Deubiquitinating activity of SARS-CoV-2 papain-like protease does not influence virus replication or innate immune responses in vivo.

The coronavirus papain-like protease (PLpro) is crucial for viral replicase polyprotein processing. Additionally, PLpro can subvert host defense mechanisms by its deubiquitinating (DUB) and deISGylating activities. To elucidate the role of these activities during SARS-CoV-2 infection, we introduced mutations that disrupt binding of PLpro to ubiquitin or ISG15. We identified several mutations that strongly reduced DUB activity of PLpro, without affecting viral polyprotein processing. In contrast, mutations that abrogated deISGylating activity also hampered viral polyprotein processing and when introduced into the virus these mutants were not viable. SARS-CoV-2 mutants exhibiting reduced DUB activity elicited a stronger interferon response in human lung cells. In a mouse model of severe disease, disruption of PLpro DUB activity did not affect lethality, virus replication, or innate immune responses in the lungs. This suggests that the DUB activity of SARS-CoV-2 PLpro is dispensable for virus replication and does not affect innate immune responses in vivo. Interestingly, the DUB mutant of SARS-CoV replicated to slightly lower titers in mice and elicited a diminished immune response early in infection, although lethality was unaffected. We previously showed that a MERS-CoV mutant deficient in DUB and deISGylating activity was strongly attenuated in mice. Here, we demonstrate that the role of PLpro DUB activity during infection can vary considerably between highly pathogenic coronaviruses. Therefore, careful considerations should be taken when developing pan-coronavirus antiviral strategies targeting PLpro.

Genome-wide identification and in-silico analysis of papain-family cysteine protease encoding genes in Tetrahymena thermophila.

Tetrahymena thermophila is a promising host for recombinant protein production, but its utilization in biotechnology is mostly limited due to the presence of intracellular and extracellular papain-family cysteine proteases (PFCPs). In this study, we employed bioinformatics approaches to investigate the T. thermophila PFCP genes and their encoded proteases (TtPFCPs), the most prominent protease family in the genome. Results from the multiple sequence alignment, protein modeling, and conserved motif analyses revealed that all TtPFCPs showed considerably high homology with mammalian cysteine cathepsins and contained conserved amino acid motifs. The total of 121 TtPFCP-encoding genes, 14 of which were classified as non-peptidase homologs, were found. Remaining 107 true TtPFCPs were divided into four distinct subgroups depending on their homology with mammalian lysosomal cathepsins: cathepsin L-like (TtCATLs), cathepsin B-like (TtCATBs), cathepsin C-like (TtCATCs), and cathepsin X-like (TtCATXs) PFCPs. The majority of true TtPFCPs (96 out of the total) were in TtCATL-like peptidase subgroup. Both phylogenetic and chromosomal localization analyses of TtPFCPs supported the hypothesis that TtPFCPs likely evolved through tandem gene duplication events and predominantly accumulated on micronuclear chromosome 5. Additionally, more than half of the identified TtPFCP genes are expressed in considerably low quantities compared to the rest of the TtPFCP genes, which are expressed at a higher level. However, their expression patterns fluctuate based on the stage of the life cycle. In conclusion, this study provides the first comprehensive in-silico analysis of TtPFCP genes and encoded proteases. The results would help designing an effective strategy for protease knockout mutant cell lines to discover biological function and to improve the recombinant protein production in T. thermophila.

Structure-based design of SARS-CoV-2 papain-like protease inhibitors.

The COVID-19 pandemic is caused by SARS-CoV-2, an RNA virus with high transmissibility and mutation rate. Given the paucity of orally bioavailable antiviral drugs to combat SARS-CoV-2 infection, there is a critical need for additional antivirals with alternative mechanisms of action. Papain-like protease (PLpro) is one of the two SARS-CoV-2 encoded viral cysteine proteases essential for viral replication. PLpro cleaves at three sites of the viral polyproteins. In addition, PLpro antagonizes the host immune response upon viral infection by cleaving ISG15 and ubiquitin from host proteins. Therefore, PLpro is a validated antiviral drug target. In this study, we report the X-ray crystal structures of papain-like protease (PLpro) with two potent inhibitors, Jun9722 and Jun9843. Subsequently, we designed and synthesized several series of analogs to explore the structure-activity relationship, which led to the discovery of PLpro inhibitors with potent enzymatic inhibitory activity and antiviral activity against SARS-CoV-2. Together, the lead compounds are promising drug candidates for further development.

Demonstrating the importance of porcine reproductive and respiratory syndrome virus papain-like protease 2 deubiquitinating activity in viral replication by structure-guided mutagenesis.

Deubiquitination of cellular substrates by viral proteases is a mechanism used to interfere with host cellular signaling processes, shared between members of the coronavirus- and arterivirus families. In the case of Arteriviruses, deubiquitinating and polyprotein processing activities are accomplished by the virus-encoded papain-like protease 2 (PLP2). Several studies have implicated the deubiquitinating activity of the porcine reproductive and respiratory syndrome virus (PRRSV) PLP2 in the downregulation of cellular interferon production, however to date, the only arterivirus PLP2 structure described is that of equine arteritis virus (EAV), a distantly related virus. Here we describe the first crystal structure of the PRRSV PLP2 domain both in the presence and absence of its ubiquitin substrate, which reveals unique structural differences in this viral domain compared to PLP2 from EAV. To probe the role of PRRSV PLP2 deubiquitinating activity in host immune evasion, we selectively removed this activity from the domain by mutagenesis and found that the viral domain could no longer downregulate cellular interferon production. Interestingly, unlike EAV, and also unlike the situation for MERS-CoV, we found that recombinant PRRSV carrying PLP2 DUB-specific mutations faces significant selective pressure to revert to wild-type virus in MARC-145 cells, suggesting that the PLP2 DUB activity, which in PRRSV is present as three different versions of viral protein nsp2 expressed during infection, is critically important for PRRSV replication.

First Identification and Pathogenicity Evaluation of an EV-G17 Strain Carrying a Torovirus Papain-like Cysteine Protease (PLCP) Gene in China.

Enterovirus G (EV-G) is prevalent in pig populations worldwide, and a total of 20 genotypes (G1 to G20) have been confirmed. Recently, recombinant EV-Gs carrying the papain-like cysteine protease (PLCP) gene of porcine torovirus have been isolated or detected, while their pathogenicity is poorly understood. In this study, an EV-G17-PLCP strain, 'EV-G/YN23/2022', was isolated from the feces of pigs with diarrhea, and the virus replicated robustly in numerous cell lines. The isolate showed the highest complete genome nucleotide (87.5%) and polyprotein amino acid (96.6%) identity in relation to the G17 strain 'IShi-Ya4' (LC549655), and a possible recombination event was detected at the 708 and 3383 positions in the EV-G/YN23/2022 genome. EV-G/YN23/2022 was nonlethal to piglets, but mild diarrhea, transient fever, typical skin lesions, and weight gain deceleration were observed. The virus replicated efficiently in multiple organs, and the pathological lesions were mainly located in the small intestine. All the challenged piglets showed seroconversion for EV-G/YN23/2022 at 6 to 9 days post-inoculation (dpi), and the neutralization antibody peaked at 15 dpi. The mRNA expression levels of IL-6, IL-18, IFN-α, IFN-ß, and ISG-15 in the peripheral blood mononuclear cells (PBMCs) were significantly up-regulated during viral infection. This is the first documentation of the isolation and pathogenicity evaluation of the EV-G17-PLCP strain in China. The results may advance our understanding of the evolution characteristics and pathogenesis of EV-G-PLCP.

Origin and Early Diversification of the Papain Family of Cysteine Peptidases.

Peptidases of the papain family play a key role in protein degradation, regulated proteolysis, and the host-pathogen arms race. Although the papain family has been the subject of many studies, knowledge about its diversity, origin, and evolution in Eukaryota, Bacteria, and Archaea is limited; thus, we aimed to address these long-standing knowledge gaps. We traced the origin and expansion of the papain family with a phylogenomic analysis, using sequence data from numerous prokaryotic and eukaryotic proteomes, transcriptomes, and genomes. We identified the full complement of the papain family in all prokaryotic and eukaryotic lineages. Analysis of the papain family provided strong evidence for its early diversification in the ancestor of eukaryotes. We found that the papain family has undergone complex and dynamic evolution through numerous gene duplications, which produced eight eukaryotic ancestral paralogous C1A lineages during eukaryogenesis. Different evolutionary forces operated on C1A peptidases, including gene duplication, horizontal gene transfer, and gene loss. This study challenges the current understanding of the origin and evolution of the papain family and provides valuable insights into their early diversification. The findings of this comprehensive study provide guidelines for future structural and functional studies of the papain family.

The substrate selectivity of papain-like proteases from human-infecting coronaviruses correlates with innate immune suppression.

Coronaviruses that can infect humans can cause either common colds (HCoV-NL63, HCoV-229E, HCoV-HKU1, and HCoV-OC43) or severe respiratory symptoms (SARS-CoV-2, SARS-CoV, and MERS-CoV). The papain-like proteases (PLPs) of SARS-CoV, SARS-CoV-2, MERS-CoV, and HCoV-NL63 function in viral innate immune evasion and have deubiquitinating (DUB) and deISGylating activities. We identified the PLPs of HCoV-229E, HCoV-HKU1, and HCoV-OC43 and found that their enzymatic properties correlated with their ability to suppress innate immune responses. A conserved noncatalytic aspartic acid residue was critical for both DUB and deISGylating activities, but the PLPs had differing ubiquitin (Ub) chain cleavage selectivities and binding affinities for Ub, K48-linked diUb, and interferon-stimulated gene 15 (ISG15) substrates. The crystal structure of HKU1-PLP2 in complex with Ub revealed binding interfaces that accounted for the unusually high binding affinity between this PLP and Ub. In cellular assays, the PLPs from the severe disease-causing coronaviruses strongly suppressed innate immune IFN-I and NF-κB signaling and stimulated autophagy, whereas the PLPs from the mild disease-causing coronaviruses generally showed weaker effects on immune suppression and autophagy induction. In addition, a PLP from a SARS-CoV-2 variant of concern showed increased suppression of innate immune signaling pathways. Overall, these results demonstrated that the DUB and deISGylating activities and substrate selectivities of these PLPs differentially contribute to viral innate immune evasion and may affect viral pathogenicity.

Identification, expression profiling, and functional characterization of cystatin C from big-belly seahorse (Hippocampus abdominalis).

Cystatins are natural inhibitors of lysosomal cysteine proteases, including cathepsins B, L, H, and S. Cystatin C (CSTC) is a member of the type 2 cystatin family and is an essential biomarker in the prognosis of several diseases. Emerging evidence suggests the immune regulatory roles of CSTC in antigen presentation, the release of different inflammatory mediators, and apoptosis in various pathophysiologies. In this study, the 390-bp cystatin C (HaCSTC) cDNA from big-belly seahorse (Hippocampus abdominalis) was cloned and characterized by screening the pre-established cDNA library. Based on similarities in sequence, HaCSTC is a homolog of the teleost type 2 cystatin family with putative catalytic cystatin domains, signal peptides, and disulfide bonds. HaCSTC transcripts were ubiquitously expressed in all tested big-belly seahorse tissues, with the highest expression in ovaries. Immune challenge with lipopolysaccharides, polyinosinic:polycytidylic acid, Edwardsiella tarda, and Streptococcus iniae caused significant upregulation in HaCSTC transcript levels. Using a pMAL-c5X expression vector, the 14.29-kDa protein of recombinant HaCSTC (rHaCSTC) was expressed in Escherichia coli BL21 (DE3), and its protease inhibitory activity against papain cysteine protease was determined with the aid of a protease substrate. Papain was competitively blocked by rHaCSTC in a dose-dependent manner. In response to viral hemorrhagic septicemia virus (VHSV) infection, HaCSTC overexpression strongly decreased the expression of VHSV transcripts, pro-inflammatory cytokines, and pro-apoptotic genes; while increasing the expression of anti-apoptotic genes in fathead minnow (FHM) cells. Furthermore, HaCSTC overexpression protected VHSV-infected FHM cells against VHSV-induced apoptosis and increased cell viability. Our findings imply the profound role of HaCSTC against pathogen infections by modulating fish immune responses.

Discovery of novel papain-like protease inhibitors for potential treatment of COVID-19.

The recent emergence of different SARS-CoV-2 variants creates an urgent need to develop more effective therapeutic agents to prevent COVID-19 outbreaks. Among SARS-CoV-2 essential proteases is papain-like protease (SARS-CoV-2 PLpro), which plays multiple roles in regulating SARS-CoV-2 viral spread and innate immunity such as deubiquitinating and deISG15ylating (interferon-induced gene 15) activities. Many studies are currently focused on targeting this protease to tackle SARS-CoV-2 infection. In this context, we performed a phenotypic screening using an in-house pilot compounds collection possessing a diverse skeleta against SARS-CoV-2 PLpro. This screen identified SIMR3030 as a potent inhibitor of SARS-CoV-2. SIMR3030 has been shown to exhibit deubiquitinating activity and inhibition of SARS-CoV-2 specific gene expression (ORF1b and Spike) in infected host cells and possessing virucidal activity. Moreover, SIMR3030 was demonstrated to inhibit the expression of inflammatory markers, including IFN-α, IL-6, and OAS1, which are reported to mediate the development of cytokine storms and aggressive immune responses. In vitro absorption, distribution, metabolism, and excretion (ADME) assessment of the drug-likeness properties of SIMR3030 demonstrated good microsomal stability in liver microsomes. Furthermore, SIMR3030 demonstrated very low potency as an inhibitor of CYP450, CYP3A4, CYP2D6 and CYP2C9 which rules out any potential drug-drug interactions. In addition, SIMR3030 showed moderate permeability in Caco2-cells. Critically, SIMR3030 has maintained a high in vivo safety profile at different concentrations. Molecular modeling studies of SIMR3030 in the active sites of SARS-CoV-2 and MERS-CoV PLpro were performed to shed light on the binding modes of this inhibitor. This study demonstrates that SIMR3030 is a potent inhibitor of SARS-CoV-2 PLpro that forms the foundation for developing new drugs to tackle the COVID-19 pandemic and may pave the way for the development of novel therapeutics for a possible future outbreak of new SARS-CoV-2 variants or other Coronavirus species.

Comparative analysis of two cathepsin L genes in Asiatic hard clam (Meretrix meretrix): Similar in sequence features, different in expression profiles.

Cathepsin L is widely found in eukaryotes and prokaryotes, and it plays important roles in innate immunity. In the present study, we cloned two cathepsin L genes (designated as MmCTSL1 and MmCTSL2, respectively) from Asiatic hard clam (Meretrix meretrix). The complete sequence of MmCTSL1 cDNA contained a 5' untranslated region (UTR) of 31 bp, a 3' UTR of 228 bp with a poly (A) tail, and an open reading frame (ORF) of 1005 bp encoding 334 amino acids with predicted molecular weight of 37.5 kDa and theoretical isoelectric point of 5.27, and contained a signal peptide (from M1 to A16), a protease inhibitor I29 family domain (from W27 to F87), and a papain family cysteine protease domain (from L118 to T333). The complete sequence of MmCTSL2 cDNA contained a 5' UTR of 50 bp, a 3' UTR of 162 bp with a poly (A) tail, and an ORF of 996 bp encoding a polypeptide of 331 amino acids with predicted molecular weight of 36.8 kDa and theoretical isoelectric point of 7.07. It contained a signal peptide (from M1 to A16), a protease inhibitor I29 family domain (from W30 to F89), and a papain family cysteine protease domain (from L115 to T330). Real-time quantitative PCR analysis demonstrated that MmCTSL1 and MmCTSL2 were widely expressed in all the tested tissues, including adductor muscle, foot, gill, hemocytes, hepatopancreas and mantle, with the highest mRNA expression level in hepatopancreas and hemocytes, respectively. After Vibrio splendidus challenge, the mRNA expression levels of MmCTSL1 and MmCTSL2 in hemocytes and hepatopancreas were both significantly up-regulated with different expression profiles. In hemocytes, the expression levels of MmCTSL1 and MmCTSL2 reached their respective peaks (3.4-fold and 13.0-fold compared with the control, respectively) at 12 h after bacterial challenge, and MmCTSL2 responds earlier than MmCTSL1. In hepatopancreas, the expression levels of MmCTSL1 and MmCTSL2 reached their respective peaks at 6 h (9.0-fold compared with the control) and 24 h (2.8-fold compared with the control) after bacterial challenge, meaning that MmCTSL1 responds earlier than MmCTSL2. At the same time, whether in hepatopancreas or hemocytes, MmCTSL1 persist for a while after the bacterial challenge peak, while MmCTSL2 would quickly return to the initial level after the bacterial challenge peak. These results indicate that cathepsin L may be involved in the immune process of hard clam against V. splendidus with different potential roles.

A computational essential dynamics approach to investigate structural influences of ligand binding on Papain like protease from SARS-CoV-2.

Papain like protease (PLpro) is a cysteine protease from the coronaviridae family of viruses. Coronaviruses possess a positive sense, single-strand RNA, leading to the translation of two viral polypeptides containing viral structural, non-structural and accessory proteins. PLpro is responsible for the cleavage of nsp1-3 from the viral polypeptide. PLpro also possesses deubiquitinating and deISGlyating activity, which sequesters the virus from the host's immune system. This indispensable attribute of PLpro makes it a protein of interest as a drug target. The present study aims to analyze the structural influences of ligand binding on PLpro. First, PLpro was screened against the ZINC-in-trials library, from which four lead compounds were identified based on estimated binding affinity and interaction patterns. Next, based on molecular docking results, ZINC000000596945, ZINC000064033452 and VIR251 (control molecule) were subjected to molecular dynamics simulation. The study evaluated global and essential dynamics analyses utilising principal component analyses, dynamic cross-correlation matrix, free energy landscape and time-dependant essential dynamics to predict the structural changes observed in PLpro upon ligand binding in a simulated environment. The MM/PBSA-based binding free energy calculations of the two selected molecules, ZINC000000596945 (-41.23 ± 3.70 kcal/mol) and ZINC000064033452 (-25.10 ± 2.65 kcal/mol), displayed significant values which delineate them as potential inhibitors of PLpro from SARS-CoV-2.

Heterologous expression of a papain-like protease inhibitor (SnuCalCpI17) in the E. coli and its mode of inhibition.

The effect of the Escherichia coli (E. coli) Rosetta (DE3) system on the expression of recombinant papain-like cysteine protease inhibitors (SnuCalCpIs) was evaluated, and the inhibition mode of the expressed inhibitor was determined. SnuCalCpI08 and SnuCalCpI17, which previously had not been expressed in the E. coli BL21 (DE3) system due to rare codons of more than 10%, were successfully expressed in E. coli Rosetta (DE3) since the strain provides tRNAs for six rare codons. Initially, both inhibitors were expressed as inclusion bodies; however, the water solubility of SnuCalCpI17 could be improved by lowering the incubation temperature, reducing the IPTG concentration, and increasing the induction time. In contrast, the other inhibitor could not be solubilized in water. To validate whether the inhibitor was expressed with correct protein folding, a papain inhibition assay was performed with SnuCalCpI17. SnuCalCpI17 showed a half-maximal inhibitory concentration (IC50) of 105.671 ± 9.857 µg/mL and a slow-binding inhibition mode against papain at pH 7.0 with a Kiapp of 75.80 µg/mL. The slow-binding inhibitor has a slow dissociation from the inhibitor-target complex, resulting in a long residence time in vivo, and thus can effectively inhibit the target at doses far below the IC50 of the inhibitor. KEY POINTS: • Propeptide inhibitor (SnuCalCpI17) containing rare codons was expressed in E. coli Rosetta (DE3). • The slow-binding inhibition was shown by plotting the apparent first-order rate constant (kobs). • Protein-protein interaction between SnuCalCpIs and papain was verified by docking simulation.

Investigation of the interaction of a papain-like cysteine protease (RD19c) with selenium-binding protein 1 (SBP1) in Arabidopsis thaliana.

AtRD19c is a member of the papain-like cysteine proteases known for its participation in anther development after its maturation by ßVPE (vacuolar processing enzyme). This papain-like cysteine protease was identified as an interacting protein of AtSBP1 (selenium binding protein 1) in a yeast two-hybrid screening. To confirm this interaction, we studied AtRD19c with respect to its expression and ability to interact with AtSBP1. The highest gene expression levels of AtRD19c were observed in the roots of 10-day-old seedlings, whereas minimum levels appeared in the hypocotyls of 10-day-old seedlings and flowers. AtRD19c expression was upregulated by selenium, and analysis of its promoter activity showed colocalization of a reporter gene (GUS) with AtSBP1. Additionally, the AtRD19c expression pattern was upregulated in the presence of selenite, indicating its participation in the Se response network. Confocal fluorescence microscopy revealed that AtRD19c localizes in the root tip, lateral roots, and leaf trichomes. Finally, we confirmed the physical interaction between AtRD19c and AtSBP1 and showed the importance of the first 175 aa of the AtSBP1 polypeptide in this interaction. Importantly, the AtRD19c-AtSBP1 interaction was also demonstrated in planta by employing bimolecular fluorescent complementation (BiFC) in a protoplast system.

Genome-wide survey of soybean papain-like cysteine proteases and their expression analysis in root nodule symbiosis.

BACKGROUND: Plant papain-like cysteine proteases (PLCPs) are a large class of proteolytic enzymes and play important roles in root nodule symbiosis (RNS), while the whole-genome studies of PLCP family genes in legume are quite limited, and the roles of Glycine max PLCPs (GmPLCPs) in nodulation, nodule development and senescence are not fully understood. RESULTS: In the present study, we identified 97 GmPLCPs and performed a genome-wide survey to explore the expansion of soybean PLCP family genes and their relationships to RNS. Nineteen paralogous pairs of genomic segments, consisting of 77 GmPLCPs, formed by whole-genome duplication (WGD) events were identified, showing a high degree of complexity in duplication. Phylogenetic analysis among different species showed that the lineage differentiation of GmPLCPs occurred after family expansion, and large tandem repeat segment were specifically in soybean. The expression patterns of GmPLCPs in symbiosis-related tissues and nodules identified RNS-related GmPLCPs and provided insights into their putative symbiotic functions in soybean. The symbiotic function analyses showed that a RNS-related GmPLCP gene (Glyma.04G190700) really participate in nodulation and nodule development. CONCLUSIONS: Our findings improved our understanding of the functional diversity of legume PLCP family genes, and provided insights into the putative roles of the legume PLCPs in nodulation, nodule development and senescence.

Cellular poly(C) binding protein 2 interacts with porcine epidemic diarrhea virus papain-like protease 1 and supports viral replication.

Porcine epidemic diarrhea virus (PEDV) belongs to the Alphacoronavirus genus in the Coronaviridae family. Similar to other coronaviruses, PEDV encodes two papain-like proteases. Papain-like protease (PLP)2 has been proposed to play a key role in antagonizing host innate immunity. However, the function of PLP1 remains unclear. In this study, we found that overexpression of PLP1 significantly promoted PEDV replication and inhibited production of interferon-ß. Immunoprecipitation and mass spectrometry were used to identify cellular interaction partners of PLP1. Host cell poly(C) binding protein 2 (PCBP2) was determined to bind and interact with PLP1. Both endogenous and overexpressed PCBP2 co-localized with PLP1 in the cytoplasm. Overexpression of PLP1 upregulated expression of PCBP2. Furthermore, overexpression of PCBP2 promoted PEDV replication. Silencing of endogenous PCBP2 using small interfering RNAs attenuated PEDV replication. Taken together, these data demonstrated that PLP1 negatively regulated the production of type 1 interferon by interacting with PCBP2 and promoted PEDV replication.

Genetic diversity of enterovirus G detected in faecal samples of wild boars in Japan: identification of novel genotypes carrying a papain-like cysteine protease sequence.

The genetic diversity of enterovirus G (EV-G) was investigated in the wild-boar population in Japan. EV-G-specific reverse transcription PCR demonstrated 30 (37.5 %) positives out of 80 faecal samples. Of these, viral protein 1 (VP1) fragments of 20 samples were classified into G1 (3 samples), G4 (1 sample), G6 (2 samples), G8 (4 samples), G11 (1 sample), G12 (7 samples), G14 (1 sample) and G17 (1 sample), among which 11 samples had a papain-like cysteine protease (PL-CP) sequence, believed to be the first discoveries in G1 (2 samples) or G17 (1 sample) wild-boar EV-Gs, and in G8 (2 samples) or G12 (6 samples) EV-Gs from any animals. Sequences of the non-structural protein regions were similar among EV-Gs possessing the PL-CP sequence (PL-CP EV-Gs) regardless of genotype or origin, suggesting the existence of a common ancestor for these strains. Interestingly, for the two G8 and two G12 samples, the genome sequences contained two versions, with or without the PL-CP sequence, together with the homologous 2C/PL-CP and PL-CP/3A junction sequences, which may explain how the recombination and deletion of the PL-CP sequences occured in the PL-CP EV-G genomes. These findings shed light on the genetic plasticity and evolution of EV-G.

An exclusive 42 amino acid signature in pp1ab protein provides insights into the evolutive history of the 2019 novel human-pathogenic coronavirus (SARS-CoV-2).

The city of Wuhan, Hubei province, China, was the origin of a severe pneumonia outbreak in December 2019, attributed to a novel coronavirus (severe acute respiratory syndrome coronavirus 2 [SARS-CoV-2]), causing a total of 2761 deaths and 81109 cases (25 February 2020). SARS-CoV-2 belongs to genus Betacoronavirus, subgenus Sarbecovirus. The polyprotein 1ab (pp1ab) remains unstudied thoroughly since it is similar to other sarbecoviruses. In this short communication, we performed phylogenetic-structural sequence analysis of pp1ab protein of SARS-CoV-2. The analysis showed that the viral pp1ab has not changed in most isolates throughout the outbreak time, but interestingly a deletion of 8 aa in the virulence factor nonstructural protein 1 was found in a virus isolated from a Japanese patient that did not display critical symptoms. While comparing pp1ab protein with other betacoronaviruses, we found a 42 amino acid signature that is only present in SARS-CoV-2 (AS-SCoV2). Members from clade 2 of sarbecoviruses have traces of this signature. The AS-SCoV2 located in the acidic-domain of papain-like protein of SARS-CoV-2 and bat-SL-CoV-RatG13 guided us to suggest that the novel 2019 coronavirus probably emerged by genetic drift from bat-SL-CoV-RaTG13. The implication of this amino acid signature in papain-like protein structure arrangement and function is something worth to be explored.

Inhibition of Plasmodium falciparum cysteine protease falcipain-2 by a human cross-class inhibitor serpinB3: A mechanistic insight.

Falcipain-2(FP2), a cysteine protease from Plasmodium falciparum, cleaves host erythrocyte hemoglobin and specific membrane skeleton components during the parasite life cycle. Therefore its inhibition has been considered as an attractive approach to combat the disease. SerpinB3 (SPB3) belongs to the ovalbumin-serpin family and is a potent cross-class inhibitor of cysteine cathepsins L, K, S and papain. This study explored the possibility of inhibition of FP2 by SPB3. It turned out that general proteolytic activities as well as specific hemoglobinolytic activity of FP2 have been inhibited by SPB3. Furthermore, studies have been designed to investigate and characterize the mechanism of inhibition in comparison with proteases Cathepsin L (CTSL) and papain. The Ki value of inhibition for FP2, measured against its specific substrate (VLK-pNA), is 338.11 nM and stoichiometry (I/E ratio) of inhibition is 1. These values are comparable to CTSL and papain. Analytical gel filtration profile and CD spectroscopy data confirm FP2-SPB3 complex formation. Our studies revealed that interaction of SPB3 with FP2 is non-covalent type like that of CTSL and papain but unlike other serine protease-inhibiting serpins. An in-silico docking and simulation study have been performed with FP2 as well as CTSL and results suggest different binding mode for FP2 and CTSL, though both the complexes are stable with significant contribution from electrostatic energy of interaction. We further showed a disease state mutant SPB3-Gly351Ala performed better anti-protease activity against FP2. This study, for the first time, has shown a serpin family inhibitor from human could efficiently inhibit activity of FP2.

Analysis of Coronavirus Temperature-Sensitive Mutants Reveals an Interplay between the Macrodomain and Papain-Like Protease Impacting Replication and Pathogenesis.

Analysis of temperature-sensitive (ts) mutant viruses is a classic method allowing researchers to identify genetic loci involved in viral replication and pathogenesis. Here, we report genetic analysis of a ts strain of mouse hepatitis virus (MHV), tsNC11, focusing on the role of mutations in the macrodomain (MAC) and the papain-like protease 2 (PLP2) domain of nonstructural protein 3 (nsp3), a component of the viral replication complex. Using MHV reverse genetics, we generated a series of mutant viruses to define the contributions of macrodomain- and PLP2-specific mutations to the ts phenotype. Viral replication kinetics and efficiency-of-plating analysis performed at permissive and nonpermissive temperatures revealed that changes in the macrodomain alone were both necessary and sufficient for the ts phenotype. Interestingly, mutations in the PLP2 domain were not responsible for the temperature sensitivity but did reduce the frequency of reversion of macrodomain mutants. Coimmunoprecipitation studies are consistent with an interaction between the macrodomain and PLP2. Expression studies of the macrodomain-PLP2 portion of nsp3 indicate that the ts mutations enhance proteasome-mediated degradation of the protein. Furthermore, we found that during virus infection, the replicase proteins containing the MAC and PLP2 mutations were more rapidly degraded at the nonpermissive temperature than were the wild-type proteins. Importantly, we show that the macrodomain and PLP2 mutant viruses trigger production of type I interferon in vitro and are attenuated in mice, further highlighting the importance of the macrodomain-PLP2 interplay in viral pathogenesis.IMPORTANCE Coronaviruses (CoVs) are emerging human and veterinary pathogens with pandemic potential. Despite the established and predicted threat these viruses pose to human health, there are currently no approved countermeasures to control infections with these viruses in humans. Viral macrodomains, enzymes that remove posttranslational ADP-ribosylation of proteins, and viral multifunctional papain-like proteases, enzymes that cleave polyproteins and remove polyubiquitin chains via deubiquitinating activity, are two important virulence factors. Here, we reveal an unanticipated interplay between the macrodomain and the PLP2 domain that is important for replication and antagonizing the host innate immune response. Targeting the interaction of these enzymes may provide new therapeutic opportunities to treat CoV disease.

Hepatitis E Virus Cysteine Protease Has Papain Like Properties Validated by in silico Modeling and Cell-Free Inhibition Assays.

Hepatitis E virus (HEV) has emerged as a global health concern during the last decade. In spite of a high mortality rate in pregnant women with fulminant hepatitis, no antiviral drugs or licensed vaccine is available in India. HEV-protease is a pivotal enzyme responsible for ORF1 polyprotein processing leading to cleavage of the non-structural enzymes involved in virus replication. HEV-protease region encoding 432-592 amino acids of Genotype-1 was amplified, expressed in Sf21 cells and purified in its native form. The recombinant enzyme was biochemically characterized using SDS-PAGE, Western blotting and Immunofluorescence. The enzyme activity and the inhibition studies were conducted using Zymography, FTC-casein based protease assay and ORF1 polyprotein digestion. To conduct ORF1 digestion assay, the polyprotein, natural substrate of HEV-protease, was expressed in E. coli and purified. Cleavage of 186 kDa ORF1 polyprotein by the recombinant HEV-protease lead to appearance of non-structural proteins viz. Methyltransferase, Protease, Helicase and RNA dependent RNA polymerase which were confirmed through immunoblotting using antibodies generated against specific epitopes of the enzymes. FTC-casein substrate was used for kinetic studies to determine Km and Vmax of the enzyme and also the effect of different metal ions and other protease inhibitors. A 95% inhibition was observed with E-64 which was validated through in silico analysis. The correlation coefficient between inhibition and docking score of Inhibitors was found to have a significant value of r2 = 0.75. The predicted 3D model showed two domain architecture structures similar to Papain like cysteine protease though they differed in arrangements of alpha helices and beta sheets. Hence, we propose that HEV-protease has characteristics of "Papain-like cysteine protease," as determined through structural homology, active site residues and class-specific inhibition. However, conclusive nature of the enzyme remains to be established.