Activation of extracellular signal-regulated kinase (ERK) or mitogen-activated protein kinase by MEK (mitogen-activated protein kinase or extracellular signal-regulated kinase kinase) is an essential event in the mitogenic growth factor-induced signal transduction pathway. Phosphorylation of MEKs correlates with their ability to phosphorylate and activate ERKs. MEK1 and MEK2 can also be activated by autophosphorylation. Lipopolysaccharide activates many of the MAPK family members of the immediate upstream MAPK activator MEK1, MEK2, and MEK3. In plants, MEK can phosphorylate and activate MAPK, and that Tyr phosphorylation is critical for the catalytic activity of MAPK in plants.