Activation of the pseudokinase mixed lineage kinase domain-like (MLKL) upon its phosphorylation by the protein kinase RIPK3 triggers necroptosis, a form of programmed cell death in which rupture of cellular membranes yields release of intracellular components. We report that MLKL also associated with endosomes and controlled the transport of endocytosed proteins, thereby enhancing degradation of receptors and ligands, modulating their induced signaling and facilitating the generation of extracellular vesicles. This role was exerted on two quantitative grades: a constitutive one independent of RIPK3, and an enhanced one, triggered by RIPK3, where the association of MLKL with the endosomes was enhanced, and it was found to bind endosomal sorting complexes required for transport (ESCRT) proteins and the flotillins and to be excluded, together with them, from cells within vesicles. We suggest that release of phosphorylated MLKL within extracellular vesicles serves as a mechanism for self-restricting the necroptotic activity of this protein. When phosphorylated by RIPK3, MLKL triggers necroptotic death. Yoon et al. show that MLKL also contributes to endosomal trafficking and generation of extracellular vesicles. This function is independent of RIPK3 but can be enhanced by it, yielding phospho-MLKL release within the vesicles, thereby apparently withholding death mediation by MLKL.
Yoon, S., Kovalenko, A., Bogdanov, K., & Wallach, D. (2017). MLKL, the Protein that Mediates Necroptosis, Also Regulates Endosomal Trafficking and Extracellular Vesicle Generation. Immunity, 47(1), 51-65.e7. https://doi.org/10.1016/j.immuni.2017.06.001