Definition of the immune evasion-replication interface of rabies virus P protein

In vitro characterization of the antiviral activity of Bat Interferon-Induced protein with tetratricopeptide repeats 5 (bat IFIT5) against bat-associated rabies virus

Bats are important reservoirs of zoonotic viruses, including the rabies virus (RABV), which causes rabies, a significant and fatal disease. In Brazil, RABV has been detected in several bat species. Interferon-induced protein with tetratricopeptide repeats 5 (IFIT5) is part of a group of interferon-stimulated genes (ISGs) known for their antiviral activity. This study investigated the interaction between batIFIT5 and different genetic lineages of RABV. The batIFIT5 was expressed in HEK-293T cells, which were infected with RABV genetic lineages isolated from Eptesicus furinalis (IP 964/06) and Tadarida brasiliensis (IP 3214/19), at varying infectious doses (pure, 100, 10, and 1). Direct immunofluorescence was performed to assess the effect of batIFIT5 on virus replication through the counting of fluorescent foci. Subsequently, after the expression of batIFIT5, 1 MOI was selected and used to evaluate the potential antiviral effect. Immunofluorescence was performed 24 and 48 h after infection. As a result, the viral concentration remained similar in the presence of batIFIT5 across distinct infectious doses. After infection with 1 MOI, a 30% reduction in infection rates was observed, particularly for the IP 3214/19 isolate after 24 h. These results highlight the potential antiviral role of IFIT5 against RABV infection.

Analysis of mechanisms of the rabies virus P protein-nucleocapsid interaction using engineered N-protein peptides and potential applications in antivirals design

The Phosphoprotein (P protein) of the rabies virus has multiple roles in virus replication. A critical function is to act as a cofactor in genome replication and mRNA production through binding via its N-terminal region to the L protein, the essential enzyme for mRNA and genome synthesis/processing, and via its C-terminal domain (PCTD) to the N protein and viral RNA (N-RNA) ribonucleoprotein complex. The binding site of the PCTD on the N protein is a disordered loop that is expected to be phosphorylated at Ser389. This interface may provide novel targets for antiviral approaches. Following an alanine scan of the peptide we selected two single site mutations that showed improved affinity and combined these mutations with a phosphomimetic (S389E) to produce double and triple mutants in the context of linear and cyclic peptides of the disordered loop, with the goal of generating a competitive peptide against the N-RNA complex. To assess the binding properties of the peptides we characterized their thermodynamics identifying complex properties of improved enthalpy but with compensating entropy for mutants and cyclized peptides. Nevertheless, a triple mutant shows 3.5-fold stronger affinity for PCTD than the full-length S389E N protein. Structural characterization of the triple mutant suggests the improved affinity may be due to trapping a favoured β-strand structure for binding to the PCTD. This novel peptide may serve as a template for the future design of antivirals.

Engineering G protein-coupled receptors for stabilization

G protein-coupled receptors (GPCRs) are one of the most important families of targets for drug discovery. One of the limiting steps in the study of GPCRs has been their stability, with significant and time-consuming protein engineering often used to stabilize GPCRs for structural characterization and drug screening. Unfortunately, computational methods developed using globular soluble proteins have translated poorly to the rational engineering of GPCRs. To fill this gap, we propose GPCR-tm, a novel and personalized structurally driven web-based machine learning tool to study the impacts of mutations on GPCR stability. We show that GPCR-tm performs as well as or better than alternative methods, and that it can accurately rank the stability changes of a wide range of mutations occurring in various types of class A GPCRs. GPCR-tm achieved Pearson's correlation coefficients of 0.74 and 0.46 on 10-fold cross-validation and blind test sets, respectively. We observed that the (structural) graph-based signatures were the most important set of features for predicting destabilizing mutations, which points out that these signatures properly describe the changes in the environment where the mutations occur. More specifically, GPCR-tm was able to accurately rank mutations based on their effect on protein stability, guiding their rational stabilization. GPCR-tm is available through a user-friendly web server at https://biosig.lab.uq.edu.au/gpcr_tm/.

A bias of Asparagine to Lysine mutations in SARS-CoV-2 outside the receptor binding domain affects protein flexibility

Introduction

COVID-19 pandemic has been threatening public health and economic development worldwide for over two years. Compared with the original SARS-CoV-2 strain reported in 2019, the Omicron variant (B.1.1.529.1) is more transmissible. This variant has 34 mutations in its Spike protein, 15 of which are present in the Receptor Binding Domain (RBD), facilitating viral internalization via binding to the angiotensin-converting enzyme 2 (ACE2) receptor on endothelial cells as well as promoting increased immune evasion capacity.

Methods

Herein we compared SARS-CoV-2 proteins (including ORF3a, ORF7, ORF8, Nucleoprotein (N), membrane protein (M) and Spike (S) proteins) from multiple ancestral strains. We included the currently designated original Variant of Concern (VOC) Omicron, its subsequent emerged variants BA.1, BA2, BA3, BA.4, BA.5, the two currently emerging variants BQ.1 and BBX.1, and compared these with the previously circulating VOCs Alpha, Beta, Gamma, and Delta, to better understand the nature and potential impact of Omicron specific mutations.

Results

Only in Omicron and its subvariants, a bias toward an Asparagine to Lysine (N to K) mutation was evident within the Spike protein, including regions outside the RBD domain, while none of the regions outside the Spike protein domain were characterized by this mutational bias. Computational structural analysis revealed that three of these specific mutations located in the central core region, contribute to a preference for the alteration of conformations of the Spike protein. Several mutations in the RBD which have circulated across most Omicron subvariants were also analysed, and these showed more potential for immune escape.

Conclusion

This study emphasizes the importance of understanding how specific N to K mutations outside of the RBD region affect SARS-CoV-2 conformational changes and the need for neutralizing antibodies for Omicron to target a subset of conformationally dependent B cell epitopes.

IRF2 Cooperates with Phosphoprotein of Spring Viremia of Carp Virus to Suppress Antiviral Response in Zebrafish

IFN regulatory factor (IRF) 2 belongs to the IRF1 subfamily, and its functions are not yet fully understood. In this study, we showed that IRF2a was a negative regulator of the interferon (IFN) response induced by spring viremia of carp virus (SVCV). Irf2a^-/- knockout zebrafish were less susceptible to SVCV than wild-type fish. Transcriptomic analysis reveals that differentially expressed genes (DEGs) in the irf2a^-/- and irf2a^+/+ cells derived caudal fins were mainly involved in cytokine-cytokine receptor interaction, mitogen-activated protein kinase (MAPK) signaling pathway, and transforming growth factor-beta (TGF-beta) signaling pathway. Interestingly, the basal expression levels of interferon stimulating genes (ISGs), including pkz, mx, apol, and stat1 were higher in the irf2a^-/- cells than irf2a^+/+ cells, suggesting that they may contribute to the increased viral resistance of the irf2a^-/- cells. Overexpression of IRF2a inhibited the activation of ifnφ1 and ifnφ3 induced by SVCV and poly(I:C) in the epithelioma papulosum cyprini (EPC) cells. Further, it was found that SVCV phosphoprotein (SVCV-P) could interact with IRF2a to promote IRF2a nuclear translocation and protein stability via suppressing K48-linked ubiquitination of IRF2a. Both IRF2a and SVCV-P not only destabilized STAT1a but reduced its translocation into the nucleus. Our work demonstrates that IRF2a cooperates with SVCV-P to suppress host antiviral response against viral infection in zebrafish. IMPORTANCE Interferon regulatory factors (IRFs) are central in the regulation of interferon-mediated antiviral immunity. Here, we reported that IRF2a suppressed interferon response and promoted virus replication in zebrafish. The suppressive effects were enhanced by the phosphoprotein of the spring viremia of carp virus (SVCV) via inhibition of K48-linked ubiquitination of IRF2a. IRF2a and SVCV phosphoprotein cooperated to degrade STAT1 and block its nuclear translocation. Our work demonstrated that IRFs and STATs were targeted by the virus through posttranslational modifications to repress interferon-mediated antiviral response in lower vertebrates.

Substitution of S179P in the Lyssavirus Phosphoprotein Impairs Its Interferon Antagonistic Function

Interferon (IFN) and the IFN-induced cellular antiviral response constitute the first line of defense against viral invasion. Evading host innate immunity, especially IFN signaling, is the key step required for lyssaviruses to establish infection.

The Amino Acid at Position 95 in the Matrix Protein of Rabies Virus Is Involved in Antiviral Stress Granule Formation in Infected Cells

Stress granules (SGs) are dynamic structures that store cytosolic messenger ribonucleoproteins. SGs have recently been shown to serve as a platform for activating antiviral innate immunity; however, several pathogenic viruses suppress SG formation to evade innate immunity. In this study, we investigated the relationship between rabies virus (RABV) virulence and SG formation, using viral strains with different levels of virulence. We found that the virulent Nishigahara strain did not induce SG formation, but its avirulent offshoot, the Ni-CE strain, strongly induced SG formation. Furthermore, we demonstrated that the amino acid at position 95 in the RABV matrix protein (M95), a pathogenic determinant for the Nishigahara strain, plays a key role in inhibiting SG formation, followed by protein kinase R (PKR)-dependent phosphorylation of the α subunit of eukaryotic initiation factor 2α (eIF2α). M95 was also implicated in the accumulation of RIG-I, a viral RNA sensor protein, in SGs and in the subsequent acceleration of interferon induction. Taken together, our findings strongly suggest that M95-related inhibition of SG formation contributes to the pathogenesis of RABV by allowing the virus to evade the innate immune responses of the host. IMPORTANCE Rabies virus (RABV) is a neglected zoonotic pathogen that causes lethal infections in almost all mammalian hosts, including humans. Recently, RABV has been reported to induce intracellular formation of stress granules (SGs), also known as platforms that activate innate immune responses. However, the relationship between SG formation capacity and pathogenicity of RABV has remained unclear. In this study, by comparing two RABV strains with completely different levels of virulence, we found that the amino acid mutation from valine to alanine at position 95 of matrix protein (M95), which is known to be one of the amino acid mutations that determine the difference in virulence between the strains, plays a major role in SG formation. Importantly, M95 was involved in the accumulation of RIG-I in SGs and in promoting interferon induction. These findings are the first report of the effect of a single amino acid substitution associated with SGs on viral virulence.

Deactivation of the antiviral state by rabies virus through targeting and accumulation of persistently phosphorylated STAT1

Antagonism of the interferon (IFN)-mediated antiviral state is critical to infection by rabies virus (RABV) and other viruses, and involves interference in the IFN induction and signaling pathways in infected cells, as well as deactivation of the antiviral state in cells previously activated by IFN. The latter is required for viral spread in the host, but the precise mechanisms involved and roles in RABV pathogenesis are poorly defined. Here, we examined the capacity of attenuated and pathogenic strains of RABV that differ only in the IFN-antagonist P protein to overcome an established antiviral state. Importantly, P protein selectively targets IFN-activated phosphorylated STAT1 (pY-STAT1), providing a molecular tool to elucidate specific roles of pY-STAT1. We find that the extended antiviral state is dependent on a low level of pY-STAT1 that appears to persist at a steady state through ongoing phosphorylation/dephosphorylation cycles, following an initial IFN-induced peak. P protein of pathogenic RABV binds and progressively accumulates pY-STAT1 in inactive cytoplasmic complexes, enabling recovery of efficient viral replication over time. Thus, P protein-pY-STAT1 interaction contributes to ‘disarming’ of the antiviral state. P protein of the attenuated RABV is defective in this respect, such that replication remains suppressed over extended periods in cells pre-activated by IFN. These data provide new insights into the nature of the antiviral state, indicating key roles for residual pY-STAT1 signaling. They also elucidate mechanisms of viral deactivation of antiviral responses, including specialized functions of P protein in selective targeting and accumulation of pY-STAT1.

Following viral infection, the host activates multiple antiviral defenses. The ability of viruses to overcome these defenses is critical to disease. The earliest antiviral response involves the production of interferon messenger molecules. Interferons act on infected cells to inhibit viral proliferation, as well as on non-infected cells to establish an antiviral state before infection and so limit viral spread through the host organism. Many strategies used by viruses to overcome the former are well understood, but mechanisms important to the latter, and their importance to disease, are less well defined. In this study, we investigated how rabies virus overcomes a pre-established antiviral state in target cells. We found that the capacity to disable the antiviral state correlates with the ability to cause disease, and involves binding of a viral protein to cellular signaling proteins, which our data indicate are responsible for the maintenance of a prolonged antiviral state. This advances our understanding of antiviral responses, and identifies a key step in lethal infection by rabies virus that causes approximately 60,000 human deaths per year. The findings may contribute to new approaches for the development of vaccines or antivirals.

Molecular Basis of Functional Effects of Phosphorylation of the C-Terminal Domain of the Rabies Virus P Protein

Rabies virus P protein is a multifunctional protein with critical roles in replication and manipulation of host-cell processes, including subversion of immunity. This functional diversity involves interactions of several P protein isoforms with the cell nucleus and microtubules.

Research Advances on the Interactions between Rabies Virus Structural Proteins and Host Target Cells: Accrued Knowledge from the Application of Reverse Genetics Systems

Rabies is a lethal zoonotic disease caused by lyssaviruses, such as rabies virus (RABV), that results in nearly 100% mortality once clinical symptoms appear. There are no curable drugs available yet. RABV contains five structural proteins that play an important role in viral replication, transcription, infection, and immune escape mechanisms. In the past decade, progress has been made in research on the pathogenicity of RABV, which plays an important role in the creation of new recombinant RABV vaccines by reverse genetic manipulation. Here, we review the latest advances on the interaction between RABV proteins in the infected host and the applied development of rabies vaccines by using a fully operational RABV reverse genetics system. This article provides a background for more in-depth research on the pathogenic mechanism of RABV and the development of therapeutic drugs and new biologics.

Implication of the nuclear trafficking of rabies virus P3 protein in viral pathogenicity

Although the majority of viruses of the family Mononegvirales replicate exclusively in the host cell cytoplasm, many of these viruses encode proteins that traffic between the nucleus and cytoplasm, which is believed to enable accessory functions in modulating the biology of the infected host cell. Among these, the P3 protein of rabies virus localizes to the nucleus through the activity of several specific nuclear localization and nuclear export signals. The major defined functions of P3 are in evasion of interferon (IFN)-mediated antiviral responses, including through inhibition of DNA-binding by IFN-activated STAT1. P3 also localizes to nucleoli and promyelocytic leukemia (PML) nuclear bodies, and interacts with nucleolin and PML protein, indicative of several intranuclear roles. The relationship of P3 nuclear localization with pathogenicity, however, is unresolved. We report that nucleocytoplasmic localization of P3 proteins from a pathogenic RABV strain, Nishigahara (Ni) and a non-pathogenic Ni-derived strain, Ni-CE, differs significantly, with nuclear accumulation defective for Ni-CE-P3. Molecular mapping indicates that altered localization derives from a coordinated effect, including two residue substitutions that independently disable nuclear localization and augment nuclear export signals, collectively promoting nuclear exclusion. Intriguingly, this appears to relate to effects on protein conformation or regulatory mechanisms, rather than direct modification of defined trafficking signal sequences. These data provide new insights into the role of regulated nuclear trafficking of a viral protein in the pathogenicity of a virus that replicates in the cytoplasm.