Ubiquitin-Mediated Regulation of RIPK1 Kinase Activity Independent of IKK and MK2

Tumor necrosis factor (TNF) can drive inflammation, cell survival, and death. While ubiquitylation-, phosphorylation-, and nuclear factor κB (NF-κB)-dependent checkpoints suppress the cytotoxic potential of TNF, it remains unclear whether ubiquitylation can directly repress TNF-induced death. Here, we show that ubiquitylation regulates RIPK1’s cytotoxic potential not only via activation of downstream kinases and NF-kB transcriptional responses, but also by directly repressing RIPK1 kinase activity via ubiquitin-dependent inactivation. We find that the ubiquitin-associated (UBA) domain of cellular inhibitor of apoptosis (cIAP)1 is required for optimal ubiquitin-lysine occupancy and K48 ubiquitylation of RIPK1. Independently of IKK and MK2, cIAP1-mediated and UBA-assisted ubiquitylation suppresses RIPK1 kinase auto-activation and, in addition, marks it for proteasomal degradation. In the absence of a functional UBA domain of cIAP1, more active RIPK1 kinase accumulates in response to TNF, causing RIPK1 kinase-mediated cell death and systemic inflammatory response syndrome. These results reveal a direct role for cIAP-mediated ubiquitylation in controlling RIPK1 kinase activity and preventing TNF-mediated cytotoxicity. [Display omitted] •Ubiquitylation directly controls RIPK1 kinase activity in TNF signaling•UBA-dependent ubiquitylation of RIPK1 represses its kinase activity and cell death•The UBA contributes to optimal occupancy of ubiquitin-acceptor lysines in RIPK1•UBA-dependent ubiquitylation of RIPK1 also targets it for proteasomal degradation Annibaldi et al. show that cIAP-mediated ubiquitylation of RIPK1 kinase suppresses its auto-activation and, in addition, marks it for proteasomal degradation. These results reveal a direct role for ubiquitin in controlling RIPK1 kinase activity and suppressing TNF-mediated cytotoxicity.