Fig. 7: Model for a role of RIPK1 in necroptosis downstream of MLKL activation.

A The cell death induced by forced dimerisation of the human MLKL NTD involves two distinct processes. In the first, MLKL can self-associate into discrete oligomers on addition of doxycycline. In the second, addition of coumermycin promotes the assembly of large multimeric complexes. AMG-47a is unable to inhibit the second process, thus we propose it prevents membrane attachment, translocation, or oligomerisation of the human MLKL NTD through its activity as a kinase inhibitor. As AMG-47a binds RIPK1, and as RIPK1 is required for this form of cell death, we propose the primary target of AMG-47a is RIPK1 in this context. B When a cell receives a necroptotic stimulus, a number of key events occur, beginning with assembly of a necrosome containing RIPK1, RIPK3, and MLKL. This triggers a number of events, beginning with the phosphorylation of MLKL by RIPK3, which enables a conformational change in MLKL that allows oligomerisation, translocation to cellular membranes, and ultimately cell death. Our data suggest an additional role for RIPK1 in human necroptosis by supporting the conformational change, oligomerisation, or membrane translocation of MLKL following phosphorylation by RIPK3.