SHARPIN is a component of the NF-κB-activating linear ubiquitin chain assembly complex

Cpdm (chronic proliferative dermatitis) mice develop chronic dermatitis and an immunodeficiency with increased serum IgM(1-3), symptoms that resemble those of patients with X-linked hyper-IgM syndrome and hypohydrotic ectodermal dysplasia (XHM-ED), which is caused by mutations in NEMO (NF-kappa B essential modulator; also known as IKBKG)(4-6). Spontaneous null mutations in the Sharpin (SHANK-associated RH domain interacting protein in postsynaptic density)(7) gene are responsible for the cpdm phenotype in mice(8). SHARPIN shows significant similarity to HOIL-1L (also known as RBCK1)(8,9), a component of linear ubiquitin chain assembly complex (LUBAC), which induces NF-kappa B activation through conjugation of linear polyubiquitin chains to NEMO10-13. Here, we identify SHARPIN as an additional component of LUBAC. SHARPIN-containing complexes can linearly ubiquitinate NEMO and activated NF-kappa B. Thus, we re-define LUBAC as a complex containing SHARPIN, HOIL-1L, and HOIP (also known as RNF31). Deletion of SHARPIN drastically reduced the amount of LUBAC, which resulted in attenuated TNF-alpha- and CD40-mediated activation of NF-kappa B in mouse embryonic fibroblasts (MEFs) or B cells from cpdm mice. Considering the pleomorphic phenotype of cpdm mice, these results confirm the predicted role of LUBAC-mediated linear polyubiquitination in NF-kappa B activation induced by various stimuli, and strongly suggest the involvement of LUBAC-induced NF-kappa B activation in various disorders.