Mitogen-activated protein kinase involved in a signal transduction pathway that is activated by changes in the osmolarity of the extracellular environment. Controls osmotic regulation of transcription of target genes. Also involved in response UV radiations and mediates the sensitivity to fludioxonil, an agricultural fungicide. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Activated by tyrosine and threonine phosphorylation. Predominantly cytoplasmic in unstressed cells but rapidly concentrates within the nucleus in response to hyperosmotic conditions and phosphorylation. The TXY motif contains the threonine and tyrosine residues whose phosphorylation activates the MAP kinases. Dually phosphorylated on Thr-171 and Tyr-173, which activates the enzyme (By similarity). Phosphorylated by PBS2 after osmotic stress. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. MAP kinase subfamily. HOG1 sub-subfamily.