Transcription factor genes of Schizophyllum commune involved in regulation of mushroom formation

Mushrooms represent the most conspicuous structures of fungi. Their development is being studied in the model basidiomycete Schizophyllum commune. The genome of S. commune contains 472 genes encoding predicted transcription factors. Of these, fst3 and fst4 were shown to inhibit and induce mushroom development respectively. Here, we inactivated five additional transcription factor genes. This resulted in absence of mushroom development (in the case of deletion of bri1 and hom2), in arrested development at the stage of aggregate formation (in the case of c2h2) and in the formation of more but smaller mushrooms (in the case of hom1 and gat1). Moreover, strains in which hom2 and bri1 were inactivated formed symmetrical colonies instead of irregular colonies like the wild type. A genome-wide expression analysis identified several gene classes that were differentially expressed in the strains in which either hom2 or fst4 was inactivated. Among the genes that were downregulated in these strains were c2h2 and hom1. Based on these results, a regulatory model of mushroom development in S. commune is proposed. This model most likely also applies to other mushroom-forming fungi and will serve as a basis to understand mushroom formation in nature and to enable and improve commercial mushroom production.

Characterization of aPleurotus ostreatus fruiting body-specific hydrophobin gene,Po.hyd

Hydrophobins are a family of small, moderately hydrophobic proteins with eight cysteine residues arranged in a conserved pattern. A full-length cDNA, designated Po.hyd, corresponding to a hydrophobin gene of Pleurotus ostreatus was obtained in our previous work. The Po.hyd gene contains a 333 bp open reading frame (ORF), which is interrupted by two typical classI introns. There was no consensus signal for a polyA tail detected in the 3'untranslated region. However, an analogous T- or TG-rich motif was observed that probably influence the formation of the mRNA 3' end. We assign the putative Po.HYD protein to the classI hydrophobins since its sequence arrangement and hydropathy pattern has a high consensus to other known class I hydrophobins. Northern analysis showed that the Po.hyd gene was abundantly expressed throughout the fruiting process (from primordium to mature fruiting body) but silenced during vegetative growth of the mycelium. Southern blot analysis showed Po.hyd to be a single copy gene in the genome of dikaryotic strain likely to locate at the same locus within the two parental genomes.

Repellents have functionally replaced hydrophobins in mediating attachment to a hydrophobic surface and in formation of hydrophobic aerial hyphae in Ustilago maydis

Ustilago maydis contains one repellent and two class I hydrophobin genes in its genome. The repellent gene rep1 has been described previously. It encodes 11 secreted repellent peptides that result from the cleavage of a precursor protein at KEX2 recognition sites. The hydrophobin gene hum2 encodes a typical class I hydrophobin of 117 aa, while hum3 encodes a hydrophobin that is preceded by 17 repeat sequences. These repeats are separated, like the repellent peptides, by KEX2 recognition sites. Gene hum2, but not hum3, was shown to be expressed in a cross of two compatible wild-type strains, suggesting a role of the former hydrophobin gene in aerial hyphae formation. Indeed, aerial hyphae formation was reduced in a Delta hum2 cross. However, the reduction in aerial hyphae formation was much more dramatic in the Delta rep1 cross. Moreover, colonies of the Delta rep1 cross were completely wettable, while surface hydrophobicity was unaffected and only slightly reduced in the Delta hum2 and the Delta hum2 Delta hum3 cross, respectively. It was also shown that the repellents and not the hydrophobins are involved in attachment of hyphae to hydrophobic Teflon. Deleting either or both hydrophobin genes in the Delta rep1 strains did not further affect aerial hyphae formation, surface hydrophobicity and attachment. From these data it is concluded that hydrophobins of U. maydis have been functionally replaced, at least partially, by repellents.

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.

Hydrophobins and the interactions between fungi and plants

Summary Hydrophobins are small proteins thought to be ubiquitous in filamentous fungi. They are usually secreted and are found on the outer surfaces of cell walls of hyphae and conidia where they mediate interactions between the fungus and the environment. We review here what is currently known about the primary and secondary structure of these proteins, as well as their post-translational modifications. We also discuss the diverse functions of hydrophobins in biology and development, with particular attention to fungi involved in pathogenesis and symbiosis.

Identification of a hydrophobin gene that is developmentally regulated in the ectomycorrhizal fungus Tricholoma terreum

The symbiosis between ectomycorrhizal fungi and trees is an essential part of forest ecology and depends entirely on the communication between the two partners for establishing and maintaining the relationship. The identification and characterization of differentially expressed genes is a step to identifying such signals and to understanding the regulation of this process. We determined the role of hydrophobins produced by Tricholoma terreum in mycorrhiza formation and hyphal development. A hydrophobin was purified from culture supernatant, and the corresponding gene was identified. The gene is expressed in aerial mycelium and in mycorrhiza. By using a heterologous antiserum directed against a hydrophobin found in the aerial mycelium of Schizophyllum commune, we detected a hydrophobin in the symbiosis between T. terreum and its native pine host Pinus sylvestris. The hydrophobin was found in aerial mycelium of the hyphal mantle and also in the Hartig net hyphae, which form the interface between both partners. Interestingly, this was not the case in the interaction of T. terreum with a host of low compatibility, the spruce Picea abies. The differential expression with respect to host was verified at the transcriptional level by competitive PCR. The differential protein accumulation pattern with respect to host compatibility seen by immunofluorescence staining can thus be attributed at least in part to transcriptional control of the hyd1 gene.

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.

Hydrophobins: multipurpose proteins

Class I and class II hydrophobins are small secreted fungal proteins that play a role in a broad range of processes in the growth and development of filamentous fungi. For instance, they are involved in the formation of aerial structures and in the attachment of hyphae to hydrophobic surfaces. The mechanisms by which hydrophobins fulfill these functions are based on their property to self-assemble at hydrophilic-hydrophobic interfaces into a 10 nm-thin highly amphipathic film. Complementation studies have shown that class I hydrophobins belong to a closely related group of morphogenetic proteins, but that they have evolved to function at specific interfaces. Recent evidence indicates that hydrophobins do not only function by self-assembly. Monomeric hydrophobin has been implicated in cell-wall assembly, but the underlying mechanism is not yet clear. In addition, hydrophobin monomers could act as toxins and elicitors.

Immunolocalization of hydrophobin HYDPt-1 from the ectomycorrhizal basidiomycete Pisolithus tinctorius during colonization of Eucalyptus globulus roots

•  The immunolocalization of one of the hydrophobins of Pisolithustinctorius (HYDPt-1) is reported. Hydrophobin proteins play key roles in adhesion and aggregation of fungal hyphae, and it is already known that formation of ectomycorrhizas on eucalypt roots enhances the accumulation of hydrophobin mRNAs in the mycelium of Pisolithus tinctorius. •  Purification of SDS-insoluble proteins from the mycelium of P. tinctorius showed the presence of a 13 kDa polypeptide with properties of class I hydrophobin. •  Polyconal antibodies were raised against a recombinant HYDPt-1 polypeptide, and these were used for immunofluorescence-coupled transmission electron microscopy. •  HYDPt-1 is a cell wall protein located at the surface of the hyphae with no preferential accumulation in the fungal cells of the different tissues of the ectomycorrhiza (i.e. extraradical hyphae, mantle or Hartig net).

Hydrophobin gene expression affects hyphal wall composition in Schizophyllum commune

Disruption of the SC3 hydrophobin gene of Schizophyllum commune (DeltaSC3 strain) affected the composition of the cell wall. Compared to a wild-type strain the amount of mucilage (i.e., water-soluble (1-3)beta-glucan with single glucose residues attached by (1-6)beta-linkages) increased considerably, while the amount of alkali-resistant glucan (linked to chitin) decreased. Reintroduction of the SC3 gene or other hydrophobins genes expressed behind the SC3 promotor restored wild-type cell wall composition. However, addition of purified SC3 protein to the medium or growing the DeltaSC3 strain in spent medium of the wild-type strain had no effect. In young cultures of wild-type strains of S.commune, not yet expressing SC3, the amount of mucilage was also relatively high. These data show that hydrophobins not only function at hydrophilic/hydrophobic interfaces, as shown previously, but also affect wall composition.

SC3 and SC4 hydrophobins have distinct roles in formation of aerial structures in dikaryons of Schizophyllum commune

Two monokaryons of Schizophyllum commune can form a fertile dikaryon when the mating-type genes differ. Monokaryons form sterile aerial hyphae, while dikaryons also form fruiting bodies that function in sexual reproduction. The SC3 hydrophobin gene is expressed both in monokaryons and in dikaryons. The SC4 hydrophobin is dikaryon specific. In the monokaryon, SC3 lowers the water surface tension, coats aerial hyphae with a hydrophobic layer and mediates attachment of hyphae to hydrophobic surfaces. The SC4 protein lines gas channels within fruiting bodies with a hydrophobic membrane. Using gene disruptions, in this study, we show that in dikaryons SC3 fulfils the same roles as in monokaryons. SC4, on the other hand, has a role within fruiting bodies. In contrast to gas channels in fruiting bodies of the wild type, those of a DeltaSC4 strain easily filled with water. Thus, SC4 prevents gas channels filling with water under wet conditions, probably serving uninterrupted gas exchange. Other dikaryon-specific hydrophobin genes, SC1 and SC6, apparently do not substitute for the SC4 gene. In addition, by expressing the SC4 gene behind the SC3 promoter in a DeltaSC3 monokaryon, it was shown that SC4 cannot fully substitute for SC3, indicating that both hydrophobins evolved to fulfil specific functions.

Structural proteins involved in emergence of microbial aerial hyphae

Filamentous fungi and filamentous bacteria (i.e., the streptomycetes) belong to different kingdoms that diverged early in evolution. Yet, they adopted similar lifestyles. After a submerged feeding mycelium has been established, hyphae grow into the air and form aerial structures from which (a)sexual spores can develop. These spores are dispersed and can give rise to a new mycelium. Some of the key processes involved in the formation of aerial hyphae by these microbes appear to be very similar. In both cases molecules that lower the surface tension are secreted into the aqueous environment, thereby enabling hyphae to grow into the air. Aerial hyphae are then covered with a hydrophobic film. In fungi, this film is characterized by a mosaic of parallel rodlets, while similar rodlets have also been observed on aerial structures of filamentous bacteria. Although the erection of aerial hyphae in both filamentous fungi and filamentous bacteria is dependent upon (poly)peptides that are structurally unrelated, they can, at least partially, functionally substitute for each other.

Extracellular proteins in fungi: a cytological and molecular perspective

Protein secretion is a vital process in fungi. For many, the secretion of hydrolytic enzymes provides a crucial step in their nutrition in nature. However, in recent years the list of different types of secreted proteins that have been discovered has extended significantly. These have been shown to have a diversity of functions including toxic molecule transport and control of desiccation. The majority of secreted proteins are glycosylated and our understanding of this aspect of fungal biochemistry has also extended in recent years. This review addresses the process of protein secretion from the cytological, biochemical and genetical standpoints. Advances in technology in many areas of scientific approach have enabled a better and understanding of this important process in fungi.

Introns are necessary for mRNA accumulation in Schizophyllum commune

The cDNA coding sequence of the Agaricus bisporus hydrophobin gene ABH1 under the regulation sequences of the Schizophyllum commune SC3 hydrophobin gene gave no expression in S. commune. In contrast, the genomic coding sequence (containing three introns) produced high levels of ABH1 mRNA when transformed to S. commune in the same configuration. Apparently, introns were needed for the accumulation of mRNAs from the ABH1 gene. When the effect of intron deletion on expression of the homologous genes SC3 and SC6 was examined, it was observed that only the genomic coding sequences were expressed in S. commune. Run-on analysis with nuclei harbouring intron-containing and intronless SC6 showed that this effect did not occur at the level of transcription initiation: genomic and cDNA sequences were equally active in this respect. When a 50 bp artificial intron containing the consensus splice and branch sites of S. commune introns, in addition to random-generated sequences, was introduced in the right orientation into the intronless SC3 transcriptional unit, accumulation of SC3 mRNA was restored. By polymerase chain reaction amplification, no unspliced SC3 mRNA species could be detected. Furthermore, the addition of an intron into the transcriptional unit of the gene for green fluorescent protein (GFP) effected clear fluorescence of the transgenic hyphae. Apparently, splicing is required for the normal processing of primary transcripts in S. commune.

Regulation of dikaryon-expressed genes by FRT1 in the basidiomycete Schizophyllum commune

The gene FRT1 has previously been shown to induce homokaryotic fruiting in transformation recipients of the basidiomycete Schizophyllum commune. In this paper, we demonstrate by gene disruption experiments that FRT1 is dispensable for dikaryotic fruiting. Nonfruiting homokaryotic FRT1 disruptant strains exhibited enhanced aerial growth of mycelia compared to wild type. Introduction of a functional FRT1 allele into the disruptant restored the wild-type colony morphology. Transcript abundance of the dikaryon-expressed SC1 and SC4 hydrophobin genes and the SC7 gene were greatly elevated in homokaryotic FRT1 disruptant strains. Growth of the disruptant strains under continuous light was found to inhibit the elevation of SC1 and SC4 transcript levels, but not of SC7 mRNA. These data suggest that the role of FRT1 in vegetatively growing homokaryons is to act as a negative regulator of dikaryon-expressed genes.

Identification of three differentially expressed hydrophobins in Pleurotus ostreatus (oyster mushroom)

Three proteins with characteristic features of class I hydrophobins, designated POH1, POH2 and POH3, were isolated from the basidiomycete Pleurotus ostreatus. Based on N-terminal sequence analyses, their cDNAs were isolated using RT-PCR; the cDNAs and corresponding genes were sequenced and their regulation studied. POH1 is expressed in the fruiting bodies but not in vegetative mycelium. The regulation of POH2 and POH3 is tightly correlated. Both genes are switched off in the fruiting bodies but abundantly expressed in the vegetative mycelium of both monokaryon and dikaryon. POH2 and POH3 were isolated from the culture medium and from aerial hyphae. Co-purified POH2 and POH3 assembled in vitro into a protein membrane with a typical rodlet pattern as found previously with other hydrophobins. Similar structures were detected on the surface of aerial hyphae.

A hydrophobin (ABH3) specifically secreted by vegetatively growing hyphae of Agaricus bisporus (common white button mushroom)

Aerial mycelium and hyphal strands of Agaricus bisporus, strain U1, exhibited a rodlet pattern at their surfaces characteristic for assembled class I hydrophobins. An SDS-insoluble/trifluoroacetic-acid-soluble fraction from strands was found to contain one abundant protein with an apparent molecular mass on gel of 19 kDa. Two sequences for this protein (ABH3), typical of class I hydrophobins, could be deduced by sequencing cDNA clones obtained by RT-PCR. The two forms of the protein could be assigned to different alleles present in the two homokaryons that constitute the heterokaryotic U1 strain. ABH3 displays all the in vitro properties of a typical class I hydrophobin such as SC3 from Schizophyllum commune but is not glycosylated or otherwise post-translationally modified because the molecular mass values deduced from the amino acid sequence (9228 and 9271 Da) and derived from mass spectrometry were in good agreement. The ABH3 transcript was found to be present in the vegetative mycelium of both primary and secondary mycelium but not in the fruiting bodies, whereas the reverse was found for the ABH1 hydrophobin. Using an S. commune mutant with a disrupted SC3 gene it was found that ABH3 can substitute for SC3 in inducing formation of aerial hyphae, suggesting a role of ABH3 in the emergence of aerial hyphae and strands in A. bisporus.

Positioning of nuclei in the secondary Mycelium of Schizophyllum commune in relation to differential gene expression

In this paper we propose a novel type of gene regulation in the MATA|l4 MATB|l4 heterokaryon of Schizophyllum commune by means of differential positioning of the nuclei. It was found that binucleate hyphae with juxtaposed nuclei secrete SC4 hydrophobin (abundant during fruit-body formation), while SC3 (abundant during aerial hyphae formation in both mono- and dikaryons) appeared to be absent. Certain growth conditions disrupted the binucleate state in that the compatible nuclei became separated at a considerable distance. Under these conditions SC4 was not secreted while SC3 was secreted to a high degree. Disruption of the binucleate state was earlier observed in developing aerial hyphae which secrete SC3. Apparently, when the nuclei are in close proximity the dikaryon-expressed genes are switched on by interaction of the products of the MATA and MATB mating-type genes, while SC3 is suppressed by interacting products of the MATB genes, as occurs in the common MATA heterokaryon (MATA= MATB|l4). Growth conditions that lead to disruption of the binucleate state apparently result in abolishment of interaction between the MATB mating-type genes. Under these conditions, dikaryon-specific mRNAs do not accumulate in the MATA|l4 MATB|l4 heterokaryon, while SC3 mRNA becomes highly abundant.

Differential expression of the vegetative and spore-bound hydrophobins of Trichoderma reesei--cloning and characterization of the hfb2 gene

The hfb2 gene encoding the hydrophobin HFBII of the filamentous fungus Trichoderma reesei was isolated by heterologous hybridization using the vegetative hydrophobin I, hfb1, gene of T. reesei as a probe. The hfb2 gene codes for a typical fungal secreted hydrophobin of 71 amino acids containing eight cysteine residues. The amino acid similarity towards HFBI is 69%. The HFBII protein was isolated from the fungal spores by extraction with trifluoroacetic acid/acetonitrile solution, and by bubbling from the lactose-based culture medium. Expression of the hfb1 and hfb2 genes is divergent. hfb1 expression was only observed in vegetative cultures on glucose-containing and sorbitol-containing media. It was not expressed on media containing complex plant polysaccharides, cellulose, xylan, cellobiose or lactose, whereas hfb2 was highly expressed in vegetative cultures on these media. Expression of hfb2 was also strongly induced by N and C starvation, by light and in conidiating cultures.

Expression of two closely linked hydrophobin genes of Coprinus cinereus is monokaryon-specific and down-regulated by the oid-1 mutation

A protein with characteristic properties of a fungal hydrophobin (CoH1) was isolated from the monokaryotic stage of the basidiomycete Coprinus cinereus. A cosmid clone containing the corresponding gene (coH1) was identified using a cDNA probe derived by RT-PCR. Hybridization and sequence analysis identified a second gene, coH2, just 4.1 kb downstream of coH1 encoding a hydrophobin (CoH2) with 64% sequence identity. Both coH1 and coH2 are subject to developmental regulation. They are expressed in vegetative monokaryotic cells but not in the asexual oidia produced on the surface of monokaryons. Transcripts of the genes were barely detected in dikaryotic mycelium and were absent from fruit bodies. Loss of aerial growth due to a mutation known as oid-1 was correlated with lack of both hydrophobins.

Cerato-ulmin, a hydrophobin secreted by the causal agents of Dutch elm disease, is a parasitic fitness factor

Dutch elm disease is caused by the aggressive Ophiostoma novo-ulmi and the nonaggressive O. ulmi. Both secrete the protein cerato-ulmin (CU). To determine what role CU plays in the pathology of Dutch elm disease, we constructed a CU overexpression mutant of the nonaggressive O. ulmi H5. Stable integration of a single copy of the cu gene from the aggressive O. novo-ulmi into the genome of the nonaggressive isolate resulted in increased secretion of CU protein. Trials with American elm, Ulmus americana, suggested no alteration of virulence of this overexpressing transformant. Using aggressive and nonaggressive wild types, the cu overexpressing mutant, and our cu- mutant (Bowden et al., 1996), we have demonstrated that CU production is correlated with an altered phenotype and more hydrophobic and adherent yeast-like cells. Our results also demonstrate that CU has a role in protecting infectious propagules from desiccation. These biological roles for CU would affect transmission of Dutch elm disease, and we therefore propose that this hydrophobin acts as a parasitic fitness factor.

HCF-1, a hydrophobin from the tomato pathogen Cladosporium fulvum

A hydrophobin, named HCF-1, was isolated from the culture medium of Cladosporium fulvum, the causal agent of tomato leaf mould. The protein forms insoluble aggregates when the medium is vigorously aerated. These aggregates can be dissociated by trifluoroacetic acid into monomers which migrate as 10-kDa molecules on SDS-PAGE. HCf-1 is encoded by a single gene, HCf-1. The gene contains two small introns and is translated into a 105-amino acid protein which is then processed to give a mature 83-amino acid protein. The position of the eight cysteine residues and the predicted hydrophobicity profile are typical of fungal hydrophobins. HCf-1 RNA is expressed in growing mycelium and conidia but its quantity diminishes transiently after germination; its abundance does not change when the fungus is grown on nitrogen- or carbon-deficient medium. This is the first step in evaluating the role of hydrophobins in establishment of basic compatibility between C. fulvum and tomato.

An abundant hydrophobin (ABH1) forms hydrophobic rodlet layers in Agaricus bisporus fruiting bodies

The SDS-insoluble protein fraction of Agaricus bisporus fruiting bodies was solubilized with trifluoroacetic acid. On SDS-PAGE this fraction was found to contain one abundant protein with an apparent M(r) of 16 kDa. The N-terminal amino acid sequence of this protein was determined and RT-PCR used to isolate a cDNA clone which upon sequencing identified the protein as a typical class I hydrophobin (ABH1). The gene (ABH1) was isolated and sequenced, and a second hydrophobin gene (ABH2) was found about 2.5 kbp downstream of ABH1. Purified ABH1 self-assembled at hydrophobic-hydrophilic interfaces, producing the typical rodlet layer known from other hydrophobins. Similar rodlets were observed on the surface of the fruiting body, while immunological localization showed the hydrophobin to be particularly abundant at the outer surface of fruiting bodies, in the veil and in the core tissue of the stipe. Transcripts of ABH1 were found only in fruiting-body hyphae. The ABH1 hydrophobin is probably solely responsible for the hydrophobicity of the fruiting-body surface but may also line air channels within fruiting bodies.