Fusarium graminearum cerato-platanin proteins weaken cellulosic materials and enhance cellulase activity in an expansin-like manner

Cerato-platanin proteins (CPPs) belong to a family of small secreted non-catalytic fungal proteins with phytotoxic activity. CPPs have been recently classified as expansin-like proteins because of structural and functional features related to plant expansins, small secreted proteins able to loosen and disrupt the non-covalent bonding networks of plant cell wall polysaccharides without enzymatic activity. The genome of Fusarium graminearum, the causal agent of Fusarium head blight disease of wheat and other cereal grains, contains two genes putatively encoding for CPPs (FgCPPs). To characterize their role, the two proteins have been heterologously expressed in yeast. Enzymatic assays have shown the ability of the recombinant FgCPPs to reduce the viscosity of a cellulose soluble derivate (carboxymethyl cellulose, CMC) mainly with a non-enzymatic activity. Indeed, differently from other fungal CPPs and similarly to expansins, FgCPPs seem trapped by cellulose and not by chitin, thus suggesting that they could interact with cellulose. The incubation of CMC with a cellulase in presence or absence of the two recombinant proteins has shown that the FgCPPs enhance cellulase activity. A double knock-out mutant deleted of both FgCPPs encoding genes produces higher cellulase activity when grown on CMC, thus suggesting that the absence of FgCPPs forces the fungus to produce more cellulase activity to compensate for the lack of expansin-like activity. Finally, the preliminary demonstration that the FgCPPs act also loosening filter paper, a natural insoluble cellulose, could suggest a possible future biotechnological application in second-generation biofuels production from agricultural lignocellulosic biomasses rich in cellulose.