Dissecting Cellular Function and Distribution of β-Glucosidases in Trichoderma reesei

Trichoderma reesei has 11 putative β-glucosidases in its genome, playing key parts in the induction and production of cellulase. Nevertheless, the reason why the T. reesei genome encodes so many β-glucosidases and the distinct role each β-glucosidase plays in cellulase production remain unknown. In the present study, the cellular function and distribution of 10 known β-glucosidases (CEL3B, CEL3E, CEL3F, CEL3H, CEL3J, CEL1A, CEL3C, CEL1B, CEL3G, and CEL3D) were explored in T. reesei, leaving out BGL1 (CEL3A), which has been well investigated. We found that the overexpression of cel3b or cel3g significantly enhanced extracellular β-glucosidase production, whereas the overexpression of cel1b severely inhibited cellulase production by cellulose, resulting in nearly no growth of T. reesei Four types of cellular distribution patterns were observed for β-glucosidases in T. reesei: (i) CEL3B, CEL3E, CEL3F, and CEL3G forming clearly separated protein secretion vesicles in the cytoplasm; (ii) CEL3H and CEL3J diffusing the whole endomembrane as well as the cell membrane with protein aggregation, like a reticular network; (iii) CEL1A and CEL3D in vacuoles; (iv) and CEL3C in the nucleus. β-glucosidases CEL1A, CEL3B, CEL3E, CEL3F, CEL3G, CEL3H, and CEL3J were identified as extracellular, CEL3C and CEL3D as intracellular, and CEL1B as unknown. The extracellular β-glucosidases CEL3B, CEL3E, CEL3F, CEL3H, and CEL3G were secreted through a tip-directed conventional secretion pathway, and CEL1A, via a vacuole-mediated pathway that was achieved without any signal peptide, while CEL3J was secreted via an unconventional protein pathway bypassing the endoplasmic reticulum (ER) and Golgi.IMPORTANCE Although β-glucosidases play an important role in fungal cellulase induction and production, our current understanding does not provide a global perspective on β-glucosidase function. This work comprehensively studies all the β-glucosidases regarding their effect on cellulase production and their cellular distribution and secretion. Overexpression of cel3b or cel3g significantly enhanced β-glucosidase production, whereas overexpression of cel1b severely inhibited cellulase production on cellulose. In addition, overexpression of cel3b, cel3e, cel3f, cel3h, cel3j, cel3c, or cel3g delayed endoglucanase (EG) production. We first identified four cellular distribution patterns of β-glucosidases in Trichoderma reesei Specially, CEL3C was located in the nucleus. CEL3J was secreted through the nonclassical protein secretion pathway bypassing endoplasmic reticulum (ER) and Golgi. CEL1A was secreted via a vacuole-mediated conventional secretion route without a signal peptide. These findings advance our understanding of β-glucosidase properties and secretory pathways in filamentous fungi, holding key clues for future study.