Figure 10.

Effects of USP20 Ser334 phosphorylation in IL-1R signaling. Activation with IL-1β promotes dimerization of the IL-1R and subsequent recruitment and activation of the IL-1R-associated kinase1 (IRAK1), which is a key mediator of IL-1R signaling. Activated IRAK1 binds to and activates the adaptor called tumor necrosis factor associated factor 6 (TRAF6). TRAF6 autoubiquitinates as part of its activation process, and subsequently ubiquitinates NFκB essential modulator (NEMO) as well as the TAB2 subunit of the TAK1 kinase complex—both by covalently attaching K63-linked polyubiquitin and by synthesizing free K63-linked polyubiquitins that serve as substrates for generating hybrid K63/linear ubiquitin chains that bind to NEMO and TAB2 via their ubiquitin-binding domains and subsequently augment their activity (not shown). TAK1 then autophosphorylates, activates and then phosphorylates and activates IκB kinase-β (IKK). Phosphorylation of IκBα by IKK provokes K48-linked polyubiquitination and 26S proteasomal degradation of IκBα. Subsequently, the now-uninhibited protein subunits of NFκB translocate to the nucleus and trigger gene transcription that promotes SMC proliferation and migration. In this signaling cascade, IKK also phosphorylates the p65 subunit of NFκB (and thereby increases its transcriptional activity (4)). Unphosphorylated USP20 (gray inset) deubiquitinates and thereby deactivates TRAF6 and thus diminishes downstream activation of NFκB. When IRAK1 phosphorylates USP20 Ser334 (schema at left, yellow-shaded background), the association of USP20 with TRAF6 is weakened and therefore USP20-mediated deubiquitination and deactivation of TRAF6 is prevented; signaling to NFκB activation proceeds. Preventing USP20 Ser334 phosphorylation (as in USP20-S334A) augments the binding of USP20 to TRAF6, augments TRAF6 deubiquitination, and diminishes NFκB activation (either by deubiquitinating and thereby inactivating a subset of TRAF6 proteins or by reducing the ubiquitination level of most or all TRAF6 proteins). UUU denotes K63-linked polyubiquitin; ^U_U^U_U denotes K48-linked polyubiquitin. IL-1, interleukin-1; SMC, smooth muscle cell; USP20, ubiquitin-specific peptidase 20.