Necroptosis is an inflammatory form of programmed cell death, the final steps of which are mediated by the pseudokinase protein Mixed Lineage Kinase domain-Like (MLKL). MLKL acts as a molecular switch in this pathway. Phosphorylation of conserved residues in the pseudokinase domain by RIPK3 triggers the transformation of MLKL from a dormant cytoplasmic monomer, to a membrane-associated oligomer, that disrupts the plasma membrane ultimately leading to the death of the cell.
The crystal structure of full-length mouse MLKL was published in 20131, it revealed that MLKL is comprised of an N-terminal 4-helical bundle (4HB) domain, followed by two brace helices, which form a link to the C-terminal pseudokinase domain. Whilst it is agreed that the 4HB is primarily responsible for membrane permeabilisation and the pseudokinase is responsible for regulating this function, so far, the role of the brace helices remains unknown. Therefore, we set out to examine the function of the brace helices of mouse and human MLKL through biochemical and cell-based experiments. Here we show that the first brace helix plays an important role in communicating activation signals through MLKL, and that the second brace helix is required for homo-oligomerisation, a key step in MLKL’s executioner function.