Structural basis of mismatch recognition by a SARS-CoV-2 proofreading enzyme

Phosphorylation Changes SARS-CoV-2 Nucleocapsid Protein's Structural Dynamics and Its Interaction With RNA

The SARS-CoV-2 nucleocapsid protein, or N-protein, is a structural protein that plays an important role in the SARS-CoV-2 life cycle. The N-protein takes part in the regulation of viral RNA replication and drives highly specific packaging of full-length genomic RNA prior to virion formation. One regulatory mechanism that is proposed to drive the switch between these two operating modes is the phosphorylation state of the N-protein. Here, we assess the dynamic behavior of non-phosphorylated and phosphorylated versions of the N-protein homodimer through atomistic molecular dynamics simulations. We show that the introduction of phosphorylation yields a more dynamic protein structure and decreases the binding affinity between the N-protein and RNA. Furthermore, we find that secondary structure is essential for the preferential binding of particular RNA elements from the 5' UTR of the viral genome to the N-terminal domain of the N-protein. Altogether, we provide detailed molecular insights into N-protein dynamics, N-protein:RNA interactions, and phosphorylation. Our results corroborate the hypothesis that phosphorylation of the N-protein serves as a regulatory mechanism that determines N-protein function.

Mutagenesis-Guided Target Identification Reveals the Protein-Binding Domain of Nsp14 in Coronaviruses as the Target of a Labdane-Oxindole Compound

The non-structural protein (nsp) 14 of coronaviruses plays an important role in maintaining the genomic stability of the virus during viral replication. This had garnered significant interest towards the identification and development of inhibitors against nsp14, specifically its exoribonuclease (ExoN) domain. However, no inhibitors have been successfully developed to date. The bioactivity of the nsp14-ExoN is governed through a complex formation with its co-factor nsp10. This provides opportunities to target the protein assembly as an antiviral modality. In this study, a labdane-oxindole compound (OX18) was identified as a promising new antiviral agent against coronaviruses. Through a combination of FRET- and BRET-based approaches, OX18 was found to target the nsp10-binding domain of nsp14. A key escape mutation to OX18 in nsp14 was also identified in our study, albeit compromising its exoribonuclease activity. To our knowledge, OX18 is the first small molecule to target the nsp14/10 protein assembly. As such, our work paves the way for the development of future inhibitors of the nsp14-ExoN with increased potency and complexity.

Structural insights into the assembly and regulation of 2'-O RNA methylation by SARS-CoV-2 nsp16/nsp10

2'-O-ribose methylation of the first transcribed base (adenine or A1 in SARS-CoV-2) of viral RNA mimics host RNAs and subverts the innate immune response. How nsp16, with partner nsp10, assembles on the 5'-end of SARS-CoV-2 mRNA to methylate A1 is not fully understood. We present a ∼2.4 Å crystal structure of the heterotetrameric complex formed by the cooperative assembly of two nsp16/nsp10 heterodimers with one 10-mer Cap-1 RNA (product) bound to each. An aromatic zipper-like motif in nsp16 and the N-terminal regions of nsp10 and nsp16 orchestrate oligomeric assembly for efficient methylation. The front catalytic pocket of nsp16 stabilizes the upstream portion of the RNA while downstream RNA remains unresolved, likely due to flexibility. An inverted nsp16 dimer extends the positively charged surface for longer RNA to influence catalysis. Additionally, a non-specific nucleotide-binding pocket on the backside of nsp16 plays a critical role in catalysis, contributing to enzymatic activity.

The coronavirus nsp14 exoribonuclease interface with the cofactor nsp10 is essential for efficient virus replication and enzymatic activity

Coronaviruses (CoVs) encode non-structural proteins (nsp's) 1-16, which assemble to form replication-transcription complexes that function in viral RNA synthesis. All CoVs encode a proofreading 3'-5' exoribonuclease in non-structural protein 14 (nsp14-ExoN) that mediates proofreading and high-fidelity replication and is critical for other roles in replication and pathogenesis. The in vitro enzymatic activity of nsp14-ExoN is enhanced in the presence of the cofactor nsp10. We introduced alanine substitutions in nsp14 of murine hepatitis virus (MHV) at the nsp14-nsp10 interface and recovered mutant viruses with a range of impairments in replication and in vitro biochemical exonuclease activity. Two of these substitutions, nsp14 K7A and D8A, had impairments intermediate between wild type-MHV nsp14 and the known ExoN(-) D89A/E91A nsp14 catalytic inactivation mutant. All introduced nsp14-nsp10 interface alanine substitutions impaired in vitro exonuclease activity. Passage of the K7A and D8A mutant viruses selected second-site non-synonymous mutations in nsp14 associated with improved mutant virus replication and exonuclease activity. These results confirm the essential role of the nsp14-nsp10 interaction for efficient enzymatic activity and virus replication, identify proximal and long-distance determinants of nsp14-nsp10 interaction, and support targeting the nsp14-nsp10 interface for viral inhibition and attenuation.IMPORTANCECoronavirus replication requires assembly of a replication transcription complex composed of nsp's, including polymerase, helicase, exonuclease, capping enzymes, and non-enzymatic cofactors. The coronavirus nsp14 exoribonuclease mediates several functions in the viral life cycle including genomic and subgenomic RNA synthesis, RNA recombination, RNA proofreading and high-fidelity replication, and native resistance to many nucleoside analogs. The nsp-14 exonuclease activity in vitro requires the non-enzymatic cofactor nsp10, but the determinants and importance of the nsp14-nsp10 interactions during viral replication have not been defined. Here we show that for the coronavirus murine hepatitis virus, nsp14 residues at the nsp14-nsp10 interface are essential for efficient viral replication and in vitro exonuclease activity. These results shed new light on the requirements for protein interactions within the coronavirus replication transcription complex, and they may reveal novel non-active-site targets for virus inhibition and attenuation.

Structural and Phylogenetic Analysis on the Proofreading Activity of SARS-CoV-2

Maintenance of genomic integrity is a fundamental characteristic of viruses, however, RNA-dependent RNA Polymerase (RdRp) lacks exonuclease activity for proofreading. To facilitate genomic proofreading in viruses, an independent exonuclease domain assists RdRp to maintain fidelity during replication. In contrast to high fidelity in DNA viruses, RNA viruses have to evolve into new variants through comparatively delicate mutagenesis activity for genetic diversity. Coronavirideae, a family of single positive-stranded RNA (+ ssRNA) viruses, meticulously sustain a balance between genetic diversity and large-size RNA genome. In coronaviruses, the proofreading activity is accomplished by an exonuclease (ExoN) domain located at the N-terminal of non-structural protein 14 (nsp14). ExoN is responsible for the new variants and antiviral resistance towards nucleotide analogs. Here, we provide an evolutionary characterization of ExoN by using a well-defined phylogenetic pipeline and structural analysis based on host and habitat. We carried out a phylogenetic analysis on ExoN, methyltransferase domain, nsp14, and whole genomes of ExoN-containing viruses. Furthermore, a three-dimensional structural comparison of the ExoN domain from various sources is also carried out to understand structural preservation. Our study has unveiled the evolutionary trajectories and structural conservation of the ExoN domain within the Coronaviridae family, highlighting its distinct evolutionary path independent of other domains. Structural analyses revealed minimal variance in RMSD values, underscoring the conserved nature of ExoN despite diverse ecological settings.

RNAproDB: A Webserver and Interactive Database for Analyzing Protein-RNA Interactions

We present RNAproDB (https://rnaprodb.usc.edu/), a new webserver, analysis pipeline, database, and highly interactive visualization tool, designed for protein-RNA complexes, and applicable to all forms of nucleic acid containing structures. RNAproDB computes several mapping schemes to place nucleic acid components and present protein-RNA interactions appropriately. Various structural annotations are computed including non-canonical base-pairing geometries, hydrogen bonds, and protein-RNA and RNA-RNA water-mediated interactions. This information is presented through integrated visualization and data tools. Subgraph selection facilitates studying smaller components of the interface. Molecular surface electrostatic potential can be visualized. RNAproDB enables analyzing and exploring experimentally determined, predicted, and designed protein-nucleic acid complexes. We present a quantitative analysis of pre-analyzed protein-RNA structures in RNAproDB revealing statistical patterns of molecular binding and recognition.

A High-Throughput Screening Pipeline to Identify Methyltransferase and Exonuclease Inhibitors of SARS-CoV-2 NSP14

SARS-CoV-2 infections led to a worldwide pandemic in 2020. As of 2024, therapeutics against SARS-CoV-2 have continued to be desirable. NSP14 is a dual-function methyltransferase (MTase) and exonuclease (ExoN) with key roles in SARS-CoV-2 genome propagation and host immune system evasion. In this work, we developed high-throughput screening (HTS) assays for NSP14 MTase and ExoN activities. We screened both activities against a collection of 40,664 compounds. A total of 1677 initial hit compounds were identified, cherrypicked, counterscreened for assay interference, and screened for off-target selectivity. We identified 396 and 174 high-quality hits against the MTase and ExoN activities, respectively. Along with inhibitors for individual activities, we identified dual-activity inhibitors, including a novel inhibitor that is not competitive with any substrate and interacts with a putative allosteric binding site. This study represents the largest published screen of SARS-CoV-2 NSP14 MTase and ExoN activities to date and culminates in a pipeline for the NSP14 drug discovery.

Two Years of SARS-CoV-2 Omicron Genomic Evolution in Brazil (2022-2024): Subvariant Tracking and Assessment of Regional Sequencing Efforts

SARS-CoV-2, the virus responsible for COVID-19, has undergone significant genetic evolution since its emergence in 2019. This study examines the genomic diversity of SARS-CoV-2 in Brazil after the worst phase of the pandemic, the wider adoption of routine vaccination, and the abolishment of other non-pharmacological preventive measures from July 2022 to July 2024 using 55,951 sequences retrieved from the GISAID database. The analysis focuses on the correlation between confirmed COVID-19 cases, sequencing efforts across Brazilian states, and the distribution and evolution of viral lineages. Our findings reveal significant regional disparities in genomic surveillance, with São Paulo and Rio de Janeiro recovering the largest number of genomes, while Tocantins and Amazonas showed higher sequencing rates relative to their reported case numbers, indicating proactive surveillance efforts. We identified 626 distinct SARS-CoV-2 lineages circulating in Brazil, with dominant subvariants shifting over time from BA.5 in 2022 to XBB and JN.1 in 2023-2024. The emergence of new subvariants in this new epidemiological scenario underscores the importance of ongoing genomic surveillance to track viral evolution and inform public health strategies, providing valuable information to update vaccines and implement other measures, such as lockdowns, mask usage, social distancing, health education, and self-testing.

Small-molecule inhibition of SARS-CoV-2 NSP14 RNA cap methyltransferase

Coronavirus disease 2019 (COVID-19) is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)1. The rapid development of highly effective vaccines2,3 against SARS-CoV-2 has altered the trajectory of the pandemic, and antiviral therapeutics4 have further reduced the number of COVID-19 hospitalizations and deaths. Coronaviruses are enveloped, positive-sense, single-stranded RNA viruses that encode various structural and non-structural proteins, including those critical for viral RNA replication and evasion from innate immunity5. Here we report the discovery and development of a first-in-class non-covalent small-molecule inhibitor of the viral guanine-N7 methyltransferase (MTase) NSP14. High-throughput screening identified RU-0415529, which inhibited SARS-CoV-2 NSP14 by forming a unique ternary S-adenosylhomocysteine (SAH)-bound complex. Hit-to-lead optimization of RU-0415529 resulted in TDI-015051 with a dissociation constant (Kd) of 61 pM and a half-maximal effective concentration (EC50) of 11 nM, inhibiting virus infection in a cell-based system. TDI-015051 also inhibited viral replication in primary small airway epithelial cells and in a transgenic mouse model of SARS CoV-2 infection with an efficacy comparable with the FDA-approved reversible covalent protease inhibitor nirmatrelvir6. The inhibition of viral cap methylases as an antiviral strategy is also adaptable to other pandemic viruses.

The zymogenic form of SARS-CoV-2 main protease: A discrete target for drug discovery

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) main protease (Mpro) autocatalytically releases itself out of the viral polyprotein to form a fully active mature dimer in a manner that is not fully understood. Here, we introduce several tools to help elucidate differences between cis (intramolecular) and trans (intermolecular) proteolytic processing and to evaluate inhibition of precursor Mpro. We found that many mutations at the P1 position of the N-terminal autoprocessing site do not block cis autoprocessing but do inhibit trans processing. Notably, substituting the WT glutamine at the P1 position with isoleucine retains Mpro in an unprocessed precursor form that can be purified and further studied. We also developed a cell-based reporter assay suitable for compound library screening and evaluation in HEK293T cells. This assay can detect both overall Mpro inhibition and the fraction of uncleaved precursor form of Mpro through separable fluorescent signals. We observed that inhibitory compounds preferentially block mature Mpro. Bofutrelvir and a novel compound designed in-house showed the lowest selectivity between precursor and mature Mpro, indicating that inhibition of both forms may be possible. Additionally, we observed positive modulation of precursor activity at low concentrations of inhibitors. Our findings help expand understanding of the SARS-CoV-2 viral life cycle and may facilitate development of strategies to target precursor form of Mpro for inhibition or premature activation of Mpro.

Structural Insights into the Assembly and Regulation of 2'- O RNA Methylation by SARS-CoV-2 nsp16/nsp10

2'- O -ribose methylation of the first transcribed base (adenine or A 1 in SARS-CoV-2) of viral RNA mimics the host RNAs and subverts the innate immune response. How nsp16, with its obligate partner nsp10, assembles on the 5'-end of SARS-CoV-2 mRNA to methylate the A 1 has not been fully understood. We present a ∼ 2.4 Å crystal structure of the heterotetrameric complex formed by the cooperative assembly of two nsp16/nsp10 heterodimers with one 10-mer Cap-1 RNA (product) bound to each. An aromatic zipper-like motif in nsp16 and the N-terminal regions of nsp10 and nsp16 orchestrate an oligomeric assembly for efficient methylation. The front catalytic pocket of nsp16 stabilizes the upstream portion of the RNA while the downstream RNA remains unresolved, likely due to its flexibility. An inverted nsp16 dimer extends the positively charged surface area for longer RNA to influence the catalysis. Additionally, a non-specific nucleotide-binding pocket on the backside of nsp16 plays a critical role in catalysis, further contributing to its enzymatic activity.

Bat RNA viruses employ viral RHIMs orchestrating species-specific cell death programs linked to Z-RNA sensing and ZBP1-RIPK3 signaling

RHIM is a protein motif facilitating the assembly of large signaling complexes triggering regulated cell death. A few DNA viruses employ viral RHIMs mimicking host RHIMs and counteract cell death by interacting with host RHIM-proteins to alleviate antiviral defenses. Whether RNA viruses operate such viral RHIMs remains unknown. Here, we identified viral RHIMs in Nsp13 of SARS-CoV-2 and other bat RNA viruses, providing the basis for bats as the hosts for their evolution. Nsp13 promoted viral RHIM and RNA-binding channel-dependent cell death. However, Nsp13 viral RHIM is more critical for human cell death than in bat-derived Tb1 Lu cells, suggesting species-specific regulation. Nsp13 showed RHIM-dependent interactions with ZBP1 and RIPK3, forming large complexes and promoting ZBP1-RIPK3 signaling-mediated cell death. Intriguingly, the SARS-CoV-2 genome consisted of Z-RNA-forming segments promoting Nsp13-dependent cell death. Our findings reveal the functional viral RHIMs of bat-originated RNA viruses regulating host cell death associated with ZBP1-RIPK3 signaling, indicating possible mechanisms of cellular damage and cytokine storm in bat-originated RNA virus infections.

SARS-CoV-2 Nsp14 binds Tollip and activates pro-inflammatory pathways while downregulating interferon-α and interferon-γ receptors

SARS coronavirus 2 (SARS-CoV-2) non-structural protein 14 (Nsp14) possesses an N-terminal exonuclease (ExoN) domain that provides a proofreading function for the viral RNA-dependent RNA polymerase and a C-terminal N7-methyltransferase (N7-MTase) domain that methylates viral mRNA caps. Nsp14 also modulates host functions. This includes the activation of NF-κB and downregulation of interferon alpha/beta receptor 1 (IFNAR1). Here we demonstrate that Nsp14 exerts broader effects, activating not only NF-κB responses but also ERK, p38 and JNK MAP kinase (MAPK) signaling, promoting cytokine production. Further, Nsp14 downregulates not only IFNAR1 but also IFN-γ receptor 1 (IFNGR1), impairing cellular responses to both IFNα and IFNγ. IFNAR1 and IFNGR1 downregulation is via a lysosomal pathway and also occurs in SARS-CoV-2 infected cells. Analysis of a panel of Nsp14 mutants reveals a consistent pattern. Mutants that disable ExoN function remain active, whereas N7-MTase mutations impair both pro-inflammatory pathway activation and IFN receptor downregulation. Innate immune modulating functions also require the presence of both the ExoN and N7-MTase domains likely reflecting the need for the ExoN domain for N7-MTase activity. We further identify multi-functional host protein Tollip as an Nsp14 interactor. Interaction requires the phosphoinositide-binding C2 domain of Tollip and sequences C-terminal to the C2 domain. Full length Tollip or regions encompassing the Nsp14 interaction domain are sufficient to counteract both Nsp14-mediated and Nsp14-independent activation of NF-κB. Knockdown of Tollip partially reverses IFNAR1 and IFNGR1 downregulation in SARS-CoV-2 infected cells, suggesting relevance of Nsp14-Tollip interaction for Nsp14 innate immune evasion functions.

Biochemical simulation of mutation synthesis and repair during SARS-CoV-2 RNA polymerization

We biochemically simulated the mutation synthesis process of SARS-CoV-2 RNA-dependent RNA polymerase (RdRp) complex (nsp7/nsp8/nsp12) involving two sequential mechanistic steps that occur during genomic replication: misinsertion (incorporation of incorrect nucleotides) and mismatch extension. Then, we also simulated mismatch repair process catalyzed by the viral nsp10/nsp14 ExoN complex. In these mechanistic simulations, while SARS-CoV-2 RdRp displays efficient mutation synthesis capability, the viral ExoN complex was able to effectively repair the mismatch primers generated during the mutation synthesis. Also, we observed that the delayed RNA synthesis induced by mutation synthesis process was rescued by the viral ExoN activity. Collectively, our biochemical simulations suggest that SARS-CoV-2 ExoN complex may contribute to both maintenance of proper viral genetic diversity levels and successful completion of the viral large RNA genome replication by removing mismatches generated by the viral RdRp.

Dynamic expedition of leading mutations in SARS-CoV-2 spike glycoproteins

The continuous evolution of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which caused the recent pandemic, has generated countless new variants with varying fitness. Mutations of the spike glycoprotein play a particularly vital role in shaping its evolutionary trajectory, as they have the capability to alter its infectivity and antigenicity. We present a time-resolved statistical method, Dynamic Expedition of Leading Mutations (deLemus), to analyze the evolutionary dynamics of the SARS-CoV-2 spike glycoprotein. The proposed L -index of the deLemus method is effective in quantifying the mutation strength of each amino acid site and outlining evolutionarily significant sites, allowing the comprehensive characterization of the evolutionary mutation pattern of the spike glycoprotein.

Utilization of the EpiMed Coronabank Chemical Collection to identify potential SARS-CoV-2 antivirals: in silico studies targeting the nsp14 ExoN domain and PLpro naphthalene binding site

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) genome encodes 29 proteins including four structural, 16 nonstructural (nsps), and nine accessory proteins (https://epimedlab.org/sars-cov-2-proteome/). Many of these proteins contain potential targetable sites for the development of antivirals. Despite the widespread use of vaccinations, the emergence of variants necessitates the investigation of new therapeutics and antivirals. Here, the EpiMed Coronabank Chemical Collection (https://epimedlab.org/crl/) was utilized to investigate potential antivirals against the nsp14 exoribonuclease (ExoN) domain. Molecular docking was performed to evaluate the binding characteristics of our chemical library against the nsp14 ExoN site. Based on the initial screen, trisjuglone, ararobinol, corilagin, and naphthofluorescein were identified as potential lead compounds. Molecular dynamics (MD) simulations were subsequently performed, with the results highlighting the stability of the lead compounds in the nsp14 ExoN site. Protein-RNA docking revealed the potential for the lead compounds to disrupt the interaction with RNA when bound to the ExoN site. Moreover, hypericin, cyanidin-3-O-glucoside, and rutin were previously identified as lead compounds targeting the papain-like protease (PLpro) naphthalene binding site. Through performing MD simulations, the stability and interactions of lead compounds with PLpro were further examined. Overall, given the critical role of the exonuclease activity of nsp14 in ensuring viral fidelity and the multifunctional role of PLpro in viral pathobiology and replication, these nsps represent important targets for antiviral drug development. Our databases can be utilized for in silico studies, such as the ones performed here, and this approach can be applied to other potentially druggable SARS-CoV-2 protein targets.

The significance of recurrent de novo amino acid substitutions that emerged during chronic SARS-CoV-2 infection: an observational study

BACKGROUND

De novo amino acid substitutions (DNS) frequently emerge among immunocompromised patients with chronic SARS-CoV-2 infection. While previous studies have reported these DNS, their significance has not been systematically studied.

METHODS

We performed a review of DNS that emerged during chronic SARS-CoV-2 infection. We searched PubMed until June 2023 using the keywords "(SARS-CoV-2 or COVID-19) and (mutation or sequencing) and ((prolonged infection) or (chronic infection) or (long term))". We included patients with chronic SARS-CoV-2 infection who had SARS-CoV-2 sequencing performed for at least 3 time points over at least 60 days. We also included 4 additional SARS-CoV-2 patients with chronic infection of our hospital not reported previously. We determined recurrent DNS that has appeared in multiple patients and determined the significance of these mutations among epidemiologically-significant variants.

FINDINGS

A total of 34 cases were analyzed, including 30 that were published previously and 4 from our hospital. Twenty two DNS appeared in ≥3 patients, with 14 (64%) belonging to lineage-defining mutations (LDMs) of epidemiologically-significant variants and 10 (45%) emerging among chronically-infected patients before the appearance of the corresponding variant. Notably, nsp9-T35I substitution (Orf1a T4175I) emerged in all three patients with BA.2.2 infection in 2022 before the appearance of Variants of Interest that carry nsp9-T35I as LDM (EG.5 and BA.2.86/JN.1). Structural analysis suggests that nsp9-T35I substitution may affect nsp9-nsp12 interaction, which could be critical for the function of the replication and transcription complex.

INTERPRETATION

DNS that emerges recurrently in different chronically-infected patients may be used as a marker for potential epidemiologically-significant variants.

FUNDING

Theme-Based Research Scheme [T11/709/21-N] of the Research Grants Council (See acknowledgements for full list).

Difference in TMPRSS2 usage by Delta and Omicron variants of SARS-CoV-2: Implication for a sudden increase among children

It has been postulated from a combination of evidence that a sudden increase in COVID-19 cases among pediatric patients after onset of the Omicron wave was attributed to a reduced requirement for TMPRSS2-mediated entry in pediatric airways with lower expression levels of TMPRSS2. Epidemic strains were isolated from the indigenous population in an area, and the levels of TMPRSS2 required for Delta and Omicron variants were assessed. As a result, Delta variants proliferated fully in cultures of TMPRSS2-positive Vero cells but not in TMPRSS2-negative Vero cell culture (350-fold, Delta vs 9.6-fold, Omicron). There was no obvious age-dependent selection of Omicron strains affected by the TMPRSS2 (9.6-fold, Adults vs. 12-fold, Children). A phylogenetic tree was generated and Blast searches (up to 100 references) for the spread of strains in the study area showed that each strain had almost identical homology (>99.5%) with foreign isolates, although indigenous strains had obvious differences from each other. This suggested that the differences had been present abroad for a long period. Therefore, the lower requirement for TMPRSS2 by Omicron strains might be applicable to epidemic strains globally. In conclusion, the property of TMPRSS2-independent cleavage makes Omicron proliferate with ease and allows epidemics among children with fewer TMPRSS2 on epithelial surfaces of the respiratory organs.

New perspective of small-molecule antiviral drugs development for RNA viruses

High variability and adaptability of RNA viruses allows them to spread between humans and animals, causing large-scale infectious diseases which seriously threat human and animal health and social development. At present, AIDS, viral hepatitis and other viral diseases with high incidence and low cure rate are still spreading around the world. The outbreaks of Ebola, Zika, dengue and in particular of the global pandemic of COVID-19 have presented serious challenges to the global public health system. The development of highly effective and broad-spectrum antiviral drugs is a substantial and urgent research subject to deal with the current RNA virus infection and the possible new viral infections in the future. In recent years, with the rapid development of modern disciplines such as artificial intelligence technology, bioinformatics, molecular biology, and structural biology, some new strategies and targets for antivirals development have emerged. Here we review the main strategies and new targets for developing small-molecule antiviral drugs against RNA viruses through the analysis of the new drug development progress against several highly pathogenic RNA viruses, to provide clues for development of future antivirals.

SARS-CoV-2 biology and host interactions

The zoonotic emergence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and the ensuing coronavirus disease 2019 (COVID-19) pandemic have profoundly affected our society. The rapid spread and continuous evolution of new SARS-CoV-2 variants continue to threaten global public health. Recent scientific advances have dissected many of the molecular and cellular mechanisms involved in coronavirus infections, and large-scale screens have uncovered novel host-cell factors that are vitally important for the virus life cycle. In this Review, we provide an updated summary of the SARS-CoV-2 life cycle, gene function and virus-host interactions, including recent landmark findings on general aspects of coronavirus biology and newly discovered host factors necessary for virus replication.

Viral RNA Replication Suppression of SARS-CoV-2: Atomistic Insights into Inhibition Mechanisms of RdRp Machinery by ddhCTP

The nonstructural protein 12, known as RNA-dependent RNA polymerase (RdRp), is essential for both replication and repair of the viral genome. The RdRp of SARS-CoV-2 has been used as a promising candidate for drug development since the inception of the COVID-19 spread. In this work, we performed an in silico investigation on the insertion of the naturally modified pyrimidine nucleobase ddhCTP into the SARS-CoV-2 RdRp active site, in a comparative analysis with the natural one (CTP). The modification in ddhCTP involves the removal of the 3'-hydroxyl group that prevents the addition of subsequent nucleotides into the nascent strand, acting as an RNA chain terminator inhibitor. Quantum mechanical investigations helped to shed light on the mechanistic source of RdRp activity on the selected nucleobases, and comprehensive all-atom simulations provided insights about the structural rearrangements occurring in the active-site region when inorganic pyrophosphate (PPi) is formed. Subsequently, the intricate pathways for the release of PPi, the catalytic product of RdRp, were investigated using Umbrella Sampling simulations. The results are in line with the available experimental data and contribute to a more comprehensive point of view on such an important viral enzyme.

SARS-CoV-2 NSP14 governs mutational instability and assists in making new SARS-CoV-2 variants

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the rapidly evolving RNA virus behind the COVID-19 pandemic, has spawned numerous variants since its 2019 emergence. The multifunctional Nonstructural protein 14 (NSP14) enzyme, possessing exonuclease and messenger RNA (mRNA) capping capabilities, serves as a key player. Notably, single and co-occurring mutations within NSP14 significantly influence replication fidelity and drive variant diversification. This study comprehensively examines 120 co-mutations, 68 unique mutations, and 160 conserved residues across NSP14 homologs, shedding light on their implications for phylogenetic patterns, pathogenicity, and residue interactions. Quantitative physicochemical analysis categorizes 3953 NSP14 variants into three clusters, revealing genetic diversity. This research underscoresthe dynamic nature of SARS-CoV-2 evolution, primarily governed by NSP14 mutations. Understanding these genetic dynamics provides valuable insights for therapeutic and vaccine development.

Informatics and Computational Approaches for the Discovery and Optimization of Natural Product-Inspired Inhibitors of the SARS-CoV-2 2'-O-Methyltransferase

The urgent need for new classes of orally available, safe, and effective antivirals─covering a breadth of emerging viruses─is evidenced by the loss of life and economic challenges created by the HIV-1 and SARS-CoV-2 pandemics. As frontline interventions, small-molecule antivirals can be deployed prophylactically or postinfection to control the initial spread of outbreaks by reducing transmissibility and symptom severity. Natural products have an impressive track record of success as prototypic antivirals and continue to provide new drugs through synthesis, medicinal chemistry, and optimization decades after discovery. Here, we demonstrate an approach using computational analysis typically used for rational drug design to identify and develop natural product-inspired antivirals. This was done with the goal of identifying natural product prototypes to aid the effort of progressing toward safe, effective, and affordable broad-spectrum inhibitors of Betacoronavirus replication by targeting the highly conserved RNA 2'-O-methyltransferase (2'-O-MTase). Machaeriols RS-1 (7) and RS-2 (8) were identified using a previously outlined informatics approach to first screen for natural product prototypes, followed by in silico-guided synthesis. Both molecules are based on a rare natural product group. The machaeriols (3-6), isolated from the genus Machaerium, endemic to Amazonia, inhibited the SARS-CoV-2 2'-O-MTase more potently than the positive control, Sinefungin (2), and in silico modeling suggests distinct molecular interactions. This report highlights the potential of computationally driven screening to leverage natural product libraries and improve the efficiency of isolation or synthetic analog development.

An exonuclease-resistant chain-terminating nucleotide analogue targeting the SARS-CoV-2 replicase complex

Nucleotide analogues (NA) are currently employed for treatment of several viral diseases, including COVID-19. NA prodrugs are intracellularly activated to the 5'-triphosphate form. They are incorporated into the viral RNA by the viral polymerase (SARS-CoV-2 nsp12), terminating or corrupting RNA synthesis. For Coronaviruses, natural resistance to NAs is provided by a viral 3'-to-5' exonuclease heterodimer nsp14/nsp10, which can remove terminal analogues. Here, we show that the replacement of the α-phosphate of Bemnifosbuvir 5'-triphosphate form (AT-9010) by an α-thiophosphate renders it resistant to excision. The resulting α-thiotriphosphate, AT-9052, exists as two epimers (RP/SP). Through co-crystallization and activity assays, we show that the Sp isomer is preferentially used as a substrate by nucleotide diphosphate kinase (NDPK), and by SARS-CoV-2 nsp12, where its incorporation causes immediate chain-termination. The same -Sp isomer, once incorporated by nsp12, is also totally resistant to the excision by nsp10/nsp14 complex. However, unlike AT-9010, AT-9052-RP/SP no longer inhibits the N-terminal nucleotidylation domain of nsp12. We conclude that AT-9052-Sp exhibits a unique mechanism of action against SARS-CoV-2. Moreover, the thio modification provides a general approach to rescue existing NAs whose activity is hampered by coronavirus proofreading capacity.

All-trans retinoic acid acts as a dual-purpose inhibitor of SARS-CoV-2 infection and inflammation

Coronavirus disease 2019 (COVID-19) was an epidemic that effected human health caused by SARS-CoV-2 infection. All-trans retinoic acid (ATRA) has anti-inflammatory capability. In this article, we evaluated the effectiveness and revealed the molecular mechanism of ATRA for treating SARS-CoV-2 using deep learning, in vitro studies, multi-scale molecular modeling, and network pharmacology. The DeepDTA model suggested that ATRA would be effective against COVID-19. In vitro studies confirmed the antiviral activity of ATRA. Subsequently, multi-scale molecular modeling indicated that ATRA could binding to angiotensin converting enzyme 2 (ACE2), 3C-like protease (3CLpro), RNA dependent RNA polymerase (RdRp), helicase, and 3'-to-5' exonuclease by non-covalent interactions. Additionally, network pharmacology suggested that ATRA alleviated inflammatory response by regulating the IL-17 signaling pathway and binding with TNF, PTGS2, and MAPK1 directly. In summary, our findings provide the first evidence that ATRA suppresses the entry and replication of SARS-CoV-2, and regulates inflammatory response of host cells.

Drug Discovery in Low Data Regimes: Leveraging a Computational Pipeline for the Discovery of Novel SARS-CoV-2 Nsp14-MTase Inhibitors

The COVID-19 pandemic, caused by the SARS-CoV-2 virus, has led to significant global morbidity and mortality. A crucial viral protein, the non-structural protein 14 (nsp14), catalyzes the methylation of viral RNA and plays a critical role in viral genome replication and transcription. Due to the low mutation rate in the nsp region among various SARS-CoV-2 variants, nsp14 has emerged as a promising therapeutic target. However, discovering potential inhibitors remains a challenge. In this work, we introduce a computational pipeline for the rapid and efficient identification of potential nsp14 inhibitors by leveraging virtual screening and the NCI open compound collection, which contains 250,000 freely available molecules for researchers worldwide. The introduced pipeline provides a cost-effective and efficient approach for early-stage drug discovery by allowing researchers to evaluate promising molecules without incurring synthesis expenses. Our pipeline successfully identified seven promising candidates after experimentally validating only 40 compounds. Notably, we discovered NSC620333, a compound that exhibits a strong binding affinity to nsp14 with a dissociation constant of 427 ± 84 nM. In addition, we gained new insights into the structure and function of this protein through molecular dynamics simulations. We identified new conformational states of the protein and determined that residues Phe367, Tyr368, and Gln354 within the binding pocket serve as stabilizing residues for novel ligand interactions. We also found that metal coordination complexes are crucial for the overall function of the binding pocket. Lastly, we present the solved crystal structure of the nsp14-MTase complexed with SS148 (PDB:8BWU), a potent inhibitor of methyltransferase activity at the nanomolar level (IC50 value of 70 ± 6 nM). Our computational pipeline accurately predicted the binding pose of SS148, demonstrating its effectiveness and potential in accelerating drug discovery efforts against SARS-CoV-2 and other emerging viruses.

A universal fluorescence polarization high throughput screening assay to target the SAM-binding sites of SARS-CoV-2 and other viral methyltransferases

SARS-CoV-2 has caused a global pandemic with significant humanity and economic loss since 2020. Currently, only limited options are available to treat SARS-CoV-2 infections for vulnerable populations. In this study, we report a universal fluorescence polarization (FP)-based high throughput screening (HTS) assay for SAM-dependent viral methyltransferases (MTases), using a fluorescent SAM-analogue, FL-NAH. We performed the assay against a reference MTase, NSP14, an essential enzyme for SARS-CoV-2 to methylate the N7 position of viral 5'-RNA guanine cap. The assay is universal and suitable for any SAM-dependent viral MTases such as the SARS-CoV-2 NSP16/NSP10 MTase complex and the NS5 MTase of Zika virus (ZIKV). Pilot screening demonstrated that the HTS assay was very robust and identified two candidate inhibitors, NSC 111552 and 288387. The two compounds inhibited the FL-NAH binding to the NSP14 MTase with low micromolar IC50. We used three functional MTase assays to unambiguously verified the inhibitory potency of these molecules for the NSP14 N7-MTase function. Binding studies indicated that these molecules are bound directly to the NSP14 MTase with similar low micromolar affinity. Moreover, we further demonstrated that these molecules significantly inhibited the SARS-CoV-2 replication in cell-based assays at concentrations not causing cytotoxicity. Furthermore, NSC111552 significantly synergized with known SARS-CoV-2 drugs including nirmatrelvir and remdesivir. Finally, docking suggested that these molecules bind specifically to the SAM-binding site on the NSP14 MTase. Overall, these molecules represent novel and promising candidates to further develop broad-spectrum inhibitors for the management of viral infections.

Pervasive aggregation and depletion of host and viral proteins in response to cysteine-reactive electrophilic compounds

Protein homeostasis is tightly regulated, with damaged or misfolded proteins quickly eliminated by the proteasome and autophagosome pathways. By co-opting these processes, targeted protein degradation technologies enable pharmacological manipulation of protein abundance. Recently, cysteine-reactive molecules have been added to the degrader toolbox, which offer the benefit of unlocking the therapeutic potential of 'undruggable' protein targets. The proteome-wide impact of these molecules remains to be fully understood and given the general reactivity of many classes of cysteine-reactive electrophiles, on- and off-target effects are likely. Using chemical proteomics, we identified a cysteine-reactive small molecule degrader of the SARS-CoV-2 nonstructural protein 14 (nsp14), which effects degradation through direct modification of cysteines in both nsp14 and in host chaperones together with activation of global cell stress response pathways. We find that cysteine-reactive electrophiles increase global protein ubiquitylation, trigger proteasome activation, and result in widespread aggregation and depletion of host proteins, including components of the nuclear pore complex. Formation of stress granules was also found to be a remarkably ubiquitous cellular response to nearly all cysteine-reactive compounds and degraders. Collectively, our study sheds light on complexities of covalent target protein degradation and highlights untapped opportunities in manipulating and characterizing proteostasis processes via deciphering the cysteine-centric regulation of stress response pathways.

A Proofreading Mutation with an Allosteric Effect Allows a Cluster of SARS-CoV-2 Viruses to Rapidly Evolve

The RNA-dependent RNA polymerase of the severe acute respiratory syndrome coronavirus 2 virus is error prone, with errors being corrected by the exonuclease (NSP14) proofreading mechanism. However, the mutagenesis and subsequent evolutionary trajectory of the virus is mediated by the delicate interplay of replicase fidelity and environmental pressures. Here, we have shown that a single, distal mutation (F60S) in NSP14 can have a profound impact upon proofreading with an increased accumulation of mutations and elevated evolutionary rate being observed. Understanding the implications of these changes is crucial, as these underlying mutational processes may have important implications for understanding the population-wide evolution of the virus. This study underscores the urgent need for continued research into the replicative mechanisms of this virus to combat its continued impact on global health, through the re-emergence of immuno-evasive variants.

Current understanding of nucleoside analogs inhibiting the SARS-CoV-2 RNA-dependent RNA polymerase

Since the outbreak of the COVID-19 pandemic, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) RNA-dependent RNA polymerase (RdRp) has become a main target for antiviral therapeutics due to its essential role in viral replication and transcription. Thus, nucleoside analogs structurally resemble the natural RdRp substrate and hold great potential as inhibitors. Until now, extensive experimental investigations have been performed to explore nucleoside analogs to inhibit the RdRp, and concerted efforts have been made to elucidate the underlying molecular mechanisms further. This review begins by discussing the nucleoside analogs that have demonstrated inhibition in the experiments. Second, we examine the current understanding of the molecular mechanisms underlying the action of nucleoside analogs on the SARS-CoV-2 RdRp. Recent findings in structural biology and computational research are presented through the classification of inhibitory mechanisms. This review summarizes previous experimental findings and mechanistic investigations of nucleoside analogs inhibiting SARS-CoV-2 RdRp. It would guide the rational design of antiviral medications and research into viral transcriptional mechanisms.

Detection of SARS-CoV-2 in saliva by a low-cost LSPR-based sensor

The SARS-CoV-2 pandemic started more than 3 years ago, but the containment of the spread is still a challenge. Screening is imperative for informed decision making by government authorities to contain the spread of the virus locally. The access to screening tests is disproportional, due to the lack of access to reagents, equipment, finances or because of supply chain disruptions. Low and middle-income countries have especially suffered with the lack of these resources. Here, we propose a low cost and easily constructed biosensor device based on localized surface plasmon resonance, or LSPR, for the screening of SARS-CoV-2. The biosensor device, dubbed "sensor" for simplicity, was constructed in two modalities: (1) viral detection in saliva and (2) antibody against COVID in saliva. Saliva collected from 18 patients were tested in triplicates. Both sensors successfully classified all COVID positive patients (among hospitalized and non-hospitalized). From the COVID negative patients 7/8 patients were correctly classified. For both sensors, sensitivity was determined as 100% (95% CI 79.5-100) and specificity as 87.5% (95% CI 80.5-100). The reagents and equipment used for the construction and deployment of this sensor are ubiquitous and low-cost. This sensor technology can then add to the potential solution for challenges related to screening tests in underserved communities.

Analogs of the Catechol Derivative Dynasore Inhibit HIV-1 Ribonuclease H, SARS-CoV-2 nsp14 Exoribonuclease, and Virus Replication

Viral replication often depends on RNA maturation and degradation processes catalyzed by viral ribonucleases, which are therefore candidate targets for antiviral drugs. Here, we synthesized and studied the antiviral properties of a novel nitrocatechol compound (1c) and other analogs that are structurally related to the catechol derivative dynasore. Interestingly, compound 1c strongly inhibited two DEDD box viral ribonucleases, HIV-1 RNase H and SARS-CoV-2 nsp14 3'-to-5' exoribonuclease (ExoN). While 1c inhibited SARS-CoV-2 ExoN activity, it did not interfere with the mRNA methyltransferase activity of nsp14. In silico molecular docking placed compound 1c in the catalytic pocket of the ExoN domain of nsp14. Finally, 1c inhibited SARS-CoV-2 replication but had no toxicity to human lung adenocarcinoma cells. Given its simple chemical synthesis from easily available starting materials, these results suggest that 1c might be a lead compound for the design of new antiviral compounds that target coronavirus nsp14 ExoN and other viral ribonucleases.

Structure-Based Discovery of Inhibitors of the SARS-CoV-2 Nsp14 N7-Methyltransferase

An under-explored target for SARS-CoV-2 is the S-adenosyl methionine (SAM)-dependent methyltransferase Nsp14, which methylates the N7-guanosine of viral RNA at the 5'-end, allowing the virus to evade host immune response. We sought new Nsp14 inhibitors with three large library docking strategies. First, up to 1.1 billion lead-like molecules were docked against the enzyme's SAM site, leading to three inhibitors with IC50 values from 6 to 50 μM. Second, docking a library of 16 million fragments revealed 9 new inhibitors with IC50 values from 12 to 341 μM. Third, docking a library of 25 million electrophiles to covalently modify Cys387 revealed 7 inhibitors with IC50 values from 3.5 to 39 μM. Overall, 32 inhibitors encompassing 11 chemotypes had IC50 values < 50 μM and 5 inhibitors in 4 chemotypes had IC50 values < 10 μM. These molecules are among the first non-SAM-like inhibitors of Nsp14, providing starting points for future optimization.

Comprehensive Understanding of the Kinetic Behaviors of Main Protease from SARS-CoV-2 and SARS-CoV: New Data and Comparison to Published Parameters

The main protease (M^pro) is a promising drug target for inhibiting the coronavirus due to its conserved properties and lack of homologous genes in humans. However, previous studies on M^pro's kinetic parameters have been confusing, hindering the selection of accurate inhibitors. Therefore, obtaining a clear view of M^pro's kinetic parameters is necessary. In our study, we investigated the kinetic behaviors of M^pro from SARS-CoV-2 and SARS-CoV using both FRET-based cleavage assay and the LC-MS method, respectively. Our findings indicate that the FRET-based cleavage assay could be used for preliminary screening of M^pro inhibitors, while the LC-MS method should be applied to select the effective inhibitors with higher reliability. Furthermore, we constructed the active site mutants (H41A and C145A) and measured the kinetic parameters to gain a deeper understanding of the atomic-level enzyme efficiency reduction compared to the wild type. Overall, our study provides valuable insights for inhibitor screening and design by offering a comprehensive understanding of M^pro's kinetic behaviors.

Therapeutic strategies for COVID-19: progress and lessons learned

The coronavirus disease 2019 (COVID-19) pandemic has stimulated tremendous efforts to develop therapeutic strategies that target severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and/or human proteins to control viral infection, encompassing hundreds of potential drugs and thousands of patients in clinical trials. So far, a few small-molecule antiviral drugs (nirmatrelvir-ritonavir, remdesivir and molnupiravir) and 11 monoclonal antibodies have been marketed for the treatment of COVID-19, mostly requiring administration within 10 days of symptom onset. In addition, hospitalized patients with severe or critical COVID-19 may benefit from treatment with previously approved immunomodulatory drugs, including glucocorticoids such as dexamethasone, cytokine antagonists such as tocilizumab and Janus kinase inhibitors such as baricitinib. Here, we summarize progress with COVID-19 drug discovery, based on accumulated findings since the pandemic began and a comprehensive list of clinical and preclinical inhibitors with anti-coronavirus activities. We also discuss the lessons learned from COVID-19 and other infectious diseases with regard to drug repurposing strategies, pan-coronavirus drug targets, in vitro assays and animal models, and platform trial design for the development of therapeutics to tackle COVID-19, long COVID and pathogenic coronaviruses in future outbreaks.

A lung-specific mutational signature enables inference of viral and bacterial respiratory niche

Exposure to different mutagens leaves distinct mutational patterns that can allow inference of pathogen replication niches. We therefore investigated whether SARS-CoV-2 mutational spectra might show lineage-specific differences, dependent on the dominant site(s) of replication and onwards transmission, and could therefore rapidly infer virulence of emergent variants of concern (VOCs). Through mutational spectrum analysis, we found a significant reduction in G>T mutations in the Omicron variant, which replicates in the upper respiratory tract (URT), compared to other lineages, which replicate in both the URT and lower respiratory tract (LRT). Mutational analysis of other viruses and bacteria indicates a robust, generalizable association of high G>T mutations with replication within the LRT. Monitoring G>T mutation rates over time, we found early separation of Omicron from Beta, Gamma and Delta, while mutational patterns in Alpha varied consistent with changes in transmission source as social restrictions were lifted. Mutational spectra may be a powerful tool to infer niches of established and emergent pathogens.

Exploring the pharmacological aspects of natural phytochemicals against SARS-CoV-2 Nsp14 through an in silico approach

The severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), possesses an important bifunctional nonstructural protein (nsp14) with a C-terminal N7-methyltransferase (N7-MTase) domain and an N-terminal domain with exoribonuclease (ExoN) activity that is required for maintaining high-fidelity viral replication. Viruses use the error-prone replication mechanism, which results in high mutation rates, to adapt quickly to stressful situations. The efficiency with which nsp14 removes mismatched nucleotides due to the presence of ExoN activity protects viruses from mutagenesis. We investigated the pharmacological role of the phytochemicals (Baicalein, Bavachinin, Emodin, Kazinol F, Lycorine, Sinigrin, Procyanidin A2, Tanshinone IIA, Tanshinone IIB, Tomentin A, and Tomentin E) against the highly conserved nsp14 protein using docking-based computational analyses in search of new potential natural drug targets. The selected eleven phytochemicals failed to bind the active site of N7-Mtase in the global docking study, while the local docking study identified the top five phytochemicals with high binding energy scores ranging from - 9.0 to - 6.4 kcal/mol. Procyanidin A2 and Tomentin A showed the highest docking score of - 9.0 and - 8.1 kcal/mol, respectively. Local docking of isoform variants was also conducted, yielding the top five phytochemicals, with Procyanidin A1 having the highest binding energy value of - 9.1 kcal/mol. The phytochemicals were later tested for pharmacokinetics and pharmacodynamics analysis for Absorption, Distribution, Metabolism, Excretion, and Toxicity (ADMET) which resulted in choosing Tomentin A as a potential candidate. The molecular dynamics simulations studies of nsp14 revealed significant conformational changes upon complex formation with the identified compound, implying that these phytochemicals could be used as safe nutraceuticals which will impart long-term immunological competence in the human population against CoVs.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s40203-023-00143-7.

Activity of nsp14 Exonuclease from SARS-CoV-2 towards RNAs with Modified 3'-Termini

The COVID-19 pandemic has shown the urgent need for new treatments for coronavirus infections. Nucleoside analogs were successfully used to inhibit replication of some viruses through the incorporation into the growing DNA or RNA chain. However, the replicative machinery of coronaviruses contains nsp14, a non-structural protein with a 3'→5'-exonuclease activity that removes misincorporated and modified nucleotides from the 3' end of the growing RNA chain. Here, we studied the efficiency of hydrolysis of RNA containing various modifications in the 3'-terminal region by SARS-CoV-2 nsp14 exonuclease and its complex with the auxiliary protein nsp10. Single-stranded RNA was a preferable substrate compared to double-stranded RNA, which is consistent with the model of transfer of the substrate strand to the exonuclease active site, which was proposed on the basis of structural analysis. Modifications of the phosphodiester bond between the penultimate and last nucleotides had the greatest effect on nsp14 activity.

Fidelity of Ribonucleotide Incorporation by the SARS-CoV-2 Replication Complex

The SARS-CoV-2 coronavirus has caused a global pandemic. Despite the initial success of vaccines at preventing infection, genomic variation has led to the proliferation of variants capable of higher infectivity. Mutations in the SARS-CoV-2 genome are the consequence of replication errors, highlighting the importance of understanding the determinants of SARS-CoV-2 replication fidelity. The RNA-dependent RNA polymerase (RdRp) is the central catalytic subunit for SARS-CoV-2 RNA replication and genome transcription. Here, we report the fidelity of ribonucleotide incorporation by SARS-CoV-2 RdRp (nsp12), along with its co-factors nsp7/nsp8, using steady-state kinetic analysis. Our analysis suggests that in the absence of the proofreading subunit (nsp14), the nsp12/7/8 complex has a surprisingly low base substitution fidelity (10-1-10-3). This is orders of magnitude lower than the fidelity reported for other coronaviruses (10-6-10-7), highlighting the importance of proofreading for faithful SARS-CoV-2 replication. We performed a mutational analysis of all reported SARS-CoV-2 genomes and identified mutations in both nsp12 and nsp14 that appear likely to lower viral replication fidelity through mechanisms that include impairing the nsp14 exonuclease activity or its association with the RdRp. Our observations provide novel insight into the mechanistic basis of replication fidelity in SARS-CoV-2 and the potential effect of nsp12 and nsp14 mutations on replication fidelity, informing the development of future antiviral agents and SARS-CoV-2 vaccines.

Direct observation of backtracking by influenza A and B polymerases upon consecutive incorporation of the nucleoside analog T1106

The antiviral pseudo-base T705 and its de-fluoro analog T1106 mimic adenine or guanine and can be competitively incorporated into nascent RNA by viral RNA-dependent RNA polymerases. Although dispersed, single pseudo-base incorporation is mutagenic, consecutive incorporation causes polymerase stalling and chain termination. Using a template encoding single and then consecutive T1106 incorporation four nucleotides later, we obtained a cryogenic electron microscopy structure of stalled influenza A/H7N9 polymerase. This shows that the entire product-template duplex backtracks by 5 nt, bringing the singly incorporated T1106 to the +1 position, where it forms an unexpected T1106:U wobble base pair. Similar structures show that influenza B polymerase also backtracks after consecutive T1106 incorporation, regardless of whether prior single incorporation has occurred. These results give insight into the unusual mechanism of chain termination by pyrazinecarboxamide base analogs. Consecutive incorporation destabilizes the proximal end of the product-template duplex, promoting irreversible backtracking to a more energetically favorable overall configuration.

Interfering with nucleotide excision by the coronavirus 3'-to-5' exoribonuclease

Some of the most efficacious antiviral therapeutics are ribonucleos(t)ide analogs. The presence of a 3'-to-5' proofreading exoribonuclease (ExoN) in coronaviruses diminishes the potency of many ribonucleotide analogs. The ability to interfere with ExoN activity will create new possibilities for control of SARS-CoV-2 infection. ExoN is formed by a 1:1 complex of nsp14 and nsp10 proteins. We have purified and characterized ExoN using a robust, quantitative system that reveals determinants of specificity and efficiency of hydrolysis. Double-stranded RNA is preferred over single-stranded RNA. Nucleotide excision is distributive, with only one or two nucleotides hydrolyzed in a single binding event. The composition of the terminal basepair modulates excision. A stalled SARS-CoV-2 replicase in complex with either correctly or incorrectly terminated products prevents excision, suggesting that a mispaired end is insufficient to displace the replicase. Finally, we have discovered several modifications to the 3'-RNA terminus that interfere with or block ExoN-catalyzed excision. While a 3'-OH facilitates hydrolysis of a nucleotide with a normal ribose configuration, this substituent is not required for a nucleotide with a planar ribose configuration such as that present in the antiviral nucleotide produced by viperin. Design of ExoN-resistant, antiviral ribonucleotides should be feasible.

Crystal structures and fragment screening of SARS-CoV-2 NSP14 reveal details of exoribonuclease activation and mRNA capping and provide starting points for antiviral drug development

NSP14 is a dual function enzyme containing an N-terminal exonuclease domain (ExoN) and C-terminal Guanine-N7-methyltransferase (N7-MTase) domain. Both activities are essential for the viral life cycle and may be targeted for anti-viral therapeutics. NSP14 forms a complex with NSP10, and this interaction enhances the nuclease but not the methyltransferase activity. We have determined the structure of SARS-CoV-2 NSP14 in the absence of NSP10 to 1.7 Å resolution. Comparisons with NSP14/NSP10 complexes reveal significant conformational changes that occur within the NSP14 ExoN domain upon binding of NSP10, including helix to coil transitions that facilitate the formation of the ExoN active site and provide an explanation of the stimulation of nuclease activity by NSP10. We have determined the structure of NSP14 in complex with cap analogue 7MeGpppG, and observe conformational changes within a SAM/SAH interacting loop that plays a key role in viral mRNA capping offering new insights into MTase activity. We perform an X-ray fragment screen on NSP14, revealing 72 hits bound to sites of inhibition in the ExoN and MTase domains. These fragments serve as excellent starting point tools for structure guided development of NSP14 inhibitors that may be used to treat COVID-19 and potentially other future viral threats.

Analysis of critical protein-protein interactions of SARS-CoV-2 capping and proofreading molecular machineries towards designing dual target inhibitory peptides

In recent years, the emergence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), as the cause of the coronavirus disease (COVID-19) global pandemic, and its variants, especially those with higher transmissibility and substantial immune evasion, have highlighted the imperative for developing novel therapeutics as sustainable solutions other than vaccination to combat coronaviruses (CoVs). Beside receptor recognition and virus entry, members of the SARS-CoV-2 replication/transcription complex are promising targets for designing antivirals. Here, the interacting residues that mediate protein-protein interactions (PPIs) of nsp10 with nsp16 and nsp14 were comprehensively analyzed, and the key residues' interaction maps, interaction energies, structural networks, and dynamics were investigated. Nsp10 stimulates both nsp14's exoribonuclease (ExoN) and nsp16's 2'O-methyltransferase (2'O-MTase). Nsp14 ExoN is an RNA proofreading enzyme that supports replication fidelity. Nsp16 2'O-MTase is responsible for the completion of RNA capping to ensure efficient replication and translation and escape from the host cell's innate immune system. The results of the PPIs analysis proposed crucial information with implications for designing SARS-CoV-2 antiviral drugs. Based on the predicted shared protein-protein interfaces of the nsp16-nsp10 and nsp14-nsp10 interactions, a set of dual-target peptide inhibitors was designed. The designed peptides were evaluated by molecular docking, peptide-protein interaction analysis, and free energy calculations, and then further optimized by in silico saturation mutagenesis. Based on the predicted evolutionary conservation of the interacted target residues among CoVs, the designed peptides have the potential to be developed as dual target pan-coronavirus inhibitors.

A review on structural, non-structural, and accessory proteins of SARS-CoV-2: Highlighting drug target sites

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent of COVID-19, is a highly transmittable and pathogenic human coronavirus that first emerged in China in December 2019. The unprecedented outbreak of SARS-CoV-2 devastated human health within a short time leading to a global public health emergency. A detailed understanding of the viral proteins including their structural characteristics and virulence mechanism on human health is very crucial for developing vaccines and therapeutics. To date, over 1800 structures of non-structural, structural, and accessory proteins of SARS-CoV-2 are determined by cryo-electron microscopy, X-ray crystallography, and NMR spectroscopy. Designing therapeutics to target the viral proteins has several benefits since they could be highly specific against the virus while maintaining minimal detrimental effects on humans. However, for ongoing and future research on SARS-CoV-2, summarizing all the viral proteins and their detailed structural information is crucial. In this review, we compile comprehensive information on viral structural, non-structural, and accessory proteins structures with their binding and catalytic sites, different domain and motifs, and potential drug target sites to assist chemists, biologists, and clinicians finding necessary details for fundamental and therapeutic research.

Potential targets of severe acute respiratory syndrome coronavirus 2 of clinical drug fluvoxamine: Docking and molecular dynamics studies to elucidate viral action

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has continued evolving for survival and adaptation by mutating itself into different variants of concern, including omicron. Several studies and clinical trials found fluvoxamine, an Food and Drug Administration-approved antidepressant drug, to be effective at preventing mild coronavirus disease 2019 (COVID-19) from progressing to severe diseases. However, the mechanism of fluvoxamine's direct antiviral action against COVID-19 is still unknown. Fluvoxamine was docked with 11 SARS-CoV-2 targets and subjected to stability, conformational changes, and binding free energy analyses to explore its mode of action. Of the targets, nonstructural protein 14 (NSP14), main protease (Mpro), and papain-like protease (PLpro) had the best docking scores with fluvoxamine. Consistent with the docking results, it was confirmed by molecular dynamics simulations that the NSP14 N7-MTase ((N7-guanine)-methyltransferase)-fluvoxamine, Mpro-fluvoxamine, and PLpro-fluvoxamine complexes are stable, with the lowest binding free energies of -105.1, -82.7, and - 38.5 kJ/mol, respectively. A number of hotspot residues involved in the interaction were also identified. These include Glu166, Asp187, His41, and Cys145 in Mpro, Gly163 and Arg166 in PLpro, and Glu302, Gly333, and Phe426 in NSP14, which could aid in the development of better antivirals against SARS-CoV-2.

Effects of natural RNA modifications on the activity of SARS-CoV-2 RNA-dependent RNA polymerase

RNA-dependent RNA polymerase (RdRp) plays a key role in the replication of RNA viruses, including SARS-CoV-2. Processive RNA synthesis by RdRp is crucial for successful genome replication and expression, especially in the case of very long coronaviral genomes. Here, we analysed the activity of SARS-CoV-2 RdRp (the nsp12-nsp7-nsp8 complex) on synthetic primer-templates of various structures, including substrates with mismatched primers or template RNA modifications. It has been shown that RdRp cannot efficiently extend RNA primers containing mismatches and has no intrinsic RNA cleavage activity to remove the primer 3'-end, thus necessitating the action of exoribonuclease for proofreading. Similar to DNA-dependent RNA polymerases, RdRp can perform processive pyrophosphorolysis of the nascent RNA product but this reaction is also blocked in the presence of mismatches. Furthermore, we have demonstrated that several natural post-transcriptional modifications in the RNA template, which do not prevent complementary interactions (N6-methyladenosine, 5-methylcytosine, inosine and pseudouridine), do not change RdRp processivity. At the same time, certain modifications of RNA bases and ribose residues strongly block RNA synthesis, either prior to nucleotide incorporation (3-methyluridine and 1-methylguanosine) or immediately after it (2'-O-methylation). The results demonstrate that the activity of SARS-CoV-2 RdRp can be strongly inhibited by common modifications of the RNA template suggesting a way to design novel antiviral compounds.

On the Evolutionary Trajectory of SARS-CoV-2: Host Immunity as a Driver of Adaptation in RNA Viruses

Host immunity can exert a complex array of selective pressures on a pathogen, which can drive highly mutable RNA viruses towards viral escape. The plasticity of a virus depends on its rate of mutation, as well as the balance of fitness cost and benefit of mutations, including viral adaptations to the host's immune response. Since its emergence, SARS-CoV-2 has diversified into genetically distinct variants, which are characterised often by clusters of mutations that bolster its capacity to escape human innate and adaptive immunity. Such viral escape is well documented in the context of other pandemic RNA viruses such as the human immunodeficiency virus (HIV) and influenza virus. This review describes the selection pressures the host's antiviral immunity exerts on SARS-CoV-2 and other RNA viruses, resulting in divergence of viral strains into more adapted forms. As RNA viruses obscure themselves from host immunity, they uncover weak points in their own armoury that can inform more comprehensive, long-lasting, and potentially cross-protective vaccine coverage.

Substrate Specificity and Kinetics of RNA Hydrolysis by SARS-CoV-2 NSP10/14 Exonuclease

Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), the virus that causes COVID-19, continues to evolve resistance to vaccines and existing antiviral therapies at an alarming rate, increasing the need for new direct-acting antiviral drugs. Despite significant advances in our fundamental understanding of the kinetics and mechanism of viral RNA replication, there are still open questions regarding how the proofreading exonuclease (NSP10/NSP14 complex) contributes to replication fidelity and resistance to nucleoside analogs. Through single turnover kinetic analysis, we show that the preferred substrate for the exonuclease is double-stranded RNA without any mismatches. Double-stranded RNA containing a 3'-terminal remdesivir was hydrolyzed at a rate similar to a correctly base-paired cognate nucleotide. Surprisingly, single-stranded RNA or duplex RNA containing a 3'-terminal mismatch was hydrolyzed at rates 125- and 45-fold slower, respectively, compared to the correctly base-paired double-stranded RNA. These results define the substrate specificity and rate of removal of remdesivir for the exonuclease and outline rigorous kinetic assays that could help in finding next-generation exonuclease inhibitors or nucleoside analogs that are able to evade excision. These results also raise important questions about the role of the polymerase/exonuclease complex in proofreading during viral replication. Addressing these questions through rigorous kinetic analysis will facilitate the search for desperately needed antiviral drugs to combat COVID-19.

Nucleoside analogs for management of respiratory virus infections: mechanism of action and clinical efficacy

The COVID-19 pandemic has accelerated the development of nucleoside analogs to treat respiratory virus infections, with remdesivir being the first compound to receive worldwide authorization and three other nucleoside analogs (i.e. favipiravir, molnupiravir, and bemnifosbuvir) in the pipeline. Here, we summarize the current knowledge concerning their clinical efficacy in suppressing the virus and reducing the need for hospitalization or respiratory support. We also mention trials of favipiravir and lumicitabine, for influenza and respiratory syncytial virus, respectively. Besides, we outline how nucleoside analogs interact with the polymerases of respiratory viruses, to cause lethal virus mutagenesis or disturbance of viral RNA synthesis. In this way, we aim to convey the key findings on this rapidly evolving class of respiratory virus medication.

Viral proteases as therapeutic targets

Some medically important viruses-including retroviruses, flaviviruses, coronaviruses, and herpesviruses-code for a protease, which is indispensable for viral maturation and pathogenesis. Viral protease inhibitors have become an important class of antiviral drugs. Development of the first-in-class viral protease inhibitor saquinavir, which targets HIV protease, started a new era in the treatment of chronic viral diseases. Combining several drugs that target different steps of the viral life cycle enables use of lower doses of individual drugs (and thereby reduction of potential side effects, which frequently occur during long term therapy) and reduces drug-resistance development. Currently, several HIV and HCV protease inhibitors are routinely used in clinical practice. In addition, a drug including an inhibitor of SARS-CoV-2 main protease, nirmatrelvir (co-administered with a pharmacokinetic booster ritonavir as Paxlovid®), was recently authorized for emergency use. This review summarizes the basic features of the proteases of human immunodeficiency virus (HIV), hepatitis C virus (HCV), and SARS-CoV-2 and discusses the properties of their inhibitors in clinical use, as well as development of compounds in the pipeline.