THE PROTEIN-KINASE PAK3 POSITIVELY REGULATES RAF-1 ACTIVITY THROUGH PHOSPHORYLATION OF SERINE-338

The pathway involving the signalling: protein p21(Ras) propagates a ra nge of extracellular signals from receptors on the cell membrane to th e cytoplasm and nucleus(1). The Ras proteins regulate many effecters, including members of the Raf family of protein kinases. Ras-dependent activation of Raf-1 at the plasma membrane involves phosphorylation ev ents, protein-protein interactions and structural changes(2-8). Phosph orylation of serine residues 338 or 339 in the catalytic domain of Raf -1 regulates its activation in response to Ras, Src and epidermal grow th factor(9,10). Here we show that the p21-activated protein kinase Pa k3 phosphorylates Raf-1 on serine 338 in vitro and in vivo. The p21-ac tivated protein kinases are regulated by the Rho-family GTPases Rac an d Cdc42 (ref, 11). Our results indicate that signal transduction throu gh 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 Ra s-dependent phosphatidylinositol-3-OH kinase(12,13), a mechanism could exist through which one Ras effector pathway can be influenced by ano ther.