trans-Dominant suppressor mutations of the H-ras oncogene

Site-directed mutagenesis of the conserved sequence motifs of p21 generated a group of mutant p21s defective in GTP binding. Some of these mutants were highly transforming, whereas others were transformation defective. Among the latter group, we found two mutants, derived from the v-H-ras oncogene by substituting the asparagine-116 with tyrosine and isoleucine, that exhibited a trans-dominant activity of suppressing the transformed phenotype of NIH3T3 cells induced by a long terminal repeat-linked c-H-ras and a wild-type v-H-ras. They caused reduction of the colony-forming efficiency in soft agar (78% in c-ras-transformed cells; 55% in v-ras cells) and morphological reversion of ras transformants. Subclones of revertants expressed a great excess of mutant p21 relative to the c-ras p21 present in these cells. These mutants were not lethal to NIH3T3 cells. Apparently, defective proteins encoded by suppressor mutants sequestered vital targets for ras function. Suppressor mutants also induced morphological reversion of NIH3T3 cells transformed by src, fes/flp, sis, and fms oncogenes, suggesting that these oncogenes function upstream to ras in the signaling pathways. Cells transformed by mos and a chemical carcinogen were unaffected.