Four human Ras homologs differ in their abilities to activate Raf-1, induce transformation, and stimulate cell motility

Human cells contain four homologous Ras proteins, but it is unknown whether each of these Ras proteins participates in distinct signal transduction ca scades or has different biological functions. To directly address these iss ues, we assessed the relative ability of constitutively active (G12V) versi ons of each of the four Ras homologs to activate the effector protein Raf-l in vivo. In addition, we compared their relative abilities to induce trans formed foci, enable anchorage-independent growth, and stimulate cell migrat ion. We found a distinct hierarchy between the four Ras homologs in each of the parameters studied. The hierarchies were as follows: for Raf-l activat ion, Ki-Ras 4B > Ki-Ras 4A >>> N-Ras > Ha-Ras; for focus formation, Ha-Ras greater than or equal to Ki-Ras 4A >>> N-Ras = Ki-Ras 4B; for anchorage-ind ependent growth, Ki-Ras 4A greater than or equal to N-Ras >>> Ki-Ras 4B = H a-Ras = no growth; and for cell migration, Ki-Ras 4B >>> Ha-Ras > N-Ras = K i-Ras 4A = no migration. Our results indicate that the four Ras homologs si gnificantly differ in their abilities to activate Raf-l and induce distinct ly different biological responses. These studies, in conjunction with our p revious report that demonstrated that the Ras homologs can be differentiall y activated by upstream guanine nucleotide exchange factors (Jones, M. K., and Jackson, J. H. (1998) J. Biol. Chem. 273, 1782-1787), indicate that eac h of the four Ras proteins may qualitatively or quantitatively participate in distinct signaling cascades and have significantly different biological roles in vivo. Importantly, these studies also suggest for the first time t hat the distinct and likely cooperative biological functions of the Ki-ras- encoded Ki-ras 4A and Ki-Ras 4B proteins may help explain why constitutivel y activating mutations of Ki-ras, but not N-ras or Ha-ras, are frequently d etected in human carcinomas.