Research highlights
Cell signalling: Free ubiquitin!
Nature Reviews Molecular Cell Biology 10, 10 (01 October 2009) | doi:10.1038/nrm2765
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The E3 ubiquitin ligase TNF receptor-associated factor 6 (TRAF6) is known to activate nuclear factor-κB (NF-κB) downstream of the interleukin-1 and Toll-like receptors. Together with the ubiquitin-conjugating E2-type enzymes UBC13 and UEV1A (also known as UBE2V1), TRAF6 catalyses Lys63-linked ubiquitylation, which stimulates phosphorylation and activation of TGFβ-activated kinase 1 (TAK1; also known as MAP3K7), consequent phosphorylation of IκB kinase (IKK) and activation of NF-κB. In Nature, Zhijian Chen and colleagues now report on the mechanism of TRAF6-mediated NF-κB activation: TRAF6 generates unanchored polyubiquitin chains that bind to the regulatory subunits of the TAK1 and IKK complexes, leading to the activation of these kinases.
PHOTOALTOThe E3 ubiquitin ligase TNF receptor-associated factor 6 (TRAF6) is known to activate nuclear factor-κB (NF-κB) downstream of the interleukin-1 and Toll-like receptors. Together with the ubiquitin-conjugating E2-type enzymes UBC13 and UEV1A (also known as UBE2V1), TRAF6 catalyses Lys63-linked ubiquitylation, which stimulates phosphorylation and activation of TGFβ-activated kinase 1 (TAK1; also known as MAP3K7), consequent phosphorylation of IκB kinase (IKK) and activation of NF-κB. In Nature, Zhijian Chen and colleagues now report on the mechanism of TRAF6-mediated NF-κB activation: TRAF6 generates unanchored polyubiquitin chains that bind to the regulatory subunits of the TAK1 and IKK complexes, leading to the activation of these kinases.
To determine the requirement for TAK1 activation in vitro, purified TRAF6 was incubated with ubiquitin, an E1 enzyme, UBC13–UEV1A and a complex comprising TAK1, TAK1-binding protein 1 (TAB1; also known as MAP3K7IP1) and TAB2 (also known as MAP3K7IP2). Phosphorylation of TAK1 depended on UBC13–UEV1A, TRAF6 and Lys63-linked ubiquitin. These data suggest that TRAF6-catalysed Lys63-linked ubiquitylation is necessary and sufficient for TAK1 activation. Intriguingly, when the in vitro assay was reconstituted without TAK1–TAB1–TAB2 and then biochemically fractionated, the reaction mixture without the ubiquitin ligase components was able to maximally activate TAK1. Further fractionation revealed that unanchored Lys63-linked polyubiquitin chains, but not monomeric ubiquitin, activated TAK1 in a dose-dependent manner.
TRAF6 activates NF-κB by generating free Lys63-linked polyubiquitin chains
How do free Lys63-linked polyubiquitin chains lead to TAK1 activation? In interleukin-1β-stimulated cells, unanchored polyubiquitin chains were detected in association with TAB2 and NF-κB essential modulator (NEMO). Mutation of the ubiquitin-binding domains in TAB2 or NEMO blocked IKK activation. Furthermore, as an autophosphorylation-deficient mutant of TAK1 was nonetheless phosphorylated, the authors speculate that several TAB2 proteins might bind to a single polyubiquitin chain, bringing discrete TAK1 kinases in close proximity to facilitate their trans-phosphorylation. However, the possibility that ubiquitin binding promotes allosteric activation of TAK1–TAB1–TAB2 cannot be ruled out.
Thus, TRAF6 activates NF-κB by generating free Lys63-linked polyubiquitin chains that bind and stimulate the TAK1 and IKK complexes. It will be interesting to see whether this novel mechanism is used more widely in cellular processes.
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ORIGINAL RESEARCH PAPER
- Xia, Z.-P et al. Direct activation of protein kinases by unanchored polyubiquitin chains. Nature 12 Aug 2009 (doi:10.1038/nature08247) | Article |
