Structural basis for the prion-like MAVS filaments in antiviral innate immunity.

eLife Sciences (Impact Factor: 8.52). 02/2014; 3:e01489. DOI: 10.7554/eLife.01489
Source: PubMed

ABSTRACT Mitochondrial antiviral signaling (MAVS) protein is required for innate immune responses against RNA viruses. In virus-infected cells MAVS forms prion-like aggregates to activate antiviral signaling cascades, but the underlying structural mechanism is unknown. Here we report cryo-electron microscopic structures of the helical filaments formed by both the N-terminal caspase activation and recruitment domain (CARD) of MAVS and a truncated MAVS lacking part of the proline-rich region and the C-terminal transmembrane domain. Both structures are left-handed three-stranded helical filaments, revealing specific interfaces between individual CARD subunits that are dictated by electrostatic interactions between neighboring strands and hydrophobic interactions within each strand. Point mutations at multiple locations of these two interfaces impaired filament formation and antiviral signaling. Super-resolution imaging of virus-infected cells revealed rod-shaped MAVS clusters on mitochondria. These results elucidate the structural mechanism of MAVS polymerization, and explain how an α-helical domain uses distinct chemical interactions to form self-perpetuating filaments. DOI:

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    ABSTRACT: In higher vertebrates, recognition of the non-self signature of invading viruses by genome-encoded pattern recognition receptors initiates antiviral innate immunity. Retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs) detect viral RNA as a non-self pattern in the cytoplasm and activate downstream signaling. Detection of viral RNA also activates stress responses resulting in stress granule-like aggregates, which facilitate RLR-mediated antiviral immunity. Among the three RLR family members RIG-I and melanoma differentiation-associated gene 5 (MDA5) recognize distinct viral RNA species with differential molecular machinery and activate signaling through mitochondrial antiviral signaling (MAVS, also known as IPS-1/VISA/Cardif), which leads to the expression of cytokines including type I and III interferons (IFNs) to restrict viral propagation. In this review, we summarize recent knowledge regarding RNA recognition and signal transduction by RLRs and MAVS/IPS-1. Copyright © 2015 Elsevier Ltd. All rights reserved.
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    ABSTRACT: Helical polymers are found throughout biology and account for a substantial fraction of the protein in a cell. These filaments are very attractive for three-dimensional reconstruction from electron micrographs due to the fact that projections of these filaments show many different views of identical subunits in identical environments. However, ambiguities exist in defining the symmetry of a helical filament when one has limited resolution, and mistakes can be made. Until one reaches a near-atomic level of resolution, there are not necessarily reality-checks that can distinguish between correct and incorrect solutions. A recent paper in eLife (Xu et al., 2014) almost certainly imposed an incorrect helical symmetry and this can be seen using filament images posted by Xu et al. A comparison between the atomic model proposed and the published three-dimensional reconstruction should have suggested that an incorrect solution was found.
    eLife Sciences 12/2014; 3. DOI:10.7554/eLife.04969 · 8.52 Impact Factor
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    ABSTRACT: RIG-I and MDA5 are the major intracellular immune receptors that recognize viral RNA species and undergo a series of conformational transitions leading to the activation of the interferon-mediated antiviral response. However, to date, full-length RLRs have resisted crystallographic efforts and a molecular description of their activation pathways remains hypothetical. Here we employ hydrogen/deuterium exchange coupled with mass spectrometry (HDX-MS) to probe the apo states of RIG-I and MDA5 and to dissect the molecular details with respect to distinct RNA species recognition, ATP binding and hydrolysis and CARDs activation. We show that human RIG-I maintains an auto-inhibited resting state owing to the intra-molecular HEL2i-CARD2 interactions while apo MDA5 lacks the analogous intra-molecular interactions and therefore adopts an extended conformation. Our work demonstrates that RIG-I binds and responds differently to short triphosphorylated RNA and long duplex RNA and that sequential addition of RNA and ATP triggers specific allosteric effects leading to RIG-I CARDs activation. We also present a high-resolution protein surface mapping technique that refines the cooperative oligomerization model of neighboring MDA5 molecules on long duplex RNA. Taken together, our data provide a high-resolution view of RLR activation in solution and offer new evidence for the molecular mechanism of RLR activation. © The Author(s) 2014. Published by Oxford University Press on behalf of Nucleic Acids Research.
    Nucleic Acids Research 12/2014; 43(2). DOI:10.1093/nar/gku1329 · 8.81 Impact Factor

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