The p97-UFD1L-NPL4 Protein Complex Mediates Cytokine-Induced IκBα Proteolysis

I kappa B alpha is an inhibitor of NF-kappa B, a family of transcription factors that transactivate genes related to inflammation. Upon inflammatory stimuli, I kappa B alpha is rapidly degraded via the ubiquitin-proteasome pathway. While it is very clear that the SCF beta-TRCP ubiquitin ligase ubiquitinates I kappa B alpha upon stimulation, little is known about the postubiquitinational events of I kappa B alpha proteolysis. Here, we report that p97, a valosin-containing protein (also called VCP), plays an essential role in the postubiquitinational regulation of I kappa B alpha turnover after tumor necrosis factor alpha (TNF-alpha) or interleukin-1 beta (IL-1 beta) treatment. The ATPase activity of p97 is essential for its role in I kappa B alpha proteolysis. Moreover, we found that UFD1L and NPL4, two cofactors of p97, assist p97 to control the postubiquitinational regulation of I kappa B alpha. The p97-UFD1L-NPL4 protein complex specifically associates with ubiquitinated I kappa B alpha via the interactions between p97 and the SCF beta-TRCP ubiquitin ligase and between the polyubiquitin binding domain of UFD1L and polyubiquitinated I kappa B alpha. Furthermore, we observed that the postubiquitinational regulation of I kappa B alpha by the p97-UFD1L-NPL4 complex is important for NF-kappa B activation under stimuli.