Viperin triggers ribosome collision-dependent translation inhibition to restrict viral replication.

Innate immune responses induce hundreds of interferon-stimulated genes (ISGs). Viperin, a member of the radical S-adenosyl methionine (SAM) superfamily of enzymes, is the product of one such ISG that restricts the replication of a broad spectrum of viruses. Here, we report a previously unknown antiviral mechanism in which viperin activates a ribosome collision-dependent pathway that inhibits both cellular and viral RNA translation. We found that the radical SAM activity of viperin is required for translation inhibition and that this is mediated by viperin's enzymatic product, 3'-deoxy-3',4'-didehydro-CTP (ddhCTP). Viperin triggers ribosome collisions and activates the MAPKKK ZAK pathway that in turn activates the GCN2 arm of the integrated stress response pathway to inhibit translation. The study illustrates the importance of translational repression in the antiviral response and identifies viperin as a translation regulator in innate immunity.

Viperin interaction with mitochondrial antiviral signaling protein (MAVS) limits viperin-mediated inhibition of the interferon response in macrophages.

Viperin is an antiviral protein that is upregulated by interferons and by ligands for a variety of innate immune receptors. It possesses diverse capabilities and functions in an array of viral infections. Studies have shown that it appears to be particularly important in defence against RNA viruses, such as West Nile, Dengue, and Chikungunya viruses, although the specific mechanisms involved are not well understood at the molecular level. Here we identify the mitochondrial antiviral signalling protein MAVS as a novel viperin interaction partner, most likely in mitochondria associated membranes, and characterize a more central, overarching role of viperin as a negative regulator of the interferon response, an ability that can be regulated by the viperin-MAVS interaction. This suggests a novel mechanism of viperin action in immune defence against RNA viruses by which it may prevent pathology from excessive immune responses.

Melanosomal formation of PMEL core amyloid is driven by aromatic residues.

PMEL is a pigment cell protein that forms physiological amyloid in melanosomes. Many amyloids and/or their oligomeric precursors are toxic, causing or contributing to severe, incurable diseases including Alzheimer's and prion diseases. Striking similarities in intracellular formation pathways between PMEL and various pathological amyloids including Aß and PrPSc suggest PMEL is an excellent model system to study endocytic amyloid. Learning how PMEL fibrils assemble without apparent toxicity may help developing novel therapies for amyloid diseases. Here we identify the critical PMEL domain that forms the melanosomal amyloid core (CAF). An unbiased alanine-scanning screen covering the entire region combined with quantitative electron microscopy analysis of the full set of mutants uncovers numerous essential residues. Many of these rely on aromaticity for function suggesting a role for π-stacking in melanosomal amyloid assembly. Various mutants are defective in amyloid nucleation. This extensive data set informs the first structural model of the CAF and provides insights into how the melanosomal amyloid core forms.

Critical residues in the PMEL/Pmel17 N-terminus direct the hierarchical assembly of melanosomal fibrils.

PMEL (also called Pmel17 or gp100) is a melanocyte/melanoma-specific glycoprotein that plays a critical role in melanosome development by forming a fibrillar amyloid matrix in the organelle for melanin deposition. Although ultimately not a component of mature fibrils, the PMEL N-terminal region (NTR) is essential for their formation. By mutational analysis we establish a high-resolution map of this domain in which sequence elements and functionally critical residues are assigned. We show that the NTR functions in cis to drive the aggregation of the downstream polycystic kidney disease (PKD) domain into a melanosomal core matrix. This is essential to promote in trans the stabilization and terminal proteolytic maturation of the repeat (RPT) domain-containing MαC units, precursors of the second fibrillogenic fragment. We conclude that during melanosome biogenesis the NTR controls the hierarchical assembly of melanosomal fibrils.