Linear polyubiquitination: a new regulator of NF-kappaB activation

Abstract

The ubiquitin-conjugation system regulates a vast range of biological phenomena by affecting protein function mostly through polyubiquitin conjugation. The type of polyubiquitin chain that is generated seems to determine how conjugated proteins are regulated, as they are recognized specifically by proteins that contain chain-specific ubiquitin-binding motifs. An enzyme complex that catalyses the formation of newly described linear polyubiquitin chains--known as linear ubiquitin chain-assembly complex (LUBAC)--has recently been characterized, as has a particular ubiquitin-binding domain that specifically recognizes linear chains. Both have been shown to have crucial roles in the canonical nuclear factor-kappaB (NF-kappaB)-activation pathway. The ubiquitin system is intimately involved in regulating the NF-kappaB pathway, and the regulatory roles of K63-linked chains have been studied extensively. However, the role of linear chains in this process is only now emerging. This article discusses the possible mechanisms underlying linear polyubiquitin-mediated activation of NF-kappaB, and the different roles that K63-linked and linear chains have in NF-kappaB activation. Future directions for linear polyubiquitin research are also discussed.

Figure 1

Schematic structure of LUBAC subunits HOIL-1L and HOIP. The NZF motifs of LUBAC have been shown to have ubiquitin-binding activity, although they are dispensable for ubiquitin-chain formation. HOIL-1L, longer isoform of haem-oxidized iron-regulatory protein ubiquitin ligase 1; HOIP, HOIL-1L interacting protein; LUBAC, linear ubiquitin chain assembly complex; NEMO, NF-κB essential modulator.

Figure 2

The canonical NF-κB-activation pathway. Upon activation by various stimuli, IκBα is phosphorylated by the IKK complex. The K48-linked polyubiquitination of phosphorylated IκBα leads to its degradation. Subsequently, free NF-κB translocates into the nucleus and induces the expression of target genes. Although signal-induced K63-linked polyubiquitination has been proposed to have important roles in this pathway, the crucial role of the LUBAC-mediated linear polyubiquitination of NEMO in NF-κB activation is now clear. HOIL-1L, longer isoform of haem-oxidized iron-regulatory protein ubiquitin ligase 1; HOIP, HOIL-1L interacting protein; IκBα, inhibitor of κBα; IKK, inhibitor of κB kinase; LUBAC, linear ubiquitin chain assembly complex; NEMO, NF-κB essential modulator; NF-κB, nuclear factor-κB; Ub, ubiquitin.

Figure 3

Possible relationships between K63-linked and linear polyubiquitin chains in NF-κB activation. (A) K63-linked chains generated by UBC13 recruit TAK1, which triggers JNK activation, but not NF-κB activation, in TNF-α signalling. (B) K63-linked chains generated by UBC13 are involved in both JNK and NF-κB activation in T-cell receptor signalling. Considering the importance of linear chains in NF-κB activation, UBC13-catalysed K63-linked chains might be crucial for recruitment of LUBAC. (C) Signals stimulating NF-κB activation might be generated independently of K63-linked chains because the IKK complex has been proposed to associate with CD40 before UBC13 associates with CD40. HOIL 1L, longer isoform of haem-oxidized iron-regulatory protein ubiquitin ligase 1; HOIP, HOIL-1L interacting protein; IKK, inhibitor of κB kinase; JNK, Jun amino-terminal kinase; LUBAC, linear ubiquitin chain assembly complex; NEMO, NF-κB essential modulator; NF-κB, nuclear factor-κB; TAK1, transforming growth factor β-activating kinase 1; TNF-α, tumour necrosis factor-α; Ub, ubiquitin; UBC13, ubiquitin conjugating enzyme 13.

Figure 4

Possible mechanisms underlying linear polyubiquitin-mediated IKK activation. (A) Linear polyubiquitin conjugated onto NEMO might be recognized by the UBAN motif of NEMO in another IKK complex, which induces multimerization of the IKK complex and trans-autophosphorylation of IKKs. (B) Subtle conformational changes in the UBAN motif of NEMO triggered by the recognition of linear chains might affect the spatial position of the IKKs, which leads to trans-phosphorylation of IKKs. (C) Linear polyubiquitination of NEMO might function as a scaffold to recruit an unknown kinase to phosphorylate IKKs. IKK, inhibitor of κB kinase; NEMO, NF-κB essential modulator; Ub, ubiquitin; UBAN, ubiquitin-binding in ABIN and NEMO.

Kazuhiro Iwai

Fuminori Tokunaga