UC Berkeley

Liquid fuels derived from plant biomass have the potential to contribute to sustainability efforts. However, the conversion of plant biomass is very difficult due to its complex nature. Cellulose, the primary component of plant cell walls, is a recalcitrant polysaccharide comprised of glucose. In nature, various fungi and bacteria enzymatically depolymerize cellulose for consumption. Understanding how cellulases are produced and function will be invaluable to the realization of cellulosic biofuels. The work here describes how a prominent cellulase interacts with cellulose during depolymerization, and how the cellulolytic fungus Neurospora crassa in part signals for the production of cellulase enzymes.

The interactions of cellulases with cellulose are not well understood due to the recalcitrance and heterogeneity of the solid substrate. Cellobiohydrolase I, the most prominent cellulase in many fungal systems, was fluorescently labeled and monitored on the surface of crystalline cellulose. Four classes of behaviors were observed for this enzyme on the surface of cellulose: productive and linear movement, nonproductive surface diffusion, stationary, and transient association. In parallel, changes to the surface of crystalline cellulose were observed with atomic force microscopy to characterize how this enzyme modifies its substrate after degradation.

The processes that initiate cellulase induction in Neurospora crassa have yet to be identified. A screen of deletion strains grown on cellulose was performed to isolate regulators of cellulase production. A kinase (NCU07399) was identified as a regulator of the transcriptional activation of several cellulase genes. Deletion of this kinase in Neurosporareduces the upregulation of cellulases by as much as 50-fold and also induces the secretion of a metalloprotease. The results here provide a basis for exploring the pathways involved in N. crassa cellulase production.