Highly pathogenic avian influenza viruses (HPAIVs) derive from H5 and H7 low pathogenic avian influenza viruses (LPAIVs). Although insertion of a furin-cleavable multibasic cleavage site (MBCS) in the hemagglutinin gene was identified decades ago as the genetic basis for the LPAIV-to-HPAIV transition, the mechanisms underlying the occurrence of insertion are unknown. Here, we show that transient H5 RNA structures, predicted to trap the influenza virus polymerase on purine-rich sequences, drive nucleotide insertions, providing empirical evidence of RNA structure involvement in MBCS acquisition. Introduction of H5-like sequences and structures into an H6 hemagglutinin resulted in MBCS-yielding insertions. Our results show that nucleotide insertions that underlie H5 HPAIV emergence result from an RNA structure-driven diversity-generating mechanism, which could also occur in other RNA viruses.

Influenza virus mRNAs are stable and competent for nuclear export and translation because they receive a 5' cap(1) structure in a process called cap snatching1. During cap snatching, the viral RNA-dependent RNA polymerase (FluPol) binds to host RNA polymerase II (Pol II) and the emerging transcript2,3. The FluPol endonuclease then cleaves a capped RNA fragment that subsequently acts as a primer for the transcription of viral genes4,5. Here we present the cryogenic electron microscopy structure of FluPol bound to a transcribing Pol II in complex with the elongation factor DSIF in the pre-cleavage state. The structure shows that FluPol directly interacts with both Pol II and DSIF, positioning the FluPol endonuclease domain near the RNA exit channel of Pol II. These interactions are important for the endonuclease activity of FluPol and FluPol activity in cells. A second structure, trapped after cap snatching, shows that the cleaved capped RNA rearranges within FluPol, directing the capped RNA 3' end toward the FluPol polymerase active site for viral transcription initiation. Together, our results provide the molecular mechanisms of co-transcriptional cap snatching by FluPol.

Inflammation is a defense mechanism that restores tissue damages and eliminates pathogens. Among the Pattern Recognition Receptors that recognize danger or pathogenic signals, Nucleotide Oligomerization Domain 1 and 2 (NOD1/2) have been identified to play an important role in innate immunity responses and inhibition of NOD1 could be interesting to treat severe infections and inflammatory diseases. In this work, we identified the first selective NOD1 vs NOD2 pathway inhibitors at the nanomolar range based on a 4-anilinoquinazoline scaffold. We demonstrated that NOD1 inhibition occurs through the inhibition of Receptor Interacting Protein Kinase 2 (RIPK2), involved in its downstream signaling pathways. Compound 37 demonstrates no cytotoxicity, a selectivity for RIPK2 over Epithelial and Vascular Endothelial Growth Factor Receptors (EGFR/VEGFR) and a capacity to reduce pro-inflammatory cytokine IL-8 secretion. The structure of the RIPK2-compound 37 complex was resolved by crystallography. The 4-anilinoquinazoline scaffold offers novel perspectives to design NOD1-RIPK2 inhibitors, potentially useful to treat inflammatory diseases.

Tilapia Lake virus (TiLV) belongs to the Amnoonviridae family within the Articulavirales order of segmented negative-strand RNA viruses and is highly diverged from more familiar orthomyxoviruses, such as influenza. The viral nucleoprotein (NP), a key component of the replication machinery, packages the viral genome into protective ribonucleoprotein particles. Here we describe the electron cryo-microscopy (cryo-EM) structure of TiLV-NP bound to RNA within in vitro reconstituted, small ring-like, pseudo-symmetrical oligomers. Although TiLV-NP is considerably smaller than its influenza counterpart and unrelated in sequence, it maintains the same topology and domain organisation. This comprises a head and body domain between which is a positively charged groove, where single-stranded RNA binds. In addition, an oligomerisation loop inserts into a hydrophobic pocket in the neighbouring NP, the flexible hinges of which allow variable orientation of adjacent NPs. Focused cryo-EM maps unambiguously define the 5' to 3' direction of the bound RNA, confirmed by double stranded, A-form RNA regions that extrude out from some of the NP-NP interfaces. This is the first fully resolved description of how single-stranded and stem-loop RNA binds to an articulaviral NP assembly. Superposition with orthomyxoviral NPs suggest that the mode of RNA binding is likely similar across the Articulavirales order.

The nuclear cap-binding complex (CBC) and its partner Arsenite-Resistance Protein 2 (ARS2) regulate the fate of RNA polymerase II transcripts via mutually exclusive interactions with RNA effectors. One such effector is PHAX, which mediates the nuclear export of U-rich small nuclear RNAs (snRNAs). Here we present the cryo-electron microscopy structure of the human snRNA export complex comprising phosphorylated PHAX, CBC, CRM1-RanGTP and capped RNA. The central region of PHAX bridges CBC to the export factor CRM1-RanGTP, while also reinforcing cap dinucleotide binding. Additionally, PHAX interacts with a distant region of CRM1, facilitating contacts of the essential phosphorylated region of PHAX with the prominent basic surface of RanGTP. CBC engagement within the snRNA export complex is incompatible with its binding to other RNA effectors such as ALYREF or NCBP3. We demonstrate that snRNA export complex formation requires synergistic binding of all its components, which in turn displaces ARS2 from CBC and commits the complex for export.

Influenza viruses are a major cause of respiratory illness, with significant public health impact due to their ability to cause pandemics. This dialogue brought together leading experts including Professors George Fu Gao, Stephen Cusack, Mark von Itzstein, Ervin Fodor, Jonathan Grimes, Aartjan J.W. te Velthuis, and Tao Deng to decode the pressing scientific challenges and future directions in influenza research. They discussed how structural studies of the influenza polymerase have advanced our understanding of viral RNA transcription and replication. These insights are crucial for developing new antiviral drugs, with a particular focus on targeting the polymerase and its interactions with host factors like acidic nuclear phosphoprotein 32 (ANP32). The dialogue also highlighted the potential of AI to assist in designing small molecule drugs, offering new strategies for combating influenza. Future research will continue to unravel the complexities of the polymerase’s role in replication, aiming to translate these findings into effective therapies and resilient public health strategies.

The current model is that the influenza virus polymerase (FluPol) binds either to host RNA polymerase II (RNAP II) or to the acidic nuclear phosphoprotein 32 (ANP32), which drives its conformation and activity towards transcription or replication of the viral genome, respectively. Here, we provide evidence that the FluPol-RNAP II binding interface, beyond its well-acknowledged function in cap-snatching during transcription initiation, has also a pivotal role in replication of the viral genome. Using a combination of cell-based and in vitro approaches, we show that the RNAP II C-terminal-domain, jointly with ANP32, enhances FluPol replication activity. We observe successive conformational changes to switch from a transcriptase to a replicase conformation in the presence of the bound RNPAII C-terminal domain and propose a model in which the host RNAP II is the anchor for transcription and replication of the viral genome. Our data open new perspectives on the spatial coupling of viral transcription and replication and the coordinated balance between these two activities.

Inflammation is a defense mechanism that restores tissue damage and eliminates pathogens. Among the pattern recognition receptors that recognize danger or pathogenic signals, nucleotide oligomerization domains 1 and 2 (NOD1/2) have been identified to play an important role in innate immunity responses, and inhibition of NOD1 could be interesting to treat severe infections and inflammatory diseases. In this work, we identified the first selective NOD1 versus NOD2 pathway inhibitors at the nanomolar range based on a 4-anilinoquinazoline scaffold. We demonstrated that NOD1 inhibition occurs through the inhibition of receptor interacting protein kinase 2 (RIPK2), which is involved in its downstream signaling pathways. Compound 37 demonstrates no cytotoxicity, a selectivity for RIPK2 over epithelial and vascular endothelial growth factor receptors (EGFR/VEGFR), and a capacity to reduce pro-inflammatory cytokine IL-8 secretion. The structure of the RIPK2-compound 37 complex was resolved by crystallography. The 4-anilinoquinazoline scaffold offers novel perspectives to design NOD1-RIPK2 signaling inhibitors.

Replication of influenza viral RNA depends on at least two viral polymerases, a parental replicase and an encapsidase, and cellular factor ANP32. ANP32 comprises an LRR domain and a long C-terminal low complexity acidic region (LCAR). Here we present evidence suggesting that ANP32 is recruited to the replication complex as an electrostatic chaperone that stabilises the encapsidase moiety within apo-polymerase symmetric dimers that are distinct for influenza A and B polymerases. The ANP32 bound encapsidase, then forms the asymmetric replication complex with the replicase, which is embedded in a parental ribonucleoprotein particle (RNP). Cryo-EM structures reveal the architecture of the influenza A and B replication complexes and the likely trajectory of the nascent RNA product into the encapsidase. The cryo-EM map of the FluB replication complex shows extra density attributable to the ANP32 LCAR wrapping around and stabilising the apo-encapsidase conformation. These structures give new insight into the various mutations that adapt avian strain polymerases to use the distinct ANP32 in mammalian cells.

Genome-wide approaches have significantly advanced our knowledge of the repertoire of RNA-binding proteins (RBPs) that associate with cellular polyadenylated mRNAs within eukaryotic cells. Recent studies focusing on the RBP interactomes of viral mRNAs, notably SARS-Cov-2, have revealed both similarities and differences between the RBP profiles of viral and cellular mRNAs. However, the RBPome of influenza virus mRNAs remains unexplored. Herein, we identify RBPs that associate with the viral mRNA encoding the nucleoprotein (NP) of an influenza A virus. Focusing on TDP-43, we show that it binds several influenza mRNAs beyond the NP-mRNA, and that its depletion results in lower levels of viral mRNAs and proteins within infected cells, and a decreased yield of infectious viral particles. We provide evidence that the viral polymerase recruits TDP-43 onto viral mRNAs through a direct interaction with the disordered C-terminal domain of TDP-43. Notably, other RBPs found to be associated with influenza virus mRNAs also interact with the viral polymerase, which points to a role of the polymerase in orchestrating the assembly of viral messenger ribonucleoproteins.

TRIM25 is an RNA-binding ubiquitin E3 ligase with central but poorly understood roles in the innate immune response to RNA viruses. The link between TRIM25's RNA binding and its role in innate immunity has not been established. Thus, we utilized a multitude of biophysical techniques to identify key RNA-binding residues of TRIM25 and developed an RNA-binding deficient mutant (TRIM25-m9). Using iCLIP2 in virus-infected and uninfected cells, we identified TRIM25's RNA sequence and structure specificity, that it binds specifically to viral RNA, and that the interaction with RNA is critical for its antiviral activity. The RNA-binding ubiquitin E3 ligase TRIM25 plays a critical role in antiviral immunity. Here the authors identify key RNA-binding residues of TRIM25, link RNA binding to antiviral activity, reveal RNA structural and sequence preferences, and investigate binding to the viral genome.

Summary: The nuclear cap-binding complex (CBC) coordinates co-transcriptional maturation, transport, or degradation of nascent RNA polymerase II (Pol II) transcripts. CBC with its partner ARS2 forms mutually exclusive complexes with diverse “effectors” that promote either productive or destructive outcomes. Combining AlphaFold predictions with structural and biochemical validation, we show how effectors NCBP3, NELF-E, ARS2, PHAX, and ZC3H18 form competing binary complexes with CBC and how PHAX, NCBP3, ZC3H18, and other effectors compete for binding to ARS2. In ternary CBC-ARS2 complexes with PHAX, NCBP3, or ZC3H18, ARS2 is responsible for the initial effector recruitment but inhibits their direct binding to the CBC. We show that in vivo ZC3H18 binding to both CBC and ARS2 is required for nuclear RNA degradation. We propose that recruitment of PHAX to CBC-ARS2 can lead, with appropriate cues, to competitive displacement of ARS2 and ZC3H18 from the CBC, thus promoting a productive rather than a degradative RNA fate.

ABSTRACT Severe fever with thrombocytopenia syndrome virus (SFTSV) is a human pathogen that is now endemic to several East Asian countries. The viral large (L) protein catalyzes viral transcription by stealing host mRNA caps via a process known as cap-snatching. Here, we establish an in vitro cap-snatching assay and present three high-quality electron cryo-microscopy (cryo-EM) structures of the SFTSV L protein in biologically relevant, transcription-specific states. In a priming-state structure, we show capped RNA bound to the L protein cap-binding domain (CBD). The L protein conformation in this priming structure is significantly different from published replication-state structures, in particular the N- and C-terminal domains. The capped-RNA is positioned in a way that it can feed directly into the RNA-dependent RNA polymerase (RdRp) ready for elongation. We also captured the L protein in an early-elongation state following primer-incorporation demonstrating that this priming conformation is retained at least in the very early stages of primer extension. This structural data is complemented by in vitro biochemical and cell-based assays. Together, these insights further our mechanistic understanding of how SFTSV and other bunyaviruses incorporate stolen host mRNA fragments into their viral transcripts thereby allowing the virus to hijack host cell translation machinery.

Pneumonia is a worldwide threat to public health, demanding novel preventative and therapeutic strategies. The lung epithelium is a critical environmental interface that functions as a physical barrier to pathogen invasion while also actively sensing and responding to pathogens. We have reported that stimulating lung epithelial cells with a combination therapeutic consisting of a diacylated lipopeptide and a synthetic CpG oligodeoxynucleotide (ODN) induces synergistic pneumonia protection against a wide range of pathogens. We report here that mice deficient in TLR9, the previously described receptor for ODN, still displayed partial ODN-induced protection. This prompted us to seek an alternate ODN receptor, and we discovered by mass spectroscopy that the RNA sensor RIG-I could also bind DNA-like ODN. ODN binding by RIG-I resulted in MAVSdependent pneumonia-protective signaling events. While RIG-I is essential to native defenses against viral infections, we report that therapeutic RIG-I activation with ODN promoted pathogen killing and host survival following both viral and bacterial challenges. These data indicate that maximal ODN-induced pneumonia protection requires activation of both the TLR9/MyD88 and RIG-I/MAVS signaling pathways. These findings not only identify what we believe to be a novel pattern recognition receptor for DNA-like molecules, but reveal a potential therapeutic strategy to protect susceptible individuals against lethal pneumonias during periods of peakvulnerability.

During annual influenza epidemics, influenza B viruses (IBVs) co-circulate with influenza A viruses (IAVs), can become predominant and cause severe morbidity and mortality. Phylogenetic analyses suggest that IAVs (primarily avian viruses) and IBVs (primarily human viruses) have diverged over long time scales. Identifying their common and distinctive features is an effective approach to increase knowledge about the molecular details of influenza infection. The virus-encoded RNA-dependent RNA polymerases (FluPolB and FluPolA) are PB1-PB2-PA heterotrimers that perform transcription and replication of the viral genome in the nucleus of infected cells. Initiation of viral mRNA synthesis requires a direct association of FluPol with the host RNA polymerase II (RNAP II), in particular the repetitive C-terminal domain (CTD) of the major RNAP II subunit, to enable "cap-snatching" whereby 5'-capped oligomers derived from nascent RNAP II transcripts are pirated to prime viral transcription. Here, we present the first high-resolution co-crystal structure of FluPolB bound to a CTD mimicking peptide at a binding site crossing from PA to PB2. By performing structure-based mutagenesis of FluPolB and FluPolA followed by a systematic investigation of FluPol-CTD binding, FluPol activity and viral phenotype, we demonstrate that IBVs and IAVs have evolved distinct binding interfaces to recruit the RNAP II CTD, despite the CTD sequence being highly conserved across host species. We find that the PB2 627 subdomain, a major determinant of FluPol-host cell interactions and IAV host-range, is involved in CTD-binding for IBVs but not for IAVs, and we show that FluPolB and FluPolA bind to the host RNAP II independently of the CTD. Altogether, our results suggest that the CTD-binding modes of IAV and IBV may represent avian- and human-optimized binding modes, respectively, and that their divergent evolution was shaped by the broader interaction network between the FluPol and the host transcriptional machinery.

Influenza polymerase, comprising subunits PA, PB1 and PB2, transcribes the negative-sense genomic viral RNA (vRNA) into mRNA or replicates it first into complementary RNA (cRNA) and then back to vRNA. Here we investigate the mechanism of de novo (unprimed) initiation of vRNA to cRNA replication. We present a high-resolution structure of A/little-yellow-shouldered-bat/H17N10 polymerase with the 3' end of the template in the RNA synthesis active site, both in the apo-state and after soaking with GTP and CTP. The priming GTP and incoming CTP are observed to base-pair to template nucleotides C2 and G3 at the -1 and +1 positions respectively, thus representing a replication initiation-like state. This configuration is stabilised by partial stacking of the tip of the priming loop on the GTP:C2 base-pair and the interaction of PB1/H649 and dibasic motif residues PA/R658 and K659 with the triphosphate of the priming GTP. The dibasic motif is universally conserved in orthomyxovirus PA subunits. Trans-complementation assays in cells using mutants of PA/K659 show that the dibasic motif is specifically essential for replication. These results shed light on the mechanism of replication initiation even though vRNA to cRNA replication is expected to be terminally initiated, with priming ATP and incoming GTP base-pairing to template nucleotides U1 and C2 at the -1 and +1 positions respectively, implying a different position of the template.

Tilapia Lake Virus (TiLV), a recently discovered pathogen of tilapia fish, belongs to the Amnoonviridae family from the Articulavirales order. Its ten genome segments have characteristic conserved ends and encode proteins with no known homologues, apart from the segment 1, which encodes an orthomyxo-like RNA-dependent-RNA polymerase core subunit. Here we show that segments 1-3 encode respectively the PB1, PB2 and PA-like subunits of an active heterotrimeric polymerase that maintains all domains found in the distantly related influenza polymerase, despite an unprecedented overall size reduction of 40%. Multiple high-resolution cryo-EM structures of TiLV polymerase in pre-initiation, initiation and active elongation states, show how it binds the vRNA and cRNA promoters and performs RNA synthesis, with both transcriptase and replicase configurations being characterised. However, the highly truncated endonuclease-like domain appears inactive and the putative cap-binding domain is autoinhibited, emphasising that many functional aspects of TiLV polymerase remain to be elucidated.

RIPK2 is an essential adaptor for NOD signalling and its kinase domain is a drug target for NOD-related diseases, such as inflammatory bowel disease. However, recent work indicates that the phosphorylation activity of RIPK2 is dispensable for signalling and that inhibitors of both RIPK2 activity and RIPK2 ubiquitination prevent the essential interaction between RIPK2 and the BIR2 domain of XIAP, the key RIPK2 ubiquitin E3 ligase. Moreover, XIAP BIR2 antagonists also block this interaction. To reveal the molecular mechanisms involved, we combined native mass spectrometry, NMR, and cryo-electron microscopy to determine the structure of the RIPK2 kinase BIR2 domain complex and validated the interface with in cellulo assays. The structure shows that BIR2 binds across the RIPK2 kinase antiparallel dimer and provides an explanation for both inhibitory mechanisms. It also highlights why phosphorylation of the kinase activation loop is dispensable for signalling while revealing the structural role of RIPK2-K209 residue in the RIPK2-XIAP BIR2 interaction. Our results clarify the features of the RIPK2 conformation essential for its role as a scaffold protein for ubiquitination.