Dissociation of the multi-enzyme complex of the brown-rot fungusPostia placenta

In vitro cell composition identification of wood decay fungi by Fourier transform infrared spectroscopy

Decay pathogens follow dissimilar metabolic mechanisms to cause irreversible damage to woody tissues. The objective of this study is to perform inter- and intra-species microbial cell structural comparison using attenuated total reflectance Fourier transform infrared spectroscopy. Representative fungi species, causing brown rot and white rot, namely, Postia placenta and Trametes versicolor, respectively, were cultured in laboratory conditions. In vitro spectral measurements were performed at periodic two week intervals of fungal growth. The study shows structural differences for both species of fungi. The prominent presence of protein amide, carbohydrate and carboxyl bands was of interest. Spectral deconvolution of the infrared broadband around approximately 3300 cm-1 produced peaks at four different wavenumbers. The hydrogen bond energy obtained at the four wavenumbers, from deconvolution, varied from approximately 41 kJ mol-1 to approximately 7 kJ mol-1, indicating the presence of strong and weak forces in microbial cell structure. The hydrogen bond distance, obtained at the deconvoluted wavenumbers, varied between 2.7 Å-2.8 Å, indicating the presence of short and long-distance forces within microbial cells. Microscopic observation showed mycelium colonization, hyphal tip and lateral branching.

A Fungal Secretome Adapted for Stress Enabled a Radical Wood Decay Mechanism

Brown rot fungi release massive amounts of carbon from forest deadwood, particularly at high latitudes. These fungi degrade wood by generating small reactive oxygen species (ROS) to loosen lignocellulose, to then selectively remove carbohydrates. The ROS mechanism has long been considered the key adaptation defining brown rot wood decomposition, but recently, we found preliminary evidence that fungal glycoside hydrolases (GHs) implicated in early cell wall loosening might have been adapted to tolerate ROS stress and to synergize with ROS to loosen woody lignocellulose. In the current study, we found more specifically that side chain hemicellulases that help in the early deconstruction of the lignocellulosic complex are significantly more tolerant of ROS in the brown rot fungus Rhodonia placenta than in a white rot fungus (Trametes versicolor) and a soft rot fungus (Trichoderma reesei). Using proteomics to understand the extent of tolerance, we found that significant oxidation of secreted R. placenta proteins exposed to ROS was less than half of the oxidation observed for T. versicolor or T. reesei. The principal oxidative modifications observed in all cases were monooxidation and dioxidation/trioxidation (mainly in methionine and tryptophan residues), some of which were critical for enzyme activity. At the peptide level, we found that GHs in R. placenta were the least ROS affected among our tested fungi. These results confirm and describe underlying mechanisms of tolerance in early-secreted brown rot fungal hemicellulases. These enzymatic adaptations may have been as important as nonenzymatic ROS pathway adaptations in brown rot fungal evolution.

Proteomic and functional analysis of the cellulase system expressed by Postia placenta during brown rot of solid wood

Brown rot basidiomycetes have an important ecological role in lignocellulose recycling and are notable for their rapid degradation of wood polymers via oxidative and hydrolytic mechanisms. However, most of these fungi apparently lack processive (exo-acting) cellulases, such as cellobiohydrolases, which are generally required for efficient cellulolysis. The recent sequencing of the Postia placenta genome now permits a proteomic approach to this longstanding conundrum. We grew P. placenta on solid aspen wood, extracted proteins from the biodegrading substrate, and analyzed tryptic digests by shotgun liquid chromatography-tandem mass spectrometry. Comparison of the data with the predicted P. placenta proteome revealed the presence of 34 likely glycoside hydrolases, but only four of these--two in glycoside hydrolase family 5, one in family 10, and one in family 12--have sequences that suggested possible activity on cellulose. We expressed these enzymes heterologously and determined that they all exhibited endoglucanase activity on phosphoric acid-swollen cellulose. They also slowly hydrolyzed filter paper, a more crystalline substrate, but the soluble/insoluble reducing sugar ratios they produced classify them as nonprocessive. Computer simulations indicated that these enzymes produced soluble/insoluble ratios on reduced phosphoric acid-swollen cellulose that were higher than expected for random hydrolysis, which suggests that they could possess limited exo activity, but they are at best 10-fold less processive than cellobiohydrolases. It appears likely that P. placenta employs a combination of oxidative mechanisms and endo-acting cellulases to degrade cellulose efficiently in the absence of a significant processive component.

Degradation of cellulose and hemicelluloses by the brown rot fungus Piptoporus betulinus – production of extracellular enzymes and characterization of the major cellulases

Piptoporus betulinus is a common wood-rotting fungus parasitic for birch (Betula species). It is able to cause fast mass loss of birch wood or other lignocellulose substrates. When grown on wheat straw, P. betulinus caused 65 % loss of dry mass within 98 days, and it produced endo-1,4-β-glucanase (EG), endo-1,4-β-xylanase, endo-1,4-β-mannanase, 1,4-β-glucosidase (BG), 1,4-β-xylosidase, 1,4-β-mannosidase and cellobiohydrolase activities. The fungus was not able to efficiently degrade crystalline cellulose. The major glycosyl hydrolases, endoglucanase EG1 and β-glucosidase BG1, were purified. EG1 was a protein of 62 kDa with a pI of 2.6–2.8. It cleaved cellulose internally, produced cellobiose and glucose from cellulose and cellooligosaccharides, and also showed β-xylosidase and endoxylanase activities. The K m for carboxymethylcellulose was 3.5 g l−1, with the highest activity at pH 3.5 and 70 °C. BG1 was a protein of 36 kDa with a pI around 2.6. It was able to produce glucose from cellobiose and cellooligosaccharides, but also produced galactose, mannose and xylose from the respective oligosaccharides and showed some cellobiohydrolase activity. The K m for p-nitrophenyl-1,4-β-glucoside was 1.8 mM, with the highest activity at pH 4 and 60 °C, and the enzyme was competitively inhibited by glucose (K i=5.8 mM). The fungus produced mainly β-glucosidase and β-mannosidase activity in its fruit bodies, while higher activities of endoglucanase, endoxylanase and β-xylosidase were found in fungus-colonized wood.