GUANINE-NUCLEOTIDE EXCHANGE PROTEIN C3G ACTIVATES JNK1 BY A RAS-INDEPENDENT MECHANISM - JNK1 ACTIVATION INHIBITED BY KINASE-NEGATIVE FORMS OF MLK3 AND DLK MIXED LINEAGE KINASES

Recently we have reported that the adaptor protein Crk transmits signa ls to c-Jun kinase (JNK) through C3G, a guanine-nucleotide exchange pr otein for the Ras family of small G proteins. Transient expression of C3G in 293T cells induced JNK1 activation without a significant effect on extracellular signal-related kinase 1 (ERK1), whereas mSos1 activa ted equally both JNK1 and ERK1. Coexpression of the dominant negative form of Ras-N17 did not suppress C3G-induced JNK1 activation but reduc ed the activity of JNK1 induced by mSos1, suggesting that Res is not r equired for JNK activation by C3G. Ras-independent activation of JNK: was supported by the finding that C3G-induced JNK activation was not i nhibited by the dominant negative forms of Rac or Pak, which are compo nents of the signaling pathway from Ras leading to JNK activation. In contrast, C3G-induced JNK1 activation was strongly inhibited by coexpr ession of the kinase negative forms of the mixed lineage kinase (MLK) family of proteins, MLK3 and dual leucine zipper kinase (DLK). In addi tion, MLK3-induced JNK1 activation was found to be suppressed by the k inase negative form of DLK, which bound to MLK3. These results suggest that C3G activates JNK1 through a pathway involving the MLK family of proteins.