Imaging of Ras-MAP kinase signal transduction cascade

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The recent development of a technology based on GFP and fluorescence resonance energy transfer (FRET) has enabled us to monitor conformational changes in signaling molecules as they occur in living cells. We have successfully developed probes to monitor the activity change of Ras-family GTPases and shown the dynamic change of Ras activity in living cells. A key signaling molecule, c-Raf, is situated downstream from Ras and upstream from the mitogen-activated protein kinase (MAPK) kinase (MEK). Here, we studied the mechanism underlying the signal transduction from Ras to MEK by using probes based on the principle of fluorescence resonance energy transfer (FRET). In agreement with previous models, it was found that c-Raf adopted two conformations: open active and closed inactive. Ras binding induced the c-Raf transition from closed to open conformation, which enabled c-Raf to bind to MEK. In the presence of a cytosolic Ras mutant, c-Raf bound to, but failed to phosphorylate, MEK in the cytoplasm. In contrast, the cytosolic Ras mutant significantly enhanced MEK phosphorylation by a membrane-targeted c-Raf. These results demonstrated the essential role of Ras-induced conformational change in MEK activation by c-Raf.