Different Roles of the HOG Pathway in Stress Responses of Yeast: Challenges in Understanding Osmostress Responses by Saccharomyces cerevisiae

摘要

Baker's yeast, Saccharomyces cerevisiae, responds to decreases in extracellular water activity by increasing the production and retention of glycerol to match the higher external osmolarity. Increased glycerol retention is mediated by closing of a glycerol channel made of the protein Fpsl while the increased glycerol production response is mediated by activation of the High Osmolarity Glycerol response (HOG) pathway, an osmosensing pathway that regulates expression of glycerol synthesis enzymes. S. cerevisiae responds to an increase in extracellular water activity by activating the cell integrity pathway, a signaling pathway related to the HOG pathway. Each of these signaling pathways, containing a cascade of protein kinases, appears to be conserved among other eukaryotes, especially fungi and animals. Both the HOG and cell integrity pathways appear to react not only to osmotic stress but also to other stress signals. Thus, these two pathways together with other stress sensing pathways, e.g., the cyclic AMP pathway, coordinate the response of yeast to the complex effects of different stress. Our goal in this paper is to discuss and analyze several important findings and challenges that have arisen during the investigation of the yeast osmostress response. In this paper, we will refer to Saccharomyces cerevisiae as yeast. Also, increases in extracellular water activity will be called a hypotonic stress, decreases in extracellular water activity a hypertonic stress.