Smad2/3-mediated transforming growth factor β signalling and the Ras-Raf-Mek-Erk cascade have important roles in stem cell and developmentand tissue homeostasis. However, it remains unknown whether Raf kinases directly crosstalk with Smad2/3 signalling and how this would regulate embryonic development. Here we show that Arafantagonizesmesendoderm induction and patterning activity of Nodal/Smad2 signals in vertebrate embryos by directly inhibiting Smad2 signalling. Knockdown of araf in zebrafish embryos leads to an increase of activated Smad2 with a decrease in linker phosphorylation; consequently, the embryos have excess mesendoderm precursors and are dorsalized. Mechanistically, Araf physically binds to and phosphorylates Smad2 in the linkerregion with S253 being indispensable in a Mek/Erk-independent manner, thereby attenuatingSmad2 signalling by accelerating degradation of activated Smad2. Our findings open avenues for investigating the potential significance of Raf regulation of transforming growth factor β signalling in versatile biological and pathological processes in the future.

Liu XF, Xiong C, Jia SJ, Zhang Y, Chen YG, Wang Q, Meng AM*. (2013) Araf kinase antagonizes Nodal-Smad2 activity in mesendoderm development by directly phosphorylating the Smad2 linker region. Nature Communications, 4:1728.