The regulation of necroptosis by post-translational modifications

Abstract

Necroptosis is a caspase-independent, lytic form of programmed cell death whose errant activation has been widely implicated in many pathologies. The pathway relies on the assembly of the apical protein kinases, RIPK1 and RIPK3, into a high molecular weight cytoplasmic complex, termed the necrosome, downstream of death receptor or pathogen detector ligation. The necrosome serves as a platform for RIPK3-mediated phosphorylation of the terminal effector, the MLKL pseudokinase, which induces its oligomerization, translocation to, and perturbation of, the plasma membrane to cause cell death. Over the past 10 years, knowledge of the post-translational modifications that govern RIPK1, RIPK3 and MLKL conformation, activity, interactions, stability and localization has rapidly expanded. Here, we review current knowledge of the functions of phosphorylation, ubiquitylation, GlcNAcylation, proteolytic cleavage, and disulfide bonding in regulating necroptotic signaling. Post-translational modifications serve a broad array of functions in modulating RIPK1 engagement in, or exclusion from, cell death signaling, whereas the bulk of identified RIPK3 and MLKL modifications promote their necroptotic functions. An enhanced understanding of the modifying enzymes that tune RIPK1, RIPK3, and MLKL necroptotic functions will prove valuable in efforts to therapeutically modulate necroptosis. Facts • Post-translational modifications of the RIPK1 and RIPK3 kinases and MLKL pseudokinase regulate their localization, activity, conformation, oligomerization, and protein interactions • To date, principally activating modifications have been reported for RIPK3 and MLKL How do RIPK3 and MLKL modifications control their protein interactions and prompt cell death?

Location, location, location: A compartmentalized view of TNF-induced necroptotic signaling

“…Signaling is diverted from NF-B when cIAP1/2 activity is low during TNFR1 stimulation, which, in turn, reduces K63-linked ubiquitylation of RIPK1 within Complex I, promotes the CYLD (Cylindromatosis)-mediated removal of Met1linked ubiquitin from Complex I, and thereby favors the formation of a potent death-induced signaling complex (Fig. 2) (60)(61)(62)(63). Complex I signaling is also governed by its subcellular location (27,28,(42)(43)(44)(45)(46).…”

“…The activation of MLKL within the necrosome then allows it to mediate downstream necroptotic events. and ubiquitination (60,78,89). Transphosphorylation-and/or autophosphoryl ation of RIPK1 and RIPK3 within the necrosome is critical for TNF-induced necroptosis (Fig.…”

There’s more to death than life: Noncatalytic functions in kinase and pseudokinase signaling

“…MLKL is the terminal effector in the necroptosis cell death pathway, which is a lytic cell death modality that, unlike the cousin pathway apoptosis, does not rely on the proteolytic functions of Caspases (reviewed in ( 48 )). Instead, necroptosis arises following an insult, such as inflammatory death receptor signaling or activation of innate pathogen sensors, which leads to oligomerization and activation of the receptor-interacting protein kinase-3 (RIPK3) effector kinase by autophosphorylation (reviewed in ( 49 )). Activated RIPK3 can then phosphorylate its substrate, MLKL, on the activation loop of its pseudokinase domain, to induce a conformational change that leads to exposure of the killer N-terminal four-helix bundle domain, MLKL oligomerization, and translocation to the plasma membrane ( Fig.…”

UbiComb: A Hybrid Deep Learning Model for Predicting Plant-Specific Protein Ubiquitylation Sites

“…Protein post-translational modifications (PTMs) are fundamental to cellular regulatory processes that control behavior, including cellular signaling, cell maintenance, cell development, and cell modification [ 1 , 2 , 3 ]. In the PTM process, a modification group is added to one or more amino acids to alter the physical and chemical properties of the proteins [ 4 ].…”