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

Nature. 1998 Nov 12;396(6707):180-3. doi: 10.1038/24184.


The pathway involving the signalling protein p21Ras propagates a range of extracellular signals from receptors on the cell membrane to the cytoplasm and nucleus. 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. 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. 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. 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, a mechanism could exist through which one Ras effector pathway can be influenced by another.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Sequence
  • Animals
  • COS Cells
  • Enzyme Activation
  • Escherichia coli
  • Humans
  • Molecular Sequence Data
  • Phosphorylation
  • Precipitin Tests
  • Protein-Serine-Threonine Kinases / isolation & purification
  • Protein-Serine-Threonine Kinases / metabolism*
  • Proto-Oncogene Proteins c-raf / metabolism*
  • Rats
  • Recombinant Proteins / metabolism
  • Serine / metabolism*
  • Signal Transduction
  • Spleen / enzymology
  • p21-Activated Kinases


  • Recombinant Proteins
  • Serine
  • PAK3 protein, human
  • Pak3 protein, rat
  • Protein-Serine-Threonine Kinases
  • Proto-Oncogene Proteins c-raf
  • p21-Activated Kinases