ROLE OF THE RAF MITOGEN-ACTIVATED PROTEIN-KINASE PATHWAY IN P21(RAS) DESENSITIZATION/

Desensitization of p21(ras) after stimulation of cells by growth facto rs and phorbol 12-myristate 13-acetate (PMA) correlates with hyperphos phorylation of the guanine nucleotide exchange factor Son-of-sevenless (Sos) and its dissociation from the adaptor protein Grb2 (Cherniack, A., Klarlund, J. K., Conway, B. R., and Czech, M. P. (1995) J. Biol. C hem. 270, 1485-1488). To test the role of the Raf/mitogen-activated pr otein (MAP) kinase pathway, we utilized cells expressing a chimera com posed of the catalytic domain of p74Raf-1 and the hormone binding doma in of the estradiol receptor (Delta Raf-1:ER), Estradiol markedly stim ulated Delta Raf-1:ER and the downstream MEK and MAP kinases in these cells as well as Sos phosphorylation, However, the dissociation of Grb 2 from Sos observed in response to PMA was not apparent upon Delta Raf -1:ER activation, Furthermore, stimulation of Delta Raf-1:ER did not i mpair GTP loading of p21(ras) in response to platelet derived growth f actor or epidermal growth factor, We conclude that activation of the R af/MAP kinase pathway alone in these cells is insufficient to cause di sassembly of Sos from Grb2 or to interrupt the ability of Sos to catal yze activation of p21(ras).