The ras proto-oncogene family codes for guanine nucleotide binding proteins with low intrinsic GTPase activity, which is increased by interaction with a GTPase activating protein (GAP). Three consensus sequences conserved in the GTP binding proteins (GXXXXGK, DXXG and NKXD) are critical in the interaction of ras proteins with the GTP or GDP nucleotides. Mutations in these regions produce p2l-ras proteins defective in GTP binding. However, depending on which amino acid is modified, the mutants can be highly transforming, while others lose their transforming activity. Transformation-defective mutants have been generated in our Laboratory and shown to interfere with the normal ras activity. Using these defective mutants (i.e., v-Ha-ras N116Y and N116I), we are trying to elucidate the signal transduction pathways in which ras proteins are involved. Interference by these defective mutants could be through sequestering the target protein(s) from the system or by switching on or off growth factor pathways. Biochemical studies are in progress to determine the nature and the biological meaning of this interference by the ras defective mutants.