Temperature-sensitive mutants and revertants in the coronavirus nonstructural protein 5 protease (3CLpro) define residues involved in long-distance communication and regulation of protease activity

Biodiversity, Biochemical Profiling, and Pharmaco-Commercial Applications of Withania somnifera: A Review

Withania somnifera L. Dunal (Ashwagandha), a key medicinal plant native to India, is used globally to manage various ailments. This review focuses on the traditional uses, botany, phytochemistry, and pharmacological advances of its plant-derived constituents. It has been reported that at least 62 crucial and 48 inferior primary and secondary metabolites are present in the W. somnifera leaves, and 29 among these found in its roots and leaves are chiefly steroidal compounds, steroidal lactones, alkaloids, amino acids, etc. In addition, the whole shrub parts possess various medicinal activities such as anti-leukotriene, antineoplastic, analgesic, anti-oxidant, immunostimulatory, and rejuvenating properties, mainly observed by in vitro demonstration. However, the course of its medical use remains unknown. This review provides a comprehensive understanding of W. somnifera, which will be useful for mechanism studies and potential medical applications of W. somnifera, as well as for the development of a rational quality control system for W. somnifera as a therapeutic material in the future.

A Temperature-Sensitive Recombinant of Avian Coronavirus Infectious Bronchitis Virus Provides Complete Protection against Homologous Challenge

Avian coronavirus infectious bronchitis virus (IBV) is the etiological agent of infectious bronchitis, an acute highly contagious economically relevant respiratory disease of poultry. Vaccination is used to control IBV infections, with live-attenuated vaccines generated via serial passage of a virulent field isolate through embryonated hens' eggs. A fine balance must be achieved between attenuation and the retention of immunogenicity. The exact molecular mechanism of attenuation is unknown, and vaccines produced in this manner present a risk of reversion to virulence as few consensus level changes are acquired. Our previous research resulted in the generation of a recombinant IBV (rIBV) known as M41-R, based on a pathogenic strain M41-CK. M41-R was attenuated in vivo by two amino acid changes, Nsp10-Pro85Leu and Nsp14-Val393Leu; however, the mechanism of attenuation was not determined. Pro85 and Val393 were found to be conserved among not only IBV strains but members of the wider coronavirus family. This study demonstrates that the same changes are associated with a temperature-sensitive (ts) replication phenotype at 41°C in vitro, suggesting that the two phenotypes may be linked. Vaccination of specific-pathogen-free chickens with M41-R induced 100% protection against clinical disease, tracheal ciliary damage, and challenge virus replication following homologous challenge with virulent M41-CK. Temperature sensitivity has been used to rationally attenuate other viral pathogens, including influenza, and the identification of amino acid changes that impart both a ts and an attenuated phenotype may therefore offer an avenue for future coronavirus vaccine development. IMPORTANCE Infectious bronchitis virus is a pathogen of economic and welfare concern for the global poultry industry. Live-attenuated vaccines against are generated by serial passage of a virulent isolate in embryonated eggs until attenuation is achieved. The exact mechanisms of attenuation are unknown, and vaccines produced have a risk of reversion to virulence. Reverse genetics provides a method to generate vaccines that are rationally attenuated and are more stable with respect to back selection due to their clonal origin. Genetic populations resulting from molecular clones are more homogeneous and lack the presence of parental pathogenic viruses, which generation by multiple passage does not. In this study, we identified two amino acids that impart a temperature-sensitive replication phenotype. Immunogenicity is retained and vaccination results in 100% protection against homologous challenge. Temperature sensitivity, used for the development of vaccines against other viruses, presents a method for the development of coronavirus vaccines.

Proteolytic Processing of the Coronavirus Replicase Nonstructural Protein 14 Exonuclease Is Not Required for Virus Replication but Alters RNA Synthesis and Viral Fitness

Coronaviruses (CoVs) initiate replication by translation of the positive-sense RNA genome into the replicase polyproteins connecting 16 nonstructural protein domains (nsp1-16), which are subsequently processed by viral proteases to yield mature nsp. For the betacoronavirus murine hepatitis virus (MHV), total inhibition of translation or proteolytic processing of replicase polyproteins results in rapid cessation of RNA synthesis. The nsp5-3CLpro (Mpro) processes nsps7-16, which assemble into functional replication-transcription complexes (RTCs), including the enzymatic nsp12-RdRp and nsp14-exoribonuclease (ExoN)/N7-methyltransferase. The nsp14-ExoN activity mediates RNA-dependent RNA proofreading, high-fidelity RNA synthesis, and replication. To date, the solved partial RTC structures, biochemistry, and models use or assume completely processed, mature nsp. Here, we demonstrate that in MHV, engineered deletion of the cleavage sites between nsp13-14 and nsp14-15 allowed recovery of replication-competent virus. Compared to wild-type (WT) MHV, the nsp13-14 and nsp14-15 cleavage deletion mutants demonstrated delayed replication kinetics, impaired genome production, altered abundance and patterns of recombination, and impaired competitive fitness. Further, the nsp13-14 and nsp14-15 mutant viruses demonstrated mutation frequencies that were significantly higher than with the WT. The results demonstrate that cleavage of nsp13-14 or nsp14-15 is not required for MHV viability and that functions of the RTC/nsp14-ExoN are impaired when assembled with noncleaved intermediates. These data will inform future genetic, structural, biochemical, and modeling studies of coronavirus RTCs and nsp 13, 14, and 15 and may reveal new approaches for inhibition or attenuation of CoV infection.

IMPORTANCE Coronavirus replication requires proteolytic maturation of the nonstructural replicase proteins to form the replication-transcription complex. Coronavirus replication-transcription complex models assume mature subunits; however, mechanisms of coronavirus maturation and replicase complex formation have yet to be defined. Here, we show that for the coronavirus murine hepatitis virus, cleavage between the nonstructural replicase proteins nsp13-14 and nsp14-15 is not required for replication but does alter RNA synthesis and recombination. These results shed new light on the requirements for coronavirus maturation and replication-transcription complex assembly, and they may reveal novel therapeutic targets and strategies for attenuation.

Analysis of SARS-CoV-2 mutations in the main viral protease (NSP5) and its implications on the vaccine designing strategies

SARS-CoV-2 (Severe Acute Respiratory Syndrome), an etiolating agent of novel COVID-19 (coronavirus 2019) pandemic, rapidly spread worldwide, creating an unprecedented public health crisis globally. NSP5, the main viral protease, is a highly conserved protein, encoded by the genome of SARS-CoV-2 and plays an important role in the viral replication cycle. In the present study, we detected a total of 33 mutations from 675 sequences submitted from India in the month of March 2020 to April 2021. Out of 33 mutations, we selected 8 frequent mutations (K236R, N142L, K90R, A7V, L75F, C22N, H246Y and I43V) for further analysis. Subsequently, protein models were constructed, revealing significant alterations in the 3-D structure of NSP5 protein when compared to the wild type protein sequence which also altered the secondary structure of NSP5 protein. Further, we identified 9 B-cell, 10 T-cell and 6 MHC-I promising epitopes using predictive tools of immunoinformatics, out of these epitopes some were non-allergenic as well as highly immunogenic. Results of our study, however, revealed that 10 B-cell epitopes reside in the mutated region of NSP5. Additionally, hydrophobicity, physiochemical properties, toxicity and stability of NSP5 protein were estimated to demonstrate the specificity of the multiepitope candidates. Taken together, variations arising as a consequence of multiple mutations may cause alterations in the structure and function of NSP5 which generate crucial insights to better understand structural aspects of SARS-CoV-2. Our study also revealed, NSP5, a main protease, can be a potentially good target for the design and development of vaccine candidate against SARS-CoV-2.

Dynamic Allostery Highlights the Evolutionary Differences between the CoV-1 and CoV-2 Main Proteases

The SARS-CoV-2 coronavirus has become one of the most immediate and widely-studied systems since its identification and subsequent global outbreak from 2019-2021. In an effort to understand the biophysical changes as a result of mutations, the mechanistic details of multiple different proteins within the SARS-CoV-2 virus have been studied and compared with SARS-CoV-1. Focusing on the main protease (mPro), we first explored the long-range dynamics using the Dynamic Coupling Index (DCI) to investigate the dynamic coupling between the catalytic site residues and the rest of the protein, both inter and intra chain, for the CoV-1 and CoV-2 mPro. We found that there is significant cross-chain coupling between these active sites and specific distal residues in the CoV-2 mPro not present in CoV-1. The enhanced long distance interactions, particularly between the two chains, suggest subsequently enhanced cooperativity for CoV-2. A further comparative analysis of the dynamic flexibility using the Dynamic Flexibility Index (DFI) between the CoV-1 and CoV-2 mPros shows that the inhibitor binding near active sites induces change in flexibility to a distal region of the protein, opposite in behavior between the two systems; this region becomes more flexible upon inhibitor binding in CoV-1 while it becomes less flexible in the CoV-2 mPro. Upon inspection, we show that, on average, the dynamic flexibility of the sites substituted from CoV-1 to CoV-2 changes significantly less than the average calculated across all residues within the structure, indicating that the differences in behaviors between the two systems is likely the result of allosteric influence, where the new substitutions in CoV-2 induce flexibility and dynamical changes elsewhere in the structure.

Identification of Hypericin as a Candidate Repurposed Therapeutic Agent for COVID-19 and Its Potential Anti-SARS-CoV-2 Activity

The COVID-19 pandemic has had an unprecedented impact on the global economy and public health. Its etiologic agent, the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is highly transmissible, pathogenic and has a rapid global spread. Currently, the increase in the number of new confirmed cases has been slowed down due to the increase of vaccination in some regions of the world. Still, the rise of new variants has influenced the detection of additional waves of rising cases that some countries have experienced. Since the virus replication cycle is composed of many distinct stages, some viral proteins related to them, as the main-protease (Mpro) and RNA dependent RNA polymerase (RdRp), constitute individual potential antiviral targets. In this study, we challenged the mentioned enzymes against compounds pre-approved by health regulatory agencies in a virtual screening and later in Molecular Mechanics/Poisson–Bolzmann Surface Area (MM/PBSA) analysis. Our results showed that, among the identified potential drugs with anti-SARS-CoV-2 properties, Hypericin, an important component of the Hypericum perforatum that presents antiviral and antitumoral properties, binds with high affinity to viral Mpro and RdRp. Furthermore, we evaluated the activity of Hypericin anti-SARS-CoV-2 replication in an in vitro model of Vero-E6 infected cells. Therefore, we show that Hypericin inhibited viral replication in a dose dependent manner. Moreover, the cytotoxicity of the compound, in cultured cells, was evaluated, but no significant activity was found. Thus, the results observed in this study indicate that Hypericin is an excellent candidate for repurposing for the treatment of COVID-19, with possible inhibition of two important phases of virus maturation.

Analysis of SARS-CoV-2 mutations in the main viral protease (NSP5) and its implications on the vaccine designing strategies

SARS-CoV-2 (Severe Acute Respiratory syndrome), an etiolating agent of novel COVID-19 (coronavirus 2019) pandemic, rapidly spread worldwide and creating an unprecedented public health crisis globally. NSP5, the main viral protease, is a highly conserved protein, encoded by the genome of SARS-CoV-2 and plays an important role in the viral replication cycle. In the present study, we detected a total of 33 mutations from 675 sequences submitted from India in the month of March 2020 to April 2021. Out of 33 mutations, we selected 8 frequent mutations (K236R, N142L, K90R, A7V, L75F, C22N, H246Y, and I43V) for further analysis. Subsequently, protein models were constructed, revealing significant alterations in the 3-D structure of NSP5 proteins when compared to the wild type protein sequence which also altered the secondary structure of NSP5 protein. Further, we identified 9 B-cell, 10 T-cell and 6 MHC-I promising epitopes using predictive tools of immunoinformatics and some were non-allergenic as well as highly immunogenic. Results of our study, however, revealed that 10 B-cell epitopes reside in the mutated region of NSP5. Additionally, hydrophobicity, physiochemical properties, toxicity and stability of NSP5 protein were estimated to demonstrate the specificity of the multiepitope candidates. Taken together, variations arising as a consequence of multiple mutations may cause alterations in the structure and function of NSP5 which generate crucial insights to better understand structural aspects of SARS-CoV-2. Our study also revealed, NSP5, a main protease, can be a potentially good target for the design and development of vaccine candidate against SARS-CoV-2.

Structure-Function Characteristics of SARS-CoV-2 Proteases and Their Potential Inhibitors from Microbial Sources

The COVID-19 pandemic, caused by Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), is considered the greatest challenge to the global health community of the century as it continues to expand. This has prompted immediate urgency to discover promising drug targets for the treatment of COVID-19. The SARS-CoV-2 viral proteases, 3-chymotrypsin-like protease (3CLpro) and papain-like cysteine protease (PLpro), have become the promising target to study due to their essential functions in spreading the virus by RNA transcription, translation, protein synthesis, processing and modification, virus replication, and infection of the host. As such, understanding of the structure and function of these two proteases is unavoidable as platforms for the development of inhibitors targeting this protein which further arrest the infection and spread of the virus. While the abundance of reports on the screening of natural compounds such as SARS-CoV-2 proteases inhibitors are available, the microorganisms-based compounds (peptides and non-peptides) remain less studied. Indeed, microorganisms-based compounds are also one of the potent antiviral candidates against COVID-19. Microbes, especially bacteria and fungi, are other resources to produce new drugs as well as nucleosides, nucleotides, and nucleic acids. Thus, we have compiled various reported literature in detail on the structures, functions of the SARS-CoV-2 proteases, and potential inhibitors from microbial sources as assistance to other researchers working with COVID-19. The compounds are also compared to HIV protease inhibitors which suggested the microorganisms-based compounds are advantageous as SARS-CoV2 proteases inhibitors. The information should serve as a platform for further development of COVID-19 drug design strategies.

Selection of SARS-CoV-2 main protease inhibitor using structure-based virtual screening

Background: Conserved domains within SARS coronavirus 2 nonstructural proteins represent key targets for the design of novel inhibitors. Methods: The authors aimed to identify potential SARS coronavirus 2 NSP5 inhibitors using the ZINC database along with structure-based virtual screening and molecular dynamics simulation. Results: Of 13,840 compounds, 353 with robust docking scores were initially chosen, of which ten hit compounds were selected as candidates for detailed analyses. Three compounds were selected as coronavirus NSP5 inhibitors after passing absorption, distribution, metabolism, excretion and toxicity study; root and mean square deviation; and radius of gyration calculations. Conclusion: ZINC000049899562, ZINC000169336666 and ZINC000095542577 are potential NSP5 protease inhibitors that warrant further experimental studies.

Preclinical characterization of an intravenous coronavirus 3CL protease inhibitor for the potential treatment of COVID19

COVID-19 caused by the SARS-CoV-2 virus has become a global pandemic. 3CL protease is a virally encoded protein that is essential across a broad spectrum of coronaviruses with no close human analogs. PF-00835231, a 3CL protease inhibitor, has exhibited potent in vitro antiviral activity against SARS-CoV-2 as a single agent. Here we report, the design and characterization of a phosphate prodrug PF-07304814 to enable the delivery and projected sustained systemic exposure in human of PF-00835231 to inhibit coronavirus family 3CL protease activity with selectivity over human host protease targets. Furthermore, we show that PF-00835231 has additive/synergistic activity in combination with remdesivir. We present the ADME, safety, in vitro, and in vivo antiviral activity data that supports the clinical evaluation of PF-07304814 as a potential COVID-19 treatment.

Computational assessment of select antiviral phytochemicals as potential SARS-Cov-2 main protease inhibitors: molecular dynamics guided ensemble docking and extended molecular dynamics

Covid-19 caused by novel coronavirus, 2019-nCoV or SARS-CoV-2 has become most severe pandemic of this century. No specific therapies are available to treat Covid-19 so far. Recently, main protease (M^pro), a potential drug target from SARS-CoV-2 has been successfully crystallised. The present study is aimed at assessment of bioactive antiviral phytochemicals as potential SARS-COV-2 M^pro inhibitors, using ensemble docking, molecular dynamics and MM-PBSA calculations. Ensemble docking studies were performed with Autodock vina program. The top 5 compounds having highest binding free energy were subjected to 100 ns molecular dynamics simulations with Gromacs. The resulting trajectories of converged period of MD were further exploited in MM-PBSA calculations to derive accurate estimates of binding free energies. The MD results were analysed with respect to RMSD, RMSF and hydrogen bond formation and occupancy parameters. The drugs remdesivir and nelfinavir were used as standard drugs for comparative studies. In the docking studies five phytochemicals, dalpanitin, amentoflavone, naringin, hinokiflavone, and rutin were found having lowest binding free energies (< − 10 kcal mol⁻¹) which is lower than standard drugs. MD studies suggested that the complexes of these five phytochemicals with M^pro stabilize with well accepted RMSD. Amongst these phytochemicals, hinokiflavove, amentoflavone and naringin were found having better binding affinity with ΔG_binging than the standard drug remdesivir. Investigations and validation of these inhibitors against SARS-CoV-2 would be helpful in bring these molecules at the clinical settings.

Two Inhibitors Against the 3C-Like Proteases of Swine Coronavirus and Feline Coronavirus

Coronaviruses (CoVs) are important human and animal pathogens that cause respiratory and gastrointestinal diseases. Porcine epidemic diarrhoea (PED), characterized by severe diarrhoea and vomiting in pigs, is a highly lethal disease caused by porcine epidemic diarrhoea virus (PEDV) and causes substantial losses in the swine industry worldwide. However, currently available commercial drugs have not shown great therapeutic effects. In this study, a fluorescence resonance energy transfer (FRET)-based assay was applied to screen a library containing 1,590 compounds and identified two compounds, 3-(aminocarbonyl)-1-phenylpyridinium and 2,3-dichloronaphthoquinone, that target the 3C-like protease (3CL^pro) of PEDV. These compounds are of low molecular weight (MW) and greatly inhibited the activity of this enzyme (IC₅₀ values were obtained in this study). Furthermore, these compounds exhibited antiviral capacity against another member of the CoV family, feline infectious peritonitis virus (FIPV). Here, the inhibitory effects of these compounds against CoVs on Vero cells and feline kidney cells were identified (with EC₅₀ values) and cell viability assays were performed. The results of putative molecular docking models indicate that these compounds, labeled compound 1 and compound 2, contact the conserved active sites (Cys144, Glu165, Gln191) of 3CL^pro via hydrogen bonds. These findings provide insight into the antiviral activities of compounds 1 and 2 that may facilitate future research on anti-CoV drugs.

Targeting novel structural and functional features of coronavirus protease nsp5 (3CLpro, Mpro) in the age of COVID-19

Coronavirus protease nsp5 (Mpro, 3CLpro) remains a primary target for coronavirus therapeutics due to its indispensable and conserved role in the proteolytic processing of the viral replicase polyproteins. In this review, we discuss the diversity of known coronaviruses, the role of nsp5 in coronavirus biology, and the structure and function of this protease across the diversity of known coronaviruses, and evaluate past and present efforts to develop inhibitors to the nsp5 protease with a particular emphasis on new and mostly unexplored potential targets of inhibition. With the recent emergence of pandemic SARS-CoV-2, this review provides novel and potentially innovative strategies and directions to develop effective therapeutics against the coronavirus protease nsp5.

Crystallographic models of SARS-CoV-2 3CLpro: in-depth assessment of structure quality and validation

The appearance at the end of 2019 of the new SARS-CoV-2 coronavirus led to an unprecedented response by the structural biology community, resulting in the rapid determination of many hundreds of structures of proteins encoded by the virus. As part of an effort to analyze and, if necessary, remediate these structures as deposited in the Protein Data Bank (PDB), this work presents a detailed analysis of 81 crystal structures of the main protease 3CLpro, an important target for the design of drugs against COVID-19. The structures of the unliganded enzyme and its complexes with a number of inhibitors were determined by multiple research groups using different experimental approaches and conditions; the resulting structures span 13 different polymorphs representing seven space groups. The structures of the enzyme itself, all determined by molecular replacement, are highly similar, with the exception of one polymorph with a different inter-domain orientation. However, a number of complexes with bound inhibitors were found to pose significant problems. Some of these could be traced to faulty definitions of geometrical restraints for ligands and to the general problem of a lack of such information in the PDB depositions. Several problems with ligand definition in the PDB itself were also noted. In several cases extensive corrections to the models were necessary to adhere to the evidence of the electron-density maps. Taken together, this analysis of a large number of structures of a single, medically important protein, all determined within less than a year using modern experimental tools, should be useful in future studies of other systems of high interest to the biomedical community.

Discovery of a Novel Inhibitor of Coronavirus 3CL Protease for the Potential Treatment of COVID-19

COVID-19 caused by the SARS-CoV-2 virus has become a global pandemic. 3CL protease is a virally encoded protein that is essential across a broad spectrum of coronaviruses with no close human analogs. The designed phosphate prodrug PF-07304814 is metabolized to PF-00835321 which is a potent inhibitor in vitro of the coronavirus family 3CL pro, with selectivity over human host protease targets. Furthermore, PF-00835231 exhibits potent in vitro antiviral activity against SARS-CoV-2 as a single agent and it is additive/synergistic in combination with remdesivir. We present the ADME, safety, in vitro , and in vivo antiviral activity data that supports the clinical evaluation of this compound as a potential COVID-19 treatment.

Coronaviruses: An Updated Overview of Their Replication and Pathogenesis

Coronaviruses (CoVs), enveloped positive-sense RNA viruses, are characterized by club-like spikes that project from their surface, an unusually large RNA genome, and a unique replication strategy. CoVs cause a variety of diseases in mammals and birds ranging from enteritis in cows and pigs, and upper respiratory tract and kidney disease in chickens to lethal human respiratory infections. Most recently, the novel coronavirus, SARS-CoV-2, which was first identified in Wuhan, China in December 2019, is the cause of a catastrophic pandemic, COVID-19, with more than 8 million infections diagnosed worldwide by mid-June 2020. Here we provide a brief introduction to CoVs discussing their replication, pathogenicity, and current prevention and treatment strategies. We will also discuss the outbreaks of the highly pathogenic Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV) and Middle Eastern Respiratory Syndrome Coronavirus (MERS-CoV), which are relevant for understanding COVID-19.

Temperature Sensitivity: A Potential Method for the Generation of Vaccines against the Avian Coronavirus Infectious Bronchitis Virus

The Gammacoronavirus infectious bronchitis virus (IBV) is a highly contagious economically important respiratory pathogen of domestic fowl. Reverse genetics allows for the molecular study of pathogenic determinants to enable rational vaccine design. The recombinant IBV (rIBV) Beau-R, a molecular clone of the apathogenic Beaudette strain, has previously been investigated as a vaccine platform. To determine tissues in which Beau-R could effectively deliver antigenic genes, an in vivo study in chickens, the natural host, was used to compare the pattern of viral dissemination of Beau-R to the pathogenic strain M41-CK. Replication of Beau-R was found to be restricted to soft tissue within the beak, whereas M41-CK was detected in beak tissue, trachea and eyelid up to seven days post infection. In vitro assays further identified that, unlike M41-CK, Beau-R could not replicate at 41 °C, the core body temperature of a chicken, but is able to replicate a 37 °C, a temperature relatable to the very upper respiratory tract. Using a panel of rIBVs with defined mutations in the structural and accessory genes, viral replication at permissive and non-permissive temperatures was investigated, identifying that the Beau-R replicase gene was a determinant of temperature sensitivity and that sub-genomic mRNA synthesis had been affected. The identification of temperature sensitive allelic lesions within the Beau-R replicase gene opens up the possibility of using this method of attenuation in other IBV strains for future vaccine development as well as a method to investigate the functions of the IBV replicase proteins.

Identification of bioactive molecule from Withania somnifera (Ashwagandha) as SARS-CoV-2 main protease inhibitor

SARS-CoV-2 is the causative agent of COVID-19 and has been declared as pandemic disease by World Health Organization. Lack of targeted therapeutics and vaccines for COVID-2019 have triggered the scientific community to develop new vaccines or drugs against this novel virus. Many synthetic compounds and antimalarial drugs are undergoing clinical trials. The traditional medical practitioners widely use Indian medicinal plant Withania somnifera (Ashwagandha) natural constituents, called withanolides for curing various diseases. The main protease (M^pro) of SARS-CoV-2 plays a vital role in disease propagation by processing the polyproteins which are required for its replication. Hence, it denotes a significant target for drug discovery. In the present study, we evaluate the potential of 40 natural chemical constituents of Ashwagandha to explore a possible inhibitor against main protease of SARS-CoV-2 by adopting the computational approach. The docking study revealed that four constituents of Ashwagandha; Withanoside II (-11.30 Kcal/mol), Withanoside IV (-11.02 Kcal/mol), Withanoside V (-8.96 Kcal/mol) and Sitoindoside IX (-8.37 Kcal/mol) exhibited the highest docking energy among the selected natural constituents. Further, MD simulation study of 100 ns predicts Withanoside V possess strong binding affinity and hydrogen-bonding interactions with the protein active site and indicates its stability in the active site. The binding free energy score also correlates with the highest score of -87.01 ± 5.01 Kcal/mol as compared to other selected compounds. In conclusion, our study suggests that Withanoside V in Ashwagandha may be serve as a potential inhibitor against M^pro of SARS-CoV-2 to combat COVID-19 and may have an antiviral effect on nCoV.Communicated by Ramaswamy H. Sarma.

Identification of potential natural inhibitors of SARS-CoV2 main protease by molecular docking and simulation studies

Coronaviruses are contagious pathogens primarily responsible for respiratory and intestinal infections. Research efforts to develop antiviral agents against coronavirus demonstrated the main protease (Mpro) protein may represent effective drug target. X-ray crystallographic structure of the SARS-CoV2 Mpro protein demonstrated the significance of Glu166, Cys141, and His41 residues involved in protein dimerization and its catalytic function. We performed in silico screening of compounds from Curcuma longa L. (Zingiberaceae family) against Mpro protein inhibition. Employing a combination of molecular docking, scoring functions, and molecular dynamics simulations, 267 compounds were screened by docking on Mpro crystallographic structure. Docking score and interaction profile analysis exhibited strong binding on the Mpro catalytic domain with compounds C1 (1E,6E)-1,2,6,7-tetrahydroxy-1,7-bis(4-hydroxy-3-methoxyphenyl)hepta-1,6-diene-3,5-dione) and C2 (4Z,6E)‐1,5‐dihydroxy‐1,7‐bis(4‐hydroxyphenyl)hepta‐4,6‐dien‐3‐one as lead agents. Compound C1 and C2 showed minimum binding score (–9.08 and –8.07 kcal/mole) against Mpro protein in comparison to shikonin and lopinavir (≈ −5.4 kcal/mole) a standard Mpro inhibitor. Furthermore, principal component analysis, free energy landscape and protein-ligand energy calculation studies revealed that these two compounds strongly bind to the catalytic core of the Mpro protein with higher efficacy than lopinavir, a standard antiretroviral of the protease inhibitor class. Taken together, this structure based optimization has provided lead on two natural Mpro inhibitors for further testing and development as therapeutics against human coronavirus.

Communicated by Ramaswamy H. Sarma

The crystal structure of main protease from mouse hepatitis virus A59 in complex with an inhibitor

Mouse hepatitis virus A59 (MHV-A59) is a representative member of the genus betacoronavirus within the subfamily Coronavirinae, which infects the liver, brain and respiratory tract. Through different inoculation routes, MHV-A59 can provide animal models for encephalitis, hepatitis and pneumonia to explore viral life machinery and virus-host interactions. In viral replication, non-structural protein 5 (Nsp5), also termed main protease (M^pro), plays a dominant role in processing coronavirus-encoded polyproteins and is thus recognized as an ideal target of anti-coronavirus agents. However, no structure of the MHV-A59 M^pro has been reported, and molecular exploration of the catalysis mechanism remains hindered. Here, we solved the crystal structure of the MHV-A59 M^pro complexed with a Michael acceptor-based inhibitor, N3. Structural analysis revealed that the Cβ of the vinyl group of N3 covalently bound to C145 of the catalytic dyad of M^pro, which irreversibly inactivated cysteine protease activity. The lactam ring of the P1 side chain and the isobutyl group of the P2 side chain, which mimic the conserved residues at the same positions of the substrate, fit well into the S1 and S2 pockets. Through a comparative study with M^pro of other coronaviruses, we observed that the substrate-recognition pocket and enzyme inhibitory mechanism is highly conservative. Altogether, our study provided structural features of MHV-A59 M^pro and indicated that a Michael acceptor inhibitor is an ideal scaffold for antiviral drugs.

Development and application of nsp5-ELISA for the detection of antibody to infectious bronchitis virus

Infectious bronchitis virus (IBV) continues to be one of the most important poultry pathogens worldwide. The current commercially available enzyme-linked immunosorbent assay (ELISA) kits for IBV specific antibody detection are mostly based on the whole virion, and few serological tests based on nonstructural proteins of IBV have been developed. Herein, an alternative indirect ELISA for detection of IBV antibody was developed with IBV nonstructural protein 5 (nsp5) produced by Escherichia coli. Using an indirect immunofluorescence assay (IFA) and a commercial ELISA kit as reference, we optimized the nsp5-ELISA and determined its cut-off as 0.12. The diagnostic sensitivity (DSN), specificity (DSP) and accuracy of the nsp5-ELISA were 93.11%, 95.38% and 93.33%, respectively, compared with IFA in 660 field serum samples, and were 98.11%, 95.00% and 97.62%, respectively, compared with the commercial IBV ELISA kit (IDEXX) in 126 field sera samples. Furthermore, a time course of IBV specific antibody level detected by nsp5-ELISA following IBV infection and vaccination is consistent with that of IBV antibody detected by the commercial ELISA kit. The results presented in this study indicate that nsp5-ELISA has the potential to serve as a rapid, reliable and cost-effective method for IBV antibody detection. This study is the first to report the development of an nsp-based ELISA to detect an antibody to IBV.

Extensive Positive Selection Drives the Evolution of Nonstructural Proteins in Lineage C Betacoronaviruses

Middle East respiratory syndrome-related coronavirus (MERS-CoV) spreads to humans via zoonotic transmission from camels. MERS-CoV belongs to lineage C of betacoronaviruses (betaCoVs), which also includes viruses isolated from bats and hedgehogs. A large portion of the betaCoV genome consists of two open reading frames (ORF1a and ORF1b) that are translated into polyproteins. These are cleaved by viral proteases to generate 16 nonstructural proteins (nsp1 to nsp16) which compose the viral replication-transcription complex. We investigated the evolution of ORF1a and ORF1b in lineage C betaCoVs. Results indicated widespread positive selection, acting mostly on ORF1a. The proportion of positively selected sites in ORF1a was much higher than that previously reported for the surface-exposed spike protein. Selected sites were unevenly distributed, with nsp3 representing the preferential target. Several pairs of coevolving sites were also detected, possibly indicating epistatic interactions; most of these were located in nsp3. Adaptive evolution at nsp3 is ongoing in MERS-CoV strains, and two selected sites (G720 and R911) were detected in the protease domain. While position 720 is variable in camel-derived viruses, suggesting that the selective event does not represent a specific adaptation to humans, the R911C substitution was observed only in human-derived MERS-CoV isolates, including the viral strain responsible for the recent South Korean outbreak. It will be extremely important to assess whether these changes affect host range or other viral phenotypes. More generally, data herein indicate that CoV nsp3 represents a major selection target and that nsp3 sequencing should be envisaged in monitoring programs and field surveys.

IMPORTANCE

Both severe acute respiratory syndrome coronavirus (SARS-CoV) and MERS-CoV originated in bats and spread to humans via an intermediate host. This clearly highlights the potential for coronavirus host shifting and the relevance of understanding the molecular events underlying the adaptation to new host species. We investigated the evolution of ORF1a and ORF1b in lineage C betaCoVs and in 87 sequenced MERS-CoV isolates. Results indicated widespread positive selection, stronger in ORF1a than in ORF1b. Several selected sites were found to be located in functionally relevant protein regions, and some of them corresponded to functional mutations in other coronaviruses. The proportion of selected sites we identified in ORF1a is much higher than that for the surface-exposed spike protein. This observation suggests that adaptive evolution in ORF1a might contribute to host shifts or immune evasion. Data herein also indicate that genetic diversity at nonstructural proteins should be taken into account when antiviral compounds are developed.

Ligand-induced Dimerization of Middle East Respiratory Syndrome (MERS) Coronavirus nsp5 Protease (3CLpro): IMPLICATIONS FOR nsp5 REGULATION AND THE DEVELOPMENT OF ANTIVIRALS

All coronaviruses, including the recently emerged Middle East respiratory syndrome coronavirus (MERS-CoV) from the β-CoV subgroup, require the proteolytic activity of the nsp5 protease (also known as 3C-like protease, 3CL^pro) during virus replication, making it a high value target for the development of anti-coronavirus therapeutics. Kinetic studies indicate that in contrast to 3CL^pro from other β-CoV 2c members, including HKU4 and HKU5, MERS-CoV 3CL^pro is less efficient at processing a peptide substrate due to MERS-CoV 3CL^pro being a weakly associated dimer. Conversely, HKU4, HKU5, and SARS-CoV 3CL^pro enzymes are tightly associated dimers. Analytical ultracentrifugation studies support that MERS-CoV 3CL^pro is a weakly associated dimer (K_d ∼52 μm) with a slow off-rate. Peptidomimetic inhibitors of MERS-CoV 3CL^pro were synthesized and utilized in analytical ultracentrifugation experiments and demonstrate that MERS-CoV 3CL^pro undergoes significant ligand-induced dimerization. Kinetic studies also revealed that designed reversible inhibitors act as activators at a low compound concentration as a result of induced dimerization. Primary sequence comparisons and x-ray structural analyses of two MERS-CoV 3CLpro and inhibitor complexes, determined to 1.6 Å, reveal remarkable structural similarity of the dimer interface with 3CL^pro from HKU4-CoV and HKU5-CoV. Despite this structural similarity, substantial differences in the dimerization ability suggest that long range interactions by the nonconserved amino acids distant from the dimer interface may control MERS-CoV 3CL^pro dimerization. Activation of MERS-CoV 3CL^pro through ligand-induced dimerization appears to be unique within the genogroup 2c and may potentially increase the complexity in the development of MERS-CoV 3CL^pro inhibitors as antiviral agents.

Analysis of human rotaviruses from a single location over an 18-year time span suggests that protein coadaption influences gene constellations

Rotaviruses (RVs) are 11-segmented, double-stranded RNA viruses that cause severe gastroenteritis in children. In addition to an error-prone genome replication mechanism, RVs can increase their genetic diversity by reassorting genes during host coinfection. Such exchanges allow RVs to acquire advantageous genes and adapt in the face of selective pressures. However, reassortment may also impose fitness costs if it unlinks genes/proteins that have accumulated compensatory, coadaptive mutations and that operate best when kept together. To better understand human RV evolutionary dynamics, we analyzed the genome sequences of 135 strains (genotype G1/G3/G4-P[8]-I1-C1-R1-A1-N1-T1-E1-H1) that were collected at a single location in Washington, DC, during the years 1974 to 1991. Intragenotypic phylogenetic trees were constructed for each viral gene using the nucleotide sequences, thereby defining novel allele level gene constellations (GCs) and illuminating putative reassortment events. The results showed that RVs with distinct GCs cocirculated during the vast majority of the collection years and that some of these GCs persisted in the community unchanged by reassortment. To investigate the influence of protein coadaptation on GC maintenance, we performed a mutual information-based analysis of the concatenated amino acid sequences and identified an extensive covariance network. Unexpectedly, amino acid covariation was highest between VP4 and VP2, which are structural components of the RV virion that are not thought to directly interact. These results suggest that GCs may be influenced by the selective constraints placed on functionally coadapted, albeit noninteracting, viral proteins. This work raises important questions about mutation-reassortment interplay and its impact on human RV evolution.

IMPORTANCE

Rotaviruses are devastating human pathogens that cause severe diarrhea and kill >450,000 children each year. The virus can evolve by accumulating mutations and by acquiring new genes from other strains via a process called reassortment. However, little is known about the relationship between mutation accumulation and gene reassortment for rotaviruses and how it impacts viral evolution. In this study, we analyzed the genome sequences of human strains found in clinical fecal specimens that were collected at a single hospital over an 18-year time span. We found that many rotaviruses did not reassort their genes but instead maintained them as specific sets (i.e., constellations). By analyzing the encoded proteins, we discovered concurrent amino acid changes among them, which suggests that they are functionally coadapted to operate best when kept together. This study increases our understanding of how rotaviruses evolve over time in the human population.

Competitive fitness in coronaviruses is not correlated with size or number of double-membrane vesicles under reduced-temperature growth conditions

Positive-stranded viruses synthesize their RNA in membrane-bound organelles, but it is not clear how this benefits the virus or the host. For coronaviruses, these organelles take the form of double-membrane vesicles (DMVs) interconnected by a convoluted membrane network. We used electron microscopy to identify murine coronaviruses with mutations in nsp3 and nsp14 that replicated normally while producing only half the normal amount of DMVs under low-temperature growth conditions. Viruses with mutations in nsp5 and nsp16 produced small DMVs but also replicated normally. Quantitative reverse transcriptase PCR (RT-PCR) confirmed that the most strongly affected of these, the nsp3 mutant, produced more viral RNA than wild-type virus. Competitive growth assays were carried out in both continuous and primary cells to better understand the contribution of DMVs to viral fitness. Surprisingly, several viruses that produced fewer or smaller DMVs showed a higher fitness than wild-type virus at the reduced temperature, suggesting that larger and more numerous DMVs do not necessarily confer a competitive advantage in primary or continuous cell culture. For the first time, this directly demonstrates that replication and organelle formation may be, at least in part, studied separately during infection with positive-stranded RNA virus. IMPORTANCE The viruses that cause severe acute respiratory syndrome (SARS), poliomyelitis, and hepatitis C all replicate in double-membrane vesicles (DMVs). The big question about DMVs is why they exist in the first place. In this study, we looked at thousands of infected cells and identified two coronavirus mutants that made half as many organelles as normal and two others that made typical numbers but smaller organelles. Despite differences in DMV size and number, all four mutants replicated as efficiently as wild-type virus. To better understand the relative importance of replicative organelles, we carried out competitive fitness experiments. None of these viruses was found to be significantly less fit than wild-type, and two were actually fitter in tests in two kinds of cells. This suggests that viruses have evolved to have tremendous plasticity in the ability to form membrane-associated replication complexes and that large and numerous DMVs are not exclusively associated with efficient coronavirus replication.

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.

Chimeric exchange of coronavirus nsp5 proteases (3CLpro) identifies common and divergent regulatory determinants of protease activity

Human coronaviruses (CoVs) such as severe acute respiratory syndrome CoV (SARS-CoV) and Middle East respiratory syndrome CoV (MERS-CoV) cause epidemics of severe human respiratory disease. A conserved step of CoV replication is the translation and processing of replicase polyproteins containing 16 nonstructural protein domains (nsp's 1 to 16). The CoV nsp5 protease (3CLpro; Mpro) processes nsp's at 11 cleavage sites and is essential for virus replication. CoV nsp5 has a conserved 3-domain structure and catalytic residues. However, the intra- and intermolecular determinants of nsp5 activity and their conservation across divergent CoVs are unknown, in part due to challenges in cultivating many human and zoonotic CoVs. To test for conservation of nsp5 structure-function determinants, we engineered chimeric betacoronavirus murine hepatitis virus (MHV) genomes encoding nsp5 proteases of human and bat alphacoronaviruses and betacoronaviruses. Exchange of nsp5 proteases from HCoV-HKU1 and HCoV-OC43, which share the same genogroup, genogroup 2a, with MHV, allowed for immediate viral recovery with efficient replication albeit with impaired fitness in direct competition with wild-type MHV. Introduction of MHV nsp5 temperature-sensitive mutations into chimeric HKU1 and OC43 nsp5 proteases resulted in clear differences in viability and temperature-sensitive phenotypes compared with MHV nsp5. These data indicate tight genetic linkage and coevolution between nsp5 protease and the genomic background and identify differences in intramolecular networks regulating nsp5 function. Our results also provide evidence that chimeric viruses within coronavirus genogroups can be used to test nsp5 determinants of function and inhibition in common isogenic backgrounds and cell types.

Presentation overrides specificity: probing the plasticity of alphaviral proteolytic activity through mutational analysis

Semliki Forest virus (genus Alphavirus) is an important model for studying regulated nonstructural (ns) polyprotein processing. In this study, we evaluated the strictness of the previously outlined cleavage rules, accounting for the timing and outcome of each of three cleavages within the ns polyprotein P1234, and assessed the significance of residues P6 to P4 within the cleavage sites using an alanine scanning approach. The processing of the 1/2 and 3/4 sites was most strongly affected following changes in residues P5 and P4, respectively. However, none of the mutations had a detectable effect on the processing of the 2/3 site. An analysis of recombinant viruses bearing combinations of mutations in cleavage sites revealed tolerance toward the cooccurrence of native and mutated cleavage sites within the same polyprotein, suggesting a remarkable plasticity of the protease recognition pocket. Even in a virus in which all of the cleavage sequences were replaced with alanines in the P6, P5, and P4 positions, the processing pattern was largely preserved, without leading to reversion of cleavage site mutations. Instead, the emergence of second-site mutations was identified, among which Q706R/L in nsP2 was confirmed to be associated with the recognition of the P4 position within the modified cleavage sites. Our results imply that the spatial arrangement of the viral replication complex inherently contributes to scissile-site presentation for the protease, alleviating stringent sequence recognition requirements yet ensuring the precision and the correct order of processing events. Obtaining a proper understanding of the consequences of cleavage site manipulations may provide new tools for taming alphaviruses.