The ERK1/2 (p44/42) MAPK signaling pathway can be activated in response to a diverse range of extracellular stimuli including mitogens, growth factors, and cytokines. Upon stimulation, a sequential three-part MAP kinase cascade is initiated, consisting of a MAP kinase kinase kinase (MAPKKK), a MAP kinase kinase (MAPKK), and a MAP kinase (MAPK). Activation of the MAPKs, ERK1 and ERK2, leads to phosphorylation of activation loop residues Thr-202/Tyr-204 and Thr-185/Tyr-187, respectively. In addition to dual phosphorylation, ERK1 and 2 are autophosphorylated on Thr-207 or Thr-188, respectively. This phosphorylation is required for nuclear translocation of ERK, and leads to phosphorylation of several nuclear proteins involved in cardiac hypertrophy. Mouse models with mutation of Thr-188 in ERK2 show that this site is critical for ERK-mediated cardiac hypertrophy. Thus, phosphorylation of Thr-188 in ERK2 may be important for controlling the nuclear functions of activated ERK1 and ERK2.

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