Characterization of a natural larger form of the antifungal protein (AFP) from Aspergillus giganteus

The Epichloë festucae Antifungal Protein Efe-AfpA Protects Creeping Bentgrass (Agrostis stolonifera) from the Plant Pathogen Clarireedia jacksonii, the Causal Agent of Dollar Spot Disease

Dollar spot disease, caused by the fungal pathogen Clarireedia jacksonii, is a major problem in many turfgrass species, particularly creeping bentgrass (Agrostis stolonifera). It is well-established that strong creeping red fescue (Festuca rubra subsp. rubra) exhibits good dollar spot resistance when infected by the fungal endophyte Epichloë festucae. This endophyte-mediated disease resistance is unique to the fine fescues and has not been observed in other grass species infected with other Epichloë spp. The mechanism underlying the unique endophyte-mediated disease resistance in strong creeping red fescue has not yet been established. We pursued the possibility that it may be due to the presence of an abundant secreted antifungal protein produced by E. festucae. Here, we compare the activity of the antifungal protein expressed in Escherichia coli, Pichia pastoris, and Penicillium chrysogenum. Active protein was recovered from all systems, with the best activity being from Pe. chrysogenum. In greenhouse assays, topical application of the purified antifungal protein to creeping bentgrass and endophyte-free strong creeping red fescue protected the plants from developing severe symptoms caused by C. jacksonii. These results support the hypothesis that Efe-AfpA is a major contributor to the dollar spot resistance observed with E. festucae-infected strong creeping red fescue in the field, and that this protein could be developed as an alternative or complement to fungicides for the management of this disease on turfgrasses.

A closer look at Aspergillus: online monitoring via scattered light enables reproducible phenotyping

BACKGROUND

Filamentously growing microorganisms offer unique advantages for biotechnological processes, such as extraordinary secretion capacities, going along with multiple obstacles due to their complex morphology. However, limited experimental throughput in bioprocess development still hampers taking advantage of their full potential. Miniaturization and automation are powerful tools to accelerate bioprocess development, but so far the application of such technologies has mainly been focused on non-filamentous systems. During cultivation, filamentous fungi can undergo remarkable morphological changes, creating challenging cultivation conditions. Depending on the process and product, only one specific state of morphology may be advantageous to achieve e.g. optimal productivity or yield. Different approaches to control morphology have been investigated, such as microparticle enhanced cultivation. However, the addition of solid microparticles impedes the optical measurements typically used by microbioreactor systems and thus alternatives are needed.

RESULTS

Aspergillus giganteus IfGB 0902 was used as a model system to develop a time-efficient and robust workflow allowing microscale cultivation with increased throughput. The effect of microtiter plate geometry, shaking frequency and medium additives (talc and calcium chloride) on homogeneity of culture morphology as well as reproducibility were analyzed via online biomass measurement, microscopic imaging and cell dry weight. While addition of talc severely affected online measurements, 2% (w v-1) calcium chloride was successfully applied to obtain a highly reproducible growth behavior with homogenous morphology. Furthermore, the influence of small amounts of complex components was investigated for the applied model strain. By correlation to cell dry weight, it could be shown that optical measurements are a suitable signal for biomass concentration. However, each correlation is only applicable for a specific set of cultivation parameters. These optimized conditions were used in micro as well as lab-scale bioreactor cultivation in order to verify the reproducibility and scalability of the setup.

CONCLUSION

A robust workflow for A. giganteus was developed, allowing for reproducible microscale cultivation with online monitoring, where calcium chloride is an useful alternative to microparticle enhanced cultivation in order to control the morphology. Independent of the cultivation volume, comparable phenotypes were observed in microtiter plates and in lab-scale bioreactor.

Strain identification and metabolites isolation of Aspergillus capensis CanS-34A from Brassica napus

An isolate (CanS-34A) of Aspergillus from a healthy plant of oilseed rape (Brassica napus) was identified based on morphological characterization and multi-locus phylogeny using the sequences of internal transcribed spacer (ITS)-5.8S rDNA region, BenA (for β-tubulin), CaM (for calmodulin) and RPB2 (for RNA polymerase II). The results showed that CanS-34A belongs to Aspergillus capensis Hirooka et al. The antifungal metabolites produced by CanS-34A in potato dextrose broth (PDB) were extracted with chloroform. Three antifungal metabolites were isolated and purified from the chloroform extract of the PDB cultural filtrates of CanS-34A, and chemically identified as methyl dichloroasterrate, penicillither and rosellichalasin. They all showed antifungal activity against the plant pathogenic fungi Botrytis cinerea, Monilinia fructicola, Sclerotinia sclerotiorum and Sclerotinia trifoliorum with the EC₅₀ values ranging from 2.46 to 65.00 μg/mL. To our knowledge, this is the first report about production of penicillither by Aspergillus and about the antifungal activity of methyl dichloroasterrate, penicillither and rosellichalasin against the four plant pathogenic fungi.

A Computational Modeling Approach Predicts Interaction of the Antifungal Protein AFP from Aspergillus giganteus with Fungal Membranes via Its γ-Core Motif

Fungal pathogens kill more people per year globally than malaria or tuberculosis and threaten international food security through crop destruction. New sophisticated strategies to inhibit fungal growth are thus urgently needed. Among the potential candidate molecules that strongly inhibit fungal spore germination are small cationic, cysteine-stabilized proteins of the AFP family secreted by a group of filamentous Ascomycetes. Its founding member, AFP from Aspergillus giganteus, is of particular interest since it selectively inhibits the growth of filamentous fungi without affecting the viability of mammalian, plant, or bacterial cells. AFPs are also characterized by their high efficacy and stability. Thus, AFP can serve as a lead compound for the development of novel antifungals. Notably, all members of the AFP family comprise a γ-core motif which is conserved in all antimicrobial proteins from pro- and eukaryotes and known to interfere with the integrity of cytoplasmic plasma membranes. In this study, we used classical molecular dynamics simulations combined with wet laboratory experiments and nuclear magnetic resonance (NMR) spectroscopy to characterize the structure and dynamical behavior of AFP isomers in solution and their interaction with fungal model membranes. We demonstrate that the γ-core motif of structurally conserved AFP is the key for its membrane interaction, thus verifying for the first time that the conserved γ-core motif of antimicrobial proteins is directly involved in protein-membrane interactions. Furthermore, molecular dynamic simulations suggested that AFP does not destroy the fungal membrane by pore formation but covers its surface in a well-defined manner, using a multistep mechanism to destroy the membranes integrity.IMPORTANCE Fungal pathogens pose a serious danger to human welfare since they kill more people per year than malaria or tuberculosis and are responsible for crop losses worldwide. The treatment of fungal infections is becoming more complicated as fungi develop resistances against commonly used fungicides. Therefore, discovery and development of novel antifungal agents are of utmost importance.

Usage of the Heterologous Expression of the Antimicrobial Gene afp From Aspergillus giganteus for Increasing Fungal Resistance in Olive

The antifungal protein (AFP) produced by Aspergillus giganteus, encoded by the afp gene, has been used to confer resistance against a broad range of fungal pathogens in several crops. In this research, transgenic olive plants expressing the afp gene under the control of the constitutive promoter CaMV35S were generated and their disease response against two root infecting fungal pathogens, Verticillium dahliae and Rosellinia necatrix, was evaluated. Embryogenic cultures derived from a mature zygotic embryo of cv. 'Picual' were used for A. tumefaciens transformation. Five independent transgenic lines were obtained, showing a variable level of afp expression in leaves and roots. None of these transgenic lines showed enhanced resistance to Verticillium wilt. However, some of the lines displayed a degree of incomplete resistance to white root rot caused by R. necatrix compared with disease reaction of non-transformed plants or transgenic plants expressing only the GUS gene. The level of resistance to this pathogen correlated with that of the afp expression in root and leaves. Our results indicate that the afp gene can be useful for enhanced partial resistance to R. necatrix in olive, but this gene does not protect against V. dahliae.

New Antimicrobial Potential and Structural Properties of PAFB: A Cationic, Cysteine-Rich Protein from Penicillium chrysogenum Q176

Small, cysteine-rich and cationic proteins with antimicrobial activity are produced by diverse organisms of all kingdoms and represent promising molecules for drug development. The ancestor of all industrial penicillin producing strains, the ascomycete Penicillium chryosgenum Q176, secretes the extensively studied antifungal protein PAF. However, the genome of this strain harbours at least two more genes that code for other small, cysteine-rich and cationic proteins with potential antifungal activity. In this study, we characterized the pafB gene product that shows high similarity to PgAFP from P. chrysogenum R42C. Although abundant and timely regulated pafB gene transcripts were detected, we could not identify PAFB in the culture broth of P. chrysogenum Q176. Therefore, we applied a P. chrysogenum-based expression system to produce sufficient amounts of recombinant PAFB to address unanswered questions concerning the structure and antimicrobial function. Nuclear magnetic resonance (NMR)-based analyses revealed a compact β-folded structure, comprising five β-strands connected by four solvent exposed and flexible loops and an “abcabc” disulphide bond pattern. We identified PAFB as an inhibitor of growth of human pathogenic moulds and yeasts. Furthermore, we document for the first time an anti-viral activity for two members of the small, cysteine-rich and cationic protein group from ascomycetes.

Optimization and partial characterization of intracellular anticandidal protein from Aspergillus giganteus MTCC 8408 using taguchi DOE

A new intracellular antifungal protein (afp) production with average molecular weight 24.3 kDa and yield of 0.65 ± 0.1 mg/gram dry cell weight (gdcw) of mycelia in submerged fermentation of Aspergillus giganteus MTCC 8408 was optimized. Taguchi's DOE (design of experiment) L₂₇ orthogonal array (OA) was constructed using Qualitek-4 software with 8 most influensive factors namely, culture pH, temperature, slant age, inoculum volume, agitation and KH₂PO₄. Scanning electron microscopy (SEM) was used to correlate the effect of selected factors on fungal cell morphology and afp production. The crude protein purification was accomplished using pure ammonium sulfate fractionation followed by carboxymethyl cellulose (CMC) ion-exchange chromatography and sephadex G-100 gel filtration. The average molecular mass of the purified protein was figured by silver stained SDS (sodium dodecylsulphate)-PAGE (poly-acryl amide gel electrophoresis). In vitro antifungal susceptibility assay was profiled against Candida albicans NCIM 3471 and minimum inhibitory concentrations (MICs) were in the range 3 to 4 µg/ml. Characterization of protein was observed with FTIR (Fourier transform infrared spectroscopy) analysis. The optimal production condition for crude afp was obtained as follows: soluble starch: 20 g/l; Corn steep liquor (CSL, 2%) + proteose peptone (PP, 1%): 30 g/l; pH: 5.8; temperature: 25°C; slant age: 3 d; inoculum size: 5% (v/v); agitation: 180 rpm; KH₂PO₄: 0.1 g/l. The validation experiments using optimized conditions confirmed an improvement in afp production by 59.4% against the expected enhancement of afp production by 61.22%. The present statistical optimization study revealed an opportunity to promote economical design at the industrial level for future scale up of effective antifungal agent against opportunistic oral and vaginal infection.

The Epichloë festucae antifungal protein has activity against the plant pathogen Sclerotinia homoeocarpa, the causal agent of dollar spot disease

Epichloë spp. are naturally occurring fungal endophytic symbionts of many cool-season grasses. Infection by the fungal endophytes often confers biotic and abiotic stress tolerance to their hosts. Endophyte-mediated disease resistance is well-established in the fine fescue grass Festuca rubra subsp. rubra (strong creeping red fescue) infected with E. festucae. Resistance to fungal pathogens is not an established effect of endophyte infection of other grass species, and may therefore be unique to the fine fescues. The underlying mechanism of the disease resistance is unknown. E. festucae produces a secreted antifungal protein that is highly expressed in the infected plant tissues and may therefore be involved in the disease resistance. Most Epichloë spp. do not have a gene for a similar antifungal protein. Here we report the characterization of the E. festucae antifungal protein, designated Efe-AfpA. The antifungal protein partially purified from the apoplastic proteins of endophyte-infected plant tissue and the recombinant protein expressed in the yeast Pichia pastoris was found to have activity against the important plant pathogen Sclerotinia homoeocarpa. Efe-AfpA may therefore be a component of the disease resistance seen in endophyte-infected strong creeping red fescue.

Structural determinants of Neosartorya fischeri antifungal protein (NFAP) for folding, stability and antifungal activity

The recent global challenges to prevent and treat fungal infections strongly demand for the development of new antifungal strategies. The structurally very similar cysteine-rich antifungal proteins from ascomycetes provide a feasible basis for designing new antifungal molecules. The main structural elements responsible for folding, stability and antifungal activity are not fully understood, although this is an essential prerequisite for rational protein design. In this study, we used the Neosartorya fischeri antifungal protein (NFAP) to investigate the role of the disulphide bridges, the hydrophobic core, and the N-terminal amino acids in the formation of a highly stable, folded, and antifungal active protein. NFAP and its mutants carrying cysteine deletion (NFAPΔC), hydrophobic core deletion (NFAPΔh), and N-terminal amino acids exchanges (NFAPΔN) were produced in Pichia pastoris. The recombinant NFAP showed the same features in structure, folding, stability and activity as the native protein. The data acquired with mass spectrometry, structural analyses and antifungal activity assays of NFAP and its mutants proved the importance of the disulphide bonding, the hydrophobic core and the correct N-terminus for folding, stability and full antifungal function. Our findings provide further support to the comprehensive understanding of the structure-function relationship in members of this protein group.

A Penicillium chrysogenum-based expression system for the production of small, cysteine-rich antifungal proteins for structural and functional analyses

Background

Small, cysteine-rich and cationic antifungal proteins (APs) from filamentous ascomycetes, such as NFAP from Neosartorya fischeri and PAF from Penicillium chrysogenum, are promising candidates for novel drug development. A prerequisite for their application is a detailed knowledge about their structure–function relation and mode of action, which would allow protein modelling to enhance their toxicity and specificity. Technologies for structure analyses, such as electronic circular dichroism (ECD) or NMR spectroscopy, require highly purified samples and in case of NMR milligrams of uniformly ¹⁵N-/¹³C-isotope labelled protein. To meet these requirements, we developed a P. chrysogenum-based expression system that ensures sufficient amount and optimal purity of APs for structural and functional analyses.

Results

The APs PAF, PAF mutants and NFAP were expressed in a P. chrysogenum ∆paf mutant strain that served as perfect microbial expression factory. This strain lacks the paf-gene coding for the endogenous antifungal PAF and is resistant towards several APs from other ascomycetes. The expression of the recombinant proteins was under the regulation of the strong paf promoter, and the presence of a paf-specific pre-pro sequence warranted the secretion of processed proteins into the supernatant. The use of defined minimal medium allowed a single-step purification of the recombinant proteins. The expression system could be extended to express PAF in the related fungus Penicillium digitatum, which does not produce detectable amounts of APs, demonstrating the versatility of the approach. The molecular masses, folded structures and antifungal activity of the recombinant proteins were analysed by ESI–MS, ECD and NMR spectroscopy and growth inhibition assays.

Conclusion

This study demonstrates the implementation of a paf promoter driven expression cassettes for the production of cysteine-rich, cationic, APs in different Penicillium species. The system is a perfect tool for the generation of correctly folded proteins with high quality for structure–function analyses.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-016-0586-4) contains supplementary material, which is available to authorized users.

EST mining identifies proteins putatively secreted by the anthracnose pathogen Colletotrichum truncatum

Background

Colletotrichum truncatum is a haploid, hemibiotrophic, ascomycete fungal pathogen that causes anthracnose disease on many economically important leguminous crops. This pathogen exploits sequential biotrophic- and necrotrophic- infection strategies to colonize the host. Transition from biotrophy to a destructive necrotrophic phase called the biotrophy-necrotrophy switch is critical in symptom development. C. truncatum likely secretes an arsenal of proteins that are implicated in maintaining a compatible interaction with its host. Some of them might be transition specific.

Results

A directional cDNA library was constructed from mRNA isolated from infected Lens culinaris leaflet tissues displaying the biotrophy-necrotrophy switch of C. truncatum and 5000 expressed sequence tags (ESTs) with an average read of > 600 bp from the 5-prime end were generated. Nearly 39% of the ESTs were predicted to encode proteins of fungal origin and among these, 162 ESTs were predicted to contain N-terminal signal peptides (SPs) in their deduced open reading frames (ORFs). The 162 sequences could be assembled into 122 tentative unigenes comprising 32 contigs and 90 singletons. Sequence analyses of unigenes revealed four potential groups: hydrolases, cell envelope associated proteins (CEAPs), candidate effectors and other proteins. Eleven candidate effector genes were identified based on features common to characterized fungal effectors, i.e. they encode small, soluble (lack of transmembrane domain), cysteine-rich proteins with a putative SP. For a selected subset of CEAPs and candidate effectors, semiquantitative RT-PCR showed that these transcripts were either expressed constitutively in both in vitro and in planta or induced during plant infection. Using potato virus X (PVX) based transient expression assays, we showed that one of the candidate effectors, i. e. contig 8 that encodes a cerato-platanin (CP) domain containing protein, unlike CP proteins from other fungal pathogens was unable to elicit a hypersensitive response (HR).

Conclusions

The current study catalogues proteins putatively secreted at the in planta biotrophy-necrotrophy transition of C. truncatum. Some of these proteins may have a role in establishing compatible interaction with the host plant.

A highly thermostable antimicrobial peptide from Aspergillus clavatus ES1: biochemical and molecular characterization

Antimicrobial peptides (AMPs) are extremely attractive candidates as therapeutic agents due to their wide spectrum of antimicrobial activity and mechanism of action, which differs from that of small-molecule antibiotics. In this study, a 6.0-kDa antimicrobial peptide from Aspergillus clavatus ES1, designated as AcAMP, was isolated by a one-step heat treatment. AcAMP was sensitive to proteolytic enzymes, stable between pH 5.0 and 10.0, and heat resistant (15 min at 100 degrees C). The acamp gene encoding AcAMP peptide was isolated by reverse-transcriptase polymerase chain reaction (RT-PCR) and cloned in pCRII-TOPO vector. Sequence analysis of the complementary DNA (cDNA) acamp gene revealed an open reading frame of 282 bp encoding a peptide of 94 amino acid residues consisting of a 21-aa signal peptide, a 22-aa pro-peptide, and a 51-aa mature peptide. The deduced amino acid sequence showed high identity with other ascomycete antifungal peptides. AcAMP belongs to the group of small, cysteine-rich, basic proteins with antimicrobial activity. In addition to its antifungal activity, AcAMP is the first fungal peptide exhibiting antibacterial activity against several Gram-positive and Gram-negative bacteria. Based on all these features, AcAMP can be considered as a promising new member of the restraint family of ascomycete antimicrobial peptides that might be used in biological control of plant diseases and also for potential applications in food preservation.

Production of the biotechnologically relevant AFP from Aspergillus giganteus in the yeast Pichia pastoris

The mould Aspergillus giganteus produces a basic, low molecular weight protein (AFP) showing in vitro and in vivo antifungal properties against important plant pathogens. AFP is secreted as an inactive precursor containing an amino-terminal extension of six amino acids (lf-AFP) which is later removed to produce the active protein. The molecular basis to explain this behavior and the features that determine the fungal specificity of this protein are not completely solved. In this work, the mature AFP (AFP *) and a version of AFP with an extended amino-terminal (proAFP) have been cloned and produced in the yeast Pichia pastoris. The two proteins have been purified to homogeneity and characterized from structural and functional points of view. Recombinant AFP * produced is practically indistinguishable from the natural fungal protein in terms of its spectroscopic and antifungal properties while proAFP is mostly inactive under identical assay conditions. The availability of an active AFP protein produced in P. pastoris will permit investigation of the mode of action and targeting specificity of AFP by using site-directed mutagenesis approaches.

Molecular cloning, structural analysis and modelling of the AcAFP antifungal peptide from Aspergillus clavatus

An abundantly secreted thermostable peptide (designed AcAFP) with a molecular mass of 5777 Da was isolated and purified in a previous work from a local strain of A. clavatus (VR1). Based on the N-terminal amino acid (aa) sequence of the AcAFP peptide, an oligonucleotide probe was derived and allowed the amplification of the encoding cDNA by RT-PCR. This cDNA fragment encodes a pre-pro-protein of 94 aa which appears to be processed to a mature product of 51 aa cys-rich protein. The deduced aa sequence of the pre-pro-sequence reveals high similarity with ascomycetes antifungal peptide. Comparison of the nucleotide sequence of the genomic fragment and the cDNA clone revealed the presence of an open reading frame of 282 bp interrupted by two small introns of 89 and 56 bp with conserved splice site. The three-dimensional (3D) structure modeling of AcAFP exhibits a compact structure consisting of five anti-parallel beta barrel stabilized by four internal disulfide bridges. The folding pattern revealed also a cationic site and spatially adjacent hydrophobic stretch. The antifungal mechanism was investigated by transmission and confocal microscopy. AcAFP cause cell wall altering in a dose-dependent manner against the phytopathogenic fungus Fusarium oxysporum.

First isolation of a novel thermostable antifungal peptide secreted by Aspergillus clavatus

A novel antifungal peptide produced by an indigenous fungal strain (VR) of Aspergillus clavatus was purified. The antifungal peptide was enriched in the supernatant after heat treatment at 70 degrees C. The thermostable character was exploited in the first purification step, as purified peptide was obtained after ultrafiltration and reverse phase-HPLC on C18 column application. The purified peptide named "AcAFP" for A. clavatus antifungal peptide, has molecular mass of 5773Da determined by MALDI-ToF spectrometry. The N-terminal sequence showed a notable identity to the limited family of antifungal peptides produced by ascomycetes fungi. The AcAFP activity remains intact even after heat treatment at 100 degrees C for 1h confirming its thermostability. It exhibits a strong inhibitory activity against mycelial growth of several serious human and plant pathogenic fungi: Fusariuym oxysporum, Fusarium solani, Aspergillus niger, Botrytis cinerea, Alternaria solani, whereas AcAFP did not affect yeast and bacterial growth.

The yeast signal sequence trap identifies secreted proteins of the hemibiotrophic corn pathogen Colletotrichum graminicola

The hemibiotroph Colletotrichum graminicola is the causal agent of stem rot and leaf anthracnose on Zea mays. Following penetration of epidermal cells, the fungus enters a short biotrophic phase, followed by a destructive necrotrophic phase of pathogenesis. During both phases, secreted fungal proteins are supposed to determine progress and success of the infection. To identify genes encoding such proteins, we constructed a yeast signal sequence trap (YSST) cDNA-library from RNA extracted from mycelium grown in vitro on corn cell walls and leaf extract. Of the 103 identified unigenes, 50 showed significant similarities to genes with a reported function, 25 sequences were similar to genes without a known function, and 28 sequences showed no similarity to entries in the databases. Macroarray hybridization and quantitative reverse-transcriptase polymerase chain reaction confirmed that most genes identified by the YSST screen are expressed in planta. Other than some genes that were constantly expressed, a larger set showed peaks of transcript abundances at specific phases of pathogenesis. Another set exhibited biphasic expression with peaks at the biotrophic and necrotrophic phase. Transcript analyses of in vitro-grown cultures revealed that several of the genes identified by the YSST screen were induced by the addition of corn leaf components, indicating that host-derived factors may have mimicked the host milieu.

A small protein that fights fungi: AFP as a new promising antifungal agent of biotechnological value

As fungal infections are becoming more prevalent in the medical or agricultural fields, novel and more efficient antifungal agents are badly needed. Within the scope of developing new strategies for the management of fungal infections, antifungal compounds that target essential fungal cell wall components are highly preferable. Ideally, newly developed antimycotics should also combine major aspects such as sustainability, high efficacy, limited toxicity and low costs of production. A naturally derived molecule that possesses all the desired characteristics is the antifungal protein (AFP) secreted by the filamentous ascomycete Aspergillus giganteus. AFP is a small, basic and cysteine-rich peptide that exerts extremely potent antifungal activity against human- and plant-pathogenic fungi without affecting the viability of bacteria, yeast, plant and mammalian cells. This review summarises the current knowledge of the structure, mode of action and expression of AFP, and highlights similarities and differences concerning these issues between AFP and its related proteins from other Ascomycetes. Furthermore, the potential use of AFP in the combat against fungal contaminations and infections will be discussed.

Biotechnologically relevant enzymes and proteins. Antifungal mechanism of the Aspergillus giganteus AFP against the rice blast fungus Magnaporthe grisea

The mold Aspergillus giganteus produces a basic, low molecular weight protein showing antifungal properties against economically important plant pathogens, the AFP (Antifungal Protein). In this study, we investigated the mechanisms by which AFP exerts its antifungal activity against Magnaporthe grisea. M. grisea is the causal agent of rice blast, one of the most devastating diseases of cultivated rice worldwide. AFP was purified from the extracellular medium of A. giganteus cultures. The AFP protein was found to induce membrane permeabilization in M. grisea cells. Electron microscopy studies revealed severe cellular degradation and damage of plasma membranes in AFP-treated fungal cells. AFP however failed to induce membrane permeabilization on rice or human HeLa cells. Furthermore, AFP enters the fungal cell and targets to the nucleus, as revealed by co-localization experiments of Alexa-labeled AFP with the SYTOX Green dye. Finally, AFP binds to nucleic acids, including M. grisea DNA. Our results suggest that the combination of fungal cell permeabilization, cell-penetrating ability and nucleic acid-binding activity of AFP determines its potent antifungal activity against M. grisea. These results are discussed in relation to the potential of the AFP protein to enhance crop protection against fungal diseases.

Peptides and proteins from fungi

The peptides and proteins secreted by fungi are reviewed in this article. They include ribosome inactivating peptides and proteins, antifungal peptides and proteins, lectins, ubiquitin-like peptides and proteins, peptides and proteins with nucleolytic activity, proteases, xylanases, cellulases, sugar oxidoreductases, laccases, invertases, trehalose phosphorylases, and various enzymes with applications in food industry, chemical production and the medical sector.

The antifungal protein AFP of Aspergillus giganteus is an oligonucleotide/oligosaccharide binding (OB) fold-containing protein that produces condensation of DNA

The antifungal protein AFP is a small polypeptide of 51 amino acid residues secreted by Aspergillus giganteus. Its potent activity against phytopathogenic fungi converts AFP in a promising tool in plant protection. However, no data have been reported regarding the mode of action of AFP. The three-dimensional structure of this protein, a small and compact beta barrel composed of five highly twisted antiparallel beta strands, displays the characteristic features of the oligonucleotide/oligosaccharide binding (OB fold) structural motif. A comparison of the structures of AFP and OB fold-containing proteins shows this structural similarity despite the absence of any significant sequence similarity. AFP, like most OB fold-containing proteins, binds nucleic acids. The protein promotes charge neutralization and condensation of DNA as demonstrated by electrophoretic mobility shift and ethidium bromide displacement assays. Nucleic acid produces quenching of the protein fluorescence emission. This nucleic acid interacting ability of AFP may be related to the antifungal activity of this small polypeptide.

A protein from the mold Aspergillus giganteus is a potent inhibitor of fungal plant pathogens

A purified preparation of antifungal protein (AFP) from Aspergillus giganteus exhibited potent antifungal activity against the phytopathogenic fungi Magnaporthe grisea and Fusarium moniliforme, as well as the oomycete pathogen Phytophthora infestans. Under conditions of total inhibition of fungal growth, no toxicity of AFP toward rice protoplasts was observed. Additionally, application of AFP on rice plants completely inhibited M. grisea growth. These results are discussed in relation to the potential of the afp gene to enhance crop protection against fungal pathogens in transgenic plants.

Purification and characterization of three antifungal proteins from cheeseweed (Malva parviflora)

Three potent antimicrobial proteins were purified from cheeseweed (Malva parviflora) seeds. These antimicrobial proteins, named CW-3, CW-4, and CW-5, showed different antimicrobial spectrum and potency compared to the two heterologous antimicrobial proteins (CW-1 and CW-2) purified previously. CW-3 and CW-4 possess antimicrobial activities against Phytophthora infestans (Pi), but not Fusarium graminearum (Fg). A database search indicated that CW-3 shares high homology to cotton vicilin, an abundant seed storage protein. CW-4 shares homology to 2S albumin, another seed storage protein from cotton. CW-5 has antimicrobial activity against Fg, but no activity against Pi was observed at protein concentration up to 50 ppm. Under low salt condition, CW-5 showed potent antimicrobial activity against Fg, but under high salt condition, the antimicrobial activity was drastically diminished. Database search indicated that CW-5 has high homology to a lipid transfer protein from grape. The IC(50) values of the three purified antimicrobial proteins under both low and high salt conditions were determined. The isolation of five antimicrobial proteins for the first time from a single plant source provides further understanding of the plant innate defense system and insight on how plants evolve their complex and complementary antimicrobial system that is important in the early stage of development.

Isolation and characterization of viridin, a new 65 kDa antifungal protein from the mould Trichoderma viride

A new extracellular antifungal protein with a yield of 10 mg per liter was isolated from the culture medium of the mould Trichoderma viride. The protein, which we named viridin, was purified by carboxymethyl-cellulose cation-exchange chromatography and Superose 12 HR 10/30 high-performance liquid chromatography. Viridin, a basic protein of approximately 65 kDa as determined by SDS-PAGE, inhibits the growth of the cotton pathogen Verticillum dahliae, the IC50 being 6 microM.

Purification of alpha-sarcin and an antifungal protein from Aspergillus giganteus by blue sepharose CL-6B affinity chromatography

A simple method for preparation of alpha-sarcin and an antifungal protein (AFP) from the mold Aspergillus giganteus MDH 18894 has been developed. alpha-Sarcin and AFP were purified simultaneously by carboxymethylcellulose 52 cation-exchange chromatography and Blue Sepharose CL-6B affinity chromatography. By this method, 4.2 mg of alpha-sarcin and 6.8 mg of AFP were obtained from 2 liters of medium. Compared with other purification methods such as gel-filtration chromatography, this procedure was simple and specific. The purified alpha-sarcin and AFP were homogeneous characterized on SDS-polyacrylamide gel. The enzymatic activity of several ribosome-inactivating proteins such as alpha-sarcin, trichosanthin, and cinnamomin was significantly inhibited by the dye Cibacron blue F3GA. In 50 microliter of reaction mixture, 10 microM of the dye could inhibit 50% activity of cinnamomin (7 x 10(-9) M), whereas 50% inhibition of the enzymatic activity of trichosanthin (7 x 10(-9) M) and alpha-sarcin (1 x 10(-7) M) required 100 and 50 microM of the dye, respectively.