Fig. 2: Regulation of RIPK1 cytotoxicity by phosphorylation.

Upon TNFR1 ligation, RIPK1 is recruited to membrane-associated complex-I and is ubiquitinated and then functions as a scaffold to interact with IKK and its related kinases that play essential roles in NF-κB and p38 MAPK activation. In addition to the role of the upstream kinases for NF-κB signaling, TAK1 and IKKs directly phosphorylate the ubiquitinated RIPK1 bound to complex-I, which suppresses the cytotoxic potential of RIPK1 via an unknown mechanism, possibly by preventing the translocation of RIPK1 into complex-IIb. In addition to TAK1 and IKKs, TBK1/IKKε and MK2 also inhibit the full activation of RIPK1 via direct phosphorylation of RIPK1 that is bound to complex-I and of the cytosolic pool of RIPK1, respectively. If the phosphorylation-mediated early cell death checkpoint is disrupted by inactivation of these kinases, RIPK1 is autophosphorylated via an allosteric effect and switches to an active death-inducing mode where it subsequently phosphorylates necrosome-associated RIPK3 to initiate necroptosis. The key kinases that phosphorylate RIPK1 at distinct sites are indicated. A broken arrow indicates indirect phosphorylation sites on RIPK1 for TBK1/IKKε. Abbreviations: TNFR1 tumor necrosis factor receptor 1, RIPK1/3 receptor-interacting serine threonine protein kinase 1/3, MAPK mitogen-activated protein kinase, TAK1 transforming growth factor β activated kinase 1, IKK inhibitory kappa B kinase, TBK1/IKKε TANK-binding kinase 1/IKK-epsilon, MK2 MAPK-activated protein kinase 2.