Intracellular β-glucosidase regulates cellulase expression and development in Aspergillus nidulans.

β-Glucosidases (EC 3.2.1.21) are essential enzymes involved in biomass degradation and metabolic regulation, but the physiological roles of intracellular β-glucosidases in filamentous fungi remain incompletely understood. In this study, we characterized CbgA (AN10124) and CbgB (AN10375), two intracellular glycoside hydrolase family 1 β-glucosidases, in Aspergillus nidulans. Gene deletion and biochemical analyses demonstrated that CbgA is the predominant intracellular β-glucosidase. Loss of cbgA led to overactivation of cellulases, cellobiose-dependent accumulation of reddish-brown secondary metabolites, and a significant reduction in conidiation. Crucially, deletion of the cellobiose transporter gene cltB in the ΔcbgA background markedly attenuated these phenotypes, providing direct genetic evidence that the ΔcbgA-associated defects are driven by intracellular cellobiose accumulation rather than energy deficiency. Our findings identify CbgA as a critical "signal gatekeeper" that modulates the intensity of cellobiose-dependent induction. By maintaining the intracellular cellobiose pool within a physiological range, CbgA prevents secretory overload and maintains the metabolic balance between primary development and secondary metabolism. This study clarifies the coordination between nutrient transport and intracellular metabolism in shaping global regulatory outputs, suggesting that the targeted modulation of CbgA activity represents a potential strategy for optimizing cellulase production in fungal cell factories. KEY POINTS: CbgA negatively regulates cellulase expression by controlling intracellular cellobiose levels. Loss of cbgA leads to hyperpigmentation and reduced conidiation on cellobiose. Deletion of cltB alleviates ΔcbgA phenotypes, confirming the role of intracellular cellobiose.