Atomic resolution structure of the HFBII hydrophobin, a self-assembling amphiphile

Simple Method to Assess Foam Structure and Stability using Hydrophobin and BSA as Model Systems

The properties and arrangement of surface-active molecules at air-water interfaces influence foam stability and bubble shape. Such multiscale-relationships necessitate a well-conducted analysis of mesoscopic foam properties. We introduce a novel automated and precise method to characterize bubble growth, size distribution and shape based on image analysis and using the machine learning algorithm Cellpose. Studying the temporal evolution of bubble size and shape facilitates conclusions on foam stability. The addition of two sets of masks, for tiny bubbles and large bubbles, provides for a high precision of analysis. A python script for analysis of the evolution of bubble diameter, circularity and dispersity is provided in the Supporting Information. Using foams stabilized by bovine serum albumin (BSA), hydrophobin (HP), and blends thereof, we show how this technique can be used to precisely characterize foam structures. Foams stabilized by HP show a significantly increased foam stability and rounder bubble shape than BSA-stabilized foams. These differences are induced by the different molecular structure of the two proteins. Our study shows that the proposed method provides an efficient way to analyze relevant foam properties in detail and at low cost, with higher precision than conventional methods of image analysis.

A new hydrophobin candidate from Cladosporium macrocarpum with super-hydrophobic surface

Hydrophobins are amphipathic proteins with small molecular weights produced in filamentous fungi. These proteins are highly stable due to the disulfide bonds formed between the protected cysteine residues. They have great potential for usage in many different fields such as surface modifications, tissue engineering, and drug transport systems because hydrophobins are surfactants and soluble in harsh mediums. In this study, it was aimed to determine the hydrophobin proteins responsible for the hydrophobicity of the super-hydrophobic fungi isolates in the culture medium and to carry out the molecular characterization of the hydrophobin producer species. As a result of measuring surface hydrophobicity by determining the water contact angle, five different fungi with the highest hydrophobicity were classified as Cladosporium by classical and molecular (ITS and D1-D2 regions) methods. Also, protein extraction according to the recommended method for obtaining hydrophobins from spores of these Cladosporium species indicated that the isolates have similar protein profiles. Ultimately, the isolate named A5 with the highest water contact angle was identified as Cladosporium macrocarpum, and the 7 kDa band was appointed as a hydrophobin since it was the most abundant protein in protein extraction for this species.

Protein-Based Biological Materials: Molecular Design and Artificial Production

Polymeric materials produced from fossil fuels have been intimately linked to the development of industrial activities in the 20th century and, consequently, to the transformation of our way of living. While this has brought many benefits, the fabrication and disposal of these materials is bringing enormous sustainable challenges. Thus, materials that are produced in a more sustainable fashion and whose degradation products are harmless to the environment are urgently needed. Natural biopolymers─which can compete with and sometimes surpass the performance of synthetic polymers─provide a great source of inspiration. They are made of natural chemicals, under benign environmental conditions, and their degradation products are harmless. Before these materials can be synthetically replicated, it is essential to elucidate their chemical design and biofabrication. For protein-based materials, this means obtaining the complete sequences of the proteinaceous building blocks, a task that historically took decades of research. Thus, we start this review with a historical perspective on early efforts to obtain the primary sequences of load-bearing proteins, followed by the latest developments in sequencing and proteomic technologies that have greatly accelerated sequencing of extracellular proteins. Next, four main classes of protein materials are presented, namely fibrous materials, bioelastomers exhibiting high reversible deformability, hard bulk materials, and biological adhesives. In each class, we focus on the design at the primary and secondary structure levels and discuss their interplays with the mechanical response. We finally discuss earlier and the latest research to artificially produce protein-based materials using biotechnology and synthetic biology, including current developments by start-up companies to scale-up the production of proteinaceous materials in an economically viable manner.

Plant Defense Elicitation by the Hydrophobin Cerato-Ulmin and Correlation with Its Structural Features

Cerato-ulmin (CU) is a 75-amino-acid-long protein that belongs to the hydrophobin family. It self-assembles at hydrophobic-hydrophilic interfaces, forming films that reverse the wettability properties of the bound surface: a capability that may confer selective advantages to the fungus in colonizing and infecting elm trees. Here, we show for the first time that CU can elicit a defense reaction (induction of phytoalexin synthesis and ROS production) in non-host plants (Arabidopsis) and exerts its eliciting capacity more efficiently when in its soluble monomeric form. We identified two hydrophobic clusters on the protein's loops endowed with dynamical and physical properties compatible with the possibility of reversibly interconverting between a disordered conformation and a β-strand-rich conformation when interacting with hydrophilic or hydrophobic surfaces. We propose that the plasticity of those loops may be part of the molecular mechanism that governs the protein defense elicitation capability.

Biodegradation of highly crystallized poly(ethylene terephthalate) through cell surface codisplay of bacterial PETase and hydrophobin

The process of recycling poly(ethylene terephthalate) (PET) remains a major challenge due to the enzymatic degradation of high-crystallinity PET (hcPET). Recently, a bacterial PET-degrading enzyme, PETase, was found to have the ability to degrade the hcPET, but with low enzymatic activity. Here we present an engineered whole-cell biocatalyst to simulate both the adsorption and degradation steps in the enzymatic degradation process of PETase to achieve the efficient degradation of hcPET. Our data shows that the adhesive unit hydrophobin and degradation unit PETase are functionally displayed on the surface of yeast cells. The turnover rate of the whole-cell biocatalyst toward hcPET (crystallinity of 45%) dramatically increases approximately 328.8-fold compared with that of purified PETase at 30 °C. In addition, molecular dynamics simulations explain how the enhanced adhesion can promote the enzymatic degradation of PET. This study demonstrates engineering the whole-cell catalyst is an efficient strategy for biodegradation of PET.

Orientation and Conformation of Hydrophobin at the Oil-Water Interface: Insights from Molecular Dynamics Simulations

Hydrophobins, a new class of potential protein emulsifiers, have been extensively employed in the food, pharmaceutical, and chemical industries. However, the knowledge of the underlying molecular mechanism of protein adsorption at the oil-water interface remains elusive. In this study, all-atom molecular dynamics simulations were performed to probe the adsorption orientation and conformation change of class II hydrophobin HFBI at the cyclohexane-water interface. It was proposed that a hydrophobic dipole of the protein could be used to quantitatively predict the orientation of the adsorbed HFBI. Simulation results revealed that HFBI adsorbed at the interface with the patch-up orientation toward the oil phase, regardless of its initial orientations. HFBI's secondary structure was maintained to be intact in the course of simulations despite relatively significant variations in the tertiary structure observed, which could well preserve the bioactivity of HFBI. From the energy analysis, the driving force for interface adsorption was primarily determined by van der Waals interactions between HFBI and cyclohexane. Further analysis indicated that the adsorption orientation and conformation of HFBI at the oil-water interface were typically regulated by the hydrophobic patch and some key residues. This study provides some insights into the orientation, conformation, and adsorption mechanism of proteins at the oil-water interface and theoretical guidelines for the design and development of novel biological emulsifiers involved in the food, pharmaceutical, and chemical industries.

Catalytically inactive lytic polysaccharide monooxygenase PcAA14A enhances the enzyme-mediated hydrolysis of polyethylene terephthalate

The massive accumulation of polyethylene terephthalate (PET) in the global ecosystem is a growing environmental crisis. Development of environmental friendly strategies to achieve enzyme-catalyzed PET degradation has attracted tremendous attention. In this study, we demonstrated the synergistic effects of combining a specific PET-degrading enzyme IsPETase^EHA variant from PET-assimilating bacterium Ideonella sakaiensis and a lytic polysaccharide monooxygenase from a white-rot fungus Pycnoporus coccineus (PcAA14A) in PET degradation. We found that the presence of PcAA14A alone did not result in PET hydrolysis, but its presence could stimulate IsPETase^EHA-mediated hydrolytic efficiency by up to 1.3-fold. Notably, the stimulatory effects of PcAA14A on IsPETase^EHA-catalyzed PET hydrolysis were found to be independent of monooxygenase activity. Dose-effects of IsPETase^EHA and PcAA14A on PET hydrolysis were observed, with the optimal concentrations being determined to 25 μg/mL and 0.25 μg/mL, respectively. In the 5-day PET hydrolysis experiment, 1097 μM hydrolysis products were produced by adding the optimized concentrations of IsPETase^EHA and PcAA14A, which was 27.7% higher than those were produced by IsPETase^EHA alone. Our study reports the first time that PcAA14A could stimulate the IsPETase^EHA-mediated PET hydrolysis through a monooxygenase activity independent manner.

Improved extraction and purification of the hydrophobin HFBI

Hydrophobins (HFBs) are a group of highly functional, low molecular weight proteins with the ability to self-assemble at hydrophobic-hydrophilic interfaces. The surface active, cysteine-rich proteins are found in filamentous fungi such as Trichoderma reesei. In the present study multiple extraction solvents and conditions were screened for the mycelium bound hydrophobin HFBI and the effects on the total amount of extracted proteins, HFBI recovery and HFBI gushing activity were investigated to gain a more thorough scientific insight on the extraction efficiency and selectivity. Results indicated the enhanced selectivity for HFBI extraction from the fungal biomass using 60% ethanol compared to solutions containing 1% sodium dodecyl sulphate (SDS). Complementing the higher selectivity, HFBI recovery was increased from 6.9 ± 0.6 mg HFBI (1% SDS) to 9.4 ± 0.4 mg HFBI per gram dry fungal biomass for extracts containing 60% ethanol. Furthermore, subsequent to HPLC purification, Cold Induced Phase Separation (CIPS) of acetonitrile-water systems was investigated at different pH levels. CIPS at pH 2.0 was found to effectively remove the majority of sorbicillinoid pigments from the purified HFBI fraction. The improved method resulted in a recovery of 85.4% of the extracted HFBI after final purification.

Genetically Encodable Scaffolds for Optimizing Enzyme Function

Enzyme engineering is an indispensable tool in the field of synthetic biology, where enzymes are challenged to carry out novel or improved functions. Achieving these goals sometimes goes beyond modifying the primary sequence of the enzyme itself. The use of protein or nucleic acid scaffolds to enhance enzyme properties has been reported for applications such as microbial production of chemicals, biosensor development and bioremediation. Key advantages of using these assemblies include optimizing reaction conditions, improving metabolic flux and increasing enzyme stability. This review summarizes recent trends in utilizing genetically encodable scaffolds, developed in line with synthetic biology methodologies, to complement the purposeful deployment of enzymes. Current molecular tools for constructing these synthetic enzyme-scaffold systems are also highlighted.

At least three families of hyphosphere small secreted cysteine-rich proteins can optimize surface properties to a moderately hydrophilic state suitable for fungal attachment

The secretomes of filamentous fungi contain a diversity of small secreted cysteine‐rich proteins (SSCPs) that have a variety of properties ranging from toxicity to surface activity. Some SSCPs are recognized by other organisms as indicators of fungal presence, but their function in fungi is not fully understood. We detected a new family of fungal surface‐active SSCPs (saSSCPs), here named hyphosphere proteins (HFSs). An evolutionary analysis of the HFSs in Pezizomycotina revealed a unique pattern of eight single cysteine residues (C‐CXXXC‐C‐C‐C‐C‐C) and a long evolutionary history of multiple gene duplications and ancient interfungal lateral gene transfers, suggesting their functional significance for fungi with different lifestyles. Interestingly, recombinantly produced saSSCPs from three families (HFSs, hydrophobins and cerato‐platanins) showed convergent surface‐modulating activity on glass and on poly(ethylene‐terephthalate), transforming their surfaces to a moderately hydrophilic state, which significantly favoured subsequent hyphal attachment. The addition of purified saSSCPs to the tomato rhizosphere had mixed effects on hyphal attachment to roots, while all tested saSSCPs had an adverse effect on plant growth in vitro. We propose that the exceptionally high diversity of saSSCPs in Trichoderma and other fungi evolved to efficiently condition various surfaces in the hyphosphere to a fungal‐beneficial state.

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.

Formation of Amphipathic Amyloid Monolayers from Fungal Hydrophobin Proteins

The fungal hydrophobins are small proteins that are able to self-assemble spontaneously into amphipathic monolayers at hydrophobic:hydrophilic interfaces. These protein monolayers can reverse the wettability of a surface, making them suitable for increasing the biocompatibility of many hydrophobic nanomaterials. One subgroup of this family, the class I hydrophobins, forms monolayers that are composed of extremely robust amyloid-like fibrils, called rodlets. Here, we describe the protocols for the production and purification of recombinant hydrophobins and oxidative refolding to a biologically active, soluble, monomeric form. We describe methods to trigger the self-assembly into the fibrillar rodlet state and techniques to characterize the physicochemical properties of the polymeric forms.

Cell surface display of proteins on filamentous fungi

Protein display approaches have been useful to endow the cell surface of yeasts with new catalytic activities so that they can act as enhanced whole-cell biocatalysts. Despite their biotechnological potential, protein display technologies remain poorly developed for filamentous fungi. The lignocellulolytic character of some of them coupled to the cell surface biosynthesis of valuable molecules by a single or a cascade of several displayed enzymes is an appealing prospect. Cell surface protein display consists in the co-translational fusion of a functional protein (passenger) to an anchor one, usually a cell-wall-resident protein. The abundance, spacing, and local environment of the displayed enzymes-determined by the relationship of the anchor protein with the structure and dynamics of the engineered cell wall-are factors that influence the performance of display-based biocatalysts. The development of protein display strategies in filamentous fungi could be based on the field advances in yeasts; however, the unique composition, structure, and biology of filamentous fungi cell walls require the customization of the approach to those microorganisms. In this prospective review, the cellular bases, the design principles, and the available tools to foster the development of cell surface protein display technologies in filamentous fungi are discussed.

Dynamic Assembly of Class II Hydrophobins from T. reesei at the Air-Water Interface

Class II hydrophobins are amphiphilic proteins produced by filamentous fungi. One of their typical features is the tendency to accumulate at the interface between an aqueous phase and a hydrophobic phase, such as the air-water interface. The kinetics of the interfacial self-assembly of wild-type hydrophobins HFBI and HFBII and some of their engineered variants at the air-water interface were measured by monitoring the accumulated mass at the interface via nondestructive ellipsometry measurements. The resulting mass vs time curves revealed unusual kinetics for a monolayer formation that did not follow a typical Langmuir-type of behavior but had a rather coverage-independent rate instead. Typically, the full surface coverage was obtained at masses corresponding to a monolayer. The formation of multilayers was not observed. Atomic force microscopy revealed formation and growth of non-fusing protein clusters at the interface. The mechanism of the adsorption was studied by varying the structure or charges of the protein or the ionic strength of the subphase, revealing that the lateral interactions between the hydrophobins play a role in their interfacial assembly. Additionally, a theoretical model was introduced to identify the underlying mechanism of the unconventional adsorption kinetics.

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.

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.

Adhesion Properties of Freestanding Hydrophobin Bilayers

Hydrophobins are a family of small-sized proteins featuring a distinct hydrophobic patch on the protein's surface, rendering them amphiphilic. This particularity allows hydrophobins to self-assemble into monolayers at any hydrophilic/hydrophobic interface. Moreover, stable pure protein bilayers can be created from two interfacial hydrophobin monolayers by contacting either their hydrophobic or their hydrophilic sides. In this study, this is achieved via a microfluidic approach, in which also the bilayers' adhesion energy can be determined. This enables us to study the origin of the adhesion of hydrophobic and hydrophilic core bilayers made from the class II hydrophobins HFBI and HFBII. Using different fluid media in this setup and introducing genetically modified variants of the HFBI molecule, the different force contributions to the adhesion of the bilayer sheets are studied. It was found that in the hydrophilic contact situation, the adhesive interaction was higher than that in the hydrophobic contact situation and could be even enhanced by reducing the contributions of electrostatic interactions. This effect indicates that the van der Waals interaction is the dominant contribution that explains the stability of the observed bilayers.

Single-Molecule Force Spectroscopy Reveals Self-Assembly Enhanced Surface Binding of Hydrophobins

Hydrophobins have raised lots of interest as powerful surface adhesives. However, it remains largely unexplored how their strong and versatile surface adhesion is linked to their unique amphiphilic structural features. Here, we develop an AFM-based single-molecule force spectroscopy assay to quantitatively measure the binding strength of hydrophobin to various types of surfaces both in isolation and in preformed protein films. We find that individual class II hydrophobins (HFBI) bind strongly to hydrophobic surfaces but weakly to hydrophilic ones. After self-assembly into protein films, they show much stronger binding strength to both surfaces due to the cooperativity of different interactions at nanoscale. Such self-assembly enhanced surface binding may serve as a general design principle for synthetic bioactive adhesives.

Effective Bioactivity Retention of Low-Concentration Antibodies on HFBI-Modified Fluorescence ICTS for Sensitive and Rapid Detection of PSA

Nowadays, increasing analytical sensitivity is still a big challenge in constructing membrane-based fluorescence immunochromatography test strips (FICTS). However, the bioactivity of antibody (Ab) immobilized on the test line (T line) of porous nitrocellulose membrane (PNM), which directly influences the analytical sensitivity, is less studied. In this work, a novel amphiphilic hydrophobin (HFBI) protein was introduced to modify the T line to effectively retain the Abs' bioactivity. The results indicated that HFBI could self-assemble on the PNM and immobilize the Abs in the "stand-up" orientation. Compared with the conventional FICTS, the HFBI-modified FICTS with only 0.2 mg/mL of monoclonal Abs on T line enable more accurate quantitative detection and better sensitivity (0.06 ng/mL for prostate specific antigen), which is more than 2 orders of magnitude lower than that of the conventional FICTS with the same concentration of monoclonal Abs on T line. Furthermore, the accuracy of this HFBI-modified FICTS was investigated by testing 150 clinical serum samples and the detection results were coincident with those by electrochemiluminescence immunoassay. Our results provide a novel and promising strategy of Ab immobilization on FICTS for near-patient and point-of-care application.

Dual-functional protein for one-step production of a soluble and targeted fluorescent dye

Low water solubility and poor selectivity are two fundamental limitations that compromise applications of near-infrared (NIR) fluorescent probes. Methods: Here, a simple strategy that can resolve these problems simultaneously was developed by using a novel hybrid protein named RGD-HFBI that is produced by fusion of hydrophobin HFBI and arginine-glycine-aspartic acid (RGD) peptide. This unique hybrid protein inherits self-assembly and targeting functions from HFBI and RGD peptide respectively. Results: Boron-dipyrromethene (BODIPY) used as a model NIR dye can be efficiently dispersed in the RGD-HFBI solution by simple mixing and sonication for 30 min. The data shows that self-assembled RGD-HFBI forms a protein nanocage by using the BODIPY as the assembly template. Cell uptake assay proves that RGD-HFBI/BODIPY can efficiently stain αvβ3 integrin-positive cancer cells. Finally, in vivo affinity tests fully demonstrate that the soluble RGD-HFBI/BODIPY complex selectively targets and labels tumor sites of tumor-bearing mice due to the high selectivity of the RGD peptide. Conclusion: Our one-step strategy using dual-functional RGD-HFBI opens a novel route to generate soluble and targeted NIR fluorescent dyes in a very simple and efficient way and may be developed as a general strategy to broaden their applications.

Pichia pastoris is a Suitable Host for the Heterologous Expression of Predicted Class I and Class II Hydrophobins for Discovery, Study, and Application in Biotechnology

The heterologous expression of proteins is often a crucial first step in not only investigating their function, but also in their industrial application. The functional assembly and aggregation of hydrophobins offers intriguing biotechnological applications from surface modification to drug delivery, yet make developing systems for their heterologous expression challenging. In this article, we describe the development of Pichia pastoris KM71H strains capable of solubly producing the full set of predicted Cordyceps militaris hydrophobins CMil1 (Class IA), CMil2 (Class II), and CMil3 (IM) at mg/L yields with the use of 6His-tags not only for purification but for their detection. This result further demonstrates the feasibility of using P. pastoris as a host organism for the production of hydrophobins from all Ascomycota Class I subdivisions (a classification our previous work defined) as well as Class II. We highlight the specific challenges related to the production of hydrophobins, notably the challenge in detecting the protein that will be described, in particular during the screening of transformants. Together with the literature, our results continue to show that P. pastoris is a suitable host for the soluble heterologous expression of hydrophobins with a wide range of properties.

Role of Hydrophobins in Aspergillus fumigatus

Resistance of Aspergillus fumigatus conidia to desiccation and their capacity to reach the alveoli are partly due to the presence of a hydrophobic layer composed of a protein from the hydrophobin family, called RodA, which covers the conidial surface. In A. fumigatus there are seven hydrophobins (RodA-RodG) belonging to class I and III. Most of them have never been studied. We constructed single and multiple hydrophobin-deletion mutants until the generation of a hydrophobin-free mutant. The phenotype, immunogenicity, and virulence of the mutants were studied. RODA is the most expressed hydrophobin in sporulating cultures, whereas RODB is upregulated in biofilm conditions and in vivo Only RodA, however, is responsible for rodlet formation, sporulation, conidial hydrophobicity, resistance to physical insult or anionic dyes, and immunological inertia of the conidia. None of the hydrophobin plays a role in biofilm formation or its hydrophobicity. RodA is the only needed hydrophobin in A. fumigatus, conditioning the structure, permeability, hydrophobicity, and immune-inertia of the cell wall surface in conidia. Moreover, the defect of rodlets on the conidial cell wall surface impacts on the drug sensitivity of the fungus.

Evaluating the potential of natural surfactants in the petroleum industry: the case of hydrophobins

Enhancing oil recovery from currently available reservoirs is a major issue for petroleum companies. Among the possible strategies towards this, chemical flooding through injection of surfactants into the wells seems to be particularly promising, thanks to their ability to reduce oil/water interfacial tension that promotes oil mobilization. Environmental concerns about the use of synthetic surfactants led to a growing interest in their replacement with surfactants of biological origin, such as lipopeptides and glycolipids produced by several microorganisms. Hydrophobins are small amphiphilic proteins produced by filamentous fungi with high surface activity and good emulsification properties, and may represent a novel sustainable tool for this purpose. We report here a thorough study of their stability and emulsifying performance towards a model hydrocarbon mixture, in conditions that mimic those of real oil reservoirs (high salinity and high temperature). Due to the moderate interfacial tension reduction induced in such conditions, the application of hydrophobins in enhanced oil recovery techniques does not appear feasible at the moment, at least in absence of co-surfactants. On the other hand, the obtained results showed the potential of hydrophobins in promoting the formation of a gel-like emulsion ‘barrier’ at the oil/water interface.

Molecular Structure of Hydrophobins Studied with Site-Directed Mutagenesis and Vibrational Sum-Frequency Generation Spectroscopy

Hydrophobins are surface-active fungal proteins that adsorb to the water-air interface and self-assemble into amphiphilic, water-repelling films that have a surface elasticity that is an order of magnitude higher than other molecular films. Here we use surface-specific sum-frequency generation spectroscopy (VSFG) and site-directed mutagenesis to study the properties of class I hydrophobin (HFBI) films from Trichoderma reesei at the molecular level. We identify protein specific HFBI signals in the frequency region 1200-1700 cm-1 that have not been observed in previous VSFG studies on proteins. We find evidence that the aspartic acid residue (D30) next to the hydrophobic patch is involved in lateral intermolecular protein interactions, while the two aspartic acid residues (D40, D43) opposite to the hydrophobic patch are primarily interacting with the water solvent.

Structure-Dependent Interfacial Properties of Chaplin F from Streptomyces coelicolor

Chaplin F (Chp F) is a secreted surface-active peptide involved in the aerial growth of Streptomyces. While Chp E demonstrates a pH-responsive surface activity, the relationship between Chp F structure, function and the effect of solution pH is unknown. Chp F peptides were found to self-assemble into amyloid fibrils at acidic pH (3.0 or the isoelectric point (pI) of 4.2), with ~99% of peptides converted into insoluble fibrils. In contrast, Chp F formed short assemblies containing a mixture of random coil and β-sheet structure at a basic pH of 10.0, where only 40% of the peptides converted to fibrils. The cysteine residues in Chp F did not appear to play a role in fibril assembly. The interfacial properties of Chp F at the air/water interface were altered by the structures adopted at different pH, with Chp F molecules forming a higher surface-active film at pH 10.0 with a lower area per molecule compared to Chp F fibrils at pH 3.0. These data show that the pH responsiveness of Chp F surface activity is the reverse of that observed for Chp E, which could prove useful in potential applications where surface activity is desired over a wide range of solution pH.

Coating Nanoparticles with Plant-Produced Transferrin-Hydrophobin Fusion Protein Enhances Their Uptake in Cancer Cells

The encapsulation of drugs to nanoparticles may offer a solution for targeted delivery. Here, we set out to engineer a self-assembling targeting ligand by combining the functional properties of human transferrin and fungal hydrophobins in a single fusion protein. We showed that human transferrin can be expressed in Nicotiana benthamiana plants as a fusion with Trichoderma reesei hydrophobins HFBI, HFBII, or HFBIV. Transferrin-HFBIV was further expressed in tobacco BY-2 suspension cells. Both partners of the fusion protein retained their functionality; the hydrophobin moiety enabled migration to a surfactant phase in an aqueous two-phase system, and the transferrin moiety was able to reversibly bind iron. Coating porous silicon nanoparticles with the fusion protein resulted in uptake of the nanoparticles in human cancer cells. This study provides a proof-of-concept for the functionalization of hydrophobin coatings with transferrin as a targeting ligand.

Observation of pH-Induced Protein Reorientation at the Water Surface

Hydrophobins are surface-active proteins that form a hydrophobic, water-repelling film around aerial fungal structures. They have a compact, particle-like structure, in which hydrophilic and hydrophobic regions are spatially separated. This surface property renders them amphiphilic and is reminiscent of synthetic Janus particles. Here we report surface-specific chiral and nonchiral vibrational sum-frequency generation spectroscopy (VSFG) measurements of hydrophobins adsorbed to their natural place of action, the air-water interface. We observe that hydrophobin molecules undergo a reversible change in orientation (tilt) at the interface when the pH is varied. We explain this local orientation toggle from the modification of the interprotein interactions and the interaction of hydrophobin with the water solvent, following the pH-induced change of the charge state of particular amino acids.

Characterization of a Basidiomycota hydrophobin reveals the structural basis for a high-similarity Class I subdivision

Class I hydrophobins are functional amyloids secreted by fungi. They self-assemble into organized films at interfaces producing structures that include cellular adhesion points and hydrophobic coatings. Here, we present the first structure and solution properties of a unique Class I protein sequence of Basidiomycota origin: the Schizophyllum commune hydrophobin SC16 (hyd1). While the core β-barrel structure and disulphide bridging characteristic of the hydrophobin family are conserved, its surface properties and secondary structure elements are reminiscent of both Class I and II hydrophobins. Sequence analyses of hydrophobins from 215 fungal species suggest this structure is largely applicable to a high-identity Basidiomycota Class I subdivision (IB). To validate this prediction, structural analysis of a comparatively distinct Class IB sequence from a different fungal order, namely the Phanerochaete carnosa PcaHyd1, indicates secondary structure properties similar to that of SC16. Together, these results form an experimental basis for a high-identity Class I subdivision and contribute to our understanding of functional amyloid formation.

The dynamics of multimer formation of the amphiphilic hydrophobin protein HFBII

Hydrophobins are surface-active proteins produced by filamentous fungi. They have amphiphilic structures and form multimers in aqueous solution to shield their hydrophobic regions. The proteins rearrange at interfaces and self-assemble into films that can show a very high degree of structural order. Little is known on dynamics of multimer interactions in solution and how this is affected by other components. In this work we examine the multimer dynamics by stopped-flow fluorescence measurements and Förster Resonance Energy Transfer (FRET) using the class II hydrophobin HFBII. The half-life of exchange in the multimer state was 0.9s at 22°C with an activation energy of 92kJ/mol. The multimer exchange process of HFBII was shown to be significantly affected by the closely related HFBI hydrophobin, lowering both activation energy and half-life for exchange. Lower molecular weight surfactants interacted in very selective ways, but other surface active proteins did not influence the rates of exchange. The results indicate that the multimer formation is driven by specific molecular interactions that distinguish different hydrophobins from each other.

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 the response of protein N–H vibrations in vibrational sum-frequency generation spectroscopy of aqueous protein films

We study the response of protein N–H vibrations in aqueous hydrophobin films using vibrational sum- frequency generation spectroscopy.

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.

Self-Assembly and Conformational Changes of Hydrophobin Classes at the Air-Water Interface

We use surface-specific vibrational sum-frequency generation spectroscopy (VSFG) to study the structure and self-assembling mechanism of the class I hydrophobin SC3 from Schizophyllum commune and the class II hydrophobin HFBI from Trichoderma reesei. We find that both hydrophobins readily accumulate at the water-air interface and form rigid, highly ordered protein films that give rise to prominent VSFG signals. We identify several resonances that are associated with β-sheet structures and assign them to the central β-barrel core present in both proteins. Differences between the hydrophobin classes are observed in their interfacial self-assembly. For HFBI, we observe no changes in conformation upon adsorption to the water surface. For SC3, we observe an increase in β-sheet-specific signals that supports a surface-driven self-assembly mechanism in which the central β-barrel remains intact and stacks into a larger-scale architecture, amyloid-like rodlets.