Distribution of Crystalline Cellulose-Binding Domain CBM104 in Wood Rotting Fungi

Wood rotting fungi play a crucial role in biodeterioration of wood. Many brown-rot fungi are known to rapidly degrade cellulose in wood despite lacking enzymes with cellulose-binding domains. This has led to the hypothesis that they rely on a non-enzymatic degradation system. We recently discovered a novel cellulose-binding domain, CBM104, in the brown-rot fungus Gloeophyllum trabeum. In this study, we analysed the function of this domain, GtAA9-CBM104. Adsorption assays revealed that GtAA9-CBM104 specifically binds to native crystalline cellulose (cellulose Iα and Iβ) but does not bind to artificially modified crystalline cellulose (cellulose II and IIII) or amorphous cellulose (phosphoric acid swollen cellulose). Further investigation of its binding sites suggested that, unlike CBM1, which interacts with the hydrophobic surface of cellulose, GtAA9-CBM104 preferentially binds to hydrophilic cellulose regions. To determine whether other CBM104 homologs exhibit similar adsorption properties, we tested six selected CBM104 variants with different appended domains. Five of these homologs exhibited specific adsorption to cellulose I, suggesting that the adsorption property similar to GtAA9-CBM104 is widely conserved. Structural modelling based on amino acid sequences suggested that these five CBM104s contain two disulfide bonds stabilising two α-helices, whereas the CBM104 that did not bind to any cellulose lacked these disulfide bonds. In order to investigate the distribution of CBM104, we conducted a tBLASTn search against publicly available fungal genomes. Our results identified 144 of CBM104 sequences in 39 species of basidiomycetes, primarily wood-decaying fungi. These findings suggest that certain wood rotting fungi may utilise CBM104 in a previously unknown cellulose degradation mechanism. This study provides new insights into the degradation strategies employed by wood rotting fungi and expands our understanding of fungal cellulose metabolism.