Structural Insights into MLKL-Mediated Necroptosis: Emerging Mechanisms.

Necroptosis is a regulated form of lytic cell death that plays important roles in inflammation, host defense, and disease. Central to this pathway is mixed lineage kinase domain-like protein (MLKL), the terminal effector responsible for membrane disruption. Upon phosphorylation by receptor-interacting protein kinase 3 (RIPK3), MLKL undergoes conformational changes, oligomerizes, and translocates to cellular membranes. Despite extensive study, however, the detailed molecular mechanism by which MLKL mediates necroptosis remains incompletely understood. Multiple models have been proposed, including pore formation, cation channel activity, and higher-order or amyloid-like assemblies, yet each is supported by limited and often indirect evidence. Notably, high-resolution structures of membrane-associated, active MLKL assemblies are still lacking, hindering a unified mechanistic understanding. In addition, emerging studies suggest that membrane permeabilization and terminal membrane rupture may be mechanistically distinct processes, potentially involving additional effectors such as SIGLEC12. In this review, we summarize current structural and mechanistic insights into MLKL, including its domain architecture, activation mechanisms, and species-specific divergence. We then evaluate competing models of MLKL-mediated membrane disruption and highlight key challenges and future directions. We emphasize the need to directly visualize the active, oligomeric MLKL assemblies in the membrane-bound state and to integrate structural, biochemical, and cellular approaches for a comprehensive understanding of MLKL-mediated membrane permeabilization. Resolving these questions will be essential for understanding necroptosis and for developing therapeutic strategies targeting MLKL-mediated cell death.