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

Abstract

The pathway involving the signalling protein p21^Ras propagates a range of extracellular signals from receptors on the cell membrane to the cytoplasm and nucleus¹. 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 factor⁹,¹⁰. 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 kinase¹²,¹³, a mechanism could exist through which one Ras effector pathway can be influenced by another.