Cellobiohydrolase II is the main conidial-bound cellulase in Trichoderma reesei and other Trichoderma strains

Extracellular vesicles carry cellulases in the industrial fungus Trichoderma reesei

BACKGROUND

Trichoderma reesei is the most important industrial producer of lignocellulolytic enzymes. These enzymes play an important role in biomass degradation leading to novel applications of this fungus in the biotechnology industry, specifically biofuel production. The secretory pathway of fungi is responsible for transporting proteins addressed to different cellular locations involving some cellular endomembrane systems. Although protein secretion is an extremely efficient process in T. reesei, the mechanisms underlying protein secretion have remained largely uncharacterized in this organism.

RESULTS

Here, we report for the first time the isolation and characterization of T. reesei extracellular vesicles (EVs). Using proteomic analysis under cellulose culture condition, we have confidently identified 188 vesicular proteins belonging to different functional categories. Also, we characterized EVs production using transmission electron microscopy in combination with light scattering analysis. Biochemical assays revealed that T. reesei extracellular vesicles have an enrichment of filter paper (FPase) and β-glucosidase activities in purified vesicles from 24, 72 and 96, and 72 and 96 h, respectively. Furthermore, our results showed that there is a slight enrichment of small RNAs inside the vesicles after 96 h and 120 h, and presence of hsp proteins inside the vesicles purified from T. reesei grown in the presence of cellulose.

CONCLUSIONS

This work points to important insights into a better understanding of the cellular mechanisms underlying the regulation of cellulolytic enzyme secretion in this fungus.

The impact of a single-nucleotide mutation of bgl2 on cellulase induction in a Trichoderma reesei mutant

BACKGROUND

The filamentous fungus Trichoderma reesei (anamorph of Hypocrea jecorina) produces increased cellulase expression when grown on cellulose or its derivatives as a sole carbon source. It has been believed that β-glucosidases of T. reesei not only metabolize cellobiose but also contribute in the production of inducers of cellulase gene expression by their transglycosylation activity. The cellulase hyper-producing mutant PC-3-7 developed in Japan has enhanced cellulase production ability when cellobiose is used as the inducer. The comparative genomics analysis of PC-3-7 and its parent revealed a single-nucleotide mutation within the bgl2 gene encoding intracellular β-glucosidase II (BGLII/Cel1a), giving rise to an amino acid substitution in PC-3-7, which could potentially account for the enhanced cellulase expression when these strains are cultivated on cellulose and cellobiose.

RESULTS

To analyze the effects of the BGLII mutation in cellulase induction, we constructed both a bgl2 revertant and a disruptant. Enzymatic analysis of the transformant lysates showed that the strain expressing mutant BGLII exhibited weakened cellobiose hydrolytic activity, but produced some transglycosylation products, suggesting that the SNP in bgl2 strongly diminished cellobiase activity, but did not result in complete loss of function of BGLII. The analysis of the recombinant BGLII revealed that transglycosylation products might be oligosaccharides, composed probably of glucose linked β-1,4, β-1,3, or a mixture of both. PC-3-7 revertants of bgl2 exhibited reduced expression and inducibility of cellulase during growth on cellulose and cellobiose substrates. Furthermore, the effect of this bgl2 mutation was reproduced in the common strain QM9414 in which the transformants showed cellulase production comparable to that of PC-3-7.

CONCLUSION

We conclude that BGLII plays an important role in cellulase induction in T. reesei and that the bgl2 mutation in PC-3-7 brought about enhanced cellulase expression on cellobiose. The results of the investigation using PC-3-7 suggested that other mutation(s) in PC-3-7 could also contribute to cellulase induction. Further investigation is essential to unravel the mechanism responsible for cellulase induction in T. reesei.

Microbial diversity in bioaerosol samples causing ODTS compared to reference bioaerosol samples as measured using Illumina sequencing and MALDI-TOF

The importance of the microbial diversity of bioaerosols in relation to occupational exposure and work related health symptoms is not known. The aim of this paper is to gain knowledge on the bacterial and fungal communities in dust causing organic dust toxic syndrome (ODTS) and in reference dust not causing ODTS. Bacterial and fungal communities were described in personal exposure samples from grass seed workers developing ODTS, in dust generated from grass seeds causing ODTS and in dust generated from reference seeds not causing ODTS. Amplicon sequencing of the bacterial 16S rRNA gene and the fungal ITS region, as well as matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) were used for identification of fungi and bacteria in personal exposure samples and in dust samples from grass seeds causing ODTS and in dust from reference grass seeds. Furthermore, activities of enzymes were measured in the same samples. The sequencing data revealed more than 150 bacterial and 25 fungal genera present in each sample. Streptomyces spp., Aspergillus fumigatus and Rhizopus microsporus were dominating in the dust causing ODTS but not in the reference dust. The dustiness in terms of Mucor sp. and R. microsporus were 100-1000 times higher for problematic seeds compared to reference seeds. The bacterial species in the dust causing ODTS included pathogenic species such as Klebsiella pneumonia and Streptomyces pneumonia, and it contained increased concentrations of total protein, serine protease, chitinase, and β-glucosidase. Twenty-three bacterial genera covered more than 50% of the total reads in the personal and problematic seed dust. These 23 genera accounted for less than 7% of the total reads in the reference seed dust. The microbial community of the dust from the problematic seeds showed great similarities to that from the personal air samples from the workers. In conclusion, we have shown for the first time a shift in the microbial community in aerosol samples that caused ODTS compared to the reference samples that did not cause the ODTS. Furthermore, elevated enzyme activities were found in the dust causing ODTS.

Comparison of the heterologous expression of Trichoderma reesei endoglucanase II and cellobiohydrolase II in the yeasts Pichia pastoris and Yarrowia lipolytica

The sequences encoding the genes for endoglucanase II and cellobiohydrolase II from the fungus Trichoderma reesei QM9414 were successfully cloned and expressed in Yarrowia lipolytica under the control of the POX2 or TEF promoters, and using either the native or preproLip2 secretion signals. The expression level of both recombinant enzymes was compared with that obtained using Pichia pastoris, under the control of the AOX1 promoter to evaluate the utility of Y. lipolytica as a host strain for recombinant EGII and CBHII production. Extracellular endoglucanase activity was similar between TEF-preoproLip2-eglII expressed in Y. lipolytica and P. pastoris induced by 0.5 % (v/v) methanol, but when recombinant protein expression in P. pastoris was induced with 3 % (v/v) methanol, the activity was increased by about sevenfold. In contrast, the expression level of cellobiohydrolase from the TEF-preproLip2-cbhII cassette was higher in Y. lipolytica than in P. pastoris. Transformed Y. lipolytica produced up to 15 mg/l endoglucanase and 50 mg/l cellobiohydrolase, with the specific activity of both proteins being greater than their homologs produced by P. pastoris. Partial characterization of recombinant endoglucanase II and cellobiohydrolase II expressed in both yeasts revealed their optimum pH and temperature, and their pH and temperature stabilities were identical and hyperglycosylation had little effect on their enzymatic activity and properties.

Regulation of cellulase and hemicellulase gene expression in fungi

Research on regulation of cellulases and hemicellulases gene expression may be very useful for increasing the production of these enzymes in their native producers. Mechanisms of gene regulation of cellulase and hemicellulase expression in filamentous fungi have been studied, mainly in Aspergillus and Trichoderma. The production of these extracellular enzymes is an energy-consuming process, so the enzymes are produced only under conditions in which the fungus needs to use plant polymers as an energy and carbon source. Moreover, production of many of these enzymes is coordinately regulated, and induced in the presence of the substrate polymers. In addition to induction by mono- and oligo-saccharides, genes encoding hydrolytic enzymes involved in plant cell wall deconstruction in filamentous fungi can be repressed during growth in the presence of easily metabolizable carbon sources, such as glucose. Carbon catabolite repression is an important mechanism to repress the production of plant cell wall degrading enzymes during growth on preferred carbon sources. This manuscript reviews the recent advancements in elucidation of molecular mechanisms responsible for regulation of expression of cellulase and hemicellulase genes in fungi.

Differential involvement of β-glucosidases from Hypocrea jecorina in rapid induction of cellulase genes by cellulose and cellobiose

Appropriate perception of cellulose outside the cell by transforming it into an intracellular signal ensures the rapid production of cellulases by cellulolytic Hypocrea jecorina. The major extracellular β-glucosidase BglI (CEL3a) has been shown to contribute to the efficient induction of cellulase genes. Multiple β-glucosidases belonging to glycosyl hydrolase (GH) family 3 and 1, however, exist in H. jecorina. Here we demonstrated that CEL1b, like CEL1a, was an intracellular β-glucosidase displaying in vitro transglycosylation activity. We then found evidence that these two major intracellular β-glucosidases were involved in the rapid induction of cellulase genes by insoluble cellulose. Deletion of cel1a and cel1b significantly compromised the efficient gene expression of the major cellulase gene, cbh1. Simultaneous absence of BglI, CEL1a, and CEL1b caused the induction of the cellulase gene by cellulose to further deteriorate. The induction defect, however, was not observed with cellobiose. The absence of the three β-glucosidases, rather, facilitated the induced synthesis of cellulase on cellobiose. Furthermore, addition of cellobiose restored the productive induction on cellulose in the deletion strains. The results indicate that the three β-glucosidases may not participate in transforming cellobiose beyond hydrolysis to provoke cellulase formation in H. jecorina. They may otherwise contribute to the accumulation of cellobiose from cellulose as inducing signals.

Metabolic engineering of inducer formation for cellulase and hemicellulase gene expression in Trichoderma reesei

The filamentous fungus T. reeseiis today a paradigm for the commercial scale production of different plant cell wall degrading enzymes mainly cellulases and hemicellulases. Its enzymes have a long history of safe use in industry and well established applications are found within the pulp, paper, food, feed or textile processing industries. However, when these enzymes are to be used for the saccharification of cellulosic plant biomass to simple sugars which can be further converted to biofuels or other biorefinery products, and thus compete with chemicals produced from fossil sources, additional efforts are needed to reduce costs and maximize yield and efficiency of the produced enzyme mixtures. One approach to this end is the use of genetic engineering to manipulate the biochemical and regulatory pathways that operate during enzyme production and control enzyme yield. This review aims at a description of the state of art in this area.

Expression of biomass-degrading enzymes is a major event during conidium development in Trichoderma reesei

The conidium plays a critical role in the life cycle of many filamentous fungi, being the primary means for survival under unfavorable conditions. To investigate the transcriptional changes taking place during the transition from growing hyphae to conidia in Trichoderma reesei, microarray experiments were performed. A total of 900 distinct genes were classified as differentially expressed, relative to their expression at time zero of conidiation, at least at one of the time points analyzed. The main functional categories (FunCat) overrepresented among the upregulated genes were those involving solute transport, metabolism, transcriptional regulation, secondary metabolite synthesis, lipases, proteases, and, particularly, cellulases and hemicellulases. Categories overrepresented among the downregulated genes were especially those associated with ribosomal and mitochondrial functions. The upregulation of cellulase and hemicellulase genes was dependent on the function of the positive transcriptional regulator XYR1, but XYR1 exerted no influence on conidiation itself. At least 20% of the significantly regulated genes were nonrandomly distributed within the T. reesei genome, suggesting an epigenetic component in the regulation of conidiation. The significant upregulation of cellulases and hemicellulases during this process, and thus cellulase and hemicellulase content in the spores of T. reesei, contributes to the hypothesis that the ability to hydrolyze plant biomass is a major trait of this fungus enabling it to break dormancy and reinitiate vegetative growth after a period of facing unfavorable conditions.

Dehydrogenase GRD1 represents a novel component of the cellulase regulon in Trichoderma reesei (Hypocrea jecorina)

Trichoderma reesei (Hypocrea jecorina) is nowadays the most important industrial producer of cellulase and hemicellulase enzymes, which are used for pretreatment of cellulosic biomass for biofuel production. In this study, we introduce a novel component, GRD1 (glucose-ribitol dehydrogenase 1), which shows enzymatic activity on cellobiose and positively influences cellulase gene transcription, expression, and extracellular endo-1,4-β-D-glucanase activity. grd1 is differentially transcribed upon growth on cellulose and the induction of cellulase gene expression by sophorose. The transcription of grd1 is coregulated with that of cel7a (cbh1) under inducing conditions. GRD1 is further involved in carbon source utilization on several carbon sources, such as those involved in lactose and D-galactose catabolism, in several cases in a light-dependent manner. We conclude that GRD1 represents a novel enhancer of cellulase gene expression, which by coregulation with the major cellulase may act via optimization of inducing mechanisms.

Metabolic engineering strategies for the improvement of cellulase production by Hypocrea jecorina

Hypocrea jecorina (= Trichoderma reesei) is the main industrial source of cellulases and hemicellulases used to depolymerise plant biomass to simple sugars that are converted to chemical intermediates and biofuels, such as ethanol. Cellulases are formed adaptively, and several positive (XYR1, ACE2, HAP2/3/5) and negative (ACE1, CRE1) components involved in this regulation are now known. In addition, its complete genome sequence has been recently published, thus making the organism susceptible to targeted improvement by metabolic engineering. In this review, we summarise current knowledge about how cellulase biosynthesis is regulated, and outline recent approaches and suitable strategies for facilitating the targeted improvement of cellulase production by genetic engineering.

Degradation of cellulose by the bean-pathogenic fungus Colletotrichum lindemuthianum. Production of extracellular cellulolytic enzymes by cellulose induction

Colletotrichum lindemuthianum was able to grow and produce extracellular cellulolytic activity in a defined medium containing cellulose as the main carbon substrate. As measured either by the hydrolysis of 4-methylumbelliferyl-beta-D -cellotrioside or the release of glucose from carboxymethylcellulose, activity reached a peak after 13 days of incubation and then declined whereas growth markedly increased afterwards. Detection of glucose in carboxymethylcellulose hydrolysates suggested the concerted operation of endo-1,4-beta-glucanase, cellobiohydrolase (exo-1,4-beta-glucanase) and beta-glucosidase activities. The highest levels of cellulolytic activity were obtained in media supplemented with cellulose and glutamate. Other carbon and nitrogen sources markedly influenced growth and enzyme production. Oligonucleotides homologous to specific regions of the cellobiohydrolase-encoding cbhII gene from Trichoderma reesei were used to isolate a C. lindemuthianum cbhII-DNA fragment whose sequence revealed homologies of 98% and 92% with the nucleotide and the deduced amino acid sequences of the corresponding cbhII-DNA of T. reesei, respectively. RT-PCR and Southern blot analyses of total RNA samples obtained from cellulose-grown but not from glucose-grown mycelium revealed the expression of the corresponding cbhII transcript. The cbhII-cDNA fragment was cloned and sequenced.

Cellulose degrading enzymes and their potential industrial applications

Bioconversion of cellulose to soluble sugars and glucose is catalyzed by a group of enzymes called cellulases. Microorganisms including fungi, bacteria and actinomycetes produce mainly three types of cellulase components--endo-1,4-beta-D-glucanase, exo-1,4-beta-D-glucanase and beta-glucosidase--either separately or in the form of a complex. Over the last several decades, cellulases have become better understood at a fundamental level; nevertheless, much remains to be learnt. The tremendous commercial potential of cellulases in a variety of applications remains the driving force for research in this area. This review summarizes the present state of knowledge on microbial cellulases and their applications.

Regulation of Trichoderma cellulase formation: lessons in molecular biology from an industrial fungus. A review

The present article reviews the current understanding of regulation of cellulase gene transcription in Hypocrea jecorina (= Trichoderma reesei). Special emphasis is put on the mechanism of action of low molecular weight inducers of cellulase formation, the presence and role of recently identified transactivating proteins (Ace1, Ace2, Hap2/3/5), and the role of the carbon catabolite repressor Cre1. We also report on some recent genomic approaches towards understanding how cellulase inducers signal their presence to the transcriptional apparatus.

Accumulation of stress and inducer-dependent plant-cell-wall-degrading enzymes during asexual development in Aspergillus nidulans

Determination and interpretation of fungal gene expression profiles based on digital reconstruction of expressed sequenced tags (ESTs) are reported. A total of 51,524 DNA sequence files processed with PipeOnline resulted in 9775 single and 5660 contig unique ESTs, 31.2% of a typical fungal transcriptome. Half of the unique ESTs shared homology with genes in public databases, 35.8% of which are functionally defined and 64.2% are unclear or unknown. In Aspergillus nidulans 86% of transcripts associate with intermediate metabolism functions, mainly related to carbohydrate, amino acid, protein, and peptide biosynthesis. During asexual development, A. nidulans unexpectedly accumulates stress response and inducer-dependent transcripts in the absence of an inducer. Stress response genes in A. nidulans ESTs total 1039 transcripts, contrasting with 117 in Neurospora crassa, a 14.3-fold difference. A total of 5.6% of A. nidulans ESTs implicate inducer-dependent cell wall degradation or amino acid acquisition, 3.5-fold higher than in N. crassa. Accumulation of stress response and inducer-dependent transcripts suggests general derepression of cis-regulation during terminal asexual development.

Molecular characterization of a cellulase-negative mutant of Hypocrea jecorina

The "cbh2 activating element," CAE, consisting of two separate boxes (ATTGG = CCAAT and GTAATA, respectively) is essential for cellobiohydrolase II gene expression in the filamentous fungus Hypcrea jecorina. Here we report that cell-free extracts from a cellulase-negative mutant form CAE-protein complexes with higher mobility and lower binding-strength compared to the wild type. EMSA analysis demonstrated an increased mobility of the GTAATA-binding protein complex and, supported by in vivo footprinting, a lowered binding strength of the HAP2/3/5 proteins. However, the hap2/hap3/hap5 genes of the mutant are unaltered and transcribed normally. A nucleotide fragment of the cbh1 promoter containing a (GG)CTAATA motif without an adjacent CCAAT box is also bound by cell-free extracts of H. jecorina, and the protein-DNA complex of the mutant shows the characteristic increase in mobility. We conclude that this mutant is defective in the functional formation of the CAE-protein complexes but not in their binding to the target sequences itself.

Role of four major cellulases in triggering of cellulase gene expression by cellulose in Trichoderma reesei

The relative contributions of four major cellulases of Trichoderma reesei (1,4-beta-D-glucan cellobiohydrolase I [CBH I], CBH II, endo-1,4-beta-D-glucanase I [EG I], and EG II) to the generation of the cellulase inducer from cellulose were studied with isogenic strains in which the corresponding genes (cbh1, cbh2, egl1, and egl2) had been deleted by insertion of the Aspergillus nidulans amdS marker gene. During growth on lactose (a soluble carbon source provoking cellulase gene expression), these strains showed no significant alterations in their ability to express the respective other cellulase genes, with the exception of the strain containing delta cbh1, which exhibited an increased steady-state level of cbh2 mRNA. On crystalline cellulose as the only carbon source, however, significant differences were apparent: strains in which cbh2 and egl2, respectively, had been deleted showed no expression of the other cellulase genes, whereas strains carrying the cbh1 or egl1 deletion showed these transcripts. The delta cbh1-containing strain also showed enhanced cbh2 mRNA levels under these conditions. A strain in which both cbh1 and cbh2 had been deleted, however, was unable to initiate growth on cellulose. Addition of 2 mM sophorose, a putative inducer of cellulase gene expression, to such cultures induced the transcription of egl1 and egl2 and restored the ability to grow on cellulose. We conclude that CBH II and EG II are of major importance for the efficient formation of the inducer from cellulose in T. reesei and that removal of both cellobiohydrolases renders T. reesei unable to attack crystalline cellulose.

Regulation of cellulase gene expression in the filamentous fungus Trichoderma reesei

Basic features of regulation of expression of the genes encoding the cellulases of the filamentous fungus Trichoderma reesei QM9414, the genes cbh1 and cbh2 encoding cellobiohydrolases and the genes egl1, egl2 and egl5 encoding endoglucanases, were studied at the mRNA level. The cellulase genes were coordinately expressed under all conditions studied, with the steady-state mRNA levels of cbh1 being the highest. Solka floc cellulose and the disaccharide sophorose induced expression to almost the same level. Moderate expression was observed when cellobiose or lactose was used as the carbon source. It was found that glycerol and sorbitol do not promote expression but, unlike glucose, do not inhibit it either, because the addition of 1 to 2 mM sophorose to glycerol or sorbitol cultures provokes high cellulase expression levels. These carbon sources thus provide a useful means to study cellulase regulation without significantly affecting the growth of the fungus. RNA slot blot experiments showed that no expression could be observed on glucose-containing medium and that high glucose levels abolish the inducing effect of sophorose. The results clearly show that distinct and clear-cut mechanisms of induction and glucose repression regulate cellulase expression in an actively growing fungus. However, derepression of cellulase expression occurs without apparent addition of an inducer once glucose has been depleted from the medium. This expression seems not to arise simply from starvation, since the lack of carbon or nitrogen as such is not sufficient to trigger significant expression.

The biological degradation of cellulose

Cellulolytic microorganisms play an important role in the biosphere by recycling cellulose, the most abundant carbohydrate produced by plants. Cellulose is a simple polymer, but it forms insoluble, crystalline microfibrils, which are highly resistant to enzymatic hydrolysis. All organisms known to degrade cellulose efficiently produce a battery of enzymes with different specificities, which act together in synergism. The study of cellulolytic enzymes at the molecular level has revealed some of the features that contribute to their activity. In spite of a considerable diversity, sequence comparisons show that the catalytic cores of cellulases belong to a restricted number of families. Within each family, available data suggest that the various enzymes share a common folding pattern, the same catalytic residues, and the same reaction mechanism, i.e. either single substitution with inversion of configuration or double substitution resulting in retention of the beta-configuration at the anomeric carbon. An increasing number of three-dimensional structures is becoming available for cellulases and xylanases belonging to different families, which will provide paradigms for molecular modeling of related enzymes. In addition to catalytic domains, many cellulolytic enzymes contain domains not involved in catalysis, but participating in substrate binding, multi-enzyme complex formation, or possibly attachment to the cell surface. Presumably, these domains assist in the degradation of crystalline cellulose by preventing the enzymes from being washed off from the surface of the substrate, by focusing hydrolysis on restricted areas in which the substrate is synergistically destabilized by multiple cutting events, and by facilitating recovery of the soluble degradation products by the cellulolytic organism. In most cellulolytic organisms, cellulase synthesis is repressed in the presence of easily metabolized, soluble carbon sources and induced in the presence of cellulose. Induction of cellulases appears to be effected by soluble products generated from cellulose by cellulolytic enzymes synthesized constitutively at a low level. These products are presumably converted into true inducers by transglycosylation reactions. Several applications of cellulases or hemicellulases are being developed for textile, food, and paper pulp processing. These applications are based on the modification of cellulose and hemicellulose by partial hydrolysis. Total hydrolysis of cellulose into glucose, which could be fermented into ethanol, isopropanol or butanol, is not yet economically feasible. However, the need to reduce emissions of greenhouse gases provides an added incentive for the development of processes generating fuels from cellulose, a major renewable carbon source.

Characterization of the Trichoderma reesei cbh2 promoter

A 613-bp fragment of the 5' upstream region of the Trichoderma reesei cbh2 gene (coding for the cellulolytic enzyme cellobiohydrolase II) has been isolated and sequenced. Fusion of this fragment to the E. coli uidA gene (coding for beta-glucuronidase) leads to--albeit low--expression of beta-glucuronidase activity in the presence of cellulose and upon the addition of low molecular weight inducers (sophorose, lactose) of cellobiohydrolase II. It also governed the formation of beta-glucuronidase activity during sporulation and its transport to the conidial surface. However, despite the presence of a signal peptide in the cbh2:uidA fusion, beta-glucuronidase was not secreted in T. reesei. Defined fragments of the 613-bp promoter region were isolated and used to identify areas involved in the regulation of cbh2 expression by protein-DNA binding assays. At least two binding areas--between -443/-363 and -363/-173, respectively--were identified. In both areas, the DNA-protein complex observed was appreciably larger when cell-free extracts from sophorose-induced mycelia were used. This suggests that at least one of the proteins regulating cbh2 transcription is itself induced by cellulose.

The Trichoderma cellulase regulatory puzzle: from the interior life of a secretory fungus

Novel applications for cellulases have reinitiated interest in the regulation of production of these enzymes by the soft rot fungus Trichoderma reesei and related species. This paper reviews the current state of knowledge concerning the question "How can insoluble molecules like cellulose initiate their own breakdown by a microorganism?" The evidence available--based on biochemical as well as molecular biological approaches--favors a model in which conidial bound cellobiohydrolases carry out a first exo-exo-wise attack on the cellulose molecule. The disaccharides so formed (cellobiose, alpha-cellobiono-1,5-lactone) are then taken up by the mycelia and promote further cellulase biosynthesis. Evidence available suggests that they are further metabolized to, rather than being, the "true" inducer. Speculations on the nature of the inducer are presented. The roles of the beta-glucosidases of Trichoderma in this process are discussed. The pathway of cellulase secretion is discussed on the basis of electron microscopical as well as gene sequence information.