Signal transduction from membrane to cytoplasm: growth factors and membrane-bound oncogene products increase Raf-1 phosphorylation and associated protein kinase activity

Proc Natl Acad Sci U S A. 1988 Dec;85(23):8855-9. doi: 10.1073/pnas.85.23.8855.


We have examined the phosphorylation and the serine/threonine-specific kinase activity of the protooncogene product Raf-1 (formerly c-raf) in response to oncogenic transformation or growth-factor treatment of mouse 3T3 cells. Expression of the membrane-bound oncogene products encoded by v-fms, v-src, v-sis, polyoma virus middle-sized tumor antigen, and Ha-ras increased the apparent molecular weight and phosphorylation of the Raf-1 protein, while expression of the nuclear oncogene and protooncogene products encoded by v-fos and c-myc did not. Changes in electrophoretic mobility and phosphorylation occurred rapidly in response to treatment of cells with platelet-derived growth factor, acidic fibroblast growth factor, epidermal growth factor, and the protein kinase C activator phorbol 12-myristate 13-acetate, but not insulin. The phosphorylation of the Raf-1 protein occurred primarily on serine and threonine residues. However, a subpopulation of Raf-1 molecules was phosphorylated on tyrosine residues in cells transformed by v-src or stimulated with platelet-derived growth factor. Transformation by v-src, or treatment with platelet-derived growth factor or phorbol 12-myristate 13-acetate, activated the Raf-1-associated serine/kinase activity as measured in immune-complex kinase assays. These findings suggest that proliferative signals generated at the membrane result in the phosphorylation of the Raf-1 protein and the activation of its serine/threonine kinase activity. Raf-1 activation may thus serve to transduce signals from the membrane to the cytoplasm and perhaps on to the nucleus.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Animals
  • Cell Membrane / physiology
  • Cells, Cultured
  • Cytoplasm / physiology
  • Epidermal Growth Factor / pharmacology*
  • Fibroblast Growth Factors / pharmacology*
  • Insulin / pharmacology
  • Mice
  • Peptide Mapping
  • Phosphorylation
  • Platelet-Derived Growth Factor / pharmacology*
  • Protein Kinases / metabolism*
  • Proto-Oncogene Proteins / metabolism*
  • Proto-Oncogene Proteins c-raf
  • Proto-Oncogenes*
  • Signal Transduction*
  • Tetradecanoylphorbol Acetate / pharmacology


  • Insulin
  • Platelet-Derived Growth Factor
  • Proto-Oncogene Proteins
  • Fibroblast Growth Factors
  • Epidermal Growth Factor
  • Protein Kinases
  • Proto-Oncogene Proteins c-raf
  • Tetradecanoylphorbol Acetate