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An oncogenic mutation uncouples the v-Jun oncoprotein from positive regulation by the SAPK/JNK pathway in vivo
1Beatson Institute for Cancer Research, Cancer Research Campaign Beatson Laboratories, Garscube Estate Switchback Road, Bearsden, Glasgow, G61 1BD, UK.
Abstract:
Stimulation of c-Jun transcriptional activity via phosphorylation mediated by the stress-activated or c-Jun amino-terminal (SAPK/JNK) subgroup of mitogen-activated protein kinases (MAP kinases) is thought to depend on a kinase-docking site (the delta region) within the amino-terminal activation domain, which is deleted from the oncogenic derivative, v-Jun [1] [2] [3]. This mutation markedly enhances v-Jun oncogenicity [4] [5]; however, its transcriptional consequences have not been resolved. In part, this reflects uncertainty as to whether binding of SAPK/JNK inhibits c-Jun function directly [6] [7] or, alternatively, serves to facilitate and maintain the specificity of positive regulatory phosphorylation [8]. Using a two-hybrid approach, we show that SAPK/JNK stimulates c-Jun transactivation in yeast and that this depends on both catalytic activity and physical interaction between the kinase and its substrate. Furthermore, c-Jun is active when tethered to DNA via SAPK/JNK, demonstrating that kinase binding does not preclude transactivation. Taken 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 loss-of-function model accounts for the deficit of v-Jun regulatory phosphorylation and repression of TPA response element (TRE)-dependent transcription observed in v-Jun-transformed cells and predicts that an important property of the oncoprotein is to antagonise SAPK/JNK-dependent gene expression.
Insights
Stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK) positively regulates c-Jun transcriptional activity through a docking site. Loss of this site in v-Jun uncouples it from SAPK/JNK control, explaining its oncogenicity.
Area of Science:
- Molecular Biology
- Cell Signaling
- Oncogenesis
Background:
- c-Jun transcriptional activity is regulated by phosphorylation mediated by stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK).
- A kinase-docking site (delta region) in c-Jun is crucial for SAPK/JNK mediated phosphorylation, and its deletion in v-Jun enhances oncogenicity.
- The precise role of SAPK/JNK binding in c-Jun function, whether inhibitory or facilitatory, remains unclear.
Purpose of the Study:
- To investigate the mechanism by which SAPK/JNK regulates c-Jun transcriptional activity.
- To determine the functional significance of the SAPK/JNK docking site in c-Jun.
- To elucidate the transcriptional consequences of the v-Jun mutation.
Main Methods:
- Yeast two-hybrid assays were employed to study the interaction between SAPK/JNK and c-Jun.
- Functional assays were conducted to assess c-Jun transactivation in the presence and absence of SAPK/JNK binding.
Main Results:
- SAPK/JNK stimulates c-Jun transactivation in yeast, dependent on both catalytic activity and physical interaction.
- c-Jun remains transcriptionally active when tethered to DNA via SAPK/JNK, indicating kinase binding does not inhibit function.
- Loss of the docking site in v-Jun uncouples it from SAPK/JNK mediated regulation, leading to repressed transcription.
Conclusions:
- SAPK/JNK primarily acts as a positive regulator of c-Jun transactivation.
- The delta region docking site is essential for SAPK/JNK mediated positive regulation of c-Jun.
- The v-Jun oncoprotein antagonizes SAPK/JNK-dependent gene expression due to the loss of this critical docking site.