8-02: Engineering yeasts for enhanced cellulase production

The biological conversion of cellulose to ethanol is preceded by the enzymatic conversion of the substrate to soluble sugars that can be assimilated by a fermentative organism, adding additional costs to the process.  Consolidation of these events in a single process step via a cellulolytic microorganism(s) is a promising approach to low-cost production of fuels and chemicals.  One strategy for developing a microorganism capable of such consolidated bioprocessing involves engineering the yeast Saccharomyces cerevisiae so that it expresses a heterologous enzyme system enabling crystalline cellulose utilization. 

 We have illustrated proof of concept in the laboratory through the successful expression of the three major cellulase activities (β‑glucosidase, endoglucanase and cellobiohydrolase) in S. cerevisiae laboratory strains but secreted titers were low in some cases.  The secretion machinery of yeasts is a multi-step process and each step is directed and regulated by several proteins, providing a vast array of targets that can be manipulated to enhance heterologous protein secretion.  We have illustrated that the production of different cellulases by S. cerevisiae can be enhanced by overexpressing or deleting genes encoding native target proteins involved in the secretion pathway or stress relief response in yeast.  We will also discuss the use of pooled-segregant whole genome sequencing analysis to identify currently unknown gene targets for enhancing secretion of cellulases by yeast.