Structures of the N- and C-terminal domains of MHV-A59 nucleocapsid protein corroborate a conserved RNA-protein binding mechanism in coronavirus

Epitopes recognition of SARS-CoV-2 nucleocapsid RNA binding domain by human monoclonal antibodies

Coronavirus nucleocapsid protein (NP) of SARS-CoV-2 plays a central role in many functions important for virus proliferation including packaging and protecting genomic RNA. The protein shares sequence, structure, and architecture with nucleocapsid proteins from betacoronaviruses. The N-terminal domain (NPRBD) binds RNA and the C-terminal domain is responsible for dimerization. After infection, NP is highly expressed and triggers robust host immune response. The anti-NP antibodies are not protective and not neutralizing but can effectively detect viral proliferation soon after infection. Two structures of SARS-CoV-2 NPRBD were determined providing a continuous model from residue 48 to 173, including RNA binding region and key epitopes. Five structures of NPRBD complexes with human mAbs were isolated using an antigen-bait sorting. Complexes revealed a distinct complement-determining regions and unique sets of epitope recognition. This may assist in the early detection of pathogens and designing peptide-based vaccines. Mutations that significantly increase viral load were mapped on developed, full length NP model, likely impacting interactions with host proteins and viral RNA.

Host protein kinases required for SARS-CoV-2 nucleocapsid phosphorylation and viral replication

Multiple coronaviruses have emerged independently in the past 20 years that cause lethal human diseases. Although vaccine development targeting these viruses has been accelerated substantially, there remain patients requiring treatment who cannot be vaccinated or who experience breakthrough infections. Understanding the common host factors necessary for the life cycles of coronaviruses may reveal conserved therapeutic targets. Here, we used the known substrate specificities of mammalian protein kinases to deconvolute the sequence of phosphorylation events mediated by three host protein kinase families (SRPK, GSK-3, and CK1) that coordinately phosphorylate a cluster of serine and threonine residues in the viral N protein, which is required for viral replication. We also showed that loss or inhibition of SRPK1/2, which we propose initiates the N protein phosphorylation cascade, compromised the viral replication cycle. Because these phosphorylation sites are highly conserved across coronaviruses, inhibitors of these protein kinases not only may have therapeutic potential against COVID-19 but also may be broadly useful against coronavirus-mediated diseases.

3β-Acetoxy-21α-H-hop-22(29)ene, a novel multitargeted phytocompound against SARS-CoV-2: in silico screening

The present pandemic disease COVID-19 demands an urgent need for more efficient antiviral drugs against SARS-CoV-2. Computational drug designing and discovery enable us to explore ethnomedicinal plants as a source of various lead molecules that can be used against present and future pathogens. Adiantum latifolium Lam., a common fern, is resistant to pathogens mainly due to the presence of various phytochemicals having antimicrobial properties. In our previous study, 3β-acetoxy-21α-H-hop-22(29)ene, a terpenoid has been characterized from the methanol extract of leaves of A. latifolium. The manuscript evaluates the antiviral potency of the compound against SARS-CoV-2 through molecular docking method. Proteins essential for SARS-CoV-2 multiplication in host cells are the target sites. The study revealed strong binding affinity of the compound for all the ten proteins selected, including seven nonstructural proteins, two structural proteins and one receptor protein, with a binding energy of -4.67 to -8.76 kcal/mol. MDS and MMPBSA analysis of the best ranked complex further confirmed the results. The multitargeted compound can be considered as a natural lead molecule in drug designing against COVID-19, but requires wet-lab experimentation and clinical trials.Communicated by Ramaswamy H. Sarma.

Evolution, Interspecies Transmission, and Zoonotic Significance of Animal Coronaviruses

Coronaviruses are single-stranded RNA viruses that affect humans and a wide variety of animal species, including livestock, wild animals, birds, and pets. These viruses have an affinity for different tissues, such as those of the respiratory and gastrointestinal tract of most mammals and birds and the hepatic and nervous tissues of rodents and porcine. As coronaviruses target different host cell receptors and show divergence in the sequences and motifs of their structural and accessory proteins, they are classified into groups, which may explain the evolutionary relationship between them. The interspecies transmission, zoonotic potential, and ability to mutate at a higher rate and emerge into variants of concern highlight their importance in the medical and veterinary fields. The contribution of various factors that result in their evolution will provide better insight and may help to understand the complexity of coronaviruses in the face of pandemics. In this review, important aspects of coronaviruses infecting livestock, birds, and pets, in particular, their structure and genome organization having a bearing on evolutionary and zoonotic outcomes, have been discussed.

High-resolution structure and biophysical characterization of the nucleocapsid phosphoprotein dimerization domain from the Covid-19 severe acute respiratory syndrome coronavirus 2

Unprecedented by number of casualties and socio-economic burden occurring worldwide, the coronavirus disease 2019 (Covid-19) pandemic caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the worst health crisis of this century. In order to develop adequate countermeasures against Covid-19, identification and structural characterization of suitable antiviral targets within the SARS-CoV-2 protein repertoire is urgently needed. The nucleocapsid phosphoprotein (N) is a multifunctional and highly immunogenic determinant of virulence and pathogenicity, whose main functions consist in oligomerizing and packaging the single-stranded RNA (ssRNA) viral genome. Here we report the structural and biophysical characterization of the SARS-CoV-2 N C-terminal domain (CTD), on which both N homo-oligomerization and ssRNA binding depend. Crystal structures solved at 1.44 Å and 1.36 Å resolution describe a rhombus-shape N CTD dimer, which stably exists in solution as validated by size-exclusion chromatography coupled to multi-angle light scattering and analytical ultracentrifugation. Differential scanning fluorimetry revealed moderate thermal stability and a tendency towards conformational change. Microscale thermophoresis demonstrated binding to a 7-bp SARS-CoV-2 genomic ssRNA fragment at micromolar affinity. Furthermore, a low-resolution preliminary model of the full-length SARS-CoV N in complex with ssRNA, obtained by cryo-electron microscopy, provides an initial understanding of self-associating and RNA binding functions exerted by the SARS-CoV-2 N.

Structural Insight Into the SARS-CoV-2 Nucleocapsid Protein C-Terminal Domain Reveals a Novel Recognition Mechanism for Viral Transcriptional Regulatory Sequences

Coronavirus disease 2019 (COVID-19) has caused massive disruptions to society and the economy, and the transcriptional regulatory mechanisms behind the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are poorly understood. Herein, we determined the crystal structure of the SARS-CoV-2 nucleocapsid protein C-terminal domain (CTD) at a resolution of 2.0 Å, and demonstrated that the CTD has a comparable distinct electrostatic potential surface to equivalent domains of other reported CoVs, suggesting that the CTD has novel roles in viral RNA binding and transcriptional regulation. Further in vitro biochemical assays demonstrated that the viral genomic intergenic transcriptional regulatory sequences (TRSs) interact with the SARS-CoV-2 nucleocapsid protein CTD with a flanking region. The unpaired adeno dinucleotide in the TRS stem-loop structure is a major determining factor for their interactions. Taken together, these results suggested that the nucleocapsid protein CTD is responsible for the discontinuous viral transcription mechanism by recognizing the different patterns of viral TRS during transcription.

The FDA-approved drug Alectinib compromises SARS-CoV-2 nucleocapsid phosphorylation and inhibits viral infection in vitro

While vaccines are vital for preventing COVID-19 infections, it is critical to develop new therapies to treat patients who become infected. Pharmacological targeting of a host factor required for viral replication can suppress viral spread with a low probability of viral mutation leading to resistance. In particular, host kinases are highly druggable targets and a number of conserved coronavirus proteins, notably the nucleoprotein (N), require phosphorylation for full functionality. In order to understand how targeting kinases could be used to compromise viral replication, we used a combination of phosphoproteomics and bioinformatics as well as genetic and pharmacological kinase inhibition to define the enzymes important for SARS-CoV-2 N protein phosphorylation and viral replication. From these data, we propose a model whereby SRPK1/2 initiates phosphorylation of the N protein, which primes for further phosphorylation by GSK-3a/b and CK1 to achieve extensive phosphorylation of the N protein SR-rich domain. Importantly, we were able to leverage our data to identify an FDA-approved kinase inhibitor, Alectinib, that suppresses N phosphorylation by SRPK1/2 and limits SARS-CoV-2 replication. Together, these data suggest that repurposing or developing novel host-kinase directed therapies may be an efficacious strategy to prevent or treat COVID-19 and other coronavirus-mediated diseases.

Architecture and self-assembly of the SARS-CoV-2 nucleocapsid protein

The COVID-2019 pandemic is the most severe acute public health threat of the twenty-first century. To properly address this crisis with both robust testing and novel treatments, we require a deep understanding of the life cycle of the causative agent, the SARS-CoV-2 coronavirus. Here, we examine the architecture and self-assembly properties of the SARS-CoV-2 nucleocapsid protein, which packages viral RNA into new virions. We determined a 1.4 Å resolution crystal structure of this protein's N2b domain, revealing a compact, intertwined dimer similar to that of related coronaviruses including SARS-CoV. While the N2b domain forms a dimer in solution, addition of the C-terminal spacer B/N3 domain mediates formation of a homotetramer. Using hydrogen-deuterium exchange mass spectrometry, we find evidence that at least part of this putatively disordered domain is structured, potentially forming an α-helix that self-associates and cooperates with the N2b domain to mediate tetramer formation. Finally, we map the locations of amino acid substitutions in the N protein from over 38,000 SARS-CoV-2 genome sequences. We find that these substitutions are strongly clustered in the protein's N2a linker domain, and that substitutions within the N1b and N2b domains cluster away from their functional RNA binding and dimerization interfaces. Overall, this work reveals the architecture and self-assembly properties of a key protein in the SARS-CoV-2 life cycle, with implications for both drug design and antibody-based testing.

Structural characterization of the C-terminal domain of SARS-CoV-2 nucleocapsid protein

The newly emerging severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has resulted in a global human health crisis. The CoV nucleocapsid (N) protein plays essential roles both in the viral genomic RNA packaging and the regulation of host cellular machinery. Here, to contribute to the structural information of the N protein, we describe the 2.0 Å crystal structure of the SARS-CoV-2 N protein C-terminal domain (N-CTD). The structure indicates an extensive interaction dimer in a domain-swapped manner. The interface of this dimer was first thoroughly illustrated. Also, the SARS-CoV-2 N-CTD dimerization form was verified in solution using size-exclusion chromatography. Based on the structural comparison of the N-CTDs from alpha-, beta-, and gamma-CoVs, we demonstrate the common and specific characteristics of the SARS-CoV-2 N-CTD. Furthermore, we provide evidence that the SARS-CoV-2 N-CTD possesses the binding ability to single-stranded RNA, single-stranded DNA as well as double-stranded DNA in vitro. In conclusion, this study could potentially accelerate research to understand the complete biological functions of the new CoV N protein.

Identification of potential inhibitors against SARS-CoV-2 by targeting proteins responsible for envelope formation and virion assembly using docking based virtual screening, and pharmacokinetics approaches

WHO has declared the outbreak of COVID-19 as a public health emergency of international concern. The ever-growing new cases have called for an urgent emergency for specific anti-COVID-19 drugs. Three structural proteins (Membrane, Envelope and Nucleocapsid protein) play an essential role in the assembly and formation of the infectious virion particles. Thus, the present study was designed to identify potential drug candidates from the unique collection of 548 anti-viral compounds (natural and synthetic anti-viral), which target SARS-CoV-2 structural proteins. High-end molecular docking analysis was performed to characterize the binding affinity of the selected drugs-the ligand, with the SARS-CoV-2 structural proteins, while high-level Simulation studies analyzed the stability of drug-protein interactions. The present study identified rutin, a bioflavonoid and the antibiotic, doxycycline, as the most potent inhibitor of SARS-CoV-2 envelope protein. Caffeic acid and ferulic acid were found to inhibit SARS-CoV-2 membrane protein while the anti-viral agent's simeprevir and grazoprevir showed a high binding affinity for nucleocapsid protein. All these compounds not only showed excellent pharmacokinetic properties, absorption, metabolism, minimal toxicity and bioavailability but were also remain stabilized at the active site of proteins during the MD simulation. Thus, the identified lead compounds may act as potential molecules for the development of effective drugs against SARS-CoV-2 by inhibiting the envelope formation, virion assembly and viral pathogenesis.

Architecture and self-assembly of the SARS-CoV-2 nucleocapsid protein

The COVID-2019 pandemic is the most severe acute public health threat of the twenty-first century. To properly address this crisis with both robust testing and novel treatments, we require a deep understanding of the life cycle of the causative agent, the SARS-CoV-2 coronavirus. Here, we examine the architecture and self-assembly properties of the SARS-CoV-2 nucleocapsid protein, which packages viral RNA into new virions. We determined a 1.4 Å resolution crystal structure of this protein's N2b domain, revealing a compact, intertwined dimer similar to that of related coronaviruses including SARS-CoV. While the N2b domain forms a dimer in solution, addition of the C-terminal spacer B/N3 domain mediates formation of a homotetramer. Using hydrogen-deuterium exchange mass spectrometry, we find evidence that at least part of this putatively disordered domain is structured, potentially forming an α-helix that self-associates and cooperates with the N2b domain to mediate tetramer formation. Finally, we map the locations of amino acid substitutions in the N protein from over 38,000 SARS-CoV-2 genome sequences. We find that these substitutions are strongly clustered in the protein's N2a linker domain, and that substitutions within the N1b and N2b domains cluster away from their functional RNA binding and dimerization interfaces. Overall, this work reveals the architecture and self-assembly properties of a key protein in the SARS-CoV-2 life cycle, with implications for both drug design and antibody-based testing.

Nucleocapsid Protein Recruitment to Replication-Transcription Complexes Plays a Crucial Role in Coronaviral Life Cycle

CoVs have long been regarded as relatively harmless pathogens for humans. Severe respiratory tract infection outbreaks caused by severe acute respiratory syndrome CoV and Middle East respiratory syndrome CoV, however, have caused high pathogenicity and mortality rates in humans. These outbreaks highlighted the relevance of being able to control CoV infections. We used a model CoV, MHV, to investigate the importance of the recruitment of N protein, a central component of CoV virions, to intracellular platforms where CoVs replicate, transcribe, and translate their genomes. By identifying the principal binding partner at these intracellular platforms and generating a specific mutant, we found that N protein recruitment to these locations is crucial for promoting viral RNA synthesis. Moreover, blocking this recruitment strongly inhibits viral infection. Thus, our results explain an important aspect of the CoV life cycle and reveal an interaction of viral proteins that could be targeted in antiviral therapies.

Coronavirus (CoV) nucleocapsid (N) proteins are key for incorporating genomic RNA into progeny viral particles. In infected cells, N proteins are present at the replication-transcription complexes (RTCs), the sites of CoV RNA synthesis. It has been shown that N proteins are important for viral replication and that the one of mouse hepatitis virus (MHV), a commonly used model CoV, interacts with nonstructural protein 3 (nsp3), a component of the RTCs. These two aspects of the CoV life cycle, however, have not been linked. We found that the MHV N protein binds exclusively to nsp3 and not other RTC components by using a systematic yeast two-hybrid approach, and we identified two distinct regions in the N protein that redundantly mediate this interaction. A selective N protein variant carrying point mutations in these two regions fails to bind nsp3 in vitro, resulting in inhibition of its recruitment to RTCs in vivo. Furthermore, in contrast to the wild-type N protein, this N protein variant impairs the stimulation of genomic RNA and viral mRNA transcription in vivo and in vitro, which in turn leads to impairment of MHV replication and progeny production. Altogether, our results show that N protein recruitment to RTCs, via binding to nsp3, is an essential step in the CoV life cycle because it is critical for optimal viral RNA synthesis.

IMPORTANCE CoVs have long been regarded as relatively harmless pathogens for humans. Severe respiratory tract infection outbreaks caused by severe acute respiratory syndrome CoV and Middle East respiratory syndrome CoV, however, have caused high pathogenicity and mortality rates in humans. These outbreaks highlighted the relevance of being able to control CoV infections. We used a model CoV, MHV, to investigate the importance of the recruitment of N protein, a central component of CoV virions, to intracellular platforms where CoVs replicate, transcribe, and translate their genomes. By identifying the principal binding partner at these intracellular platforms and generating a specific mutant, we found that N protein recruitment to these locations is crucial for promoting viral RNA synthesis. Moreover, blocking this recruitment strongly inhibits viral infection. Thus, our results explain an important aspect of the CoV life cycle and reveal an interaction of viral proteins that could be targeted in antiviral therapies.

Nucleocapsid proteins from other swine enteric coronaviruses differentially modulate PEDV replication

Porcine epidemic diarrhea virus (PEDV), transmissible gastroenteritis virus (TGEV) and porcine deltacoronavirus (PDCoV) share tropism for swine intestinal epithelial cells. Whether mixing of viral components during co-infection alters pathogenic outcomes or viral replication is not known. In this study, we investigated how different coronavirus nucleocapsid (CoV N) proteins interact and affect PEDV replication. We found that PDCoV N and TGEV N can competitively interact with PEDV N. However, the presence of PDCoV or TGEV N led to very different outcomes on PEDV replication. While PDCoV N significantly suppresses PEDV replication, overexpression of TGEV N, like that of PEDV N, increases production of PEDV RNA and virions. Despite partial interchangeability in nucleocapsid oligomerization and viral RNA synthesis, endogenous PEDV N cannot be replaced in the production of infectious PEDV particles. Results from this study give insights into functional compatibilities and evolutionary relationship between CoV viral proteins during viral co-infection and co-evolution.

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PDCoV N and TGEV N interact with PEDV N in a competitive, RNA-dependent manner.

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PEDV replication in cell culture is enhanced by overexpression of TGEV or PEDV N but strongly suppressed by that of PDCoV N.

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Both TGEV and PDCoV N can partially rescue viral RNA and protein synthesis functions of PEDV N, albeit to different degrees.

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Neither TGEV nor PDCoV N can completely replace PEDV N in the production of PEDV infectious virions.

Porcine Deltacoronavirus Nucleocapsid Protein Suppressed IFN-β Production by Interfering Porcine RIG-I dsRNA-Binding and K63-Linked Polyubiquitination

Porcine deltacoronavirus (PDCoV) is a newly detected porcine coronavirus causing serious vomiting and diarrhea in piglets, especially newborn piglets. There has been an outbreak of PDCoV in worldwide since 2014, causing significant economic losses in the pig industry. The interferon (IFN)-mediated antiviral response is an important component of virus-host interactions and plays an essential role in inhibiting virus infection. However, the mechanism of PDCoV escaping the porcine immune surveillance is unclear. In the present study, we demonstrated that the PDCoV nucleocapsid (N) protein antagonizes porcine IFN-β production after vesicular stomatitis virus (VSV) infection or poly(I:C) stimulation. PDCoV N protein also suppressed the activation of porcine IFN-β promoter when it was stimulated by porcine RLR signaling molecules. PDCoV N protein targeted porcine retinoic acid-inducible gene I (pRIG-I) and porcine TNF receptor associated factor 3 (pTRAF3) by directly interacting with them. The N-terminal region (1–246 aa) of PDCoV N protein was important for interacting with pRIG-I and interfere its function. We confirmed that PDCoV N antagonizes IFN-β production by associating with pRIG-I to impede it from binding double-stranded RNA. Furthermore, porcine Riplet (pRiplet) was an important activator for pRIG-I by mediating the K63-linked polyubiquitination. However, PDCoV N protein restrained the pRiplet binding pRIG-I to inhibit pRIG-I K63-linked polyubiquitination. Taken together, our results revealed a novel mechanism by which PDCoV N protein interferes with the early activation of pRIG-I in the host antiviral response. The novel findings provide a new insight into PDCoV on evading the host innate immune response and may provide new therapeutic targets and more efficacious vaccines strategies for PDCoV infections.

Two critical N-terminal epitopes of the nucleocapsid protein contribute to the cross-reactivity between porcine epidemic diarrhea virus and porcine transmissible gastroenteritis virus

Both porcine epidemic diarrhoea virus (PEDV) and porcine transmissible gastroenteritis virus (TGEV), which cause high mortality in piglets and produce similar clinical symptoms and histopathological morphology, belong to the genus Alphacoronavirus. Serological diagnosis plays an important role in distinguishing pathogen species. Together with the spike (S) protein, the nucleocapsid (N) protein is one of the immunodominant regions among coronaviruses. In this study, two-way antigenic cross-reactivity between the N proteins of PEDV and TGEV was observed by indirect immunofluorescence assay (IFA) and Western blot analysis. Furthermore, the PEDV N protein harbouring truncations of amino acids (aa) 1 to 170 or aa 125 to 301 was demonstrated to cross-react with the anti-TGEV N polyclonal antibody (PAb), whereas the truncation-expressing aa 302 to 401 resulted in a specific reaction with the anti-PEDV N PAb but not with the anti-TGEV N PAb. Mutants of the PEDV N protein were generated based on sequence alignment and structural analysis; we then confirmed that the N-terminal residues 58-RWRMRRGERIE-68 and 78-LGTGPHAD-85 contributed to the cross-reactivity. All the results provide vital clues for the development of precise diagnostic assays for porcine coronaviruses.

Structure and oligomerization state of the C-terminal region of the Middle East respiratory syndrome coronavirus nucleoprotein

Middle East respiratory syndrome coronavirus (MERS-CoV) is a human pathogen responsible for a severe respiratory illness that emerged in 2012. Structural information about the proteins that constitute the viral particle is scarce. In order to contribute to a better understanding of the nucleoprotein (N) in charge of RNA genome encapsidation, the structure of the C-terminal domain of N from MERS-CoV obtained using single-crystal X-ray diffraction is reported here at 1.97 Å resolution. The molecule is present as a dimer in the crystal structure and this oligomerization state is confirmed in solution, as measured by additional methods including small-angle X-ray scattering measurements. Comparisons with the structures of the C-terminal domains of N from other coronaviruses reveals a high degree of structural conservation despite low sequence conservation, and differences in electrostatic potential at the surface of the protein.

Nucleocapsid protein-dependent assembly of the RNA packaging signal of Middle East respiratory syndrome coronavirus

BACKGROUND

Middle East respiratory syndrome coronavirus (MERS-CoV) consists of a positive-sense, single-stranded RNA genome and four structural proteins: the spike, envelope, membrane, and nucleocapsid protein. The assembly of the viral genome into virus particles involves viral structural proteins and is believed to be mediated through recognition of specific sequences and RNA structures of the viral genome.

METHODS AND RESULTS

A culture system for the production of MERS coronavirus-like particles (MERS VLPs) was determined and established by electron microscopy and the detection of coexpressed viral structural proteins. Using the VLP system, a 258-nucleotide RNA fragment, which spans nucleotides 19,712 to 19,969 of the MERS-CoV genome (designated PS258(19712-19969)_ME), was identified to function as a packaging signal. Assembly of the RNA packaging signal into MERS VLPs is dependent on the viral nucleocapsid protein. In addition, a 45-nucleotide stable stem-loop substructure of the PS258(19712-19969)_ME interacted with both the N-terminal domain and the C-terminal domain of the viral nucleocapsid protein. Furthermore, a functional SARS-CoV RNA packaging signal failed to assemble into the MERS VLPs, which indicated virus-specific assembly of the RNA genome.

CONCLUSIONS

A MERS-oV RNA packaging signal was identified by the detection of GFP expression following an incubation of MERS VLPs carrying the heterologous mRNA GFP-PS258(19712-19969)_ME with virus permissive Huh7 cells. The MERS VLP system could help us in understanding virus infection and morphogenesis.

The nucleocapsid proteins of mouse hepatitis virus and severe acute respiratory syndrome coronavirus share the same IFN-β antagonizing mechanism: attenuation of PACT-mediated RIG-I/ MDA5 activation

Coronaviruses (CoVs) are a huge threat to both humans and animals and have evolved elaborate mechanisms to antagonize interferons (IFNs). Nucleocapsid (N) protein is the most abundant viral protein in CoV-infected cells, and has been identified as an innate immunity antagonist in several CoVs, including mouse hepatitis virus (MHV) and severe acute respiratory syndrome (SARS)-CoV. However, the underlying molecular mechanism(s) remain unclear. In this study, we found that MHV N protein inhibited Sendai virus and poly(I:C)-induced IFN-β production by targeting a molecule upstream of retinoic acid-induced gene I (RIG-I) and melanoma differentiation gene 5 (MDA5). Further studies showed that both MHV and SARS-CoV N proteins directly interacted with protein activator of protein kinase R (PACT), a cellular dsRNA-binding protein that can bind to RIG-I and MDA5 to activate IFN production. The N–PACT interaction sequestered the association of PACT and RIG-I/MDA5, which in turn inhibited IFN-β production. However, the N proteins from porcine epidemic diarrhea virus (PEDV) and porcine reproductive and respiratory syndrome virus (PRRSV), which are also classified in the order Nidovirales, did not interact and counteract with PACT. Taken together, our present study confirms that both MHV and SARS-CoV N proteins can perturb the function of cellular PACT to circumvent the innate antiviral response. However, this strategy does not appear to be used by all CoVs N proteins.

Coronavirus nucleocapsid proteins assemble constitutively in high molecular oligomers

Coronaviruses (CoV) are enveloped viruses and rely on their nucleocapsid N protein to incorporate the positive-stranded genomic RNA into the virions. CoV N proteins form oligomers but the mechanism and relevance underlying their multimerization remain to be fully understood. Using in vitro pull-down experiments and density glycerol gradients, we found that at least 3 regions distributed over its entire length mediate the self-interaction of mouse hepatitis virus (MHV) and severe acute respiratory syndrome coronavirus (SARS-CoV) N protein. The fact that these regions can bind reciprocally between themselves provides a possible molecular basis for N protein oligomerization. Interestingly, cytoplasmic N molecules of MHV-infected cells constitutively assemble into oligomers through a process that does not require binding to genomic RNA. Based on our data, we propose a model where constitutive N protein oligomerization allows the optimal loading of the genomic viral RNA into a ribonucleoprotein complex via the presentation of multiple viral RNA binding motifs.

Supramolecular Architecture of the Coronavirus Particle

Coronavirus particles serve three fundamentally important functions in infection. The virion provides the means to deliver the viral genome across the plasma membrane of a host cell. The virion is also a means of escape for newly synthesized genomes. Lastly, the virion is a durable vessel that protects the genome on its journey between cells. This review summarizes the available X-ray crystallography, NMR, and cryoelectron microscopy structural data for coronavirus structural proteins, and looks at the role of each of the major structural proteins in virus entry and assembly. The potential wider conservation of the nucleoprotein fold identified in the Arteriviridae and Coronaviridae families and a speculative model for the evolution of corona-like virus architecture are discussed.

Structural characterization of the N-terminal part of the MERS-CoV nucleocapsid by X-ray diffraction and small-angle X-ray scattering

The N protein of coronaviruses is a multifunctional protein that is organized into several domains. The N‐terminal part is composed of an intrinsically disordered region (IDR) followed by a structured domain called the N‐terminal domain (NTD). In this study, the structure determination of the N‐terminal region of the MERS‐CoV N protein via X‐ray diffraction measurements is reported at a resolution of 2.4 Å. Since the first 30 amino acids were not resolved by X‐ray diffraction, the structural study was completed by a SAXS experiment to propose a structural model including the IDR. This model presents the N‐terminal region of the MERS‐CoV as a monomer that displays structural features in common with other coronavirus NTDs.

Structure-based virtual screening and experimental validation of the discovery of inhibitors targeted towards the human coronavirus nucleocapsid protein

Nucleocapsid protein (NP), an essential RNA-binding viral protein in human coronavirus (CoV)-infected cells, is an antiviral target for drug discovery.

Recent insights into the development of therapeutics against coronavirus diseases by targeting N protein

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Coronavirus nucleocapsid proteins are appealing drug targets against coronavirus-induced diseases.

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A variety of compounds targeting the coronavirus nucleocapsid protein have been developed.

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Many of these compounds show potential antiviral activity.

The advent of severe acute respiratory syndrome (SARS) in the 21st century and the recent outbreak of Middle-East respiratory syndrome (MERS) highlight the importance of coronaviruses (CoVs) as human pathogens, emphasizing the need for development of novel antiviral strategies to combat acute respiratory infections caused by CoVs. Recent studies suggest that nucleocapsid (N) proteins from coronaviruses and other viruses can be useful antiviral drug targets against viral infections. This review aims to provide readers with a concise survey of the structural features of coronavirus N proteins and how these features provide insights into structure-based development of therapeutics against coronaviruses. We will also present our latest results on MERS-CoV N protein and its potential as an antiviral drug target.

Crystallographic analysis of the N-terminal domain of Middle East respiratory syndrome coronavirus nucleocapsid protein

The N-terminal domain of the nucleocapsid protein from Middle East respiratory syndrome coronavirus (MERS-CoV NP-NTD) contains many positively charged residues and has been identified to be responsible for RNA binding during ribonucleocapsid formation by the virus. In this study, the crystallization and crystallographic analysis of MERS-CoV NP-NTD (amino acids 39-165), with a molecular weight of 14.7 kDa, are reported. MERS-CoV NP-NTD was crystallized at 293 K using PEG 3350 as a precipitant and a 94.5% complete native data set was collected from a cooled crystal at 77 K to 2.63 Å resolution with an overall Rmerge of 9.6%. The crystals were monoclinic and belonged to space group P21, with unit-cell parameters a = 35.60, b = 109.64, c = 91.99 Å, β = 101.22°. The asymmetric unit contained four MERS-CoV NP-NTD molecules.

SHAPE analysis of the RNA secondary structure of the Mouse Hepatitis Virus 5' untranslated region and N-terminal nsp1 coding sequences

SHAPE technology was used to analyze RNA secondary structure of the 5' most 474 nts of the MHV-A59 genome encompassing the minimal 5' cis-acting region required for defective interfering RNA replication. The structures generated were in agreement with previous characterizations of SL1 through SL4 and two recently predicted secondary structure elements, S5 and SL5A. SHAPE provided biochemical support for four additional stem-loops not previously functionally investigated in MHV. Secondary structure predictions for 5' regions of MHV-A59, BCoV and SARS-CoV were similar despite high sequence divergence. The pattern of SHAPE reactivity of in virio genomic RNA, ex virio genomic RNA, and in vitro synthesized RNA was similar, suggesting that binding of N protein or other proteins to virion RNA fails to protect the RNA from reaction with lipid permeable SHAPE reagent. Reverse genetic experiments suggested that SL5C and SL6 within the nsp1 coding sequence are not required for viral replication.

The coronavirus nucleocapsid is a multifunctional protein

The coronavirus nucleocapsid (N) is a structural protein that forms complexes with genomic RNA, interacts with the viral membrane protein during virion assembly and plays a critical role in enhancing the efficiency of virus transcription and assembly. Recent studies have confirmed that N is a multifunctional protein. The aim of this review is to highlight the properties and functions of the N protein, with specific reference to (i) the topology; (ii) the intracellular localization and (iii) the functions of the protein.

Structural basis for the identification of the N-terminal domain of coronavirus nucleocapsid protein as an antiviral target

Coronaviruses (CoVs) cause numerous diseases, including Middle East respiratory syndrome and severe acute respiratory syndrome, generating significant health-related and economic consequences. CoVs encode the nucleocapsid (N) protein, a major structural protein that plays multiple roles in the virus replication cycle and forms a ribonucleoprotein complex with the viral RNA through the N protein’s N-terminal domain (N-NTD). Using human CoV-OC43 (HCoV-OC43) as a model for CoV, we present the 3D structure of HCoV-OC43 N-NTD complexed with ribonucleoside 5′-monophosphates to identify a distinct ribonucleotide-binding pocket. By targeting this pocket, we identified and developed a new coronavirus N protein inhibitor, N-(6-oxo-5,6-dihydrophenanthridin-2-yl)(N,N-dimethylamino)acetamide hydrochloride (PJ34), using virtual screening; this inhibitor reduced the N protein’s RNA-binding affinity and hindered viral replication. We also determined the crystal structure of the N-NTD–PJ34 complex. On the basis of these findings, we propose guidelines for developing new N protein-based antiviral agents that target CoVs.

Recognition of the murine coronavirus genomic RNA packaging signal depends on the second RNA-binding domain of the nucleocapsid protein

The coronavirus nucleocapsid (N) protein forms a helical ribonucleoprotein with the viral positive-strand RNA genome and binds to the principal constituent of the virion envelope, the membrane (M) protein, to facilitate assembly and budding. Besides these structural roles, N protein associates with a component of the replicase-transcriptase complex, nonstructural protein 3, at a critical early stage of infection. N protein has also been proposed to participate in the replication and selective packaging of genomic RNA and the transcription and translation of subgenomic mRNA. Coronavirus N proteins contain two structurally distinct RNA-binding domains, an unusual characteristic among RNA viruses. To probe the functions of these domains in the N protein of the model coronavirus mouse hepatitis virus (MHV), we constructed mutants in which each RNA-binding domain was replaced by its counterpart from the N protein of severe acute respiratory syndrome coronavirus (SARS-CoV). Mapping of revertants of the resulting chimeric viruses provided evidence for extensive intramolecular interactions between the two RNA-binding domains. Through analysis of viral RNA that was packaged into virions we identified the second of the two RNA-binding domains as a principal determinant of MHV packaging signal recognition. As expected, the interaction of N protein with M protein was not affected in either of the chimeric viruses. Moreover, the SARS-CoV N substitutions did not alter the fidelity of leader-body junction formation during subgenomic mRNA synthesis. These results more clearly delineate the functions of N protein and establish a basis for further exploration of the mechanism of genomic RNA packaging.

IMPORTANCE

This work describes the interactions of the two RNA-binding domains of the nucleocapsid protein of a model coronavirus, mouse hepatitis virus. The main finding is that the second of the two domains plays an essential role in recognizing the RNA structure that allows the selective packaging of genomic RNA into assembled virions.

The SARS coronavirus nucleocapsid protein--forms and functions

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Coronavirus N proteins share the same modular organization.

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Structures of SARS-CoV N protein provide insight into nucleocapsid formation.

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N protein binds to nucleic acid at multiple sites in a coupled-allostery manner.

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A RNP packaging model highlighting the importance of disorder and modularity is proposed.

The nucleocapsid phosphoprotein of the severe acute respiratory syndrome coronavirus (SARS-CoV N protein) packages the viral genome into a helical ribonucleocapsid (RNP) and plays a fundamental role during viral self-assembly. It is a protein with multifarious activities. In this article we will review our current understanding of the N protein structure and its interaction with nucleic acid. Highlights of the progresses include uncovering the modular organization, determining the structures of the structural domains, realizing the roles of protein disorder in protein–protein and protein–nucleic acid interactions, and visualizing the ribonucleoprotein (RNP) structure inside the virions. It was also demonstrated that N-protein binds to nucleic acid at multiple sites with a coupled-allostery manner. We propose a SARS-CoV RNP model that conforms to existing data and bears resemblance to the existing RNP structures of RNA viruses. The model highlights the critical role of modular organization and intrinsic disorder of the N protein in the formation and functions of the dynamic RNP capsid in RNA viruses. This paper forms part of a symposium in Antiviral Research on “From SARS to MERS: 10 years of research on highly pathogenic human coronaviruses.”

Characterization of a critical interaction between the coronavirus nucleocapsid protein and nonstructural protein 3 of the viral replicase-transcriptase complex

The coronavirus nucleocapsid protein (N) plays an essential structural role in virions through a network of interactions with positive-strand viral genomic RNA, the envelope membrane protein (M), and other N molecules. Additionally, N protein participates in at least one stage of the complex mechanism of coronavirus RNA synthesis. We previously uncovered an unanticipated interaction between N and the largest subunit of the viral replicase-transcriptase complex, nonstructural protein 3 (nsp3). This was found through analysis of revertants of a severely defective mutant of murine hepatitis virus (MHV) in which the N gene was replaced with that of its close relative, bovine coronavirus (BCoV). In the work reported here, we constructed BCoV chimeras and other mutants of MHV nsp3 and obtained complementary genetic evidence for its association with N protein. We found that the N-nsp3 interaction maps to the amino-terminal ubiquitin-like domain of nsp3, which is essential for the virus. The interaction does not require the adjacent acidic domain of nsp3, which is dispensable. In addition, we demonstrated a complete correspondence between N-nsp3 genetic interactions and the ability of N protein to enhance the infectivity of transfected coronavirus genomic RNA. The latter function of N was shown to depend on both of the RNA-binding domains of N, as well as on the serine- and arginine-rich central region of N, which binds nsp3. Our results support a model in which the N-nsp3 interaction serves to tether the genome to the newly translated replicase-transcriptase complex at a very early stage of infection.

The cellular interactome of the coronavirus infectious bronchitis virus nucleocapsid protein and functional implications for virus biology

The coronavirus nucleocapsid (N) protein plays a multifunctional role in the virus life cycle, from regulation of replication and transcription and genome packaging to modulation of host cell processes. These functions are likely to be facilitated by interactions with host cell proteins. The potential interactome of the infectious bronchitis virus (IBV) N protein was mapped using stable isotope labeling with amino acids in cell culture (SILAC) coupled to a green fluorescent protein-nanotrap pulldown methodology and liquid chromatography-tandem mass spectrometry. The addition of the SILAC label allowed discrimination of proteins that were likely to specifically bind to the N protein over background binding. Overall, 142 cellular proteins were selected as potentially binding to the N protein, many as part of larger possible complexes. These included ribosomal proteins, nucleolar proteins, translation initiation factors, helicases, and hnRNPs. The association of selected cellular proteins with IBV N protein was confirmed by immunoblotting, cosedimentation, and confocal microscopy. Further, the localization of selected proteins in IBV-infected cells as well as their activity during virus infection was assessed by small interfering RNA-mediated depletion, demonstrating the functional importance of cellular proteins in the biology of IBV. This interactome not only confirms previous observations made with other coronavirus and IBV N proteins with both overexpressed proteins and infectious virus but also provides novel data that can be exploited to understand the interaction between the virus and the host cell.

Crystal structure-based exploration of the important role of Arg106 in the RNA-binding domain of human coronavirus OC43 nucleocapsid protein

Human coronavirus OC43 (HCoV-OC43) is a causative agent of the common cold. The nucleocapsid (N) protein, which is a major structural protein of CoVs, binds to the viral RNA genome to form the virion core and results in the formation of the ribonucleoprotein (RNP) complex. We have solved the crystal structure of the N-terminal domain of HCoV-OC43 N protein (N-NTD) (residues 58 to 195) to a resolution of 2.0Å. The HCoV-OC43 N-NTD is a single domain protein composed of a five-stranded β-sheet core and a long extended loop, similar to that observed in the structures of N-NTDs from other coronaviruses. The positively charged loop of the HCoV-OC43 N-NTD contains a structurally well-conserved positively charged residue, R106. To assess the role of R106 in RNA binding, we undertook a series of site-directed mutagenesis experiments and docking simulations to characterize the interaction between R106 and RNA. The results show that R106 plays an important role in the interaction between the N protein and RNA. In addition, we showed that, in cells transfected with plasmids that encoded the mutant (R106A) N protein and infected with virus, the level of the matrix protein gene was decreased by 7-fold compared to cells that were transfected with the wild-type N protein. This finding suggests that R106, by enhancing binding of the N protein to viral RNA plays a critical role in the viral replication. The results also indicate that the strength of N protein/RNA interactions is critical for HCoV-OC43 replication.

Functions of proteins of porcine epidemic diarrhea virus

Porcine epidemic diarrhea (PED) is caused by porcine epidemic diarrhea virus (PEDV), which belongs to the order Nidovirales in the family Coronaviridae. Since PED was first discovered in Europe in 1987, it has become a severe infectious disease in Asian countries (including China, Korea, Japan, Philippines and Thailand) and has caused great economic losses in swine industry. Although bivalent vaccine for PEDV and transmissible gastroenteritis virus (TGEV, another porcine coronavirus) can decrease the incidence of both diseases, PED is still increasing annually in China. Unfortunately, there have been few studies on viral proteins and pathogenesis of PEDV. This review aims to provide some functional insights on PEDV viral proteins based on the available information from coronaviruse studies.

Purification, crystallization and preliminary X-ray analysis of nonstructural protein 2 (nsp2) from avian infectious bronchitis virus

Nonstructural protein 2 from avian infectious bronchitis virus has been overexpressed in E. coli, purified and crystallized. Diffraction data were collected to 2.8 Å resolution.

Avian infectious bronchitis virus (IBV) is a member of the group III coronaviruses, which differ from the other groups of coronaviruses in that they do not encode the essential pathogenic factor nonstructural protein 1 (nsp1) and instead start with nsp2. IBV nsp2 is one of the first replicase proteins to be translated and processed in the viral life cycle; however, it has an entirely unknown function. In order to better understand the structural details and functional mechanism of IBV nsp2, the recombinant protein was cloned, overexpressed in Escherichia coli, purified and crystallized. The crystals diffracted to 2.8 Å resolution and belonged to space group P2₁, with unit-cell parameters a = 57.0, b = 192.3, c = 105.7 Å, β = 90.8°. Two molecules were found in the asymmetric unit; the Matthews coefficient was 3.9 ų Da⁻¹, corresponding to a solvent content of 68.2%.

Functional transcriptional regulatory sequence (TRS) RNA binding and helix destabilizing determinants of murine hepatitis virus (MHV) nucleocapsid (N) protein

Coronavirus (CoV) nucleocapsid (N) protein contains two structurally independent RNA binding domains. These are denoted N-terminal domain (NTD) and C-terminal domain and are joined by a charged linker region rich in serine and arginine residues (SR linker). In mouse hepatitis virus (MHV), the NTD binds the transcriptional regulatory sequence (TRS) RNA, a conserved hexanucleotide sequence required for subgenomic RNA synthesis. The NTD is also capable of disrupting a short RNA duplex. We show here that three residues on the β3 (Arg-125 and Tyr-127) and β5 (Tyr-190) strands play key roles in TRS RNA binding and helix destabilization with Ala substitutions of these residues lethal to the virus. NMR studies of the MHV NTD·TRS complex revealed that this region defines a major RNA binding interface in MHV with site-directed spin labeling studies consistent with a model in which the adenosine-rich 3'-region of TRS is anchored by Arg-125, Tyr-127, and Tyr-190 in a way that is critical for efficient subgenomic RNA synthesis in MHV. Characterization of CoV N NTDs from infectious bronchitis virus and from severe acute respiratory syndrome CoV revealed that, although detailed NTD-TRS determinants are distinct from those of MHV NTD, rapid helix destabilization activity of CoV N NTDs is most strongly correlated with CoV function and virus viability.