Activation of the Hog1 MAPK by the Ssk2/Ssk22 MAP3Ks, in the absence of the osmosensors, is not sufficient to trigger osmostress adaptation in Saccharomyces cerevisiae

• Published in: The FEBS journal 2018-03, Vol.285 (6), p.1079-1096
• Main Authors: Vázquez‐Ibarra, Araceli, Subirana, Laia, Ongay‐Larios, Laura, Kawasaki, Laura, Rojas‐Ortega, Eréndira, Rodríguez‐González, Miriam, Nadal, Eulàlia, Posas, Francesc, Coria, Roberto
• Format: Article
• Language: English
• Subjects: Activation; Adaptation, Physiological - genetics; Baking yeast; Cell cycle; Chromatin; Deactivation; Gene Expression Regulation, Fungal; Glycerol; Glycerol - metabolism; Hog1; Hog1 protein; hyperosmotic stress; Inactivation; Internalization; Kinases; MAP kinase; MAP Kinase Kinase Kinases - genetics; MAP Kinase Kinase Kinases - metabolism; Mitogen-Activated Protein Kinases - genetics; Mitogen-Activated Protein Kinases - metabolism; Mutation; Nuclei; Nuclei (cytology); Osmolar Concentration; Osmolarity; Osmotic Pressure; phosphatase; Phosphorylation; Protein kinase; Saccharomyces cerevisiae; Saccharomyces cerevisiae - genetics; Saccharomyces cerevisiae - metabolism; Saccharomyces cerevisiae Proteins - genetics; Saccharomyces cerevisiae Proteins - metabolism; Stress, Physiological; Stresses; Transcription; yeast
• ISSN: 1742-464X; 1742-4658
• DOI: 10.1111/febs.14385

Yeast cells respond to hyperosmotic stress by activating the high‐osmolarity glycerol (HOG) pathway, which consists of two branches, Hkr1/Msb2‐Sho1 and Sln1, which trigger phosphorylation and nuclear internalization of the Hog1 mitogen‐activated protein kinase. In the nucleus, Hog1 regulates gene transcription and cell cycle progression, which allows the cell to respond and adapt to hyperosmotic conditions. This study demonstrates that the uncoupling of the known sensors of both branches of the pathway at the level of Ssk1 and Ste11 impairs cell growth in hyperosmotic medium. However, under these conditions, Hog1 was still phosphorylated and internalized into the nucleus, suggesting the existence of an alternative Hog1 activation mechanism. In the ssk1ste11 mutant, phosphorylated Hog1 failed to associate with chromatin and to activate transcription of canonical hyperosmolarity‐responsive genes. Accordingly, Hog1 also failed to induce glycerol production at the levels of a wild‐type strain. Inactivation of the Ptp2 phosphatase moderately rescued growth impairment of the ssk1ste11 mutant under hyperosmotic conditions, indicating that downregulation of the HOG pathway only partially explains the phenotypes displayed by the ssk1ste11 mutant. Cell cycle defects were also observed in response to stress when Hog1 was phosphorylated in the ssk1ste11 mutant. Taken together, these observations indicate that Hog1 phosphorylation by noncanonical upstream mechanisms is not sufficient to trigger a protective response to hyperosmotic stress. Upon hyperosmotic stress the MAPK Hog1 becomes phosphorylated in a mutant where both the Sho1 and Sln1 branches are inactivated. This noncanonical Hog1 phosphorylation is dependent on the Ssk2 activity. In this condition Hog1 is transported into the nucleus but is unable to associate with chromatin and to activate hyperosmotic‐responsive genes. Hog1 phosphorylation by noncanonical upstream mechanisms is not sufficient to trigger a protective response to hyperosmotic stress.