Wood-rotting basidiomycetes are the major organisms decomposing wood in nature. They are classified into two groups based on their decay modes; white-rot fungi and brown-rot fungi. White-rot fungi secrete various cellulolytic enzymes during the wood degradation process. The enzymes are known to be often appended with a cellulose binding domain (CBD) which assists the activity of catalytic domain. On the other hand, most brown-rot fungi lack enzymes that continuously degrade crystalline cellulose and CBD. Therefore, it has been considered that brown-rot fungi do not have an enzymatic degradation system capable of efficiently degrading crystalline cellulose in wood. In this study, we found a novel CBD from the brown-rot fungus Gloeophyllum trabeum. The deduced amino acid sequence of this novel CBD (GtCBD) showed no homology to any known CBD. The three-dimensional structure model of GtCBD predicted from the amino acid sequence suggested the structure consisted of two α-helices, which was quite different from that of CBM1, typical fungal CBD. The present study also showed that GtCBD specifically bound towards native crystalline cellulose (cellulose I) with higher adsorption efficiency than those of CBM1, while it did not adsorb to artificially modified crystalline cellulose and amorphous cellulose to which CBM1 binds well. These results clearly indicate that GtCBD has a different adsorption mechanism from known CBM1s, which hydrophobically interacts with the exposed hydrophobic surface of cellulose. The finding obtained in this study that G. trabeum has the novel CBD provides a new perspective on the enzymatic degradation mechanism of brown-rot fungi.

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