Cytoplasmic localization of the mitogen-activated protein kinase activator MEK.

The mitogen-activated protein kinase (MAPK) or extracellular signal-regulated kinase (ERK) is phosphorylated and activated by an upstream activator kinase, MEK (MAPK or ERK kinase), in response to mitogenic growth factors. ERKs translocate into the nucleus upon mitogen stimulation, suggesting that the subcellular redistribution of ERK may play a critical role in signal transfer from cytoplasm to the nucleus. We demonstrated in this report that MEK was exclusively localized in cytoplasm in several cell lines, including Swiss 3T3, HeLa, COS, and PC12. Immunofluorescence analysis of both native and transiently expressed MEK with a MEK-specific antibody revealed that both MEK1 and MEK2 were localized only in the cytoplasm. The cytoplasmic localization of MEK was further supported by subcellular fractionations as well as detergent permeabilization experiments. In contrast to ERK, mitogen stimulation did not cause any nuclear accumulation of MEK. These data suggest that ERK is phosphorylated and activated in the cytoplasm. The activated ERK could subsequently translocate into the nucleus and phosphorylate its nuclear substrates.