Following transfer to phosphate-free medium, the wild-type cell is capable of increasing the rate of synthesis of alkaline phosphatase molecules within 30-45 min.

Arrangement of the genes coding for ribosomal ribonucleic acids in Neurospora crassa

We have cloned and characterized Neurospora crassa ribosomal deoxyribonucleic acid (rDNA). The rDNA is found as a tandemly repeated 6.0-megadalton sequence. We have mapped a portion of the rDNA repeat unit with respect to its sites for 13 restriction endonucleases and defined those regions coding for the 5. 8S, 17S, and 26S ribosomal ribonucleic acids (rRNA's). We have also isolated several clones containing 5S rRNA sequences. The 5S rRNA coding sequences are not found within the rDNA repeat unit. We found that the sequences surrounding the 5S rRNA coding regions are highly heterogeneous.