Introns and splicing elements of five diverse fungi

Genomic sequences and expressed sequence tag data for a diverse group of fungi (Saccharomyces cerevisiae, Schizosaccharomyces pombe, Aspergillus nidulans, Neurospora crassa, and Cryptococcus neoformans) provided the opportunity to accurately characterize conserved intronic elements. An examination of large intron data sets revealed that fungal introns in general are short, that 98% or more of them belong to the canonical splice site (ss) class (5'GU...AG3'), and that they have polypyrimidine tracts predominantly in the region between the 5' ss and the branch point. Information content is high in the 5' ss, branch site, and 3' ss regions of the introns but low in the exon regions adjacent to the introns in the fungi examined. The two yeasts have broader intron length ranges and correspondingly higher intron information content than the other fungi. Generally, as intron length increases in the fungi, so does intron information content. Homologs of U2AF spliceosomal proteins were found in all species except for S. cerevisiae, suggesting a nonconventional role for U2AF in the absence of canonical polypyrimidine tracts in the majority of introns. Our observations imply that splicing in fungi may be different from that in vertebrates and may require additional proteins that interact with polypyrimidine tracts upstream of the branch point. Theoretical protein homologs for Nam8p and TIA-1, two proteins that require U-rich regions upstream of the branch point to function, were found. There appear to be sufficient differences between S. cerevisiae and S. pombe introns and the introns of two filamentous members of the Ascomycota and one member of the Basidiomycota to warrant the development of new model organisms for studying the splicing mechanisms of fungi.     

Detection and characterization of fungal-specific proteins in Saccharomyces cerevisiae

In this study, I searched for fungal-specific proteins in the genome of the budding yeast Saccharomyces cerevisiae, inferred from a comparison of amino acid sequences. I used the GTOP (Genomes to Protein structures and functions) database of the DDBJ (DNA Data Bank of Japan), which consists of 21 genomes from Archaea, 203 genomes from Bacteria, and 50 genomes from Eucarya (including 18 fungal genomes). Among 5,874 proteins of S. cerevisiae, 1,551 have homologs only in Eucarya, and 504 of the 1,551 have homologs only in fungi. To find fungal-specific proteins, homologs of the homologs have been searched repeatedly. As a result, 132 of the 504 are characterized as fungal-specific proteins. The genes encoding the 132 fungal-specific proteins are not included in the list of essential genes for viability in the S. cerevisiae genome deletion project. Among the 132 proteins, 99 are S. cerevisiae-specific, and no protein that is distributed among 10 or more of the 18 fungal species exists. In addition, most of the fungal-specific proteins are very small and functionally unknown. My results show that the fungal-specific proteins have short evolutionary histories, suggesting that S. cerevisiae produces novel proteins and that ancestral fungi also produced small proteins most of which have disappeared or have been combined with other proteins during fungal evolution.     

真菌几丁质合酶的研究进展

真菌细胞壁几丁质的合成是一个复杂的过程, 其关键酶为几丁质合酶(CS)。近年来, 丝状真菌中的CS研究有了大的突破, 与酿酒酵母中只有3种CS不同, 丝状真菌中存在7种类别的CS。大部分临床和农业中重要的病原真菌都是丝状真菌, 文中对真菌中7种类别CS的结构和功能作了概述, 重点讨论了丝状真菌中重要的CS类别, 并介绍了CS作为抗真菌药物有效靶标的研究现状, 旨在为研究真菌CS及其抑制剂提供参考。     

真菌几丁质合酶的研究进展

真菌细胞壁几丁质的合成是一个复杂的过程,其关键酶为几丁质合酶(CS).近年来,丝状真菌中的CS研究有了大的突破,与酿酒酵母中只有3种CS不同,丝状真菌中存在7种类别的CS.大部分临床和农业中重要的病原真菌都是丝状真菌,文中对真菌中7种类别CS的结构和功能作了概述,重点讨论了丝状真菌中重要的CS类别,并介绍了CS作为抗真菌药物有效靶标的研究现状,旨在为研究真菌CS及其抑制剂提供参考.     

Fungal apoptosis: function, genes and gene function

Cells of all living organisms are programmed to self-destruct under certain conditions. The most well known form of programmed cell death is apoptosis, which is essential for proper development in higher eukaryotes. In fungi, apoptotic-like cell death occurs naturally during aging and reproduction, and can be induced by environmental stresses and exposure to toxic metabolites. The core apoptotic machinery in fungi is similar to that in mammals, but the apoptotic network is less complex and of more ancient origin. Only some of the mammalian apoptosis-regulating proteins have fungal homologs, and the number of protein families is drastically reduced. Expression in fungi of animal proteins that do not have fungal homologs often affects apoptosis, suggesting functional conservation of these components despite the absence of protein-sequence similarity. Functional analysis of Saccharomyces cerevisiae apoptotic genes, and more recently of those in some filamentous species, has revealed partial conservation, along with substantial differences in function and mode of action between fungal and human proteins. It has been suggested that apoptotic proteins might be suitable targets for novel antifungal treatments. However, implementation of this approach requires a better understanding of fungal apoptotic networks and identification of the key proteins regulating apoptotic-like cell death in fungi.     

Isolation and characterization of a novel ribosome-inactivating protein from root cultures of pokeweed and its mechanism of secretion from roots

Ribosome-inactivating proteins are N-glycosidases that remove a specific adenine from the sarcin/ricin loop of the large rRNA, thus arresting protein synthesis at the translocation step. In the present study, a novel type I ribosome-inactivating protein, termed PAP-H, was purified from Agrobacterium rhizogenes-transformed hairy roots of pokeweed (Phytolacca americana). The protein was purified by anion- and cation-exchange chromatography. PAP-H has a molecular mass of 29.5 kD as detected by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and its isoelectric point was determined to be 7.8. Yeast (Saccharomyces cerevisiae) ribosomes incubated with PAP-H released the 360-nucleotide diagnostic fragment from the 26S rRNA upon aniline treatment, an indication of its ribosome-inactivating activity. Using immunofluorescence microscopy, PAP-H was found to be located in the cell walls of hairy roots and root border cells. PAP-H was determined to be constitutively secreted as part of the root exudates, with its secretion enhanced by a mechanism mediated by ethylene induction. Purified PAP-H did not show in vitro antifungal activity against soil-borne fungi. In contrast, root exudates containing PAP-H as well as additional chitinase, beta-1,3-glucanase, and protease activities did inhibit the growth of soil-borne fungi. We found that PAP-H depurinates fungal ribosomes in vitro and in vivo, suggesting an additive mechanism that enables PAP-H to penetrate fungal cells.     

Morphogenesis, adhesive properties, and antifungal resistance depend on the Pmt6 protein mannosyltransferase in the fungal pathogen candida albicans

Protein mannosyltransferases (Pmt proteins) initiate O glycosylation of secreted proteins in fungi. We have characterized PMT6, which encodes the second Pmt protein of the fungal pathogen Candida albicans. The residues of Pmt6p are 21 and 42% identical to those of C. albicans Pmt1p and S. cerevisiae Pmt6p, respectively. Mutants lacking one or two PMT6 alleles grow normally and contain normal Pmt enzymatic activities in cell extracts but show phenotypes including a partial block of hyphal formation (dimorphism) and a supersensitivity to hygromycin B. The morphogenetic defect can be suppressed by overproduction of known components of signaling pathways, including Cek1p, Cph1p, Tpk2p, and Efg1p, suggesting a specific Pmt6p target protein upstream of these components. Mutants lacking both PMT1 and PMT6 are viable and show pmt1 mutant phenotypes and an additional sensitivity to the iron chelator ethylenediamine-di(o-hydroxyphenylacetic acid). The lack of Pmt6p significantly reduces adherence to endothelial cells and overall virulence in a mouse model of systemic infection. The results suggest that Pmt6p regulates a more narrow subclass of proteins in C. albicans than Pmt1p, including secreted proteins responsible for morphogenesis and antifungal sensitivities.     

Discovery of cercosporamide, a known antifungal natural product, as a selective Pkc1 kinase inhibitor through high-throughput screening

The Pkc1-mediated cell wall integrity-signaling pathway is highly conserved in fungi and is essential for fungal growth. We thus explored the potential of targeting the Pkc1 protein kinase for developing broad-spectrum fungicidal antifungal drugs through a Candida albicans Pkc1-based high-throughput screening. We discovered that cercosporamide, a broad-spectrum natural antifungal compound, but previously with an unknown mode of action, is actually a selective and highly potent fungal Pkc1 kinase inhibitor. This finding provides a molecular explanation for previous observations in which Saccharomyces cerevisiae cell wall mutants were found to be highly sensitive to cercosporamide. Indeed, S. cerevisiae mutant cells with reduced Pkc1 kinase activity become hypersensitive to cercosporamide, and this sensitivity can be suppressed under high-osmotic growth conditions. Together, the results demonstrate that cercosporamide acts selectively on Pkc1 kinase and, thus, they provide a molecular mechanism for its antifungal activity. Furthermore, cercosporamide and a beta-1,3-glucan synthase inhibitor echinocandin analog, by targeting two different key components of the cell wall biosynthesis pathway, are highly synergistic in their antifungal activities. The synergistic antifungal activity between Pkc1 kinase and beta-1,3-glucan synthase inhibitors points to a potential highly effective combination therapy to treat fungal infections.     

Structures of the glycosylphosphatidylinositol membrane anchors from Aspergillus fumigatus membrane proteins

Glycosylphosphatidylinositol (GPI)-anchored proteins have been identified in all eukaryotes. In fungi, structural and biosynthetic studies of GPIs have been restricted to the yeast Saccharomyces cerevisiae. In this article, four GPI-anchored proteins were purified from a membrane preparation of the human filamentous fungal pathogen Aspergillus fumigatus. Using new methodology applied to western blot protein bands, the GPI structures were characterized by ES-MS, fluorescence labeling, HPLC, and specific enzymatic digestions. The phosphatidylinositol moiety of the A. fumigatus GPI membrane anchors was shown to be an inositol-phosphoceramide containing mainly phytosphingosine and monohydroxylated C24:0 fatty acid. In constrast to yeast, only ceramide was found in the GPI anchor structures of A. fumigatus, even for Gel1p, a homolog of Gas1p in S. cerevisiae that contains diacylglycerol. The A. fumigatus GPI glycan moiety is mainly a linear pentomannose structure linked to a glucosamine residue: Manalpha1-3Manalpha1-2Manalpha1-2Manalpha1-6Manalpha1-4GlcN.     

Isolation and characterization from pathogenic fungi of genes encoding ammonium permeases and their roles in dimorphism

Nutrient sensing plays important roles in fungal development in general, and specifically in critical aspects of pathogenicity and virulence, for both animal and plant pathogens. Dimorphic pathogens such as the phytopathogenic smut fungi, Ustilago maydis and Microbotryum violaceum, must switch from a yeast-like to a filamentous form in order to cause disease. Two genes encoding methylammonium permeases (MEPs) were identified from each of these latter fungi and all the encoded proteins were most similar to Mep2p, the high-affinity permease from Saccharomyces cerevisiae that plays a direct role in pseudohyphal or filamentous growth for that organism. This is the first report of MEPs from pathogenic fungi. The two genes from U. maydis and one of the genes from M. violaceum were expressed in diploid S. cerevisiae mutants deleted for all three mep genes (mep1mep2mep3). Each of the heterologous genes could complement the severe growth defect of the S. cerevisiae mutant on low ammonium. Moreover, the U. maydis ump2 gene, initially detected as an upregulated gene in budding cells, was also able to complement the pseudohyphal defect characteristic of the mutant yeast. This gene is thus one of few heterologous MEP genes capable of efficiently restoring pseudohyphal growth in yeast. For U. maydis, disruption of ump2 eliminated the filamentous phenotype of haploid cells on low ammonium, while ump1 disruption only slightly reduced methylamine uptake. The most significant drop in methylamine uptake was seen for the ump2 and the ump1ump2 double mutants. Moreover, when grown in liquid medium, the ump1ump2 double mutant aggregated and sedimented. Also, the importance of a putative site for phosphorylation by protein kinase A was investigated in both Mep2p and Ump2p via site-directed mutagenesis of the respective genes. A mutation predicted to prevent phosphorylation of either protein, still allowed each to provide growth on low ammonium, but eliminated their abilities to provide pseudohyphal growth for the S. cerevisiae triple mutant. These findings allow us to present a model of how ammonium transporters play a role in regulating dimorphic growth in fungi.     

Survival strategies of yeast and filamentous fungi against the antifungal protein AFP

The activities of signaling pathways are critical for fungi to survive antifungal attack and to maintain cell integrity. However, little is known about how fungi respond to antifungals, particularly if these interact with multiple cellular targets. The antifungal protein AFP is a very potent inhibitor of chitin synthesis and membrane integrity in filamentous fungi and has so far not been reported to interfere with the viability of yeast strains. With the hypothesis that the susceptibility of fungi toward AFP is not merely dependent on the presence of an AFP-specific target at the cell surface but relies also on the cell's capacity to counteract AFP, we used a genetic approach to decipher defense strategies of the naturally AFP-resistant strain Saccharomyces cerevisiae. The screening of selected strains from the yeast genomic deletion collection for AFP-sensitive phenotypes revealed that a concerted action of calcium signaling, TOR signaling, cAMP-protein kinase A signaling, and cell wall integrity signaling is likely to safeguard S. cerevisiae against AFP. Our studies uncovered that the yeast cell wall gets fortified with chitin to defend against AFP and that this response is largely dependent on calcium/Crz1p signaling. Most importantly, we observed that stimulation of chitin synthesis is characteristic for AFP-resistant fungi but not for AFP-sensitive fungi, suggesting that this response is a successful strategy to protect against AFP. We finally propose the adoption of the damage-response framework of microbial pathogenesis for the interactions of antimicrobial proteins and microorganisms in order to comprehensively understand the outcome of an antifungal attack.     

Candida albicans Ssa1/2p is the cell envelope binding protein for human salivary histatin 5

Salivary histatins are a family of small histidine-rich peptides with potent antifungal activity. We previously identified a 70-kDa cell envelope protein in Candida albicans and Saccharomyces cerevisiae that mediates binding of histatin (Hst) 5. Isolation of Hst 5-binding protein followed by matrix-assisted laser desorption ionization mass spectrometry analysis identified this protein as the heat shock protein Ssa1p. Ssa protein and Hst 5-binding protein were found to be co-localized on immunoblots of yeast beta-mercaptoethanol cell wall extracts and cytosolic fractions. Yeast two-hybrid analysis showed strong interactions between Ssa1p and both Hst 3 and Hst 5. To assess functional roles of Ssa proteins in the Hst 5 antifungal mechanism in vivo, both binding and fungicidal assays were carried out using S. cerevisiae isogenic SSA1/SSA2 mutants. 125I-Hst 5 binding assays showed saturable binding (Kd = 2.57 x 10(-6) m) with the wild-type SSA1/SSA2 strain; however, Hst 5 binding with the Deltassa1ssa2 double mutant was reduced (Kd = 1.25 x 10(-6) m). Cell wall HSP70 proteins were also diminished, but still detectable, in S. cerevisiae Deltassa1ssa2 cells and are likely to be Ssa3p or Ssa4p. Hst 5 (31 microm) killed 80% of the wild-type cells in fungicidal assays at room temperature. However, only 50-60% killing of the single mutants (Deltassa1 and Deltassa2) was observed, and fungicidal activity was further reduced to 20-30% in the Deltassa1ssa2 double mutant. Incubation of cells under heat shock conditions increased the sensitivity of cells to Hst 5, which correlated with increased Hst 5-binding activity in Deltassa1ssa2 cells, but not in wild-type cells. This study provides evidence for a novel function for yeast Ssa1/2 proteins as cell envelope binding receptors for Hst 5 that mediate fungicidal activity.     

Targeting antioxidative signal transduction and stress response system: control of pathogenic Aspergillus with phenolics that inhibit mitochondrial function

AIMS: The aim of this study was to show whether antioxidative response systems are potentially useful molecular targets for control of Aspergillus fumigatus and Aspergillus flavus. Selected phenolic agents are used in target-gene-based bioassays to determine their impact on mitochondrial respiration. METHODS AND RESULTS: Vanillyl acetone, vanillic acid, vanillin, cinnamic acid, veratraldehyde, m-coumaric acid (phenolic agents to which Saccharomyces cerevisiae sod2delta mutant showed sensitivity), carboxin (inhibits complex II of the mitochondrial respiratory chain), strobilurins/antimycin A (inhibits complex III of the mitochondrial respiratory chain) and fludioxonil/fenpiclonil [antifungals potentiated by mitogen-activated protein kinase (MAPK)] were examined in A. fumigatus, A. flavus and S. cerevisiae. Individual or combined application of phenolics with inhibitors of mitochondrial respiration showed some of the phenolics effectively inhibited fungal growth. Target-gene bioassays were performed using a sakAdelta (MAPK deletion) strain of A. fumigatus and a complementation analysis using the mitochondrial superoxide dismutase (Mn-SOD) gene (sodA) of A. flavus in the ortholog mutant, sod2delta, of S. cerevisiae. The results demonstrated that mitochondrial antioxidative stress system plays important roles in fungal response to antifungal agents tested. CONCLUSIONS: Antioxidative response systems of fungi can be an efficient molecular target of phenolics for pathogen control. Combined application of phenolics with inhibitors of mitochondrial respiration can effectively suppress the growth of fungi. SIGNIFICANCE AND IMPACT OF THE STUDY: Natural compounds that do not pose any significant medical or environmental risks could serve as useful alternatives or additives to conventional antifungals. Identifying the antioxidative response systems in other pathogens could improve methods for fungal control.