Identification of NAP1, a Regulatory Subunit of IκB Kinase-Related Kinases That Potentiates NF-κB Signaling

The IκB kinase (IKK)-related kinase NAK (also known as TBK of T2K) contributes to the activation of NF-κB-dependent gene expression. Here we identify NAP1 (for NAK-associated protein 1), a protein that interacts with NAK and its relative IKKε (also known as IKKi). NAP1 activates NAK and facilitates its oligomerization. Interestingly, the NAK-NAP1 complex itself effectively phosphorylated serine 536 of the p65/RelA subunit of NF-κB, and this activity was stimulated by tumor necrosis factor alpha (TNF-α). Overexpression of NAP1 specifically enhanced cytokine induction of an NF-κB-dependent, but not an AP-1-dependent, reporter. Depletion of NAP1 reduced NF-κB-dependent reporter gene expression and sensitized cells to TNF-α-induced apoptosis. These results define NAP1 as an activator of IKK-related kinases and suggest that the NAK-NAP1 complex may protect cells from TNF-α-induced apoptosis by promoting NF-κB activation.