Linear polyubiquitin chains

Abstract

The ubiquitin conjugation system regulates a wide variety of biological phenomena, including protein degradation and signal transduction, by regulating protein function via polyubiquitin conjugation in most cases. Several types of polyubiquitin chains exist in cells, and the type of polyubiquitin chain conjugated to a protein seems to determine how that protein is regulated. We identified a novel linear polyubiquitin chain and the ubiquitin-protein ligase complex that assembles it, designated LUBAC. Both were shown to have crucial roles in the canonical NFκB activation pathway. This year, three groups, including our laboratory, identified SHARPIN as a new subunit of LUBAC. Of great interest, Sharpin was identified as a causative gene of chronic proliferative dermatitis in mice (cpdm), which is characterized by numerous inflammatory symptoms including chronic dermatitis, arthritis and immune disorders. Deletion of SHARPIN drastically reduces the amount of LUBAC and attenuates signal-induced NFκB activation. The pleomorphic symptoms of cpdm mice suggest that LUBAC-mediated NFκB activation may play critical roles in mammals and be involved in various disorders. A forward look into the linear polyubiquitin research is also discussed.

Acknowledgments

Work in my laboratory was supported by Grants-in-Aid from the Ministry of Education, Culture, Sports, Science and Technology of Japan and the CREST Japan Science Technology Corporation.

Figures and Tables

Figure 1 The NFκB activation pathway. NFκB (p65-p50 heterodimer) resides in the cytoplasm in resting cells by binding to the inhibitor protein IκBα. Upon activation by various stimuli, IκBα is phosphorylated by the IKK complex. K48-linked polyubiquitination of phosphorylated IκBα leads to its degradation. Subsequently, free NFκB translocates into the nucleus and induces the expression of target genes.

Figure 2 The ubiquitin conjugation system. Ubiquitin conjugation is a reversible post-translational modification that regulates numerous biological phenomena by conjugating ubiquitin polymers to proteins. Polyubiquitin chains are generated by the repetition of the cascade of reactions catalyzed by three enzymes, E1 ubiquitin activating enzymes, E2 ubiquitin conjugating enzymes and E3 ubiquitin-protein ligases, on target proteins specifically recognized by E3s (A). Polyubiquitin chains have been thought to be generated via the Lys residues of ubiquitin. Polyubiquitin chains that function as degradation signals are generated via the Lys 48 of ubiquitin (B). K63-linked chains are involved in DNA repair and signal transduction and do not function as degradation signals (C). A new type of polyubiquitin chain was identified, a linear polyubiquitin (M1-linked) chain in which the C-terminal of ubiquitin is bound to the α-amino group of another ubiquitin. Linear polyubiquitin chains play crucial roles in NFκB activation (D).

Figure 3 Involvement of LUBAC-mediated linear ubiquitination in NFκB activation and mechanism underlying attenuated NFκB activation by loss of SHARPIN. Upon stimulation by inflammatory cytokines including TNFα and IL-1β, LUBAC, which is composed of HOIL-1L, HOIP and SHARPIN, recognizes and linearly polyubiquitinates NEMO, which induces IKK activation and leads to the degradation of IκBα. Free NFκB translocates into the nucleus and activates the transcription of target genes (A). In cpdm mice, the linear polyubiquitination of NEMO is attenuated because of the drastic reduction in the amount of LUBAC, composed solely of HOIL-1L and HOIP due to lack of SHARPIN, resulting in attenuated linear polyubiquitination of NEMO and attenuated NFκB activation (B).

Figure 4 Possible roles of different polyubiquitin chains in NFκB activation. (A) LUBAC-mediated linear polyubiquitinated NEMO can activate IKK. A hypothesis is shown according to which linearly polyubiquitinated NEMO can be recognized by other NEMO molecules, which induces phosphorylation of IKKβ. K63-linked chains or K11-linked chains generated by TRAF6 or cIAP1, respectively, are also recognized by NEMO, which may induce phosphorylation of IKKβ. (B) K63-linked chains generated by TRAF6 and K11-linked chains by cIAP1 may recruit LUBAC to the IL-1 receptor or the TNF receptor, respectively, and induce linear polyubiquitination of NEMO. Linearly polyubiquitinated NEMO induces IKKβ phosphorylation and NFκB activation.

Figure 5 Chronic dermatitis in cpdm mice. 5-mo-old wild-type (left) and cpdm mice (right) are shown.