Dual specificity protein kinase which acts as an essential component of the MAP kinase signal transduction pathway. Binding of extracellular ligands such as growth factors, cytokines and hormones to their cell-surface receptors activates the MAPK/ERK cascade, ultimately leading to phosphorylation of a threonine and a tyrosine residue in a Thr-Glu-Tyr sequence located in MAP kinases. Depending on the cellular context, this pathway mediates diverse biological functions such as cell growth, adhesion, survival and differentiation predominantly through the regulation of transcription, metabolism and cytoskeletal rearrangements (By similarity). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] ATP + L-tyrosyl-[protein] = ADP + H(+) + O-phospho-L-tyrosyl-[protein] Expressed in the central nervous system, kidney, liver, intestine and the hematopoietic system. Activated by phosphorylation on Ser/Thr catalyzed by MAP kinase kinase kinases (RAF or MOS). Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. MAP kinase kinase subfamily.