p120 GAP modulates Ras activation of Jun kinases and transformation

Although recent evidence demonstrates that Ras causes transformation by activation of multiple downstream pathways, the specific role of non-Raf effector pathways is presently unknown. Although Ras causes activation of the Jun NH2-terminal kinases (JNKs) via a Raf-independent pathway, the contribution of JNK activation to Pas transformation and the effector that mediates JNK activation have not been established. We observed that a dominant negative mutant of SEK1/JNKK, an activator of JNKs, selectively inhibited oncogenic Pas activation of JNK and Pas transformation, but not Pas activation of the p42 mitogen activated protein kinase. In contrast, overexpression of wild type SEK1 enhanced Pas activation of JNK and transforming activity. Thus, JNK activation promotes Pas transformation. Furthermore, a dominant negative mutant of p120 GAP (designated N-GAP), a candidate Ras effector, blocked Ras, but not Raf, transformation and blocked Pas, but not Pac, activation of JNK. Since N-GAP overexpression reduced the association of p190 Rac/Rho GAP with endogenous p120 GAP, N-GAP may form nonproductive complexes with components critical for p120 GAP function. In summary, p120 GAP may function as an effector for Ras activation of JNK and Pas transformation.