Interaction of cellulolytic enzymes with fungal cell wall polysaccharides

Wood rotting fungi are the primary agents responsible for the decomposition of wood in natural environments. These fungi secrete a variety of enzymes to degrade the chemically and structurally resistant components of the wood cell wall. Some of these enzymes possess a carbohydrate-binding module (CBM), which enhances enzymatic efficiency by increasing their affinity to the substrate. Previously, wood cell wall-degrading enzymes have been considered to function as free entities acting independently on cellulose and lignin polymers. Wood rotting fungi often form an extracellular matrix (ECM) during the decay process. The ECM is mainly composed of a polysaccharide mixture, including β-1,3-glucan and β-1,6-glucan, which may form a gel-like structure that facilitates adhesion to the wood cell wall. Previous studies have suggested that the ECM fills the space between the fungal hyphae and the wood cell wall and may serve as a reaction site for extracellular enzymes. However, the mechanisms underlying the interactions between ECM and wood-degrading enzymes remain poorly understood. In this study, we hypothesised that the ECM retains degrading enzymes by interacting with the binding domains. Based on this hypothesis, to investigate the interactions between fungal cell wall polysaccharides and degrading enzymes, we conducted binding experiments using recombinant proteins in which CBMs were fused to red fluorescent protein (RFP). Furthermore, to examine the role of ECM in the enzymatic degradation system of wood cell wall, we compared enzyme activity profiles between the culture supernatant and the fungal cell wall extract.