Identification of amino acid residues of Ras protein that are essential for signal‐transducing activity but not for enhancement of GTPase activity by GAP

To determine the amino acid residues required for the signal-transducing activity of the human c-Ha-Ras protein, we introduced point mutations at residues 45-54 near the 'effector region' (residues 32-40). We transfected PC12 cells with these mutant genes and also micro-injected the mutant proteins, bound with an unhydrolyzable GTP analog, into PC12 cells. Both procedures showed that Val45----Glu and Gly48----Cys mutations impaired the ability of the Ras protein to induce morphological change of PC12 cells. These mutations did not affect the guanine nucleotide-binding activity or GTPase activity in the absence or presence of bovine GTPase-activating protein (GAP). Therefore, the Val45 and Gly48 residues should be included by definition in the effector region responsible for the signal transduction, while only a subset of the effector-region residues is required for enhancement of the GTPase activity by GAP.