Crystal Structures of Flavivirus NS5 Guanylyltransferase Reveal a GMP-Arginine Adduct

Tertiary Structures of Haseki Tick Virus Nonstructural Proteins Are Similar to Those of Orthoflaviviruses

Currently, a large number of novel tick-borne viruses potentially pathogenic to humans are discovered. Studying many of them by classical methods of virology is difficult due to the absence of live viral particles or a sufficient amount of their genetic material. In this case, the use of modern methods of bioinformatics and synthetic and structural biology can help. Haseki tick virus (HSTV) is a recently discovered tick-borne unclassified ssRNA(+) virus. HSTV-positive patients experienced fever and an elevated temperature. However, at the moment, there is no information on the tertiary structure and functions of its proteins. In this work, we used AlphaFold 3 and other bioinformatic tools for the annotation of HSTV nonstructural proteins, based on the principle that the tertiary structure of a protein is inextricably linked with its molecular function. We were the first to obtain models of tertiary structures and describe the putative functions of HSTV nonstructural proteins (NS3 helicase, NS3 protease, NS5 RNA-dependent RNA-polymerase, and NS5 methyltransferase), which play a key role in viral genome replication. Our results may help in further taxonomic identification of HSTV and the development of direct-acting antiviral drugs, POC tests, and vaccines.

Mapping glycoprotein structure reveals Flaviviridae evolutionary history

Viral glycoproteins drive membrane fusion in enveloped viruses and determine host range, tissue tropism and pathogenesis1. Despite their importance, there is a fragmentary understanding of glycoproteins within the Flaviviridae2, a large virus family that include pathogens such as hepatitis C, dengue and Zika viruses, and numerous other human, animal and emergent viruses. For many flaviviruses the glycoproteins have not yet been identified, for others, such as the hepaciviruses, the molecular mechanisms of membrane fusion remain uncharacterized3. Here we combine phylogenetic analyses with protein structure prediction to survey glycoproteins across the entire Flaviviridae. We find class II fusion systems, homologous to the Orthoflavivirus E glycoprotein in most species, including highly divergent jingmenviruses and large genome flaviviruses. However, the E1E2 glycoproteins of the hepaciviruses, pegiviruses and pestiviruses are structurally distinct, may represent a novel class of fusion mechanism, and are strictly associated with infection of vertebrate hosts. By mapping glycoprotein distribution onto the underlying phylogeny, we reveal a complex evolutionary history marked by the capture of bacterial genes and potentially inter-genus recombination. These insights, made possible through protein structure prediction, refine our understanding of viral fusion mechanisms and reveal the events that have shaped the diverse virology and ecology of the Flaviviridae.

The Flavivirus Non-Structural Protein 5 (NS5): Structure, Functions, and Targeting for Development of Vaccines and Therapeutics

Flaviviruses, including dengue (DENV), Zika (ZIKV), West Nile (WNV), Japanese encephalitis (JEV), yellow fever (YFV), and tick-borne encephalitis (TBEV) viruses, pose a significant global emerging threat. With their potential to cause widespread outbreaks and severe health complications, the development of effective vaccines and antiviral therapeutics is imperative. The flaviviral non-structural protein 5 (NS5) is a highly conserved and multifunctional protein that is crucial for viral replication, and the NS5 protein of many flaviviruses has been shown to be a potent inhibitor of interferon (IFN) signalling. In this review, we discuss the functions of NS5, diverse NS5-mediated strategies adopted by flaviviruses to evade the host antiviral response, and how NS5 can be a target for the development of vaccines and antiviral therapeutics.

PfAgo-Based Zika Virus Detection

As a mosquito-borne flavivirus, Zika virus (ZIKV) has been identified as a global health threat. The virus has been linked to severe congenital disabilities, including microcephaly and other congenital malformations, resulting in fatal intrauterine death. Therefore, developing sensitive and specific methods for the early detection and accurate diagnosis of the ZIKV is essential for controlling its spread and mitigating its impact on public health. Herein, we set up a novel nucleic acid detection system based on Pyrococcus furiosus Argonaute (PfAgo)-mediated nucleic acid detection, targeting the non-structural protein 5 (NS5) region of the ZIKV genome (abbreviated ZIKV-PAND). Without preamplification with the polymerase chain reaction (PCR), the minimum detection concentration (MDC) of ZIKV-PAND was about 10 nM. When introducing an amplification step, the MDC can be dramatically decreased to the aM level (8.3 aM), which is comparable to qRT-PCR assay (1.6 aM). In addition, the diagnostic findings from the analysis of simulated clinical samples or Zika virus samples using ZIKV-PAND show a complete agreement of 100% with qRT-PCR assays. This correlation can aid in the implementation of molecular testing for clinical diagnoses and the investigation of ZIKV infection on an epidemiological scale.

Targeting cap1 RNA methyltransferases as an antiviral strategy

Methylation is one of the critical modifications that regulates numerous biological processes. Guanine capping and methylation at the 7th position (m7G) have been shown to mature mRNA for increased RNA stability and translational efficiency. The m7G capped cap0 RNA remains immature and requires additional methylation at the first nucleotide (N1-2'-O-Me), designated as cap1, to achieve full maturation. This cap1 RNA with N1-2'-O-Me prevents its recognition by innate immune sensors as non-self. Viruses have also evolved various strategies to produce self-like capped RNAs with the N1-2'-O-Me that potentially evades the antiviral response and establishes an efficient replication. In this review, we focus on the importance of the presence of N1-2'-O-Me in viral RNAs and discuss the potential for drug development by targeting host and viral N1-2'-O-methyltransferases.

Structural and functional basis of low-affinity SAM/SAH-binding in the conserved MTase of the multi-segmented Alongshan virus distantly related to canonical unsegmented flaviviruses

Alongshan virus (ALSV), a newly discovered member of unclassified Flaviviridae family, is able to infect humans. ALSV has a multi-segmented genome organization and is evolutionarily distant from canonical mono-segmented flaviviruses. The virus-encoded methyltransferase (MTase) plays an important role in viral replication. Here we show that ALSV MTase readily binds S-adenosyl-L-methionine (SAM) and S-adenosyl-L-homocysteine (SAH) but exhibits significantly lower affinities than canonical flaviviral MTases. Structures of ALSV MTase in the free and SAM/SAH-bound forms reveal that the viral enzyme possesses a unique loop-element lining side-wall of the SAM/SAH-binding pocket. While the equivalent loop in flaviviral MTases half-covers SAM/SAH, contributing multiple hydrogen-bond interactions; the pocket-lining loop of ALSV MTase is of short-length and high-flexibility, devoid of any physical contacts with SAM/SAH. Subsequent mutagenesis data further corroborate such structural difference affecting SAM/SAH-binding. Finally, we also report the structure of ALSV MTase bound with sinefungin, an SAM-analogue MTase inhibitor. These data have delineated the basis for the low-affinity interaction between ALSV MTase and SAM/SAH and should inform on antiviral drug design.

Dengue virus serotypic replacement of NS3 protease or helicase domain causes chimeric viral attenuation but can be recovered by a compensated mutation at helicase domain or NS2B, respectively

Mosquito-borne dengue viruses (DENVs) have evolved to four serotypes with 69%-78% amino acid identities, resulting in incomplete immunity, where one serotype's infection does not cross-protect against secondary infections by other serotypes. Despite the amino acid differences, structural and nonstructural (NS) proteins among serotypes play similar functions. NS3 is an enzyme complex: NS3 has N-terminal protease (PRO) and C-terminal helicase (HEL) activities in addition to 5' RNA triphosphatase (5'RTP), which is involved in the RNA capping process. In this study, the effects of NS3 replacements among serotypes were tested. The replacement of NS3 full-length (FULL), PRO or HEL region suppressed viral replication in BHK-21 mammalian cells, while the single compensatory mutation improved the viral replications; P364S mutation in HEL revived PRO (DENV3)-replaced DENV1, while S68T alteration in NS2B recovered HEL (DENV1)-replaced DENV2. The results suggest that the interactions between PRO and HEL as well as HEL and NS2B are required for replication competence. Lower-frequency mutations also appeared at various locations in viral proteins, although after infecting C6/36 mosquito cells, the mutations' frequencies changed, and/or new mutations appeared. In contrast, the inter-domain region (INT, 12 amino acids)-replaced chimera quickly replicated without mutation in BHK-21 cells, although extended cell culture accumulated various mutations. These results suggest that NS3 variously interacts with DENV proteins, in which the chimeric NS3 domain replacements induced amino acid mutations, irrespective of replication efficiency. However, the viral sequences are further adjusted for replication efficiency, to fit in both mammalian cells and mosquito cells. IMPORTANCE Enzyme activities for replicating DENV 5' cap positive (+) sense RNA have been shown to reside in NS3 and NS5. However, it remains unknown how these enzymes coordinately synthesize negative (-) sense RNA, from which abundant 5' cap (+) sense RNA is produced. We previously revealed that NS5 dimerization and NS5 methyltransferase(MT)-NS3HEL interaction are important for DENV replication. Here, we found that replication incompetence due to NS3PRO or HEL replacement was compensated by a mutation at HEL or NS2B, respectively, suggesting that the interactions among NS2B, NS3PRO, and HEL are critical for DENV replication.