Generation and physiological roles of linear ubiquitin chains

Abstract

Ubiquitination now ranks with phosphorylation as one of the best-studied post-translational modifications of proteins with broad regulatory roles across all of biology. Ubiquitination usually involves the addition of ubiquitin chains to target protein molecules, and these may be of eight different types, seven of which involve the linkage of one of the seven internal lysine (K) residues in one ubiquitin molecule to the carboxy-terminal diglycine of the next. In the eighth, the so-called linear ubiquitin chains, the linkage is between the amino-terminal amino group of methionine on a ubiquitin that is conjugated with a target protein and the carboxy-terminal carboxy group of the incoming ubiquitin. Physiological roles are well established for K48-linked chains, which are essential for signaling proteasomal degradation of proteins, and for K63-linked chains, which play a part in recruitment of DNA repair enzymes, cell signaling and endocytosis. We focus here on linear ubiquitin chains, how they are assembled, and how three different avenues of research have indicated physiological roles for linear ubiquitination in innate and adaptive immunity and suppression of inflammation.

Figure 1

Schematic representation of the LUBAC components, SHARPIN, HOIP and HOIL-1. There is significant sequence homology (45% identity) between the carboxyl terminus of SHARPIN and the amino terminus of HOIL-1, each of which contains a UBL and an NZF motif. HOIP is the catalytic subunit of the tripartite LUBAC with SHARPIN and HOIL-1 as accessory factors that bind via their respective UBL domains to the NZF2 and UBA domains of HOIP, respectively. HOIP, SHARPIN and HOIL-1 also bind to ubiquitin chains through NZF-mediated interactions. The functions of the ZnF domain of HOIP and the coiled-coil domain of SHARPIN are currently unknown. The RBR domain of HOIP, but not of HOIL-1, is responsible for linear ubiquitin chain generation by LUBAC. Arrows indicate confirmed interactions between the proteins. Abbreviations: ZnF, zinc finger; NZF, Npl4 zinc finger; UBL, ubiquitin-like domain; UBA, ubiquitin-associated domain; IBR, in-between RING domain; RBR, RING-IBR-RING domain.

Figure 2

Model of TNFR1 signaling with and without LUBAC activity. Binding of trimeric TNF crosslinks the extracellular domains of three TNFR1 molecules and induces the formation of the TNF-RSC (also referred to as complex I). The tripartite LUBAC (ochre) is recruited to the TNF-RSC in a TRADD-, TRAF2- and cIAP-dependent manner (left panel) [16,19]. LUBAC activity in the TNF-RSC results in linear ubiquitination of RIP1 and NEMO [19] and enables the NF-κB and MAPK pathways to be activated to their full physiological extent. After a delay, and probably as a consequence of deubiquitination events at the membrane-bound TNF-RSC, the composition of the complex changes, and a second complex, complex II, appears in the cytosol [45]. Complex II (not shown) recruits FADD and caspase- 8, which are responsible for the induction of apoptosis, and includes RIP1 and RIP3, which mediate necroptosis. In the presence of LUBAC, however, the induction of cell death is prevented, probably by both stabilization of complex I by linear ubiquitination and the actions of genes induced by the NF-κB and MAPK pathways [16]. In the absence of SHARPIN (right panel), the other two LUBAC components are also drastically diminished, TNF-induced gene activation is attenuated and the TNF-RSC is destabilised, resulting in enhanced complex II formation and, consequently, cell death induction by apoptosis and necroptosis. Note that we have drawn the ubiquitin chains as diubiquitins. The actual length of the individual ubiquitin chains attached to components of the TNF-RSC - or indeed to components of any other signaling complex - is currently unknown.