MURINE KSR INTERACTS WITH MEK AND INHIBITS RAS-INDUCED TRANSFORMATION

Background: Ksr (kinase supressor of Pas) was identified as a regulato r of the Pas-MAP kinase (mitogen-activated protein kinase) pathway by genetic screens in Drosophila and Caenorhabditis elegans, Ksr is a kin ase with similarities to the three conserved regions of Raf kinases, e specially within the kinase domain. To investigate whether these struc tural similarities correlated with common functional properties, we: e xamined the ability of mKsr-1, the murine homolog of Ksr, to interact with components of the vertebrate MAP kinase pathway. Results: In the yeast two-hybrid interaction assay, mKsr-1 did not bind to either Pas, B-Raf or Raf-1, but interacted strongly with both MEK-1 and MEK-2, ac tivators of MAP kinase. The Ksr-MEK interaction was confirmed by co-im munoprecipitation experiments. Ectopically expressed mKsr-1 co-precipi tated with endogenous MEK-1 in COS-l cells, and endogenous Ksr and MEK cc-precipitated from PC12 cells, Phosphorylation of MEK by mKsr-1 was not detected, however, In contrast, the MEK subpopulation complexed w ith mKsr-1 in COS-1 cells or PC12 cells did not display kinase activit y. This ability of Ksr to block MEK in an inactive form correlated wit h a biological response: mKsr-1 did not transform NIH3T3 cells, and, f urthermore, mKsr-1 reduced Pas-induced transformation, Similarly, mKsr -1 inhibited the proliferation of embryonic neuroretina cells induced by Pas and B-Raf but not that induced by MEK. Conclusions: Our results suggest a novel mechanism for Ksr in regulating the MAP kinase pathwa y, at least in part through an ability to interact with MEK.