MAPK pathways of the yeast Saccharomyces cerevisiae: Cell integrity and filamentation/invasion pathway interaction with the hog pathway
Davenport, Kenneth Ray
Gustin, Michael C.
Doctor of Philosophy
Mitogen Activated Protein Kinase (MAPK) cascades are frequently used signal transduction mechanisms in eukaryotes. Of the five MAPK cascade containing signaling pathways in the budding yeast Saccharomyces cerevisiae , the High Osmolarity Glycerol response (HOG) pathway functions to sense and respond to hypertonic stress. Mutants in the members of a second MAPK pathway, the cell integrity pathway, are sensitive to conditions of low osmolarity raising the possibility that this pathway may respond to hypotonic stress. We demonstrate that the cell integrity pathway MAPK Slt2p is phosphorylated in response to hypotonic stress in a rapid (<15 seconds), transient, and solute independent manner. The phosphorylation of Slt2p following hypotonic stress requires the upstream components of the cell integrity pathway as well as calcium, though a calcium influx into the cell is not sufficient to produce Slt2p phosphorylation. Interestingly, the HOG pathway MAPK Hog1p is rapidly de-phosphorylated in response to hypotonic stress in a cell integrity pathway dependent manner. This may indicate a cell integrity mediated regulation of the HOG pathway. It is possible that other pathways and proteins influence HOG pathway signaling or growth in various osmotic conditions. To investigate this possibility we utilized a mutant in the HOG pathway, pbs2-3, in a high copy suppressor screen to identify proteins that modulate growth on high osmolarity media. Three high copy suppressors of pbs2-3 osmosensitivity were identified: MSG5, CAK1, and TRX1. Msg5p is a dual specificity phosphatase that was previously demonstrated to dephosphorylate MAPKs in yeast. Deletions of the putative MAPK targets of Msg5p revealed that kss1Delta could suppress the osmosensitivity of pbs2-3. The filamentation/invasion pathway MAPK Kss1p is phosphorylated in response to hyperosmotic shock in a pbs2-3 strain, but not in a wild type strain or in a pbs2-3 strain overexpressing MSG5. Both TEC1 and FRE::lacZ expression is activated in strains lacking a functional HOG pathway during osmotic stress in a filamentation/invasion pathway dependent manner. Finally, the cellular projections formed by a pbs2-3 mutant on high osmolarity are absent in strains lacking KSS11 or STE7. These data suggest that the loss of filamentation/invasion pathway repression contributes to the HOG mutant phenotype.
Genetics; Cell biology; Biochemistry