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1.  Induced Fit in Protein Multimerization: The HFBI Case 
PLoS Computational Biology  2016;12(11):e1005202.
Hydrophobins, produced by filamentous fungi, are small amphipathic proteins whose biological functions rely on their unique surface-activity properties. Understanding the mechanistic details of the multimerization process is of primary importance to clarify the interfacial activity of hydrophobins. We used free energy calculations to study the role of a flexible β-hairpin in the multimerization process in hydrophobin II from Trichoderma reesei (HFBI). We characterized how the displacement of this β-hairpin controls the stability of the monomers/dimers/tetramers in solution. The regulation of the oligomerization equilibrium of HFBI will necessarily affect its interfacial properties, fundamental for its biological function and for technological applications. Moreover, we propose possible routes for the multimerization process of HFBI in solution. This is the first case where a mechanism by which a flexible loop flanking a rigid patch controls the protein-protein binding equilibrium, already known for proteins with charged binding hot-spots, is described within a hydrophobic patch.
Author Summary
Fungi proliferate by creating a complex hyphal network growing within a wet environment. However, for most fungi to colonize new territories, they must produce spores carried by aerial hyphae and spread them into the air. Aerial structures need to overcome the surface tension of the surrounding water in order to grow into the air. This process requires hydrophobins, a remarkable class of self-associating fungal proteins which lower the surface tension at the air/water interface by creating a thin amphipathic layer. In solution they form multimers in equilibrium with the interfacial layer. Due to their unique surface-activity properties, hydrophobins have been used for a variety of biotechnical applications. We used enhanced sampling molecular dynamics simulations methods to study the multimerization process in solution of a hydrophobin from Trichoderma reesei (HFBI). We clarified the fundamental role of a small flexible region within the HFBI monomer involved in the formation of multimers. A flexible loop flanking a rigid interaction patch is able to fine-tune the interaction energy. This mechanism, already known for charged binding patches, is described here for hydrophobic hot-spots. This result is remarkably important in order to clarify the mechanism of arranging at the interface and enhancing hydrophobin-based technological applications.
doi:10.1371/journal.pcbi.1005202
PMCID: PMC5104427  PMID: 27832079
2.  Hydrophobin Film Structure for HFBI and HFBII and Mechanism for Accelerated Film Formation 
PLoS Computational Biology  2014;10(7):e1003745.
Hydrophobins represent an important group of proteins from both a biological and nanotechnological standpoint. They are the means through which filamentous fungi affect their environment to promote growth, and their properties at interfaces have resulted in numerous applications. In our study we have combined protein docking, molecular dynamics simulation, and electron cryo-microscopy to gain atomistic level insight into the surface structure of films composed of two class II hydrophobins: HFBI and HFBII produced by Trichoderma reesei. Together our results suggest a unit cell composed of six proteins; however, our computational results suggest P6 symmetry, while our experimental results show P3 symmetry with a unit cell size of 56 Å. Our computational results indicate the possibility of an alternate ordering with a three protein unit cell with P3 symmetry and a smaller unit cell size, and we have used a Monte Carlo simulation of a spin model representing the hydrophobin film to show how this alternate metastable structure may play a role in increasing the rate of surface coverage by hydrophobin films, possibly indicating a mechanism of more general significance to both biology and nanotechnology.
Author Summary
Filamentous fungi release a specific type of protein, belonging to a protein family known as “hydrophobins” into their environment to control interfaces in a fashion that promotes growth. Such protein coatings are the mechanism that allows for the mycelia to grow out of the water and into the air. When these hydrophobins form films at the air-water interface and on the surface of solid objects immersed in water, they impart properties to those surfaces that has led to their use in a wide range of industrial applications. Of particular interest is the properties they impart to air liquid interfaces, and as a mechanism to bring protective materials to coat nanoparticles in nanotechnology applications. A more detailed knowledge of the structure of these surfaces will allow for augmentation of their function that is possible through genetic engineering of the hydrophobins themselves. In this study we have combined computational and experimental methods to develop atomistic level insight into the structure of this surface for two important hydrophobins: HFBI and HFBII of Trichoderma reesei. In addition to insight into the surface structure, we have uncovered an intriguing possible new mechanism for film formation, which may explain some of the striking properties of hydrophobin films, and could be extended to a more general mechanism.
doi:10.1371/journal.pcbi.1003745
PMCID: PMC4117420  PMID: 25079355
3.  Expression of a Fungal Hydrophobin in the Saccharomyces cerevisiae Cell Wall: Effect on Cell Surface Properties and Immobilization 
The aim of this work was to modify the cell surface properties of Saccharomyces cerevisiae by expression of the HFBI hydrophobin of the filamentous fungus Trichoderma reesei on the yeast cell surface. The second aim was to study the immobilization capacity of the modified cells. Fusion to the Flo1p flocculin was used to target the HFBI moiety to the cell wall. Determination of cell surface characteristics with contact angle and zeta potential measurements indicated that HFBI-producing cells are more apolar and slightly less negatively charged than the parent cells. Adsorption of the yeast cells to different commercial supports was studied. A twofold increase in the binding affinity of the hydrophobin-producing yeast to hydrophobic silicone-based materials was observed, while no improvement in the interaction with hydrophilic carriers could be seen compared to that of the parent cells. Hydrophobic interactions between the yeast cells and the support are suggested to play a major role in attachment. Also, a slight increase in the initial adsorption rate of the hydrophobin yeast was observed. Furthermore, due to the engineered cell surface, hydrophobin-producing yeast cells were efficiently separated in an aqueous two-phase system by using a nonionic polyoxyethylene detergent, C12-18EO5.
doi:10.1128/AEM.68.7.3385-3391.2002
PMCID: PMC126783  PMID: 12089019
4.  Structure-Function Relationships in Hydrophobins: Probing the Role of Charged Side Chains 
Applied and Environmental Microbiology  2013;79(18):5533-5538.
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.
doi:10.1128/AEM.01493-13
PMCID: PMC3754166  PMID: 23835172
5.  Fusion to Hydrophobin HFBI Improves the Catalytic Performance of a Cytochrome P450 System 
Cytochrome P450 monooxygenases (P450) are heme-containing enzymes that oxidize a broad range of substrates in the presence of molecular oxygen and NAD(P)H. For their activity, most P450s rely on one or two redox proteins responsible for the transfer of electrons from the cofactor NAD(P)H to the heme. One of the challenges when using P450s in vitro, especially when non-physiological redox proteins are applied, is the inefficient transfer of electrons between the individual proteins resulting in non-productive consumption of NAD(P)H – referred to as uncoupling. Herein, we describe the improvement of the coupling efficiency between a P450 and its redox partner – diflavin reductase – by fusing both enzymes individually to the hydrophobin HFBI – a small self-assembling protein of the fungus Trichoderma reesei. The separated monooxygenase (BMO) and reductase (BMR) domains of P450 BM3 from Bacillus megaterium were chosen as a P450-reductase model system and individually fused to HFBI. The fusion proteins could be expressed in soluble form in Escherichia coli. When HFBI-fused BMO and BMR were mixed in vitro, substantially higher coupling efficiencies were measured as compared with the respective non-fused enzymes. Consequently, myristic acid conversion increased up to 20-fold (after 6 h) and 5-fold (after 24 h). Size exclusion chromatography demonstrated that in vitro the hydrophobin-fused enzymes build multimeric protein assemblies. Thus, the higher activity is hypothesized to be due to HFBI-mediated self-assembly arranging BMO and BMR in close spatial proximity in aqueous solution.
doi:10.3389/fbioe.2016.00057
PMCID: PMC4930934  PMID: 27458582
cytochrome P450 monooxygenase; P450 BM3; CYP102A1; diflavin reductase; fusion protein; hydrophobin; HFBI
6.  Novel Hydrophobin Fusion Tags for Plant-Produced Fusion Proteins 
PLoS ONE  2016;11(10):e0164032.
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.
doi:10.1371/journal.pone.0164032
PMCID: PMC5051927  PMID: 27706254
7.  Protein body formation in stable transgenic tobacco expressing elastin-like polypeptide and hydrophobin fusion proteins 
BMC Biotechnology  2013;13:40.
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.
doi:10.1186/1472-6750-13-40
PMCID: PMC3659085  PMID: 23663656
Protein body; Protein body formation; Elastin-like polypeptide; ELP; Hydrophobin I; HFBI; Tobacco; Transgenic expression; Molecular farming; Green fluorescent protein
8.  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.
doi:10.3389/fpls.2016.00693
PMCID: PMC4876836  PMID: 27242885
protein body; protein body formation; protein trafficking; zera; elastin-like polypeptides (ELP); hydrophobin (HFBI); Nicotiana benthamiana; confocal microscopy
9.  The Contribution of Polystyrene Nanospheres towards the Crystallization of Proteins 
PLoS ONE  2009;4(1):e4198.
Background
Protein crystallization is a slow process of trial and error and limits the amount of solved protein structures. Search of a universal heterogeneous nucleant is an effort to facilitate crystallizability of proteins.
Methodology
The effect of polystyrene nanospheres on protein crystallization were tested with three commercial proteins: lysozyme, xylanase, xylose isomerase, and with five research target proteins: hydrophobins HFBI and HFBII, laccase, sarcosine dimethylglycine N-methyltransferase (SDMT), and anti-testosterone Fab fragment 5F2. The use of nanospheres both in screening and as an additive for known crystallization conditions was studied. In screening, the addition of an aqueous solution of nanosphere to the crystallization drop had a significant positive effect on crystallization success in comparison to the control screen. As an additive in hydrophobin crystallization, the nanospheres altered the crystal packing, most likely due to the amphiphilic nature of hydrophobins. In the case of laccase, nanospheres could be used as an alternative for streak-seeding, which insofar had remained the only technique to produce high-diffracting crystals. With methyltransferase SDMT the nanospheres, used also as an additive, produced fewer, larger crystals in less time. Nanospheres, combined with the streak-seeding method, produced single 5F2 Fab crystals in shorter equilibration times.
Conclusions
All in all, the use of nanospheres in protein crystallization proved to be beneficial, both when screening new crystallization conditions to promote nucleation and when used as an additive to produce better quality crystals, faster. The polystyrene nanospheres are easy to use, commercially available and close to being inert, as even with amphiphilic proteins only the crystal packing is altered and the nanospheres do not interfere with the structure and function of the protein.
doi:10.1371/journal.pone.0004198
PMCID: PMC2615210  PMID: 19145241
10.  Modification of Hydrophilic and Hydrophobic Surfaces Using an Ionic-Complementary Peptide 
PLoS ONE  2007;2(12):e1325.
Ionic-complementary peptides are novel nano-biomaterials with a variety of biomedical applications including potential biosurface engineering. This study presents evidence that a model ionic-complementary peptide EAK16-II is capable of assembling/coating on hydrophilic mica as well as hydrophobic highly ordered pyrolytic graphite (HOPG) surfaces with different nano-patterns. EAK16-II forms randomly oriented nanofibers or nanofiber networks on mica, while ordered nanofibers parallel or oriented 60° or 120° to each other on HOPG, reflecting the crystallographic symmetry of graphite (0001). The density of coated nanofibers on both surfaces can be controlled by adjusting the peptide concentration and the contact time of the peptide solution with the surface. The coated EAK16-II nanofibers alter the wettability of the two surfaces differently: the water contact angle of bare mica surface is measured to be <10°, while it increases to 20.3±2.9° upon 2 h modification of the surface using a 29 µM EAK16-II solution. In contrast, the water contact angle decreases significantly from 71.2±11.1° to 39.4±4.3° after the HOPG surface is coated with a 29 µM peptide solution for 2 h. The stability of the EAK16-II nanofibers on both surfaces is further evaluated by immersing the surface into acidic and basic solutions and analyzing the changes in the nanofiber surface coverage. The EAK16-II nanofibers on mica remain stable in acidic solution but not in alkaline solution, while they are stable on the HOPG surface regardless of the solution pH. This work demonstrates the possibility of using self-assembling peptides for surface modification applications.
doi:10.1371/journal.pone.0001325
PMCID: PMC2117347  PMID: 18091996
11.  Surface-water Interface Induces Conformational Changes Critical for Protein Adsorption: Implications for Monolayer Formation of EAS Hydrophobin 
The class I hydrophobin EAS is part of a family of small, amphiphilic fungal proteins best known for their ability to self-assemble into stable monolayers that modify the hydrophobicity of a surface to facilitate further microbial growth. These proteins have attracted increasing attention for industrial and biomedical applications, with the aim of designing surfaces that have the potential to maintain their clean state by resisting non-specific protein binding. To gain a better understanding of this process, we have employed all-atom molecular dynamics to study initial stages of the spontaneous adsorption of monomeric EAS hydrophobin on fully hydroxylated silica, a commonly used industrial and biomedical substrate. Particular interest has been paid to the Cys3-Cys4 loop, which has been shown to exhibit disruptive behavior in solution, and the Cys7-Cys8 loop, which is believed to be involved in the aggregation of EAS hydrophobin at interfaces. Specific and water mediated interactions with the surface were also analyzed. We have identified two possible binding motifs, one which allows unfolding of the Cys7-Cys8 loop due to the surfactant-like behavior of the Cys3-Cys4 loop, and another which has limited unfolding due to the Cys3-Cys4 loop remaining disordered in solution. We have also identified intermittent interactions with water which mediate the protein adsorption to the surface, as well as longer lasting interactions which control the diffusion of water around the adsorption site. These results have shown that EAS behaves in a similar way at the air-water and surface-water interfaces, and have also highlighted the need for hydrophilic ligand functionalization of the silica surface in order to prevent the adsorption of EAS hydrophobin.
doi:10.3389/fmolb.2015.00064
PMCID: PMC4644811  PMID: 26636091
molecular dynamics; hydrophobin; anti-fouling; protein adsorption; silica surface; biofouling
12.  Purifying selection and birth-and-death evolution in the class II hydrophobin gene families of the ascomycete Trichoderma/Hypocrea 
Background
Hydrophobins are proteins containing eight conserved cysteine residues that occur uniquely in mycelial fungi. Their main function is to confer hydrophobicity to fungal surfaces in contact with air or during attachment of hyphae to hydrophobic surfaces of hosts, symbiotic partners or themselves resulting in morphogenetic signals. Based on their hydropathy patterns and solubility characteristics, hydrophobins are divided into two classes (I and II), the latter being found only in ascomycetes.
Results
We have investigated the mechanisms driving the evolution of the class II hydrophobins in nine species of the mycoparasitic ascomycetous genus Trichoderma/Hypocrea, using three draft sequenced genomes (H. jecorina = T. reesei, H. atroviridis = T. atroviride; H. virens = T. virens) an additional 14,000 ESTs from six other Trichoderma spp. (T. asperellum, H. lixii = T. harzianum, T. aggressivum var. europeae, T. longibrachiatum, T. cf. viride). The former three contained six, ten and nine members, respectively. Ten is the highest number found in any ascomycete so far. All the hydrophobins we examined had the conserved four beta-strands/one helix structure, which is stabilized by four disulfide bonds. In addition, a small number of these hydrophobins (HFBs)contained an extended N-terminus rich in either proline and aspartate, or glycine-asparagine. Phylogenetic analysis reveals a mosaic of terminal clades containing duplicated genes and shows only three reasonably supported clades. Calculation of the ratio of differences in synonymous vs. non-synonymous nucleotide substitutions provides evidence for strong purifying selection (KS/Ka >> 1). A genome database search for class II HFBs from other ascomycetes retrieved a much smaller number of hydrophobins (2–4) from each species, and most were from Sordariomycetes. A combined phylogeny of these sequences with those of Trichoderma showed that the Trichoderma HFBs mostly formed their own clades, whereas those of other Sordariomycetes occurred in shared clades.
Conclusion
Our study shows that the genus Trichoderma/Hypocrea has a proliferated arsenal of class II hydrophobins which arose by birth-and-death evolution followed by purifying selection.
doi:10.1186/1471-2148-8-4
PMCID: PMC2253510  PMID: 18186925
13.  Lack of evidence for a role of hydrophobins in conferring surface hydrophobicity to conidia and hyphae of Botrytis cinerea 
BMC Microbiology  2011;11:10.
Background
Hydrophobins are small, cysteine rich, surface active proteins secreted by filamentous fungi, forming hydrophobic layers on the walls of aerial mycelia and spores. Hydrophobin mutants in a variety of fungi have been described to show 'easily wettable' phenotypes, indicating that hydrophobins play a general role in conferring surface hydrophobicity to aerial hyphae and spores.
Results
In the genome of the grey mould fungus Botrytis cinerea, genes encoding three hydrophobins and six hydrophobin-like proteins were identified. Expression analyses revealed low or no expression of these genes in conidia, while some of them showed increased or specific expression in other stages, such as sclerotia or fruiting bodies. Bhp1 belongs to the class I hydrophobins, whereas Bhp2 and Bhp3 are members of hydrophobin class II. Single, double and triple hydrophobin knock-out mutants were constructed by consecutively deleting bhp1, bhp2 and bhp3. In addition, a mutant in the hydrophobin-like gene bhl1 was generated. The mutants were tested for germination and growth under different conditions, formation of sclerotia, ability to penetrate and infect host tissue, and for spore and mycelium surface properties. Surprisingly, none of the B. cinerea hydrophobin mutants showed obvious phenotypic defects in any of these characters. Scanning electron microscopy of the hydrophobic conidial surfaces did not reveal evidence for the presence of typical hydrophobin 'rodlet' layers.
Conclusions
These data provide evidence that in B. cinerea, hydrophobins are not involved in conferring surface hydrophobicity to conidia and aerial hyphae, and challenge their universal role in filamentous fungi. The function of some of these proteins in sclerotia and fruiting bodies remains to be investigated.
doi:10.1186/1471-2180-11-10
PMCID: PMC3032640  PMID: 21232149
14.  Controlling the dispersion of supported polyoxometalate heterogeneous catalysts: impact of hybridization and the role of hydrophilicity–hydrophobicity balance and supramolecularity 
Summary
The hybridization of polyoxometalates (POMs) through an organic–inorganic association offers several processing advantages in the design of heterogeneous catalysts. A clear understanding of the organization of these hybrid materials on solid surfaces is necessary to optimise their properties. Herein, we report for the first time the organization of Keggin phosphotungstic [PW12O40]3− and Wells–Dawson (WD) phosphomolybdic [P2Mo18O62]6− anions deposited on mica (hydrophilic), and highly oriented pyrolytic graphite (HOPG) (hydrophobic) surfaces. Next, the supramolecular organization of the organic–inorganic hybrid materials formed from the association of POM anions and dimethyldioctadecylammonium bromide (DODA) is investigated as a function of the hydrophilic or hydrophobic nature of the surfaces. The height of the Keggin-POM anions, measured with tapping mode (TM-AFM) is always in good agreement with the molecular dimension of symmetric Keggin-POM anions (ca. 1 nm). However, the asymmetric WD-POM anions form monolayer assemblies on the surfaces with the orientation of their long molecular axis (ca. 1.6 nm) depending on the hydrophilic or hydrophobic properties of the substrate. Namely, the long axis is parallel on mica, and perpendicular on HOPG. When hybridized with DODA, the organization of the hybrid material is dictated by the interaction of the alkyl side chains of DODA with the substrate surface. On HOPG, the DODA–POM hybrid forms small domains of epitaxially arranged straight nanorod structures with their orientation parallel to each other. Conversely, randomly distributed nanospheres are formed when the hybrid material is deposited on freshly cleaved mica. Finally, a UV–ozone treatment of the hybrid material allows one to obtain highly dispersed isolated POM entities on both hydrophilic and hydrophobic surfaces. The hybridization strategy to prevent the clustering of POMs on various supports would enable to develop highly dispersed POM-based heterogeneous catalysts with enhanced functionalities.
doi:10.3762/bjnano.5.185
PMCID: PMC4222433  PMID: 25383286
atomic force microscopy; heterogeneous hybrid catalyst; organic–inorganic hybrid materials; polyoxometalates; supramolecular organization
15.  Investigating the Co-Adsorption Behavior of Nucleic-Acid Base (Thymine and Cytosine) and Melamine at Liquid/Solid Interface 
The co-adsorption behavior of nucleic-acid base (thymine; cytosine) and melamine was investigated by scanning tunneling microscopy (STM) technique at liquid/solid (1-octanol/graphite) interface. STM characterization results indicate that phase separation happened after dropping the mixed solution of thymine-melamine onto highly oriented pyrolytic graphite (HOPG) surface, while the hetero-component cluster-like structure was observed when cytosine-melamine binary assembly system is used. From the viewpoints of non-covalent interactions calculated by using density functional theory (DFT) method, the formation mechanisms of these assembled structures were explored in detail. This work will supply a methodology to design the supramolecular assembled structures and the hetero-component materials composed by biological and chemical compound.
Electronic supplementary material
The online version of this article (doi:10.1186/s11671-016-1767-0) contains supplementary material, which is available to authorized users.
doi:10.1186/s11671-016-1767-0
PMCID: PMC5174008  PMID: 28000170
Binary self-assembly; Hetero-compound; Hydrogen binding; Intermolecular recognition
16.  Enhanced Cutinase-Catalyzed Hydrolysis of Polyethylene Terephthalate by Covalent Fusion to Hydrophobins 
Applied and Environmental Microbiology  2015;81(11):3586-3592.
Cutinases have shown potential for hydrolysis of the recalcitrant synthetic polymer polyethylene terephthalate (PET). We have shown previously that the rate of this hydrolysis can be enhanced by the addition of hydrophobins, small fungal proteins that can alter the physicochemical properties of surfaces. Here we have investigated whether the PET-hydrolyzing activity of a bacterial cutinase from Thermobifida cellulosilytica (Thc_Cut1) would be further enhanced by fusion to one of three Trichoderma hydrophobins, i.e., the class II hydrophobins HFB4 and HFB7 and the pseudo-class I hydrophobin HFB9b. The fusion enzymes exhibited decreased kcat values on soluble substrates (p-nitrophenyl acetate and p-nitrophenyl butyrate) and strongly decreased the hydrophilicity of glass but caused only small changes in the hydrophobicity of PET. When the enzyme was fused to HFB4 or HFB7, the hydrolysis of PET was enhanced >16-fold over the level with the free enzyme, while a mixture of the enzyme and the hydrophobins led only to a 4-fold increase at most. Fusion with the non-class II hydrophobin HFB9b did not increase the rate of hydrolysis over that of the enzyme-hydrophobin mixture, but HFB9b performed best when PET was preincubated with the hydrophobins before enzyme treatment. The pattern of hydrolysis by the fusion enzymes differed from that of Thc_Cut1 as the concentration of the product mono(2-hydroxyethyl) terephthalate relative to that of the main product, terephthalic acid, increased. Small-angle X-ray scattering (SAXS) analysis revealed an increased scattering contrast of the fusion proteins over that of the free proteins, suggesting a change in conformation or enhanced protein aggregation. Our data show that the level of hydrolysis of PET by cutinase can be significantly increased by fusion to hydrophobins. The data further suggest that this likely involves binding of the hydrophobins to the cutinase and changes in the conformation of its active center.
doi:10.1128/AEM.04111-14
PMCID: PMC4421044  PMID: 25795674
17.  Cerato-platanins: a fungal protein family with intriguing properties and application potential 
Applied Microbiology and Biotechnology  2014;98(11):4795-4803.
Cerato-platanin proteins are small, secreted proteins with four conserved cysteines that are abundantly produced by filamentous fungi with all types of lifestyles. These proteins appear to be readily recognized by other organisms and are therefore important factors in interactions of fungi with other organisms, e.g. by stimulating the induction of defence responses in plants. However, it is not known yet whether the main function of cerato-platanin proteins is associated with these fungal interactions or rather a role in fungal growth and development. Cerato-platanin proteins seem to unify several biochemical properties that are not found in this combination in other proteins. On one hand, cerato-platanins are carbohydrate-binding proteins and are able to bind to chitin and N-acetylglucosamine oligosaccharides; on the other hand, they are able to self-assemble at hydrophobic/hydrophilic interfaces and form protein layers, e.g. on the surface of aqueous solutions, thereby altering the polarity of solutions and surfaces. The latter property is reminiscent of hydrophobins, which are also small, secreted fungal proteins, but interestingly, the surface-activity-altering properties of cerato-platanins are the opposite of what can be observed for hydrophobins. The so far known biochemical properties of cerato-platanin proteins are summarized in this review, and potential biotechnological applications as well as implications of these properties for the biological functions of cerato-platanin proteins are discussed.
doi:10.1007/s00253-014-5690-y
PMCID: PMC4024134  PMID: 24687753
Cerato-platanin; Chitin; Self-assembly; Protein layer; Hydrophobin; Expansin
18.  Direct observation of photocarrier electron dynamics in C60 films on graphite by time-resolved two-photon photoemission 
Scientific Reports  2016;6:35853.
Time-resolved two-photon photoemission (TR-2PPE) spectroscopy is employed to probe the electronic states of a C60 fullerene film formed on highly oriented pyrolytic graphite (HOPG), acting as a model two-dimensional (2D) material for multi-layered graphene. Owing to the in-plane sp2-hybridized nature of the HOPG, the TR-2PPE spectra reveal the energetics and dynamics of photocarriers in the C60 film: after hot excitons are nascently formed in C60 via intramolecular excitation by a pump photon, they dissociate into photocarriers of free electrons and the corresponding holes, and the electrons are subsequently detected by a probe photon as photoelectrons. The decay rate of photocarriers from the C60 film into the HOPG is evaluated to be 1.31 × 1012 s−1, suggesting a weak van der Waals interaction at the interface, where the photocarriers tentatively occupy the lowest unoccupied molecular orbital (LUMO) of C60. The photocarrier electron dynamics following the hot exciton dissociation in the organic thin films has not been realized for any metallic substrates exhibiting strong interactions with the overlayer. Furthermore, the thickness dependence of the electron lifetime in the LUMO reveals that the electron hopping rate in C60 layers is 3.3 ± 1.2 × 1013 s−1.
doi:10.1038/srep35853
PMCID: PMC5075791  PMID: 27775005
19.  Identification of new members of hydrophobin family using primary structure analysis 
BMC Bioinformatics  2006;7(Suppl 4):S16.
Background
Hydrophobins are fungal proteins that can turn into amphipathic membranes at hydrophilic/hydrophobic interfaces by self-assembly. The assemblages by Class I hydrophobins are extremely stable and possess the remarkable ability to change the polarity of the surface. One of its most important industrial applications is its usage as paint. Without detailed knowledge of the 3D structure and self-assembly principles of hydrophobins, it is difficult to make significant progress in furthering its research.
Results
In order to provide useful information to hydrophobin researchers, we analyzed primary structure of hydrophobins to gain more insight about these proteins. In this paper, we presented an in-depth primary sequence analysis using batch BLAST search of the database, sequence filtering by programming and motif finding by MEME. We used batch BLAST to find similar sequences in the NCBI nr database. Then we used MEME to find out motifs. Based on the newly found motifs and the well-known C-CC-C-C-CC-C pattern we used MAST to search the entire nr database. At the end, domain search and phylogenetic analysis were conducted to confirm the result. After searching the nr database with the new PSSM-format motifs identified by MEME, many sequences from various species were found by MAST. Filtering process by pattern, domain and length left 9 qualified candidates.
Conclusion
All of 9 newly identified potential hydrophobins possess the common pattern and hydrophobin domain. From the multiple sequence alignment result, we can see that some of them are grouped very close to other known hydrophobins, which means their phylogenetic relationship is very close and it is highly plausible that they are indeed hydrophobin proteins.
doi:10.1186/1471-2105-7-S4-S16
PMCID: PMC1780129  PMID: 17217508
20.  Production and characterization of in planta transiently produced polygalacturanase from Aspergillus niger and its fusions with hydrophobin or ELP tags 
BMC Biotechnology  2014;14:59.
Background
Pectinases play an important role in plant cell wall deconstruction and have potential in diverse industries such as food, wine, animal feed, textile, paper, fuel, and others. The demand for such enzymes is increasing exponentially, as are the efforts to improve their production and to implement their use in several industrial processes. The goal of this study was to examine the potential of producing polygalacturonase I from Aspergillus niger in plants and to investigate the effects of subcellular compartmentalization and protein fusions on its accumulation and activity.
Results
Polygalacturonase I from Aspergillus niger (AnPGI) was transiently produced in Nicotiana benthamiana by targeting it to five different cellular compartments: apoplast, endoplasmic reticulum (ER), vacuole, chloroplast and cytosol. Accumulation levels of 2.5%, 3.0%, and 1.9% of total soluble protein (TSP) were observed in the apoplast, ER, and vacuole, respectively, and specific activity was significantly higher in vacuole-targeted AnPGI compared to the same enzyme targeted to the ER or apoplast. No accumulation was found for AnPGI when targeted to the chloroplast or cytosol. Analysis of AnPGI fused with elastin-like polypeptide (ELP) revealed a significant increase in the protein accumulation level, especially when targeted to the vacuole where the protein doubles its accumulation to 3.6% of TSP, while the hydrophobin (HFBI) fusion impaired AnPGI accumulation and both tags impaired activity, albeit to different extents. The recombinant protein showed activity against polygalacturonic acid with optimum conditions at pH 5.0 and temperature from 30 to 50°C, depending on its fusion. In vivo analysis of reducing sugar content revealed a higher release of reducing sugars in plant tissue expressing recombinant AnPGI compared to wild type N. benthamiana leaves.
Conclusion
Our results demonstrate that subcellular compartmentalization of enzymes has an impact on both the target protein accumulation and its activity, especially in the case of proteins that undergo post-translational modifications, and should be taken into consideration when protein production strategies are designed. Using plants to produce heterologous enzymes for the degradation of a key component of the plant cell wall could reduce the cost of biomass pretreatment for the production of cellulosic biofuels.
doi:10.1186/1472-6750-14-59
PMCID: PMC4083859  PMID: 24970673
Polygalacturonase; Aspergillus niger; Nicotiana benthamiana; Elastin-like polypeptide; Hydrophobin I; Cellulosic biofuels
21.  Thermally-nucleated self-assembly of water and alcohol into stable structures at hydrophobic interfaces 
Nature Communications  2016;7:13064.
At the interface with solids, the mobility of liquid molecules tends to be reduced compared with bulk, often resulting in increased local order due to interactions with the surface of the solid. At room temperature, liquids such as water and methanol can form solvation structures, but the molecules remain highly mobile, thus preventing the formation of long-lived supramolecular assemblies. Here we show that mixtures of water with methanol can form a novel type of interfaces with hydrophobic solids. Combining in situ atomic force microscopy and multiscale molecular dynamics simulations, we identify solid-like two-dimensional interfacial structures that nucleate thermally, and are held together by an extended network of hydrogen bonds. On graphite, nucleation occurs above ∼35 °C, resulting in robust, multilayered nanoscopic patterns. Our findings could have an impact on many fields where water-alcohol mixtures play an important role such as fuel cells, chemical synthesis, self-assembly, catalysis and surface treatments.
Alcohol-water mixtures are characterized by the existence of segregated clusters, whose dynamics are too fast to be investigated in bulk solution. Here, Voïtchovsky et al. show the formation of stable two-dimensional water-alcohol wire-like structures via H-bonds on graphite surface at room temperature.
doi:10.1038/ncomms13064
PMCID: PMC5059760  PMID: 27713413
22.  Interface Properties of Organic para-Hexaphenyl/α-Sexithiophene Heterostructures Deposited on Highly Oriented Pyrolytic Graphite 
Langmuir  2013;29(47):14444-14450.
It was recently reported, that heterostructures of para-hexaphenyl (p-6P) and α-sexithiophene (6T) deposited on muscovite mica exhibit the intriguing possibility to prepare lasing nanofibers of tunable emission wavelength. For p-6P/6T heterostructures, two different types of 6T emission have been observed, namely, the well-known red emission of bulk 6T crystals and additionally a green emission connected to the interface between p-6P and 6T. In this study, the origin of the green fluorescence is investigated by photoelectron spectroscopy (PES). As a prerequisite, it is necessary to prepare structurally similar organic crystals on a conductive surface, which leads to the choice of highly oriented pyrolytic graphite (HOPG) as a substrate. The similarity between p-6P/6T heterostructures on muscovite mica and on HOPG is evidenced by X-ray diffraction (XRD), scanning force microscopy (SFM), and optical spectroscopy. PES measurements show that the interface between p-6P and 6T crystals is sharp on a molecular level without any sign of interface dipole formation or chemical interaction between the molecules. We therefore conclude that the different emission colors of the two 6T phases are caused by different types of molecular aggregation.
doi:10.1021/la402242b
PMCID: PMC3842851  PMID: 24156627
23.  Hydrophobin Vmh2–glucose complexes self-assemble in nanometric biofilms 
Hydrophobins are small proteins secreted by fungi, which self-assemble into amphipathic membranes at air–liquid or liquid–solid interfaces. The physical and chemical properties of some hydrophobins, both in solution and as a biofilm, are affected by poly or oligosaccharides. We have studied the interaction between glucose and the hydrophobin Vmh2 from Pleurotus ostreatus by spectroscopic ellipsometry (SE), atomic force microscopy (AFM) and water contact angle (WCA). We have found that Vmh2–glucose complexes forms a chemically stable biofilm, obtained by drop deposition on silicon, 1.6 nm thick and containing 35 per cent of glucose, quantified by SE. AFM highlighted the presence of nanometric rodlet-like aggregates (average height, width and length being equal to 3.6, 23.8 and 40 nm, respectively) on the biofilm surface, slightly different from those obtained in the absence of glucose (4.11, 23.9 and 64 nm). The wettability of a silicon surface, covered by the organic layer of Vmh2–glucose, strongly changed: WCA decreased from 90° down to 17°.
doi:10.1098/rsif.2012.0217
PMCID: PMC3427517  PMID: 22572031
surface modification; protein biofilm; spectroscopic ellipsometry; atomic force microscopy
24.  Surface Modifications Created by Using Engineered Hydrophobins 
Hydrophobins are small (ca. 100 amino acids) secreted fungal proteins that are characterized by the presence of eight conserved cysteine residues and by a typical hydropathy pattern. Class I hydrophobins self-assemble at hydrophilic-hydrophobic interfaces into highly insoluble amphipathic membranes, thereby changing the nature of surfaces. Hydrophobic surfaces become hydrophilic, while hydrophilic surfaces become hydrophobic. To see whether surface properties of assembled hydrophobins can be changed, 25 N-terminal residues of the mature SC3 hydrophobin were deleted (TrSC3). In addition, the cell-binding domain of fibronectin (RGD) was fused to the N terminus of mature SC3 (RGD-SC3) and TrSC3 (RGD-TrSC3). Self-assembly and surface activity were not affected by these modifications. However, physiochemical properties at the hydrophilic side of the assembled hydrophobin did change. This was demonstrated by a change in wettability and by enhanced growth of fibroblasts on Teflon-coated with RGD-SC3, TrSC3, or RGD-TrSC3 compared to bare Teflon or Teflon coated with SC3. Thus, engineered hydrophobins can be used to functionalize surfaces.
doi:10.1128/AEM.68.3.1367-1373.2002
PMCID: PMC123772  PMID: 11872489
25.  Hydrophobin-Based Surface Engineering for Sensitive and Robust Quantification of Yeast Pheromones 
Detection and quantification of small peptides, such as yeast pheromones, are often challenging. We developed a highly sensitive and robust affinity-assay for the quantification of the α-factor pheromone of Saccharomyces cerevisiae based on recombinant hydrophobins. These small, amphipathic proteins self-assemble into highly stable monolayers at hydrophilic-hydrophobic interfaces. Upon functionalization of solid supports with a combination of hydrophobins either lacking or exposing the α-factor, pheromone-specific antibodies were bound to the surface. Increasing concentrations of the pheromone competitively detached the antibodies, thus allowing for quantification of the pheromone. By adjusting the percentage of pheromone-exposing hydrophobins, the sensitivity of the assay could be precisely predefined. The assay proved to be highly robust against changes in sample matrix composition. Due to the high stability of hydrophobin layers, the functionalized surfaces could be repeatedly used without affecting the sensitivity. Furthermore, by using an inverse setup, the sensitivity was increased by three orders of magnitude, yielding a novel kind of biosensor for the yeast pheromone with the lowest limit of detection reported so far. This assay was applied to study the pheromone secretion of diverse yeast strains including a whole-cell biosensor strain of Schizosaccharomyces pombe modulating α-factor secretion in response to an environmental signal.
doi:10.3390/s16050602
PMCID: PMC4883293  PMID: 27128920
analyte detection; yeast pheromone; hydrophobin; surface functionalization; whole-cell biosensor; biosensor

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