Effects of tunicamycin on secretion and enzymatic activities of cellulase fromTrichoderma reesei

Purification and biochemical characterization of two xylanases from Aspergillus sydowii SBS 45

Two xylanases were isolated and purified from crude culture filtrate of Aspergillus sydowii SBS 45 after 9 days of growth on wheat bran containing 0.5% (w/v) birch wood xylan as the carbon source under solid-state fermentation. After a three-step purification scheme involving ammonium sulfate precipitation, gel filtration chromatography (Sephadex G-200), and anion exchange chromatography (DEAE-Sephadex A-50), xylanase I was purified 93.41 times, and xylanase II was purified 77.40 times with yields of 4.49 and 10.46, respectively. Molecular weights of xylanase I and II were 20.1 and 43 kDa, respectively, in sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Optimum temperature was 50 degrees C, and optimum pH was 10.0 for both xylanase I and II. The Km value of xylanase I for birch wood xylan was 3.18 mg ml(-1) and for oat spelt xylan 6.45 mg ml(-1), while the Km value of xylanase II for birch wood xylan was 6.51 mg ml(-1) and for oat spelt xylan 7.69 mg ml(-1). Metal ions like Al3+, Ba2+, Ca2+, Na+, and Zn2+ enhanced the activity of xylanase I and II at 10 mM concentration. Among the additives, L-tryptophan enhanced the activity of xylanase I and II at 10-, 20-, and 30-mM concentrations. Both xylanases appeared to be glycoproteins.

Cellobiase from Termitomyces clypeatus: activity and secretion in presence of glycosylation inhibitors

In presence of the glycosylation inhibitors, 2-deoxy-D-glucose (1 mg/ml), tunicamycin (30 microg/ml), 1-deoxynojirimycin (30 microg/ml) and D-glucono-delta-lactone (1 mg/ml), total cellobiase activity, in the extracellular, intracellular and cell bound fractions, of the fungus Termitomyces clypeatus grown in 20 ml cellobiose medium (1%, w/v) increased by 50-, 1.8-, 2.4-, 1.3-fold, respectively, with respect to control medium (16.3 U). The inhibitors also stimulated secretion of 95% of the total protein in culture medium, except D-glucono-delta-lactone which released 60% of the total protein. 2-Deoxy-D-glucose (1 mg/ml) led to production of extracellular cellobiase up to 40 U/ml, whereas in absence of the inhibitors only 0.59 U/ml enzyme was detected.

Paracoccidioides brasiliensis and paracoccidioidomycosis: molecular approaches to morphogenesis, diagnosis, epidemiology, taxonomy and genetics

Paracoccidioides brasiliensis is an amenable model to study the molecular and biochemical events that lead to morphological transition in fungi, because temperature seems to be the only factor regulating this process. It is the causative agent of paracoccidioidomycosis, a systemic mycosis that affects humans and that is geographically confined to Latin America, where it constitutes one of the most prevalent deep mycoses. With the help of molecular tools, events leading to the morphological transition have been traced to genes that control cell wall glucan and chitin syntheses, and other metabolic processes such as production of heat shock proteins and ornithine decarboxylase activity. Molecular diagnosis and epidemiology of paracoccidioidomycosis are also the focus of intensive research, with several primers being proposed as specific probes for clinical and field uses. Although P. brasiliensis is refractory to cytogenetic analysis, electrophoretic methods have allowed an approximation of its genomic organization and ploidy. Finally, the recognition of P. brasiliensis as an anamorph in the phylum Ascomycota, order Onygenales, family Onygenaceae, has been accomplished by means of molecular tools. This phylogenetic placement has revised the taxonomic position of this fungus, which was traditionally included within now-abandoned higher anamorph taxa, the phylum Deuteromycota and the class Hyphomycetes.

Effect of tunicamycin on N-acetyl-beta-D-glucosaminidase produced by Trichoderma harzianum

The effect of tunicamycin, an inhibitor of protein N-glycosylation, was studied in non-growing mycelium of Trichoderma harzianum induced to secrete N-acetyl-beta-D-glucosaminidase by the addition of N-acetylglucosamine. Tunicamycin (30 microg ml(-1)) had no significant effect on growth of the fungus, or on the total protein secreted or specific activity of N-acetyl-beta-D-glucosaminidase. However, in the presence of the inhibitor an underglycosylated form of the enzyme was produced. The apparent molecular masses for this and the native enzyme were 110 and 124 kDa, respectively. Both forms of the enzyme showed the same optimum pH and temperature, but the underglycosylated form was more sensitive to inactivation by both high temperature (60 degrees C) and the proteolytic enzyme trypsin.

Molecular cloning and characterization of a glucan synthase gene from the human pathogenic fungus Paracoccidioides brasiliensis

1,3-beta-D-glucan is a fungal cell wall polymer synthesized by the multi-subunit enzyme 1,3-beta-D-glucan synthase. A subunit of this integral membrane protein was first described as the product of the FKS1 gene from Saccharomyces cerevisiae using echinocandin mutants. Other FKS1 genes were also reported for Candida albicans, Aspergillus nidulans and Cryptococcus neoformans. Here, we report the nucleotide sequence of the first homologous FKS gene cloned from the pathogenic fungus Paracoccidioides brasiliensis. An open reading frame of 5942 bp was identified in the complete sequence, interrupted by two putative introns, the first close to the 5' end and the second close to the 3' end of the gene. A promoter region is also described containing consensus sequences such as canonical TATA and CAAT boxes and, possibly, multiple sites for glucose regulation by creA protein. The deduced sequence of 1926 amino acid show more than 85% similarity to FksAp from A. nidulans, and 71% to Fks1p and Fks2p from S. cerevisiae. Computational analysis of P. brasiliensis Fks1p suggests a similar structure to transmembrane proteins, such as FksAp, with the presence of two domains composed by hydrophobic helices that limit the putative highly hydrophilic catalytic domain within the cytoplasm.

Molecular and biotechnological aspects of xylanases

Hemicellulolytic microorganisms play a significant role in nature by recycling hemicellulose, one of the main components of plant polysaccharides. Xylanases (EC 3.2.1.8) catalyze the hydrolysis of xylan, the major constituent of hemicellulose. The use of these enzymes could greatly improve the overall economics of processing lignocellulosic materials for the generation of liquid fuels and chemicals. Recently cellulase-free xylanases have received great attention in the development of environmentally friendly technologies in the paper and pulp industry. In microorganisms that produce xylanases low molecular mass fragments of xylan and their positional isomers play a key role in regulating its biosynthesis. Xylanase and cellulase production appear to be regulated separately, although the pleiotropy of mutations, which causes the elimination of both genes, suggests some linkage in the synthesis of the two enzymes. Xylanases are found in a cornucopia of organisms and the genes encoding them have been cloned in homologous and heterologous hosts with the objectives of overproducing the enzyme and altering its properties to suit commercial applications. Sequence analyses of xylanases have revealed distinct catalytic and cellulose binding domains, with a separate non-catalytic domain that has been reported to confer enhanced thermostability in some xylanases. Analyses of three-dimensional structures and the properties of mutants have revealed the involvement of specific tyrosine and tryptophan residues in the substrate binding site and of glutamate and aspartate residues in the catalytic mechanism. Many lines of evidence suggest that xylanases operate via a double displacement mechanism in which the anomeric configuration is retained, although some of the enzymes catalyze single displacement reactions with inversion of configuration. Based on a dendrogram obtained from amino acid sequence similarities the evolutionary relationship between xylanases is assessed. In addition the properties of xylanases from extremophilic organisms have been evaluated in terms of biotechnological applications.

The effects of tunicamycin on secretion, adhesion and activities of the cellulase complex of Clostridium cellulolyticum, ATCC 35319

The effects of tunicamycin, an inhibitor of N-asparagine-linked glycosylation, on the secretion, adhesion and activities of the cellulase complex produced by Clostridium cellulolyticum have been studied. Tunicamycin at 0.1 micrograms/ml slightly inhibited growth on cellobiose. Endoglucanase, p-nitrophenylcellobiosidase and avicelase activities of the "Avicel"-adsorbed fraction from a culture grown with this drug were decreased 4.4-, 1.4- and 12.2-fold, respectively. During growth on cellulose, tunicamycin considerably inhibited growth and adhesion of cells on their substrate (only 28% of the cells were bound to cellulose). SDS-PAGE mobilities of some proteins excreted during growth with the drug were different from those of proteins from control cultures; the native Avicel-adsorbed fraction (PH2O) consisted of three major components of molecular weights about 135, 90 and 68 kDa, whereas in the presence of tunicamycin (0.1 micrograms/ml), the Avicel-adsorbed fraction (PH2OT) contained only a major band of 105 kDa, and the proteins of 135 and 68 kDa appeared weakly. By using the "Dig Glycan Detection" kit, some proteins appeared to be glycosylated, such as the 135-, 95-, 47- and 40-kDa proteins. Moreover, the affinity for Avicel and the avicelase activity decreased dramatically for the Avicel-adsorbed fraction from a culture grown with the drug. The remaining avicelase activity of the PH2O fraction in the presence of specific P135 antiserum was 50% of the initial activity, whereas CMCase and pNPCbase were not affected. The glycosylated protein of 135 kDa played a prominent role in the adhesion and avicelase activity of C. cellulolyticum. Moreover, the endoglucanase activity in a culture broth from tunicamycin-grown cells was more thermolabile and protease-sensitive than that from control cultures.

Protein secretion in filamentous fungi--trying to understand a highly productive black box

Protein secretion is important in all fungi. The majority of proteins by fungi are thought to be glycosylated and many of them are structurally associated with the cell envelope, the plasma membrane and the cell wall. Many of the enzymes secreted by fungi have been incorporated into commercial processes and used in a range of industries. The existence of strains producing very high levels of secreted enzymes stimulated interest in the use of fungi as hosts for the expression of recombinant proteins. Despite the attention that protein secretion in fungi has attracted, and the multifaceted importance of the process, our understanding of the cellular mechanisms involved is still minimal and, for the most part, it is necessary to extrapolate from other eukaryotic organisms. However, current research suggests that protein secretion in filamentous fungi is intimately associated with the process of growth at the hyphal tip. Such unique features merit a detailed study of this important phenomenon.

Expression and characterisation of chymosin pH optima mutants produced in Trichoderma reesei

The production of chymosin mutants designed to have altered pH optima using the cellulolytic filamentous fungus Trichoderma reesei is described. The strong promoter of the gene encoding the major cellulase, cellobiohydrolase I (CBHI) has been used for the expression and secretion of active calf chymosin. Structural analysis of the hydrogen bonding network around the two active site aspartates 32 and 215 in chymosin have suggested that residues Thr 218 and Asp 303 may influence the rate and pH optima for catalysis. The chymosin mutants Thr218Ala and the double mutant Thr218Ala/Asp303Ala have been made by site-directed mutagenesis and expressed in T. reesei. Enzyme kinetics of the active enzyme T218A indicate a pH optimum of 4.2 compared to 3.8 for native chymosin B using a synthetic octa-peptide substrate, confirming the previous analysis undertaken in E. coli. The double mutant T218A/D303A exhibits a similar optimum of 4.4 to that reported for the D303A, indicating that the combination of these changes is not additive. The application of protein engineering in the rational design of specific modifications to tailor the properties of enzymes offers a new approach to the development of industrial processes.

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.

Intra- and extracellular forms of ethanol-modified O-underglycosylated galactose oxidase

The effect of ethanol and tunicamycin on synthesis and secretion of galactose oxidase was studied in resting cells of Dactylium dendroides. Ethanol promoted an overall decrease in both intra- and extracellular enzyme levels to the same extent that it inhibited [14C]glucosamine incorporation into total protein. The carbohydrate content of the intracellular enzyme was also depressed (44%) with a simultaneous decrease in O-Ser linked oligosaccharides. The intracellular galactose oxidase obtained after exposure of mycelia to ethanol plus tunicamycin lost 86% of its carbohydrate moieties, whereas the extracellular form lost only 35%. In both cases, residual sugar moieties were not eliminated by mild alkaline treatment. These data suggest that ethanol affects O-glycosylation of galactose oxidase. O-Underglycosylation did not affect the S0.5 values for galactose but diminished the molar catalytic activity. The absence of O-Ser/Thr-linked saccharides turned the intracellular enzyme into a form more susceptible to proteolysis than that devoid of N-linked sugars (tunicamycin-treated). O-Underglycosylation had a significant effect on the renaturation-reactivation of the enzyme after denaturation with 2.4 M Gdn-HCl.

Regulatory aspects of cellulase biosynthesis and secretion

The cellulase enzyme system consists of cellobiohydrolase, endoglucanase, and beta-glucosidase and has been extensively studied with respect to its biosynthesis, properties, mode of action, application, and, most recently, secretion mechanisms. A knowledge of the factors governing the biosynthesis and secretion of these enzymes at the molecular level will be useful in maximizing enzyme productivity in extracellular fluid. Among other topics, the regulatory effects of sorbose (a noninducing sugar which is not a product of cellulose hydrolysis) on cellulase synthesis and release are described. Cellulase genes have recently been cloned into a number of microorganisms with a view to understanding the gene structure and expression and to obtaining the enzyme components in pure form. The factors governing biosynthesis and secretion of cellulases in recombinant cells are also discussed. Cellulases are known to be glycoproteins, therefore, the role of O- and N-linked glycosylation on enzyme stability and secretion is also detailed.

Glycosylation of bacterial cellulases prevents proteolytic cleavage between functional domains

Glycosylated cellulases from Cellulomonas fimi were compared with their non-glycosylated counterparts synthesized in Escherichia coli from recombinant DNA. Glycosylation of the enzymes does not significantly affect their kinetic properties, or their stabilities towards heat and pH. However, the glycosylated enzymes are protected from attack by a C. fimi protease when bound to cellulose, while the non-glycosylated enzymes yield active, truncated products with greatly reduced affinity for cellulose.

Ethanol Perturbs Glycosylation and Inhibits Hypersecretion in Trichoderma reesei

The effects of ethanol and phenylethanol on the growth of and glycoprotein secretion by Trichoderma reesei were studied. Low levels (1.5%, vol/vol) of ethanol perturbed the glycosylation process, as shown by alterations in the isoelectric profile of the secreted proteins and a reduction in the rate of incorporation of mannose into oligosaccharides. In addition to these effects on posttranslational modification, ethanol drastically lowered the protein secretion level of a hypersecretory strain.