Contribution of the ERK5/MEK5 pathway to Ras/Raf signaling and growth control

The activity of the catalytic domain of the orphan MAP kinase ERK5 is increased by Ras but not Raf-l in cells, which suggests that ERK5 might mediate Raf-independent signaling by Bas. We found that Raf-1 does contribute to Ras activation of ERK5 but in a manner that does not; correlate with Raf-l catalytic activity. A clue to the mechanism of action of Raf-1 on ERK5 comes from the observation that endogenous Raf-l binds to endogenous ERK5, suggesting the involvement of regulatory protein-protein interactions. This interaction is specific because Raf-l binds only to ERK5 and not ERK2 or SAPK. Finally, we demonstrate the ERK5/MEK5 pathway is required for Raf-dependent cellular transformation and that a constitutively active form of MEK5, MEK5DD, synergizes with Raf to transform NIH 3T3 cells. These observations suggest that ERK5 plays a large role in Raf-l-mediated signal transduction.