| The activation of signal transduction pathways by growth factors, hormones and neurotransmitters is mediated through two closely related MAP kinases, p44 and p42, which are encoded by ERK 1 and ERK 2, respectively. Both proteins are regulated by dual phosphorylation on specific tyrosine and threonine residues mapping within a characteristic Thr-Glu-Tyr motif. In response to activation, both MAP kinases phosphorylate downstream components on serine and threonine. Upstream MAP kinase regulators include MAP kinase kinase (MEK), MEK kinase and Raf-1. ERK 3 is a MAP kinase-related protein. The human homolog of ERK 3 has also been cloned and shown to encode a 97 kDa protein. ERK 5 has been identified as an 815 amino acid protein that functions as a substrate for MEK-5, but not for MEK-1 or MEK-2. ERK 6 (also designated SAPK3) is highly expressed in human skeletal muscle and appears to function as a signal transducer during differentiation of myoblasts to myotubes. |
