Expression of ubiquitin-specific protease 18 (USP18) is induced by various Toll-like receptor (TLR) ligands in human monocytes and macrophages (Yang Z et al. 2015). A nuclear factor kappa B (NF-kappa-B, NF-κB) luciferase reporter gene assay showed that expression of tagged USP18 negatively regulates TLR-mediated activation of NF-kappa B in human embryonic kidney HEK293T cells. USP18 also inhibited the degradation of endogenous IκBα protein in HEK293T cells (Yang Z et al. 2015). Further, knockdown of USP18 by USP18-specific small interfering RNAs (siRNA) enhanced NF-kappaB activity in LPS- stimulated human monocyte-like THP-1 cells (Yang Z et al. 2015). Co-immunoprecipitation and immunoblot analysis revealed that USP18 targets the regulatory subunit IKBKG (NEMO) of the IKK (CHUK:IKBKB:IKBKG) complex upon co-expression of tagged proteins in HEK293T cells. Mutagenesis analysis using HEK293T cells showed that USP18 directly binds to the UBAN motif of IKBKG inhibiting K63-linked ubiquitination of IKBKG by masking the ubiquitination sites at K325 and K326 (Yang Z et al. 2015). In addition, USP18 targets the TAK1-TAB1 complex and cleaves the K63-linked polyubiquitin chains of TAK1 in a protease-dependent manner (Liu X et al. 2013; Yang Z et al. 2015). These data suggest that USP18 functions as a negative regulator of NF-kappa-B activation.

This Reactome events shows USP18 binding to IKBKG within the IKK (CHUK:IKBKB:IKBKG) complex.