The protein kinase Pak3 positively regulates Raf-1 activity through phosphorylation of serine 338

The pathway involving the signalling protein p21 Ras propagates a range of extracellular signals from receptors on the cell membrane to the cytoplasm and nucleus 1 . The Ras proteins regulate many effectors, including members of the Raf family of protein kinases. Ras-dependent activation of Raf-1 at the plasma membrane involves phosphorylation events, protein–protein interactions and structural changes 2 , 3 , 4 , 5 , 6 , 7 , 8 . Phosphorylation of serine residues 338 or 339 in the catalytic domain of Raf-1 regulates its activation in response to Ras, Src and epidermal growth factor 9 , 10 . Here we show that the p21-activated protein kinase Pak3 phosphorylates Raf-1 on serine 338 in vitro and in vivo . The p21-activated protein kinases are regulated by the Rho-family GTPases Rac and Cdc42 ( ref. 11 ). Our results indicate that signal transduction through Raf-1 depends on both Ras and the activation of the Pak pathway. As guanine-nucleotide-exchange activity on Rac can be stimulated by a Ras-dependent phosphatidylinositol-3-OH kinase 12 , 13 , a mechanism could exist through which one Ras effector pathway can be influenced by another.