Oligomerization of hydrophobin SC3 in solution: from soluble state to self-assembly

Interfacial mechanics of β-casein and albumin mixed protein assemblies at liquid-liquid interfaces

Protein emulsifiers play an important role in formulation science, from food product development to emerging applications in biotechnologies. The impact of mixed protein assemblies on surface composition and interfacial shear mechanics remains broadly unexplored, in comparison to the impact that formulation has on dilatational mechanics and surface tension or pressure. In this report, we use interfacial shear rheology to quantify the evolution of interfacial shear moduli as a function of composition in bovine serum albumin (BSA)/β-casein mixed assemblies. We present the pronounced difference in mechanics of these two protein, at oil interfaces, and observe the dominance of β-casein in regulating interfacial shear mechanics. This observation correlates well with the strong asymmetry of adsorption of these two proteins, characterised by fluorescence microscopy. Using neutron reflectometry and fluorescence recovery after photobleaching, we examine the architecture of corresponding protein assemblies and their surface diffusion, providing evidence for distinct morphologies, but surprisingly comparable diffusion profiles. Finally, we explore the impact of crosslinking and sequential protein adsorption on the interfacial shear mechanics of corresponding assemblies. Overall, this work indicates that, despite comparable surface densities, BSA and β-casein assemblies at liquid-liquid interfaces display almost 2 orders of magnitude difference in interfacial shear storage modulus and markedly different viscoelastic profiles. In addition, co-adsorption and sequential adsorption processes are found to further modulate interfacial shear mechanics. Beyond formulation science, the understanding of complex mixed protein assemblies and mechanics may have implications for the stability of emulsions and may underpin changes in the mechanical strength of corresponding interfaces, for example in tissue culture or in physiological conditions.

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.

Reshaping sub-millimetre bubbles from spheres to tori

Shape-changing objects are prized for applications ranging from acoustics to robotics. We report sub-millimetre bubbles that reversibly and rapidly change not only their shape but also their topological class, from sphere to torus, when subjected to a simple pressure treatment. Stabilized by a solid-like film of nanoscopic protein "particles", the bubbles may persist in toroidal form for several days, most of them with the relative dimensions expected of Clifford tori. The ability to cross topological classes reversibly and quickly is enabled by the expulsion of protein from the strained surfaces in the form of submicron assemblies. Compared to structural modifications of liquid-filled vesicles, for example by slow changes in solution osmolality, the rapid inducement of shape changes in bubbles by application of pressure may hasten experimental investigations of surface mechanics, even as it suggests new routes to lightweight materials with high surface areas.

Identification and characterization of a hydrophobin Vmh3 from Pleurotus ostreatus

Hydrophobins (HPs) are relatively small surface-active proteins of fungal origin. Being an industrially important protein, isolation of new molecules from GRAS (Generally Regarded as Safe) strains like mushrooms is the need of the time. In the present work, hydrophobin Vmh3-1 is isolated, purified, and identified from a culture broth and vegetative mycelia of Pleurotus ostreatus grown in a Potato dextrose broth (PDB) in static culture conditions. Purified proteins from the broth and the cell wall showed bands of 11 kDa and 17 kDa when analyzed on SDS-PAGE. Hydrophobin Vmh3-1 was identified in purified protein samples by the Orbitrap-HR-LC-MS/MS analysis with a maximum of 66% sequence coverage. The amphipathic nature of the protein was revealed by an increase in the water contact angle (WCA) of the hydrophilic surface of glass by 87% as well as a decrease in the WCA of the hydrophobic surface of Teflon by 19%. The emulsification property was tested with food-grade oils and Hexane. A maximum activity (EI ₂₄) of 87.64% was recorded for Sunflower oil. In CD (Circular dichroism) spectra, Vmh3-1 showed the typical spectra of hydrophobin with a dominance of β-sheets (51%) in the secondary structure and a minimum percentage of the α-helix (2%). The protein did not show a self-aggregating property on vigorous shaking making it suitable for numerous industrial applications. The identification of Vmh3-1 with detailed amino acid sequencing and the characterization of the protein to evaluate its potential in surface modifications for various industrial applications is demonstrated herein for the first time.

Improvement Thermal Stability of D-Lactate Dehydrogenase by Hydrophobin-1 and in Silico Prediction of Protein-Protein Interactions

Hydrophobins are small surface-active proteins. They can connect to hydrophobic or hydrophilic regions and oligomerize in solution to form massive construction. In nature, these proteins are produced by filamentous fungi at different stages of growth. So far, researchers have used them in various fields of biotechnology. In this study, recombinant hydrophobin-1 (rHFB1, 7.5 kDa) was used to stabilize recombinant D-lactate dehydrogenase (rD-LDH, 35 kDa). rD-LDH is a sensitive enzyme deactivated and oxidized by external agents such as O₂ and lights. So, its stabilization with rHFB1 can be the best index to demonstrate the positive effect of rHFB1 on preserving and improving enzyme's activity. The unique ability of rHFB1 for interacting with hydrophobic regions of rD-LDH was predicted by protein-protein docking study with ClusPro and PIC servers and confirmed by fluorescence experiments, and Colorless Native-PAGE. Measurement of thermodynamic parameters allows for authenticating the role of rHFB1 as a thermal stabilizer in the protein-protein complex (rD-LDH@rHFB1). Interaction between rHFB1 and rD-LDH improved half-life of enzyme 2.25-fold at 40 °C. Investigation of the kinetic parameters proved that the presence of rHFB1 along with the rD-LDH enhancement strongly the affinity of the enzyme for pyruvate. Furthermore, an increase of Kcat/Km for complex displayed the effect of rHFB1 for improving the enzyme's catalytic efficiency.

Concentration- and pH-Dependent Oligomerization of the Thrombin-Derived C-Terminal Peptide TCP-25

Peptide oligomerization dynamics affects peptide structure, activity, and pharmacodynamic properties. The thrombin C-terminal peptide, TCP-25 (GKYGFYTHVFRLKKWIQKVIDQFGE), is currently in preclinical development for improved wound healing and infection prevention. It exhibits turbidity when formulated at pH 7.4, particularly at concentrations of 0.3 mM or more. We used biochemical and biophysical approaches to explore whether the peptide self-associates and forms oligomers. The peptide showed a dose-dependent increase in turbidity as well as α-helical structure at pH 7.4, a phenomenon not observed at pH 5.0. By analyzing the intrinsic tryptophan fluorescence, we demonstrate that TCP-25 is more stable at high concentrations (0.3 mM) when exposed to high temperatures or a high concentration of denaturant agents, which is compatible with oligomer formation. The denaturation process was reversible above 100 µM of peptide. Dynamic light scattering demonstrated that TCP-25 oligomerization is sensitive to changes in pH, time, and temperature. Computational modeling with an active 18-mer region of TCP-25 showed that the peptide can form pH-dependent higher-order end-to-end oligomers and micelle-like structures, which is in agreement with the experimental data. Thus, TCP-25 exhibits pH- and temperature-dependent dynamic changes involving helical induction and reversible oligomerization, which explains the observed turbidity of the pharmacologically developed formulation.

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.

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.

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

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

Self-assembly of proteins into a three-dimensional multilayer system: investigation of the surface of the human fungal pathogen Aspergillus fumigatus

Hydrophobins are small surface active proteins that fulfil a wide spectrum of functions in fungal growth and development. The human fungal pathogen Aspergillus fumigatus expresses RodA hydrophobins that self-assemble on the outer conidial surface into tightly organized nanorods known as rodlets. AFM investigation of the conidial surface allows us to evidence that RodA hydrophobins self-assemble into rodlets through bilayers. Within bilayers, hydrophilic domains of hydrophobins point inward, thus making a hydrophilic core, while hydrophobic domains point outward. AFM measurements reveal that several rodlet bilayers are present on the conidial surface thus showing that proteins self-assemble into a complex three-dimensional multilayer system. The self-assembly of RodA hydrophobins into rodlets results from attractive interactions between stacked β-sheets, which conduct to a final linear cross-β spine structure. A Monte Carlo simulation shows that anisotropic interactions are the main driving forces leading the hydrophobins to self-assemble into parallel rodlets, which are further structured in nanodomains. Taken together, these findings allow us to propose a mechanism, which conducts RodA hydrophobins to a highly ordered rodlet structure. The mechanism of hydrophobin assembly into rodlets offers new prospects for the development of more efficient strategies leading to disruption of rodlet formation allowing a rapid detection of the fungus by the immune system.

Monitoring peripheral protein oligomerization on biological membranes

Peripheral proteins transiently interact with cellular membranes where they regulate important cellular events such as signal transduction. A number of peripheral proteins harbor lipid-binding modules that not only bind selectively with nanomolar affinity to biological membranes but also oligomerize on the membrane surface. In some cases, specific lipid binding or specific lipid compositions can induce peripheral protein oligomerization on cellular membranes. These oligomers serve different roles in biological signaling such as regulating protein-protein interactions, induction of membrane bending, or facilitating membrane scission. A number of technologies have been employed to study protein oligomerization with fluctuation analysis of fluorescently labeled molecules recently developed for use with commercial laser-scanning microscopes. In this chapter, the approach of raster image correlation spectroscopy coupled with number and brightness (N&B) analysis to investigate protein oligomerization on cellular membranes in live cells is presented. Important considerations are discussed for designing experiments, collecting data, and performing analysis. N&B analysis provides a robust method for assessing membrane binding and assembly properties of peripheral proteins and lipid-binding modules.

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.

Conformational changes in the DNA-binding domains of the ecdysteroid receptor during the formation of a complex with the hsp27 response element

The ecdysone receptor (EcR) and the ultraspiracle protein (Usp) form the functional receptor for ecdysteroids that initiates metamorphosis in insects. The Usp and EcR DNA-binding domains (UspDBD and EcRDBD, respectively) form a heterodimer on the natural pseudopalindromic element from the hsp27 gene promoter. The conformational changes in the protein-DNA during the formation of the UspDBD-EcRDBD-hsp27 complex were analyzed. Recombined UspDBD and EcRDBD proteins were purified and fluorescein labeled (FL) using the intein method at the C-ends of both proteins. The changes in the distances from the respective C-ends of EcRDBD and/or UspDBD to the 5'- and/or 3'-end of the response element were measured using fluorescence resonance energy transfer (FRET) methodology. The binding of EcRDBD induced a strong conformational change in UspDBD and caused the C-terminal fragment of the UspDBD molecule to move away from both ends of the regulatory element. UspDBD also induced a significant conformational change in the EcRDBD molecule. The EcRDBD C-terminus moved away from the 5'-end of the regulatory element and moved close to the 3'-end. An analysis was also done on the effect that DHR38DBD, the Drosophila ortholog of the mammalian NGFI-B, had on the interaction of UspDBD and EcRDBD with hsp27. FRET analysis demonstrated that hsp27 bending was induced by DHR38DBD. Fluorescence data revealed that hsp27 had a shorter end-to-end distance both in the presence of EcRDBD as well as in the presence of EcRDBD together with DHR38DBD, with DNA bend angles of about 36.2° and 33.6°, respectively. A model of how DHR38DBD binds to hsp27 in the presence of EcRDBD is presented.

Layer thickness of hydrophobin films leads to oscillation in wettability

In nanobiotechnology, the properties of surfaces are often key to sensor applications. If analytes possess a low tolerance or affinity regarding the sensory substrate (surface), then the setup of mediators may be indicated. Hydrophobins enable biocompatible surface functionalization without significant restrictions of the physicochemical substrate properties. Because of the imperfect formation of hydrophobin films, a high variation in surface properties is observed. In this study, we report on the relation between the film thickness of hydrophobin-coated solid surfaces and their wettability. We found that the wettability of protein-coated surfaces strictly depends on the amount of adsorbed protein, as reflected in an oscillation of the contact angles of hydrophobin-coated silicon wafers. Fusion proteins of Ccg2 and HFBI, representatives of class I and II hydrophobins, document the influence of fused peptide tags on the wettability. The orientation of the first crystal nuclei plays a decisive role in the formation of the growing hydrophobin layers. Here, a simple method of deducing the film thickness of hydrophobin assemblies on solid surfaces is presented. The determination of the static contact angle allows the prediction of which part of the protein is exposed to possible analytes.

Dissecting amelogenin protein nanospheres: characterization of metastable oligomers

Amelogenin self-assembles to form an extracellular protein matrix, which serves as a template for the continuously growing enamel apatite crystals. To gain further insight into the molecular mechanism of amelogenin nanosphere formation, we manipulated the interactions between amelogenin monomers by altering pH, temperature, and protein concentration to create isolated metastable amelogenin oligomers. Recombinant porcine amelogenins (rP172 and rP148) and three different mutants containing only a single tryptophan (Trp(161), Trp(45), and Trp(25)) were used. Dynamic light scattering and fluorescence studies demonstrated that oligomers were metastable and in constant equilibrium with monomers. Stable oligomers with an average hydrodynamic radius (R(H)) of 7.5 nm were observed at pH 5.5 between 4 and 10 mg · ml(-1). We did not find any evidence of a significant increase in folding upon self-association of the monomers into oligomers, indicating that they are disordered. Fluorescence experiments with single tryptophan amelogenins revealed that upon oligomerization the C terminus of amelogenin (around residue Trp(161)) is exposed at the surface of the oligomers, whereas the N-terminal region around Trp(25) and Trp(45) is involved in protein-protein interaction. The truncated rP148 formed similar but smaller oligomers, suggesting that the C terminus is not critical for amelogenin oligomerization. We propose a model for nanosphere formation via oligomers, and we predict that nanospheres will break up to form oligomers in mildly acidic environments via histidine protonation. We further suggest that oligomeric structures might be functional components during maturation of enamel apatite.

Unique properties of bubbles and foam films stabilized by HFBII hydrophobin

The HFBII hydrophobin is an amphiphilic protein that can irreversibly adsorb at the air/water interface. The formed protein monolayers can reach a state of two-dimensional elastic solid that exhibits a high mechanical strength as compared to adsorption layers of typical amphiphilic proteins. Bubbles formed in HFBII solutions preserve the nonspherical shape they had at the moment of solidification of their surfaces. The stirring of HFBII solutions leads to the formation of many bubbles of micrometer size. Measuring the electrophoretic mobility of such bubbles, the ζ-potential was determined. Upon compression, the HFBII monolayers form periodic wrinkles of wavelength 11.5 μm, which corresponds to bending elasticity k(c) = 1.1 × 10(-19) J. The wrinkled hydrophobin monolayers are close to a tension-free state, which prevents the Ostwald ripening and provides bubble longevity in HFBII stabilized foams. Films formed between two bubbles are studied by experiments in a capillary cell. In the absence of added electrolyte, the films are electrostatically stabilized. The appearance of protein aggregates is enhanced with the increase of the HFBII and electrolyte concentrations and at pH close to the isoelectric point. When the aggregate concentration is not too high (to block the film thinning), the films reach a state with 12 nm uniform thickness, which corresponds to two surface monolayers plus HFBII tetramers sandwiched between them. In water, the HFBII molecules can stick to each other not only by their hydrophobic moieties but also by their hydrophilic parts. The latter leads to the attachment of HFBII aggregates such as dimers, tetramers, and bigger ones to the interfacial adsorption monolayers, which provides additional stabilization of the liquid films.

Creating Surface Properties Using a Palette of Hydrophobins

Small secreted proteins called hydrophobins play diverse roles in the life cycle of filamentous fungi. For example, the hydrophobin SC3 of Schizophyllum commune is involved in aerial hyphae formation, cell-wall assembly and attachment to hydrophobic surfaces. Hydrophobins are capable of self-assembly at a hydrophilic-hydrophobic interface, resulting in the formation of an amphipathic film. This amphipathic film can make hydrophobic surfaces of a liquid or a solid material wettable, while a hydrophilic surface can be turned into a hydrophobic one. These properties, among others, make hydrophobins of interest for medical and technical applications. For instance, hydrophobins can be used to purify proteins from complex mixtures; to reduce the friction of materials; to increase the biocompatibility of medical implants; to increase the solubility of water insoluble drugs; and to immobilize enzymes, for example, biosensor surfaces.

Intrinsic disorder and coiled-coil formation in prostate apoptosis response factor 4

Prostate apoptosis response factor-4 (Par-4) is an ubiquitously expressed pro-apoptotic and tumour suppressive protein that can both activate cell-death mechanisms and inhibit pro-survival factors. Par-4 contains a highly conserved coiled-coil region that serves as the primary recognition domain for a large number of binding partners. Par-4 is also tightly regulated by the aforementioned binding partners and by post-translational modifications. Biophysical data obtained in the present study indicate that Par-4 primarily comprises an intrinsically disordered protein. Bioinformatic analysis of the highly conserved Par-4 reveals low sequence complexity and enrichment in polar and charged amino acids. The high proteolytic susceptibility and an increased hydrodynamic radius are consistent with a largely extended structure in solution. Spectroscopic measurements using CD and NMR also reveal characteristic features of intrinsic disorder. Under physiological conditions, the data obtained show that Par-4 self-associates via the C-terminal domain, forming a coiled-coil. Interruption of self-association by urea also resulted in loss of secondary structure. These results are consistent with the stabilization of the coiled-coil motif through an intramolecular association.

Self-assembled films of hydrophobin proteins HFBI and HFBII studied in situ at the air/water interface

Hydrophobins are a group of surface-active fungal proteins known to adsorb to the air/water interface and self-assemble into highly crystalline films. We characterized the self-assembled protein films of two hydrophobins, HFBI and HFBII from Trichoderma reesei, directly at the air/water interface using Brewster angle microscopy, grazing-incidence X-ray diffraction, and reflectivity. Already in zero surface pressure, HFBI and HFBII self-assembled into micrometer-sized rafts containing hexagonally ordered two-dimensional crystallites with lattice constants of 55 A and 56 A, respectively. Increasing the pressure did not change the ordering of the proteins in the crystallites. According to the reflectivity measurements, the thicknesses of the hydrophobin films were 28 A (HFBI) and 24 A (HFBII) at 20 mN/m. The stable films could also be transferred to a silicon substrate. Modeling of the diffraction data indicated that both hydrophobin films contained six molecules in the unit cell, but the ordering of the molecules was somewhat different for HFBI and HFBII, suggesting specific protein-protein interactions.

Protein HGFI from the edible mushroom Grifola frondosa is a novel 8 kDa class I hydrophobin that forms rodlets in compressed monolayers

Hydrophobins are a group of low-molecular-mass, cysteine-rich proteins that have unusual biophysical properties. They are highly surface-active and can self-assemble at hydrophobic-hydrophilic interfaces, forming surface layers that are able to reverse the hydropathy of surfaces. Here we describe a novel hydrophobin from the edible mushroom Grifola frondosa, which was named HGFI and belongs to class I. The hydrophobin gene was identified during sequencing of random clones from a cDNA library, and the corresponding protein was isolated as a hot SDS-insoluble aggregate from the cell wall. The purified HGFI was found to have 83 amino acids. The protein sequence deduced from the cDNA sequence had 107 amino acids, from which a 24 aa signal sequence had been cleaved off in the mature protein. This signal sequence was 5 aa longer than had been predicted on the basis of signal peptide analysis of the cDNA. Rodlet mosaic structures were imaged using atomic force microscopy (AFM) on mica surfaces after drying-down HGFI solutions. Using Langmuir films we were also able to take images of both the hydrophobic and hydrophilic sides of films formed at the air-water interface. No distinct structure was observed in films compressed once, but in films compressed several times rodlet structures could be seen. Most rodlets were aligned in the same direction, indicating that formation of rodlets may be promoted during compression of the monolayer.

Interactions of hydrophobin proteins in solution studied by small-angle X-ray scattering

Hydrophobins are a group of very surface-active, fungal proteins known to self-assemble on various hydrophobic/hydrophilic interfaces. The self-assembled films coat fungal structures and mediate their attachment to surfaces. Hydrophobins are also soluble in water. Here, the association of hydrophobins HFBI and HFBII from Trichoderma reesei in aqueous solution was studied using small-angle x-ray scattering. Both HFBI and HFBII exist mainly as tetramers in solution in the concentration range 0.5-10 mg/ml. The assemblies of HFBII dissociate more easily than those of HFBI, which can tolerate changes of pH from 3 to 9 and temperatures in the range 5 degrees C-60 degrees C. The self-association of HFBI and HFBII is mainly driven by the hydrophobic effect, and addition of salts along the Hofmeister series promotes the formation of larger assemblies, whereas ethanol breaks the tetramers into monomers. The possibility that the oligomers in solution form the building blocks of the self-assembled film at the air/water interface is discussed.

Crystal structures of hydrophobin HFBII in the presence of detergent implicate the formation of fibrils and monolayer films

Hydrophobins are small, amphiphilic proteins secreted by filamentous fungi. Their functionality arises from a patch of hydrophobic residues on the protein surface. Spontaneous self-assembly of hydrophobins leads to the formation of an amphiphilic layer that remarkably reduces the surface tension of water. We have determined by x-ray diffraction two new crystal structures of Trichoderma reesei hydrophobin HFBII in the presence of a detergent. The monoclinic crystal structure (2.2A resolution, R = 22, R(free) = 28) is composed of layers of hydrophobin molecules where the hydrophobic surface areas of the molecules are aligned within the layer. Viewed perpendicular to the aligned hydrophobic surface areas, the molecules in the layer pack together to form six-membered rings, thus leaving small pores in the layer. Similar packing has been observed in the atomic force microscopy images of the self-assembled layers of class II hydrophobin, indicating that the crystal structure resembles that of natural hydrophobin film. The orthorhombic crystal structure (1.0 A resolution, R = 13, R(free) = 15) is composed of fiber-like arrays of protein molecules. Rodlet structures have been observed on amphiphilic layers formed by class I hydrophobins; fibrils of class II hydrophobins appear by vigorous shaking. We propose that the structure of the fibrils and/or rodlets is similar to that observed in the crystal structure.

The relation between solution association and surface activity of the hydrophobin HFBI fromTrichoderma reesei

Hydrophobins are small fungal surface active proteins that self-assemble at interfaces into films with nanoscale structures. The hydrophobin HFBI from Trichoderma reesei has been shown to associate in solution into tetramers but the role of this association on the function of HFBI has remained unclear. We produced two HFBI variants that showed a significant shift in solution association equilibrium towards the tetramer state. However, this enhanced solution association did not alter the surface properties of the variant HFBIs. The results show that there is not a strong relationship between HFBI solution association state and surface properties such as surface activity.

Two crystal structures of Trichoderma reesei hydrophobin HFBI--the structure of a protein amphiphile with and without detergent interaction

Hydrophobins are small fungal proteins that are highly surface active and possess a unique ability to form amphiphilic membranes through spontaneous self-assembly. The first crystal structure of a hydrophobin, Trichoderma reesei HFBII, revealed the structural basis for the function of this amphiphilic protein--a patch consisting of hydrophobic side chains on the protein surface. Here, the crystal structures of a native and a variant T. reesei hydrophobin HFBI are presented, revealing the same overall structure and functional hydrophobic patch as in the HFBII structure. However, some structural flexibility was found in the native HFBI structure: The asymmetric unit contained four molecules, and, in two of these, an area of seven residues was displaced as compared to the two other HFBI molecules and the previously determined HFBII structure. This structural change is most probably induced by multimer formation. Both the native and the N-Cys-variant of HFBI were crystallized in the presence of detergents, but an association between the protein and a detergent was only detected in the variant structure. There, the molecules were arranged into an extraordinary detergent-associated octamer and the solvent content of the crystals was 75%. This study highlights the conservation of the fold of class II hydrophobins in spite of the low sequence identity and supports our previous suggestion that concealment of the hydrophobic surface areas of the protein is the driving force in the formation of multimers and monolayers in the self-assembly process.

Molecular dynamics simulations of the hydrophobin SC3 at a hydrophobic/hydrophilic interface

Hydrophobins are small ( approximately 100 aa) proteins that have an important role in the growth and development of mycelial fungi. They are surface active and, after secretion by the fungi, self-assemble into amphipathic membranes at hydrophobic/hydrophilic interfaces, reversing the hydrophobicity of the surface. In this study, molecular dynamics simulation techniques have been used to model the process by which a specific class I hydrophobin, SC3, binds to a range of hydrophobic/hydrophilic interfaces. The structure of SC3 used in this investigation was modeled based on the crystal structure of the class II hydrophobin HFBII using the assumption that the disulfide pairings of the eight conserved cysteine residues are maintained. The proposed model for SC3 in aqueous solution is compact and globular containing primarily beta-strand and coil structures. The behavior of this model of SC3 was investigated at an air/water, an oil/water, and a hydrophobic solid/water interface. It was found that SC3 preferentially binds to the interfaces via the loop region between the third and fourth cysteine residues and that binding is associated with an increase in alpha-helix formation in qualitative agreement with experiment. Based on a combination of the available experiment data and the current simulation studies, we propose a possible model for SC3 self-assembly on a hydrophobic solid/water interface.