An unexpected twist to the activation of IKKβ: TAK1 primes IKKβ for activation by autophosphorylation.

Jiazhen Zhang,Mark W Peggie,Philip Cohen,Toby Lawrence,Kristopher Clark

doi:10.1042/bj20140444

Jiazhen Zhang, Mark W Peggie + Show 3 more

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IKKβ {IκB [inhibitor of NF-κB (nuclear factor κB)] kinase β} is required to activate the transcription factor NF-κB, but how IKKβ itself is activated in vivo is still unclear. It was found to require phosphorylation by one or more ‘upstream’ protein kinases in some reports, but by autophosphorylation in others. In the present study, we resolve this contro-versy by demonstrating that the activation of IKKβ induced by IL-1 (interleukin-1) or TNF (tumour necrosis factor) in embryonic fibroblasts, or by ligands that activate Toll-like receptors in macrophages, requires two distinct phosphorylation events: first, the TAK1 [TGFβ (transforming growth factor β)-activated kinase-1]-catalysed phosphorylation of Ser177 and, secondly, the IKKβ-catalysed autophosphorylation of Ser181. The phosphorylation of Ser177 by TAK1 is a priming event required for the subsequent autophosphorylation of Ser181, which enables IKKβ to phosphorylate exogenous substrates. We also provide genetic evidence which indicates that the IL-1-stimulated, LUBAC (linear ubiquitin chain assembly complex)-catalysed formation of linear ubiquitin chains and their interaction with the NEMO (NF-κB essential modulator) component of the canonical IKK complex permits the TAK1-catalysed priming phosphorylation of IKKβ at Ser177 and IKKα at Ser176. These findings may be of general significance for the activation of other protein kinases.

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The canonical inhibitor of NF-κB (IκB) kinase (IKK) {IκB [inhibitor of nuclear factor κB (NF-κB)] kinase β} complex, consisting of the protein kinases IKKα and IKKβ and a regulatory component called NF-κB essential modulator (NEMO) (NF-κB essential modifier) [1,2], is one of the most studied of all protein kinases
We initially confirmed that IL-1 or tumour necrosis factor (TNF) stimulate the dual phosphorylation of IKKβ at Ser177 and Ser181 in IKKα-deficient mouse embryonic fibroblast (MEF), and that this was prevented by the inclusion of the IKKβ inhibitor BI605 906 in the culture medium (Figures 1A and 1B, top panel, compare lanes 1–3 with 10–12)
The activation process is initiated by the transforming growth factor β-activated kinase-1 (TAK1)-catalysed phosphorylation of IKKβ at Ser177, which is a priming event that permits IKKβ to phosphorylate itself at Ser181, which is needed before IKKβ can phosphorylate exogenous substrates, such as IκBα (Figure 5)

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Introduction

The canonical IKK {IκB [inhibitor of NF-κB (nuclear factor κB)] kinase β} complex, consisting of the protein kinases IKKα and IKKβ ( called IKK1 and IKK2) and a regulatory component called NEMO (NF-κB essential modifier) [1,2], is one of the most studied of all protein kinases. J. Zhang and others role for autophosphorylation, we found that in IKKα-deficient MEFs the specific IKKβ inhibitor BI605906 prevented the IL1- or TNF-stimulated conversion of IKKβ into the active diphosphorylated species, i.e. phosphorylated at both Ser177 and Ser181 [8].

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