c-IAP1 and UbcH5 promote K11-linked polyubiquitination of RIP1 in TNF signalling

Ubiquitin ligases are critical components of the ubiquitination process that determine substrate specificity and, in collaboration with E2 ubiquitin‐conjugating enzymes, regulate the nature of polyubiquitin chains assembled on their substrates. Cellular inhibitor of apoptosis (c‐IAP1 and c‐IAP2) proteins are recruited to TNFR1‐associated signalling complexes where they regulate receptor‐stimulated NF‐κB activation through their RING domain ubiquitin ligase activity. Using a directed yeast two‐hybrid screen, we found several novel and previously identified E2 partners of IAP RING domains. Among these, the UbcH5 family of E2 enzymes are critical regulators of the stability of c‐IAP1 protein following destabilizing stimuli such as TWEAK or CD40 signalling or IAP antagonists. We demonstrate that c‐IAP1 and UbcH5 family promote K11‐linked polyubiquitination of receptor‐interacting protein 1 (RIP1) in vitro and in vivo . We further show that TNFα‐stimulated NF‐κB activation involves endogenous K11‐linked ubiquitination of RIP1 within the TNFR1 signalling complex that is c‐IAP1 and UbcH5 dependent. Lastly, NF‐κB essential modifier efficiently binds K11‐linked ubiquitin chains, suggesting that this ubiquitin linkage may have a signalling role in the activation of proliferative cellular pathways. Canonical Lys48‐ and Lys63‐linked as well as non‐canonical linear polyubiquitin chains are involved in receptor‐mediated NF‐κB activation. This study identifies c‐IAP1‐generated Lys11‐linked chains, so far implicated only in cell‐cycle control and proteasome targeting, as an additional signal in this pathway.