Structures of dengue virus RNA replicase complexes

Characterization of SLA RNA promoter from dengue virus and its interaction with the viral non-structural NS5 protein

The Dengue virus (DENV) is the most significant arthropod-borne viral pathogen in humans with 400 million infections annually. DENV comprises four distinct serotypes (DENV-1 to -4) which complicates vaccine development. Any of the four serotypes can cause clinical illness but with distinctive infection dynamics. Variations in sequences identified within the four genomes induce structural differences in crucial RNA motifs that were suggested to be correlated to the degree of pathogenicity among DENV-1 to -4. In particular, the RNA Stem-loop A (SLA) at the 5'-end of the genome, acts as a key regulator of the viral replication cycle by interacting with the viral NS5 polymerase to initiate the minus-strand viral RNA synthesis and later to methylate and cap the synthesized RNA. The molecular details of this interaction remain not fully described. Here, we report the solution secondary structures of SLA from DENV-1 to -4. Our results highlight that the four SLA exhibit structural and dynamic differences. Secondly, to determine whether SLA RNA contains serotype-specific determinants for the recognition by the viral NS5 protein, we investigated interactions between SLA from DENV -1 to -4 and DENV2 NS5 using combined biophysical approaches. Our results show that NS5 from DENV2 is able to bind SLA from other serotypes, but that other viral or host factors may be necessary to stabilize the complex and promote the catalytically active state of the NS5. By contrast, we show that a serotype-specific binding is driven by specific interactions involving conformational changes within the SLA RNA.

Deciphering the interaction surface between the West Nile virus NS3 and NS5 proteins

West Nile virus (WNV) is the most prevalent mosquito-borne virus and the leading cause of viral encephalitis in the continental United States. It belongs to the family Flaviviridae which includes other important human pathogens such as dengue virus (DENV), Japanese encephalitis virus (JEV) and Zika viruses (ZIKV). Despite several decades of research, no specific antiviral drugs are available to treat flavivirus infections. The present study characterizes the interaction between the WNV NS3 and NS5 proteins for the purpose of identifying hotspots in the protein-protein interaction which could be targeted for the development of antiviral therapeutics. We previously developed an interaction model in silico based on data available in the literature. Here, potential interacting residues on NS3 and NS5 were mutated in a WNV replicon, and seven mutations in the NS3 protein were found to drastically reduce viral replication. In addition to being well conserved among mosquito-borne flaviviruses, these residues are located on the protein's surface in two clusters which might be interesting new targets for future drug development.

Structure of an RNA G-quadruplex from the West Nile virus genome

Potential G-quadruplex sites have been identified in the genomes of DNA and RNA viruses and proposed as regulatory elements. The genus Orthoflavivirus contains arthropod-transmitted, positive-sense, single-stranded RNA viruses that cause significant human disease globally. Computational studies have identified multiple potential G-quadruplex sites that are conserved across members of this genus. Subsequent biophysical studies established that some G-quadruplexes predicted in Zika and tickborne encephalitis virus genomes can form and known quadruplex binders reduced viral yields from cells infected with these viruses. The susceptibility of RNA to degradation and the variability of loop regions have made structure determination challenging. Despite these difficulties, we report a high-resolution structure of the NS5-B quadruplex from the West Nile virus genome. Analysis reveals two stacked tetrads that are further stabilized by a stacked triad and transient noncanonical base pairing. This structure expands the landscape of solved RNA quadruplex structures and demonstrates the diversity and complexity of biological quadruplexes. We anticipate that the availability of this structure will assist in solving further viral RNA quadruplexes and provides a model for a conserved antiviral target in Orthoflavivirus genomes.

Structural and functional insights in flavivirus NS5 proteins gained by the structure of Ntaya virus polymerase and methyltransferase

Flaviviruses are single-stranded positive-sense RNA (+RNA) viruses that are responsible for several (re)emerging diseases such as yellow, dengue, or West Nile fevers. The Zika epidemic highlighted their dangerousness when a relatively benign virus known since the 1950s turned into a deadly pathogen. The central protein for their replication is NS5 (non-structural protein 5), which is composed of the N-terminal methyltransferase (MTase) domain and the C-terminal RNA-dependent RNA-polymerase (RdRp) domain. It is responsible for both RNA replication and installation of the 5' RNA cap. We structurally and biochemically analyzed the Ntaya virus MTase and RdRp domains and we compared their properties to other flaviviral NS5s. The enzymatic centers are well conserved across Flaviviridae, suggesting that the development of drugs targeting all flaviviruses is feasible. However, the enzymatic activities of the isolated proteins were significantly different for the MTase domains.

High-throughput crystallographic fragment screening of Zika virus NS3 Helicase

The Zika virus (ZIKV), discovered in Africa in 1947, swiftly spread across continents, causing significant concern due to its recent association with microcephaly in newborns and Guillain-Barré syndrome in adults. Despite a decrease in prevalence, the potential for a resurgence remains, necessitating urgent therapeutic interventions. Like other flaviviruses, ZIKV presents promising drug targets within its replication machinery, notably the NS3 helicase (NS3Hel) protein, which plays critical roles in viral replication. However, a lack of structural information impedes the development of specific inhibitors targeting NS3Hel. Here we applied high-throughput crystallographic fragment screening on ZIKV NS3Hel, which yielded structures that reveal 3D binding poses of 46 fragments at multiple sites of the protein, including 11 unique fragments in the RNA-cleft site. These fragment structures provide templates for direct design of hit compounds and should thus assist the development of novel direct-acting antivirals against ZIKV and related flaviviruses, thus opening a promising avenue for combating future outbreaks.

A jingmenvirus RNA-dependent RNA polymerase structurally resembles the flavivirus counterpart but with different features at the initiation phase

Jingmenviruses are a category of emerging segmented viruses that have garnered global attention in recent years, and are close relatives of the flaviviruses in the Flaviviridae family. One of their genome segments encodes NSP1 homologous to flavivirus NS5. NSP1 comprises both the methyltransferase (MTase) and RNA-dependent RNA polymerase (RdRP) modules playing essential roles in viral genome replication and capping. Here we solved a 1.8-Å resolution crystal structure of the NSP1 RdRP module from Jingmen tick virus (JMTV), the type species of jingmenviruses. The structure highly resembles flavivirus NS5 RdRP despite a sequence identity less than 30%. NSP1 RdRP enzymatic properties were dissected in a comparative setting with several representative Flaviviridae RdRPs included. Our data indicate that JMTV NSP1 produces characteristic 3-mer abortive products similar to the hepatitis C virus RdRP, and exhibits the highest preference of terminal initiation and shorter-primer usage. Unlike flavivirus NS5, JMTV RdRP may require the MTase for optimal transition from initiation to elongation, as an MTase-less NSP1 construct produced more 4-5-mer intermediate products than the full-length protein. Taken together, this work consolidates the evolutionary relationship between the jingmenvirus group and the Flaviviridae family, providing a basis to the further understanding of their viral replication/transcription process.

The Dynamic Landscape of Capsid Proteins and Viral RNA Interactions in Flavivirus Genome Packaging and Virus Assembly

The Flavivirus genus of the Flaviviridae family of enveloped single-stranded RNA viruses encompasses more than 70 members, many of which cause significant disease in humans and livestock. Packaging and assembly of the flavivirus RNA genome is essential for the formation of virions, which requires intricate coordination of genomic RNA, viral structural, and nonstructural proteins in association with virus-induced, modified endoplasmic reticulum (ER) membrane structures. The capsid (C) protein, a small but versatile RNA-binding protein, and the positive single-stranded RNA genome are at the heart of the elusive flavivirus assembly process. The nucleocapsid core, consisting of the genomic RNA encapsidated by C proteins, buds through the ER membrane, which contains viral glycoproteins prM and E organized as trimeric spikes into the lumen, forming an immature virus. During the maturation process, which involves the low pH-mediated structural rearrangement of prM and E and furin cleavage of prM in the secretory pathway, the spiky immature virus with a partially ordered nucleocapsid core becomes a smooth, mature virus with no discernible nucleocapsid. This review focuses on the mechanisms of genome packaging and assembly by examining the structural and functional aspects of C protein and viral RNA. We review the current lexicon of critical C protein features and evaluate interactions between C and genomic RNA in the context of assembly and throughout the life cycle.

A conformational selection mechanism of flavivirus NS5 for species-specific STAT2 inhibition

Flaviviruses, including Zika virus (ZIKV) and Dengue virus (DENV), rely on their non-structural protein 5 (NS5) for both replication of viral genome and suppression of host IFN signaling. DENV and ZIKV NS5s were shown to facilitate proteosome-mediated protein degradation of human STAT2 (hSTAT2). However, how flavivirus NS5s have evolved for species-specific IFN-suppression remains unclear. Here we report structure-function characterization of the DENV serotype 2 (DENV2) NS5-hSTAT2 complex. The MTase and RdRP domains of DENV2 NS5 form an extended conformation to interact with the coiled-coil and N-terminal domains of hSTAT2, thereby promoting hSTAT2 degradation in cells. Disruption of the extended conformation of DENV2/ZIKV NS5, but not the alternative compact state, impaired their hSTAT2 binding. Our comparative structural analysis of flavivirus NS5s further reveals a conserved protein-interaction platform with subtle amino-acid variations likely underpinning diverse IFN-suppression mechanisms. Together, this study uncovers a conformational selection mechanism underlying species-specific hSTAT2 inhibition by flavivirus NS5.

The myriad roles of RNA structure in the flavivirus life cycle

As positive-sense RNA viruses, the genomes of flaviviruses serve as the template for all stages of the viral life cycle, including translation, replication, and infectious particle production. Yet, they encode just 10 proteins, suggesting that the structure and dynamics of the viral RNA itself helps shepherd the viral genome through these stages. Herein, we highlight advances in our understanding of flavivirus RNA structural elements through the lens of their impact on the viral life cycle. We highlight how RNA structures impact translation, the switch from translation to replication, negative- and positive-strand RNA synthesis, and virion assembly. Consequently, we describe three major themes regarding the roles of RNA structure in flavivirus infections: 1) providing a layer of specificity; 2) increasing the functional capacity; and 3) providing a mechanism to support genome compaction. While the interactions described herein are specific to flaviviruses, these themes appear to extend more broadly across RNA viruses.