Wood-rotting basidiomycetes are the primary microorganisms that decay wood in nature. They are classified as white-rot fungi and brown-rot fungi by the difference in decaying types. White-rot fungi secrete a variety of cellulolytic enzymes during wood degradation. These enzymes often have an additional cellulose-binding domain (CBD) that adsorbs to the cellulose surface and localizes the catalytic domain close to the substrate. Cellulases lacking CBD have reduced degradation efficiency on crystalline cellulose, and it has been considered that CBD is an essential factor in the degradation of crystalline cellulose. On the other hand, most brown-rot fungi lack CBD. It has been believed that brown-rot fungi do not possess an enzymatic degrading system that efficiently degrades crystalline cellulose in wood. However, we have found a novel CBD from the brown-rot fungus Gloeophyllum trabeum and named it GtCBD. GtCBD specifically bound to native crystalline cellulose (cellulose I) and showed higher adsorption efficiency than that of CBM1, while it did not adsorb to artificially modified crystalline cellulose or amorphous cellulose to which CBM1 often binds. Confocal laser microscopy and scanning electron microscopy observations suggested that the adsorption site of GtCBD is the hydrophilic corner of cellulose microfibrils, unlike any previously known CBD. These unique properties of GtCBD may be applicable to morphological evaluations of cellulose microfibrils. Therefore, to evaluate the morphology of cellulose microfibrils in wood cell walls, the localization of GtCBD on lignin-removed wood sections was observed. As a result, GtCBD was observed to be strongly localized in the pits and primary walls. This result suggested the presence of cellulose microfibrils with exposed corners of the hydrophilic surface at these sites.

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