Prediction of proteinase cleavage sites in polyproteins of coronaviruses and its applications in analyzing SARS-CoV genomes

Bats-associated beta-coronavirus detection and characterization: First report from Pakistan

Bats remains as reservoirs for highly contagious and pathogenic viral families including the Coronaviridae, Filoviridae, Paramyxoviruses, and Rhabdoviridae. Spill over of viral species (SARS-CoV, MERS-CoV & SARS-CoV2) from bats (as a possible potential reservoirs) have recently caused worst outbreaks. Early detection of viral species of pandemic potential in bats is of great importance. We detected beta coronaviruses in the studied bats population (positive samples from Rousettus leschenaultia) and performed the evolutionary analysis, amino acid sequence alignment, and analysed the 3-Dimentional protein structure. We detected the coronaviruses for the first time in bats from Pakistan. Our analysis based on RdRp partial gene sequencing suggest that the studied viral strains are closely related to MERS-CoV-like viruses as they exhibit close structure similarities (with few substitutions) and also observed a substitution in highly conserved SDD in the palm subdomain of motif C to ADD, when compared with earlier reported viral strains. It could be concluded from our study that coronaviruses are circulating among the bat's population in Pakistan. Based on the current findings, we suggest large scale screening procedures of bat virome across the country to detect potential pathogenic viral species.

Cell Entry and Unusual Replication of SARS-CoV-2

BACKGROUND

SARS-CoV-2 is the causative virus for the CoVID-19 pandemic that has frequently mutated to continue to infect and resist available vaccines. Emerging new variants of the virus have complicated notions of immunity conferred by vaccines versus immunity that results from infection. While we continue to progress from epidemic to endemic as a result of this collective immunity, the pandemic remains a morbid and mortal problem.

OBJECTIVE

The SARS-CoV-2 virus has a very complex manner of replication. The spike protein, one of the four structural proteins of the encapsulated virus, is central to the ability of the virus to penetrate cells to replicate. The objective of this review is to summarize these complex features of viral replication.

METHODS

A review of the recent literature was performed on the biology of SARS-CoV-2 infection from published work from PubMed and works reported to preprint servers, e.g., bioRxiv and medRxiv.

RESULTS AND CONCLUSION

The complex molecular and cellular biology involved in SARS-CoV-2 replication and the origination of >30 proteins from a single open reading frame (ORF) have been summarized, as well as the structural biology of spike protein, a critical factor in the cellular entry of the virus, which is a necessary feature for it to replicate and cause disease.

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.

SARS-CoV-2 3CLpro displays faster self-maturation in vitro than SARS-CoV 3CLpro due to faster C-terminal cleavage

The coronavirus (CoV) disease 2019 (COVID-19) caused by the severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2) has become a worldwide pandemic. The 3C-like protease (3CL^pro ), which cleaves 11 sites including its own N- and C-termini on the viral polyproteins, is essential for SARS-CoV-2 replication. In this study, we constructed the full-length inactive 3CL^pro with N- and C-terminal extensions as substrates for monitoring self-cleavage by wild-type 3CL^pro . We found that the rate-limiting C-terminal self-cleavage rate of SARS-CoV-2 3CL^pro was 35-fold faster than that of SARS-CoV 3CL^pro by using the Trx/GST-tagged C145A 3CL^pro substrates. Since self-cleavage of 3CL^pro is the initial step for maturation of other viral proteins, our study suggests more facile SARS-CoV-2 replication as compared to SARS-CoV.

SARS-CoV-2 3CLpro whole human proteome cleavage prediction and enrichment/depletion analysis

A novel coronavirus (SARS-CoV-2) has devastated the globe as a pandemic that has killed millions of people. Widespread vaccination is still uncertain, so many scientific efforts have been directed toward discovering antiviral treatments. Many drugs are being investigated to inhibit the coronavirus main protease, 3CLpro, from cleaving its viral polyprotein, but few publications have addressed this protease’s interactions with the host proteome or their probable contribution to virulence. Too few host protein cleavages have been experimentally verified to fully understand 3CLpro’s global effects on relevant cellular pathways and tissues. Here, I set out to determine this protease’s targets and corresponding potential drug targets. Using a neural network trained on cleavages from 392 coronavirus proteomes with a Matthews correlation coefficient of 0.985, I predict that a large proportion of the human proteome is vulnerable to 3CLpro, with 4898 out of approximately 20,000 human proteins containing at least one putative cleavage site. These cleavages are nonrandomly distributed and are enriched in the epithelium along the respiratory tract, brain, testis, plasma, and immune tissues and depleted in olfactory and gustatory receptors despite the prevalence of anosmia and ageusia in COVID-19 patients. Affected cellular pathways include cytoskeleton/motor/cell adhesion proteins, nuclear condensation and other epigenetics, host transcription and RNAi, ribosomal stoichiometry and nascent-chain detection and degradation, ubiquitination, pattern recognition receptors, coagulation, lipoproteins, redox, and apoptosis. This whole proteome cleavage prediction demonstrates the importance of 3CLpro in expected and nontrivial pathways affecting virulence, lead me to propose more than a dozen potential therapeutic targets against coronaviruses, and should therefore be applied to all viral proteases and subsequently experimentally verified.

Natural Products for the Prevention and Control of the COVID-19 Pandemic: Sustainable Bioresources

Background: The world has been unprecedentedly hit by a global pandemic which broke the record of deadly pandemics that faced humanity ever since its existence. Even kids are well-versed in the terminologies and basics of the SARS-CoV-2 virus and COVID-19 now. The vaccination program has been successfully launched in various countries, given that the huge global population of concern is still far behind to be vaccinated. Furthermore, the scarcity of any potential drug against the COVID-19-causing virus forces scientists and clinicians to search for alternative and complementary medicines on a war-footing basis. Aims and Objectives: The present review aims to cover and analyze the etiology and epidemiology of COVID-19, the role of intestinal microbiota and pro-inflammatory markers, and most importantly, the natural products to combat this deadly SARS-CoV-2 virus. Methods: A primary literature search was conducted through PubMed and Google Scholar using relevant keywords. Natural products were searched from January 2020 to November 2020. No timeline limit has been imposed on the search for the biological sources of those phytochemicals. Interactive mapping has been done to analyze the multi-modal and multi-target sources. Results and Discussion: The intestinal microbiota and the pro-inflammatory markers that can serve the prognosis, diagnosis, and treatment of COVID-19 were discussed. The literature search resulted in yielding 70 phytochemicals and ten polyherbal formulations which were scientifically analyzed against the SARS-CoV-2 virus and its targets and found significant. Retrospective analyses led to provide information about 165 biological sources that can also be screened if not done earlier. Conclusion: The interactive analysis mapping of biological sources with phytochemicals and targets as well as that of phytochemical class with phytochemicals and COVID-19 targets yielded insights into the multitarget and multimodal evidence-based complementary medicines.

Data-driven identification of SARS-CoV-2 subpopulations using PhenoGraph and binary-coded genomic data

For epidemic prevention and control, the identification of SARS-CoV-2 subpopulations sharing similar micro-epidemiological patterns and evolutionary histories is necessary for a more targeted investigation into the links among COVID-19 outbreaks caused by SARS-CoV-2 with similar genetic backgrounds. Genomic sequencing analysis has demonstrated the ability to uncover viral genetic diversity. However, an objective analysis is necessary for the identification of SARS-CoV-2 subpopulations. Herein, we detected all the mutations in 186 682 SARS-CoV-2 isolates. We found that the GC content of the SARS-CoV-2 genome had evolved to be lower, which may be conducive to viral spread, and the frameshift mutation was rare in the global population. Next, we encoded the genomic mutations in binary form and used an unsupervised learning classifier, namely PhenoGraph, to classify this information. Consequently, PhenoGraph successfully identified 303 SARS-CoV-2 subpopulations, and we found that the PhenoGraph classification was consistent with, but more detailed and precise than the known GISAID clades (S, L, V, G, GH, GR, GV and O). By the change trend analysis, we found that the growth rate of SARS-CoV-2 diversity has slowed down significantly. We also analyzed the temporal, spatial and phylogenetic relationships among the subpopulations and revealed the evolutionary trajectory of SARS-CoV-2 to a certain extent. Hence, our results provide a better understanding of the patterns and trends in the genomic evolution and epidemiology of SARS-CoV-2.

A Novel Potentially Recombinant Rodent Coronavirus with a Polybasic Cleavage Site in the Spike Protein

The emergence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has reignited global interest in animal coronaviruses and their potential for human transmission. While bats are thought to be the wildlife reservoir of SARS-CoV and SARS-CoV-2, the widespread human coronavirus OC43 is thought to have originated in rodents. Here, we sampled 297 rodents and shrews, representing eight species, from three municipalities of southern China. We report coronavirus prevalences of 23.3% and 0.7% in Guangzhou and Guilin, respectively, with samples from urban areas having significantly higher coronavirus prevalences than those from rural areas. We obtained three coronavirus genome sequences from Rattus norvegicus, including a Betacoronavirus (rat coronavirus [RCoV] GCCDC3), an Alphacoronavirus (RCoV-GCCDC5), and a novel Betacoronavirus (RCoV-GCCDC4). Recombination analysis suggests that there was a potential recombination event involving RCoV-GCCDC4, murine hepatitis virus (MHV), and Longquan Rl rat coronavirus (LRLV). Furthermore, we uncovered a polybasic cleavage site, RARR, in the spike (S) protein of RCoV-GCCDC4, which is dominant in RCoV. These findings provide further information on the potential for interspecies transmission of coronaviruses and demonstrate the value of a One Health approach to virus discovery.

IMPORTANCE Surveillance of viruses among rodents in rural and urban areas of South China identified three rodent coronaviruses, RCoV-GCCDC3, RCoV-GCCDC4, and RCoV-GCCDC5, one of which was identified as a novel potentially recombinant coronavirus with a polybasic cleavage site in the spike (S) protein. Through reverse transcription-PCR (RT-PCR) screening of coronaviruses, we found that coronavirus prevalence in urban areas is much higher than that in rural areas. Subsequently, we obtained three coronavirus genome sequences by deep sequencing. After different method-based analyses, we found that RCoV-GCCDC4 was a novel potentially recombinant coronavirus with a polybasic cleavage site in the S protein, dominant in RCoV. This newly identified coronavirus RCoV-GCCDC4 with its potentially recombinant genome and polybasic cleavage site provides a new insight into the evolution of coronaviruses. Furthermore, our results provide further information on the potential for interspecies transmission of coronaviruses and demonstrate the necessity of a One Health approach for zoonotic disease surveillance.

Discovery and Evolution of a Divergent Coronavirus in the Plateau Pika From China That Extends the Host Range of Alphacoronaviruses

Although plateau pikas are the keystone species in the plateau ecosystem of the Qinghai Province of China, little is known about their role in the evolution and transmission of viral pathogens, especially coronaviruses. Here, we describe the characterization and evolution of a novel alphacoronavirus, termed plateau pika coronavirus (PPCoV) P83, which has a prevalence of 4.5% in plateau pika fecal samples. In addition to classical gene order, the complete viral genome contains a unique nonstructural protein (NS2), several variable transcription regulatory sequences and a highly divergent spike protein. Phylogenetic analysis indicates that the newly discovered PPCoV falls into the genus Alphacoronavirus and is most closely related to rodent alphacoronaviruses. The co-speciation analysis shows that the phylogenetic trees of the alphacoronaviruses and their hosts are not always matched, suggesting inter-species transmission is common in alphacoronaviruses. And, PPCoV origin was estimated by molecular clock based on membrane and RNA-dependent RNA polymerase encoding genes, respectively, which revealed an apparent discrepancy with that of co-speciation analysis. PPCoV was detected mainly in intestinal samples, indicating a potential enteric tropism for the virus. Overall, this study extends the host range of alphacoronaviruses to a new order (Lagomorpha), indicating that plateau pikas may be the natural reservoir of PPCoV and play an important and long-term role in alphacoronavirus evolution.

An overview of potential inhibitors targeting non-structural proteins 3 (PLpro and Mac1) and 5 (3CLpro/Mpro) of SARS-CoV-2

There is an urgent need to develop effective treatments for coronavirus disease 2019 (COVID-19), which is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The rapid spread of SARS-CoV-2 has resulted in a global pandemic that has not only affected the daily lives of individuals but also had a significant impact on the global economy and public health. Although extensive research has been conducted to identify inhibitors targeting SARS-CoV-2, there are still no effective treatment strategies to combat COVID-19. SARS-CoV-2 comprises two important proteolytic enzymes, namely, the papain-like proteinase, located within non-structural protein 3 (nsp3), and nsp5, both of which cleave large replicase polypeptides into multiple fragments that are required for viral replication. Moreover, a domain within nsp3, known as the macrodomain (Mac1), also plays an important role in viral replication. Inhibition of their functions should be able to significantly interfere with the replication cycle of the virus, and therefore these key proteins may serve as potential therapeutic targets. The functions of the above viral targets and their corresponding inhibitors have been summarized in the current review. This review provides comprehensive updates of nsp3 and nsp5 inhibitor development and would help advance the discovery of novel anti-viral therapeutics against SARS-CoV-2.

Potential 3-chymotrypsin-like cysteine protease cleavage sites in the coronavirus polyproteins pp1a and pp1ab and their possible relevance to COVID-19 vaccine and drug development

Coronavirus (CoV) 3-chymotrypsin (C)-like cysteine protease (3CLpro ) is a target for anti-CoV drug development and drug repurposing because along with papain-like protease, it cleaves CoV-encoded polyproteins (pp1a and pp1ab) into nonstructural proteins (nsps) for viral replication. However, the cleavage sites of 3CLpro and their relevant nsps remain unclear, which is the subject of this perspective. Here, we address the subject from three standpoints. First, we explore the inconsistency in the cleavage sites and relevant nsps across CoVs, and investigate the function of nsp11. Second, we consider the nsp16 mRNA overlapping of the spike protein mRNA, and analyze the effect of this overlapping on mRNA vaccines. Finally, we study nsp12, whose existence depends on ribosomal frameshifting, and investigate whether 3CLpro requires a large number of inhibitors to achieve full inhibition. This perspective helps us to clarify viral replication and is useful for developing anti-CoV drugs with 3CLpro as a target in the current coronavirus disease 2019 (COVID-19) pandemic.

The identification of novel inhibitors of human angiotensin-converting enzyme 2 and main protease of Sars-Cov-2: A combination of in silico methods for treatment of COVID-19

The angiotensin-converting enzyme 2 (ACE2) and main protease (MPro), are the putative drug candidates for the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). In this study, we performed 3D-QSAR pharmacophore modeling and screened 1,264,479 ligands gathered from Pubchem and Zinc databases. Following the calculation of the ADMET properties, molecular docking was carried out. Moreover, the de novo ligand design was performed. MD simulation was then applied to survey the behavior of the ligand-protein complexes. Furthermore, MMPBSA has utilized to re-estimate the binding affinities. Then, a free energy landscape was used to find the most stable conformation of the complexes. Finally, the hybrid QM-MM method was carried out for the precise calculation of the energies.

The Hypo1 pharmacophore model was selected as the best model. Our docking results indicate that the compounds ZINC12562757 and 112,260,215 were the best potential inhibitors of the ACE2 and MPro, respectively. Furthermore, the Evo_1 compound enjoys the highest docking energy among the designed de novo ligands. Results of RMSD, RMSF, H-bond, and DSSP analyses have demonstrated that the lead compounds preserve the stability of the complexes’ conformation during the MD simulation. MMPBSA data confirmed the molecular docking results. The results of QM-MM showed that Evo_1 has a stronger potential for specific inhibition of MPro, as compared to the 112,260,215 compound.

Comparison of the binding characteristics of SARS-CoV and SARS-CoV-2 RBDs to ACE2 at different temperatures by MD simulations

Temperature plays a significant role in the survival and transmission of SARS-CoV (severe acute respiratory syndrome coronavirus) and SARS-CoV-2. To reveal the binding differences of SARS-CoV and SARS-CoV-2 receptor-binding domains (RBDs) to angiotensin-converting enzyme 2 (ACE2) at different temperatures at atomic level, 20 molecular dynamics simulations were carried out for SARS-CoV and SARS-CoV-2 RBD-ACE2 complexes at five selected temperatures, i.e. 200, 250, 273, 300 and 350 K. The analyses on structural flexibility and conformational distribution indicated that the structure of the SARS-CoV-2 RBD was more stable than that of the SARS-CoV RBD at all investigated temperatures. Then, molecular mechanics Poisson-Boltzmann surface area and solvated interaction energy approaches were combined to estimate the differences in binding affinity of SARS-CoV and SARS-CoV-2 RBDs to ACE2; it is found that the binding ability of ACE2 to the SARS-CoV-2 RBD was stronger than that to the SARS-CoV RBD at five temperatures, and the main reason for promoting such binding differences is electrostatic and polar interactions between RBDs and ACE2. Finally, the hotspot residues facilitating the binding of SARS-CoV and SARS-CoV-2 RBDs to ACE2, the key differential residues contributing to the difference in binding and the interaction mechanism of differential residues that exist at all investigated temperatures were analyzed and compared in depth. The current work would provide a molecular basis for better understanding of the high infectiousness of SARS-CoV-2 and offer better theoretical guidance for the design of inhibitors targeting infectious diseases caused by SARS-CoV-2.

A Review on Plant Bioactive Compounds and Their Modes of Action Against Coronavirus Infection

The rapid outbreak of coronavirus disease 2019 (COVID-19) has demonstrated the need for development of new vaccine candidates and therapeutic drugs to fight against the underlying virus, severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2). Currently, no antiviral treatment is available to treat COVID-19 as treatment is mostly directed to only relieving the symptoms. Retrospectively, herbal medicinal plants have been used for thousands of years as a medicinal alternative including for the treatment of various viral illnesses. However, a comprehensive description using various medicinal plants in treating coronavirus infection has not to date been described adequately, especially their modes of action. Most other reports and reviews have also only focused on selected ethnobotanical herbs such as Traditional Chinese Medicine, yet more plants can be considered to enrich the source of the anti-viral compounds. In this review, we have screened and identified potential herbal medicinal plants as anti-coronavirus medication across major literature databases without being limited to any regions or ethnobotanic criteria. As such we have successfully gathered experimentally validated in vivo, in vitro, or in silico findings of more than 30 plants in which these plant extracts or their related compounds, such as those of Artemisia annua L., Houttuynia cordata Thunb., and Sambucus formosana Nakai, are described through their respective modes of action against specific mechanisms or pathways during the viral infection. This includes inhibition of viral attachment and penetration, inhibition of viral RNA and protein synthesis, inhibition of viral key proteins such as 3-chymotrypsin-like cysteine protease (3CLpro) and papain-like protease 2 (PLpro), as well as other mechanisms including inhibition of the viral release and enhanced host immunity. We hope this compilation will help researchers and clinicians to identify the source of appropriate anti-viral drugs from plants in combating COVID-19 and, ultimately, save millions of affected human lives.

Structure-Based Screening to Discover New Inhibitors for Papain-like Proteinase of SARS-CoV-2: An In Silico Study

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) expresses a multifunctional papain-like proteinase (PLpro), which mediates the processing of the viral replicase polyprotein. Inhibition of PLpro has been shown to suppress the viral replication. This study aimed to explore new anti-PLpro candidates by applying virtual screening based on GRL0617, a known PLpro inhibitor of SARS coronavirus (SARS-CoV). The three-dimensional (3D) structure of SARS-CoV-2 PLpro was built by homology modeling, using SARS-CoV PLpro as the template. The model was refined and studied through molecular dynamic simulation. AutoDock Vina was then used to perform virtual screening where 50 chemicals with at least 65% similarity to GRL0617 were docked with the optimized SARS-CoV-2 PLpro. In this screening, 5-(aminomethyl)-2-methyl-N-[(1R)-1-naphthalen-1-ylethyl]benzamide outperformed GRL0617 in terms of binding affinity (-9.7 kcal/mol). Furthermore, 2-(4-fluorobenzyl)-5-nitro-1H-isoindole-1,3(2H)-dione (previously introduced as an inhibitor of cyclooxygenase-2), 3-nitro-N-[(1r)-1-phenylethyl]-5-(trifluoromethyl)benzamide (inhibitor against Mycobacterium tuberculosis), as well as the recently introduced SARS-CoV-2 PLpro inhibitor 5-acetamido-2-methyl-N-[(1S)-1-naphthalen-1-ylethyl]benzamide showed promising affinity for the viral proteinase. All of the identified compounds demonstrated an acceptable pharmacokinetic profile. In conclusion, our findings represent rediscovery of analgesic, anti-inflammatory, antibacterial, or antiviral drugs as promising pharmaceutical candidates against the ongoing coronavirus.

Advances in developing small molecule SARS 3CLpro inhibitors as potential remedy for corona virus infection

Originated in China, coronavirus disease 2019 (COVID-19)- the highly contagious and fatal respiratory disease caused by SARS-CoV-2 has already infected more than 29 million people worldwide with a mortality rate of 3.15% (according to World Health Organization's (WHO's) report, September 2020) and the number is exponentially increasing with no remedy whatsoever discovered till date. But it is not the first time this infectious viral disease has appeared, in 2002 SARS-CoV infected more than 8000 individuals of which 9.6% patients died and in 2012 approximately 35% of MERS-CoV infected patients have died. Literature reports indicate that a chymotripsin-like cystein protease (3CLpro) is responsible for the replication of the virus inside the host cell. Therefore, design and synthesis of 3CLpro inhibitor molecules play a great impact in drug development against this COVID-19 pandemic. In this review, we are discussing the anti-SARS effect of some small molecule 3CLpro inhibitors with their various binding modes of interactions to the target protein.

Porcine epidemic diarrhea virus: Molecular mechanisms of attenuation and vaccines

Porcine epidemic diarrhea virus (PEDV) causes an emerging and re-emerging coronavirus disease characterized by vomiting, acute diarrhea, dehydration, and up to 100% mortality in neonatal suckling piglets, leading to huge economic losses in the global swine industry. Vaccination remains the most promising and effective way to prevent and control PEDV. However, effective vaccines for PEDV are still under development. Understanding the genomic structure and function of PEDV and the influence of the viral components on innate immunity is essential for developing effective vaccines. In the current review, we systematically describe the recent developments in vaccine against PEDV and the roles of structural proteins, non-structural proteins and accessory proteins of PEDV in affecting viral virulence and regulating innate immunity, which will provide insight into the rational design of effective and safe vaccines for PEDV or other coronaviruses.

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Advances in vaccines of PEDV, such as inactivated and attenuated live vaccines, subunit vaccines, and nucleic acid vaccines.

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The application of reverse genetics in the development of live attenuated PEDV vaccines.

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The roles of PEDV proteins in affecting viral virulence and regulating innate immunity.

Blood Targets of Adjuvant Drugs Against COVID19

While waiting for the vaccine and/or the best treatment for COVID19, several drugs have been identified as potential adjuvant drugs to counteract the viral action. Several drugs, in fact, have been suggested for their ancillary antiviral role. Viral proteases and peptidases, may interact with well-known drugs such as anticoagulants, antihypertensives, antiserotoninergics and immunomodulants. We here report a basic list of these drugs that include bioflavonoids, heparinoids, ACE inhibitors, angiotensin receptor blockers, antiserotoninergics, and monoclonal antibodies against cytokines that may interact with the viral cycle.

Computational Models Identify Several FDA Approved or Experimental Drugs as Putative Agents Against SARS-CoV-2

The outbreak of a novel human coronavirus (SARS-CoV-2) has evolved into global health emergency, infecting hundreds of thousands of people worldwide. We have identified experimental data on the inhibitory activity of compounds tested against closely related (96% sequence identity, 100% active site conservation) protease of SARS-CoV and employed this data to build QSAR models for this dataset. We employed these models for virtual screening of all drugs from DrugBank, including compounds in clinical trials. Molecular docking and similarity search approaches were explored in parallel with QSAR modeling, but molecular docking failed to correctly discriminate between experimentally active and inactive compounds. As a result of our studies, we recommended 41 approved, experimental, or investigational drugs as potential agents against SARS-CoV-2 acting as putative inhibitors of Mpro. Ten compounds with feasible prices were purchased and are awaiting the experimental validation.
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Structural and Biochemical Characterization of Endoribonuclease Nsp15 Encoded by Middle East Respiratory Syndrome Coronavirus

Nonstructural protein 15 (Nsp15) encoded by coronavirus (CoV) is a nidoviral uridylate-specific endoribonuclease (NendoU) that plays an essential role in the life cycle of the virus. Structural information on this crucial protein from the Middle East respiratory syndrome CoV (MERS-CoV), which is lethally pathogenic and has caused severe respiratory diseases worldwide, is lacking. Here, we determined the crystal structure of MERS-CoV Nsp15 at a 2.7-Å resolution and performed the relevant biochemical assays to study how NendoU activity is regulated. Although the overall structure is conserved, MERS-CoV Nsp15 shows unique and novel features compared to its homologs. Serine substitution of residue F285, which harbors an aromatic side chain that disturbs RNA binding compared with that of other homologs, increases catalytic activity. Mutations of residues residing on the oligomerization interfaces that distort hexamerization, namely, N38A, Y58A, and N157A, decrease thermostability, decrease affinity of binding with RNA, and reduce the NendoU activity of Nsp15. In contrast, mutant D39A exhibits increased activity and a higher substrate binding capacity. Importantly, Nsp8 was found to interact with both monomeric and hexameric Nsp15. The Nsp7/Nsp8 complex displays a higher binding affinity for Nsp15. Furthermore, Nsp8 and the Nsp7/Nsp8 complex also enhance the NendoU activity of hexameric Nsp15 in vitro Taking the findings together, this work first provides evidence on how the activity of Nsp15 may be functionally mediated by catalytic residues, oligomeric assembly, RNA binding efficiency, or the possible association with other nonstructural proteins.IMPORTANCE The lethally pathogenic Middle East respiratory syndrome coronavirus (MERS-CoV) and the severe acute respiratory syndrome coronavirus (SARS-CoV) pose serious threats to humans. Endoribonuclease Nsp15 encoded by coronavirus plays an important role in viral infection and pathogenesis. This study determines the structure of MERS-CoV Nsp15 and demonstrates how the catalytic activity of this protein is potentially mediated, thereby providing structural and functional evidence for developing antiviral drugs. We also hypothesize that the primase-like protein Nsp8 and the Nsp7/Nsp8 complex may interact with Nsp15 and affect enzymatic activity. This contributes to the understanding of the association of Nsp15 with the viral replication and transcription machinery.

Genomic characterization and pathogenicity of a porcine hemagglutinating encephalomyelitis virus strain isolated in China

Porcine hemagglutinating encephalomyelitis virus (PHEV) is a member of the genus betacoronavirus within the family coronaviridae, which invades the central nervous system (CNS) via peripheral nervous system and causes encephalomyelitis or vomiting and wasting disease (VWD) in sucking piglets. Up to now, although few complete nucleotide sequences of PHEV have been reported, they are not annotated. This study aimed to illuminate genome characterization, phylogenesis and pathogenicity of the PHEV/2008 strain. The full length of the PHEV/2008 strain genome was 30,684 bp, with a G + C content of 37.27%. The genome included at a minimum of 11 predicted open reading frames (ORFs) flanked by 5' and 3' untranslated regions (UTR) of 211 and 289 nucleotides. The replicase polyproteins pp1a and pp1ab, which had 4382 and 7094 amino acid residues, respectively, were predicted to be cleaved into 16 subunits by two viral proteinases. Phylogenetic analysis based on the complete genome sequence revealed that PHEV/2008 strain was genetically different from other known PHEV types, which represented a novel genotype (GI-1). In addition, we found that PHEV/2008 was neurotropic and highly pathogenic to 4-week-old BALB/c mice. Taken together, this is the first detailed annotated, complete genomic sequence of a new genotype PHEV strain in China.

Detection of alpha- and betacoronaviruses in rodents from Yunnan, China

Background

Rodents represent the most diverse mammals on the planet and are important reservoirs of human pathogens. Coronaviruses infect various animals, but to date, relatively few coronaviruses have been identified in rodents worldwide. The evolution and ecology of coronaviruses in rodent have not been fully investigated.

Results

In this study, we collected 177 intestinal samples from thress species of rodents in Jianchuan County, Yunnan Province, China. Alphacoronavirus and betacoronavirus were detected in 23 rodent samples from three species, namely Apodemus chevrieri (21/98), Eothenomys fidelis (1/62), and Apodemus ilex (1/17). We further characterized the full-length genome of an alphacoronavirus from the A. chevrieri rat and named it as AcCoV-JC34. The AcCoV-JC34 genome was 27,649 nucleotides long and showed a structure similar to the HKU2 bat coronavirus. Comparing the normal transcription regulatory sequence (TRS), 3 variant TRS sequences upstream the spike (S), ORF3, and ORF8 genes were found in the genome of AcCoV-JC34. In the conserved replicase domains, AcCoV-JC34 was most closely related to Rattus norvegicus coronavirus LNRV but diverged from other alphacoronaviruses, indicating that AcCoV-JC34 and LNRV may represent a novel alphacoronavirus species. However, the S and nucleocapsid proteins showed low similarity to those of LRNV, with 66.5 and 77.4% identities, respectively. Phylogenetic analysis revealed that the S genes of AcCoV-JC34, LRNV, and HKU2 formed a distinct lineage with all known coronaviruses.

Conclusions

Both alphacoronaviruses and betacoronaviruses were detected in Apodemus chevrieri in the Yunnan Province of China, indicating that Apodemus chevrieri is an important host for coronavirus. Several new features were identified in the genome of an Apodemus chevrieri coronavirus. The phylogenetic distance to other coronaviruses suggests a variable origin and evolutionary route of the S genes of AcCoV-JC34, LRNV, and HKU2. These results indicate that the diversity of rodent coronaviruses is much higher than previously expected. Further surveillance and functional studies of these coronaviruses will help to better understand the importance of rodent as host for coronaviruses.

Electronic supplementary material

The online version of this article (doi:10.1186/s12985-017-0766-9) contains supplementary material, which is available to authorized users.

A Bat-Derived Putative Cross-Family Recombinant Coronavirus with a Reovirus Gene

The emergence of severe acute respiratory syndrome coronavirus (SARS-CoV) in 2002 and Middle East respiratory syndrome coronavirus (MERS-CoV) in 2012 has generated enormous interest in the biodiversity, genomics and cross-species transmission potential of coronaviruses, especially those from bats, the second most speciose order of mammals. Herein, we identified a novel coronavirus, provisionally designated Rousettus bat coronavirus GCCDC1 (Ro-BatCoV GCCDC1), in the rectal swab samples of Rousettus leschenaulti bats by using pan-coronavirus RT-PCR and next-generation sequencing. Although the virus is similar to Rousettus bat coronavirus HKU9 (Ro-BatCoV HKU9) in genome characteristics, it is sufficiently distinct to be classified as a new species according to the criteria defined by the International Committee of Taxonomy of Viruses (ICTV). More striking was that Ro-BatCoV GCCDC1 contained a unique gene integrated into the 3'-end of the genome that has no homologs in any known coronavirus, but which sequence and phylogeny analyses indicated most likely originated from the p10 gene of a bat orthoreovirus. Subgenomic mRNA and cellular-level observations demonstrated that the p10 gene is functional and induces the formation of cell syncytia. Therefore, here we report a putative heterologous inter-family recombination event between a single-stranded, positive-sense RNA virus and a double-stranded segmented RNA virus, providing insights into the fundamental mechanisms of viral evolution.

Naturally occurring recombination in ferret coronaviruses revealed by complete genome characterization

Ferret coronaviruses (FRCoVs) exist as an enteric and a systemic pathotype, of which the latter is highly lethal to ferrets. To our knowledge, this study provides the first full genome sequence of a FRCoV, tentatively called FRCoV-NL-2010, which was detected in 2010 in ferrets in The Netherlands. Phylogenetic analysis showed that FRCoV-NL-2010 is most closely related to mink CoV, forming a separate clade of mustelid alphacoronavirus that split off early from other alphacoronaviruses. Based on sequence homology of the complete genome, we propose that these mustelid coronaviruses may be assigned to a new species. Comparison of FRCoV-NL-2010 with the partially sequenced ferret systemic coronavirus MSU-1 and ferret enteric coronavirus MSU-2 revealed that recombination in the spike, 3c and envelope genes occurred between different FRCoVs.

Detection and characterization of diverse alpha- and betacoronaviruses from bats in China

Bats have been implicated as important reservoir hosts of alpha- and betacoronaviruses. In this study, diverse coronaviruses (CoVs) were detected in 50 of 951 (positive rate 5.3%) intestinal specimens of eight bat species collected in four provinces and the Tibet Autonomous Region of China by pan-coronavirus RT-PCR screening. Based on 400-nt RNA-dependent RNA polymerase (RdRP) sequence analysis, eight belonged to genus Alphacoronavirus and 42 to Betacoronavirus. Among the 50 positive specimens, thirteen gave rise to CoV full-length RdRP gene amplification for further sequence comparison, of which three divergent sequences (two from a unreported province) were subjected to full genome sequencing. Two complete genomes of betacoronaviruses (JTMC15 and JPDB144) and one nearly-complete genome of alphacoronavirus (JTAC2) were sequenced and their genomic organization predicted. The present study has identified additional numbers of genetically diverse bat-borne coronaviruses with a wide distribution in China. Two new species of bat CoV, identified through sequence comparison and phylogenetic analysis, are proposed.

First complete genome sequence of European turkey coronavirus suggests complex recombination history related with US turkey and guinea fowl coronaviruses

A full-length genome sequence of 27 739 nt was determined for the only known European turkey coronavirus (TCoV) isolate. In general, the order, number and size of ORFs were consistent with other gammacoronaviruses. Three points of recombination were predicted, one towards the end of 1a, a second in 1b just upstream of S and a third in 3b. Phylogenetic analysis of the four regions defined by these three points supported the previous notion that European and American viruses do indeed have different evolutionary pathways. Very close relationships were revealed between the European TCoV and the European guinea fowl coronavirus in all regions except one, and both were shown to be closely related to the European infectious bronchitis virus (IBV) Italy 2005. None of these regions of sequence grouped European and American TCoVs. The region of sequence containing the S gene was unique in grouping all turkey and guinea fowl coronaviruses together, separating them from IBVs. Interestingly the French guinea fowl virus was more closely related to the North American viruses. These data demonstrate that European turkey and guinea fowl coronaviruses share a common genetic backbone (most likely an ancestor of IBV Italy 2005) and suggest that this recombined in two separate events with different, yet related, unknown avian coronaviruses, acquiring their S-3a genes. The data also showed that the North American viruses do not share a common backbone with European turkey and guinea fowl viruses; however, they do share similar S-3a genes with guinea fowl virus.

Prediction and biochemical analysis of putative cleavage sites of the 3C-like protease of Middle East respiratory syndrome coronavirus

Coronavirus 3C-like protease (3CLpro) is responsible for the cleavage of coronaviral polyprotein 1a/1ab (pp1a/1ab) to produce the mature non-structural proteins (nsps) of nsp4-16. The nsp5 of the newly emerging Middle East respiratory syndrome coronavirus (MERS-CoV) was identified as 3CLpro and its canonical cleavage sites (between nsps) were predicted based on sequence alignment, but the cleavability of these cleavage sites remains to be experimentally confirmed and putative non-canonical cleavage sites (inside one nsp) within the pp1a/1ab awaits further analysis. Here, we proposed a method for predicting coronaviral 3CLpro cleavage sites which balances the prediction accuracy and false positive outcomes. By applying this method to MERS-CoV, the 11 canonical cleavage sites were readily identified and verified by the biochemical assays. The Michaelis constant of the canonical cleavage sites of MERS-CoV showed that the substrate specificity of MERS-CoV 3CLpro is relatively conserved. Interestingly, nine putative non-canonical cleavage sites were predicted and three of them could be cleaved by MERS-CoV nsp5. These results pave the way for identification and functional characterization of new nsp products of coronaviruses.

Human coronavirus OC43 3CL protease and the potential of ML188 as a broad-spectrum lead compound: homology modelling and molecular dynamic studies

Background

The coronavirus 3 chymotrypsin-like protease (3CL^pro) is a validated target in the design of potential anticoronavirus inhibitors. The high degree of homology within the protease’s active site and substrate conservation supports the identification of broad spectrum lead compounds. A previous study identified the compound ML188, also termed 16R, as an inhibitor of the Severe Acute Respiratory Syndrome coronavirus (SARS-CoV) 3CL^pro. This study will detail the generation of a homology model of the 3CL^pro of the human coronavirus OC43 and determine the potential of 16R to form a broad-spectrum lead compound. MODELLER was used to generate a suitable three-dimensional model of the OC43 3CL^pro and the Prime module of Schrӧdinger predicted the binding conformation and free energy of binding of 16R within the 3CL^pro active site. Molecular dynamics further confirmed ligand stability and hydrogen bonding networks.

Results

A high quality homology model of the OC43 3CL^pro was successfully generated in an active conformation. Further studies reproduced the binding pose of 16R within the active site of the generated model, where its free energy of binding was shown to equal that of the 3CL^pro of SARS-CoV, a receptor it is experimentally proven to inhibit. The stability of the ligand was subsequently confirmed by molecular dynamics.

Conclusion

The lead compound 16R may represent a broad-spectrum inhibitor of the 3CL^pro of OC43 and potentially other coronaviruses. This study provides an atomistic structure of the 3CL^pro of OC43 and supports further experimental validation of the inhibitory effects of 16R. These findings further confirm that the 3CL^pro of coronaviruses can be inhibited by broad spectrum lead compounds.

A Brief Review: The Z-curve Theory and its Application in Genome Analysis

In theoretical physics, there exist two basic mathematical approaches, algebraic and geometrical methods, which, in most cases, are complementary. In the area of genome sequence analysis, however, algebraic approaches have been widely used, while geometrical approaches have been less explored for a long time. The Z-curve theory is a geometrical approach to genome analysis. The Z-curve is a three-dimensional curve that represents a given DNA sequence in the sense that each can be uniquely reconstructed given the other. The Z-curve, therefore, contains all the information that the corresponding DNA sequence carries. The analysis of a DNA sequence can then be performed through studying the corresponding Z-curve. The Z-curve method has found applications in a wide range of areas in the past two decades, including the identifications of protein-coding genes, replication origins, horizontally-transferred genomic islands, promoters, translational start sides and isochores, as well as studies on phylogenetics, genome visualization and comparative genomics. Here, we review the progress of Z-curve studies from aspects of both theory and applications in genome analysis.

Recent developments in anti-severe acute respiratory syndrome coronavirus chemotherapy

Severe acute respiratory syndrome coronavirus (SARS-CoV) emerged in early 2003 to cause a very severe acute respiratory syndrome, which eventually resulted in a 10% case-fatality rate. Owing to excellent public health measures that isolated focus cases and their contacts, and the use of supportive therapies, the epidemic was suppressed to the point that further cases have not appeared since 2005. However, despite intensive research since then (over 3500 publications), it remains an untreatable disease. The potential for re-emergence of the SARS-CoV or a similar virus with unknown but potentially serious consequences remains high. This is due in part to the extreme genetic variability of RNA viruses such as the coronaviruses, the many animal reservoirs that seem to be able host the SARS-CoV in which reassortment or recombination events could occur and the ability coronaviruses have to transmit relatively rapidly from species to species in a short period of time. Thus, it seems prudent to continue to explore and develop antiviral chemotherapies to treat SARS-CoV infections. To this end, the various efficacious anti-SARS-CoV therapies recently published from 2007 to 2010 are reviewed in this article. In addition, compounds that have been tested in various animal models and were found to reduce virus lung titers and/or were protective against death in lethal models of disease, or otherwise have been shown to ameliorate the effects of viral infection, are also reported.

Increase of SARS-CoV 3CL peptidase activity due to macromolecular crowding effects in the milieu composition

The 3C-like peptidase of the severe acute respiratory syndrome virus (SARS-CoV) is strictly required for viral replication, thus being a potential target for the development of antiviral agents. In contrast to monomeric picornavirus 3C peptidases, SARS-CoV 3CLpro exists in equilibrium between the monomer and dimer forms in solution, and only the dimer is proteolytically active in dilute buffer solutions. In this study, the increase of SARS-CoV 3CLpro peptidase activity in presence of kosmotropic salts and crowding agents is described. The activation followed the Hofmeister series of anions, with two orders of magnitude enhancement in the presence of Na2SO4, whereas the crowding agents polyethylene glycol and bovine serum albumin increased the hydrolytic rate up to 3 times. Kinetic determinations of the monomer dimer dissociation constant (K d) indicated that activation was a result of a more active dimer, without significant changes in K d values. The activation was found to be independent of substrate length and was derived from both k cat increase and K m decrease. The viral peptidase activation described here could be related to the crowded intracellular environment and indicates a further fine-tuning mechanism for biological control, particularly in the microenvironment of the vesicles that are induced in host cells during positive strand RNA virus infection.

Functional and genetic studies of the substrate specificity of coronavirus infectious bronchitis virus 3C-like proteinase

Coronavirus (CoV) 3C-like proteinase (3CLpro), located in nonstructural protein 5 (nsp5), processes the replicase polyproteins 1a and 1ab (pp1a and pp1ab) at 11 specific sites to produce 12 mature nonstructural proteins (nsp5 to nsp16). Structural and biochemical studies suggest that a conserved Gln residue at the P1 position is absolutely required for efficient cleavage. Here, we investigate the effects of amino acid substitution at the P1 position of 3CLpro cleavage sites of infectious bronchitis virus (IBV) on the cleavage efficiency and viral replication by in vitro cleavage assays and reverse genetic approaches. Our results demonstrated that a P1-Asn substitution at the nsp4-5/Q2779, nsp5-6/Q3086, nsp7-8/Q3462, nsp8-9/Q3672, and nsp9-10/Q3783 sites, a P1-Glu substitution at the nsp8-9/Q3672 site, and a P1-His substitution at the nsp15-16/Q6327 site were tolerated and allowed recovery of infectious mutant viruses, albeit with variable degrees of growth defects. In contrast, a P1-Asn substitution at the nsp6-7/Q3379, nsp12-13/Q4868, nsp13-14/Q5468, and nsp14-15/Q5989 sites, as well as a P1-Pro substitution at the nsp15-16/Q6327 site, abolished 3CLpro-mediated cleavage at the corresponding position and blocked the recovery of infectious viruses. Analysis of the effects of these lethal mutations on RNA synthesis suggested that processing intermediates, such as the nsp6-7, nsp12-13, nsp13-14, nsp14-15, and nsp15-16 precursors, may function in negative-stranded genomic RNA replication, whereas mature proteins may be required for subgenomic RNA (sgRNA) transcription. More interestingly, a mutant 3CLpro with either a P166S or P166L mutation was selected when an IBV infectious cDNA clone carrying the Q6327N mutation at the nsp15-16 site was introduced into cells. Either of the two mutations was proved to enhance significantly the 3CLpro-mediated cleavage efficiency at the nsp15-16 site with a P1-Asn substitution and compensate for the detrimental effects on recovery of infectious virus.

Structure and cleavage specificity of the chymotrypsin-like serine protease (3CLSP/nsp4) of Porcine Reproductive and Respiratory Syndrome Virus (PRRSV)

Biogenesis and replication of the porcine reproductive and respiratory syndrome virus (PRRSV) include the crucial step of replicative polyprotein processing by self-encoded proteases. Whole genome bioinformatics analysis suggests that nonstructural protein 4 (nsp4) is a 3C-like serine protease (3CLSP), responsible for most of the nonstructural protein processing. The gene encoding this protease was cloned and expressed in Escherichia coli in order to confirm this prediction. The purified protein was crystallized, and the structure was solved at 1.9 A resolution. In addition, the crystal structure of the Ser118Ala mutant was determined at 2.0 A resolution. The monomeric enzyme folds into three domains, similar to that of the homologous protease of equine arteritis virus, which, like PRRSV, is a member of the family Arteriviridae in the order of Nidovirales. The active site of the PRRSV 3CLSP is located between domains I and II and harbors a canonical catalytic triad comprising Ser118, His39, and Asp64. The structure also shows an atypical oxyanion hole and a partially collapsed S1 specificity pocket. The proteolytic activity of the purified protein was assessed in vitro. Three sites joining nonstructural protein domains in the PRRSV replicative polyprotein are confirmed to be processed by the enzyme. Two of them, the nsp3/nsp4 and nsp11/nsp12 junctions, are shown to be cleaved in trans, while cis cleavage is demonstrated for the nsp4/nsp5 linker. Thus, we provide structural evidence as well as enzymatic proof of the nsp4 protein being a functional 3CLSP. We also show that the enzyme has a strong preference for glutamic acid at the P1 position of the substrate.

Proteolytic processing of polyproteins 1a and 1ab between non-structural proteins 10 and 11/12 of Coronavirus infectious bronchitis virus is dispensable for viral replication in cultured cells

Coronavirus 3C-like proteinase (3CLpro) plays important roles in viral life cycle through extensive processing of the polyproteins 1a and 1ab into 12 mature, non-structural proteins (nsp5–nsp16). Structural and biochemical studies have revealed that all confirmed 3CLpro cleavage sites have a conserved Gln residue at the P1 position, which is thought to be absolutely required for efficient cleavage. Recent studies on murine hepatitis virus (MHV) showed that processing of the 1a polyprotein at the position between nsp10–nsp11 is essential for viral replication. In this report, we investigated the requirement of processing at the equivalent position for replication of avian coronavirus infectious bronchitis virus (IBV), using an infectious cloning system. The results showed that mutation of the P1 Gln to Pro or deletion of the Gln residue in the nsp10–nsp11/12 site completely abolished the 3CLpro-mediated processing, but allowed production of infectious recombinant viruses with variable degrees of growth defect, suggesting that cleavage at the nsp10–nsp11/12 site of IBV is dispensable for viral replication in cultured cells. This study would pave a way for potential vaccine development by generation of attenuated IBV from field isolates through manipulation of the nsp10–nsp11/12 cleavage site. Similar approaches would be also applicable to other human and animal coronaviruses.

Mutation of Gly-11 on the dimer interface results in the complete crystallographic dimer dissociation of severe acute respiratory syndrome coronavirus 3C-like protease: crystal structure with molecular dynamics simulations

SARS-CoV 3C-like protease (3CL(pro)) is an attractive target for anti-severe acute respiratory syndrome (SARS) drug discovery, and its dimerization has been extensively proved to be indispensable for enzymatic activity. However, the reason why the dissociated monomer is inactive still remains unclear due to the absence of the monomer structure. In this study, we showed that mutation of the dimer-interface residue Gly-11 to alanine entirely abolished the activity of SARS-CoV 3CL(pro). Subsequently, we determined the crystal structure of this mutant and discovered a complete crystallographic dimer dissociation of SARS-CoV 3CL(pro). The mutation might shorten the alpha-helix A' of domain I and cause a mis-oriented N-terminal finger that could not correctly squeeze into the pocket of another monomer during dimerization, thus destabilizing the dimer structure. Several structural features essential for catalysis and substrate recognition are severely impaired in the G11A monomer. Moreover, domain III rotates dramatically against the chymotrypsin fold compared with the dimer, from which we proposed a putative dimerization model for SARS-CoV 3CL(pro). As the first reported monomer structure for SARS-CoV 3CL(pro), the crystal structure of G11A mutant might provide insight into the dimerization mechanism of the protease and supply direct structural evidence for the incompetence of the dissociated monomer.

Crystal structure of a monomeric form of severe acute respiratory syndrome coronavirus endonuclease nsp15 suggests a role for hexamerization as an allosteric switch

Mature nonstructural protein-15 (nsp15) from the severe acute respiratory syndrome coronavirus (SARS-CoV) contains a novel uridylate-specific Mn2+-dependent endoribonuclease (NendoU). Structure studies of the full-length form of the obligate hexameric enzyme from two CoVs, SARS-CoV and murine hepatitis virus, and its monomeric homologue, XendoU from Xenopus laevis, combined with mutagenesis studies have implicated several residues in enzymatic activity and the N-terminal domain as the major determinant of hexamerization. However, the tight link between hexamerization and enzyme activity in NendoUs has remained an enigma. Here, we report the structure of a trimmed, monomeric form of SARS-CoV nsp15 (residues 28 to 335) determined to a resolution of 2.9 A. The catalytic loop (residues 234 to 249) with its two reactive histidines (His 234 and His 249) is dramatically flipped by approximately 120 degrees into the active site cleft. Furthermore, the catalytic nucleophile Lys 289 points in a diametrically opposite direction, a consequence of an outward displacement of the supporting loop (residues 276 to 295). In the full-length hexameric forms, these two loops are packed against each other and are stabilized by intimate intersubunit interactions. Our results support the hypothesis that absence of an adjacent monomer due to deletion of the hexamerization domain is the most likely cause for disruption of the active site, offering a structural basis for why only the hexameric form of this enzyme is active.

Crystal structure of nonstructural protein 10 from the severe acute respiratory syndrome coronavirus reveals a novel fold with two zinc-binding motifs

The severe acute respiratory syndrome coronavirus (SARS-CoV) possesses a large 29.7-kb positive-stranded RNA genome. The first open reading frame encodes replicase polyproteins 1a and 1ab, which are cleaved to generate 16 "nonstructural" proteins, nsp1 to nsp16, involved in viral replication and/or RNA processing. Among these, nsp10 plays a critical role in minus-strand RNA synthesis in a related coronavirus, murine hepatitis virus. Here, we report the crystal structure of SARS-CoV nsp10 at a resolution of 1.8 A as determined by single-wavelength anomalous dispersion using phases derived from hexatantalum dodecabromide. nsp10 is a single domain protein consisting of a pair of antiparallel N-terminal helices stacked against an irregular beta-sheet, a coil-rich C terminus, and two Zn fingers. nsp10 represents a novel fold and is the first structural representative of this family of Zn finger proteins found so far exclusively in coronaviruses. The first Zn finger coordinates a Zn2+ ion in a unique conformation. The second Zn finger, with four cysteines, is a distant member of the "gag-knuckle fold group" of Zn2+-binding domains and appears to maintain the structural integrity of the C-terminal tail. A distinct clustering of basic residues on the protein surface suggests a nucleic acid-binding function. Gel shift assays indicate that in isolation, nsp10 binds single- and double-stranded RNA and DNA with high-micromolar affinity and without obvious sequence specificity. It is possible that nsp10 functions within a larger RNA-binding protein complex. However, its exact role within the replicase complex is still not clear.

Prevalence and genetic diversity of coronaviruses in bats from China

Coronaviruses can infect a variety of animals including poultry, livestock, and humans and are currently classified into three groups. The interspecies transmissions of coronaviruses between different hosts form a complex ecosystem of which little is known. The outbreak of severe acute respiratory syndrome (SARS) and the recent identification of new coronaviruses have highlighted the necessity for further investigation of coronavirus ecology, in particular the role of bats and other wild animals. In this study, we sampled bat populations in 15 provinces of China and reveal that approximately 6.5% of the bats, from diverse species distributed throughout the region, harbor coronaviruses. Full genomes of four coronavirues from bats were sequenced and analyzed. Phylogenetic analyses of the spike, envelope, membrane, and nucleoprotein structural proteins and the two conserved replicase domains, putative RNA-dependent RNA polymerase and RNA helicase, revealed that bat coronaviruses cluster in three different groups: group 1, another group that includes all SARS and SARS-like coronaviruses (putative group 4), and an independent bat coronavirus group (putative group 5). Further genetic analyses showed that different species of bats maintain coronaviruses from different groups and that a single bat species from different geographic locations supports similar coronaviruses. Thus, the findings of this study suggest that bats may play an integral role in the ecology and evolution of coronaviruses.

Synthesis of glutamic acid and glutamine peptides possessing a trifluoromethyl ketone group as SARS-CoV 3CL protease inhibitors

Trifluoromethyl-β-amino alcohol 11 [(4S)-tert-butyl 4-amino-6,6,6-trifluoro-5-hydroxyhexanoate] was synthesized in five steps starting from Cbz-l-Glu-OH 5 where the key step involved the introduction of the trifluoromethyl (CF₃) group to oxazolidinone 7, resulting in the formation of silyl ether 8 [(4S,5S)-benzyl 4-(2-(tert-butoxycarbonyl)ethyl)-5-(trifluoromethyl)-5-(trimethylsilyloxy)oxazolidine-3-carboxylate]. Compound 11 was then converted into four tri- and tetra-glutamic acid and glutamine peptides (1-4) possessing a CF₃-ketone group that exhibited inhibitory activity against severe acute respiratory syndrome coronavirus protease (SARS-CoV 3CL^pro).

Anti-SARS drug screening by molecular docking

Starting from a collection of 1386 druggable compounds obtained from the 3D pharmacophore search, we performed a similarity search to narrow down the scope of docking studies. The template molecule is KZ7088 (Chou et al., 2003, Biochem Biophys Res Commun 308: 148-151). The MDL MACCS keys were used to fingerprint the molecules. The Tanimoto coefficient is taken as the metric to compare fingerprints. If the similarity threshold was 0.8, a set of 50 unique hits and 103 conformers were retrieved as a result of similarity search. The AutoDock 3.011 was used to carry out molecular docking of 50 ligands to their macromolecular protein receptors. Three compounds, i.e., C(28)H(34)O(4)N(7)Cl, C(21)H(36)O(5)N(6), and C(21)H(36)O(5)N(6), were found that may be promising candidates for further investigation. The main feature shared by these three potential inhibitors as well as the information of the involved side chains of SARS Cov Mpro may provide useful insights for the development of potent inhibitors against SARS enzyme.

Programmed ribosomal frameshifting in HIV-1 and the SARS-CoV

Ribosomal frameshifting is a mechanism of gene expression used by several RNA viruses to express replicase enzymes. This article focuses on frameshifting in two human pathogens, the retrovirus human immunodeficiency virus type 1 (HIV-1) and the coronavirus responsible for severe acute respiratory syndrome (SARS). The nature of the frameshift signals of HIV-1 and the SARS–CoV will be described and the impact of this knowledge on models of frameshifting will be considered. The role of frameshifting in the replication cycle of the two pathogens and potential antiviral therapies targeting frameshifting will also be discussed.

Coronavirus phylogeny based on a geometric approach

A novel coronavirus has been identified as the cause of the outbreak of severe acute respiratory syndrome (SARS). Previous phylogenetic analyses based on sequence alignments show that SARS-CoVs form a new group distantly related to the other three groups of previously characterized coronaviruses. In this paper, a geometric approach based on the Z-curve representation of the whole genome sequence is proposed to analyze the phylogenetic relationships of coronaviruses. The evolutionary distances are obtained through measuring the differences among the three-dimensional Z-curves. The Z-curve is approximately described by its geometric center and the associated three eigenvectors, which indicate the center position and the trend of the Z-curve, respectively. Although some information is lost due to the approximate description of the Z-curve, the phylogenetic tree constructed based on these parameters is consistent with those of previous analyses. The present method has the merits of simplicity and intuitiveness, but it is still in its premature stage. Because the phylogenetic relationships are inferred from the whole genome, instead of some individual genes, the present method represents a new direction of phylogeny study in the post-genome era.

Molecular modeling and chemical modification for finding peptide inhibitor against severe acute respiratory syndrome coronavirus main proteinase

Severe acute respiratory syndrome (SARS) is a respiratory disease caused by a newly found virus, called SARS coronavirus. In this study, the cleavage mechanism of the SARS coronavirus main proteinase (M^pro or 3CL^pro) on the octapeptide NH₂-AVLQ ↓ SGFR-COOH was investigated using molecular mechanics and quantum mechanics simulations based on the experimental structure of the proteinase. It has been observed that the catalytic dyad (His-41/Cys-145) site between domains I and II attracts the π electron density from the peptide bond Gln–Ser, increasing the positive charge on C(CO) of Gln and the negative charge on N(NH) of Ser, so as to weaken the Gln–Ser peptide bond. The catalytic functional group is the imidazole group of His-41 and the S in Cys-145. N_δ1 on the imidazole ring plays the acid–base catalytic role. Based on the “distorted key theory” [K.C. Chou, Anal. Biochem. 233 (1996) 1–14], the possibility to convert the octapeptide to a competent inhibitor has been studied. It has been found that the chemical bond between Gln and Ser will become much stronger and no longer cleavable by the SARS enzyme after either changing the carbonyl group CO of Gln to CH₂ or CF₂ or changing the NH of Ser to CH₂ or CF₂. The octapeptide thus modified might become an effective inhibitor or a potential drug candidate against SARS.

Sequence analysis and structural prediction of the severe acute respiratory syndrome coronavirus nsp5

Abstract The non‐structural proteins (nsp or replicase proteins) of coronaviruses are relatively conserved and can be effective targets for drugs. Few studies have been conducted into the function of the severe acute respiratory syndrome coronavirus (SARS‐CoV) nsp5. In this study, bioinformatics methods were employed to predict the secondary structure and construct 3‐D models of the SARS‐CoV GD strain nsp5. Sequencing and sequential comparison was performed to analyze the mutation trend of the polymerase nsp5 gene during the epidemic process using a nucleotide‐nucleotide basic local alignment search tool (BLASTN) and a protein‐protein basic local alignment search tool (BLASTP). The results indicated that the nsp5 gene was steady during the epidemic process and the protein was homologous with other coronavirus nsp5 proteins. The protein encoded by the nsp5 gene was expressed in COS‐7 cells and analyzed by sodium dodecylsulfate‐polyacrylamide gel electrophoresis (SDS‐PAGE). This study provided the foundation for further exploration of the protein's biological function, and contributed to the search for anti‐SARS‐CoV drugs.

Edited by Bing SUN

Cinanserin is an inhibitor of the 3C-like proteinase of severe acute respiratory syndrome coronavirus and strongly reduces virus replication in vitro

The 3C-like proteinase (3CLpro) of severe acute respiratory syndrome-associated coronavirus (SARS-CoV) is one of the most promising targets for anti-SARS-CoV drugs due to its crucial role in the viral life cycle. In this study, a database containing structural information of more than 8,000 existing drugs was virtually screened by a docking approach to identify potential binding molecules of SARS-CoV 3CLpro. As a target for screening, both a homology model and the crystallographic structure of the binding pocket of the enzyme were used. Cinanserin (SQ 10,643), a well-characterized serotonin antagonist that has undergone preliminary clinical testing in humans in the 1960s, showed a high score in the screening and was chosen for further experimental evaluation. Binding of both cinanserin and its hydrochloride to bacterially expressed 3CLpro of SARS-CoV and the related human coronavirus 229E (HCoV-229E) was demonstrated by surface plasmon resonance technology. The catalytic activity of both enzymes was inhibited with 50% inhibitory concentration (IC50) values of 5 microM, as tested with a fluorogenic substrate. The antiviral activity of cinanserin was further evaluated in tissue culture assays, namely, a replicon system based on HCoV-229E and quantitative test assays with infectious SARS-CoV and HCoV-229E. All assays revealed a strong inhibition of coronavirus replication at nontoxic drug concentrations. The level of virus RNA and infectious particles was reduced by up to 4 log units, with IC50 values ranging from 19 to 34 microM. These findings demonstrate that the old drug cinanserin is an inhibitor of SARS-CoV replication, acting most likely via inhibition of the 3CL proteinase.

Mining SARS-CoV protease cleavage data using non-orthogonal decision trees: a novel method for decisive template selection

MOTIVATION

Although the outbreak of the severe acute respiratory syndrome (SARS) is currently over, it is expected that it will return to attack human beings. A critical challenge to scientists from various disciplines worldwide is to study the specificity of cleavage activity of SARS-related coronavirus (SARS-CoV) and use the knowledge obtained from the study for effective inhibitor design to fight the disease. The most commonly used inductive programming methods for knowledge discovery from data assume that the elements of input patterns are orthogonal to each other. Suppose a sub-sequence is denoted as P2-P1-P1'-P2', the conventional inductive programming method may result in a rule like 'if P1 = Q, then the sub-sequence is cleaved, otherwise non-cleaved'. If the site P1 is not orthogonal to the others (for instance, P2, P1' and P2'), the prediction power of these kind of rules may be limited. Therefore this study is aimed at developing a novel method for constructing non-orthogonal decision trees for mining protease data.

RESULT

Eighteen sequences of coronavirus polyprotein were downloaded from NCBI (http://www.ncbi.nlm.nih.gov). Among these sequences, 252 cleavage sites were experimentally determined. These sequences were scanned using a sliding window with size k to generate about 50,000 k-mer sub-sequences (for short, k-mers). The value of k varies from 4 to 12 with a gap of two. The bio-basis function proposed by Thomson et al. is used to transform the k-mers to a high-dimensional numerical space on which an inductive programming method is applied for the purpose of deriving a decision tree for decision-making. The process of this transform is referred to as a bio-mapping. The constructed decision trees select about 10 out of 50,000 k-mers. This small set of selected k-mers is regarded as a set of decisive templates. By doing so, non-orthogonal decision trees are constructed using the selected templates and the prediction accuracy is significantly improved.