Hydrophobins: multipurpose proteins

Fungal Biofilms and Polymicrobial Diseases

Biofilm formation is an important virulence factor for pathogenic fungi. Both yeasts and filamentous fungi can adhere to biotic and abiotic surfaces, developing into highly organized communities that are resistant to antimicrobials and environmental conditions. In recent years, new genera of fungi have been correlated with biofilm formation. However, Candida biofilms remain the most widely studied from the morphological and molecular perspectives. Biofilms formed by yeast and filamentous fungi present differences, and studies of polymicrobial communities have become increasingly important. A key feature of resistance is the extracellular matrix, which covers and protects biofilm cells from the surrounding environment. Furthermore, to achieve cell–cell communication, microorganisms secrete quorum-sensing molecules that control their biological activities and behaviors and play a role in fungal resistance and pathogenicity. Several in vitro techniques have been developed to study fungal biofilms, from colorimetric methods to omics approaches that aim to identify new therapeutic strategies by developing new compounds to combat these microbial communities as well as new diagnostic tools to identify these complex formations in vivo. In this review, recent advances related to pathogenic fungal biofilms are addressed.

Comparative analysis of the predicted secretomes of Rosaceae scab pathogens Venturia inaequalis and V. pirina reveals expanded effector families and putative determinants of host range

BACKGROUND

Fungal plant pathogens belonging to the genus Venturia cause damaging scab diseases of members of the Rosaceae. In terms of economic impact, the most important of these are V. inaequalis, which infects apple, and V. pirina, which is a pathogen of European pear. Given that Venturia fungi colonise the sub-cuticular space without penetrating plant cells, it is assumed that effectors that contribute to virulence and determination of host range will be secreted into this plant-pathogen interface. Thus the predicted secretomes of a range of isolates of Venturia with distinct host-ranges were interrogated to reveal putative proteins involved in virulence and pathogenicity.

RESULTS

Genomes of Venturia pirina (one European pear scab isolate) and Venturia inaequalis (three apple scab, and one loquat scab, isolates) were sequenced and the predicted secretomes of each isolate identified. RNA-Seq was conducted on the apple-specific V. inaequalis isolate Vi1 (in vitro and infected apple leaves) to highlight virulence and pathogenicity components of the secretome. Genes encoding over 600 small secreted proteins (candidate effectors) were identified, most of which are novel to Venturia, with expansion of putative effector families a feature of the genus. Numerous genes with similarity to Leptosphaeria maculans AvrLm6 and the Verticillium spp. Ave1 were identified. Candidates for avirulence effectors with cognate resistance genes involved in race-cultivar specificity were identified, as were putative proteins involved in host-species determination. Candidate effectors were found, on average, to be in regions of relatively low gene-density and in closer proximity to repeats (e.g. transposable elements), compared with core eukaryotic genes.

CONCLUSIONS

Comparative secretomics has revealed candidate effectors from Venturia fungal plant pathogens that attack pome fruit. Effectors that are putative determinants of host range were identified; both those that may be involved in race-cultivar and host-species specificity. Since many of the effector candidates are in close proximity to repetitive sequences this may point to a possible mechanism for the effector gene family expansion observed and a route to diversification via transposition and repeat-induced point mutation.

The novel fungal-specific gene FUG1 has a role in pathogenicity and fumonisin biosynthesis in Fusarium verticillioides

Fusarium verticillioides is a globally important pathogen of maize, capable of causing severe yield reductions and economic losses. In addition, F. verticillioides produces toxic secondary metabolites during kernel colonization that pose significant threats to human and animal health. Fusarium verticillioides and other plant-pathogenic fungi possess a large number of genes with no known or predicted function, some of which could encode novel virulence factors or antifungal targets. In this study, we identified and characterized the novel gene FUG1 (Fungal Unknown Gene 1) in F. verticillioides through functional genetics. Deletion of FUG1 impaired maize kernel colonization and fumonisin biosynthesis. In addition, deletion of FUG1 increased sensitivity to the antimicrobial compound 2-benzoxazolinone and to hydrogen peroxide, which indicates that FUG1 may play a role in mitigating stresses associated with host defence. Transcriptional profiling via RNA-sequencing (RNA-seq) identified numerous fungal genes that were differentially expressed in the kernel environment following the deletion of FUG1, including genes involved in secondary metabolism and mycelial development. Sequence analysis of the Fug1 protein provided evidence for nuclear localization, DNA binding and a domain of unknown function associated with previously characterized transcriptional regulators. This information, combined with the observed transcriptional reprogramming in the deletion mutant, suggests that FUG1 represents a novel class of fungal transcription factors or genes otherwise involved in signal transduction.

From Folding to Function: Design of a New Switchable Biosurfactant Protein

A new anionic biosurfactant protein (SP16) capable of tuning foaming behaviour by pH or salt has been designed. This biosurfactant exhibits unique foaming behaviour with high sensitivity to pH. A good level of foaming was observed at pH 2 but not at pH 3. A further increase by one pH unit to pH 4 restored good foaming. At pH 5-8, SP16 again showed low foaming propensity, whereas the presence of salt (NaCl) was able to restore foaming again. Interfacial tension and circular dichroism investigations revealed the foaming control mechanism. The high negative charge (-16.6) at pH 6 and above restricted the ability of SP16 to fold into an α-helical conformation and also restricted surface activity. For pH 5 (-13.6), even though SP16 folds in bulk to give α-helical structure, the high charge inhibited adsorption at the air-water interface, resulting in a significant lag time of about 150-200 sec to achieve a decrease in interfacial tension. In contrast to its low foaming behaviour at pH 5-8, the presence of salt (NaCl) was found to effectively screen negative charge, thus leading to its folding and a decrease of interfacial tension. This new design offers a new strategy to control foaming behaviour, and elaborates a clear link between charge, structure and interfacial activity for biosurfactants.

Identification of effector-like proteins in Trichoderma spp. and role of a hydrophobin in the plant-fungus interaction and mycoparasitism

BACKGROUND

Trichoderma spp. can establish beneficial interactions with plants by promoting plant growth and defense systems, as well as, antagonizing fungal phytopathogens in mycoparasitic interactions. Such interactions depend on signal exchange between both participants and can be mediated by effector proteins that alter the host cell structure and function, allowing the establishment of the relationship. The main purpose of this work was to identify, using computational methods, candidates of effector proteins from T. virens, T. atroviride and T. reesei, validate the expression of some of the genes during a beneficial interaction and mycoparasitism and to define the biological function for one of them.

RESULTS

We defined a catalogue of putative effector proteins from T. virens, T. atroviride and T. reesei. We further validated the expression of 16 genes encoding putative effector proteins from T. virens and T. atroviride during the interaction with the plant Arabidopsis thaliana, and with two anastomosis groups of the phytopathogenic fungus Rhizoctonia solani. We found genes which transcript levels are modified in response to the presence of both plant fungi, as well as genes that respond only to either a plant or a fungal host. Further, we show that overexpression of the gene tvhydii1, a Class II hydrophobin family member, enhances the antagonistic activity of T. virens against R. solani AG2. Further, deletion of tvhydii1 results in reduced colonization of plant roots, while its overexpression increases it.

CONCLUSIONS

Our results show that Trichoderma is able to respond in different ways to the presence of a plant or a fungal host, and it can even distinguish between different strains of fungi of a given species. The putative effector proteins identified here may play roles in preventing perception of the fungus by its hosts, favoring host colonization or protecting it from the host's defense response. Finally, the novel effector protein TVHYDII1 plays a role in plant root colonization by T, virens, and participates in its antagonistic activity against R. solani.

HFB7 - A novel orphan hydrophobin of the Harzianum and Virens clades of Trichoderma, is involved in response to biotic and abiotic stresses

Hydrophobins are small secreted cysteine-rich proteins exclusively found in fungi. They are able to self-assemble in single molecular layers at hydrophobic-hydrophilic interfaces and can therefore be directly involved in establishment of fungi in their habitat. The genomes of filamentous mycotrophic fungi Trichoderma encode a rich diversity of hydrophobins, which are divided in several groups based on their structure and evolution. Here we describe a new member of class II hydrophobins, HFB7, that has a taxonomically restricted occurrence in Harzianum and Virens clades of Trichoderma. Evolutionary analysis reveals that HFB7 proteins form a separate clade distinct from other Trichoderma class II hydrophobins and that genes encoding them evolve under positive selection pressure. Homology modelling of HFB7 structure in comparison to T. reesei HFB2 reveals that the two large hydrophobic patches on the surface of the protein are remarkably conserved between the two hydrophobins despite significant difference in their primary structures. Expression of hfb7 gene in T. virens increases at interactions with other fungi and a plant and in response to a diversity of abiotic stress conditions, and is also upregulated during formation of aerial mycelium in a standing liquid culture. This upregulation significantly exceeds that of expression of hfb7 under a strong constitutive promoter, and T. virens strains overexpressing hfb7 thus display only changes in traits characterized by low hfb7 expression, i.e. faster growth in submerged liquid culture. The hfb7 gene is not expressed in conidia. Our data allow to conclude that this protein is involved in defence of Trichoderma against a diversity of stress factors related to the oxidative stress. Moreover, HFB7 likely helps in the establishment of the fungus in wetlands or other conditions related to high humidity.

Hydrophobins in the Life Cycle of the Ectomycorrhizal Basidiomycete Tricholoma vaccinum

Hydrophobins-secreted small cysteine-rich, amphipathic proteins-foster interactions of fungal hyphae with hydrophobic surfaces, and are involved in the formation of aerial hyphae. Phylogenetic analyses of Tricholoma vaccinum hydrophobins showed a grouping with hydrophobins of other ectomycorrhizal fungi, which might be a result of co-evolution. Further analyses indicate angiosperms as likely host trees for the last common ancestor of the genus Tricholoma. The nine hydrophobin genes in the T. vaccinum genome were investigated to infer their individual roles in different stages of the life cycle, host interaction, asexual and sexual development, and with respect to different stresses. In aerial mycelium, hyd8 was up-regulated. In silico analysis predicted three packing arrangements, i.e., ring-like, plus-like and sheet-like structure for Hyd8; the first two may assemble to rodlets of hydrophobin covering aerial hyphae, whereas the third is expected to be involved in forming a two-dimensional network of hydrophobins. Metal stress induced hydrophobin gene hyd5. In early steps of mycorrhization, induction of hyd4 and hyd5 by plant root exudates and root volatiles could be shown, followed by hyd5 up-regulation during formation of mantle, Hartig' net, and rhizomorphs with concomitant repression of hyd8 and hyd9. During fruiting body formation, mainly hyd3, but also hyd8 were induced. Host preference between the compatible host Picea abies and the low compatibility host Pinus sylvestris could be linked to a stronger induction of hyd4 and hyd5 by the preferred host and a stronger repression of hyd8, whereas the repression of hyd9 was comparable between the two hosts.

Analysis of the ionic interaction between the hydrophobin RodA and two cutinases of Aspergillus nidulans obtained via an Aspergillus oryzae expression system

Hydrophobins are amphipathic secretory proteins with eight conserved cysteine residues and are ubiquitous among filamentous fungi. In the fungus Aspergillus oryzae, the hydrophobin RolA and the polyesterase CutL1 are co-expressed when the sole available carbon source is the biodegradable polyester polybutylene succinate-co-adipate (PBSA). RolA promotes the degradation of PBSA by attaching to the particle surface, changing its structure and interacting with CutL1 to concentrate CutL1 on the PBSA surface. We previously reported that positively charged residues in RolA and negatively charged residues in CutL1 are cooperatively involved in the ionic interaction between RolA and CutL1. We also reported that hydrophobin RodA of the model fungus Aspergillus nidulans, which was obtained via an A. oryzae expression system, interacted via ionic interactions with CutL1. In the present study, phylogenetic and alignment analyses revealed that the N-terminal regions of several RolA orthologs contained positively charged residues and that the corresponding negatively charged residues on the surface of CutL1 that were essential for the RolA-CutL1 interaction were highly conserved in several CutL1 orthologs. A PBSA microparticle degradation assay, a pull-down assay using a dispersion of Teflon particles, and a kinetic analysis using a quartz crystal microbalance revealed that recombinant A. nidulans RodA interacted via ionic interactions with two recombinant A. nidulans cutinases. Together, these results imply that ionic interactions between hydrophobins and cutinases may be common among aspergilli and other filamentous fungi.

Induced Fit in Protein Multimerization: The HFBI Case

Hydrophobins, produced by filamentous fungi, are small amphipathic proteins whose biological functions rely on their unique surface-activity properties. Understanding the mechanistic details of the multimerization process is of primary importance to clarify the interfacial activity of hydrophobins. We used free energy calculations to study the role of a flexible β-hairpin in the multimerization process in hydrophobin II from Trichoderma reesei (HFBI). We characterized how the displacement of this β-hairpin controls the stability of the monomers/dimers/tetramers in solution. The regulation of the oligomerization equilibrium of HFBI will necessarily affect its interfacial properties, fundamental for its biological function and for technological applications. Moreover, we propose possible routes for the multimerization process of HFBI in solution. This is the first case where a mechanism by which a flexible loop flanking a rigid patch controls the protein-protein binding equilibrium, already known for proteins with charged binding hot-spots, is described within a hydrophobic patch.

Fungi proliferate by creating a complex hyphal network growing within a wet environment. However, for most fungi to colonize new territories, they must produce spores carried by aerial hyphae and spread them into the air. Aerial structures need to overcome the surface tension of the surrounding water in order to grow into the air. This process requires hydrophobins, a remarkable class of self-associating fungal proteins which lower the surface tension at the air/water interface by creating a thin amphipathic layer. In solution they form multimers in equilibrium with the interfacial layer. Due to their unique surface-activity properties, hydrophobins have been used for a variety of biotechnical applications. We used enhanced sampling molecular dynamics simulations methods to study the multimerization process in solution of a hydrophobin from Trichoderma reesei (HFBI). We clarified the fundamental role of a small flexible region within the HFBI monomer involved in the formation of multimers. A flexible loop flanking a rigid interaction patch is able to fine-tune the interaction energy. This mechanism, already known for charged binding patches, is described here for hydrophobic hot-spots. This result is remarkably important in order to clarify the mechanism of arranging at the interface and enhancing hydrophobin-based technological applications.

Studying of cellular interaction of hairpin-like peptide EcAMP1 from barnyard grass (Echinochloa crusgalli L.) seeds with plant pathogenic fungus Fusarium solani using microscopy techniques

An interaction of recombinant hairpin-like cationic peptide EcAMP1 with conidia of plant pathogenic fungus Fusarium solani at the cellular level was studied by a combination of microscopic methods. EcAMP1 is from barnyard grass (Echinochloa crusgalli L.), and obtained by heterologous expression in Escherichia coli system. As a result, a direct relationship between hyphal growth inhibition and increasing active peptide concentration, time of incubation and fungal physiological condition has been determined. Dynamics of accumulation and redistribution of the peptide studied on fungal cellular cover and inside the conidia cells has been shown. The dynamics are dependent on time of coupling, as well as, a dissimilarity of EcAMP1 binding with cover of fungal conidia and its stepwise accumulation and diffuse localization in the cytoplasm. Correlation between structural disruption of fungal conidia and the presence of morphological changes has also been found. The correlation was found under the influence of peptide high concentrations at concentrations above 32 μM. The results indicate the presence of a binding of EcAMP1 with the surface of fungal conidia, thus, demonstrating a main specificity for its antifungal action at the cellular level. These results, however, cannot exclude the existence of attendant EcAMP1 action based on its intracellular localization on some specific targets. SCANNING 38:591-598, 2016. © 2016 Wiley Periodicals, Inc.

Structural and functional diversity among amyloid proteins: Agents of disease, building blocks of biology, and implications for molecular engineering

Amyloids have long been associated with protein dysfunction and neurodegenerative diseases, but recent research has demonstrated that some organisms utilize the unique properties of the amyloid fold to create functional structures with important roles in biological processes. Additionally, new engineering approaches have taken advantage of amyloid structures for implementation in a wide variety of materials and devices. In this review, the role of amyloid in human disease is discussed and compared to the functional amyloids, which serve a largely structural purpose. We then consider the use of amyloid constructs in engineering applications, including their utility as building blocks for synthetic biology and molecular engineering. Biotechnol. Bioeng. 2017;114: 7-20. © 2016 Wiley Periodicals, Inc.

Marine fungi as source of new hydrophobins

Hydrophobins have been described as the most powerful surface-active proteins known. They are produced by filamentous fungi and exhibit a distinct amphiphilic structure determining their self-assembly at hydrophilic-hydrophobic interfaces and surfactant properties which have been demonstrated to be useful for several biotechnological applications. The marine environment represents a vast natural resource of new molecules produced by organisms growing in various stressful conditions. This study was focused on the screening of 100 marine fungi from Mycoteca Universitatis Taurinensis (MUT) for the identification of new hydrophobins. Four different methods were set up to extract hydrophobins of class I and II, from the mycelium or the culture broth of fungi. Six fungi were selected as the best producers of hydrophobins endowed with different characteristics. Their ability to form stable amphiphilic films and their emulsification capacity in the presence of olive oil was evaluated.

Superhydrophobic hierarchically structured surfaces in biology: evolution, structural principles and biomimetic applications

A comprehensive survey of the construction principles and occurrences of superhydrophobic surfaces in plants, animals and other organisms is provided and is based on our own scanning electron microscopic examinations of almost 20 000 different species and the existing literature. Properties such as self-cleaning (lotus effect), fluid drag reduction (Salvinia effect) and the introduction of new functions (air layers as sensory systems) are described and biomimetic applications are discussed: self-cleaning is established, drag reduction becomes increasingly important, and novel air-retaining grid technology is introduced. Surprisingly, no evidence for lasting superhydrophobicity in non-biological surfaces exists (except technical materials). Phylogenetic trees indicate that superhydrophobicity evolved as a consequence of the conquest of land about 450 million years ago and may be a key innovation in the evolution of terrestrial life. The approximate 10 million extant species exhibit a stunning diversity of materials and structures, many of which are formed by self-assembly, and are solely based on a limited number of molecules. A short historical survey shows that bionics (today often called biomimetics) dates back more than 100 years. Statistical data illustrate that the interest in biomimetic surfaces is much younger still. Superhydrophobicity caught the attention of scientists only after the extreme superhydrophobicity of lotus leaves was published in 1997. Regrettably, parabionic products play an increasing role in marketing.This article is part of the themed issue 'Bioinspired hierarchically structured surfaces for green science'.

A phenomenological description of BslA assemblies across multiple length scales

Intrinsically interfacially active proteins have garnered considerable interest recently owing to their potential use in a range of materials applications. Notably, the fungal hydrophobins are known to form robust and well-organized surface layers with high mechanical strength. Recently, it was shown that the bacterial biofilm protein BslA also forms highly elastic surface layers at interfaces. Here we describe several self-assembled structures formed by BslA, both at interfaces and in bulk solution, over a range of length scales spanning from nanometres to millimetres. First, we observe transiently stable and highly elongated air bubbles formed in agitated BslA samples. We study their behaviour in a range of solution conditions and hypothesize that their dissipation is a consequence of the slow adsorption kinetics of BslA to an air-water interface. Second, we describe elongated tubules formed by BslA interfacial films when shear stresses are applied in both a Langmuir trough and a rheometer. These structures bear a striking resemblance, although much larger in scale, to the elongated air bubbles formed during agitation. Taken together, this knowledge will better inform the conditions and applications of how BslA can be used in the stabilization of multi-phase materials.This article is part of the themed issue 'Soft interfacial materials: from fundamentals to formulation'.

Up-regulation of carbon metabolism-related glyoxylate cycle and toxin production in Beauveria bassiana JEF-007 during infection of bean bug, Riptortus pedestris (Hemiptera: Alydidae)

Beauveria bassiana (Bb) is used as an environment-friendly biopesticide. However, the molecular mechanisms of Bb-host interactions are not well understood. Herein, RNA isolated from B. bassiana (Bb JEF-007) and Riptortus pedestris (Hemiptera: Alydidae) infected with this strain were firstly subjected to high-throughput next generation sequencing (NGS) to analyze and compare transcriptomes. Due to lack of fungal and host genome information, fungal transcriptome was processed to partially exclude non-infection specific genes and host-flora. Differentially Expressed Gene (DEG) analysis showed that 2381 genes were up-regulated and 2303 genes were down-regulated upon infection. Most DEGs were classified into the categories of single-organism, cellular and metabolism processes by Gene Ontology analysis. Most DEGs were involved in metabolic pathways based on Kyoto Encyclopedia of Genes and Genomes pathway mapping. Carbon metabolism-related enzymes in the glyoxylate cycle were significantly up-regulated, suggesting a possible role for them in Bb growth in the host. Additionally, transcript levels of several fungal genes were dramatically increased after infection, such as cytotoxic lectin-like protein, bacterial-like toxin, proteins related to cell wall formation, hyphal growth, nutrient uptake, and halogenated compound synthesis. This work provides insight into how entomopathogenic B. bassiana grows in agriculturally harmful bean bug at 6 d post infection.

Formation of a Rigid Hydrophobin Film and Disruption by an Anionic Surfactant at an Air/Water Interface

Hydrophobins are amphiphilic proteins produced by fungi. Cerato-ulmin (CU) is a hydrophobin that has been associated with Dutch elm disease. Like other hydrophobins, CU stabilizes air bubbles and oil droplets through the formation of a persistent protein film at the interface. The behavior of hydrophobins at surfaces has raised interest in their potential applications, including use in surface coatings, food foams, and emulsions and as dispersants. The practical use of hydrophobins requires an improved understanding of the interfacial behavior of these proteins, alone and in the presence of added surfactants. In this study, the adsorption behavior of CU at air/water interfaces is characterized by measuring the surface tension and interfacial rheology as a function of adsorption time. CU is found to adsorb irreversibly at air/water interfaces. The magnitude of the dilatational modulus increases with adsorption time and surface pressure until CU eventually forms a rigid film. The persistence of this film is tested through the sequential addition of strong surfactant sodium dodecyl sulfate (SDS) to the bulk liquid adjacent to the interface. SDS is found to coadsorb to interfaces precoated with a CU film. At high concentrations, the addition of SDS significantly decreases the dilatational modulus, indicating disruption and displacement of CU by SDS. Sequential adsorption results in mixed layers with properties not observed in interfaces generated from complexes formed in the bulk. These results lend insight to the complex interfacial interactions between hydrophobins and surfactants.

The Diverse Structures and Functions of Surfactant Proteins

Surface tension at liquid–air interfaces is a major barrier that needs to be surmounted by a wide range of organisms; surfactant and interfacially active proteins have evolved for this purpose. Although these proteins are essential for a variety of biological processes, our understanding of how they elicit their function has been limited. However, with the recent determination of high-resolution 3D structures of several examples, we have gained insight into the distinct shapes and mechanisms that have evolved to confer interfacial activity. It is now a matter of harnessing this information, and these systems, for biotechnological purposes.

Interfacially active proteins fulfill a wide range of biological functions in organisms ranging from bacteria and fungi to mammals.

Their physicochemical properties make interfacially active proteins attractive for biotechnological applications; for example, as coatings on nanodevices or medical implants and as emulsifiers in food and personal-care products.

High-resolution 3D structures show that the mechanisms by which interfacially active proteins achieve their function are highly diverse.

Novel application of hydrophobin in medical science: a drug carrier for improving serum stability

Multiple physiological properties of glucagon-like peptide-1 (GLP-1) ensure that it is a promising drug candidate for the treatment of type 2 diabetes. However, the in vivo half-life of GLP-1 is short because of rapid degradation by dipeptidyl peptidase-IV (DPP-IV) and renal clearance. The poor serum stability of GLP-1 has significantly limited its clinical utility, although many studies are focused on extending the serum stability of this molecule. Hydrophobin, a self-assembling protein, was first applied as drug carrier to stabilize GLP-1 against protease degradation by forming a cavity. The glucose tolerance test clarified that the complex retained blood glucose clearance activity for 72 hours suggesting that this complex might be utilized as a drug candidate administered every 2–3 days. Additionally, it was found that the mutagenesis of hydrophobin preferred a unique pH condition for self-assembly. These findings suggested that hydrophobin might be a powerful tool as a drug carrier or a pH sensitive drug-release compound. The novel pharmaceutical applications of hydrophobin might result in future widespread interest in hydrophobin.

De Novo Sequencing and Transcriptome Analysis of Pleurotus eryngii subsp. tuoliensis (Bailinggu) Mycelia in Response to Cold Stimulation

Cold stimulation of Bailinggu's mycelia is the main factor that triggers primordia initiation for successful production of fruiting bodies under commercial cultivation. Yet, the molecular-level mechanisms involved in mycelia response to cold stimulation are still unclear. Here, we performed comparative transcriptomic analysis using RNA-Seq technology to better understand the gene expression regulation during different temporal stages of cold stimulation in Bailinggu. A total of 21,558 Bailinggu mycelia unigenes were de novo assembled and annotated from four libraries (control at 25 °C, plus cold stimulation treatments at -3 °C for a duration of 1-2 days, 5-6 days, and 9-10 days). GO and KEGG pathway analysis indicated that functional groups of differentially expressed unigenes associated with cell wall and membrane stabilization, calcium signaling and mitogen-activated protein kinases (MAPK) pathways, and soluble sugars and protein biosynthesis and metabolism pathways play a vital role in Bailinggu's response to cold stimulation. Six hundred and seven potential EST-based SSRs loci were identified in these unigenes, and 100 EST-SSR primers were randomly selected for validation. The overall polymorphism rate was 92% by using 10 wild strains of Bailinggu. Therefore, these results can serve as a valuable resource for a better understanding of the molecular mechanisms associated with Bailinggu's response to cold stimulation.

Recent Advances in Fungal Hydrophobin Towards Using in Industry

Fungal hydrophobin is a family of low molecular weight proteins consisting of four disulfide bridges and an extraordinary hydrophobic patch. The hydrophobic patch of hydrophobins and the molecules of gaseous CO2 may interact together and form the stable CO2-nanobubbles covered by an elastic membrane in carbonated beverages. The nanobubbles provide the required energy to provoke primary gushing. Due to the hydrophobicity of hydrophobin, this protein is used as a biosurfactant, foaming agent or encapsulating agent in food products and medicine formulations. Increasing demands for using of hydrophobins led to a challenge regarding production and purification of this product. However, the main issue to use hydrophobin in the industry is the regulatory affairs: yet there is no approved legislation for using hydrophobin in food and beverages. To comply with the legislation, establishing a consistent method for obtaining pure hydrophobins is necessary. Currently, few research teams in Europe are focusing on different aspects of hydrophobins. In this paper, an up-to-date collection of highlights from those special groups about the bio-chemical and physicochemical characteristics of hydrophobins have been studied. The recent advances of those groups concerning the production and purification, positive applications and negative function of hydrophobin are also summarised.

Secretion of small proteins is species-specific within Aspergillus sp

Small secreted proteins (SSP) have been defined as proteins containing a signal peptide and a sequence of less than 300 amino acids. In this analysis, we have compared the secretion pattern of SSPs among eight aspergilli species in the context of plant biomass degradation and have highlighted putative interesting candidates that could be involved in the degradative process or in the strategies developed by fungi to resist the associated stress that could be due to the toxicity of some aromatic compounds or reactive oxygen species released during degradation. Among these candidates, for example, some stress‐related superoxide dismutases or some hydrophobic surface binding proteins (HsbA) are specifically secreted according to the species . Since these latter proteins are able to recruit lytic enzymes to the surface of hydrophobic solid materials and promote their degradation, a synergistic action of HsbA with the degradative system may be considered and need further investigations. These SSPs could have great applications in biotechnology by optimizing the efficiency of the enzymatic systems for biomass degradation.

Hydrophobin-Based Surface Engineering for Sensitive and Robust Quantification of Yeast Pheromones

Detection and quantification of small peptides, such as yeast pheromones, are often challenging. We developed a highly sensitive and robust affinity-assay for the quantification of the α-factor pheromone of Saccharomyces cerevisiae based on recombinant hydrophobins. These small, amphipathic proteins self-assemble into highly stable monolayers at hydrophilic-hydrophobic interfaces. Upon functionalization of solid supports with a combination of hydrophobins either lacking or exposing the α-factor, pheromone-specific antibodies were bound to the surface. Increasing concentrations of the pheromone competitively detached the antibodies, thus allowing for quantification of the pheromone. By adjusting the percentage of pheromone-exposing hydrophobins, the sensitivity of the assay could be precisely predefined. The assay proved to be highly robust against changes in sample matrix composition. Due to the high stability of hydrophobin layers, the functionalized surfaces could be repeatedly used without affecting the sensitivity. Furthermore, by using an inverse setup, the sensitivity was increased by three orders of magnitude, yielding a novel kind of biosensor for the yeast pheromone with the lowest limit of detection reported so far. This assay was applied to study the pheromone secretion of diverse yeast strains including a whole-cell biosensor strain of Schizosaccharomyces pombe modulating α-factor secretion in response to an environmental signal.

Reactive oxygen species production, induced by atmospheric modification, alter conidial quality of Beauveria bassiana

AIM

The aim of this study was to determine the relationship between reactive oxygen species (ROS) production and conidial infectivity in Beauveria bassiana.

METHODS AND RESULTS

Beauveria bassiana Bb 882.5 was cultured in solid-state culture (SSC) using rice under three oxygen conditions (21%, or pulses at 16 and 26%). Hydrophobicity was determined using exclusion phase assay. Bioassays with larvae or adults of Tenebrio molitor allowed the measurements of infectivity parameters. A fluorometric method was used for ROS quantification (superoxide and total peroxides). NADPH oxidase (NOX) activity was determined by specific inhibition. Conidial hydrophobicity decreased by O2 pulses. Mortality of larvae was only achieved with conidia harvested from cultures under 21% O2 ; whereas for adult insects, the infectivity parameters deteriorated in conidia obtained after pulses at 16 and 26% O2 . At day 7, ROS production increased after 16 and 26% O2 treatments. NOX activity induced ROS production at early stages of the culture.

CONCLUSION

Modification of atmospheric oxygen increases ROS production, reducing conidial quality and infectivity.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first study in which conidial infectivity and ROS production in B. bassiana has been related, enhancing the knowledge of the effect of O2 pulses in B. bassiana.

Fungal spores for dispersion in space and time

Spores are an integral part of the life cycle of the gross majority of fungi. Their morphology and the mode of formation are both highly variable among the fungi, as is their resistance to stressors. The main aim for spores is to be dispersed, both in space, by various mechanisms or in time, by an extended period of dormancy. Some fungal ascospores belong to the most stress-resistant eukaryotic cells described to date. Stabilization is a process in which biomolecules and complexes thereof are protected by different types of molecules against heat, drought, or other molecules. This review discusses the most important compounds that are known to protect fungal spores and also addresses the biophysics of cell protection. It further covers the phenomena of dormancy, breaking of dormancy, and early germination. Germination is the transition from a dormant cell toward a vegetative cell and includes a number of specific changes. Finally, the applied aspects of spore biology are discussed.

Ectomycorrhizal fungi decompose soil organic matter using oxidative mechanisms adapted from saprotrophic ancestors

Ectomycorrhizal fungi are thought to have a key role in mobilizing organic nitrogen that is trapped in soil organic matter (SOM). However, the extent to which ectomycorrhizal fungi decompose SOM and the mechanism by which they do so remain unclear, considering that they have lost many genes encoding lignocellulose-degrading enzymes that are present in their saprotrophic ancestors. Spectroscopic analyses and transcriptome profiling were used to examine the mechanisms by which five species of ectomycorrhizal fungi, representing at least four origins of symbiosis, decompose SOM extracted from forest soils. In the presence of glucose and when acquiring nitrogen, all species converted the organic matter in the SOM extract using oxidative mechanisms. The transcriptome expressed during oxidative decomposition has diverged over evolutionary time. Each species expressed a different set of transcripts encoding proteins associated with oxidation of lignocellulose by saprotrophic fungi. The decomposition 'toolbox' has diverged through differences in the regulation of orthologous genes, the formation of new genes by gene duplications, and the recruitment of genes from diverse but functionally similar enzyme families. The capacity to oxidize SOM appears to be common among ectomycorrhizal fungi. We propose that the ancestral decay mechanisms used primarily to obtain carbon have been adapted in symbiosis to scavenge nutrients instead.

Orotate phosphoribosyl transferase MoPyr5 is involved in uridine 5'-phosphate synthesis and pathogenesis of Magnaporthe oryzae

Orotate phosphoribosyl transferase (OPRTase) plays an important role in de novo and salvage pathways of nucleotide synthesis and is widely used as a screening marker in genetic transformation. However, the function of OPRTase in plant pathogens remains unclear. In this study, we characterized an ortholog of Saccharomyces cerevisiae Ura5, the OPRTase MoPyr5, from the rice blast fungus Magnaporthe oryzae. Targeted gene disruption revealed that MoPyr5 is required for mycelial growth, appressorial turgor pressure and penetration into plant tissues, invasive hyphal growth, and pathogenicity. Interestingly, the ∆Mopyr5 mutant is also involved in mycelial surface hydrophobicity. Exogenous uridine 5'-phosphate (UMP) restored vegetative growth and rescued the defect in pathogenicity on detached barley and rice leaf sheath. Collectively, our results show that MoPyr5 is an OPRTase for UMP biosynthesis in M. oryzae and indicate that UTP biosynthesis is closely linked with vegetative growth, cell wall integrity, and pathogenicity of fungus. Our results also suggest that UMP biosynthesis would be a good target for the development of novel fungicides against M. oryzae.

Cell Adhesion on Amyloid Fibrils Lacking Integrin Recognition Motif

Amyloids are highly ordered, cross-β-sheet-rich protein/peptide aggregates associated with both human diseases and native functions. Given the well established ability of amyloids in interacting with cell membranes, we hypothesize that amyloids can serve as universal cell-adhesive substrates. Here, we show that, similar to the extracellular matrix protein collagen, amyloids of various proteins/peptides support attachment and spreading of cells via robust stimulation of integrin expression and formation of integrin-based focal adhesions. Additionally, amyloid fibrils are also capable of immobilizing non-adherent red blood cells through charge-based interactions. Together, our results indicate that both active and passive mechanisms contribute to adhesion on amyloid fibrils. The present data may delineate the functional aspect of cell adhesion on amyloids by various organisms and its involvement in human diseases. Our results also raise the exciting possibility that cell adhesivity might be a generic property of amyloids.

Surface heterogeneity: a friend or foe of protein adsorption – insights from theoretical simulations

A lack in the detailed understanding of mechanisms through which proteins adsorb or are repelled at various solid/liquid interfaces limits the capacity to rationally design and produce more sophisticated surfaces with controlled protein adsorption in both biomedical and industrial settings. To date there are three main approaches to achieve anti biofouling efficacy, namely chemically adjusting the surface hydrophobicity and introducing various degrees of surface roughness, or a combination of both. More recently, surface nanostructuring has been shown to have an effect on protein adsorption. However, the current resolution of experimental techniques makes it difficult to investigate these three phase systems at the molecular level. In this molecular dynamics study we explore in all-atom detail the adsorption process of one of the most surface active proteins, EAS hydrophobin, known for its versatile ability to self-assemble on both hydrophobic and hydrophilic surfaces forming stable monolayers that facilitate further biofilm growth. We model the adsorption of this protein on organic ligand protected silica surfaces with varying degrees of chemical heterogeneity and roughness, including fully homogenous hydrophobic and hydrophilic surfaces for comparison. We present a detailed characterisation of the functionalised surface structure and dynamics for each of these systems, and the effect the ligands have on interfacial water, the adsorption process and conformational rearrangements of the protein. Results suggest that the ligand arrangement that produces the highest hydrophilic chain mobility and the lack of significant hydrophobic patches shows the most promising anti-fouling efficacy toward hydrophobin. However, the presence on the protein surface of a flexible loop with amphipathic character (the Cys3–Cys4 loop) is seen to facilitate EAS adsorption on all surfaces by enabling the protein to match the surface pattern.

Conidiation of Penicillium camemberti in submerged liquid cultures is dependent on the nitrogen source

OBJECTIVE

To study the ability of a commercial Penicillium camemberti strain, used for Camembert type cheese ripening, to produce conidia during growth in liquid culture (LC), in media containing different sources of nitrogen as, industrially, conidia are produced by growth at the surface of a solid state culture because conidiation in stirred submerged aerobic LC is not known.

RESULTS

In complex media containing peptic digest of meat, hyphae ends did not differentiate into phialides and conidia. Contrarily, in a synthetic media containing KNO3 as sole nitrogen source, hyphae ends differentiated into phialides producing 0.5 × 10(7) conidia/ml. Conidia produced in LC were 25 % less hydrophobic than conidia produced in solid culture, and this correlates with a seven-times-lower expression of the gene rodA encoding hydrophobin RodA in the mycelium grown in LC.

CONCLUSION

Conidiation of P. camembertii is stimulated in iquid medium containing KNO3 as sole source of nitrogen and therefore opens up opportunities for using liquid medium in commercial productions.

Surface-water Interface Induces Conformational Changes Critical for Protein Adsorption: Implications for Monolayer Formation of EAS Hydrophobin

The class I hydrophobin EAS is part of a family of small, amphiphilic fungal proteins best known for their ability to self-assemble into stable monolayers that modify the hydrophobicity of a surface to facilitate further microbial growth. These proteins have attracted increasing attention for industrial and biomedical applications, with the aim of designing surfaces that have the potential to maintain their clean state by resisting non-specific protein binding. To gain a better understanding of this process, we have employed all-atom molecular dynamics to study initial stages of the spontaneous adsorption of monomeric EAS hydrophobin on fully hydroxylated silica, a commonly used industrial and biomedical substrate. Particular interest has been paid to the Cys3-Cys4 loop, which has been shown to exhibit disruptive behavior in solution, and the Cys7-Cys8 loop, which is believed to be involved in the aggregation of EAS hydrophobin at interfaces. Specific and water mediated interactions with the surface were also analyzed. We have identified two possible binding motifs, one which allows unfolding of the Cys7-Cys8 loop due to the surfactant-like behavior of the Cys3-Cys4 loop, and another which has limited unfolding due to the Cys3-Cys4 loop remaining disordered in solution. We have also identified intermittent interactions with water which mediate the protein adsorption to the surface, as well as longer lasting interactions which control the diffusion of water around the adsorption site. These results have shown that EAS behaves in a similar way at the air-water and surface-water interfaces, and have also highlighted the need for hydrophilic ligand functionalization of the silica surface in order to prevent the adsorption of EAS hydrophobin.

A cerato-platanin-like protein HaCPL2 from Heterobasidion annosum sensu stricto induces cell death in Nicotiana tabacum and Pinus sylvestris

The cerato-platanin family is a group of small secreted cysteine-rich proteins exclusive for filamentous fungi. They have been shown to be involved in the interactions between fungi and plants. Functional characterization of members from this family has been performed mainly in Ascomycota, except Moniliophthora perniciosa. Our previous phylogenetic analysis revealed that recent gene duplication of cerato-platanins has occurred in Basidiomycota but not in Ascomycota, suggesting higher functional diversification of this protein family in Basidiomycota than in Ascomycota. In this study, we identified three cerato-platanin homologues from the basidiomycete conifer pathogen Heterobasidion annosum sensu stricto. Expression of the homologues under various conditions as well as their roles in the H. annosum s.s.-Pinus sylvestris (Scots pine) pathosystem was investigated. Results showed that HaCPL2 (cerato-platanin-like protein 2) had the highest sequence similarity to cerato-platanin from Ceratocystis platani and hacpl2 was significantly induced during nutrient starvation and necrotrophic growth. The treatment with recombinant HaCPL2 induced cell death, phytoalexin production and defense gene expression in Nicotiana tabacum. Eliciting and cell death-inducing ability accompanied by retardation of apical root growth was also demonstrated in Scots pine seedlings. Our results suggest that HaCPL2 might contribute to the virulence of H. annosum s.s. by promoting plant cell death.

Structure Analysis Uncovers a Highly Diverse but Structurally Conserved Effector Family in Phytopathogenic Fungi

Phytopathogenic ascomycete fungi possess huge effector repertoires that are dominated by hundreds of sequence-unrelated small secreted proteins. The molecular function of these effectors and the evolutionary mechanisms that generate this tremendous number of singleton genes are largely unknown. To get a deeper understanding of fungal effectors, we determined by NMR spectroscopy the 3-dimensional structures of the Magnaporthe oryzae effectors AVR1-CO39 and AVR-Pia. Despite a lack of sequence similarity, both proteins have very similar 6 β-sandwich structures that are stabilized in both cases by a disulfide bridge between 2 conserved cysteins located in similar positions of the proteins. Structural similarity searches revealed that AvrPiz-t, another effector from M. oryzae, and ToxB, an effector of the wheat tan spot pathogen Pyrenophora tritici-repentis have the same structures suggesting the existence of a family of sequence-unrelated but structurally conserved fungal effectors that we named MAX-effectors (MagnaportheAvrs and ToxB like). Structure-informed pattern searches strengthened this hypothesis by identifying MAX-effector candidates in a broad range of ascomycete phytopathogens. Strong expansion of the MAX-effector family was detected in M. oryzae and M. grisea where they seem to be particularly important since they account for 5–10% of the effector repertoire and 50% of the cloned avirulence effectors. Expression analysis indicated that the majority of M. oryzae MAX-effectors are expressed specifically during early infection suggesting important functions during biotrophic host colonization. We hypothesize that the scenario observed for MAX-effectors can serve as a paradigm for ascomycete effector diversity and that the enormous number of sequence-unrelated ascomycete effectors may in fact belong to a restricted set of structurally conserved effector families.

Fungal plant pathogens are of outstanding economic and ecological importance and cause destructive diseases on many cultivated and wild plants. Effector proteins that are secreted during infection to manipulate the host and to promote disease are a key element in fungal virulence. Phytopathogenic fungi possess huge effector repertoires that are dominated by hundreds of sequence-unrelated small secreted proteins. The molecular functions of this most important class of fungal effectors and the evolutionary mechanisms that generate this tremendous numbers of apparently unrelated proteins are largely unknown. By investigating the 3-dimensional structures of effectors from the rice blast fungus M. oryzae, we discovered an effector family comprising structurally conserved but sequence-unrelated effectors from M. oryzae and the phylogenetically distant wheat pathogen Pyrenophora tritici-repentis that we named MAX-effectors (M. oryzaeAvrs and ToxB). Structure-informed searches of whole genome sequence databases suggest that MAX-effectors are present at low frequencies and with a patchy phylogenetic distribution in many ascomycete phytopathogens. They underwent strong lineage-specific expansion in fungi of the Pyriculariae family that contains M. oryzae where they seem particularly important during biotrophic plant colonization and account for 50% of the cloned Avr effectors and 5–10% of the effector repertoire. Based on our results on the MAX-effectors and the widely accepted concept that fungal effectors evolve according to a birth-and-death model we propose the hypothesis that the majority of the immense numbers of different ascomycete effectors could in fact belong to a limited set of structurally defined families whose members are phylogenetically related.

The Role of Pathogen-Secreted Proteins in Fungal Vascular Wilt Diseases

A limited number of fungi can cause wilting disease in plants through colonization of the vascular system, the most well-known being Verticillium dahliae and Fusarium oxysporum. Like all pathogenic microorganisms, vascular wilt fungi secrete proteins during host colonization. Whole-genome sequencing and proteomics screens have identified many of these proteins, including small, usually cysteine-rich proteins, necrosis-inducing proteins and enzymes. Gene deletion experiments have provided evidence that some of these proteins are required for pathogenicity, while the role of other secreted proteins remains enigmatic. On the other hand, the plant immune system can recognize some secreted proteins or their actions, resulting in disease resistance. We give an overview of proteins currently known to be secreted by vascular wilt fungi and discuss their role in pathogenicity and plant immunity.

Tricholoma vaccinum host communication during ectomycorrhiza formation

The genome sequence of Tricholoma vaccinum was obtained to predict its secretome in order to elucidate communication of T. vaccinum with its host tree spruce (Picea abies) in interkingdom signaling. The most prominent protein domains within the 206 predicted secreted proteins belong to energy and nutrition (52%), cell wall degradation (19%) and mycorrhiza establishment (9%). Additionally, we found small secreted proteins that show typical features of effectors potentially involved in host communication. From the secretome, 22 proteins could be identified, two of which showed higher protein abundances after spruce root exudate exposure, while five were downregulated in this treatment. The changes in T. vaccinum protein excretion with first recognition of the partner were used to identify small secreted proteins with the potential to act as effectors in the mutually beneficial symbiosis. Our observations support the hypothesis of a complex communication network including a cocktail of communication molecules induced long before physical contact of the partners.

Heterologous expression of a hydrophobin HFB1 and evaluation of its contribution to producing stable foam

Hydrophobins are small secreted proteins belong to filamentous fungi. These proteins possess a unique ability to self-assemble at air/water interfaces. Hydrophobins have a broad range of biotechnological applications such as stabilizing emulsions and foams, immobilizing proteins on a surface, designing biosensors, affinity tag for protein purification, and drug delivery. We have successfully expressed HFB1 from Trichoderma reesei belonged to class II of hydrophobins in Pichia pastoris. The recombinant gene was under the control of the methanol-inducible AOX1 promoter (alcohol oxidase 1) in the pPICZAα vector. The amount of secreted HFB1 was increased in 90-h using methanol induction. The recombinant HFB1 was purified based on the presence of His-tag and foam formation. Furthermore, HFB1 was able to produce macro and micro stable air bubbles in the liquid due to the presence of hydrophobic patches on its surface. The liquid medium containing HFB1 becomes turbid after shaking, and then the stable bubbles are formed and remained for three weeks.

The filamentous fungal pellet and forces driving its formation

Filamentous fungi play an important role not only in the bio-manufacturing of value-added products, but also in bioenergy and environmental research. The bioprocess manipulation of filamentous fungi is more difficult than that of other microbial species because of their different pellet morphologies and the presence of tangled mycelia under different cultivation conditions. Fungal pellets, which have the advantages of harvest ease, low fermentation broth viscosity and high yield of some proteins, have been used for a long time. Many attempts have been made to establish the relationship between pellet and product yield using quantitative approaches. Fungal pellet formation is attributed to the combination of electrostatic interactions, hydrophobicity and specific interactions from spore wall components. Electrostatic interactions result from van der Waals forces and negative charge repulsion from carboxyl groups in the spore wall structure. Electrostatic interactions are also affected by counter-ions (cations) and the physiologic conditions of spores that modify the carboxyl groups. Fungal aggregates are promoted by the hydrophobicity generated by hydrophobins, which form a hydrophobic coat that covers the spore. The specific interactions of spore wall components contribute to spore aggregation through salt bridging. A model of spore aggregation was proposed based on these forces. Additionally, some challenges were addressed, including the limitations of research techniques, the quantitative determination of forces and the complex information of biological systems, to clarify the mechanism of fungal pellet formation.

Molecular determinants of mechanical properties of V. cholerae biofilms at the air-liquid interface

Biofilm formation increases both the survival and infectivity of Vibrio cholerae, the causative agent of cholera. V. cholerae is capable of forming biofilms on solid surfaces and at the air-liquid interface, termed pellicles. Known components of the extracellular matrix include the matrix proteins Bap1, RbmA, and RbmC, an exopolysaccharide termed Vibrio polysaccharide, and DNA. In this work, we examined a rugose strain of V. cholerae and its mutants unable to produce matrix proteins by interfacial rheology to compare the evolution of pellicle elasticity in real time to understand the molecular basis of matrix protein contributions to pellicle integrity and elasticity. Together with electron micrographs, visual inspection, and contact angle measurements of the pellicles, we defined distinct contributions of the matrix proteins to pellicle morphology, microscale architecture, and mechanical properties. Furthermore, we discovered that Bap1 is uniquely required for the maintenance of the mechanical strength of the pellicle over time and contributes to the hydrophobicity of the pellicle. Thus, Bap1 presents an important matrix component to target in the prevention and dispersal of V. cholerae biofilms.