Self-assembled structures of hydrophobins HFBI and HFBII

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.

The functional role of Cys3-Cys4 loop in hydrophobin HGFI

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

Self-assembly of hydrophobin and hydrophobin/surfactant mixtures in aqueous solution

The self-assembly of the protein hydrophobin, HFBII, and its self-assembly with cationic, anionic, and nonionic surfactants hexadecylterimethyl ammonium bromide, CTAB, sodium dodecyl sulfate, SDS, and hexaethylene monododecyl ether, C(12)E(6), in aqueous solution have been studied by small-angle neutron scattering, SANS. HFBII self-assembles in solution as small globular aggregates, consistent with the formation of trimers or tetramers. Its self-assembly is not substantially affected by the pH or electrolytes. In the presence of CTAB, SDS, or C(12)E(6), HFBII/surfactant complexes are formed. The structure of the HFBII/surfactant complexes has been identified using contrast variation and is in the form of HFBII molecules bound to the outer surface of globular surfactant micelles. The binding of HFBII decreases the surfactant micelle aggregation number for increasing HFBII concentration in solution, and the number of hydrophobin molecules bound/micelle increases.

Coating of hydrophobins on three-dimensional electrospun poly(lactic-co-glycolic acid) scaffolds for cell adhesion

Surface modification with hydrophobins is very important for cell adhesion in its applications in biosensor fabrication. In this study, we modified the surface of three-dimensional electrospun poly(lactide-co-glycolide) (PLGA) scaffolds with hydrophobin HFBI and collagen, and investigated its applications for cell adhesion. We found that HFBI could not only improve the hydrophilicity of the three-dimensional electrospun PLGA scaffolds but also endow the electrospun PLGA scaffolds with water permeability. This permeability should be attributed to both the hydrophilicity of the modified PLGA surface and the large positive capillary effect induced by the microstructures. Further experiment indicated that HFBI modification could improve collagen immobilization on the electrospun PLGA scaffolds and the HFBI/collagen modified electrospun PLGA scaffolds showed higher efficiency in promoting cell adhesion than the native PLGA scaffolds. This finding should be of potential application in biosensor device fabrication.

Surface modification using a novel type I hydrophobin HGFI

Surface wettability conversion with hydrophobins is important for its applications in biodevices. In this work, the application of a type I hydrophobin HGFI in surface wettability conversion on mica, glass, and poly(dimethylsiloxane) (PDMS) was investigated. X-ray photoelectron spectroscopy (XPS) and water-contact-angle (WCA) measurements indicated that HGFI modification could efficiently change the surface wettability. Data also showed that self-assembled HGFI had better stability than type II hydrophobin HFBI. Protein patterning and the following immunoassay illustrated that surface modification with HGFI should be a feasible strategy for biosensor device fabrication.

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.

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.

Two methods for glass surface modification and their application in protein immobilization

Protein immobilization is a crucial step in protein chip, biosensor, etc. Here, two methods to immobilize proteins on glass surface were analyzed, one is silanization method using 3-aminopropyltriethoxysilane (APTES), and the other is hydrophobin HFBI coating. The modified glass surfaces were characterized with X-ray photoelectron spectroscopy (XPS), water contact angle measurement (WCA) and immunoassay. The results of XPS and WCA illustrated that the surface property of glass can be changed by both the two methods. The following immunoassay using microcontact printing (microCP) verified that both methods could help protein immobilization effectively on glass slides. Compared with the amine treatment, it is concluded that hydrophobin self-assemblies is a simple and generic way for protein immobilization on glass slides, which has potential application in protein chips and biosensors.

Structural analysis of hydrophobins

Hydrophobins are a remarkable class of small cysteine-rich proteins found exclusively in fungi. They self-assemble to form robust polymeric monolayers that are highly amphipathic and play numerous roles in fungal biology, such as in the formation and dispersal of aerial spores and in pathogenic and mutualistic interactions. The polymeric form can be reversibly disassembled and is able to reverse the wettability of a surface, leading to many proposals for nanotechnological applications over recent years. The surprising properties of hydrophobins and their potential for commercialization have led to substantial efforts to delineate their morphology and molecular structure. In this review, we summarize the progress that has been made using a variety of spectroscopic and microscopic approaches towards understanding the molecular mechanisms underlying hydrophobin structure.

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.

Surface properties of class ii hydrophobins from Trichoderma reesei and influence on bubble stability

We report the remarkable surface behavior of class II hydrophobin proteins HFBI and HFBII from Trichoderma reesei and the resulting effect that these proteins have on the stability of air bubbles to the process of disproportionation. The surface properties were studied using surface tensiometry and surface shear rheology. Surface tensiometry data show that hydrophobins are very surface active proteins, reducing the surface tension to approximately 30 mN m-1. The rate at which the hydrophobins adsorb at the surface may also be related to the self-assembly behavior in aqueous solution. We further show that hydrophobins form air/water surfaces with high elasticity, the magnitude of which is well in excess of that of surface layers formed by other common proteins used as foam or emulsion stabilizers. The measured surface properties translate to the stability of bubbles with adsorbed hydrophobin, and in this study, we demonstrate the ability of hydrophobin to have a dramatic effect on the rate of disproportionation in some simple bubble dissolution studies.

Bioactive surface modification of mica and poly(dimethylsiloxane) with hydrophobins for protein immobilization

Bioactive surfaces with appropriate hydrophilicity for protein immobilization can be achieved by hydrophobin II (HFBI) self-assembly on mica and polydimethylsiloxane (PDMS) surfaces. X-ray photoelectron spectroscopy and water contact angle measurements illustrated that the surface wettability can be changed from superhydrophobic (PDMS) or superhydrophilic (mica) to moderately hydrophilic, which is suitable for protein (chicken IgG) immobilization on both substrate surfaces. The results suggest that HFBI assembly, one kind of hydrophobin from Trichoderma reesei, may be a versatile and convenient method for the immobilization of biomolecules on diverse substrates, which may have potential applications in biosensors, immunoassays, and microfluidic networks.

Interaction and Comparison of a Class I Hydrophobin from Schizophyllum commune and Class II Hydrophobins from Trichoderma reesei

Hydrophobins fulfill a wide spectrum of functions in fungal growth and development. These proteins self-assemble at hydrophilic-hydrophobic interfaces into amphipathic membranes. Hydrophobins are divided into two classes based on their hydropathy patterns and solubility. We show here that the properties of the class II hydrophobins HFBI and HFBII of Trichoderma reesei differ from those of the class I hydrophobin SC3 of Schizophyllum commune. In contrast to SC3, self-assembly of HFBI and HFBII at the water-air interface was neither accompanied by a change in secondary structure nor by a change in ultrastructure. Moreover, maximal lowering of the water surface tension was obtained instantly or took several minutes in the case of HFBII and HFBI, respectively. In contrast, it took several hours in the case of SC3. Oil emulsions prepared with HFBI and SC3 were more stable than those of HFBII, and HFBI and SC3 also interacted more strongly with the hydrophobic Teflon surface making it wettable. Yet, the HFBI coating did not resist treatment with hot detergent, while that of SC3 remained unaffected. Interaction of all the hydrophobins with Teflon was accompanied with a change in the circular dichroism spectra, indicating the formation of an alpha-helical structure. HFBI and HFBII did not affect self-assembly of the class I hydrophobin SC3 of S. commune and vice versa. However, precipitation of SC3 was reduced by the class II hydrophobins, indicating interaction between the assemblies of both classes of hydrophobins.

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

Hydrophobins are proteins specific to filamentous fungi. Hydrophobins have several important roles in fungal physiology, for example, adhesion, formation of protective surface coatings, and the reduction of the surface tension of water, which allows growth of aerial structures. Hydrophobins show remarkable biophysical properties, for example, they are the most powerful surface-active proteins known. To this point the molecular basis of the function of this group of proteins has been largely unknown. We have now determined the crystal structure of the hydrophobin HFBII from Trichoderma reesei at 1.0 A resolution. HFBII has a novel, compact single domain structure containing one alpha-helix and four antiparallel beta-strands that completely envelop two disulfide bridges. The protein surface is mainly hydrophilic, but two beta-hairpin loops contain several conserved aliphatic side chains that form a flat hydrophobic patch that makes the molecule amphiphilic. The amphiphilicity of the HFBII molecule is expected to be a source for surface activity, and we suggest that the behavior of this surfactant is greatly enhanced by the self-assembly that is favored by the combination of size and rigidity. This mechanism of function is supported by atomic force micrographs that show highly ordered arrays of HFBII at the air water interface. The data presented show that much of the current views on structure function relations in hydrophobins must be re-evaluated.