The RHIM of the Immune Adaptor Protein TRIF Forms Hybrid Amyloids with Other Necroptosis-Associated Proteins.

TIR-domain-containing adapter-inducing interferon-β (TRIF) is an innate immune protein that serves as an adaptor for multiple cellular signalling outcomes in the context of infection. TRIF is activated via ligation of Toll-like receptors 3 and 4. One outcome of TRIF-directed signalling is the activation of the programmed cell death pathway necroptosis, which is governed by interactions between proteins that contain a RIP Homotypic Interaction Motif (RHIM).

Herpes simplex virus encoded ICP6 protein forms functional amyloid assemblies with necroptosis-associated host proteins.

The viral protein ICP6, encoded by herpes simplex virus 1 (HSV-1), harbours a RIP-homotypic interaction motif (RHIM), that plays a role in viral inhibition of host cell death pathways. Other members of the Herpesviridae family also encode RHIM-containing proteins that interfere with host-cell death pathways, including the M45 protein from murine cytomegalovirus, and ORF20 protein from varicella zoster virus. We have used amyloid assembly assays, electron microscopy and single molecule fluorescence spectroscopy to show that the ICP6 RHIM is amyloidogenic and can interact with host RHIM-containing proteins to form heteromeric amyloid complexes, in a manner similar to that of M45 and ORF20 RHIMs.

Varicella zoster virus encodes a viral decoy RHIM to inhibit cell death.

Herpesviruses are known to encode a number of inhibitors of host cell death, including RIP Homotypic Interaction Motif (RHIM)-containing proteins. Varicella zoster virus (VZV) is a member of the alphaherpesvirus subfamily and is responsible for causing chickenpox and shingles. We have identified a novel viral RHIM in the VZV capsid triplex protein, open reading frame (ORF) 20, that acts as a host cell death inhibitor.

Microbial functional amyloids serve diverse purposes for structure, adhesion and defence.

The functional amyloid state of proteins has in recent years garnered much attention for its role in serving crucial and diverse biological roles. Amyloid is a protein fold characterised by fibrillar morphology, binding of the amyloid-specific dyes Thioflavin T and Congo Red, insolubility and underlying cross-β structure. Amyloids were initially characterised as an aberrant protein fold associated with mammalian disease.

Viral M45 and necroptosis-associated proteins form heteromeric amyloid assemblies.

The murine cytomegalovirus protein M45 protects infected mouse cells from necroptotic death and, when heterologously expressed, can protect human cells from necroptosis induced by tumour necrosis factor receptor (TNFR) activation. Here, we show that the N-terminal 90 residues of the M45 protein, which contain a RIP homotypic interaction motif (RHIM), are sufficient to confer protection against TNFR-induced necroptosis. This N-terminal region of M45 drives rapid self-assembly into homo-oligomeric amyloid fibrils and interacts with the RHIMs of the human kinases RIPK1 and RIPK3, and the Z-DNA binding protein 1 (ZBP1), to form heteromeric amyloid fibrils Mutation of the tetrad residues in the M45 RHIM attenuates homo- and hetero-amyloid assembly by M45, suggesting that the amyloidogenic nature of the M45 RHIM underlies its biological activity.

RHIM-based protein:protein interactions in microbial defence against programmed cell death by necroptosis.

The Receptor-interacting protein kinase Homotypic Interaction Motif (RHIM) is an amino acid sequence that mediates multiple protein:protein interactions in the mammalian programmed cell death pathway known as necroptosis. At least one key RHIM-based complex has been shown to have a functional amyloid fibril structure, which provides a stable hetero-oligomeric platform for downstream signaling. RHIMs and related motifs are present in immunity-related proteins across nature, from viruses to fungi to metazoans.