Secretome discovery reveals lignocellulose degradation capacity of the ectomycorrhizal fungus Paxillus involutus

To improve our understanding of the role ectomycorrhizal fungi play in biomass conversion, we studied the transcriptome of P. involutus grown on glass beads in extract of soil organic matter. The mycelium was used for a cDNA library screened by Transposon-Assisted Signal Trapping (TAST*) for genes encoding secreted proteins. We identified 11 glycoside hydrolases (GH), none of them being cellulases of the GH families 6, 7 and 45, which constitute the well described enzymatic cellulose degradation system from numerous efficient cellulolytic fungi. In contrast, several predicted enzymes, namely a laccase and oxidoreductases possibly contribute to hydroxyl radical formation. The most abundant GH found was GH61, although typically described as accessory protein in the enzymatic cellulolytic apparatus. All in all, our results suggest that the cellulose degradation system of P. involutus resembles the brown rot fungi systems. In addition, GH61 apparently acts as accessory protein both in enzymatic and in radical-based cellulolysis. * Becker et al., J. Microbial Methods, 2004, 57(1), 123-33.