MEK kinase activity is not necessary for Raf-1 function

Raf-l protein kinase has been identified as an integral component of the Ras/Raf/MEK/ERK signalling pathway in mammals. Activation of Raf-l is achieved by Ras,GTP binding and other events at the plasma membrane including tyrosine phosphorylation at residues 340/341, We have used gene targeting to generate a 'knockout' of the raf-l gene in mice as well as a rafFF mutant version of endogenous Raf-l with Y340FY341F mutations. Raf-1(-/-) mice die in embryogenesis and show vascular defects in the yolk sac and placenta as well as increased apoptosis of embryonic tissues. Cell proliferation is not affected. Raf-l from cells derived from raf-1(FF/FF) mice has no detectable activity towards MEK in vitro, and yet raf-1(FF/FF) mice survive to adulthood, are fertile and have an apparently normal phenotype, In cells derived from both the raf-1(-/-) and raf-1(FF/FF) mice, ERK activation is normal. These results strongly argue that MEK kinase activity of Raf-l is not essential for normal mouse development and that Raf-l plays a key role in preventing apoptosis.