HERC4-mediated ubiquitination licenses RIPK1 to initiate TNF-induced cell death.

TNF can activate both prosurvival and prodeath signaling downstream of tumor necrosis factor receptor 1 (TNFR1). Survival signaling originates from TNFR1-containing membrane-bound complex I, while death signaling is driven by cytosolic complex II. Receptor-interacting protein kinase 1 (RIPK1) is a central component of both complexes but the molecular switch converting RIPK1 from a prosurvival scaffold in complex I to a prodeath kinase in complex II has remained elusive. Here, we identify the E3 ligase HERC4 as the molecular determinant of prodeath signaling. We show that HERC4 binds complex I-derived S166-phosphorylated, kinase-active RIPK1 and ubiquitinates it within its death domain. This enables RIPK1 oligomerization and assembly of the apoptosis-inducing RIPK1-FADD-caspase 8-containing complex IIa and, upon caspase inhibition, formation of the necroptosis-initiating RIPK1-RIPK3-containing necrosome. HERC4 deficiency protects mice from TNF-induced systemic inflammatory response syndrome and acute liver injury. Thus, HERC4 is the link enabling complex I-derived RIPK1 to initiate death signaling.