Activation of two isoforms of mitogen-activated protein kinase kinase in response to epidermal growth factor and nerve growth factor.

Mitogen-activated protein kinase kinase (MAPKK) is a dual-specificity protein kinase which phosphorylates and activates mitogen-activated protein kinase (MAPK). cDNAs encoding two isoforms of MAPKK, MAPKK1 and MAPKK2 (also known as MEK1 and MEK2), have been cloned in mammalian cells. To analyze the characteristics of MAPKK1 and MAPKK2 individually, we have produced specific anti-MAPKK serum against each isoform. MAPKK1 and MAPKK2 have apparent molecular masses of 45 kDa and 47 kDa, respectively, on SDS/polyacrylamide gel electrophoresis. In mouse tissues, MAPKK1 was highly enriched in brain, while MAPKK2 was present relatively evenly. In rat fibroblastic 3Y1 cells, epidermal growth factor (EGF) treatment induced activation of both MAPKK1 and MAPKK2. Immunoprecipitation experiments have shown that the time courses of activation and deactivation of both isoforms of MAPKK were superimposed. In PC12 cells, both MAPKK1 and MAPKK2 were activated in response to nerve growth factor (NGF) as well as EGF, and the time courses of activation and deactivation of both isoforms were indistinguishable from each other in the NGF-stimulated cells and also in the EGF-stimulated cells. Furthermore, localization of both MAPKK1 and MAPKK2 in the cytoplasm was unchanged in response to EGF and NGF. Thus, the same or quite similar mechanisms may operate in the regulation of the activation and deactivation of two isoforms of MAPKK, and both kinases might have redundant functions when expressed in the same cell.