The Aspergillus nidulans signalling mucin MsbA regulates starvation responses, adhesion and affects cellulase secretion in response to environmental cues

In the heterogeneous semi-solid environment naturally occupied by lignocellulolytic fungi the majority of nutrients are locked away as insoluble plant biomass. Hence, lignocellulolytic fungi must actively search for, and attach to, a desirable source of nutrients. During growth on lignocellulose a period of carbon deprivation provokes carbon catabolite derepression and scavenging hydrolase secretion. Subsequently, starvation and/or contact sensing was hypothesized to play a role in lignocellulose attachment and degradation. In Aspergillus nidulans the extracellular signalling mucin, MsbA, influences growth under nutrient-poor conditions including lignocellulose. Cellulase secretion and activity was affected by MsbA via a mechanism that was independent of cellulase transcription. MsbA modulated both the cell wall integrity and filamentous growth MAPK pathways influencing adhesion, biofilm formation and secretion. The constitutive activation of MsbA subsequently enhanced cellulase activity by increasing the secretion of the cellobiohydrolase, CbhA, while improved substrate attachment and may contribute to an enhanced starvation response. Starvation and/or contact sensing therefore represents a new dimension to the already multifaceted regulation of cellulase activity.