SHARPIN forms a linear ubiquitin ligase complex regulating NF-κB activity and apoptosis

Abstract

The ubiquitin conjugation system regulates the canonical (or classical) nuclear factor κB (NF-κB)-activation pathway that mediates immune responses. Linear polyubiquitin chains, in which the C-terminal glycine of ubiquitin is conjugated to the amino group of the N-terminal methionine of another ubiquitin, is generated by a unique ubiquitin ligase complex called LUBAC — the linear ubiquitin chain assembly complex. LUBAC is composed of two RING domain proteins called HOIL-1 and HOIP. Now, three complementary studies published by the laboratories of Henning Walczak, Kazuhiro Iwai and Ivan Dikic identify a novel component of the LUBAC complex called SHARPIN, which is recruited to receptor signalling complexes that form following TNF and CD40L stimulation. The LUBAC complex containing SHARPIN stimulates the formation of linear ubiquitin chains in vitro and in vivo and is required for the activation of NF-κB signalling. The ubiquitin conjugation system regulates the canonical NF-κB activation pathway, which mediates immune responses. Linear polyubiquitin chains—in which the C terminal glycine of ubiquitin is conjugated to the α-amino group of the amino-terminal methionine of another ubiquitin—are generated by a unique ubiquitin ligase complex called linear ubiquitin chain assembly complex (LUBAC) composed of two RING domain proteins called HOIL-1 and HOIP. This is one of three complementary studies identifying a novel component of the LUBAC complex called SHARPIN, which is recruited to receptor signalling complexes (RSCs) that form after TNF and CD40L stimulation. The LUBAC complex containing SHARPIN stimulates the formation of linear ubiquitin chains in vitro and in vivo and is required for the activation of NF-κB signalling. SHARPIN is a ubiquitin-binding and ubiquitin-like-domain-containing protein which, when mutated in mice, results in immune system disorders and multi-organ inflammation1,2. Here we report that SHARPIN functions as a novel component of the linear ubiquitin chain assembly complex (LUBAC) and that the absence of SHARPIN causes dysregulation of NF-κB and apoptotic signalling pathways, explaining the severe phenotypes displayed by chronic proliferative dermatitis (cpdm) in SHARPIN-deficient mice. Upon binding to the LUBAC subunit HOIP (also known as RNF31), SHARPIN stimulates the formation of linear ubiquitin chains in vitro and in vivo. Coexpression of SHARPIN and HOIP promotes linear ubiquitination of NEMO (also known as IKBKG), an adaptor of the IκB kinases (IKKs) and subsequent activation of NF-κB signalling, whereas SHARPIN deficiency in mice causes an impaired activation of the IKK complex and NF-κB in B cells, macrophages and mouse embryonic fibroblasts (MEFs). This effect is further enhanced upon concurrent downregulation of HOIL-1L (also known as RBCK1), another HOIP-binding component of LUBAC. In addition, SHARPIN deficiency leads to rapid cell death upon tumour-necrosis factor α (TNF-α) stimulation via FADD- and caspase-8-dependent pathways. SHARPIN thus activates NF-κB and inhibits apoptosis via distinct pathways in vivo.