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

IKKß {IkB [inhibitor of NF-?B (nuclear factor ?B)] kinase ß} is required to activate the transcription factor NF-?B, but how IKKß 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 IKKß 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 [TGFß (transforming growth factor ß)-activated kinase-1]-catalysed phosphorylation of Ser¹⁷⁷ and, secondly, the IKKß-catalysed autophosphorylation of Ser¹⁸¹. The phosphorylation of Ser¹⁷⁷ by TAK1 is a priming event required for the subsequent autophosphorylation of Ser¹⁸¹, which enables IKKß 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-?B essential modulator) component of the canonical IKK complex permits the TAK1-catalysed priming phosphorylation of IKKß at Ser¹⁷⁷ and IKKa at Ser¹⁷⁶. These findings may be of general significance for the activation of other protein kinases.