Identification and characterization of mutations in Ha-Ras that selectively decrease binding to cRaf-1.

The oncoprotein Ras transforms cells by binding to one or more effector proteins. Effector proteins have been identified by their ability to bind to Ras in the GTP but not GDP form, and by their requirement for the Ras effector domain for binding. The best understood Ras effectors are serine/threonine kinases of the Raf family, but other candidate Ras effectors, including a Ral guanine nucleotide dissociation stimulator and phosphatidylinositol 3-kinase (PI3 kinase) have also been identified. To investigate the mechanism of binding of cRaf-1 to Ras, and to investigate the roles of other candidate Ras effectors in transformation, we have isolated and characterized mutants of activated Ras with decreased binding to cRaf-1 relative to other candidate effectors. Examination of these mutants indicates that surface-exposed residues of Ras outside the minimal effector domain interact differentially with cRaf-1 and other Ras-binding proteins, and that fibroblast transformation correlates with cRaf-1 binding and mitogen-activated protein (MAP) kinase activation. Furthermore, activation of PI3 kinase can occur in the absence of significant MAP kinase activation, suggesting that PI3 kinase activation is a primary effect of Ras.