Catalyzes the concomitant phosphorylation of a threonine and a tyrosine residue in a Thr-Glu-Tyr sequence located in MAP kinases. Activates the ERK1 and ERK2 MAP kinases (PubMed:19219045). Activates BRAF in a KSR1 or KSR2-dependent manner; by binding to KSR1 or KSR2 releases the inhibitory intramolecular interaction between KSR1 or KSR2 protein kinase and N-terminal domains which promotes KSR1 or KSR2-BRAF dimerization and BRAF activation (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] Inhibited by serine/threonine phosphatase 2A. Interacts with MORG1 (PubMed:15118098). Interacts with SGK1 (By similarity). Interacts with KSR1 (PubMed:10409742). Interacts with KSR1 and BRAF; the interaction with KSR1 mediates KSR1-BRAF dimerization (By similarity). Interacts with GLS (By similarity). Membrane localization is probably regulated by its interaction with KSR1. Expressed in adult intestine, kidney, liver, lung, pancreas, spleen, thymus, and at high levels in the neonatal brain. Lower expression is found in adult brain and heart. Phosphorylation on Ser/Thr by MAP kinase kinase kinases (RAF or MEKK1) regulates positively the kinase activity. Phosphorylated by MAP2K1/MEK1. Low levels of autophosphorylation have been observed. Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. MAP kinase kinase subfamily.