Adsorption Capacity of CBM104s Appended to Different Types of Catalytic Domains

Filamentous fungi use various enzymes to degrade cellulose, some of which contain cellulose-binding domains (CBDs), most of which belong to carbohydrate-binding module family 1 (CBM1). We recently identified the novel fungal CBD, CBM104, from Gloeophyllum trabeum. Reportedly, CBM104 specifically binds to native crystalline cellulose, not to amorphous or artificially modified crystalline cellulose, exhibiting a unique adsorption characteristic. To gain further insights into CBM104, the adsorption properties of six different CBM104s, each appended to a different catalytic domain, were investigated. The adsorption tests illustrated that all CBM104s predicted to possess a three-dimensional structure in which two α-helices were crosslinked by disulfide bonds specifically adsorbed onto cellulose I. Conversely, CBM104 lacking these disulfide bonds failed to adsorb onto any form of cellulose used in this study, suggesting the importance of the fixed pair of α-helices for specific binding to cellulose I. To identify CBM104 homologs in which the disulfide bonds are conserved, a homology search was performed against fungal genomes, resulting in 144 hits. These CBM104 homologs were primarily appended to auxiliary activities (AA) family 9 or to domains that work cooperatively with AA9 enzyme. CBM104s were found only in certain orders of Agaricomycetes, and the majority of these fungi are suggested to have the ability to degrade plant cell walls. These results suggest that some Agaricomycetes utilize plant cell wall degradation systems involving CBM104-attached proteins. This study provides detailed insights into the structural factors involved in the adsorption capacity of CBM104, as well as its phylogenetic distribution.