The hydrophobins HFBI and HFBII from Trichoderma reesei showing efficient interactions with nonionic surfactants in aqueous two-phase systems

Flattened and Wrinkled Encapsulated Droplets: Shape Morphing Induced by Gravity and Evaporation

We report surprising morphological changes of suspension droplets (containing class II hydrophobin protein HFBI from Trichoderma reesei in water) as they evaporate with a contact line pinned on a rigid solid substrate. Both pendant and sessile droplets display the formation of an encapsulating elastic film as the bulk concentration of solute reaches a critical value during evaporation, but the morphology of the droplet varies significantly: for sessile droplets, the elastic film ultimately crumples in a nearly flattened area close to the apex while in pendant droplets, circumferential wrinkling occurs close to the contact line. These different morphologies are understood through a gravito-elastocapillary model that predicts the droplet morphology and the onset of shape changes, as well as showing that the influence of the direction of gravity remains crucial even for very small droplets (where the effect of gravity can normally be neglected). The results pave the way to control droplet shape in several engineering and biomedical applications.

Engineered Hydrophobin as a Crystallization Inhibitor for Flufenamic Acid

Hydrophobins are multifunctional, highly surface-active proteins produced in filamentous fungi. Due to their surface-active properties, resistance to degradation, and potential immunological inertness, hydrophobins have been used in many applications such as protein purification, increasing implant biocompatibility, increasing water solubility of insoluble drugs, and foam stabilizers for food products. To further explore surface-active and self-assembly properties of hydrophobins, we evaluated an engineered, recombinant hydrophobin (class II type 1, HFB1) as a potential crystallization inhibitor for maintaining drug supersaturation for an amorphous drug delivery system. A supersaturation-precipitation method was employed utilizing an ultraviolet (UV) fiber optic system for tracking precipitation kinetics of a model drug, flufenamic acid (FA), that was selected due to its low aqueous solubility in its crystalline form. The effectiveness of HFB1 as a crystallization inhibitor was compared with commonly used pharmaceutical grade polymeric crystallization inhibitors. The following polymers were selected to compare with HFB1: methocel (A4C grade), methocel (K15M grade), Kollidon vinylpyrrolidone-vinyl acetate (VA64), and hydroxypropyl methylcellulose acetate succinate (HPMCAS) (MF grade). The supersaturation-precipitation experiments concluded that HFB1 outperformed all polymers tested in this study and can potentially be used as a crystallization inhibitor at significantly lower concentrations in amorphous drug delivery systems. Dynamic light scattering (DLS) and circular dichroism (CD) results suggest a crystallization inhibition mechanism in which HFB1 functions differently depending on whether flufenamic acid is molecularly dispersed but supersaturated relative to its crystalline solubility or it has exceeded its amorphous solubility limit and exists as a phase-separated drug-rich colloid.

Soluble Expression and Efficient Purification of Recombinant Class I Hydrophobin DewA

Hydrophobins are small proteins (<20 kDa) with an amphipathic tertiary structure that are secreted by various filamentous fungi. Their amphipathic properties provide surfactant-like activity, leading to the formation of robust amphipathic layers at hydrophilic–hydrophobic interfaces, which make them useful for a wide variety of industrial fields spanning protein immobilization to surface functionalization. However, the industrial use of recombinant hydrophobins has been hampered due to low yield from inclusion bodies owing to the complicated process, including an auxiliary refolding step. Herein, we report the soluble expression of a recombinant class I hydrophobin DewA originating from Aspergillus nidulans, and its efficient purification from recombinant Escherichia coli. Soluble expression of the recombinant hydrophobin DewA was achieved by a tagging strategy using a systematically designed expression tag (ramp tag) that was fused to the N-terminus of DewA lacking the innate signal sequence. Highly expressed recombinant hydrophobin DewA in a soluble form was efficiently purified by a modified aqueous two-phase separation technique using isopropyl alcohol. Our approach for expression and purification of the recombinant hydrophobin DewA in E. coli shed light on the industrial production of hydrophobins from prokaryotic hosts.

Quantifying biomolecular hydrophobicity: Single molecule force spectroscopy of class II hydrophobins

Hydrophobins are surface-active proteins produced by filamentous fungi. The amphiphilic structure of hydrophobins is very compact, containing a distinct hydrophobic patch on one side of the molecule, locked by four intramolecular disulfide bridges. Hydrophobins form dimers and multimers in solution to shield these hydrophobic patches from water exposure. Multimer formation in solution is dynamic, and hydrophobin monomers can be exchanged between multimers. Unlike class I hydrophobins, class II hydrophobins assemble into highly ordered films at the air-water interface. In order to increase our understanding of the strength and nature of the interaction between hydrophobins, we used atomic force microscopy for single molecule force spectroscopy to explore the molecular interaction forces between class II hydrophobins from Trichoderma reesei under different environmental conditions. A genetically engineered hydrophobin variant, NCys-HFBI, enabled covalent attachment of proteins to the apex of the atomic force microscopy cantilever tip and sample surfaces in controlled orientation with sufficient freedom of movement to measure molecular forces between hydrophobic patches. The measured rupture force between two assembled hydrophobins was ∼31 pN, at a loading rate of 500 pN/s. The results indicated stronger interaction between hydrophobins and hydrophobic surfaces than between two assembling hydrophobin molecules. Furthermore, this interaction was stable under different environmental conditions, which demonstrates the dominance of hydrophobicity in hydrophobin-hydrophobin interactions. This is the first time that interaction forces between hydrophobin molecules, and also between naturally occurring hydrophobic surfaces, have been measured directly at a single-molecule level.

Lab-on-a-brane for spheroid formation

Lab-On-a-Brane (LOB) represents a class of Lab-On-a-Chip (LOC) integrating flexible, highly gas permeable and biocompatible thin membranes (TMs). Here we demonstrate the potentiality of LOBs as cell biochips promoting 3D cell growth. The human cancer cells MCF-7 were cultured into standard multiwells (MWs) and into polydimethylsiloxane (PDMS) MWs, LOCs, and LOBs of different wettability. Surface treatments based on oxygen plasma and coating deposition have been performed to produce hydrophilic, hydrophobic, and oleophobic chips. By a comparison between all these chips, we observed that 3D cell aggregation is favored in LOBs, independent of substrate wettability. This may be attributed to the TM flexibility and the high oxygen/carbon dioxide permeability. Ultimately, LOBs seem to combine the advantages of LOCs as multi-well microfluidic chips to reduce operation time for cell seeding and medium refresh, with the mechanical/morphological properties of PDMS TMs. This is convenient in the perspective of applying mechanical stimuli and monitoring cell stiffness, or studying the metabolism of molecules permeable to PDMS membrane in response to external stimuli with interesting outcomes in cellular biology.

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.

Coacervation of resilin fusion proteins containing terminal functionalities

Liquid-liquid phase transition known as coacervation of resilin-like-peptide fusion proteins containing different terminal domains were investigated. Two different modular proteins were designed and produced and their behavior were compared to a resilin-like-peptide without terminal domains. The size of the particle-like coacervates was modulated by the protein concentration, pH and temperature. The morphology and three-dimensional (3D) structural details of the coacervate particles were investigated by cryogenic transmission electron microscopy (cryo-TEM) and tomography (cryo-ET) reconstruction. Selective adhesion of the coacervates on cellulose and graphene surfaces was demonstrated.

In-solution antibody harvesting with a plant-produced hydrophobin-Protein A fusion

Purification is a bottleneck and a major cost factor in the production of antibodies. We set out to engineer a bifunctional fusion protein from two building blocks, Protein A and a hydrophobin, aiming at low-cost and scalable antibody capturing in solutions. Immunoglobulin-binding Protein A is widely used in affinity-based purification. The hydrophobin fusion tag, on the other hand, has been shown to enable purification by two-phase separation. Protein A was fused to two different hydrophobin tags, HFBI or II, and expressed transiently in Nicotiana benthamiana. The hydrophobins enhanced accumulation up to 35-fold, yielding up to 25% of total soluble protein. Both fused and nonfused Protein A accumulated in protein bodies. Hence, the increased yield could not be attributed to HFB-induced protein body formation. We also demonstrated production of HFBI-Protein A fusion protein in tobacco BY-2 suspension cells in 30 l scale, with a yield of 35 mg/l. Efficient partitioning to the surfactant phase confirmed that the fusion proteins retained the amphipathic properties of the hydrophobin block. The reversible antibody-binding capacity of the Protein A block was similar to the nonfused Protein A. The best-performing fusion protein was tested in capturing antibodies from hybridoma culture supernatant with two-phase separation. The fusion protein was able to carry target antibodies to the surfactant phase and subsequently release them back to the aqueous phase after a change in pH. This report demonstrates the potential of hydrophobin fusion proteins for novel applications, such as harvesting antibodies in solutions.

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.

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.

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.

Integrated process for the purification and immobilization of the envelope protein domain III of dengue virus type 2 expressed in Rachiplusia nu larvae and its potential application in a diagnostic assay

Dengue incidence has grown dramatically in the last years, with about 40% of the world population at risk of infection. Recently, a vaccine developed by Sanofi Pasteur has been registered, but only in a few countries. Moreover, specific antiviral drugs are not available. Thus, an efficient and accurate diagnosis is important for disease management. To develop a low-cost immunoassay for dengue diagnosis, in the present study we expressed the envelope protein domain III of dengue virus type 2 in Rachiplusia nu larvae by infection with a recombinant baculovirus. The antigen was expressed as a fusion to hydrophobin I (DomIIIHFBI) to easily purify it by an aqueous two-phase system (ATPS) and to efficiently immobilize it in immunoassay plates. A high level of recombinant DomIIIHFBI was obtained in R. nu, where yields reached 4.5 mg per g of larva. Also, we were able to purify DomIIIHFBI by an ATPS with 2% of Triton X-114, reaching a yield of 73% and purity higher than 80% in a single purification step. The recombinant DomIIIHFBI was efficiently immobilized in hydrophobic surface plates. The immunoassay we developed with the immobilized antigen was able to detect IgG specific for dengue virus type 2 in serum samples and not for other serotypes.

Binding of cellulose binding modules reveal differences between cellulose substrates

The interaction between cellulase enzymes and their substrates is of central importance to several technological and scientific challenges. Here we report that the binding of cellulose binding modules (CBM) from Trichoderma reesei cellulases Cel6A and Cel7A show a major difference in how they interact with substrates originating from wood compared to bacterial cellulose. We found that the CBM from TrCel7A recognizes the two substrates differently and as a consequence shows an unexpected way of binding. We show that the substrate has a large impact on the exchange rate of the studied CBM, and moreover, CBM-TrCel7A seems to have an additional mode of binding on wood derived cellulose but not on cellulose originating from bacterial source. This mode is not seen in double CBM (DCBM) constructs comprising both CBM-TrCel7A and CBM-TrCel6A. The linker length of DCBMs affects the binding properties, and slows down the exchange rates of the proteins and thus, can be used to analyze the differences between the single CBM. These results have impact on the cellulase research and offer new understanding on how these industrially relevant enzymes act.

Novel Hydrophobin Fusion Tags for Plant-Produced Fusion Proteins

Hydrophobin fusion technology has been applied in the expression of several recombinant proteins in plants. Until now, the technology has relied exclusively on the Trichoderma reesei hydrophobin HFBI. We screened eight novel hydrophobin tags, T. reesei HFBII, HFBIII, HFBIV, HFBV, HFBVI and Fusarium verticillioides derived HYD3, HYD4 and HYD5, for production of fusion proteins in plants and purification by two-phase separation. To study the properties of the hydrophobins, we used N-terminal and C-terminal GFP as a fusion partner. Transient expression of the hydrophobin fusions in Nicotiana benthamiana revealed large variability in accumulation levels, which was also reflected in formation of protein bodies. In two-phase separations, only HFBII and HFBIV were able to concentrate GFP into the surfactant phase from a plant extract. The separation efficiency of both tags was comparable to HFBI. When the accumulation was tested side by side, HFBII-GFP gave a better yield than HFBI-GFP, while the yield of HFBIV-GFP remained lower. Thus we present here two alternatives for HFBI as functional fusion tags for plant-based protein production and first step purification.

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.

Flattened-Top Domical Water Drops Formed through Self-Organization of Hydrophobin Membranes: A Structural and Mechanistic Study Using Atomic Force Microscopy

The Trichoderma reesei hydrophobin, HFBI, is a unique structural protein. This protein forms membranes by self-organization at air/water or water/solid interfaces. When HFBI forms a membrane at an air/water interface, the top of the water droplet is flattened. The mechanism underlying this phenomenon has not been explored. In this study, this unique phenomenon has been investigated. Self-organized HFBI membranes form a hexagonal structured membrane on the surface of water droplets; the structure was confirmed by atomic force microscopy (AFM) measurement. Assembled hexagons can form a planar sheet or a tube. Self-organized HFBI membranes on water droplets form a sheet with an array of hexagonal structures or a honeycomb structure. This membrane, with its arrayed hexagonal structures, has very high buckling strength. We hypothesized that the high buckling strength is the reason that water droplets containing HFBI form flattened domes. To test this hypothesis, the strength of the self-organized HFBI membranes was analyzed using AFM. The buckling strength of HFBI membranes was measured to be 66.9 mN/m. In contrast, the surface tension of water droplets containing dissolved HFBI is 42 mN/m. Thus, the buckling strength of a self-organized HFBI membrane is higher than the surface tension of water containing dissolved HFBI. This mechanistic study clarifies why the water droplets formed by self-organized HFBI membranes have a flattened top.

Continuous Flow Separation of Hydrophobin Fusion Proteins from Plant Cell Culture Extract

Fusion to fungal hydrophobins has proven to be a useful tool to enhance accumulation and recovery of recombinant proteins in plants. Aqueous two-phase separation (ATPS) is an attractive system to capture hydrophobin fusion proteins from plant extracts. The process can simultaneously purify and concentrate target protein with minimal background. ATPS avoids the use of chromatographic column steps, can be carried out in a short time frame, and is amenable to industrial-scale protein purification. A drawback of performing ATPS in large volumes is the lengthy time required for phase separation; however, this can be avoided by incorporating continuous systems, which are often preferred by the processing industry. This method chapter illustrates the capture of GFP-HFBI hydrophobin fusion protein from BY-2 plant cell suspension extract using a semi-continuous ATPS method.

Protein Bodies in Leaves Exchange Contents through the Endoplasmic Reticulum

Protein bodies (PBs) are organelles found in seeds whose main function is the storage of proteins that are used during germination for sustaining growth. PBs can also be induced to form in leaves when foreign proteins are produced at high levels in the endoplasmic reticulum (ER) and when fused to one of three tags: Zera®, elastin-like polypeptides (ELP), or hydrophobin-I (HFBI). In this study, we investigate the differences between ELP, HFBI and Zera PB formation, packing, and communication. Our results confirm the ER origin of all three fusion-tag-induced PBs. We show that secretory pathway proteins can be sequestered into all types of PBs but with different patterns, and that different fusion tags can target a specific protein to different PBs. Zera PBs are mobile and dependent on actomyosin motility similar to ELP and HFBI PBs. We show in vivo trafficking of proteins between PBs using GFP photoconversion. We also show that protein trafficking between ELP or HFBI PBs is faster and proteins travel further when compared to Zera PBs. Our results indicate that fusion-tag-induced PBs do not represent terminally stored cytosolic organelles, but that they form in, and remain part of the ER, and dynamically communicate with each other via the ER. We hypothesize that the previously documented PB mobility along the actin cytoskeleton is associated with ER movement rather than independent streaming of detached organelles.

Selective isolation of hydrophobin SC3 by solid-phase extraction with polytetrafluoroethylene microparticles and subsequent mass spectrometric analysis

Hydrophobins are small proteins that play a role in a number of processes during the filamentous fungi growth and development. These proteins are characterized by the self-assembly of their molecules into an amphipathic membrane at hydrophilic-hydrophobic interfaces. Isolation and purification of hydrophobins generally present a challenge in their analysis. Hydrophobin SC3 from Schizophyllum commune was selected as a representative of class I hydrophobins in this work. A novel procedure for selective and effective isolation of hydrophobin SC3 based on solid-phase extraction with polytetrafluoroethylene microparticles loaded in a small self-made microcolumn is reported. The tailored binding of hydrophobins to polytetrafluoroethylene followed by harsh elution conditions resulted in a highly specific isolation of hydrophobin SC3 from the model mixture of ten proteins. The presented isolation protocol can have a positive impact on the analysis and utilization of these proteins including all class I hydrophobins. Hydrophobin SC3 was further subjected to reduction of its highly stable disulfide bonds and to chymotryptic digestion followed by mass spectrometric analysis. The isolation and digestion protocols presented in this work make the analysis of these highly hydrophobic and compact proteins possible.

Atomistic simulation of hydrophobin HFBII conformation in aqueous and fluorous media and at the water/vacuum interface

Hydrophobins are proteins of interest for numerous applications thanks to their unique conformational and surface properties and their ability to self-assemble at interfaces. Here we report fully atomistic molecular mechanics and molecular dynamics results together with circular dichroism experimental data, aimed to study the conformational properties of the hydrophobin HFBII in a fluorinated solvent in comparison with a water solution and/or at an aqueous/vacuum interface. Both the atomistic simulations and the circular dichroism data show the remarkable structural stability of HFBII at all scales in all these environments, with no significant structural change, although a small cavity is formed in the fluorinated solvent. The combination of theoretical calculations and circular dichroism data can describe in detail the protein conformation and flexibility in different solvents and/or at an interface, and constitutes a first step towards the study of their self-assembly.

Protein body formation in leaves of Nicotiana benthamiana: a concentration-dependent mechanism influenced by the presence of fusion tags

Protein bodies (PBs) are endoplasmic reticulum (ER) derived organelles originally found in seeds whose function is to accumulate seed storage proteins. It has been shown that PB formation is not limited to seeds and green fluorescent protein (GFP) fused to either elastin-like polypeptide (ELP) or hydrophobin (HFBI) fusion tags induce the formation of PBs in leaves of N. benthamiana. In this study, we compared the ELP- and HFBI-induced PBs and showed that ELP-induced PBs are larger than HFBI-induced PBs. The size of ELP- and HFBI-induced PBs increased over time along with the accumulation levels of their fused protein. Our results show that PB formation is a concentration-dependent mechanism in which proteins accumulating at levels higher than 0.2% of total soluble protein are capable of inducing PBs in vivo. Our results show that the presence of fusion tags is not necessary for the formation of PBs, but affects the distribution pattern and size of PBs. This was confirmed by PBs induced by fluorescent proteins as well as fungal xylanases. We noticed that in the process of PB formation, secretory and ER-resident molecules are passively sequestered into the lumen of PBs. We propose to use this property of PBs as a tool to increase the accumulation levels of erythropoietin and human interleukin-10 by co-expression with PB-inducing proteins.

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.

Electrochemical properties of honeycomb-like structured HFBI self-organized membranes on HOPG electrodes

HFBI (derived from Trichoderma sp.) is a unique structural protein, which forms a self-organized monolayer at both air/water interface and water/solid interfaces in accurate two-dimensional ordered structures. We have taken advantage of the unique functionality of HFBI as a molecular carrier for preparation of ordered molecular phase on solid substrate surfaces. The HFBI molecular carrier can easily form ordered structures; however, the dense molecular layers form an electrochemical barrier between the electrode and solution phase. In this study, the electrochemical properties of HFBI self-organized membrane-covered electrodes were investigated. Wild-type HFBI has balanced positive and negative charges on its surface. Highly oriented pyrolytic graphite (HOPG) electrodes coated with HFBI molecules were investigated electrochemically. To improve the electrochemical properties of this HFBI-coated electrode, the two types of HFBI variants, with oppositely charged surfaces, were prepared genetically. All three types of HFBI-coated HOPG electrode perform electron transfer between the electrode and solution phase through the dense HFBI molecular layer. This is because the HFBI self-organized membrane has a honeycomb-like structure, with penetrating holes. In the cases of HFBI variants, the oppositely charged HFBI membrane phases shown opposite electrochemical behaviors in electrochemical impedance spectroscopy. HFBI is a molecule with a unique structure, and can easily form honeycomb-like structures on solid material surfaces such as electrodes. The molecular membrane phase can be used for electrochemical molecular interfaces.

Solid-support immobilization of a "swing" fusion protein for enhanced glucose oxidase catalytic activity

The strategic surface immobilization of a protein can add new functionality to a solid substrate; however, protein activity, e.g., enzymatic activity, can be drastically decreased on immobilization onto a solid surface. The concept of a designed and optimized "molecular interface" is herein introduced in order to address this problem. In this study, molecular interface was designed and constructed with the aim of attaining high enzymatic activity of a solid-surface-immobilized a using the hydrophobin HFBI protein in conjunction with a fusion protein of HFBI attached to glucose oxidase (GOx). The ability of HFBI to form a self-organized membrane on a solid surface in addition to its adhesion properties makes it an ideal candidate for immobilization. The developed fusion protein was also able to form an organized membrane, and its structure and immobilized state on a solid surface were investigated using QCM-D measurements. This method of immobilization showed retention of high enzymatic activity and the ability to control the density of the immobilized enzyme. In this study, we demonstrated the importance of the design and construction of molecular interface for numerous purposes. This method of protein immobilization could be utilized for preparation of high throughput products requiring structurally ordered molecular interfaces, in addition to many other applications.

Structure-function relationships in hydrophobins: probing the role of charged side chains

Hydrophobins are small fungal proteins that are amphiphilic and have a strong tendency to assemble at interfaces. By taking advantage of this property, hydrophobins have been used for a number of applications: as affinity tags in protein purification, for protein immobilization, such as in foam stabilizers, and as dispersion agents for insoluble drug molecules. Here, we used site-directed mutagenesis to gain an understanding of the molecular basis of their properties. We especially focused on the role of charged amino acids in the structure of hydrophobins. For this purpose, fusion proteins consisting of Trichoderma reesei hydrophobin I (HFBI) and the green fluorescent protein (GFP) that contained various combinations of substitutions of charged amino acids (D30, K32, D40, D43, R45, K50) in the HFBI structure were produced. The effects of the introduced mutations on binding, oligomerization, and partitioning were characterized in an aqueous two-phase system. It was found that some substitutions caused better surface binding and reduced oligomerization, while some showed the opposite effects. However, all mutations decreased partitioning in surfactant systems, indicating that the different functions are not directly correlated and that partitioning is dependent on finely tuned properties of hydrophobins. This work shows that not all functions in self-assembly are connected in a predictable way and that a simple surfactant model for hydrophobin function is insufficient.

Protein body formation in stable transgenic tobacco expressing elastin-like polypeptide and hydrophobin fusion proteins

BACKGROUND

Plants are recognized as an efficient and inexpensive system to produce valuable recombinant proteins. Two different strategies have been commonly used for the expression of recombinant proteins in plants: transient expression mediated by Agrobacterium; or stable transformation of the plant genome. However, the use of plants as bioreactors still faces two main limitations: low accumulation levels of some recombinant proteins and lack of efficient purification methods. Elastin-like polypeptide (ELP), hydrophobin I (HFBI) and Zera® are three fusion partners found to increase the accumulation levels of recombinant proteins and induce the formation of protein bodies (PBs) in leaves when targeted to the endoplasmic reticulum (ER) in transient expression assays. In this study the effects of ELP and HFBI fusion tags on recombinant protein accumulation levels and PB formation was examined in stable transgenic Nicotiana tabacum.

RESULTS

The accumulation of recombinant protein and PB formation was evaluated in two cultivars of Nicotiana tabacum transformed with green fluorescent protein (GFP) fused to ELP or HFBI, both targeted and retrieved to the ER. The ELP and HFBI tags increased the accumulation of the recombinant protein and induced the formation of PBs in leaves of stable transgenic plants from both cultivars. Furthermore, these tags induced the formation of PBs in a concentration-dependent manner, where a specific level of recombinant protein accumulation was required for PBs to appear. Moreover, agro-infiltration of plants accumulating low levels of recombinant protein with p19, a suppressor of post-transcriptional gene silencing (PTGS), increased accumulation levels in four independent transgenic lines, suggesting that PTGS might have caused the low accumulation levels in these plants.

CONCLUSION

The use of ELP and HFBI tags as fusion partners in stable transgenic plants of tobacco is feasible and promising. In a constitutive environment, these tags increase the accumulation levels of the recombinant protein and induce the formation of PBs regardless of the cultivar used. However, a specific level of recombinant protein accumulation needs to be reached for PBs to form.

Drug release from nanoparticles embedded in four different nanofibrillar cellulose aerogels

Highly porous nanocellulose aerogels prepared by freeze-drying from various nanofibrillar cellulose (NFC) hydrogels are introduced as nanoparticle reservoirs for oral drug delivery systems. Here we show that beclomethasone dipropionate (BDP) nanoparticles coated with amphiphilic hydrophobin proteins can be well integrated into the NFC aerogels. NFCs from four different origins are introduced and compared to microcrystalline cellulose (MCC). The nanocellulose aerogel scaffolds made from red pepper (RC) and MCC release the drug immediately, while bacterial cellulose (BC), quince seed (QC) and TEMPO-oxidized birch cellulose-based (TC) aerogels show sustained drug release. Since the release of the drug is controlled by the structure and interactions between the nanoparticles and the cellulose matrix, modulation of the matrix formers enable a control of the drug release rate. These nanocomposite structures can be very useful in many pharmaceutical nanoparticle applications and open up new possibilities as carriers for controlled drug delivery.

Wrinkling in the deflation of elastic bubbles

The protein hydrophobin HFBII self-assembles into very elastic films at the surface of water; these films wrinkle readily upon compression. We demonstrate and study this wrinkling instability in the context of non-planar interfaces by forming HFBII layers at the surface of bubbles whose interfaces are then compressed by deflation of the bubble. By varying the initial concentration of the hydrophobin solutions, we are able to show that buckling occurs at a critical packing fraction of protein molecules on the surface. Independent experiments show that at this packing fraction the interface has a finite positive surface tension, and not zero surface tension as is usually assumed at buckling. We attribute this non-zero wrinkling tension to the finite elasticity of these interfaces. We develop a simple geometrical model for the evolution of the wrinkle length with further deflation and show that wrinkles grow rapidly near the needle (used for deflation) towards the mid-plane of the bubble. This geometrical model yields predictions for the length of wrinkles in good agreement with experiments independently of the rheological properties of the adsorbed layer.