| Regulation of transcription factor proteins by MAPK signaling cascades is often associated with regulatory phosphorylation reactions. In addition, the ability of inactive MAP kinases to dock their transcription factor targets has been shown to have additional regulatory properties. The transcription factor c-Jun requires phosphorylation by MAPK family member JNK for full activity. The kinase binds at the c-Jun δ-domain and, upon pathway stimulation, phosphorylates Ser63 and Ser73. c-Jun was discovered as v-Jun, the transforming component of the tumor promoting retrovirus, ASV-17. The in-frame deletion of δ-domain is largely responsible for v-Jun's oncogenic properties, but loss of phosphorylation of Ser63/Ser73 is not. This has lead to the widespread belief that transformation is regulated by JNK binding but not catalytic activity. However, the model has never been directly tested.; Through mutational analysis, we were able to identify individual amino acids throughout the δ-domain that are involved in the association with JNK. These mutants enabled us to confirm the requirement of JNK binding for phosphorylation of Ser63/Ser73 and transcriptional activity. In addition, two novel regulatory functions for JNK docking were discovered: repression of c-Jun activity by the association of inactive JNK and the regulation of JNK subcellular localization. Finally, although residues involved in transformation were identified, JNK does not play a direct role in this function. The evidence suggests an unidentified regulatory molecule is responsible. |
