Coordination of the cell wall integrity and high osmolarity glycerol pathways in response to ethanol stress in Saccharomyces cerevisiae

Nisarut Udom, Pakkanan Chansongkrow, Varodom Charoensawan, Choowong Auesukaree
Applied and Environmental Microbiology 2019 May 17
During fermentation, high ethanol concentration is a major stress that influences the vitality and viability of yeast cells, which in turn lead to a termination of fermentation process. In this study, we show that the BCK1 and SLT2 genes encoding MAPKKK and MAPK of the cell wall integrity (CWI) pathway, respectively, are essential for ethanol tolerance, suggesting that the CWI pathway is involved in the response to ethanol-induced cell wall stress. Upon ethanol exposure, the CWI pathway induces the expression of specific cell wall remodeling genes including FKS2 , CRH1 , and PIR3 (encoding β-1,3 glucan synthase, chitin transglycosylase, and O-glycosylated cell wall protein, respectively), which eventually leads to the remodeling of cell wall structure. Our results revealed that, in response to ethanol stress, the high-osmolarity glycerol (HOG) pathway plays a collaborative role with the CWI pathway in inducing cell wall remodeling via the up-regulation of specific cell wall biosynthesis genes such as the CRH1 gene. Furthermore, the substantial expression of CWI-responsive genes is also triggered by external hyperosmolarity, suggesting that the adaptive changes in the cell wall are crucial for protecting yeast cells to not only cell wall stress but also osmotic stress. On the other hand, cell wall stress-inducing agent Calcofluor white has no effect on promoting expression of GPD1 , a major target gene of the HOG pathway. Collectively, these findings suggest that, during ethanol stress, the CWI and HOG pathways collaboratively regulate the transcription of specific cell wall biosynthesis genes, thereby leading to adaptive changes in the cell wall. IMPORTANCE The budding yeast Saccharomyces cerevisiae has been widely used in industrial fermentations, including production of alcoholic beverages and bioethanol. During fermentation, increased ethanol concentration is the main stress that affects yeast metabolism and inhibit ethanol production. This work presents evidence that, in response to ethanol stress, both CWI and HOG pathways cooperate to control expression of cell wall remodeling genes in order to build adaptive strength of the cell wall. These findings will contribute to better understanding of the molecular mechanisms underlying adaptive response and tolerance of yeast to ethanol stress, which is essential for successful engineering of yeast strains for improved ethanol tolerance.

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