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. Ubiquitous. Detected during gametophytic pollen development and constitutively expressed in embryogenic pollen. The TXY motif contains the threonine and tyrosine residues whose phosphorylation activates the MAP kinases. Dually phosphorylated on Thr-191 and Tyr-193, which activates the enzyme (By similarity). Very low autophosphorylation, although dramatically increased when Mn(2+) is added to the reaction instead of Mg(2+). Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. MAP kinase subfamily.