Structural studies of an antinecroptosis viral:human functional heteroamyloid M45:RIPK3 using SSNMR.

The formation of RIP-homotypic interaction motif (RHIM)-based heteromeric amyloid assemblies between effector proteins such as receptor-interacting protein kinases 1, Z-DNA Binding Protein 1, or TRIF and the kinase RIPK3 serves as regulating signals for the necroptosis process, a key element of innate immune defense. Murine cytomegalovirus expresses the M45-encoded viral inhibitor of RIP activation which inhibits necroptosis in a RHIM-dependent manner. A pivotal question is how viral M45 forms heteroamyloids with RIPK3 to effectively create an inhibitory assembly. We report a high-resolution structure of the M45:RIPK3 complex where M45 and RIPK3 alternately stack in an amyloid-state structure. Mutagenesis of the residues flanking the IQIG tetrad in M45 results in specific impacts on coassembly with RIPK3, indicating an extended interface in the heteromeric fibrils. Other key interactions support the formation of stable viral:host fibrils. The M45:RIPK3 heteroamyloid is likely to act as an antinecroptotic signal by competing with formation of other pronecroptotic species and introducing a barrier to RIPK3 autophosphorylation.