Murine Ksr interacts with MEK and inhibits Ras-induced transformation.

Ksr (kinase supressor of Ras) was identified as a regulator of the Ras-MAP kinase (mitogen-activated protein kinase) pathway by genetic screens in Drosophila and Caenorhabditis elegans. Ksr is a kinase with similarities to the three conserved regions of Raf kinases, especially within the kinase domain. To investigate whether these structural similarities correlated with common functional properties, we examined the ability of mKsr-1, the murine homolog of Ksr, to interact with components of the vertebrate MAP kinase pathway. In the yeast two-hybrid interaction assay, mKsr-1 did not bind to either Ras, B-Raf or Raf-1, but interacted strongly with both MEK-1 and MEK-2, activators of MAP kinase. The Ksr-MEK interaction was confirmed by co-immunoprecipitation experiments. Ectopically expressed mKsr-1 co-precipitated with endogenous MEK-1 in COS-1 cells, and endogenous Ksr and MEK co-precipitated from PC12 cells. Phosphorylation of MEK by mKsr-1 was not detected, however. In contrast, the MEK subpopulation complexed with mKsr-1 in COS-1 cells or PC12 cells did not display kinase activity. This ability of Ksr to block MEK in an inactive form correlated with a biological response: mKsr-1 did not transform NIH3T3 cells, and, furthermore, mKsr-1 reduced Ras-induced transformation. Similarly, mKsr-1 inhibited the proliferation of embryonic neuroretina cells induced by Ras and B-Raf but not that induced by MEK. Our results suggest a novel mechanism for Ksr in regulating the MAP kinase pathway, at least in part through an ability to interact with MEK.