Identification of the site of inhibition of mitogenic signaling by oncogenic ras-p21 by a ras effector peptide.

We have previously found that a ras switch 1 domain peptide (PNC-7, residues 35-47) selectively blocks oocyte maturation induced by oncogenic (Val 12-containing) ras-p21 protein and also blocks c-raf-induced oocyte maturation. We now find that oncogenic ras-p21 does not inhibit oocyte maturation of a constitutively activated raf protein (raf BXB) that is lacking most of the first 301 amino terminal amino acids, including the major ras binding domain and accessory ras-binding regions. We also find that a dominant negative raf that completely blocks c-raf-induced maturation likewise does not block raf-BXB-induced maturation. We conclude that PNC-7 blocks ras by binding to the amino-terminal domain of raf and that raf BXB must initiate signal transduction in the cytosol.