Tyrosine 170 is dispensable for c-Jun turnover

The turnover rate of many transcription factors such as c-Jun, a member of the AP-1 family of transcription factors, is regulated by phosphorylation events. Phosphorylation on serine residues 63 and 73 (S63/73) by the c-Jun-N-terminal kinases (JNKs) regulates c-Jun's turnover, and is critical for its ability to regulate cell death and proliferation. Recently, biochemical evidence indicated that the c-Abl and Csk kinases were able to phosphorylate the tyrosine 170 (Y170) residue of c-Jun - which lies within the recognition motif of the Itch ubiquitin ligase - and also regulate its stability independent of the JNK phosphorylation sites. We have investigated here the relevance of Y170 residue by substituting it to either an unphosphorylable phenylanine or a pseudo-phosphorylated aspartic acid residue, and re-expressing the mutants stably in c-jun(-/-) embryonic fibroblasts. Our results indicate that Y170 residue is not a critical determinant of c-Jun stability. Itch was able to bind and degrade both wild-type and the c-Jun(170F)/c-Jun(170D) mutants, albeit to varying extents. Moreover, both Csk and c-Abl were also not defective in binding to these mutants, although Csk and c-Abl were either unable to degrade or increased the steady-state levels of all c-Jun mutants respectively. Phosphorylation on S63/73 upon exposure to genotoxic stresses was also not affected by the status of Y170, although cell death and proliferation were slightly affected regardless of the substituted residue. These data therefore proposes that altering the Y170 residue does not affect c-Jun's turnover and does not abolish its functions in regulating cellular proliferation and cell survival.