Four conserved intramolecular disulphide linkages are required for secretion and cell wall localization of a hydrophobin during fungal morphogenesis

Hydrophobins are morphogenetic proteins produced by fungi during assembly of aerial hyphae, sporulation, mushroom development and pathogenesis. Eight cysteine residues are present in hydrophobins and form intramolecular disulphide bonds. Here, we show that expressing eight cysteine-alanine substitution alleles of the MPG1 hydrophobin gene from Magnaporthe grisea causes severe defects in development of aerial hyphae and spores. Immunolocalization revealed that Mpg1 hydrophobin variants, lacking intact disulphide bonds, retain the capacity to self-assemble, but are not secreted to the cell surface. This provides the first genetic evidence that disulphide bridges in a hydrophobin are dispensable for aggregation, but essential for secretion.     

The thn mutation of Schizophyllum commune, which suppresses formation of aerial hyphae, affects expression of the Sc3 hydrophobin gene.

The spontaneous and recessive mutation thn in the basidiomycete Schizophyllum commune suppresses the formation of aerial hyphae in the monokaryon and, if present as a double dose, the formation of both aerial hyphae and fruit-bodies in the dikaryon. In the monokaryon, the mutation prevents accumulation of mRNA of the Sc3 gene, and in the dikaryon it also prevents the accumulation of fruiting-specific mRNAs, including mRNAs of the Sc1 and Sc4 genes, which are homologous to the Sc3 gene. These three genes code for hydrophobins, a family of small hydrophobic cysteine-rich proteins. In the thn monokaryon, the only detectable change in synthesized proteins is the disappearance of an abundant protein of apparent Mr = 28 K from the culture medium and from the cell walls. Protein sequencing shows that this is the product of the Sc3 gene. The Sc3 hydrophobin is present in the walls of aerial hyphae as a hot-SDS-insoluble complex. Submerged hyphae excrete large amounts of the hydrophobin into the medium.     

Two hydrophobin genes from the conifer pathogen Heterobasidion annosum are expressed in aerial hyphae

Two hydrophobin genes (HAH1 and HAH2) have been identified in a Heterobasidion annosum infection-stage cDNA-library. Comparisons of their nucleotide and amino acid sequences show similarity to the coh1 hydrophobin from Coprinopsis cinerea and the sc3 hydrophobin from Schizophyllum commune. Both HAH1 and HAH2 display the amino acid consensus pattern of class I hydrophobins, including the spacing of eight conserved cysteine residues. Real-time quantitative RT-PCR showed high expression of both genes in aerial hyphae but low expression in submerged hyphae and during in vitro infection of pine seedlings. Segregation analysis of HAH1 and HAH2 in a defined cross of Heterobasidion annosum localised HAH1 to linkage group 3 but did not positioned HAH2 in the genetic linkage map. Sequence characteristics and expression patterns of HAH1 and HAH2 suggest a role in aerial growth of mycelia, but not during pathogenesis.     

Atomic composition of the hydrophobic and hydrophilic membrane sides of self-assembled SC3p hydrophobin.

The hydrophobin SC3p of Schizophyllum commune self-assembles into a 10-nm-thick amphipathic membrane at hydrophilic-hydrophobic interfaces. X-ray photoelectron spectroscopy of the hydrophobic membrane side of SC3p, assembled in vitro, showed an atomic composition similar to the calculated composition of SC3p when glycosylation was taken into account. The atomic composition measured at the hydrophilic membrane side deviated from that at the hydrophobic side and indicated the presence of a lower number of peptide bonds. High levels of S and N were detected only on mycelia carrying hydrophobic aerial hyphae, as expected with assembled SC3p present at the surface of these hyphae.     

The SC15 protein of Schizophyllum commune mediates formation of aerial hyphae and attachment in the absence of the SC3 hydrophobin

Disruption of the SC3 gene in the basidiomycete Schizophyllum commune affected not only formation of aerial hyphae but also attachment to hydrophobic surfaces. However, these processes were not completely abolished, indicating involvement of other molecules. We here show that the SC15 protein mediates formation of aerial hyphae and attachment in the absence of SC3. SC15 is a secreted protein of 191 aa with a hydrophilic N-terminal half and a highly hydrophobic C-terminal half. It is not a hydrophobin as it lacks the eight conserved cysteine residues found in these proteins. Besides being secreted into the medium, SC15 was localized in the cell wall and the mucilage that binds aerial hyphae together. In a strain in which the SC15 gene was deleted (DeltaSC15) formation of aerial hyphae and attachment were not affected. However, these processes were almost completely abolished when the SC15 gene was deleted in the DeltaSC3 background. The absence of aerial hyphae in the DeltaSC3DeltaSC15 strain can be explained by the inability of the strain to lower the water surface tension and to make aerial hyphae hydrophobic.     

Interfacial Self-Assembly of a Fungal Hydrophobin into a Hydrophobic Rodlet Layer

The Sc3p hydrophobin of the basidiomycete Schizophyllum commune is a small hydrophobic protein (100 to 101 amino acids) containing eight cysteine residues. Large amounts of the protein are excreted into the culture medium as monomers, but in the walls of aerial hyphae, the protein is present as an SDS-insoluble complex. In this study, we show that the Sc3p hydrophobin spontaneously assembles into an SDS-insoluble protein membrane on the surface of gas bubbles or when dried down on a hydrophilic surface. Electron microscopy of the assembled hydrophobin shows a surface consisting of rodlets spaced 10 nm apart, which is similar to those rodlets seen on the surface of aerial hyphae. When the purified Sc3p hydrophobin assembles on a hydrophilic surface, a surface is exposed with high hydrophobicity, similar to that of aerial hyphae. The rodlet layer, assembled in vivo and in vitro, can be disassembled by dissolution in trifluoroacetic acid and, after removal of the acid, reassembled into a rodlet layer. We propose, therefore, that the hydrophobic rodlet layer on aerial hyphae arises by interfacial self-assembly of Sc3p hydrophobin monomers, involving noncovalent interactions only. Submerged hyphae merely excrete monomers because these hyphae are not exposed to a water-air interface. The generally observed rodlet layers on fungal spores may arise in a similar way.     

Interfacial self-assembly of a hydrophobin into an amphipathic protein membrane mediates fungal attachment to hydrophobic surfaces.

The SC3p hydrophobin of Schizophyllum commune is a small hydrophobic protein (100-101 amino acids with eight cysteine residues) that self-assembles at a water/air interface and coats aerial hyphae with an SDS-insoluble protein membrane, at the outer side highly hydrophobic and with a typical rodlet pattern. SC3p monomers in water also self-assemble at the interfaces between water and oils or hydrophobic solids. These materials are then coated with a 10 nm thick SDS-insoluble assemblage of SC3p making their surfaces hydrophilic. Hyphae of S. commune growing on a Teflon surface became firmly attached and SC3p was shown to be present between the fungal cell wall and the Teflon. Decreased attachment of hyphae to Teflon was observed in strains not expressing SC3, i.e. a strain containing a targeted mutation in this gene and a regulatory mutant thn. These findings indicate that hydrophobins, in addition to forming hydrophobic wall coatings, play a role in adherence of fungal hyphae to hydrophobic surfaces.     

Complementation of the mpg1 mutant phenotype in Magnaporthe grisea reveals functional relationships between fungal hydrophobins.

The functional relationship between fungal hydrophobins was studied by complementation analysis of an mpg1(-) gene disruption mutant in Magnaporthe grisea. MPG1 encodes a hydrophobin required for full pathogenicity of the fungus, efficient elaboration of its infection structures and conidial rodlet protein production. Seven heterologous hydrophobin genes were selected which play distinct roles in conidiogenesis, fruit body development, aerial hyphae formation and infection structure elaboration in diverse fungal species. Each hydrophobin was introduced into an mpg1(-) mutant by transformation. Only one hydrophobin gene, SC1 from Schizophyllum commune, was able partially to complement mpg1(-) mutant phenotypes when regulated by its own promoter. In contrast, six of the transformants expressing hydrophobin genes controlled by the MPG1 promoter (SC1 and SC4 from S.commune, rodA and dewA from Aspergillus nidulans, EAS from Neurospora crassa and ssgA from Metarhizium anisopliae) could partially complement each of the diverse functions of MPG1. Complementation was always associated with partial restoration of a rodlet protein layer, characteristic of the particular hydrophobin being expressed, and with hydrophobin surface assembly during infection structure formation. This provides the first genetic evidence that diverse hydrophobin-encoding genes encode functionally related proteins and suggests that, although very diverse in amino acid sequence, the hydrophobins constitute a closely related group of morphogenetic proteins.     

The Cys3-Cys4 loop of the hydrophobin EAS is not required for rodlet formation and surface activity

Class I hydrophobins are fungal proteins that self-assemble into robust amphipathic rodlet monolayers on the surface of aerial structures such as spores and fruiting bodies. These layers share many structural characteristics with amyloid fibrils and belong to the growing family of functional amyloid-like materials produced by microorganisms. Although the three-dimensional structure of the soluble monomeric form of a class I hydrophobin has been determined, little is known about the molecular structure of the rodlets or their assembly mechanism. Several models have been proposed, some of which suggest that the Cys3-Cys4 loop has a critical role in the initiation of assembly or in the polymeric structure. In order to provide insight into the relationship between hydrophobin sequence and rodlet assembly, we investigated the role of the Cys3-Cys4 loop in EAS, a class I hydrophobin from Neurospora crassa. Remarkably, deletion of up to 15 residues from this 25-residue loop does not impair rodlet formation or reduce the surface activity of the protein, and the physicochemical properties of rodlets formed by this mutant are indistinguishable from those of its full-length counterpart. In addition, the core structure of the truncation mutant is essentially unchanged. Molecular dynamics simulations carried out on the full-length protein and this truncation mutant binding to an air-water interface show that, although it is hydrophobic, the loop does not play a role in positioning the protein at the surface. These results demonstrate that the Cys3-Cys4 loop does not have an integral role in the formation or structure of the rodlets and that the major determinant of the unique properties of these proteins is the amphipathic core structure, which is likely to be preserved in all hydrophobins despite the high degree of sequence variation across the family.     

Partial characterization of a hydrophobin protein Po.HYD1 purified from the oyster mushroom <Emphasis Type="Italic">Pleurotus ostreatus</Emphasis>

Hydrophobins fulfill various physiological functions in fungal development and growth, based on their mechanism of self-assembly at hydrophilic–hydrophobic interfaces into nano-scale, amphipathic membranes. One hydrophobin with an approximate molecular weight of 15 kDa, designated Po.HYD1, was purified from aerial hyphae of Pleurotus ostreatus strain Pm039. Ultrastructures of self-assembled films formed by Po.HYD1 on hydrophobic and hydrophilic substrates were revealed by atomic force microscopy (AFM). A monomolecular adsorption layer, thickness ranging from 3.2 to 3.8 nm, was observed on the surface of highly oriented pyrolytic graphite (HOPG), while a typical rodlet layer with uniform thickness of 4.2 ± 0.1 nm formed on the mica surface. Comparison of CD spectra showed significant secondary structural changes between monomeric and self-assembled states. The spectrum of monomeric Po.HYD1 had a maximum ellipticity at 190 nm and a minimum at 209 nm, and that of assemblage showed the maximum at 195 nm and the minimum shifted to 215–218 nm. Po.HYD1 showed high surface activity, based on the dramatic drop of surface tension through self-assembly at the water–air interface. Moreover, Po.HYD1 is capable of stabilizing the emulsion consisting of water and hexane.     

Properties of a hydrophobin isolated from the mycoparasitic fungus Verticillium fungicola

Verticillium fungicola, isolated from Agaricus bisporus (commercial mushroom), produced significant extracellular hydrophobin when grown for 7 days in a static liquid culture of synthetic minimal medium. The hydrophobin was purified by precipitation with ammonium sulphate (80% saturation), Sephadex G-100 gel filtration, and hydroxyapatite column chromatography. The purified protein yielded a single band in polyacrylamide gel electrophoresis under native conditions, with an apparent molecular mass of 70 +/- 4 kDa, and also another single band in SDS-PAGE, with a molecular mass of 7 +/- 3 kDa. Molecular mass determined with matrix-assisted laser desorption ionization-mass spectrometry (MALDI-MS) resulted in 7563.9 m/z. The same protein was extracted from the V. fungicola mycelium. Analysis of the amino acid composition revealed the presence of about 50% hydrophobic residues, detecting at least six cysteines, evaluated as cystines, and no free sulfhydryl groups. The protein did not show any glycosylation. On the basis of similarities in hydropathy patterns and solubility characteristics, V. fungicola hydrophobin can be included as a new member of Class II hydrophobins.     

HCf-6, a novel class II hydrophobin from Cladosporium fulvum

C. fulvum, a fungal tomato pathogen, has previously been shown to express a complex family of hydrophobin genes including four class I hydrophobins and one class II hydrophobin. Here we describe a gene for HCf-6, a sixth member of the hydrophobin family and the second class II gene. The protein is predicted to consist of a signal sequence, an N-terminus rich in glycine and asparagine and a C-terminal hydrophobic domain which bears the hall-marks of hydrophobins. In contrast to the previously described class II hydrophobin HCf-5, HCf-6 is expressed in mycelium growing in pure culture and mRNA levels do not increase during sporulation. It is down-regulated by carbon starvation but not by depletion of nitrogen in the growth medium.     

How a fungus escapes the water to grow into the air

Fungi are well known to the casual observer for producing water-repelling aerial moulds and elaborate fruiting bodies such as mushrooms and polypores. Filamentous fungi colonize moist substrates (such as wood) and have to breach the water-air interface to grow into the air. Animals and plants breach this interface by mechanical force. Here, we show that a filamentous fungus such as Schizophyllum commune first has to reduce the water surface tension before its hyphae can escape the aqueous phase to form aerial structures such as aerial hyphae or fruiting bodies. The large drop in surface tension (from 72 to 24 mJ m-2) results from self-assembly of a secreted hydrophobin (SC3) into a stable amphipathic protein film at the water-air interface. Other, but not all, surface-active molecules (that is, other class I hydrophobins and streptofactin from Streptomyces tendae) can substitute for SC3 in the medium. This demonstrates that hydrophobins not only have a function at the hyphal surface but also at the medium-air interface, which explains why fungi secrete large amounts of hydrophobin into their aqueous surroundings.     

A surface active protein involved in aerial hyphae formation in the filamentous fungus Schizophillum commune restores the capacity of a bald mutant of the filamentous bacterium Streptomyces coelicolor to erect aerial structures

The filamentous bacterium Streptomyces coelicolor undergoes a complex process of morphological differentiation involving the formation of a dense lawn of aerial hyphae that grow away from the colony surface into the air to form an aerial mycelium. Bald mutants of S. coelicolor, which are blocked in aerial mycelium formation, regain the capacity to erect aerial structures when exposed to a small hydrophobic protein called SapB, whose synthesis is temporally and spatially correlated with morphological differentiation. We now report that SapB is a surfactant that is capable of reducing the surface tension of water from 72 mJ m^?2 to 30 mJ m^?2 at a concentration of 50 μg ml^?1. We also report that SapB, like the surface-active peptide streptofactin produced by the species S. tendae, was capable of restoring the capacity of bald mutants of S. tendae to erect aerial structures. Strikingly, a member (SC3) of the hydrophobin family of fungal proteins involved in the erection of aerial hyphae in the filamentous fungus Schizophyllum commune was also capable of restoring the capacity of S. coelicolor and S. tendae bald mutants to erect aerial structures. SC3 is unrelated in structure to SapB and streptofactin but, like the streptomycetes proteins, the fungal protein is a surface active agent. Scanning electron microscopy revealed that aerial structures produced in response to both the bacterial or the fungal proteins were undifferentiated vegetative hyphae that had grown away from the colony surface but had not commenced the process of spore formation. We conclude that the production of SapB and streptofactin at the start of morphological differentiation contributes to the erection of aerial hyphae by decreasing the surface tension at the colony surface but that subsequent morphogenesis requires additional developmentally regulated events under the control of bald genes.     

The properties of citrate transport catalyzed by CitP of Lactococcus lactis ssp. lactis biovar diacetylactis

Abstract The SC3 hydrophobin gene of Schizophyllum commune was disrupted by homologous integration of an SC3 genomic fragment interrupted by a phleomycin resistance cassette. The phenotype of the mutant was particularly clear in sealed plates in which formation of aerial hyphae was blocked. In non-sealed plates aerial hyphae did form but these were hydrophilic and not hydrophobic as in wild-type strains. Complementation with a genomic SC3 clone restored formation of hydrophobic aerial hyphae in sealed plates. In a dikaryon homozygous for the SC3 mutation normal sporulating fruiting bodies were produced but aerial hyphae were hydrophilic.     

Adsorption of a fungal hydrophobin onto surfaces as mediated by the associated polysaccharide schizophyllan

The filamentous fungus Schizophyllum commune secretes three major biopolymers into liquid growth media. These include a 24 kiloDalton hydrophobin, a 17 kiloDalton protein, and a high molecular mass polysaccharide, schizophyllan. The fungal culture supernatant forms sodium dodecyl sulfate resistant coatings on both hydrophobic and hydrophilic surfaces as demonstrated by water contact angle measurements and atomic force microscopy. Specific digestion of the schizophyllan from the supernatant has little effect on coating integrity on a hydrophobic surface. By contrast, enzymatic digestion of the hydrophobin eliminates the ability of the remaining supernatant components to assemble as a stable entity on a hydrophobic surface. Digestion of either the polysaccharide or the hydrophobin prevents stable association with a hydrophilic surface. Previous studies have demonstrated the role of hydrophobin in forming stable coatings on various surfaces; however, the synergistic interaction of schizophyllan with hydrophobin was not considered. Our data form the basis for a preliminary model in which hydrophobin is stabilized in the culture supernatant by a protective hydrophilic corona. Coalescence of hydrophobin onto solid mica or Parafilm® surfaces is favored over self‐association in the presence of schizophyllan. © 1999 John Wiley & Sons, Inc. Biopoly 49: 621–633, 1999     

Toluene gas phase biofiltration by Paecilomyces lilacinus and isolation and identification of a hydrophobin protein produced thereof

Paecilomyces lilacinus consumed toluene as the sole carbon source in a gas-phase biofilter packed with perlite obtaining an average elimination capacity of 50 g m(-3) h(-1), a removal efficiency of 53%, and a final biomass of 31.6 mg biomass g dry support(-1). Hydrophobin proteins from the mycelium produced in the biofilter were purified by formic acid extraction and precipitated by electrobubbling, and the molecular weight was found to be 10.6 +/- 0.3 kDa. The peptide mass fingerprinting analysis of the purified hydrophobin by matrix-assisted laser desorption/ionization time-of-flight resulted in the identification of two peptides that presented high homology with sequences of class I hydrophobin proteins from other ascomycetous fungi when compared against the National Center for Biotechnology Information database. The yield of hydrophobin (PLHYD) from P. lilacinus was 1.1 mg PLHYD g biomass(-1). These proteins modified the hydrophobicity of Teflon by lowering the contact angle from 130.1 (+/-2) degrees to 57.0 (+/-5) degrees supporting hot sodium dodecyl sulfate washing. This work is the first report about biodegradation of toluene by the nematophagous fungus P. lilacinus in a gas-phase biofilter and the identification of its hydrophobin protein.