AN ONCOGENIC MUTATION UNCOUPLES THE V-JUN ONCOPROTEIN FROM POSITIVE REGULATION BY THE SAPK JNK PATHWAY IN-VIVO/

Stimulation of c-Jun transcriptional activity via phosphorylation medi ated by the stress-activated or c-Jun amino-terminal (SAPK/JNK) subgro up of mitogen-activated protein kinases (MAP kinases) is thought to de pend on a kinase-docking site (the delta region) within the amino-term inal activation domain, which is deleted from the oncogenic derivative , v-Jun [1-3], This mutation markedly enhances v-Jun oncogenicity [4,5 ]; however, its transcriptional consequences have not been resolved, I n part, this reflects uncertainty as to whether binding of SAPK/JNK in hibits c-Jun function directly [6,7] or, alternatively, serves to faci litate and maintain the specificity of positive regulatory phosphoryla tion [8], Using a two-hybrid approach, we shaw that SAPK/JNK stimulate s c-Jun transactivation in yeast and that this depends on both catalyt ic activity and physical interaction between the kinase and its substr ate, Furthermore, c-Jun is active when tethered to DNA via SAPK/JNK, d emonstrating that kinase binding does not preclude transactivation. Ta ken together, these results suggest that SAPK/JNK acts primarily as a positive regulator of c-Jun transactivation in situ, and that loss of the docking site physically uncouples v-Jun from this control, This lo ss-of-function model accounts for the deficit of v-Jun regulatory phos phorylation and repression of TPA response element (TRE)-dependent tra nscription observed in v-Jun-transformed cells and predicts that an im portant property of the oncoprotein is to antagonise SAPK/JNK-dependen t gene expression.