Animal cap explants from Xenopus embryos injected with a dominant negative Raf-1 mutant, termed NAF (not a functional Raf), demonstrated a complete block to basic fibroblast growth factor (FGF)-stimulated mesoderm induction. Activin induction of mesoderm was normal in embryos that expressed NAF. Injection of NAF RNA into 2-cell stage embryos blocked normal development during neurula stages and caused severe posterior truncations in tadpoles. The phenotype induced by NAF could be rescued by coinjection of wild-type raf-I RNA. The NAF mutant functioned by specifically blocking the activation of endogenous Raf kinase activity. These findings suggest that Raf-1 mediates FGF, but not activin, receptor signaling during mesoderm induction and implicate Raf-1 as a key signaling molecule in the development of posterior structure.
