Fig. 8: A model for RIPK1 K612 ubiquitination in TNFR1 and TLR3/4 signalings.

In TNFα-stimulated WT cells, RIPK1 is rapidly recruited to TNFR1 by direct binding to the death domain (DD) of TNFR1 or that of TRADD to initiate the formation of complex I. In complex I the ubiquitination of RIPK1 is modulated by multiple E3 ubiquitin ligases such as cIAP1/2 and LUBAC complex. Ubiquitination of RIPK1 mediates NF-κB activation. Ubiquitination of RIPK1 K612 modulates overall patterns of RIPK1 ubiquitination and promotes RIPK1-RIPK1 and RIPK1-TNFR1 interaction mediated by its DD, which leads to its activation and interaction with FADD/Caspase-8 to mediate RDA or with RIPK3 to mediate necroptosis when caspase-8 is inactivated. K612R mutation reduces the recruitment of RIPK1 and inhibits the activation of RIPK1 upon stimulation of TNFR1 by TNFα. RIPK1 K612R mutation inhibits cell death induced by TNFα through disrupting RIPK1 dimerization and RIPK1-TNFR1 interaction mediated by DD domain. In LPS or Poly (I:C)-stimulated WT cells, RIPK1 and RIPK3 interact with TRIF which regulates the activation of RIPK1 and RIPK3. Ubiquitination of RIPK1 K612 promotes RIPK1 ubiquitination, dimerization and interaction of RIPK1 with FADD, which in turn recruits FADD to suppress necroptosis and caspase-1 activation by restricting RIPK3 activation in response to TLR3 and TLR4. RIPK1 K612R mutation disrupts the interaction of RIPK1 and FADD to promote RIPK3-dependent necroptosis and inflammation induced by LPS or Poly(I:C).