Evidence for Transceptor Function of Cellodextrin Transporters in Neurospora crassa

Background: Mechanisms filamentous fungi use to sense cellulose in the environment remain unclear. Results:Neurospora crassa cellodextrin transporters, CDT-1 and CDT-2, are required to induce the expression and secretion of cellulases, using a mechanism distinct from that in Trichoderma reesei. Conclusion: CDT-1 and/or CDT-2 may act as transporting receptors (transceptors). Significance: CDT-1- and/or CDT-2-mediated cellodextrin signaling may be conserved in industrially useful fungi. Neurospora crassa colonizes burnt grasslands and metabolizes both cellulose and hemicellulose from plant cell walls. When switched from a favored carbon source to cellulose, N. crassa dramatically up-regulates expression and secretion of genes encoding 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 -glucosidase enzymes (3G) lacks -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. Here, we show that in N. crassa, two cellodextrin transporters, CDT-1 and CDT-2, contribute to cellulose sensing. A N. crassa mutant carrying deletions for both transporters is unable to induce cellulase gene expression in response to crystalline cellulose. Furthermore, a mutant lacking genes encoding both the -glucosidase enzymes and cellodextrin transporters (3G2T) does not induce cellulase gene expression in response to cellobiose. Point mutations that severely reduce cellobiose transport by either CDT-1 or CDT-2 when expressed individually do not greatly impact cellobiose induction of cellulase gene expression. These data suggest that the N. crassa cellodextrin transporters act as transceptors with dual functions - cellodextrin transport and receptor signaling that results in downstream activation of cellulolytic gene expression. Similar mechanisms of transceptor activity likely occur in related ascomycetes used for industrial cellulase production.