Aug 2026· PLoS Pathogens· Vol 22, pp. e1014449 - e1014449· 0 citations· 53 references
Medicine
TL;DR
Overall, the results suggest that vRNPs from influenza virus type A, B and D share the same right-handed antiparallel helical conformation and that the B/NP N-terminal tail does not participate to the helical architecture stabilization once the vRNPs are assembled.
Abstract
Influenza viruses belong to the Orthomyxoviridae family, they are categorized into four types: A, B, C, and D. Influenza B viruses co-circulate annually with influenza A strains during seasonal flu epidemics in humans, causing severe disease. The segmented RNA encapsidated by multiple copies of the nucleoprotein (NP) and attached to the heterotrimeric polymerase forms the central replicative unit called the viral ribonucleoprotein (vRNP). All influenza NP proteins share the same core domain folding, only NP of influenza B virus (B/NP) has an extended unfolded N-terminal tail of 70 amino-acids. This disordered N-terminal tail is required for nuclear localization of the protein, and may be involved in viral RNA transcription and replication regulation. In this study, we report that in absence of the extended N-terminal tail, the truncated NP maintains RNA binding ability and NP oligomeric state in vitro. We then reconstituted RNP-like particles by incubating truncated B/NP with synthetic RNA and we solved the cryo-EM structure at 4.1 Å resolution. Their morphology appears identical to native vRNPs extracted from viruses when observed by negative-stain electron microscopy. Overall, our results suggest that vRNPs from influenza virus type A, B and D share the same right-handed antiparallel helical conformation and that the B/NP N-terminal tail does not participate to the helical architecture stabilization once the vRNPs are assembled.
This work provides novel insights into paramyxoviral protein complexes, structures, and morphology in NiV by interrogating the protein:protein interactions of the main NiV structural proteins M/N/F/G.
Viraj Upadhye, Jean F. Lee, Nihan Ercanli et al.· bioRxiv· 0 citations
Vaccinia virus (VACV) is an orthopoxvirus closely related to mpox virus, which started global outbreaks in 2022. Poxvirus genomes are flanked by short, inverted complementary hairpin telomeres that feature mismatched bases and insertions essential for viral replication. In this context, a role of the late protein K4 has been proposed. K4 is present in the virion, is apparently non-essential and has a phospholipase D (PLD)-fold as has VACV F13 protein. It also shares fold and nuclease activity with its closest homologue, mammalian PLD3. We established an endonuclease activity against ssDNA and hairpin loops and bubbles in a dsDNA context while RNA is resistant to cleavage. The 2.4 Å cryo-EM structure of K4 shows an unusual octameric assembly, also present in solution. At low concentration, tetramers and dimers similar to the one of hPLD3 are also present. Despite its nuclease activity, in K4 a C-terminal extension blocks the DNA binding pockets. Using an inactive mutant, fortuitously, a DNA 19mer bound simultaneously to 2 sites of the octamer where it displaced the C-termini. DNA binding uses similar residues as the hPLD3 5'-exonuclease, despite different activities and orientations of the DNA. The role of K4 and the control of its activity by the observed auto-inhibition remain enigmatic.
Henri Gröger, Candice Trouba, Jade Barbaste et al.· Journal of Molecular Biology· 0 citations
Nairoviruses are emerging tick-borne pathogens for which effective antiviral therapies are currently unavailable. Although nucleoproteins (NPs) are essential for viral genome encapsulation and have been extensively characterized at the structural level, whether they perform additional functions during viral replication remains unclear. Here, we investigated the NP of the representative nairovirus Tacheng tick virus 1 (TcTV1). We found that the TcTV1 NP binds to nucleic acids in a sequence-independent manner and assembles into tetramer-based ribonucleoprotein complexes upon nucleic acid binding. This assembly process is accompanied by a pronounced conformational rearrangement that facilitates NP polymerization. In addition to its role in RNA encapsulation, TcTV1 NP exhibits intrinsic endonuclease activity that does not require metal ions and preferentially cleaves unstructured single-stranded RNA, while structured RNA substrates are largely resistant to cleavage. Functional analysis indicates that the stalk domain of NP plays a central role in coordinating RNA binding, oligomerization, and access to the nuclease-active site, thereby influencing whether an RNA molecule is protected or degraded. Finally, we identified a small-molecule compound that interferes with both RNA binding and nuclease activity by targeting a conserved functional region of nairovirus NP. Together, these results reveal an expanded functional repertoire of nairovirus NPs and suggest that NP-mediated RNA discrimination may contribute to viral replication. Our findings also support the feasibility of targeting NP for the development of antiviral drugs against emerging nairoviruses.
Zan Li, Shan Du, Feng Gao et al.· Proceedings of the National...· 0 citations
ABSTRACT Orthoflaviviruses are RNA viruses responsible for significant diseases in humans, domesticated animals, and wildlife. Their NS5 protein is central in viral replication, functioning both as an RNA-dependent RNA polymerase and a methyltransferase, while also modulating cellular processes, including the interferon response. Although viral replication is cytoplasmic, the NS5 protein of several mosquito-borne orthoflaviviruses cycles between the cytoplasm and the nucleus of infected human cells. However, the nuclear localization and function of NS5 of tick-borne orthoflaviviruses, such as tick-borne encephalitis virus (TBEV), remain poorly understood. Microscopy analysis and cell fractionation revealed that the NS5 protein of TBEV localized to both the cytoplasm and nucleoplasm of infected cells. Mutagenesis studies identified critical residues required for its nuclear targeting. Mutating these residues in a TBEV replicon abolished viral replication. Immunoprecipitation-mass spectrometry analyses performed in two human cell lines infected with TBEV recovered 352 NS5 partners. Among them, 187 were nuclear or partially nuclear. By integrating our interactome data with that of Powassan virus (POWV), another tick-borne orthoflavivirus, we refined a list of 20 high-confidence NS5 partners, including splicing factors and chromatin modulators. Functional analysis revealed that seven of these nuclear partners significantly modulated viral replication, further underscoring the importance of nuclear NS5 in the viral life cycle. Our work advances our understanding of the nuclear function of the NS5 proteins of tick-borne orthoflaviviruses. IMPORTANCE Tick-borne orthoflaviviruses are emerging globally, spreading across Europe, Asia, and North America, where they infect humans, domesticated animals, and wildlife. These viruses produce a protein called NS5, which drives viral replication and helps evade the innate immune response. We observed that the NS5 protein of tick-borne encephalitis virus (TBEV) localized both in the cytoplasm and nucleoplasm of infected human cells. We identified the specific residues responsible for its nuclear addressing and showed that it interacts with numerous nuclear proteins, including some involved in regulating gene expression. Seven of these nuclear partners significantly influenced viral replication, highlighting the importance of NS5’s nuclear activity. This work sheds light on how tick-borne orthoflaviviruses manipulate host cells, deepening our understanding of their replication strategies. Tick-borne orthoflaviviruses are emerging globally, spreading across Europe, Asia, and North America, where they infect humans, domesticated animals, and wildlife. These viruses produce a protein called NS5, which drives viral replication and helps evade the innate immune response. We observed that the NS5 protein of tick-borne encephalitis virus (TBEV) localized both in the cytoplasm and nucleoplasm of infected human cells. We identified the specific residues responsible for its nuclear addressing and showed that it interacts with numerous nuclear proteins, including some involved in regulating gene expression. Seven of these nuclear partners significantly influenced viral replication, highlighting the importance of NS5’s nuclear activity. This work sheds light on how tick-borne orthoflaviviruses manipulate host cells, deepening our understanding of their replication strategies.
Maxime Chazal, A. Sow, Elodie Le Seac'h et al.· Journal of Virology· 0 citations
ABSTRACT Coliphage N4 employs a unique infection and transcription strategy in which early gene expression is driven by a virion-encapsidated RNA polymerase (vRNAP) that is injected into the host cytoplasm upon infection. Despite extensive biochemical and crystallographic studies of the polymerase domain of vRNAP, the structural organization and regulatory roles of the N-terminal domain (NTD) and C-terminal domain (CTD) regions of the 3,500-residue-long whole enzyme have remained unresolved. Here, we report the cryo-electron microscopy (cryo-EM) structures of full-length N4 vRNAP in its apo state and in a transcription initiation complex (TIC) with promoter DNA and initiating nucleotides. The apo structure reveals a modular architecture in which an α-helical CTD packs against the Pol domain to stabilize an autoinhibited conformation characterized by occlusion of the nucleotide-binding site through tight contact between the plug module and motif B loop. In contrast, promoter binding induces conformational rearrangements that displace the motif B loop from the active site and separate the CTD from the Pol domain. The NTD is unresolved in both states, consistent with substantial intrinsic flexibility, and supporting its proposed role in membrane association and genome injection. Structural modeling suggests that domain segmentation and conformational plasticity may enable translocation of vRNAP through the ~30 Å wide phage tail channel during infection. Together, these results define the molecular architecture of full-length vRNAP and establish a structural framework for understanding how the conformational transition of vRNAP is coupled to its ejection, DNA injection, and early gene expression. IMPORTANCE This study investigates the structure of full-length bacteriophage N4 virion RNA polymerase (vRNAP), one of the largest known single-subunit RNA polymerases. The functions of its extensive N- and C-terminal regions remained unknown. Our work uncovers how the C-terminal domain regulates polymerase activity through a structural “switch” that locks the enzyme in an inactive state until it recognizes its promoter DNA. These findings explain how the phage prevents premature transcription and ensures precise control of early gene expression during infection. By integrating structures with the architecture of the N4 phage particle, we propose a mechanism by which this vRNAP is transported through the narrow phage tail into the host cell. Together, this work provides fundamental insight into phage transcription and viral gene regulation. This study investigates the structure of full-length bacteriophage N4 virion RNA polymerase (vRNAP), one of the largest known single-subunit RNA polymerases. The functions of its extensive N- and C-terminal regions remained unknown. Our work uncovers how the C-terminal domain regulates polymerase activity through a structural “switch” that locks the enzyme in an inactive state until it recognizes its promoter DNA. These findings explain how the phage prevents premature transcription and ensures precise control of early gene expression during infection. By integrating structures with the architecture of the N4 phage particle, we propose a mechanism by which this vRNAP is transported through the narrow phage tail into the host cell. Together, this work provides fundamental insight into phage transcription and viral gene regulation.
M. Narwal, Y. Shin, Katsuhiko S. Murakami· Journal of Bacteriology· 0 citations
Nipah virus (NiV) is a highly pathogenic, nonsegmented, negative-sense RNA virus (nsNSV) from the Mononegavirales order that causes frequent outbreaks, with no approved treatment available. Replication and transcription of its genome are carried out by a viral RNA-dependent RNA polymerase (RdRp) complex composed of the large catalytic protein (L) and the tetrameric phosphoprotein (P). Recently, structural insights into the NiV RdRp complex have emerged at an unprecedented pace. In particular, snapshots of the complex in precatalytic, early-elongation, and inhibitor-bound states have been reported. In this article, we review how these data shed light on the molecular mechanisms of RNA synthesis and inhibition in NiV and explore how these insights expand our understanding of nsNSV RdRps in general.
Fernanda A. Sala, H. S. Hillen· TIBS -Trends in Biochemical...· 0 citations