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Cytoplasmic localization of the mitogen-activated protein kinase activator MEK
1Department of Biological Chemistry, University of Michigan Medical School, Ann Arbor 48109-0606.
Abstract:
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.
Insights
Mitogen-activated protein kinase kinase (MEK) remains in the cytoplasm, while extracellular signal-regulated kinase (ERK) translocates to the nucleus. This localization difference is crucial for signal transduction pathways.
Area of Science:
- Cellular Biology
- Molecular Signaling
- Signal Transduction
Background:
- Mitogen-activated protein kinase (MAPK) pathways are critical for cellular responses to growth factors.
- Extracellular signal-regulated kinases (ERK) translocate to the nucleus upon stimulation, indicating a role in nuclear signal transfer.
- The upstream activator kinase, MEK (MAPK or ERK kinase), plays a key role in ERK activation.
Purpose of the Study:
- To investigate the subcellular localization of MEK in response to mitogenic stimulation.
- To determine if MEK undergoes nuclear translocation similar to ERK.
- To elucidate the site of ERK phosphorylation and activation within the cell.
Main Methods:
- Immunofluorescence analysis using MEK-specific antibodies in various cell lines (Swiss 3T3, HeLa, COS, PC12).
- Subcellular fractionation experiments to confirm protein localization.
- Detergent permeabilization assays to assess protein accessibility and localization.
Main Results:
- MEK (both MEK1 and MEK2) was found exclusively in the cytoplasm across all tested cell lines.
- Mitogenic stimulation did not induce any detectable nuclear accumulation of MEK.
- In contrast to ERK, MEK's localization remained cytoplasmic regardless of stimulation.
Conclusions:
- ERK is phosphorylated and activated within the cytoplasm by MEK.
- Activated ERK subsequently translocates to the nucleus to phosphorylate nuclear substrates.
- MEK's exclusive cytoplasmic localization is a key feature of the ERK signaling pathway.