P32: Transport of cellobiose and signaling to induce cellulase gene expression is mediated cellodextrin transporters in Neurospora crassa

Neurospora crassa colonizes burnt grasslands in the wild and metabolizes both cellulose and hemicellulose from plant cell walls. When switched from a favored carbon source to cellulose, N. crassa dramatically upregulates expression and secretion of a wide variety of lignocellulolytic enzymes.  However, the means by which N. crassa and other filamentous fungi sense the presence of cellulose in the environment remains unclear. Previously, we have shown that a N. crassa mutant carrying deletions of three predicted β-glucosidase enzymes lacks β-glucosidase activity, but efficiently induces cellulase gene expression and cellulolytic activity in the presence of cellobiose as the sole carbon source. These observations indicate that cellobiose, or a modified version of cellobiose, functions as an inducer of lignocellulolytic gene expression and activity in N. crassa. More recently, we have begun to examine the role of two cellodextrin transporters in cellulose sensing. Here, we show that a N. crassa mutant carrying deletions for both transporters is unable to induce cellulase gene expression in response to cellulose. Furthermore, a mutant lacking genes encoding both the β-glucosidase enzymes and cellodextrin transporters does not induce cellulase gene expression in response to either cellobiose or cellulose. By examining point mutants in the cellodextrin transporters that fail to transport cellobiose,  we show that transport is not required for the induction of cellulase gene expression. These data suggest that the cellodextrin transporters show dual functionality – that of cellodextrin transport as well as functioning similar to a receptor that results in downstream signaling and induction of cellulolytic genes.