The Discovery of Novel ER-Localized Cellobiose Transporters Involved in Cellulase Biosynthesis in Trichoderma reesei

Sugar transporters are of great importance in sensing and transporting varied sugars for cellulase biosynthesis of lignocellulolytic fungi. Nevertheless, the function and the relevant mechanism of sugar transporters in fungal cellulase biosynthesis remain to be explored. Here, putative maltose transporters Mal1, Mal2, Mal3, Mal4, and Mal5 in Trichoderma reesei were investigated. Despite that only the transcriptional abundance of Mal1 was upregulated under cellulase-generating condition, the individual deletion of Mal1, Mal2, Mal3, Mal4, and Mal5 all impaired cellulase biosynthesis. The possible reason for this is that the individual knockout of Mal2, Mal3, Mal4, and Mal5 resulted in no gene expression of Mal1 at 24 h during the cellulase production. The transcriptional analysis showed that the absence of these transporters noticeably inhibited cellulase genes at 24 h, which was then relieved. Interestingly, the individual missing of these maltose transporters significantly retarded the cellular consumption of cellobiose, rather than maltose, and they were distributed in cytoplasm, largely in endoplasmic reticulum (ER). These findings manifested that these putative maltose transporters may be in fact endomembrane cellobiose transporters, influencing fungal cellulase generation probably through Mal1 at the early stage. This research advances the knowledge of endomembrane sugar transporters in fungal cellulase biosynthesis.