Induction of p100 Processing by NF-κB-inducing Kinase Involves Docking IκB Kinase α (IKKα) to p100 and IKKα-mediated Phosphorylation.

The processing of the nfκb2 gene product p100 to generate p52 is a regulated event, which is important for the instrumental function of NF-κB. We previously demonstrated that this tightly controlled event is regulated positively by NF-κB-inducing kinase (NIK) and its downstream kinase, IκB kinase α (IKKα). However, the precise mechanisms by which NIK and IKKα induce p100 processing remain unclear. Here, we show that, besides activating IKKα, NIK also serves as a docking molecule recruiting IKKα to p100. This novel function of NIK requires two specific amino acid residues, serine 866 and serine 870, of pl00 that are known to be essential for inducible processing of p100. We also show that, after being recruited into p100 complex, activated IKKα phosphorylates specific serines located in both N- and Cterminal regions of p100 (serines 99, 108, 115, 123, and 872). The phosphorylation of these specific serines is the prerequisite for ubiquitination and subsequent processing of p100 mediated by the β-TrCP ubiquitin ligase and 26 S proteasome, respectively. These results highlight the critical but different roles of NIK and IKKα in regulating pl00 processing and shed light on the mechanisms mediating the tight control of p100 processing. These data also provide the first evidence for explaining why overexpression of IKKα or its activation by many other stimuli such as tumor necrosis factor and mitogens fails to induce p100 processing. [ABSTRACT FROM AUTHOR]