REGULATION OF THE MAP KINASE PATHWAY BY MAMMALIAN KSR THROUGH DIRECT INTERACTION WITH MEK AND ERK

Background: Genetic screens in Drosophila melanogaster and Caenorhabdi tis elegans identified the kinase suppressor of Ras, Ksr, as a new com ponent in the Ras intracellular signaling pathway. In these organisms, mutations in Ksr resulted in attenuation of Ras-mediated signaling. H omologs of Ksr have also been isolated from mice and humans; their pre cise role in Pas signaling is not well defined. Here, we present data showing interactions between the murine form of Ksr (mKsr-1) and other components of the Pas pathway. Results: To gain insight into the biol ogical function of Ksr, we used a yeast two-hybrid screen and found an interaction between the carboxy-terminal region of mKsr-1 and mitogen -activated protein (MAP) kinase kinase 1 (MAPKK-1 or MEK-1). An intera ction was also detected between MAP kinase (also called extracellular signal-regulated kinase; ERK), and the amino-terminal region of mKsr-1 . ,These interactions were recapitulated in COS-7 cells. Further, when COS-7 cells were transfected with either full-length mKsr-1 or only i ts carboxyterminal region, an inhibition of serum-stimulated MAP kinas e activation was observed. Microinjection of full-length mKsr-1 or its carboxy-terminal, but not its amino-terminal region, blacked serum-in duced DNA synthesis in rat embryo fibroblasts. Go-injection of mKsr-1 with MEK-1 reversed the blockade. Conclusions: Together with the data from genetic analyses, our findings lead us to propose that mKsr-1 may control MAP kinase signaling by serving as a scaffold protein that li nks MEK and its substrate ERK.