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The nuclear pore complex (NPC) is composed of multiple nucleoporins (NUPs) and enables the exchange of RNA and proteins between the nucleus and cytoplasm. NUP98 is one of the major components of the NPC, being involved in the RNA export pathway by interacting with several transport factors. Previous studies have suggested both proviral and antiviral functions of NUP98 in viral infection, yet little is known about its function in orthoflavivirus infection. In this study we show that NUP98 is a proviral cellular protein that is recruited to the cytoplasm during orthoflavivirus infection. We observe that NUP98 is found specifically in the vicinity of the replication vesicles during infections with tick-borne encephalitis virus, Japanese encephalitis virus, and yellow fever virus. Furthermore, using surface plasmon resonance, cross-link immunoprecipitation, and cross-link immunoprecipitation-sequencing we observe that the C-Terminal domain of NUP98 directly interacts with a conserved site of the viral RNA (vRNA) in the E coding region promoting viral replication. We identified a peptide that binds to NUP98 that is antivirally active against several orthoflaviviruses by outcompeting the binding between NUP98 and vRNA, making NUP98 an attractive target for antiviral development.

Orthoflaviviruses are RNA viruses that cause serious diseases in humans, with currently no antivirals available. Targeting host factors is emerging as an attractive antiviral approach. However, as a first step, there is a need to understand which host proteins are hijacked and for what purpose. Here, using a combination of fluorescence microscopy, knock-down, crosslinking immunoprecipitation sequencing, mass spectrometry, and in vitro and biophysical assays, we identify nucleoporin-153 (NUP153) as a proviral factor during orthoflavivirus infection. We show that NUP153 is recruited to the virus amplification site on the endoplasmic reticulum to impact the structural to nonstructural viral protein ratios. We find that NUP153 interacts with both the viral proteins NS3 and NS5, and a highly conserved G-rich motif on the viral RNA. These interactions specifically promote the production of viral structural proteins, leading to an efficient virion assembly, virus release and spread to new cells. We propose that NUP153 acts as a key regulator in viral protein ratios, a mechanism that appears conserved among orthoflaviviruses.

Deep mutational scanning (DMS) has emerged as a powerful approach for evaluating the effects of mutations on binding or function. Here, we developed a DMS by phage display protocol to define the specificity determinants of short linear motifs (SLiMs) binding to peptide-binding domains. We first designed a benchmarking DMS library to evaluate the performance of the approach on well-known ligands for 11 different peptide-binding domains, including the talin-1 PTB domain, the G3BP1 NTF2 domain, and the MDM2 SWIB domain. Comparison with a set of reference motifs from the eukaryotic linear motif (ELM) database confirmed that the DMS by phage display analysis correctly identifies known motif binding determinants and provides novel insights into specificity determinants, including defining a non-canonical talin-1 PTB binding motif with a putative extended conformation. A second DMS library was designed, aiming to provide information on the binding determinants for 19 SLiM-based interactions between human and SARS-CoV-2 proteins. The analysis confirmed the affinity determining residues of viral peptides binding to host proteins and refined the consensus motifs in human peptides binding to five domains from SARS-CoV-2 proteins, including the non-structural protein (NSP) 9. The DMS analysis further pinpointed mutations that increased the affinity of ligands for NSP3 and NSP9. An affinity-improved cell-permeable NSP9-binding peptide was found to exert stronger antiviral effects than the wild-type peptide. Our study demonstrates that DMS by phage display can efficiently be multiplexed and applied to refine binding determinants and shows how the results can guide peptide-engineering efforts.

Flavivirus infection involves extensive remodeling of the endoplasmic reticulum (ER), which is key to both the replication of the viral RNA genome as well as the assembly and release of new virions. However, little is known about how viral proteins and host factors cooperatively facilitate such a vast transformation of the ER, and how this influences the different steps of the viral life cycle. In this study, we screened for host proteins that were enriched in close proximity to the tick-borne encephalitis virus (TBEV) protein NS4B and found that the top candidates were coupled to trafficking between ER exit sites (ERES) and the Golgi. We characterized the role of ACBD3, one of the identified proteins, and showed that it promotes TBEV infection. Depletion of ACBD3 inhibited virus replication and resulted in abnormal transformation of the ER, leading to reduced virion release. ACBD3's proviral mechanism did not involve the recruitment of PI4PK as previously described for enteroviruses. Instead, productive TBEV infection required the full-length ACBD3, which localizes to ER-Golgi contact sites together with NS4B. We propose that NS4B and ACBD3 promote replication by coordinating the transformation of the ER, which is required for RNA replication and particle release. The transformation involves direct coupling to the Golgi which facilitates efficient virion transport.

IMPORTANCE: Flaviviruses like tick-borne encephalitis have significant effects on human health. During flavivirus infection, the viral particles enter the host cells and transform the endoplasmic reticulum (ER), which is a membranous organelle and the main site of cellular protein synthesis. Although this is critical for successful infection, the details of the process are unknown. Here, we found that the viral protein NS4B and the host protein ACBD facilitate this transformation by ensuring that the ER is coupled to the Golgi apparatus, the organelle responsible for transporting material out of the cell. TBEV uses ACBD3 to guarantee that the connection sites between the transformed ER and the Golgi remain functional so that RNA is replicated and the produced viral particles are exported from the cell and can infect further cells. Our work sheds light both on the basic biology of flavivirus infection, and virus-induced remodeling of membranous organelles.

Objectives: The recent detection and expansion of West Nile virus (WNV) and Usutu virus (USUV) in the Netherlands, Germany, and Austria point to the likelihood of the viruses spreading to Northern Europe. Migratory birds and ornithophilic mosquitoes may spread these viruses to new areas. We sampled birds during the spring and autumn bird migration of 2021 in Southern Sweden to investigate the risk of the introduction of mosquito-borne zoonotic avian viruses like WNV and USUV.

Methods: We collected blood samples from 1775 birds comprising 59 species and determined the seroprevalence of WNV using a competitive enzyme-linked immunosorbent assay (ELISA). WNV and USUV belong to the Japanese encephalitis serocomplex, and antibodies against both viruses are detected in the WNV ELISA. Focus-forming assays or fluorescence-based neutralization assays were performed to verify ELISA results and to differentiate between antibodies against WNV and USUV.

Results: We found nine (0.51%) samples to be WNV-antibody-positive. Cross-neutralization experiments with WNV and USUV confirmed that seven (0.41%) had WNV-neutralizing antibodies and two (0.11%) had USUV-neutralizing antibodies. Interestingly, the two samples had neutralizing antibodies of both viruses. All samples but one with anti-flavivirus antibodies came from long-distance migrants wintering in sub-Saharan Africa. Antibodies were detected in samples taken during spring and autumn and only in adult birds.

Conclusion: The findings show that migratory birds in Sweden have been exposed to WNV and USUV.

Viruses interact with numerous host factors to facilitate viral replication and to dampen antiviral defense mechanisms. We currently have a limited mechanistic understanding of how SARS-CoV-2 binds host factors and the functional role of these interactions. Here, we uncover a novel interaction between the viral NSP3 protein and the fragile X mental retardation proteins (FMRPs: FMR1, FXR1-2). SARS-CoV-2 NSP3 mutant viruses preventing FMRP binding have attenuated replication in vitro and reduced levels of viral antigen in lungs during the early stages of infection. We show that a unique peptide motif in NSP3 binds directly to the two central KH domains of FMRPs and that this interaction is disrupted by the I304N mutation found in a patient with fragile X syndrome. NSP3 binding to FMRPs disrupts their interaction with the stress granule component UBAP2L through direct competition with a peptide motif in UBAP2L to prevent FMRP incorporation into stress granules. Collectively, our results provide novel insight into how SARS-CoV-2 hijacks host cell proteins and provides molecular insight into the possible underlying molecular defects in fragile X syndrome.

Tick-borne encephalitis virus (TBEV) is a neurotropic member of the genus Orthoflavivirus (former Flavivirus) and is of significant health concern in Europe and Asia. TBEV pathogenesis may occur directly via virus-induced damage to neurons or through immunopathology due to excessive inflammation. While primary cells isolated from the host can be used to study the immune response to TBEV, it is still unclear how well these reflect the immune response elicited in vivo. Here, we compared the transcriptional response to TBEV and the less pathogenic tick-borne flavivirus, Langat virus (LGTV), in primary monocultures of neurons, astrocytes and microglia in vitro, with the transcriptional response in vivo captured by single-nuclei RNA sequencing (snRNA-seq) of a whole mouse cortex. We detected similar transcriptional changes induced by both LGTV and TBEV infection in vitro, with the lower response to LGTV likely resulting from slower viral kinetics. Gene set enrichment analysis showed a stronger transcriptional response in vivo than in vitro for astrocytes and microglia, with a limited overlap mainly dominated by interferon signaling. Together, this adds to our understanding of neurotropic flavivirus pathogenesis and the strengths and limitations of available model systems.

The virus life cycle depends on host-virus protein-protein interactions, which often involve a disordered protein region binding to a folded protein domain. Here, we used proteomic peptide phage display (ProP-PD) to identify peptides from the intrinsically disordered regions of the human proteome that bind to folded protein domains encoded by the SARS-CoV-2 genome. Eleven folded domains of SARS-CoV-2 proteins were found to bind 281 peptides from human proteins, and affinities of 31 interactions involving eight SARS-CoV-2 protein domains were determined (K D ∼ 7-300 μM). Key specificity residues of the peptides were established for six of the interactions. Two of the peptides, binding Nsp9 and Nsp16, respectively, inhibited viral replication. Our findings demonstrate how high-throughput peptide binding screens simultaneously identify potential host-virus interactions and peptides with antiviral properties. Furthermore, the high number of low-affinity interactions suggest that overexpression of viral proteins during infection may perturb multiple cellular pathways.

Viruses mimic host short linear motifs (SLiMs) to hijack and deregulate cellular functions. Studies of motif-mediated interactions therefore provide insight into virus-host dependencies, and reveal targets for therapeutic intervention. Here, we describe the pan-viral discovery of 1712 SLiM-based virus-host interactions using a phage peptidome tiling the intrinsically disordered protein regions of 229 RNA viruses. We find mimicry of host SLiMs to be a ubiquitous viral strategy, reveal novel host proteins hijacked by viruses, and identify cellular pathways frequently deregulated by viral motif mimicry. Using structural and biophysical analyses, we show that viral mimicry-based interactions have similar binding strength and bound conformations as endogenous interactions. Finally, we establish polyadenylate-binding protein 1 as a potential target for broad-spectrum antiviral agent development. Our platform enables rapid discovery of mechanisms of viral interference and the identification of potential therapeutic targets which can aid in combating future epidemics and pandemics.

Viral tropism within the brain and the role(s) of vertebrate immune response to neurotropic flaviviruses infection is largely understudied. We combine multimodal imaging (cm-nm scale) with single nuclei RNA-sequencing to study Langat virus in wildtype and interferon alpha/beta receptor knockout (Ifnar^-/-) mice to visualize viral pathogenesis and define molecular mechanisms. Whole brain viral infection is imaged by Optical Projection Tomography coregistered to ex vivo MRI. Infection is limited to grey matter of sensory systems in wildtype mice, but extends into white matter, meninges and choroid plexus in Ifnar^-/- mice. Cells in wildtype display strong type I and II IFN responses, likely due to Ifnb expressing astrocytes, infiltration of macrophages and Ifng-expressing CD8+ NK cells, whereas in Ifnar^-/-, the absence of this response contributes to a shift in cellular tropism towards non-activated resident microglia. Multimodal imaging-transcriptomics exemplifies a powerful way to characterize mechanisms of viral pathogenesis and tropism.