A Plug Release Mechanism for Membrane Permeation by MLKL

• The solution structure of the MLKL N-terminal domain has been elucidated
• It contains a four-helix bundle, a helix at the top and a sixth C-terminal helix
• The four-helix bundle is responsible for membrane permeation by MLKL
• The sixth helix likely acts as a plug that inhibits the four-helix bundle

SummaryMLKL is crucial for necroptosis, permeabilizing membranes through its N-terminal region upon phosphorylation of its kinase-like domain by RIP3. However, the mechanism underlying membrane permeabilization is unknown. The solution structure of the MLKL N-terminal region determined by nuclear magnetic resonance spectroscopy reveals a four-helix bundle with an additional helix at the top that is likely key for MLKL function, and a sixth, C-terminal helix that interacts with the top helix and with a poorly packed interface within the four-helix bundle. Fluorescence spectroscopy measurements indicate that much of the four-helix bundle inserts into membranes, but not the C-terminal helix. Moreover, we find that the four-helix bundle is sufficient to induce liposome leakage and that the C-terminal helix inhibits this activity. These results suggest that the four-helix bundle mediates membrane breakdown during necroptosis and that the sixth helix acts as a plug that prevents opening of the bundle and is released upon RIP3 phosphorylation.

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