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Mutations of Ras with three extra amino acids inserted into the phosphate-binding (P) loop have been investigated both in vitro and in vivo. Such mutants have originally been detected as oncogenes both in the ras and the TC21 genes. Biochemical experiments reveal the molecular basis of their oncogenic potential: the mutants show a strongly attenuated binding affinity for nucleotides, most notably for GDP, leading to a preference for GTP binding. Furthermore, both the intrinsic as well as the GAP-stimulated GTP hydrolysis are drastically diminished. The binding interaction with GAP is reduced, whereas binding to the Ras-binding domain of the downstream effector c-Raf1 is not altered appreciably. Microinjection into PC12 cells shows the mutants to be as potent to induce neurite outgrowth as conventional oncogenic Ras mutants. Unexpectedly, their ability to stimulate the MAP kinase pathway as measured by a reporter gene assay in RK13 cells is much higher than that of the normal oncogenic mutant G12V. This characteristic was attributed to an increased stimulation of c-Raf1 kinase activity by the insertional Ras mutants.

Members of the Ras subfamily of small GTP-binding proteins have been shown to be promiscuous towards a variety of putative effector molecules such as the protein kinase c-Raf and the Ral-specific guanine nucleotide exchange factor (Ral-GEF). To address the question of specificity of interactions we have introduced the mutations E30D and K31E into Rap and show biochemically, by X-ray structure analysis and by transfection in vivo that the identical core effector region of Ras and Rap (residues 32-40) is responsible for molecular recognition, but that residues outside this region are responsible for the specificity of the interaction. The major determinant for the switch in specificity is the opposite charge of residue 31--Lys in Rap, Glu in Ras--which creates a favourable complementary interface for the Ras-Raf interaction.