The RAS guanine nucleotide binding proteins activate multiple signaling events that regulate cell growth and differentiation. In quiescent fibroblasts, ectopic expression of activated H-RAS (H-RASV12, where V12 indicates valine-12) induces membrane ruffling, mitogen-activated protein (MAP) kinase activation, and stimulation of DNA synthesis. A mutant of activated H-RAS, H-RASV12C40 (where C40 indicates cysteine-40), was identified that was defective for MAP kinase activation and stimulation of DNA synthesis, but retained the ability to induce membrane ruffling. Another mutant of activated H-RAS, H-RASV12S35 (where S35 indicates serine-35), which activates MAP kinase, was defective for stimulation of membrane ruffling and induction of DNA synthesis. Expression of both mutants resulted in a stimulation of DNA synthesis that was comparable to that induced by H-RASV12. These results indicate that membrane ruffling and activation of MAP kinase represent distinct RAS effector pathways and that input from both pathways is required for the mitogenic activity of RAS.

The RAC guanine nucleotide binding proteins regulate multiple biological activities, including actin polymerization, activation of the Jun kinase (JNK) cascade, and cell proliferation. RAC effector loop mutants were identified that separate the ability of RAC to interact with different downstream effectors. One mutant of activated human RAC protein, RACV12H40 (with valine and histidine substituted at position 12 and 40, respectively), was defective in binding to PAK3, a Ste20-related p21-activated kinase (PAK), but bound to POR1, a RAC-binding protein. This mutant failed to stimulate PAK and JNK activity but still induced membrane ruffling and mediated transformation. A second mutant, RACV12L37 (with leucine substituted at position 37), which bound PAK but not POR1, induced JNK activation but was defective in inducing membrane ruffling and transformation. These results indicate that the effects of RAC on the JNK cascade and on actin polymerization and cell proliferation are mediated by distinct effector pathways that diverge at the level of RAC itself.

The Rac GTP-binding protein controls signal transduction pathways that are critical for mitogenesis and oncogenesis (1,2). The biochemical nature of these signaling pathways is presently unknown. Here we report that a region in Rac1 (residues 124-135), previously defined as the insert region (3), is essential for its mitogenic activity. Deletion of this region does not interfere with the ability of Rac1 to induce cytoskeletal changes or to activate the Jun kinase mitogen-activated protein kinase cascade but abrogates Rac1-induced stimulation of DNA synthesis and Rac1-mediated superoxide production in quiescent fibroblasts. Treatment of cells with agents that abolish superoxide generation inhibits specifically the mitogenic effect of Rac1. Our results identify an effector site in Rac1 that is necessary for mitogenic signaling and implicate superoxide generation as a candidate effector pathway of Rac1-dependent cell growth.