Hydrophobins, from molecular structure to multiple functions in fungal development

Functional amyloids

While amyloid has traditionally been viewed as a harmful formation, emerging evidence suggests that amyloids may also play a functional role in cell biology, contributing to normal physiological processes that have been conserved throughout evolution. Functional amyloids have been discovered in several creatures, spanning from bacteria to mammals. These amyloids serve a multitude of purposes, including but not limited to, forming biofilms, melanin synthesis, storage, information transfer, and memory. The functional role of amyloids has been consistently validated by the discovery of more functional amyloids, indicating a conceptual convergence. The biology of amyloids is well-represented by non-pathogenic amyloids, given the numerous ones already identified and the ongoing rate of new discoveries. In this chapter, functional amyloids in microorganisms, animals, and plants are described.

Use of Aureobasidium in a sustainable economy

Aureobasidium is omnipresent and can be isolated from air, water bodies, soil, wood, and other plant materials, as well as inorganic materials such as rocks and marble. A total of 32 species of this fungal genus have been identified at the level of DNA, of which Aureobasidium pullulans is best known. Aureobasidium is of interest for a sustainable economy because it can be used to produce a wide variety of compounds, including enzymes, polysaccharides, and biosurfactants. Moreover, it can be used to promote plant growth and protect wood and crops. To this end, Aureobasidium cells adhere to wood or plants by producing extracellular polysaccharides, thereby forming a biofilm. This biofilm provides a sustainable alternative to petrol-based coatings and toxic chemicals. This and the fact that Aureobasidium biofilms have the potential of self-repair make them a potential engineered living material avant la lettre. KEY POINTS: •Aureobasidium produces products of interest to the industry •Aureobasidium can stimulate plant growth and protect crops •Biofinish of A. pullulans is a sustainable alternative to petrol-based coatings •Aureobasidium biofilms have the potential to function as engineered living materials.

Genome-wide transcriptome profiling reveals molecular response pathways of Trichoderma harzianum in response to salt stress

Trichoderma harzianum exhibits a strong biological control effect on many important plant pathogens, such as Fusarium oxysporum, Botrytis cinerea, and Meloidogyne. However, its biocontrol effectiveness is weakened or reduced under salt stress. The aim of this study was to investigate the molecular response of T. harzianum to salt stress at the whole-genome level. Here, we present a 44.47 Mb near-complete genome assembly of the T. harzianum qt40003 strain for the first time, which was assembled de novo with 7.59 Gb Nanopore sequencing long reads (~170-fold) and 5.2 Gb Illumina short reads (~116-fold). The assembled qt40003 genome contains 12 contigs, with a contig N50 of 4.81 Mb, in which four of the 12 contigs were entirely reconstructed in a single chromosome from telomere to telomere. The qt40003 genome contains 4.27 Mb of repeat sequences and 12,238 protein-coding genes with a BUSCO completeness of 97.5%, indicating the high accuracy and completeness of our gene annotations. Genome-wide transcriptomic analysis was used to investigate gene expression changes related to salt stress in qt40003 at 0, 2% (T2), and 4% (T4) sodium chloride concentrations. A total of 2,937 and 3,527 differentially expressed genes (DEGs) were obtained under T2 and T4 conditions, respectively. GO enrichment analysis showed that the T2-treatment DEGs were highly enriched in detoxification (p < 0.001), while the T4 DEGs were mainly enriched in cell components, mostly in cellular detoxification, cell surface, and cell wall. KEGG metabolic pathway analysis showed that 91 and 173 DEGs were significantly enriched in the T2 and T4 treatments, respectively (p < 0.01), mainly in the glutathione metabolism pathway. We further experimentally analyzed the differentially expressed glutathione transferase genes in the glutathione metabolic pathway, most of which were downregulated (13/15). In addition, we screened 13 genes related to active oxygen clearance, including six upregulated and seven downregulated genes, alongside five fungal hydrophobic proteins, of which two genes were highly expressed. Our study provides high-quality genome information for the use of T. harzianum for biological control and offers significant insights into the molecular responses of T. harzianum under salt-stress conditions.

Hydrophobin Bilayer as Water Impermeable Protein Membrane

One of the most important properties of membranes is their permeability to water and other small molecules. A targeted change in permeability allows the passage of molecules to be controlled. Vesicles made of membranes with low water permeability are preferable for drug delivery, for example, because they are more stable and maintain the drug concentration inside. This study reports on the very low water permeability of pure protein membranes composed of a bilayer of the amphiphilic protein hydrophobin HFBI. Using a droplet interface bilayer setup, we demonstrate that HFBI bilayers are essentially impermeable to water. HFBI bilayers withstand far larger osmotic pressures than lipid membranes. Only by disturbing the packing of the proteins in the HFBI bilayer is a measurable water permeability induced. To investigate possible molecular mechanisms causing the near-zero permeability, we used all-atom molecular dynamics simulations of various HFBI bilayer models. The simulations suggest that the experimental HFBI bilayer permeability is compatible neither with a lateral honeycomb structure, as found for HFBI monolayers, nor with a residual oil layer within the bilayer or with a disordered lateral packing similar to the packing in lipid bilayers. These results suggest that the low permeabilities of HFBI and lipid bilayers rely on different mechanisms. With their extremely low but adaptable permeability and high stability, HFBI membranes could be used as an osmotic pressure-insensitive barrier in situations where lipid membranes fail such as desalination membranes.

Applying molecular and genetic methods to trees and their fungal communities

Forests provide invaluable economic, ecological, and social services. At the same time, they are exposed to several threats, such as fragmentation, changing climatic conditions, or increasingly destructive pests and pathogens. Trees, the inherent species of forests, cannot be viewed as isolated organisms. Manifold (micro)organisms are associated with trees playing a pivotal role in forest ecosystems. Of these organisms, fungi may have the greatest impact on the life of trees. A multitude of molecular and genetic methods are now available to investigate tree species and their associated organisms. Due to their smaller genome sizes compared to tree species, whole genomes of different fungi are routinely compared. Such studies have only recently started in forest tree species. Here, we summarize the application of molecular and genetic methods in forest conservation genetics, tree breeding, and association genetics as well as for the investigation of fungal communities and their interrelated ecological functions. These techniques provide valuable insights into the molecular basis of adaptive traits, the impacts of forest management, and changing environmental conditions on tree species and fungal communities and can enhance tree-breeding cycles due to reduced time for field testing. It becomes clear that there are multifaceted interactions among microbial species as well as between these organisms and trees. We demonstrate the versatility of the different approaches based on case studies on trees and fungi. KEY POINTS: • Current knowledge of genetic methods applied to forest trees and associated fungi. • Genomic methods are essential in conservation, breeding, management, and research. • Important role of phytobiomes for trees and their ecosystems.

Identification of Key Regulatory Pathways of Basidiocarp Formation in Pleurotus spp. Using Modeling, Simulation and System Biology Studies

Pleurotus (Oyster mushroom) is an important cultivated edible mushroom across the world. It has several therapeutic effects as it contains various useful bio-molecules. The cultivation and crop management of these basidiomycete fungi depends on many extrinsic and intrinsic factors such as substrate composition, growing environment, enzymatic properties, and the genetic makeup, etc. Moreover, for efficient crop production, a comprehensive understanding of the fundamental properties viz. intrinsic–extrinsic factors and genotype-environment interaction analysis is required. The present study explores the basidiocarp formation biology in Pleurotus mushroom using an in silico response to the environmental factors and involvement of the major regulatory genes. The predictive model developed in this study indicates involvement of the key regulatory pathways in the pinhead to fruit body development process. Notably, the major regulatory pathways involved in the conversion of mycelium aggregation to pinhead formation and White Collar protein (PoWC1) binding flavin-chromophore (FAD) to activate respiratory enzymes. Overall, cell differentiation and higher expression of respiratory enzymes are the two important steps for basidiocarp formation. PoWC1 and pofst genes were participate in the structural changes process. Besides this, the PoWC1 gene is also involved in the respiratory requirement, while the OLYA6 gene is the triggering point of fruiting. The findings of the present study could be utilized to understand the detailed mechanism associated with the basidiocarp formation and to cultivate mushrooms at a sustainable level.

The Colletotrichum siamense Hydrophobin CsHydr1 Interacts with the Lipid Droplet-Coating Protein CsCap20 and Regulates Lipid Metabolism and Virulence

Previous studies of the lipid droplet-coating protein Cap20 in Colletotrichum show that it plays a key role in appressorium development and virulence. In this study, the hydrophobin CsHydr1, which contains a signal peptide of 19 amino acids and a hydrophobic domain (HYDRO), was shown to interact with CsCap20 in Colletotrichum siamense. The CsHydr1 deletion mutant showed slightly enhanced mycelial growth, small conidia, slow spore germination and appressoria formation, cell wall integrity and virulence. Like CsCAP20, CsHydr1 is also localized on the lipid droplet surface of C. siamense. However, when CsCap20 was absent, some CsHydr1 was observed in other parts. Quantitative lipid determination showed that the absence of either CsHydr1 or CsCap20 reduced the content of lipids in mycelia and conidia, while the effect of CsCap20 was more obvious; these results suggest that an interaction protein CsHydr1 of CsCap20 is localized on the lipid droplet surface and involved in lipid metabolism, which affects appressorium formation and virulence in C. siamense.

Lessons From Insect Fungiculture: From Microbial Ecology to Plastics Degradation

Anthropogenic activities have extensively transformed the biosphere by extracting and disposing of resources, crossing boundaries of planetary threat while causing a global crisis of waste overload. Despite fundamental differences regarding structure and recalcitrance, lignocellulose and plastic polymers share physical-chemical properties to some extent, that include carbon skeletons with similar chemical bonds, hydrophobic properties, amorphous and crystalline regions. Microbial strategies for metabolizing recalcitrant polymers have been selected and optimized through evolution, thus understanding natural processes for lignocellulose modification could aid the challenge of dealing with the recalcitrant human-made polymers spread worldwide. We propose to look for inspiration in the charismatic fungal-growing insects to understand multipartite degradation of plant polymers. Independently evolved in diverse insect lineages, fungiculture embraces passive or active fungal cultivation for food, protection, and structural purposes. We consider there is much to learn from these symbioses, in special from the community-level degradation of recalcitrant biomass and defensive metabolites. Microbial plant-degrading systems at the core of insect fungicultures could be promising candidates for degrading synthetic plastics. Here, we first compare the degradation of lignocellulose and plastic polymers, with emphasis in the overlapping microbial players and enzymatic activities between these processes. Second, we review the literature on diverse insect fungiculture systems, focusing on features that, while supporting insects' ecology and evolution, could also be applied in biotechnological processes. Third, taking lessons from these microbial communities, we suggest multidisciplinary strategies to identify microbial degraders, degrading enzymes and pathways, as well as microbial interactions and interdependencies. Spanning from multiomics to spectroscopy, microscopy, stable isotopes probing, enrichment microcosmos, and synthetic communities, these strategies would allow for a systemic understanding of the fungiculture ecology, driving to application possibilities. Detailing how the metabolic landscape is entangled to achieve ecological success could inspire sustainable efforts for mitigating the current environmental crisis.

Dynamical shapes of droplets of cyclodextrin-surfactant solutions

We present a series of experiments with droplets of aqueous cyclodextrin-surfactant solutions, in which the volume is reduced after the equilibrium spherical shape is reached. The final shape of the drop after this perturbation is found to be dependent on the concentration of inclusion complexes in the bulk of the solution. These inclusion complexes are formed by two cyclodextrin molecules and one surfactat molecule. We propose a model to describe these dynamical processes. Dipole–dipole interactions on the surface of the drop trigger a competition between water surface tension and dipole–dipole interaction energies. The results of the model reproduce the spherical and rod-like shapes found in the experiments.

The growth of marine fungi on seaweed polysaccharides produces cerato-platanin and hydrophobin self-assembling proteins

The marine fungi Paradendryphiela salina and Talaromyces pinophilus degrade and assimilate complex substrates from plants and seaweed. Additionally, these fungi secrete surface-active proteins, identified as cerato-platanins and hydrophobins. These hydrophobic proteins have the ability to self-assemble forming amyloid-like aggregates and play an essential role in the growth and development of the filamentous fungi. It is the first time that one cerato-platanin (CP) is identified and isolated from P. salina (PsCP) and two Class I hydrophobins (HFBs) from T. pinophilus (TpHYD1 and TpHYD2). Furthermore, it is possible to extract cerato-platanins and hydrophobins using marine fungi that can feed on seaweed biomass, and through a submerged liquid fermentation process. The propensity to aggregate of these proteins has been analyzed using different techniques such as Thioflavin T fluorescence assay, Fourier-transform Infrared Spectroscopy, and Atomic Force Microscopy. Here, we show that the formation of aggregates of PsCP and TpHYD, was influenced by the carbon source from seaweed. This study highlighted the potential of these self-assembling proteins generated from a fermentation process with marine fungi and with promising properties such as conformational plasticity with extensive applications in biotechnology, pharmacy, nanotechnology, and biomedicine.

Cysteine-Rich Hydrophobin Gene Family: Genome Wide Analysis, Phylogeny and Transcript Profiling in Cordyceps militaris

Hydrophobins are a family of small secreted proteins found exclusively in fungi, and they play various roles in the life cycle. In the present study, genome wide analysis and transcript profiling of the hydrophobin family in Cordyceps militaris, a well-known edible and medicinal mushroom, were studied. The distribution of hydrophobins in ascomycetes with different lifestyles showed that pathogenic fungi had significantly more hydrophobins than saprotrophic fungi, and class II members accounted for the majority. Phylogenetic analysis of hydrophobin proteins from the species of Cordyceps s.l. indicated that there was more variability among the class II members than class I. Only a few hydrophobin-encoding genes evolved by duplication in Cordyceps s.l., which was inconsistent with the important role of gene duplication in basidiomycetes. Different transcript patterns of four hydrophobin-encoding genes during the life cycle indicated the possible different functions for each. The transcripts of Cmhyd2, 3 and 4 can respond to light and were related with the photoreceptors. CmQHYD, with four hydrophobin II domains, was first found in C. militaris, and multi-domain hydrophobins were only distributed in the species of Cordycipitaceae and Clavicipitaceae. These results could be helpful for further function research of hydrophobins and could provide valuable information for the evolution of hydrophobins.

In search of a novel chassis material for synthetic cells: emergence of synthetic peptide compartment

Giant lipid vesicles have been used extensively as a synthetic cell model to recapitulate various life-like processes, including in vitro protein synthesis, DNA replication, and cytoskeleton organization. Cell-sized lipid vesicles are mechanically fragile in nature and prone to rupture due to osmotic stress, which limits their usability. Recently, peptide vesicles have been introduced as a synthetic cell model that would potentially overcome the aforementioned limitations. Peptide vesicles are robust, reasonably more stable than lipid vesicles and can withstand harsh conditions including pH, thermal, and osmotic variations. This mini-review summarizes the current state-of-the-art in the design, engineering, and realization of peptide-based chassis materials, including both experimental and computational work. We present an outlook for simulation-aided and data-driven design and experimental realization of engineered and multifunctional synthetic cells.

Versatile biomimetic medical device surface: hydrophobin coated, nitric oxide-releasing polymer for antimicrobial and hemocompatible applications

In medical device design, there is a vital need for a coating that promotes treatment of the patient and simultaneously prevents fouling by biomacromolecules which in turn can progress to infections, thrombosis, and other device-related complications. In this work, hydrophobin SC3 (SC3), a self-assembling amphiphilic protein, was coated on a nitric oxide (NO) releasing medical grade polymer to provide an antifouling layer to work synergistically with NO's bactericidal and antiplatelet activity (SC3-NO). The contact angle of SC3 samples were ∼30% lesser than uncoated control samples and was maintained for a month in physiological conditions, demonstrating a stable, hydrophilic coating. NO release characteristics were not adversely affected by the SC3 coating and samples with SC3 coating maintained NO release. Fibrinogen adsorption was reduced over tenfold on SC3 coated samples when compared to non-SC3 coated samples. The viable cell count of adhered bacteria (Staphylococcus aureus) on SC3-NO was 79.097 ± 7.529% lesser than control samples and 49.533 ± 18.18% lesser than NO samples. Platelet adherence on SC3-NO was reduced by 73.407 ± 14.59% when compared to control samples and 53.202 ± 25.67 when compared to NO samples. Finally, the cytocompatibility of SC3-NO was tested and proved to be safe and not trigger a cytotoxic response. The overall favorable results from the physical, chemical and biological characterization analyses demonstrate the novelty and importance of a naturally-produced antifouling layer coated on a bactericidal and antiplatelet polymer, and thus will prove to be advantageous in a multitude of medical device applications.

Microbial functional amyloids serve diverse purposes for structure, adhesion and defence

The functional amyloid state of proteins has in recent years garnered much attention for its role in serving crucial and diverse biological roles. Amyloid is a protein fold characterised by fibrillar morphology, binding of the amyloid-specific dyes Thioflavin T and Congo Red, insolubility and underlying cross-β structure. Amyloids were initially characterised as an aberrant protein fold associated with mammalian disease. However, in the last two decades, functional amyloids have been described in almost all biological systems, from viruses, to bacteria and archaea, to humans. Understanding the structure and role of these amyloids elucidates novel and potentially ancient mechanisms of protein function throughout nature. Many of these microbial functional amyloids are utilised by pathogens for invasion and maintenance of infection. As such, they offer novel avenues for therapies. This review examines the structure and mechanism of known microbial functional amyloids, with a particular focus on the pathogenicity conferred by the production of these structures and the strategies utilised by microbes to interfere with host amyloid structures. The biological importance of microbial amyloid assemblies is highlighted by their ubiquity and diverse functionality.

Structure Formation in Class I and Class II Hydrophobins at the Air-Water Interface under Multiple Compression/Expansion Cycles

Hydrophobins are small amphiphilic fungal proteins empirically divided into two classes. We investigated the self-assembled structures of class I SC3 from S. commune and class II HFBII from T. reesei transferred to mica from the air-water interface by using the Langmuir-Schaefer (LS) technique and atomic force microscopy (AFM). The main focus is the influence of areal constraint and multiple compressions and expansions on the morphology of the protein films. SC3 shows a rather homogenous coverage of the mica surface, with fibrillary structures. Multiple compressions to a surface pressure of 13 mn m^-1 led to a shortening of the fibrils. HFBII exhibits multilayered structures of varying thickness at higher surface pressures. Multiple compressions led to a variety of large, multilayer aggregates. Several compressions and expansions homogenized the films of both types. Both proteins showed similar dendritic structures with relevant length scales of at least several hundred nanometers at pressures of 13 mn m^-1 and above, although the primary structures they assemble into are usually different in size and type, and range from fibrils to hexagonally ordered films. These dendritic structures may stem from a combination of mechanical influences, such as compressions, expansions, and the drying effect during LS transfer, which may simulate processes during physiological applications of hydrophobins, such as encapsulation or release of spores.

Secondary metabolism in the lichen symbiosis

Lichens, which are defined by a core symbiosis between a mycobiont (fungal partner) and a photobiont (photoautotrophic partner), are in fact complex assemblages of microorganisms that constitute a largely untapped source of bioactive secondary metabolites. Historically, compounds isolated from lichens have predominantly been those produced by the dominant fungal partner, and these continue to be of great interest for their unique chemistry and biotechnological potential. In recent years it has become apparent that many photobionts and lichen-associated bacteria also produce a range of potentially valuable molecules. There is evidence to suggest that the unique nature of the symbiosis has played a substantial role in shaping many aspects of lichen chemistry, for example driving bacteria to produce metabolites that do not bring them direct benefit but are useful to the lichen as a whole. This is most evident in studies of cyanobacterial photobionts, which produce compounds that differ from free living cyanobacteria and are unique to symbiotic organisms. The roles that these and other lichen-derived molecules may play in communication and maintaining the symbiosis are poorly understood at present. Nonetheless, advances in genomics, mass spectrometry and other analytical technologies are continuing to illuminate the wealth of biological and chemical diversity present within the lichen holobiome. Implementation of novel biodiscovery strategies such as metagenomic screening, coupled with synthetic biology approaches to reconstitute, re-engineer and heterologously express lichen-derived biosynthetic gene clusters in a cultivable host, offer a promising means for tapping into this hitherto inaccessible wealth of natural products.

The Gushing Experience—A Quick Overview

Beer lovers all over the world like to get their drink with a certain volume of stabile foam, which mainly depends on the beer style. However, sometimes this foam comes in form of a sudden, eruptive, and uncontrolled over-foaming (gushing) of beer. Gushing occurs after the bottle has been opened, without previously being treated inappropriately (exposure to high temperatures, shaking, or any other kind of agitation). According to recent scientific and professional literature, gushing may be induced by many factors, but fungal proteins are directly connected to this phenomenon. Gushing caused by fungal proteins—hydrophobins—is called primary gushing, and depends solely on raw material quality. Other reasons for extensive foaming after the bottle has been opened can be of chemical or technological nature in the course of the brewing process. This is called secondary gushing, which can be influenced and reduced by applying good manufacturing practice protocols.

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.

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.

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 functional role of Cys3-Cys4 loop in hydrophobin HGFI

Hydrophobins are a large group of low-molecular weight proteins. These proteins are highly surface-active and can form amphipathic membranes by self-assembling at hydrophobic-hydrophilic interfaces. Based on physical properties and hydropathy profiles, hydrophobins are divided into two classes. Upon the analysis of amino acid sequences and higher structures, some models suggest that the Cys3-Cys4 loop regions in class I and II hydrophobins can exhibit remarkable difference in their alignment and conformation, and have a critical role in the rodlets structure formation. To examine the requirement for the Cys3-Cys4 loop in class I hydrophobins, we used protein fusion technology to obtain a mutant protein HGFI-AR by replacing the amino acids between Cys3 and Cys4 of the class I hydrophobin HGFI from Grifola frondosa with those ones between Cys3 and Cys4 of the class II hydrophobin HFBI from Trichoderma reesei. The gene of the mutant protein HGFI-AR was successfully expressed in Pichia pastoris. Water contact angle (WCA) and X-ray photoelectron spectroscopy (XPS) measurements demonstrated that the purified HGFI-AR could form amphipathic membranes by self-assembling at mica and hydrophobic polystyrene surfaces. This property enabled them to alter the surface wettabilities of polystyrene and mica and change the elemental composition of siliconized glass. In comparison to recombinant class I hydrophobin HGFI (rHGFI), the membranes formed on hydrophobic surfaces by HGFI-AR were not robust enough to resist 1 % hot SDS washing. Atomic force microscopy (AFM) measurements indicated that unlike rHGFI, no rodlet structure was observed on the mutant protein HGFI-AR coated mica surface. In addition, when compared to rHGFI, no secondary structural change was detected by Circular Dichroism (CD) spectroscopy after HGFI-AR self-assembled at the water-air interface. HGFI-AR could not either be deemed responsible for the fluorescence intensity increase of Thioflavin T (THT) and the Congo Red (CR) absorption spectra shift (after the THT(CR)/HGFI-AR mixed aqueous solution was drastically vortexed). Remarkably, replacement of the Cys3-Cys4 loop could impair the rodlet formation of the class I hydrophobin HGFI. So, it could be speculated that the Cys3-Cys4 loop plays an important role in conformation and functionality, when the class I hydrophobin HGFI self-assembles at hydrophobic-hydrophilic interfaces.

Spontaneous self-assembly of SC3 hydrophobins into nanorods in aqueous solution

Hydrophobins are small surface active proteins secreted by filamentous fungi. Because of their ability to self-assemble at hydrophilic-hydrophobic interfaces, hydrophobins play a key role in fungal growth and development. In the present work, the organization in aqueous solution of SC3 hydrophobins from the fungus Schizophyllum commune was assessed using Dynamic Light Scattering, Atomic Force Microscopy and fluorescence spectroscopy. These complementary approaches have demonstrated that SC3 hydrophobins are able not only to spontaneously self-assemble at the air-water interface but also in pure water. AFM experiments evidenced that hydrophobins self-assemble in solution into nanorods. Fluorescence assays with thioflavin T allowed establishing that the mechanism governing SC3 hydrophobin self-assembly into nanorods involves β-sheet stacking. SC3 assembly was shown to be strongly influenced by ionic strength and solution pH. The presence of a very low ionic strength significantly favoured the protein self-assembly but a further increase of ions in solution disrupted the protein assembly. It was assessed that solution pH had a significant effect on the SC3 hydrophobins organization. In peculiar, the self-assembly process was considerably reduced at acidic pH. Our findings demonstrate that the self-assembly of SC3 hydrophobins into nanorods of well-defined length can be directly controlled in solution. Such control allows opening the way for the development of new smart self-assembled structures for targeted applications.

Cerato-platanins: a fungal protein family with intriguing properties and application potential

Cerato-platanin proteins are small, secreted proteins with four conserved cysteines that are abundantly produced by filamentous fungi with all types of lifestyles. These proteins appear to be readily recognized by other organisms and are therefore important factors in interactions of fungi with other organisms, e.g. by stimulating the induction of defence responses in plants. However, it is not known yet whether the main function of cerato-platanin proteins is associated with these fungal interactions or rather a role in fungal growth and development. Cerato-platanin proteins seem to unify several biochemical properties that are not found in this combination in other proteins. On one hand, cerato-platanins are carbohydrate-binding proteins and are able to bind to chitin and N-acetylglucosamine oligosaccharides; on the other hand, they are able to self-assemble at hydrophobic/hydrophilic interfaces and form protein layers, e.g. on the surface of aqueous solutions, thereby altering the polarity of solutions and surfaces. The latter property is reminiscent of hydrophobins, which are also small, secreted fungal proteins, but interestingly, the surface-activity-altering properties of cerato-platanins are the opposite of what can be observed for hydrophobins. The so far known biochemical properties of cerato-platanin proteins are summarized in this review, and potential biotechnological applications as well as implications of these properties for the biological functions of cerato-platanin proteins are discussed.

PfaH2: a novel hydrophobin from the ascomycete Paecilomyces farinosus

The pfah2 gene coding for a novel hydrophobin PfaH2 from the ascomycete Paecilomyces farinosus was identified during sequencing of random clones from a cDNA library. The corresponding protein sequence of PfaH2 deduced from the cDNA comprised 134 amino acids (aa). A 16 aa signal sequence preceded the N-terminus of the mature protein. PfaH2 belonged to the class Ia hydrophobins. The protein was isolated using trifluoroacetic acid extraction and purified via SDS-PAGE and high-performance liquid chromatography. The surface activity of the recently described PfaH1 and of PfaH2 was compared by the determination of contact angles (CAs) on glass slides and Teflon tape, and the CA of distilled water droplets was measured on glass slides coated with hydrophobin PfaH1 or PfaH2. Surprisingly, both hydrophobins adsorbed to hydrophilic surfaces and changed their physicochemical properties to a similar quantitative extent, although little aa sequence homology was found.

Kinetic and equilibrium aspects of adsorption and desorption of class II hydrophobins HFBI and HFBII at silicon oxynitride/water and air/water interfaces

Hydrophobins are relatively small globular proteins produced by filamentous fungi. They display unusual high surface activity and are implied as mediators of attachment to surfaces, which has resulted in high scientific and technological interest. In this work we focus on kinetic and equilibrium aspects of adsorption and desorption properties of two representatives of class II hydrophobins, namely HFBI and HFBII, at a negatively charged hydrophilic solid/water interface and at the air/water interface. The layers formed at the air/liquid interface were examined in a Langmuir trough, whereas layers formed at the solid/liquid interface were studied using dual polarization interferometry (DPI) under different flow conditions. For comparison, another globular protein, lysozyme, was also investigated. It was found that both the adsorbed amount and the adsorption kinetics were different for HFBI and HFBII, and the adsorption behavior of both hydrophobins on the negatively charged surface displayed some unusual features. For instance, even though the adsorption rate for HFBI was slowed down with increasing adsorbed amount as expected from packing constraints at the interface, the adsorption kinetics curves for HFBII displayed a region indicating adsorption cooperativity. Further, it was found that hydrophobin layers formed under flow partly desorbed when the flow was stopped, and the desorption rate for HFBII was enhanced in the presence of hydrophobins in solution.

Hydrophobin genes of the entomopathogenic fungus, Metarhizium brunneum, are differentially expressed and corresponding mutants are decreased in virulence

Hydrophobins are small, cysteine-rich, secreted proteins, ubiquitously produced by filamentous fungi that are speculated to function in fungal growth, cell surface properties, and development, although this has been rigorously tested for only a few species. Herein, we report identification of three hydrophobin genes from the entomopathogenic fungus, Metarhizium brunneum, and functional characterization of strains lacking these genes. One gene (HYD1/ssgA) encodes a class I hydrophobin identified previously. Two new genes, HYD3 and HYD2, encode a class I and class II hydrophobin, respectively. To examine function, we deleted all three separately, from the M. brunneum strain KTU-60 genome, using Agrobacterium tumefaciens-mediated transformation. Deletion strains were screened for alterations in developmental phenotypes including growth, sporulation, pigmentation, colony surface properties, and virulence to insects. All deletion strains were reduced in their ability to sporulate and showed alterations in wild-type pigmentation, but all retained wild-type hydrophobicity, except for one individual hyd3 mutant. Complementation with the wild-type HYD3 gene restored hydrophobicity. Each gene, present as a single copy in the genome, showed differential expression patterns dependent on the developmental stage of the fungus. When Spodoptera exigua (beet armyworm) larvae were treated with either conidia or blastospores of each hyd mutant, reductions in virulence and delayed mortality were observed as compared to WT. Together, these results suggest that hydrophobins are differentially expressed and may have distinct, but compensating roles, in conidiation, pigmentation, hydrophobicity, and virulence.

Recombinant production of an Aspergillus nidulans class I hydrophobin (DewA) in Hypocrea jecorina (Trichoderma reesei) is promoter-dependent

Fungal hydrophobins have potential for several applications because of their abilities to change the hydrophobicity of different surfaces. Yet because of their tendency for aggregation and attachment to interfacial areas only few production processes have so far been reported. Towards the development of a heterologous production system, we report here the expression of a class I hydrophobin DewA of Aspergillus nidulans in Hypocrea jecorina (Trichoderma reesei). Using the H. jecorina hfb2 (class II hydrophobin-encoding) promoter and lactose as a carbon source, only a minor fraction of the DewA remained cell-wall-bound and the majority of it secreted into the medium with up to 15% of the total secreted protein. N-terminal amino acid sequencing showed that it was correctly processed. In contrast, no DewA was produced under the cel7A (cellobiohydrolase I) promoter, although its mRNA was abundantly detected in the cells. This lack of secretion is not due to trapping in the cell wall or to its degradation because of the unfolded protein response. Recombinant DewA could be conveniently precipitated from the culture filtrate, and its bioactivity proven by its ability to stably bind to hydrophilic and hydrophobic surfaces (glass and Teflon, respectively). We thus consider H. jecorina as a promising host for further optimization of DewA production.

Prokaryotic expression, purification, and polyclonal antibody production of a hydrophobin from Grifola frondosa

Hydrophobins are small fungal proteins that self-assemble spontaneously at hydrophilic-hydrophobic interfaces and change the polar nature of the surfaces to which they attach. A new hydrophobin gene hgfI was identified recently from the edible mushroom Grifola frondosa. In this paper, the cloning, expression, purification, and polyclonal antibody preparation of the HGFI were described. The hgfI gene was cloned into pET-28a expression plasmid at the EcoRI and NdeI restriction sites and then transformed into Escherichia coli BL21 strain. SDSPAGE analysis showed that recombinant HGFI protein was satisfactorily expressed by optimizing the concentration and induction time of IPTG. The expressed recombinant HGFI protein was purified by electroelution because its inclusion body was insoluble in traditional processing method. After a desalting procedure with Sephadex G-25, the recombinant HGFI protein was used to immunize adult rabbits following standard protocol. ELISA and western blot analysis indicated that the produced antiserum could detect both HGFI protein expressed in the prokaryotic (E. coli) and in the eukaryotic cells (G. frondosa). Furthermore, the antiserum was used to determine the localization of HGFI protein in G. frondosa cells using an immunofluorescence technique. The results demonstrated that HGFI protein was localized in the cell wall, especially at the budding position of hypha. The polyclonal antibody against HGFI will facilitate further production and functional study of HGFI protein.