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Our innate immune responses to viral RNA are vital defenses. Long cytosolic double-stranded RNA (dsRNA) is recognized by MDA5. The ATPase activity of MDA5 contributes to its dsRNA binding selectivity. Mutations that reduce RNA selectivity can cause autoinflammatory disease. Here, we show how the disease-associated MDA5 variant M854K perturbs MDA5-dsRNA recognition. M854K MDA5 constitutively activates interferon signaling in the absence of exogenous RNA. M854K MDA5 lacks ATPase activity and binds more stably to synthetic Alu:Alu dsRNA. CryoEM structures of MDA5-dsRNA filaments at different stages of ATP hydrolysis show that the K854 sidechain forms polar bonds that constrain the conformation of MDA5 subdomains, disrupting key steps in the ATPase cycle- RNA footprint expansion and helical twist modulation. The M854K mutation inhibits ATP-dependent RNA proofreading via an allosteric mechanism, allowing MDA5 to form signaling complexes on endogenous RNAs. This work provides insights on how MDA5 recognizes dsRNA in health and disease., (© 2021. The Author(s).)

The RNA sensor MDA5 recruits the signaling adaptor MAVS to initiate type I interferon signaling and downstream antiviral responses, a process that requires K63-linked polyubiquitin chains. Here, we examined the mechanisms whereby K63-polyUb chain regulate MDA5 activation. Only long unanchored K63-polyUb n (n ≥ 8) could mediate tetramerization of the caspase activation and recruitment domains of MDA5 ( MDA5 CARDs). Cryoelectron microscopy structures of a polyUb 13 -bound MDA5 CARDs tetramer and a polyUb 11 -bound MDA5 CARDs- MAVS CARD assembly revealed a tower-like formation, wherein eight Ubs tethered along the outer rim of the helical shell, bridging MDA5 CARDs and MAVS CARD tetramers into proximity. ATP binding and hydrolysis promoted the stabilization of RNA-bound MDA5 prior to MAVS activation via allosteric effects on CARDs-polyUb complex. Abundant ATP prevented basal activation of apo MDA5. Our findings reveal the ordered assembly of a MDA5 signaling complex competent to recruit and activate MAVS and highlight differences with RIG-I in terms of CARD orientation and Ub sensing that suggest different abilities to induce antiviral responses., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2021 Elsevier Inc. All rights reserved.)

Human respiratory syncytial virus (RSV) nonstructural protein 2 (NS2) inhibits host interferon (IFN) responses stimulated by RSV infection by targeting early steps in the IFN-signaling pathway. But the molecular mechanisms related to how NS2 regulates these processes remain incompletely understood. To address this gap, here we solved the X-ray crystal structure of NS2. This structure revealed a unique fold that is distinct from other known viral IFN antagonists, including RSV NS1. We also show that NS2 directly interacts with an inactive conformation of the RIG-I-like receptors (RLRs) RIG-I and MDA5. NS2 binding prevents RLR ubiquitination, a process critical for prolonged activation of downstream signaling. Structural analysis, including by hydrogen-deuterium exchange coupled to mass spectrometry, revealed that the N terminus of NS2 is essential for binding to the RIG-I caspase activation and recruitment domains. N-terminal mutations significantly diminish RIG-I interactions and result in increased IFNβ messenger RNA levels. Collectively, our studies uncover a previously unappreciated regulatory mechanism by which NS2 further modulates host responses and define an approach for targeting host responses., Competing Interests: The authors declare no competing interest., (Copyright © 2021 the Author(s). Published by PNAS.)

Cytoplasmic RIG-I-like receptor (RLR) proteins in mammalian cells recognize viral RNA and initiate an antiviral response that results in IFN-β induction. Melanoma differentiation-associated protein 5 (MDA5) forms fibers along viral dsRNA and propagates an antiviral response via a signaling domain, the tandem CARD. The most enigmatic RLR, laboratory of genetics and physiology (LGP2), lacks the signaling domain but functions in viral sensing through cooperation with MDA5. However, it remains unclear how LGP2 coordinates fiber formation and subsequent MDA5 activation. We utilized biochemical and biophysical approaches to observe fiber formation and the conformation of MDA5. LGP2 facilitated MDA5 fiber assembly. LGP2 was incorporated into the fibers with an average inter-molecular distance of 32 nm, suggesting the formation of hetero-oligomers with MDA5. Furthermore, limited protease digestion revealed that LGP2 induces significant conformational changes on MDA5, promoting exposure of its CARDs. Although the fibers were efficiently dissociated by ATP hydrolysis, MDA5 maintained its active conformation to participate in downstream signaling. Our study demonstrated the coordinated actions of LGP2 and MDA5, where LGP2 acts as an MDA5 nucleator and requisite partner in the conversion of MDA5 to an active conformation. We revealed a mechanistic basis for LGP2-mediated regulation of MDA5 antiviral innate immune responses., (© The Author(s) 2020. Published by Oxford University Press on behalf of Nucleic Acids Research.)

IFIH1 gain-of-function has been reported as a cause of a type I interferonopathy encompassing a spectrum of autoinflammatory phenotypes including Aicardi-Goutières syndrome and Singleton Merten syndrome. Ascertaining patients through a European and North American collaboration, we set out to describe the molecular, clinical and interferon status of a cohort of individuals with pathogenic heterozygous mutations in IFIH1. We identified 74 individuals from 51 families segregating a total of 27 likely pathogenic mutations in IFIH1. Ten adult individuals, 13.5% of all mutation carriers, were clinically asymptomatic (with seven of these aged over 50 years). All mutations were associated with enhanced type I interferon signaling, including six variants (22%) which were predicted as benign according to multiple in silico pathogenicity programs. The identified mutations cluster close to the ATP binding region of the protein. These data confirm variable expression and nonpenetrance as important characteristics of the IFIH1 genotype, a consistent association with enhanced type I interferon signaling, and a common mutational mechanism involving increased RNA binding affinity or decreased efficiency of ATP hydrolysis and filament disassembly rate., (© 2020 The Authors. Human Mutation published by Wiley Periodicals, Inc.)

MDA5 belongs to the RIG-I-like receptor family, which is involved in innate immunity. During viral infection, MDA5 generates an antiviral response by recognizing the ligand to activate interferon. However, the role and mechanism of MDA5 in canine influenza virus (CIV) infection are unclear. To understand the mechanism of canine MDA5-mediated innate immunity during CIV infection, we detected the distribution of MDA5 in beagles, and the structural prediction showed that MDA5 was mainly composed of a CARD domain, RD domain, and DExD/H helix structure. Moreover, we found that MDA5 inhibits CIV replication. Furthermore, in the dual luciferase assay, we revealed that the CARD region of MDA5 strongly activated the IFN-β promoter and mainly transmitted signals through the CARD region. Overexpression of the CARD region of MDA5 revealed that the MDA5-mediated signaling pathway could transmit signals by activating the IRF3/NF-κB and IRF3 promoters, promoting the expression of antiviral proteins and cytokine release, thereby inhibiting CIV replication. Upon silencing of MDA5, cytokine production decreased, while the replication ability of CIV was increased. Thus, this study revealed a novel mechanism by which MDA5 mediated CIV infection and provided new avenues for the development of antiviral strategies.

The innate immune system is the first line of defense against invading pathogens. The retinoic acid-inducible gene I (RIG-I) like receptors (RLRs), RIG-I and melanoma differentiation-associated protein 5 (MDA5), are critical for host recognition of viral RNAs. These receptors contain a pair of N-terminal tandem caspase activation and recruitment domains (2CARD), an SF2 helicase core domain, and a C-terminal regulatory domain. Upon RLR activation, 2CARD associates with the CARD domain of MAVS, leading to the oligomerization of MAVS, downstream signaling and interferon induction. Unanchored K63-linked polyubiquitin chains (polyUb) interacts with the 2CARD domain, and in the case of RIG-I, induce tetramer formation. However, the nature of the MDA5 2CARD signaling complex is not known. We have used sedimentation velocity analytical ultracentrifugation to compare MDA5 2CARD and RIG-I 2CARD binding to polyUb and to characterize the assembly of MDA5 2CARD oligomers in the absence of polyUb. Multi-signal sedimentation velocity analysis indicates that Ub 4 binds to RIG-I 2CARD with a 3:4 stoichiometry and cooperatively induces formation of an RIG-I 2CARD tetramer. In contrast, Ub 4 and Ub 7 interact with MDA5 2CARD weakly and form complexes with 1:1 and 2:1 stoichiometries but do not induce 2CARD oligomerization. In the absence of polyUb, MDA5 2CARD self-associates to forms large oligomers in a concentration-dependent manner. Thus, RIG-I and MDA5 2CARD assembly processes are distinct. MDA5 2CARD concentration-dependent self-association, rather than polyUb binding, drives oligomerization and MDA5 2CARD forms oligomers larger than tetramer. We propose a mechanism where MDA5 2CARD oligomers, rather than a stable tetramer, function to nucleate MAVS polymerization., (© 2019 The Protein Society.)

MDA5 is a cytoplasmic viral double-stranded RNA recognition receptor that plays a pivotal role in the aquatic animal innate immune system. To decipher the role of MDA5 of Squaliobarbus curriculus (ScMDA5) in the immune response, full-length cDNA of ScMDA5 was cloned using the RACE technology, mRNA and protein expression levels of ScMDA5 signalling pathway members in response to stimulation were detected and effects of overexpression of ScMDA5 on the immune response were investigated. ScMDA5 comprises 3597 bp and is composed of an open reading frame (2958 nucleotides long) that translates into a putative peptide of 985 amino acid residues. ScMDA5 possesses two N-terminal caspase-recruiting domains, DEAD-like helicases superfamily, helicase superfamily C-terminal and RIG-I&#95;C-RD domains, and differences in these domains among species were mainly observed with respect to their length and location. ScMDA5 was closely clustered with those of Carassius auratus, Ctenopharyngodon idellus and Mylopharyngodon piceus. ScMDA5 transcripts were most abundant in the spleen and the lowest in the liver. Expression levels of ScMDA5 in healthy tissues were significantly correlated with those of ScIRF3, ScIRF7 and ScIFN. Besides, mRNA expression levels of ScIRF3 were significantly correlated with those of ScIRF7 (0.956, P < 0.01). Expression level changes, including downregulation, upregulation and initial upregulation followed by downregulation, were found in ScMDA5 signalling pathway molecules in tissues after grass carp reovirus infection. Protein levels of ScMDA5 were the highest in the liver and the lowest in the spleen in detected healthy tissues. Overexpression of ScMDA5 led to significantly enhanced CiIRF7 and CiMx transcription in grass carp ovary cells (P < 0.05). The results of this study helped to clarify the role of ScMDA5 in the immune reaction against grass carp reovirus and provided fundamental information for fish breeding to achieve strong resistance to infection., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

1. Melanoma differentiation-associated gene 5 (MDA5) is a critical member of cytosolic pattern recognition receptors (PRRs) that recognise viral RNA and mediate type I interferon secretion in host cells. 2. The objective of the present study was to identify and characterise the structure and expression of pigeon MDA5. 3. The full-length MDA5 cDNA was cloned from pigeon spleen using RT-PCR and RACE. The distribution and expression level of pigeon MDA5 in different tissues were determined by QRT-PCR. 4. The results showed that the full-length pigeon MDA5 cDNA had 3858 nucleotides (containing a 210-bp 5'-UTR, a 3030-bp open reading frame and a 618-bp 3'-UTR) encoding a polypeptide of 1009 amino acids. The deduced amino acid sequence contained six conserved structural domains typical of RIG-I-like receptor (RLR), including two tandem arranged N-terminal caspase activation and recruitment domains (CARDs), a DEAH/DEAD box helicase domain (DExDc), a helicase superfamily c-terminal domain (HELICc), a type III restriction enzyme (ResIII) and a C-terminal regulatory domain (RD). 5. The pigeon MDA5 showed 84.8%, 87.3%, 87.9% and 87.2% amino acid sequence identities with previously described homologues from chicken, duck, goose and Muscovy ducks, respectively, and phylogenetic analysis revealed a close relationship among these MDA5. 6. Pigeon MDA5 transcript was ubiquitously expressed in all seven tissues tested in healthy pigeons and showed a high level in the thymus gland and kidney. 7. These findings lay the foundation for further research on the function and mechanism of MDA5 in innate immune responses related to vaccinations and infectious diseases in the pigeon.

Singleton-Merten syndrome, a critical and rare multifactorial disorder that is closely linked to R516Q mutation in MDA5 protein associated with an enhanced interferon response in the affected individual. In the present study, we provide conclusive key evidence on R516Q mutation and their connectivity towards sequence-structural basis dysfunction of MDA5 protein. Among the various mutations, we found R516Q is the most pathogenic mutation based on mutational signature Q-A-[RE]-G-R-[GA]-R-A-[ED]-[DE]-S-[ST]-Y-[TSAV]-L-V designed from our work. Further, we derived a distant ortholog for this mutational signature from which we identified 343 intra-residue interactions that fall communally in the position required to maintain the structural and functional integration of protein architecture. This identification served us to understand the critical role of hot spots in residual aggregation that holds a native form of folding conformation in the functional region. In addition, the long-range molecular dynamics simulation demarcated the residual dependencies of conformational transition in distinct regions ( L29 360-370 α18 , α19 380-410 L31 , α21 430-480 L33-α22-L35 and α24 510-520 L38 ) occurring upon R516Q mutation. Together, our results emphasise that the dislocation of functional hot spots Pro229, Arg414, Val498, Met510, Ala513, Gly515 and Arg516 in MDA5 protein which is important for interior structural packing and fold arrangements. In a nutshell, our findings are perfectly conceded with other experimental reports and will have potential implications in immune therapeutical advancement for rare singleton-merten syndrome.

RIG-I and MDA5 receptors are key sensors of pathogen-associated molecular pattern (PAMP)-containing viral RNA and transduce downstream signals to activate an antiviral and immunomodulatory response. Fifteen years of research have put them at the center of an ongoing hunt for novel pharmacological pan-antivirals, vaccine adjuvants, and antitumor strategies. Current knowledge testifies to the redundant, but also distinct, functions mediated by RIG-I and MDA5, opening opportunities for the use of specific and potent nucleic acid agonists. We critically discuss the evidence and remaining knowledge gaps that have an impact on the choice and design of optimal RNA ligands to achieve an appropriate immunostimulatory response, with limited adverse effects, for prophylactic and therapeutic interventions against viruses and cancer in humans., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Background: Infections, mostly of viral origin, may contribute to the seasonal variation in the onset of type 1 diabetes mellitus (T1DM). The rs1990760 (A>G, Ala946Thr) polymorphism (GG genotype) of the interferon induced helicase (IFIH1), a virus recognition receptor, confers a modest protection for T1DM. The aim of our study was to evaluate a possible association between this IFIH1 polymorphism and the seasonal variation in the onset of T1DM., Materials and Methods: The IFIH1 rs1990760 polymorphism was genotyped in 1055 patients of Central-Eastern European ancestry with T1DM (median age at diagnosis: 8.2 [interquartile range, IQR 4.8-11.8] years). T1DM onset was recorded in monthly intervals., Results: The IFIH1 genotype distribution was the following: 436 patients (41.3%) had AA genotype, 483 patients (45.8%) had AG genotype, and 136 patients (12.9%) had GG genotype. Significant seasonal variation in manifestation of T1DM (highest rate in winter and lowest rate in summer period) was observed in the total cohort (n = 1055), irrespective of gender. The disease predisposing AA genotype was more frequently found among new cases with onset in summer vs in those with onset in winter (44.3% vs 37.9%); conversely, the protective GG genotype was less frequent (9.3% vs 12.9%, respectively; P = .0268 for trend). Significant effect of genotype (P = .0418) was found on the seasonal variability of T1DM onset in the total cohort., Conclusions: The IFIH1 rs1990760 polymorphism seems to be associated with the seasonal manifestation of T1DM. Our findings suggest that this virus receptor gene may contribute to T1DM manifestation primarily in the summer period., (© 2017 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

Self/non-self-discrimination by the innate immune system relies on germline-encoded, non-rearranging receptors expressed by innate immune cells recognizing conserved pathogen-associated molecular patterns. The natural killer group 2D (NKG2D) receptor is a potent immune-activating receptor that binds human genome-encoded ligands, whose expression is negligible in normal tissues, but increased in stress and disease conditions for reasons that are incompletely understood. Here it is not clear how the immune system reconciles receptor binding of self-proteins with self/non-self-discrimination to avoid autoreactivity. We now report that increased expression of NKG2D ligands after virus infection depends on interferon response factors activated by the detection of viral double-stranded RNA by pattern-recognition receptors (RIG-I/MDA-5) and that NKG2D ligand up-regulation can be blocked by the expression of viral dsRNA-binding proteins. Thus, innate immunity-mediated recognition of viral nucleic acids triggers the infected cell to release interferon for NK cell recruitment and to express NKG2D ligands to become more visible to the immune system. Finally, the observation that NKG2D-ligand induction is a consequence of signaling by pattern-recognition receptors that have been selected over evolutionary time to be highly pathogen-specific explains how the risks of autoreactivity in this system are minimized., (© 2017 by The American Society for Biochemistry and Molecular Biology, Inc.)

Innate immunity is the first line against the invasion of pathogenic microorganisms. Previous reports only demonstrated production traits of commercial importance were often negatively correlated with innate disease resistance. However, whether different purpose of artificial selection influences innate immunity have not been understood. In this study, we cloned exon1, exon6 of IFIH1 and exon2 of IFIT5 by molecular biology techniques in seven different chicken breeds to detect the potential effect of artificial selection for commercial traits on disease resistance for the first time. In total, 8 single nucleotide polymorphisms(SNPs) of IFIH1 gene exon1 and exon6, 19 SNPs of IFIT5 gene exon2 were detected. We found all native chicken breeds had a relatively close relationship to broiler breeds but a remote relationship to layer breed. A great difference between CB and LLH with different selected purpose were observed. The allele frequencies of these two positive antiviral genes were associated with different purpose of artificial selection. Our experiment constituted the foundation for the interaction between commercial traits and immune trait., (Copyright © 2017 Elsevier B.V. All rights reserved.)

Retinoic acid-inducible gene-I (RIG-I)-like receptors (RLRs), including melanoma differentiation-associated gene 5 (MDA5) and RIG-I, are crucial for host recognition of non-self RNAs, especially viral RNA. Thus, the expression and activation of RLRs play fundamental roles in eliminating the invading RNA viruses and maintaining immune homeostasis. However, how RLR expression is tightly regulated remains to be further investigated. In this study, we identified a major histocompatibility complex (MHC)-encoded gene, tripartite interaction motif 40 (TRIM40), as a suppressor of RLR signaling by directly targeting MDA5 and RIG-I. TRIM40 binds to MDA5 and RIG-I and promotes their K27- and K48-linked polyubiquitination via its E3 ligase activity, leading to their proteasomal degradation. TRIM40 deficiency enhances RLR-triggered signaling. Consequently, TRIM40 deficiency greatly enhances antiviral immune responses and decreases viral replication in vivo. Thus, we demonstrate that TRIM40 limits RLR-triggered innate activation, suggesting TRIM40 as a potential therapeutic target for the control of viral infection., (Copyright © 2017 The Author(s). Published by Elsevier Inc. All rights reserved.)

Nucleic acids carrying pathogen-associated molecular patterns trigger innate immune responses and are used to activate host immunity. Although synthetic nucleic acids have been used for that purpose, they have shown limitations for in vivo and clinical applications. To address this issue, we tested a naturally occurring dsRNA extracted from rice bran (rb-dsRNA) and characterized it as a potent ligand of TLR3 and MDA5. In this study, intranasal administration of rb-dsRNA induced production of type I IFNs by alveolar macrophages and protected mice from morbidity and mortality resulting from respiratory virus infection, such as influenza A virus. This protection was completely absent in mice lacking both TRIF and MDA5, indicating the essential role of TLR3- and MDA5-dependent pathways. Interestingly, IFNAR1-deficient mice retained residual antiviral protection, which was abolished by pharmacological inhibition of caspase 1, but not IL-1β signaling. In fact, rb-dsRNA activated caspase 1 via TRIF, resulting in the release of IL-1β and LDH. In addition to the direct antiviral activity, rb-dsRNA modulated the immune cell population in the lungs by repopulating virus-depleted alveolar macrophages. Our data demonstrate that rb-dsRNA orchestrates IFN-dependent and -independent direct antiviral protection and that it is a potent immune stimulator modulating antiviral immunity in the lungs. These findings open doors to a range of precise immune-modulating studies and therapeutic options., (Copyright © 2017 by The American Association of Immunologists, Inc.)

MDA5 plays a critical role in antiviral innate immunity by functioning as a cytoplasmic double-stranded RNA sensor that can activate type I interferon signaling pathways, but the mechanism for the activation of MDA5 is poorly understood. Here, we show that TRIM65 specifically interacts with MDA5 and promotes K63-linked ubiquitination of MDA5 at lysine 743, which is critical for MDA5 oligomerization and activation. Trim65 deficiency abolishes MDA5 agonist or encephalomyocarditis virus (EMCV)-induced interferon regulatory factor 3 (IRF3) activation and type I interferon production but has no effect on retinoic acid-inducible I (RIG-I), Toll-like receptor 3 (TLR3), or cyclic GMP-AMP synthase signaling pathways. Importantly, Trim65 -/- mice are more susceptible to EMCV infection than controls and cannot produce type I interferon in vivo. Collectively, our results identify TRIM65 as an essential component for the MDA5 signaling pathway and provide physiological evidence showing that ubiquitination is important for MDA5 oligomerization and activation., (© 2017 Lang et al.)

The function of the RIG-I-like receptors (RLRs; including RIG-I, MDA5, and LGP2) as key cytoplasmic sensors of viral pathogen-associated molecular patterns (PAMPs) has been subjected to numerous pathogenic challenges and has undergone a dynamic evolution. We found evolutionary evidence that RIG-I was lost in the Chinese tree shrew lineage. Along with the loss of RIG-I, both MDA5 (tMDA5) and LGP2 (tLGP2) have undergone strong positive selection in the tree shrew. tMDA5 or tMDA5/tLGP2 could sense Sendai virus (an RNA virus posed as a RIG-I agonist) for inducing type I IFN, although conventional RIG-I and MDA5 were thought to recognize distinct RNA structures and viruses. tMDA5 interacted with adaptor tMITA (STINGTMEM173/ERIS), which was reported to bind only with RIG-I. The positively selected sites in tMDA5 endowed the substitute function for the lost RIG-I. These findings provided insights into the adaptation and functional diversity of innate antiviral activity in vertebrates., Competing Interests: The authors declare no conflict of interest.

In this study, we sequenced and characterized melanoma differentiation-associated antigen 5 (LcMDA5), laboratory of genetics and physiology 2 (LcLGP2) and mitochondrial antiviral signaling protein (LcMAVS) from large yellow croaker (Larimichthys crocea). The LcMDA5 encodes 969 amino acids and contains two caspase-associated and recruitment domains (CARDs), a DExDc (DExD/H box-containing domain), a HELICc (helicase superfamily C-terminal domain) and a C-terminal regulatory domain (RD). The LcLGP2 encodes 679 amino acids and contains a DExDc, a HELICc and a RD. The LcMAVS encodes 512 amino acids and contains a CARD, a proline-rich domain, a transmembrane helix domain and a putative TRAF2-binding motif ((269)PVQDT(273)). Phylogenetic analyses showed that all the three genes of large yellow croaker are clustered together with their counterparts from other teleost fishes. The Real-time PCR analyses showed that all the three genes were found to be constitutively expressed in all examined tissues in large yellow croaker, but all with relatively low expression levels. Expression analyses showed that the three genes were all rapidly and significantly upregulated in vivo after poly (I:C) challenge in peripheral blood, liver, spleen and head kidney tissues. The results indicate that the LcMDA5, LcLGP2 and LcMAVS might play important roles in antiviral immune responses., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

RIG-I and MDA5 sense virus-derived short 5'ppp blunt-ended or long dsRNA, respectively, causing interferon production. Non-signaling LGP2 appears to positively and negatively regulate MDA5 and RIG-I signaling, respectively. Co-crystal structures of chicken (ch) LGP2 with dsRNA display a fully or semi-closed conformation depending on the presence or absence of nucleotide. LGP2 caps blunt, 3' or 5' overhang dsRNA ends with 1 bp longer overall footprint than RIG-I. Structures of 1:1 and 2:1 complexes of chMDA5 with short dsRNA reveal head-to-head packing rather than the polar head-to-tail orientation described for long filaments. chLGP2 and chMDA5 make filaments with a similar axial repeat, although less co-operatively for chLGP2. Overall, LGP2 resembles a chimera combining a MDA5-like helicase domain and RIG-I like CTD supporting both stem and end binding. Functionally, RNA binding is required for LGP2-mediated enhancement of MDA5 activation. We propose that LGP2 end-binding may promote nucleation of MDA5 oligomerization on dsRNA., (Copyright © 2016 The Author(s). Published by Elsevier Inc. All rights reserved.)