Engineering hydrophobin DewA to generate surfaces that enhance adhesion of human but not bacterial cells

Cytotoxicity and Inflammatory Effects of Chitin Nanofibrils Isolated from Fungi

Fungal nanochitin can assist the transition from the linear fossil-based economy to a circular biobased economy given its environmental benefits over conventional crustacean-nanochitin. Its real-world implementation requires carefully assessing its toxicity so that unwanted human health and environmental issues are avoided. Accordingly, the cytotoxicity and inflammatory effects of chitin nanofibrils (ChNFs) from white mushroom is assessed. ChNFs are few nanometers in diameter, with a 75.8% N-acetylation degree, a crystallinity of 59.1%, and present a 44:56 chitin/glucan weight ratio. Studies are conducted for aqueous colloidal ChNF dispersions (0-5 mg·mL-1) and free-standing films having physically entangled ChNFs. Aqueous dispersions of chitin nanocrystals (ChNCs) isolated via hydrochloric acid hydrolysis of α-chitin powder are also evaluated for comparison. Cytotoxicity studies conducted in human fibroblasts (MRC-5 cells) and murine brain microglia (BV-2 cells) reveal a comparatively safer behavior over related biobased nanomaterials. However, a strong inflammatory response was observed when BV-2 cells were cultured in the presence of colloidal ChNFs. These novel cytotoxicity and inflammatory studies shed light on the potential of fungal ChNFs for biomedical applications.

Innovative surface bio-functionalization by fungal hydrophobins and their engineered variants

Research on innovative surface functionalization strategies to develop materials with high added value is particularly challenging since this process is a crucial step in a wide range of fields (i.e., biomedical, biosensing, and food packaging). Up to now, the main applied derivatization methods require hazardous and poorly biocompatible reagents, harsh conditions of temperature and pressure, and are time consuming and cost effective. The discovery of biomolecules able to adhere by non-covalent bonds on several surfaces paves the way for their employment as a replacement of chemical processes. A simple, fast, and environment-friendly method of achieving modification of chemically inert surfaces is offered by hydrophobins, small amphiphilic proteins produced by filamentous fungi. Due to their structural characteristics, they form stable protein layers at interfaces, serving as anchoring points that can strongly bind molecules of interest. In addition, genetic engineering techniques allow the production of hydrophobins fused to a wide spectrum of relevant proteins, providing further benefits in term of time and ease of the process. In fact, it is possible to bio-functionalize materials by simply dip-casting, or by direct deposition, rendering them exploitable, for example, in the development of biomedical and biosensing platforms.

Aspergillus Hydrophobins: Physicochemical Properties, Biochemical Properties, and Functions in Solid Polymer Degradation

Hydrophobins are small amphipathic proteins conserved in filamentous fungi. In this review, the properties and functions of Aspergillus hydrophobins are comprehensively discussed on the basis of recent findings. Multiple Aspergillus hydrophobins have been identified and categorized in conventional class I and two non-conventional classes. Some Aspergillus hydrophobins can be purified in a water phase without organic solvents. Class I hydrophobins of Aspergilli self-assemble to form amphipathic membranes. At the air–liquid interface, RolA of Aspergillus oryzae self-assembles via four stages, and its self-assembled films consist of two layers, a rodlet membrane facing air and rod-like structures facing liquid. The self-assembly depends mainly on hydrophobin conformation and solution pH. Cys4–Cys5 and Cys7–Cys8 loops, disulfide bonds, and conserved Cys residues of RodA-like hydrophobins are necessary for self-assembly at the interface and for adsorption to solid surfaces. AfRodA helps Aspergillus fumigatus to evade recognition by the host immune system. RodA-like hydrophobins recruit cutinases to promote the hydrolysis of aliphatic polyesters. This mechanism appears to be conserved in Aspergillus and other filamentous fungi, and may be beneficial for their growth. Aspergilli produce various small secreted proteins (SSPs) including hydrophobins, hydrophobic surface–binding proteins, and effector proteins. Aspergilli may use a wide variety of SSPs to decompose solid polymers.

Class I hydrophobin fusion with cellulose binding domain for its soluble expression and facile purification

Hydrophobins, highly surface-active proteins, have the ability to reverse surface hydrophobicity through self-assembly at the hydrophilic-hydrophobic interfaces. Their unique structure and interfacial activity lead hydrophobins to have potential applications on surface functional modifications. However, class I hydrophobins are prone to self-assemble into highly insoluble amyloid-like rodlets structure. Recombinant hydrophobins could be produced by Escherichia coli but generally as an insoluble inclusion body. To overcome this insoluble expression limitation, cellulose-binding domain (CBD) from Clostridium thermocellum was fused to the N-terminal of class I hydrophobin HGFI to enhance its soluble expression in E. coli. Approximately, 94% of expressed CBD fused HGFI (CBD-HGFI) was found as soluble protein. The fused CBD could also bind specifically onto bacterial cellulose (BC) nanofibrils produced by Komagataeibacter xylinus to facilitate rapid isolation and purification of HGFI from crude extract. Lysostaphin (Lst), known as GlyGly endopeptidase could successfully cleave the flexible linker (GGGGS)₂ between CBD and HGFI to recover HGFI from BC-bound CBD-HGFI. CBD-HGFI purified by immobilized metal-chelated affinity chromatography (IMAC) and Lst cleaved BC-CBD-HGFI still retained interfacial activity of hydrophobin and its effect on accelerating PETase hydrolysis against poly(ethylene terephthalate) (PET) fiber.

Surface display of HFBI and DewA hydrophobins on Saccharomyces cerevisiae modifies tolerance to several adverse conditions and biocatalytic performance

Hydrophobins are relatively small proteins produced naturally by filamentous fungi with interesting biotechnological and biomedical applications given their self-assembly capacity, efficient adherence to natural and artificial surfaces, and to introduce modifications on the hydrophobicity/hydrophilicity of surfaces. In this work we demonstrate the efficient expression on the S. cerevisiae cell surface of class II HFBI of Trichoderma reesei and class I DewA of Aspergillus nidulans, a hydrophobin not previously exposed, using the Yeast Surface Display a-agglutinin (Aga1-Aga2) system. We show that the resulting modifications affect surface properties, and also yeast cells' resistance to several adverse conditions. The fact that viability of the engineered strains increases under heat and osmotic stress is particularly interesting. Besides, improved biocatalytic activity toward the reduction of ketone 1-phenoxypropan-2-one takes place in the reactions carried out at both 30 °C and 40 °C, within a concentration range between 0.65 and 2.5 mg/mL. These results suggest interesting potential applications for hydrophobin-exposing yeasts. KEY POINTS : • Class I hydrophobin DewA can be efficiently exposed on S. cerevisiae cell surfaces. • Yeast exposure of HFBI and DewA increases osmotic and heat resistance. • Engineered strains show modified biocatalytic behavior.

Cell-free expression of natively folded hydrophobins

Hydrophobins are a family of cysteine-rich proteins unique to filamentous fungi. The proteins are produced in a soluble form but self-assemble into organised amphipathic layers at hydrophilic:hydrophobic interfaces. These layers contribute to transitions between wet and dry environments, spore dispersal and attachment to surfaces for growth and infection. Hydrophobins are characterised by four disulphide bonds that are critical to their structure and function. Thus, obtaining correctly folded, soluble and functional hydrophobins directly from bacterial recombinant expression is challenging and in most cases, initial denaturation from inclusion bodies followed by oxidative refolding are required to obtain folded proteins. Here, we report the use of cell-free expression with E. coli cell lysate to directly obtain natively folded hydrophobins. All six of the hydrophobins tested could be expressed after optimisation of redox conditions. For some hydrophobins, the inclusion of the disulfide isomerase DsbC further enhanced expression levels. We are able to achieve a yield of up to 1 mg of natively folded hydrophobin per mL of reaction. This has allowed the confirmation of the correct folding of hydrophobins with the use of ¹⁵N-cysteine and ¹⁵N-¹H nuclear magnetic resonance experiments within 24 h of starting from plasmid stocks.

Surface Functionalization by Hydrophobin-EPSPS Fusion Protein Allows for the Fast and Simple Detection of Glyphosate

Glyphosate, the most widely used pesticide worldwide, is under debate due to its potentially cancerogenic effects and harmful influence on biodiversity and environment. Therefore, the detection of glyphosate in water, food or environmental probes is of high interest. Currently detection of glyphosate usually requires specialized, costly instruments, is labor intensive and time consuming. Here we present a fast and simple method to detect glyphosate in the nanomolar range based on the surface immobilization of glyphosate's target enzyme 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) via fusion to the hydrophobin Ccg2 and determination of enzyme activity with a malachite green assay, which is a common photometric technique to measure inorganic phosphate (Pi). The assay demonstrates a new approach for a fast and simple detection of pesticides.

Fungal hydrophobins render stones impermeable for water but keep them permeable for vapor

The conservation of architectural heritage is a big challenge in times with increasing air pollution with aggressive gases. A second major threat to buildings is the combination of water and air contaminants which may be used by microorganisms for their metabolism. Hence, myriads of different bacteria and fungi populate stone surfaces and penetrate into the fine pores and cracks. Whereas epoxid-based paintings (or other paintings) may protect the coated surfaces from water and aggressive gases, these chemicals seal the stone surface and prevent also the evaporation of vapor from the inside of the buildings. Here, we tested a natural, fungal protein-based coating method. Fungi use small, amphiphilic proteins to turn their surfaces hydrophobic. We found that Aspergillus nidulans hydrophobin DewA and Trichoderma reesei HFBI confer hydrophobicity to stones but keep their pores open. The effect resembles “Gore-tex” fabric material.

Hydrophobins: multifunctional biosurfactants for interface engineering

Hydrophobins are highly surface-active proteins that have versatile potential as agents for interface engineering. Due to the large and growing number of unique hydrophobin sequences identified, there is growing potential to engineer variants for particular applications using protein engineering and other approaches. Recent applications and advancements in hydrophobin technologies and production strategies are reviewed. The application space of hydrophobins is large and growing, including hydrophobic drug solubilization and delivery, protein purification tags, tools for protein and cell immobilization, antimicrobial coatings, biosensors, biomineralization templates and emulsifying agents. While there is significant promise for their use in a wide range of applications, developing new production strategies is a key need to improve on low recombinant yields to enable their use in broader applications; further optimization of expression systems and yields remains a challenge in order to use designed hydrophobin in commercial applications.

Comparative analysis of surface coating properties of five hydrophobins from Aspergillus nidulans and Trichoderma reseei

Fungal hydrophobins are small amphiphilic proteins that self-assemble into monolayers on hydrophobic:hydrophilic interfaces and can be used for surface coatings. Because e.g. Aspergillus nidulans contains six different hydrophobins, it is likely that they have different properties and are used for different “applications” in the fungus. We established a method for recombinant production of different class hydrophobins in Escherichia coli. We produced DewA, DewC, DewD, DewE from A. nidulans and HFBI from Trichoderma reesei and compared surface coating properties of these hydrophobins. All tested proteins formed coatings on glass, strongly increasing the hydrophobicity of the surface, and showed emulsion-stabilizing properties. But whereas the typical class I hydrophobin DewA formed the most stable coating on glass, the intermediate class hydrophobins DewE and DewD were more effective in stabilization of oil:water emulsions. This work gives insights into correlations between structural characteristics of hydrophobins and their behaviour as surface binding agents. It could help with the clarification of their biological functions and lead to novel biotechnological applications.

Probing Structural Changes during Self-assembly of Surface-Active Hydrophobin Proteins that Form Functional Amyloids in Fungi

Hydrophobins are amphiphilic proteins secreted by filamentous fungi in a soluble form, which can self-assemble at hydrophilic/hydrophobic or water/air interfaces to form amphiphilic layers that have multiple biological roles. We have investigated the conformational changes that occur upon self-assembly of six hydrophobins that form functional amyloid fibrils with a rodlet morphology. These hydrophobins are present in the cell wall of spores from different fungal species. From available structures and NMR chemical shifts, we established the secondary structures of the monomeric forms of these proteins and monitored their conformational changes upon amyloid rodlet formation or thermal transitions using synchrotron radiation circular dichroism and Fourier-transform infrared spectroscopy (FT-IR). Thermal transitions were followed by synchrotron radiation circular dichroism in quartz cells that allowed for microbubbles and hence water/air interfaces to form and showed irreversible conformations that differed from the rodlet state for most of the proteins. In contrast, thermal transitions on hermetic calcium fluoride cells showed reversible conformational changes. Heating hydrophobin solutions with a water/air interface on a silicon crystal surface in FT-IR experiments resulted in a gain in β-sheet content typical of amyloid fibrils for all except one protein. Rodlet formation was further confirmed by electron microscopy. FT-IR spectra of pre-formed hydrophobin rodlet preparations also showed a gain in β-sheet characteristic of the amyloid cross-β structure. Our results indicate that hydrophobins are capable of significant conformational plasticity and the nature of the assemblies formed by these surface-active proteins is highly dependent on the interface at which self-assembly takes place.

Applications of Functional Amyloids from Fungi: Surface Modification by Class I Hydrophobins

Class I hydrophobins produced from fungi are amongst the first proteins recognized as functional amyloids. They are amphiphilic proteins involved in the formation of aerial structures such as spores or fruiting bodies. They form chemically robust layers which can only be dissolved in strong acids. These layers adhere to different surfaces, changing their wettability, and allow the binding of other proteins. Herein, the modification of diverse types of surfaces with Class I hydrophobins is reported, highlighting the applications of the coated surfaces. Indeed, these coatings can be exploited in several fields, spanning from biomedical to industrial applications, which include biosensing and textile manufacturing.

Immobilization of LccC Laccase from Aspergillus nidulans on Hard Surfaces via Fungal Hydrophobins

Fungal hydrophobins are small amphiphilic proteins that can be used for coatings on hydrophilic and hydrophobic surfaces. Through the formation of monolayers, they change the hydrophobicity of a given surface. Especially, the class I hydrophobins are interesting for biotechnology, because their layers are stable at high temperatures and can only be removed with strong solvents. These proteins self-assemble into monolayers under physiological conditions and undergo conformational changes that stabilize the layer structure. Several studies have demonstrated how the fusion of hydrophobins with short peptides allows the specific modification of the properties of a given surface or have increased the protein production levels through controlled localization of hydrophobin molecules inside the cell. Here, we fused the Aspergillus nidulans laccase LccC to the class I hydrophobins DewA and DewB and used the fusion proteins to functionalize surfaces with immobilized enzymes. In contrast to previous studies with enzymes fused to class II hydrophobins, the DewA-LccC fusion protein is secreted into the culture medium. The crude culture supernatant was directly used for coatings of glass and polystyrene without additional purification steps. The highest laccase surface activity was achieved after protein immobilization on modified hydrophilic polystyrene at pH 7. This study presents an easy-to-use alternative to classical enzyme immobilization techniques and can be applied not only for laccases but also for other biotechnologically relevant enzymes.

IMPORTANCE

Although fusion with small peptides to modify hydrophobin properties has already been performed in several studies, fusion with an enzyme presents a more challenging task. Both protein partners need to remain in active form so that the hydrophobins can interact with one another and form layers, and so the enzyme (e.g., laccase) will remain active at the same time. Also, because of the amphiphilic nature of hydrophobins, their production and purification remain challenging so far and often include steps that would irreversibly disrupt most enzymes. In our study, we present the first functional fusion proteins of class I hydrophobins from A. nidulans with a laccase. The resulting fusion enzyme is directly secreted into the culture medium by the fungus and can be used for the functionalization of hard surfaces.

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.

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.

Applications of hydrophobins: current state and perspectives

Hydrophobins are proteins exclusively produced by filamentous fungi. They self-assemble at hydrophilic-hydrophobic interfaces into an amphipathic film. This protein film renders hydrophobic surfaces of gas bubbles, liquids, or solid materials wettable, while hydrophilic surfaces can be turned hydrophobic. These properties, among others, make hydrophobins of interest for medical and technical applications. For instance, hydrophobins can be used to disperse hydrophobic materials; to stabilize foam in food products; and to immobilize enzymes, peptides, antibodies, cells, and anorganic molecules on surfaces. At the same time, they may be used to prevent binding of molecules. Furthermore, hydrophobins have therapeutic value as immunomodulators and can been used to produce recombinant proteins.

Six hydrophobins are involved in hydrophobin rodlet formation in Aspergillus nidulans and contribute to hydrophobicity of the spore surface

Hydrophobins are amphiphilic proteins able to self-assemble at water-air interphases and are only found in filamentous fungi. In Aspergillus nidulans two hydrophobins, RodA and DewA, have been characterized, which both localize on the conidiospore surface and contribute to its hydrophobicity. RodA is the constituent protein of very regularly arranged rodlets, 10 nm in diameter. Here we analyzed four more hydrophobins, DewB-E, in A. nidulans and found that all six hydrophobins contribute to the hydrophobic surface of the conidiospores but only deletion of rodA caused loss of the rodlet structure. Analysis of the rodlets in the dewB-E deletion strains with atomic force microscopy revealed that the rodlets appeared less robust. Expression of DewA and DewB driven from the rodA promoter and secreted with the RodA secretion signal in a strain lacking RodA, restored partly the hydrophobicity. DewA and B were able to form rodlets to some extent but never reached the rodlet structure of RodA. The rodlet-lacking rodA-deletion strain opens the possibility to systematically study rodlet formation of other natural or synthetic hydrophobins.

Crossover fungal pathogens: the biology and pathogenesis of fungi capable of crossing kingdoms to infect plants and humans

The outbreak of fungal meningitis associated with contaminated methylprednisolone acetate has thrust the importance of fungal infections into the public consciousness. The predominant pathogen isolated from clinical specimens, Exserohilum rostratum (teleomorph: Setosphaeria rostrata), is a dematiaceous fungus that infects grasses and rarely humans. This outbreak highlights the potential for fungal pathogens to infect both plants and humans. Most crossover or trans-kingdom pathogens are soil saprophytes and include fungi in Ascomycota and Mucormycotina phyla. To establish infection, crossover fungi must overcome disparate, host-specific barriers, including protective surfaces (e.g. cuticle, skin), elevated temperature, and immune defenses. This review illuminates the underlying mechanisms used by crossover fungi to cause infection in plants and mammals, and highlights critical events that lead to human infection by these pathogens. Several genes including veA, laeA, and hapX are important in regulating biological processes in fungi important for both invasive plant and animal infections.

Oxygen plasma functionalization of parylene C coating for implants surface: nanotopography and active sites for drug anchoring

The effect of oxygen plasma treatment (t=0.1-60 min, pO2=0.2 mbar, P=50 W) of parylene C implant surface coating was investigated in order to check its influence on morphology (SEM, AFM observations), chemical composition (XPS analysis), hydrophilicity (contact angle measurements) and biocompatibility (MG-63 cell line and Staphylococcus aureus 24167 DSM adhesion screening). The modification procedure leads to oxygen insertion (up to 20 at.%) into the polymer matrix and together with surface topography changes has a dramatic impact on wettability (change of contact angle from θ=78±2 to θ=33±1.9 for unmodified and 60 min treated sample, respectively). As a result, the hydrophilic surface of modified parylene C promotes MG-63 cells growth and at the same time does not influence S. aureus adhesion. The obtained results clearly show that the plasma treatment of parylene C surface provides suitable polar groups (C=O, C-O, O-C=O, C-O-O and O-C(O)-O) for further development of the coating functionality.

Analysis of the structure and conformational states of DewA gives insight into the assembly of the fungal hydrophobins

The hydrophobin DewA from the fungus Aspergillus nidulans is a highly surface-active protein that spontaneously self-assembles into amphipathic monolayers at hydrophobic:hydrophilic interfaces. These monolayers are composed of fibrils that are a form of functional amyloid. While there has been significant interest in the use of DewA for a variety of surface coatings and as an emulsifier in biotechnological applications, little is understood about the structure of the protein or the mechanism of self-assembly. We have solved the solution NMR structure of DewA. While the pattern of four disulfide bonds that is a defining feature of hydrophobins is conserved, the arrangement and composition of secondary-structure elements in DewA are quite different to what has been observed in other hydrophobin structures. In addition, we demonstrate that DewA populates two conformations in solution, both of which are assembly competent. One conformer forms a dimer at high concentrations, but this dimer is off-pathway to fibril formation and may represent an assembly control mechanism. These data highlight the structural differences between fibril-forming hydrophobins and those that form amorphous monolayers. This work will open up new opportunities for the engineering of hydrophobins with novel biotechnological applications.