Serine 165 Phosphorylation of SHARPIN regulates the Activation of NF-κB

The adaptor SHARPIN composes, together with the E3 ligases HOIP and HOIL1, the linear ubiquitin chain assembly complex. This enzymatic complex catalyzes and stamps atypical linear ubiquitin chains onto substrates to modify their fate, and has been linked to the regulation of the NF-B pathway downstream most immunoreceptors, inflammation and cell death. However, how this signaling complex is regulated is not fully understood. Here, we report that a portion of SHARPIN is constitutively phosphorylated on the serine in position 165 in lymphoblastoid cells, and can be further induced following antigen receptor stimulation. Analysis of a phosphorylation-resistant mutant of SHARPIN revealed that this mark is dispensable for the generation of linear ubiquitin chains. However, phosphorylation allows the optimal activation of NF-B in response to TNFα and T-cell receptor engagement. These results identify a new layer of regulation of the LUBAC, and unveil new strategies to modulate its action