Molecular cloning and sequencing of the cDNA and gene for a novel elastinolytic metalloproteinase from Aspergillus fumigatus and its expression in Escherichia coli

Functional Characterization of the M36 Metalloprotease FgFly1 in Fusarium graminearum

Fungalysin metallopeptidase (M36), a hydrolase, catalyzes the hydrolysis of alanine, glycine, etc. Normally, it is considered to play an important role in the progress of fungal infection. However, the function of fungalysin metallopeptidase (M36) in Fusarium graminearum has not been reported. In this study, we explored the biological functions of FgFly1, a fungalysin metallopeptidase (M36) of F. graminearum. We found that ΔFgFly1 did not affect the ability to produce DON toxin, although it inhibited spore germination during asexual reproduction and reduction in pathogenicity compared with PH-1. Therefore, we speculated that FgFly1 affects the pathogenicity of F.graminearum by affecting pathways related to wheat disease resistance. Target protein TaCAMTA (calmodulin-binding transcription activator) was selected by a yeast two-hybrid (Y2H) system. Then, the interaction between FgFly1 and TaCAMTA was verified by bimolecular fluorescent complimentary (BiFC) and luciferase complementation assay (LCA). Furthermore, compared with wild-type Arabidopsis thaliana, the morbidity level of ΔAtCAMTA was increased after infection with F.graminearum, and the expression level of NPR1 was significantly reduced. Based on the above results, we concluded that FgFly1 regulated F. graminearum pathogenicity by interacting with host cell CAMTA protein.

Biochemical Characteristics and Allergenic Activity of Common Fungus Allergens

Fungi form a large kingdom with more than 1.5 million species. Fungal spores are universal atmospheric components and are generally recognized as important causes of allergic disorders, including allergic rhinitis, allergic rhinosinusitis, asthma, and allergic bronchopulmonary aspergillosis. The 4 genera which have the closest connection with allergic disorder are Cladosporium, Alternaria, Aspergillus and Penicillium. The cDNA sequences of many fungi allergens and the amino acids involved in their immunoglobulin E binding and T-cell activation have already been elucidated. Until now, 111 allergens from 29 fungal genera have been approved by the International Allergen Nomenclature Sub-committee. This review mainly focuses on the biochemical characteristics and allergenic activity of important allergens from common environmental fungi.

Genomic, transcriptomic, and proteomic insights into the symbiosis of deep-sea tubeworm holobionts

Deep-sea hydrothermal vents and methane seeps are often densely populated by animals that host chemosynthetic symbiotic bacteria, but the molecular mechanisms of such host-symbiont relationship remain largely unclear. We characterized the symbiont genome of the seep-living siboglinid Paraescarpia echinospica and compared seven siboglinid-symbiont genomes. Our comparative analyses indicate that seep-living siboglinid endosymbionts have more virulence traits for establishing infections and modulating host-bacterium interaction than the vent-dwelling species, and have a high potential to resist environmental hazards. Metatranscriptome and metaproteome analyses of the Paraescarpia holobiont reveal that the symbiont is highly versatile in its energy use and efficient in carbon fixation. There is close cooperation within the holobiont in production and supply of nutrients, and the symbiont may be able to obtain nutrients from host cells using virulence factors. Moreover, the symbiont is speculated to have evolved strategies to mediate host protective immunity, resulting in weak expression of host innate immunity genes in the trophosome. Overall, our results reveal the interdependence of the tubeworm holobiont through mutual nutrient supply, a pathogen-type regulatory mechanism, and host-symbiont cooperation in energy utilization and nutrient production, which is a key adaptation allowing the tubeworm to thrive in deep-sea chemosynthetic environments.

Aspergillus fumigatus conidial metalloprotease Mep1p cleaves host complement proteins

Innate immunity in animals including humans encompasses the complement system, which is considered an important host defense mechanism against Aspergillus fumigatus, one of the most ubiquitous opportunistic human fungal pathogens. Previously, it has been shown that the alkaline protease Alp1p secreted from A. fumigatus mycelia degrades the complement components C3, C4, and C5. However, it remains unclear how the fungal spores (i.e. conidia) defend themselves against the activities of the complement system immediately after inhalation into the lung. Here, we show that A. fumigatus conidia contain a metalloprotease Mep1p, which is released upon conidial contact with collagen and inactivates all three complement pathways. In particular, Mep1p efficiently inactivated the major complement components C3, C4, and C5 and their activation products (C3a, C4a, and C5a) as well as the pattern-recognition molecules MBL and ficolin-1, either by directly cleaving them or by cleaving them to a form that is further broken down by other proteases of the complement system. Moreover, incubation of Mep1p with human serum significantly inhibited the complement hemolytic activity and conidial opsonization by C3b and their subsequent phagocytosis by macrophages. Together, these results indicate that Mep1p associated with and released from A. fumigatus conidia likely facilitates early immune evasion by disarming the complement defense in the human host.

Investigation of the Fusarium virguliforme Transcriptomes Induced during Infection of Soybean Roots Suggests that Enzymes with Hydrolytic Activities Could Play a Major Role in Root Necrosis

Sudden death syndrome (SDS) is caused by the fungal pathogen, Fusarium virguliforme, and is a major threat to soybean production in North America. There are two major components of this disease: (i) root necrosis and (ii) foliar SDS. Root symptoms consist of root necrosis with vascular discoloration. Foliar SDS is characterized by interveinal chlorosis and leaf necrosis, and in severe cases by flower and pod abscission. A major toxin involved in initiating foliar SDS has been identified. Nothing is known about how root necrosis develops. In order to unravel the mechanisms used by the pathogen to cause root necrosis, the transcriptome of the pathogen in infected soybean root tissues of a susceptible cultivar, 'Essex', was investigated. The transcriptomes of the germinating conidia and mycelia were also examined. Of the 14,845 predicted F. virguliforme genes, we observed that 12,017 (81%) were expressed in germinating conidia and 12,208 (82%) in mycelia and 10,626 (72%) in infected soybean roots. Of the 10,626 genes induced in infected roots, 224 were transcribed only following infection. Expression of several infection-induced genes encoding enzymes with oxidation-reduction properties suggests that degradation of antimicrobial compounds such as the phytoalexin, glyceollin, could be important in early stages of the root tissue infection. Enzymes with hydrolytic and catalytic activities could play an important role in establishing the necrotrophic phase. The expression of a large number of genes encoding enzymes with catalytic and hydrolytic activities during the late infection stages suggests that cell wall degradation could be involved in root necrosis and the establishment of the necrotrophic phase in this pathogen.

Fungal inhibitors of proteolytic enzymes: classification, properties, possible biological roles, and perspectives for practical use

Peptidase inhibitors are ubiquitous regulatory proteins controlling catalytic activity of proteolytic enzymes. Interest in these proteins increased substantially after it became clear that they can be used for therapy of various important diseases including cancer, malaria, and autoimmune and neurodegenerative diseases. In this review we summarize available data on peptidase inhibitors from fungi, emphasizing their properties, biological role, and possible practical applications of these proteins in the future. A number of fungal peptidase inhibitors with unique structure and specificity of action have no sequence homology with other classes of peptidase inhibitors, thus representing new and specific candidates for therapeutic use. The main classifications of inhibitors in current use are considered. Available data on structure, mechanisms and conditions of action, and diversity of functions of peptidase inhibitors of fungi are analyzed. It is mentioned that on one side the unique properties of some inhibitors can be used for selective inhibition of peptidases responsible for initiation and development of pathogenic processes. On the other side, general inhibitory activity of other inhibitors towards peptidases of various catalytic classes might be able to provide efficient defense of transgenic plants against insect pests by overcoming compensatory synthesis of new peptidases by these pests in response to introduction of a fungal inhibitor. Together, the data analyzed in this review reveal that fungal inhibitors extend the spectrum of known peptidase inhibitors potentially suitable for use in medicine and agriculture.

A functional and structural study of the major metalloprotease secreted by the pathogenic fungusAspergillus fumigatus

Fungalysins are secreted fungal peptidases with the ability to degrade the extracellular matrix proteins elastin and collagen and are thought to act as virulence factors in diseases caused by fungi. Fungalysins constitute a unique family among zinc-dependent peptidases that bears low sequence similarity to known bacterial peptidases of the thermolysin family. The crystal structure of the archetype of the fungalysin family,Aspergillus fumigatusmetalloprotease (AfuMep), has been obtained for the first time. The 1.8 Å resolution structure of AfuMep corresponds to that of an autoproteolyzed proenzyme with separate polypeptide chains corresponding to the N-terminal prodomain in a binary complex with the C-terminal zinc-bound catalytic domain. The prodomain consists of a tandem of cystatin-like folds whose C-terminal end is buried into the active-site cleft of the catalytic domain. The catalytic domain harbouring the key catalytic zinc ion and its ligands, two histidines and one glutamic acid, undergoes a conspicuous rearrangement of its N-terminal end during maturation. One key positively charged amino-acid residue and the C-terminal disulfide bridge appear to contribute to its structural–functional properties. Thus, structural, biophysical and biochemical analysis were combined to provide a deeper comprehension of the underlying properties ofA. fumigatusfungalysin, serving as a framework for the as yet poorly known metallopeptidases from pathogenic fungi.

Identification of a chitinase-modifying protein from Fusarium verticillioides: truncation of a host resistance protein by a fungalysin metalloprotease

Chitinase-modifying proteins (cmps) are proteases secreted by fungal pathogens that truncate the plant class IV chitinases ChitA and ChitB during maize ear rot. cmp activity has been characterized for Bipolaris zeicola and Stenocarpella maydis, but the identities of the proteases are not known. Here, we report that cmps are secreted by multiple species from the genus Fusarium, that cmp from Fusarium verticillioides (Fv-cmp) is a fungalysin metalloprotease, and that it cleaves within a sequence that is conserved in class IV chitinases. Protein extracts from Fusarium cultures were found to truncate ChitA and ChitB in vitro. Based on this activity, Fv-cmp was purified from F. verticillioides. N-terminal sequencing of truncated ChitA and MALDI-TOF-MS analysis of reaction products showed that Fv-cmp is an endoprotease that cleaves a peptide bond on the C-terminal side of the lectin domain. The N-terminal sequence of purified Fv-cmp was determined and compared with a set of predicted proteins, resulting in its identification as a zinc metalloprotease of the fungalysin family. Recombinant Fv-cmp also truncated ChitA, confirming its identity, but had reduced activity, suggesting that the recombinant protease did not mature efficiently from its propeptide-containing precursor. This is the first report of a fungalysin that targets a nonstructural host protein and the first to implicate this class of virulence-related proteases in plant disease.

What makes Aspergillus fumigatus a successful pathogen? Genes and molecules involved in invasive aspergillosis

Aspergillus fumigatus is an opportunistic pathogen that causes 90% of invasive aspergillosis (IA) due to Aspergillus genus, with a 50-95% mortality rate. It has been postulated that certain virulence factors are characteristic of A. fumigatus, but the "non-classical" virulence factors seem to be highly variable. Overall, published studies have demonstrated that the virulence of this fungus is multifactorial, associated with its structure, its capacity for growth and adaptation to stress conditions, its mechanisms for evading the immune system and its ability to cause damage to the host. In this review we intend to give a general overview of the genes and molecules involved in the development of IA. The thermotolerance section focuses on five genes related with the capacity of the fungus to grow at temperatures above 30°C (thtA, cgrA, afpmt1, kre2/afmnt1, and hsp1/asp f 12). The following sections discuss molecules and genes related to interaction with the host and with the immune responses. These sections include β-glucan, α-glucan, chitin, galactomannan, galactomannoproteins (afmp1/asp f 17 and afmp2), hydrophobins (rodA/hyp1 and rodB), DHN-melanin, their respective synthases (fks1, rho1-4, ags1-3, chsA-G, och1-4, mnn9, van1, anp1, glfA, pksP/alb1, arp1, arp2, abr1, abr2, and ayg1), and modifying enzymes (gel1-7, bgt1, eng1, ecm33, afpigA, afpmt1-2, afpmt4, kre2/afmnt1, afmnt2-3, afcwh41 and pmi); several enzymes related to oxidative stress protection such as catalases (catA, cat1/catB, cat2/katG, catC, and catE), superoxide dismutases (sod1, sod2, sod3/asp f 6, and sod4), fatty acid oxygenases (ppoA-C), glutathione tranferases (gstA-E), and others (afyap1, skn7, and pes1); and efflux transporters (mdr1-4, atrF, abcA-E, and msfA-E). In addition, this review considers toxins and related genes, such as a diffusible toxic substance from conidia, gliotoxin (gliP and gliZ), mitogillin (res/mitF/asp f 1), hemolysin (aspHS), festuclavine and fumigaclavine A-C, fumitremorgin A-C, verruculogen, fumagillin, helvolic acid, aflatoxin B1 and G1, and laeA. Two sections cover genes and molecules related with nutrient uptake, signaling and metabolic regulations involved in virulence, including enzymes, such as serine proteases (alp/asp f 13, alp2, and asp f 18), metalloproteases (mep/asp f 5, mepB, and mep20), aspartic proteases (pep/asp f 10, pep2, and ctsD), dipeptidylpeptidases (dppIV and dppV), and phospholipases (plb1-3 and phospholipase C); siderophores and iron acquisition (sidA-G, sreA, ftrA, fetC, mirB-C, and amcA); zinc acquisition (zrfA-H, zafA, and pacC); amino acid biosynthesis, nitrogen uptake, and cross-pathways control (areA, rhbA, mcsA, lysF, cpcA/gcn4p, and cpcC/gcn2p); general biosynthetic pathway (pyrG, hcsA, and pabaA), trehalose biosynthesis (tpsA and tpsB), and other regulation pathways such as those of the MAP kinases (sakA/hogA, mpkA-C, ste7, pbs2, mkk2, steC/ste11, bck1, ssk2, and sho1), G-proteins (gpaA, sfaD, and cpgA), cAMP-PKA signaling (acyA, gpaB, pkaC1, and pkaR), His kinases (fos1 and tcsB), Ca(2+) signaling (calA/cnaA, crzA, gprC and gprD), and Ras family (rasA, rasB, and rhbA), and others (ace2, medA, and srbA). Finally, we also comment on the effect of A. fumigatus allergens (Asp f 1-Asp f 34) on IA. The data gathered generate a complex puzzle, the pieces representing virulence factors or the different activities of the fungus, and these need to be arranged to obtain a comprehensive vision of the virulence of A. fumigatus. The most recent gene expression studies using DNA-microarrays may be help us to understand this complex virulence, and to detect targets to develop rapid diagnostic methods and new antifungal agents.

Heterotrimeric G protein mediated regulation of proteinase production in Aspergillus nidulans

Extracellular proteinase production induced by carbon starvation was studied in a series of heterotrimeric G protein signaling pathway mutants of Aspergillus nidulans. All the mutants tested--including deltafadA (Galpha), deltasfaD (Gbeta), deltagpgA (Ggamma) and deltasfgA (regulator of FadA signaling)--showed an elevated proteinase production after glucose depletion. Our results strongly support the view that during growth, FadA/SfaD/GpgA G protein signaling inhibits proteinase production via both Galpha and Gbetagamma subunits, and all conditions, which are not sufficient to support vegetative growth and, hence, inhibit this type of G protein signaling, elevate extracellular proteinase activities.

Post-antifungal effects of the antifungal compound 2-(3,4-dimethyl-2,5-dihydro-1H-pyrrol-2-yl)-1-methylethyl pentanoate on Aspergillus fumigatus

The post-antifungal effect (PAFE) of the antifungal compound 2-(3,4-dimethyl-2,5-dihydro-1H-pyrrol-2-yl)-1-methylethyl pentanoate (DHP) upon Aspergillus fumigatus was investigated. The conidia of A. fumigatus were exposed to DHP at concentrations of 1× and 4× MIC90 for variable times at 37 °C. Amphotericin B (AmB)-treated or drug-free controls were included in the study. DHP as well as AmB exposure resulted in prolonged lag phases of the turbidimetric growth curves. Both the treatments gave rise to delayed growth, with lag phases of 11 h upon treatment with a concentration of 4× MIC90 for 4 h. Furthermore, it was observed that DHP inhibited the expression of three A. fumigatus secretory proteins of 18, 42 and 55 kDa. One protein of 42 kDa was found to be a metalloprotease, which is an important virulence factor. Analysis of time-dependent antigenic profiles showed the early expression of high-molecular-mass antigens. Expression of low-molecular-mass antigens started after 24 h culture. The antigens of A. fumigatus that are expressed during the early phase of growth were observed to be adversely affected after treatment with DHP. Although the mechanism of action of DHP to inhibit these proteins/antigens is unknown, the observations may be valuable to understand their role in the virulence of the pathogen, as well as the antigen-mediated responses caused by A. fumigatus.

Characterization of the zinc-containing metalloprotease encoded by zpx and development of a species-specific detection method for Enterobacter sakazakii

Enterobacter sakazakii causes a severe form of neonatal meningitis that occurs as sporadic cases as well as outbreaks. The disease has been epidemiologically associated with consumption of reconstituted, dried infant formulas. Very little information is available regarding pathogenicity of the organism and production of virulence factors. Clinical and environmental strains were screened for production of factors which have activity against Chinese hamster ovary (CHO) cells in tissue culture. Polymyxin B lysate and sonicate preparations but not culture supernatants from the strains caused "rounding" of CHO cells. Subsequent studies showed that the CHO cell-rounding factor is a proteolytic enzyme that has activity against azocasein. The cell-bound protease was isolated by using a combination of polymyxin B lysis, followed by sonication of cells harvested from tryptone broth. The protease was purified to homogeneity by sequential ammonium sulfate precipitation, gel filtration chromatography with Sephadex G-100, hydrophobic interaction chromatography with phenyl-Sepharose CL-4B, and a second gel filtration with Sephadex G-100. In addition to activity against azocasein, the purified protease also exhibits activity against azocoll and insoluble casein but not elastin. The protease has a molecular weight of 38,000 and an isoelectric point of 4.4. It is heat labile and for maximal activity against azocasein has an optimum temperature of 37 degrees C and a pH range of 5 to 7. Proteolytic activity is inhibited by ortho-phenanthroline and Zincov but is not affected by phenylmethylsulfonyl fluoride, N-ethylmaleimide, and trypsin inhibitors, which demonstrates that the protease is a zinc-containing metalloprotease. The metalloprotease does not hemagglutinate chicken or sheep erythrocytes. Twenty-three to 27 of the first 42 N-terminal amino acid residues of the metalloprotease are identical to proteases produced by Serratia proteamaculans, Pectobacterium carotovorum, and Anabaena sp. PCR analysis using primers designed from a consensus nucleotide sequence showed that 135 E. sakazakii strains possessed the metalloprotease gene, zpx, and 25 non-E. sakazakii strains did not. The cloned zpx gene of strain 29544 consists of 1,026 nucleotides, and the deduced amino acid sequence of the metalloprotease has 341 amino acid residues, which corresponds to a theoretical protein size of 37,782 with a theoretical pI of 5.23. The sequence possesses three well-characterized zinc-binding and active-site motifs present in other bacterial zinc metalloproteases.

Sedolisins, a new class of secreted proteases from Aspergillus fumigatus with endoprotease or tripeptidyl-peptidase activity at acidic pHs

The secreted proteolytic activity of Aspergillus fumigatus is of potential importance as a virulence factor and in the industrial hydrolysis of protein sources. The A. fumigatus genome contains sequences that could encode a five-member gene family that produces proteases in the sedolisin family (MEROPS S53). Four putative secreted sedolisins with a predicted 17- to 20-amino-acid signal sequence were identified and termed SedA to SedD. SedA produced heterologously in Pichia pastoris was an acidic endoprotease. Heterologously produced SedB, SedC, and SedD were tripeptidyl-peptidases (TPP) with a common specificity for tripeptide-p-nitroanilide substrates at acidic pHs. Purified SedB hydrolyzed the peptide Ala-Pro-Gly-Asp-Arg-Ile-Tyr-Val-His-Pro-Phe to Arg-Pro-Gly, Asp-Arg-Ile, and Tyr-Val-His-Pro-Phe, thereby confirming TPP activity of the enzyme. SedB, SedC, and SedD were detected by Western blotting in culture supernatants of A. fumigatus grown in a medium containing hemoglobin as the sole nitrogen source. A degradation product of SedA also was observed. A search for genes encoding sedolisin homologues in other fungal genomes indicates that sedolisin gene families are widespread among filamentous ascomycetes.

Genes and molecules involved in Aspergillus fumigatus virulence

Aspergillus fumigatus causes a wide range of diseases that include mycotoxicosis, allergic reactions and systemic diseases (invasive aspergillosis) with high mortality rates. Pathogenicity depends on immune status of patients and fungal strain. There is no unique essential virulence factor for development of this fungus in the patient and its virulence appears to be under polygenetic control. The group of molecules and genes associated with the virulence of this fungus includes many cell wall components, such as beta-(1-3)-glucan, galactomannan, galactomannanproteins (Afmp1 and Afmp2), and the chitin synthetases (Chs; chsE and chsG), as well as others. Some genes and molecules have been implicated in evasion from the immune response, such as the rodlets layer (rodA/hyp1 gene) and the conidial melanin-DHN (pksP/alb1 gene). The detoxifying systems for Reactive Oxygen Species (ROS) by catalases (Cat1p and Cat2p) and superoxide dismutases (MnSOD and Cu, ZnSOD), had also been pointed out as essential for virulence. In addition, this fungus produces toxins (14 kDa diffusible substance from conidia, fumigaclavin C, aurasperon C, gliotoxin, helvolic acid, fumagilin, Asp-hemolysin, and ribotoxin Asp fI/mitogilin F/restrictocin), allergens (Asp f1 to Asp f23), and enzymatic proteins as alkaline serin proteases (Alp and Alp2), metalloproteases (Mep), aspartic proteases (Pep and Pep2), dipeptidyl-peptidases (DppIV and DppV), phospholipase C and phospholipase B (Plb1 and Plb2). These toxic substances and enzymes seems to be additive and/or synergistic, decreasing the survival rates of the infected animals due to their direct action on cells or supporting microbial invasion during infection. Adaptation ability to different trophic situations is an essential attribute of most pathogens. To maintain its virulence attributes A. fumigatus requires iron obtaining by hydroxamate type siderophores (ornitin monooxigenase/SidA), phosphorous obtaining (fos1, fos2, and fos3), signal transductional falls that regulate morphogenesis and/or usage of nutrients as nitrogen (rasA, rasB, rhbA), mitogen activated kinases (sakA codified MAP-kinase), AMPc-Pka signal transductional route, as well as others. In addition, they seem to be essential in this field the amino acid biosynthesis (cpcA and homoaconitase/lysF), the activation and expression of some genes at 37 degrees C (Hsp1/Asp f12, cgrA), some molecules and genes that maintain cellular viability (smcA, Prp8, anexins), etc. Conversely, knowledge about relationship between pathogen and immune response of the host has been improved, opening new research possibilities. The involvement of non-professional cells (endothelial, and tracheal and alveolar epithelial cells) and professional cells (natural killer or NK, and dendritic cells) in infection has been also observed. Pathogen Associated Molecular Patterns (PAMP) and Patterns Recognizing Receptors (PRR; as Toll like receptors TLR-2 and TLR-4) could influence inflammatory response and dominant cytokine profile, and consequently Th response to infec tion. Superficial components of fungus and host cell surface receptors driving these phenomena are still unknown, although some molecules already associated with its virulence could also be involved. Sequencing of A. fumigatus genome and study of gene expression during their infective process by using DNA microarray and biochips, promises to improve the knowledge of virulence of this fungus.

EST analysis of genes expressed by the zygomycete pathogen Conidiobolus coronatus during growth on insect cuticle

Conidiobolus coronatus (Zygomycota) is a facultative saprobe that is a pathogen of many insect species. Almost 2000 expressed sequence tag (EST) cDNA clones were sequenced to analyse gene expression during growth on insect cuticle. Sixty percent of the ESTs that could be clustered into functional groups (E<or=10(-5)) had their best BLAST hits among fungal sequences. These included chitinases and multiple subtilisins, trypsin, metalloprotease and aspartyl protease activities with the potential to degrade host tissues and disable anti-microbial peptides. Otherwise, compared to the ascomycete entomopathogen Metarhizium anisopliae, Con. coronatus produced many fewer types of hydrolases (e.g. no phospholipases), antimicrobial agents, toxic secondary metabolites and no ESTs with putative roles in the generation of antibiotics. Instead, Con. coronatus produced a much higher proportion of ESTs encoding ribosomal proteins and enzymes of intermediate metabolism that facilitate its rapid growth. These results are consistent with Con. coronatus having adapted a modification of the saprophytic ruderal-selected strategy, using rapid growth to overwhelm the host and exploit the cadaver before competitors overrun it. This strategy does not preclude specialization to pathogenicity, as Con. coronatus produces the greatest complexity of proteases on insect cuticle, indicating an ability to respond to conditions in the cuticle.

Secreted proteases from pathogenic fungi

Many species of human pathogenic fungi secrete proteases in vitro or during the infection process. Secreted endoproteases belong to the aspartic proteases of the pepsin family, serine proteases of the subtilisin family, and metalloproteases of two different families. To these proteases has to be added the non-pepsin-type aspartic protease from Aspergillus niger and a unique chymotrypsin-like protease from Coccidioides immitis. Pathogenic fungi also secrete aminopeptidases, carboxypeptidases and dipeptidyl-peptidases. The function of fungal secreted proteases and their importance in infections vary. It is evident that secreted proteases are important for the virulence of dermatophytes since these fungi grow exclusively in the stratum corneum, nails or hair, which constitutes their sole nitrogen and carbon sources. The aspartic proteases secreted by Candida albicans are involved in the adherence process and penetration of tissues, and in interactions with the immune system of the infected host. For Aspergillus fumigatus, the role of proteolytic activity has not yet been proved. Although the secreted proteases have been intensively investigated as potential virulence factors, knowledge on protease substrate specificities is rather poor and few studies have focused on the research of inhibitors. Knowledge of substrate specificities will increase our understanding about the action of each protease secreted by pathogenic fungi and will help to determine their contribution to virulence.

The Aspergillus nidulans xprF gene encodes a hexokinase-like protein involved in the regulation of extracellular proteases

The extracellular proteases of Aspergillus nidulans are produced in response to limitation of carbon, nitrogen, or sulfur, even in the absence of exogenous protein. Mutations in the A. nidulans xprF and xprG genes have been shown to result in elevated levels of extracellular protease in response to carbon limitation. The xprF gene was isolated and sequence analysis indicates that it encodes a 615-amino-acid protein, which represents a new type of fungal hexokinase or hexokinase-like protein. In addition to their catalytic role, hexokinases are thought to be involved in triggering carbon catabolite repression. Sequence analysis of the xprF1 and xprF2 alleles showed that both alleles contain nonsense mutations. No loss of glucose or fructose phosphorylating activity was detected in xprF1 or xprF2 mutants. There are two possible explanations for this observation: (1) the xprF gene may encode a minor hexokinase or (2) the xprF gene may encode a protein with no hexose phosphorylating activity. Genetic evidence suggests that the xprF and xprG genes are involved in the same regulatory pathway. Support for this hypothesis was provided by the identification of a new class of xprG(-) mutation that suppresses the xprF1 mutation and results in a protease-deficient phenotype.

The immunologically protective P-4 antigen of Leishmania amastigotes. A developmentally regulated single strand-specific nuclease associated with the endoplasmic reticulum

The purified membrane-associated Leishmania pifanoi amastigote protein P-4 has been shown to induce protective immunity against infection and to elicit preferentially a T helper 1-like response in peripheral blood mononuclear cells of patients with American cutaneous leishmaniasis. As this molecule is potentially important for future vaccine studies, the L. pifanoi gene encoding the P-4 membrane protein was cloned and sequenced. Southern blot analyses indicate the presence of six tandemly arrayed copies of the P-4 gene in L. pifanoi; homologues of the P-4 gene are found in all other species of the genus Leishmania examined. DNA-derived protein sequence data indicated an identity to the P1 zinc-dependent nuclease of Penicillium citrinum (20.8%) and the C-terminal domain of the 3' nucleotidase of Leishmania donovani (33.7%). Consistent with these sequence analyses, purified L. pifanoi P-4 protein possesses single strand nuclease (DNA and RNA) and phosphomonoesterase activity, with a preference for UMP > TMP > AMP >> CMP. Double-labeling immunofluorescence microscopic analyses employing anti-binding protein antibodies revealed that the P-4 protein is localized in the endoplasmic reticulum of the amastigote. Northern blot analyses indicated that the gene is selectively expressed in the intracellular amastigote stage (mammalian host) but not in the promastigote stage (insect) of the parasite. Based upon its subcellular localization and single-stranded specific nuclease activity, possible roles of the P-4 nuclease in the amastigote in RNA stability (gene expression) or DNA repair are discussed.

Multiple forms of the serine protease Alp of Aspergillus fumigatus

Alkaline proteases were produced by a virulent strain of Aspergillus fumigatus during growth on media containing glucose and proteins or peptides. After isoelectric focusing, six bands of proteolytic activity were detected with synthetic substrates after blotting on nitrocellulose membranes. The main protease (pI = 8.6) corresponded to the known subtilisin-like protease Alp of A. fumigatus and five minor components had lower isoelectric points (8.1 to 5.2). All proteases were produced on different media and in various phases of growth with only small quantitative variations. They also had identical pH optima, were denatured above 45 degrees C and stabilized by Ca2+ ions, were affected by the inhibitors of serine proteases only and had nearly identical substrate specificity against 13 synthetic substrates. On gel chromatography the three most acidic components had higher molecular weights than the main enzyme Alp. It remains to be determined if the enzymes under study arise through posttranslational processing of the main protease or are true isoenzymes, products of a gene family.

Requirement for either a host- or pectin-induced pectate lyase for infection of Pisum sativum by Nectria hematococca

Fungal pathogens usually have multiple genes that encode extracellular hydrolytic enzymes that may degrade the physical barriers in their hosts during the invasion process. Nectria hematococca, a plant pathogen, has two inducible pectate lyase (PL) genes (pel) encoding PL that can help degrade the carbohydrate barrier in the host. pelA is induced by pectin, whereas pelD is induced only in planta. We show that the disruption of either the pelA or pelD genes alone causes no detectable decrease in virulence. Disruption of both pelA and pelD drastically reduces virulence. Complementation of the double disruptant with pelD gene, or supplementation of the infection droplets of the double disruptant with either purified enzyme, PLA, or PLD, caused a recovery in virulence. These results show that PL is a virulence factor. Thus, we demonstrate that disruption of all functionally redundant genes is required to demonstrate the role of host barrier-degrading enzymes in pathogenesis and that dismissal of the role of such enzymes based on the effects of single-gene disruption may be premature.

Zinc-regulated biosynthesis of immunodominant antigens from Aspergillus spp

ASPND1 and ASPF2 are immunodominant antigens from Aspergillus nidulans and A. fumigatus, respectively, that are readily synthesized in infections in the human host, as demonstrated by their reactivity with more than 80% of sera from patients with aspergilloma or allergic bronchopulmonary aspergillosis. We demonstrate here that both antigens are exclusively produced under situations of low bioavailability of free Zn2+. Addition of micromolar concentrations of Zn2+ to the culture medium strongly stimulated Aspergillus growth but totally inhibited ASPND1 or ASPF2 production. This effect was specific, since other divalent metals had no effect. Removal of endogenous Zn2+ by a chelator also stimulated ASPND1 production, and the effect was specifically reversed by Zn2+. These results suggest a possible role of these antigens in the survival of the fungus in the lungs.

Aspergillus fumigatus and aspergillosis

Aspergillus fumigatus is one of the most ubiquitous of the airborne saprophytic fungi. Humans and animals constantly inhale numerous conidia of this fungus. The conidia are normally eliminated in the immunocompetent host by innate immune mechanisms, and aspergilloma and allergic bronchopulmonary aspergillosis, uncommon clinical syndromes, are the only infections observed in such hosts. Thus, A. fumigatus was considered for years to be a weak pathogen. With increases in the number of immunosuppressed patients, however, there has been a dramatic increase in severe and usually fatal invasive aspergillosis, now the most common mold infection worldwide. In this review, the focus is on the biology of A. fumigatus and the diseases it causes. Included are discussions of (i) genomic and molecular characterization of the organism, (ii) clinical and laboratory methods available for the diagnosis of aspergillosis in immunocompetent and immunocompromised hosts, (iii) identification of host and fungal factors that play a role in the establishment of the fungus in vivo, and (iv) problems associated with antifungal therapy.

Molecular and biotechnological aspects of microbial proteases

Proteases represent the class of enzymes which occupy a pivotal position with respect to their physiological roles as well as their commercial applications. They perform both degradative and synthetic functions. Since they are physiologically necessary for living organisms, proteases occur ubiquitously in a wide diversity of sources such as plants, animals, and microorganisms. Microbes are an attractive source of proteases owing to the limited space required for their cultivation and their ready susceptibility to genetic manipulation. Proteases are divided into exo- and endopeptidases based on their action at or away from the termini, respectively. They are also classified as serine proteases, aspartic proteases, cysteine proteases, and metalloproteases depending on the nature of the functional group at the active site. Proteases play a critical role in many physiological and pathophysiological processes. Based on their classification, four different types of catalytic mechanisms are operative. Proteases find extensive applications in the food and dairy industries. Alkaline proteases hold a great potential for application in the detergent and leather industries due to the increasing trend to develop environmentally friendly technologies. There is a renaissance of interest in using proteolytic enzymes as targets for developing therapeutic agents. Protease genes from several bacteria, fungi, and viruses have been cloned and sequenced with the prime aims of (i) overproduction of the enzyme by gene amplification, (ii) delineation of the role of the enzyme in pathogenecity, and (iii) alteration in enzyme properties to suit its commercial application. Protein engineering techniques have been exploited to obtain proteases which show unique specificity and/or enhanced stability at high temperature or pH or in the presence of detergents and to understand the structure-function relationships of the enzyme. Protein sequences of acidic, alkaline, and neutral proteases from diverse origins have been analyzed with the aim of studying their evolutionary relationships. Despite the extensive research on several aspects of proteases, there is a paucity of knowledge about the roles that govern the diverse specificity of these enzymes. Deciphering these secrets would enable us to exploit proteases for their applications in biotechnology.

Virulence of an aspergillopepsin-deficient mutant of Aspergillus fumigatus and evidence for another aspartic proteinase linked to the fungal cell wall

A gene replacement was performed to produce mutants of Aspergillus fumigatus deficient in the aspergillopepsin PEP (E.C. 3.4.23.18). The correct replacement of the PEP gene was confirmed by PCR and Southern hybridization experiments, whereas the absence of PEP production was demonstrated by Western blots. The culture supernatant of the transformants showed no detectable acid proteinase activity, suggesting that there is only one acid proteinase secreted by the fungus. The wild-type strain and the PEP-deficient mutants invaded tissues to a similar extent and produced comparable mortality in guinea pigs. As PEP represents a third secretory proteinase of A. fumigatus and the other two proteinases also did not show significant influence on fungal invasiveness, it is probable that secreted proteinases do not contribute decisively to tissue invasion in the pathogenesis of systemic aspergillosis. However, immunofluorescence on A. fumigatus colonies using polyclonal antibodies to PEP showed a similar pattern for the wild-type and for the mutants, with a bright fluorescence on young conidiophores, on submerged mycelium and on the tips of growing aerial mycelium. Conidia and mature aerial hyphae were not fluorescent. This pattern could reflect the existence of another crossreacting aspartic proteinase (PEP2) which was found to be sensitive to pepstatin but tightly linked to the fungal cell wall.

Characterization of the Aspergillus nidulans aspnd1 gene demonstrates that the ASPND1 antigen, which it encodes, and several Aspergillus fumigatus immunodominant antigens belong to the same family

For the first time, an immunodominant Aspergillus nidulans antigen (ASPND1) consistently reactive with serum samples from aspergilloma patients has been purified and characterized, and its coding gene (aspnd1) has been cloned and sequenced. ASPND1 is a glycoprotein with four N-glycosidically-bound sugar chains (around 2.1 kDa each) which are not necessary for reactivity with immune human sera. The polypeptide part is synthesized as a 277-amino-acid precursor of 30.6 kDa that after cleavage of a putative signal peptide of 16 amino acids, affords a mature protein of 261 amino acids with a molecular mass of 29 kDa and a pI of 4.24 (as deduced from the sequence). The ASPND1 protein is 53.1% identical to the AspfII allergen from Aspergillus fumigatus and 48% identical to an unpublished Candida albicans antigen. All of the cysteine residues and most of the glycosylation sites are perfectly conserved in the three proteins, suggesting a similar but yet unknown function. Analysis of the primary structure of the ASPND1 coding gene (aspnd1) has allowed the establishment of a clear relationship between several previously reported A. fumigatus and A. nidulans immunodominant antigens.

Virulence factors of medically important fungi

Human fungal pathogens have become an increasingly important medical problem with the explosion in the number of immunocompromised patients as a result of cancer, steroid therapy, chemotherapy, and AIDS. Additionally, the globalization of travel and expansion of humankind into previously undisturbed habitats have led to the reemergence of old fungi and new exposure to previously undescribed fungi. Until recently, relatively little was known about virulence factors for the medically important fungi. With the advent of molecular genetics, rapid progress has now been made in understanding the basis of pathogenicity for organisms such as Aspergillus species and Cryptococcus neoformans. The twin technologies of genetic transformation and "knockout" deletion construction allowed for genetic tests of virulence factors in these organisms. Such knowledge will prove invaluable for the rational design of antifungal therapies. Putative virulence factors and attributes are reviewed for Aspergillus species, C. neoformans, the dimorphic fungal pathogens, and others, with a focus upon a molecular genetic approach. Candida species are excluded from coverage, having been the subject of numerous recent reviews. This growing body of knowledge about fungal pathogens and their virulence factors will significantly aid efforts to treat the serious diseases they cause.

Disruption of the serine proteinase gene (sep) in Aspergillus flavus leads to a compensatory increase in the expression of a metalloproteinase gene (mep20)

The serine proteinase gene (sep) in Aspergillus flavus was disrupted by homologous recombination with a hygromycin resistance gene as the marker. The gene-disrupted mutant GR-2 contained a single-copy insertion of the marker gene and did not express the sep gene. Serine proteinase activity, 36-kDa protein labeled by 3H-diisopropylfluorophosphate, and immunologically detectable proteinase were not detected in the culture fluid of GR-2. Despite the absence of the serine proteinase, the total elastinolytic activity levels in the mutant and the wild-type A.flavus were comparable. Immunoblots revealed that the mutant secreted greater amounts of an elastinolytic metalloproteinase gene (mep20) product than did the wild type. Furthermore, mep20 mRNA levels, measured by RNase protection assay, in the mutant were higher than those in the wild type. Inhibition of the serine proteinase by Streptomyces subtilisin inhibitor (SSI) in the culture medium of wild-type A.flavus also resulted in an elevation of mep20 gene products. Although no serine proteinase activity could be detected, the level of elastinolytic activity of the SSI-treated culture was comparable to that of the control. Immunoblots revealed that the addition of SSI caused an elevation in the levels of metalloproteinase and its mRNA. These results suggest that the expression of the genes encoding serine and metalloproteinases are controlled by a common regulatory system and the fungus has a mechanism to sense the status of extracellular proteolytic activities.

Specific inhibition of mature fungal serine proteinases and metalloproteinases by their propeptides

The function of the long propeptides of fungal proteinases is not known. Aspergillus fumigatus produces a 33-kDa serine proteinase of the subtilisin family and a 42-kDa metalloproteinase of the thermolysin family. These extracellular enzymes are synthesized as preproenzymes containing large amino-terminal propeptides. Recombinant propeptides were produced in Escherichia coli as soluble fusion proteins with glutathione S-transferase or thioredoxin and purified by affinity chromatography. A. fumigatus serine proteinase propeptide competitively inhibited serine proteinase, with a Ki of 5.3 x 10(-6) M, whereas a homologous serine proteinase from A. flavus was less strongly inhibited and subtilisin was not inhibited. Binding of metalloproteinase propeptide from A. fumigatus to the mature metalloenzyme was demonstrated. This propeptide strongly inhibited its mature enzyme, with a Ki of 3 x 10(-9) M, whereas thermolysin and a metalloproteinase from A. flavus were not inhibited by this propeptide. Enzymatically inactive metalloproteinase propeptide complex could be completely activated by trypsin treatment. These results demonstrate that the propeptides of the fungal proteinases bind specifically and inhibit the respective mature enzymes, probably reflecting a biological role of keeping these extracellular enzymes inactive until secretion.

Molecular cloning and expression of a recombinant Aspergillus fumigatus protein Asp f II with significant immunoglobulin E reactivity in allergic bronchopulmonary aspergillosis

The cDNA of Aspergillus fumigatus encoding an allergen was cloned and expressed in Escherichia coli. The 987 bp long cDNA clone expressed a recombinant protein Asp f II of 34 kd. This protein exhibited binding to immunoglobulin E (IgE) in the serum samples from patients with allergic bronchopulmonary aspergillosis (ABPA). The patients with ABPA and central bronchiectasis demonstrated high levels of serum IgE antibodies, whereas patients with ABPA without central bronchiectasis, patients with asthma and Aspergillus skin test reactivity but no evidence of ABPA, and patients with aspergilloma showed only low levels of IgE antibody to Asp f II. In two-dimensional electrophoresis, a native antigen electroeluted from an A. fumigatus culture filtrate antigen preparation showed an isoelectric point and molecular weight similar to that of Asp f II. In a competitive enzyme-linked immunosorbent assay (ELISA), the IgE antibody reactivity of Asp f II with patient serum samples could be significantly inhibited by culture filtrate antigens of A. fumigatus. These results indicate that Asp f II has immunologic reactivities comparable to those of native A. fumigatus antigens. The recombinant Asp f II can be expressed in E. coli in large quantities and should prove useful as a standardized allergen for sensitive and specific immunodiagnosis of ABPA, especially in patients with central bronchiectasis.

Expression of PZ-peptidases by cultures of several pathogenic fungi. Purification and characterization of a collagenase from Trichophyton schoenleinii

Peptidolytic activity was studied in the broken-cell extracts of 17 isolates of pathogenic fungi tested with phenylazobenzyloxycarbonyl-Pro-Leu-Gly-Pro-Arg (PZ-PLGPA) as a substrate. All the fungi studied except Candida spp., Cryptococcus neoformans and two actinomycetes hydrolyzed the substrate and therefore contained a so-called PZ-peptidase activity. Of all the positive strains, Trichophyton schoenleinii, a pathogenic fungus showed the highest activity and was therefore chosen as a source for PZ-peptidase purification. The four chromatographic steps, a 'negative' dye column, a 'positive' dye column, hydroxyapatite Ultrogel, and modified TSK (HW 55), gave a highly purified peptidase with a 12% overall yield. Inhibitor studies suggested that the 82 000 M(r) PZ-peptidase is a metalloproteinase. Moreover it cleaved native rat type I collagen. Partial peptide sequencing showed a strong sequence homology to regions of two metalloproteinases previously identified in the yeast Saccharomyces cerevisiae and in rat.

Cloning and characterization of the cDNAs and genes (mep20) encoding homologous metalloproteinases from Aspergillus flavus and A. fumigatus

Aspergillus fumigatus (Afu) and A. flavus (Afl), two causative agents of invasive aspergillosis, produce highly homologous serine proteinases. In addition, the former produces a 42-kDa metalloproteinase (MEP), whereas the latter produces a 23-kDa MEP. The cDNA and the gene encoding the 42-kDa MEP were cloned and sequenced. Here, we report the cloning of the cDNA and the gene encoding the 23-kDa MEP from Afl and a homologous gene from the Afu. Using degenerate primers based on the amino acid (aa) sequence of A. oryzae (Ao) MEP and thermolysin-like proteinases, a 282-bp fragment of the 23-kDa MEP-encoding gene of Afl was cloned by PCR. A 6.5-kb KpnI fragment of Afl genomic DNA containing the complete gene was cloned. The open reading frame (ORF) in this gene encodes a protein of 381 aa. Since the mature enzyme from this and other aspergilli would have a theoretical molecular mass of about 20 kDa, this MEP-encoding gene is designated mep20. A Western blot of the protein in the culture filtrate of Afl with polyclonal antibodies prepared against the MEP showed a single band at 23 kDa. The N-terminal sequence of the extracellular MEP20, TKVAS, was found at aa 194-198 within the ORF. Thus, the primary translation product has a putative 19-aa signal and a pro region of 174 aa. A homologous gene cloned from a genomic DNA library of Afu showed an ORF encoding 365 aa. Comparison of the nucleotide (nt) sequences of the cDNAs cloned by RT-PCR with their respective genes showed that there are no introns in the ORF of mep20 in Afl, but there is a 59-bp intron in the gene from Afu. The MEP20 of Afl and Afu have 68% identity and show weak immunological cross reactivity. MEP20 from both these fungi share about 60% sequence identity with the penicillolysin of Penicillium citrinum and the neutral protease II of Ao. MEP20 of Afl and Afu show only the conserved sequence, HEFTHA, but not the two other conserved sequences seen in thermolysins and similar MEP.

Molecular cloning of the cDNA and gene for an elastinolytic aspartic proteinase from Aspergillus fumigatus and evidence of its secretion by the fungus during invasion of the host lung

Hydrolysis of structural proteins in the lung by extracellular proteinases secreted by Aspergillus fumigatus is thought to play a significant role in invasive aspergillosis. This fungus was found previously to secrete an elastinolytic serine proteinase and a metalloproteinase. We report that A. fumigatus also secretes an aspartic proteinase (aspergillopepsin F) that can catalyze hydrolysis of the major structural proteins of basement membrane, elastin, collagen, and laminin. The pH optimum for the enzymatic activity was 5.0 with elastin-Congo red as the substrate, and the activity was not significantly inhibited by pepstatin A, diazoacetyl norleucine methylester, and 1,2-epoxy-3-(p-nitrophenoxy) propane. The cDNA and gene encoding this aspartic proteinase were cloned and sequenced. The open reading frame, interrupted by three introns, would encode a protein of 393 amino acids composed of a putative 21-amino-acid signal peptide and a 49-amino-acid propeptide preceding the 323-amino-acid mature protein. The amino acid sequence of A. fumigatus aspartic proteinase has 70, 66, and 67% homology to the sequences of those from Aspergillus oryzae, Aspergillus awamori, and Aspergillus saitoi, respectively. The active-site motif (DTG) and the catalytic aspartic residues characteristic of aspartic proteinases are found in the presently described enzyme, indicating that it belongs to a family of aspartic proteinases. Polyclonal antibodies were produced in rabbits against both the mature and precursor forms of the aspartic proteinase expressed in Escherichia coli. Immunoblotting with both antibodies detected a 39-kDa mature enzyme in the culture supernatant of A. fumigatus. The aspartic proteinase activity was inhibited by the antibodies, suggesting that the aspartic proteinase in the culture supernatant corresponds to the product of the cloned gene. Immunogold electron microscopy showed that the aspartic proteinase was secreted by A. fumigatus invading neutropenic mouse lung and its secretion was directed toward the germ tubes of penetrating hyphae.