Processing of the nf(kappa)b2 gene product p100 to generate p52 is an important step in NF-kappaB regulation. We show that this step is negatively regulated by a processing-inhibitory domain (PID) within p100 and positively regulated by the NF-kappaB-inducing kinase (NIK). While the PID suppresses the constitutive processing of p100, NIK induces p100 processing by stimulating site-specific phosphorylation and ubiquitination of this precursor protein. Further, a natural mutation of the gene encoding NIK in alymphoplasia (aly) mice cripples the function of NIK in p100 processing, causing a severe defect in p52 production. These data suggest that NIK is a specific kinase regulating p100 processing and explain why the aly and nf(kappa)b2 knockout mice exhibit similar immune deficiencies.

A20 negatively regulates inflammation by inhibiting the nuclear factor κB (NF-κB) transcription factor in the tumor necrosis factor receptor (TNFR) and Toll-like receptor (TLR) pathways. A20 contains deubiquitinase and E3 ligase domains and thus has been proposed to function as a ubiquitin-editing enzyme downstream of TNFR1 by inactivating ubiquitinated RIP1. However, it remains unclear how A20 terminates NF-κB signaling downstream of TLRs. We have shown that A20 inhibited the E3 ligase activities of TRAF6, TRAF2, and cIAP1 by antagonizing interactions with the E2 ubiquitin conjugating enzymes Ubc13 and UbcH5c. A20, together with the regulatory molecule TAX1BP1, interacted with Ubc13 and UbcH5c and triggered their ubiquitination and proteasome-dependent degradation. These findings suggest a mechanism of A20 action in the inhibition of inflammatory signaling pathways.

The NF-B family of transcription factors plays a pivotal role in regulation of diverse biological processes, including immune responses, cell growth, and apoptosis. Activation of NF-B is mediated by both canonical and noncanonical signaling pathways. Although the canonical pathway has been extensively studied, the mechanism mediating the noncanonical pathway is still poorly understood. Recent studies have identified the NF-B-inducing kinase (NIK) as a key component of the noncanonical pathway of NF-B activation; however, how the signaling function of NIK is regulated remains unknown. We report here that one important mechanism of NIK regulation is through its dynamic interaction with the tumor necrosis factor receptor-associated factor 3 (TRAF3). TRAF3 physically associates with NIK via a specific sequence motif located in the N-terminal region of NIK; this molecular interaction appears to target NIK for degradation by the proteasome. Interestingly, induction of noncanonical NF-B signaling by extracellular signals involves degradation of TRAF3 and the concomitant enhancement of NIK expression. These results suggest that induction of noncanonical NF-B signaling may involve the rescue of NIK from TRAF3-mediated negative regulation.

IκB kinase (IKK) is a key mediator of NF‐κB activation induced by various immunological signals. In T cells and most other cell types, the primary target of IKK is a labile inhibitor of NF‐κB, IκBα, which is responsible for the canonical NF‐κB activation. Here, we show that in T cells infected with the human T‐cell leukemia virus (HTLV), IKKα is targeted to a novel signaling pathway that mediates processing of the nfκb2 precursor protein p100, resulting in active production of the NF‐κB subunit, p52. This pathogenic action is mediated by the HTLV‐encoded oncoprotein Tax, which appears to act by physically recruiting IKKα to p100, triggering phosphorylation‐dependent ubiquitylation and processing of p100. These findings suggest a novel mechanism by which Tax modulates the NF‐κB signaling pathway.