vIRA Inhibition of Antiviral Necroptosis and RIPK3 Binding Are Separable Events.

Necroptosis is an antiviral form of programmed cell death modulated by proteins that interact via RIP Homotypic Interaction Motifs (RHIMs). The result of the signaling pathways depends on which RHIM-containing proteins are involved: although both host and viral proteins contain RHIMs, virally encoded RHIM proteins, such as murine cytomegalovirus (MCMV)-encoded viral inhibitor of RIP activation (vIRA) serve to prevent cell death. Although every RHIM contains the same core four-amino-acid pattern, there are variations in individual sequences that we hypothesized would determine the differential outcomes in necroptotic signaling. As such, we replaced the RHIM in vIRA with the RHIMs from other proteins involved in the signaling cascade (RIPK1, RIPK3, ZBP1, ICP6) to assess the effect on necroptosis during MCMV infection. Although these RHIM-swap vIRA constructs remained able to bind to RIPK3, in the context of MCMV infection, they lost the ability to prevent necroptosis. These results are consistent with other studies that demonstrate that RHIM-containing proteins form amyloid fibrils unique to the proteins interfacing. Our results provide biological context for the growing model that the outcome of RHIM-based signaling is influenced by the specific amyloid fibril structures that are driven by the unique amino-acid sequences of each RHIM present.