Nuclear accumulation of cRel following C-terminal phosphorylation by TBK1/IKKε

The NF-kappa B transcription factors are key regulators of immunomodulatory, cell cycle, and developmental gene regulation. NF-kappa B activity is mainly regulated through the phosphorylation of I kappa B by the I kappa B kinase (IKK) complex IKK alpha beta gamma, leading to proteasome-mediated degradation of I kappa B, nuclear translocation of NF-kappa B dimers, DNA binding, and gene induction. Additionally, direct posttranslational modifications of NF-kappa B p65 and cRel subunits involving C-terminal phosphorylation has been demonstrated. The noncanonical IKK-related homologs, TNFR-associated factor family member-associated NF-kappa B activator (TANK)-binding kinase (TBK)1 and IKK epsilon, are also thought to play a role in NF-kappa B regulation, but their functions remain unclear. TBK1 and IKK epsilon were recently described as essential regulators of IFN gene activation through direct phosphorylation of the IFN regulatory factor-3 and -7 transcription factors. In the present study, we sought to determine whether IKK epsilon and TBK1 could modulate cRel activity via phosphorylation. TBK1 and IKK epsilon directly phosphorylate the C-terminal domain of cRel in vitro and in vivo and regulate nuclear accumulation of cRel, independently of the classical I kappa B/IKK pathway. I kappa B alpha degradation is not affected, but rather IKK epsilon-mediated phosphorylation of cRel leads to dissociation of the I kappa B alpha-cRel complex. These results illustrate a previously unrecognized Aspect of cRel regulation, controlled by direct IKK epsilon/TBK1 phosphorylation.