SARS-CoV-2 Nsp1 suppresses host but not viral translation through a bipartite mechanism

Deletion of 82-85 N-Terminal Residues in SARS-CoV-2 Nsp1 Restricts Virus Replication

Non-structural protein 1 (Nsp1) represents one of the most crucial SARS-CoV-2 virulence factors by inhibiting the translation of host mRNAs and promoting their degradation. We selected naturally occurring virus lineages with specific Nsp1 deletions located at both the N- and C-terminus of the protein. Our data provide new insights into how Nsp1 coordinates these functions on host and viral mRNA recognition. Residues 82-85 in the N-terminal part of Nsp1 likely play a role in docking the 40S mRNA entry channel, preserving the inhibition of host gene expression without affecting cellular mRNA decay. Furthermore, this domain prevents viral mRNAs containing the 5'-leader sequence to escape translational repression. These findings support the presence of distinct domains within the Nsp1 protein that differentially modulate mRNA recognition, translation and turnover. These insights have implications for the development of drugs targeting viral proteins and provides new evidences of how specific mutations in SARS-CoV-2 Nsp1 could attenuate the virus.

CryoREAD: de novo structure modeling for nucleic acids in cryo-EM maps using deep learning

DNA and RNA play fundamental roles in various cellular processes, where their three-dimensional structures provide information critical to understanding the molecular mechanisms of their functions. Although an increasing number of nucleic acid structures and their complexes with proteins are determined by cryogenic electron microscopy (cryo-EM), structure modeling for DNA and RNA remains challenging particularly when the map is determined at a resolution coarser than atomic level. Moreover, computational methods for nucleic acid structure modeling are relatively scarce. Here, we present CryoREAD, a fully automated de novo DNA/RNA atomic structure modeling method using deep learning. CryoREAD identifies phosphate, sugar and base positions in a cryo-EM map using deep learning, which are traced and modeled into a three-dimensional structure. When tested on cryo-EM maps determined at 2.0 to 5.0 Å resolution, CryoREAD built substantially more accurate models than existing methods. We also applied the method to cryo-EM maps of biomolecular complexes in severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).

High-Confidence Placement of Fragments into Electron Density Using Anomalous Diffraction-A Case Study Using Hits Targeting SARS-CoV-2 Non-Structural Protein 1

The identification of multiple simultaneous orientations of small molecule inhibitors binding to a protein target is a common challenge. It has recently been reported that the conformational heterogeneity of ligands is widely underreported in the Protein Data Bank, which is likely to impede optimal exploitation to improve affinity of these ligands. Significantly less is even known about multiple binding orientations for fragments (<300 Da), although this information would be essential for subsequent fragment optimisation using growing, linking or merging and rational structure-based design. Here, we use recently reported fragment hits for the SARS-CoV-2 non-structural protein 1 (nsp1) N-terminal domain to propose a general procedure for unambiguously identifying binding orientations of 2-dimensional fragments containing either sulphur or chloro substituents within the wavelength range of most tunable beamlines. By measuring datasets at two energies, using a tunable beamline operating in vacuum and optimised for data collection at very low X-ray energies, we show that the anomalous signal can be used to identify multiple orientations in small fragments containing sulphur and/or chloro substituents or to verify recently reported conformations. Although in this specific case we identified the positions of sulphur and chlorine in fragments bound to their protein target, we are confident that this work can be further expanded to additional atoms or ions which often occur in fragments. Finally, our improvements in the understanding of binding orientations will also serve to improve the rational optimisation of SARS-CoV-2 nsp1 fragment hits.

A viral pan-end RNA element and host complex define a SARS-CoV-2 regulon

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent of COVID-19, generates multiple protein-coding, subgenomic RNAs (sgRNAs) from a longer genomic RNA, all bearing identical termini with poorly understood roles in regulating viral gene expression. Insulin and interferon-gamma, two host-derived, stress-related agents, and virus spike protein, induce binding of glutamyl-prolyl-tRNA synthetase (EPRS1), within an unconventional, tetra-aminoacyl-tRNA synthetase complex, to the sgRNA 3'-end thereby enhancing sgRNA expression. We identify an EPRS1-binding sarbecoviral pan-end activating RNA (SPEAR) element in the 3'-end of viral RNAs driving agonist-induction. Translation of another co-terminal 3'-end feature, ORF10, is necessary for SPEAR-mediated induction, independent of Orf10 protein expression. The SPEAR element enhances viral programmed ribosomal frameshifting, thereby expanding its functionality. By co-opting noncanonical activities of a family of essential host proteins, the virus establishes a post-transcriptional regulon stimulating global viral RNA translation. A SPEAR-targeting strategy markedly reduces SARS-CoV-2 titer, suggesting a pan-sarbecoviral therapeutic modality.

Optimization of 5'UTR to evade SARS-CoV-2 Nonstructural protein 1-directed inhibition of protein synthesis in cells

Maximizing the expression level of therapeutic proteins in cells is the general goal for DNA/mRNA therapies. It is particularly challenging to achieve efficient protein expression in the cellular contexts with inhibited translation machineries, such as in the presence of cellular Nonstructural protein 1 (Nsp1) of coronaviruses (CoVs) that has been reported to inhibit overall protein synthesis of host genes and exogenously delivered mRNAs/DNAs. In this study, we thoroughly examined the sequence and structure contexts of viral and non-viral 5'UTRs that determine the protein expression levels of exogenously delivered DNAs and mRNAs in cells expressing SARS-CoV-2 Nsp1. It was found that high 5'-proximal A/U content promotes an escape from Nsp1-directed inhibition of protein synthesis and results in selective protein expression. Furthermore, 5'-proximal Cs were found to significantly enhance the protein expression in an Nsp1-dependent manner, while Gs located at a specific window close to the 5'-end counteract such enhancement. The distinct protein expression levels resulted from different 5'UTRs were found correlated to Nsp1-induced mRNA degradations. These findings ultimately enabled rational designs for optimized 5'UTRs that lead to strong expression of exogenous proteins regardless of the translationally repressive Nsp1. On the other hand, we have also identified several 5'-proximal sequences derived from host genes that are capable of mediating the escapes. These results provided novel perspectives to the optimizations of 5'UTRs for DNA/mRNA therapies and/or vaccinations, as well as shedding light on the potential host escapees from Nsp1-directed translational shutoffs. KEY POINTS: • The 5'-proximal SL1 and 5a/b derived from SARS-CoV-2 genomic RNA promote exogenous protein synthesis in cells expressing Nsp1 comparing with non-specific 5'UTRs. • Specific 5'-proximal sequence contexts are the key determinants of the escapes from Nsp1-directed translational repression and thereby enhance protein expressions. • Systematic mutagenesis identified optimized 5'UTRs that strongly enhance protein expression and promote resistance to Nsp1-induced translational repression and RNA degradation.

Ribosome biogenesis in disease: new players and therapeutic targets

The ribosome is a multi-unit complex that translates mRNA into protein. Ribosome biogenesis is the process that generates ribosomes and plays an essential role in cell proliferation, differentiation, apoptosis, development, and transformation. The mTORC1, Myc, and noncoding RNA signaling pathways are the primary mediators that work jointly with RNA polymerases and ribosome proteins to control ribosome biogenesis and protein synthesis. Activation of mTORC1 is required for normal fetal growth and development and tissue regeneration after birth. Myc is implicated in cancer development by enhancing RNA Pol II activity, leading to uncontrolled cancer cell growth. The deregulation of noncoding RNAs such as microRNAs, long noncoding RNAs, and circular RNAs is involved in developing blood, neurodegenerative diseases, and atherosclerosis. We review the similarities and differences between eukaryotic and bacterial ribosomes and the molecular mechanism of ribosome-targeting antibiotics and bacterial resistance. We also review the most recent findings of ribosome dysfunction in COVID-19 and other conditions and discuss the consequences of ribosome frameshifting, ribosome-stalling, and ribosome-collision. We summarize the role of ribosome biogenesis in the development of various diseases. Furthermore, we review the current clinical trials, prospective vaccines for COVID-19, and therapies targeting ribosome biogenesis in cancer, cardiovascular disease, aging, and neurodegenerative disease.

SARS-CoV-2 modulation of RIG-I-MAVS signaling: Potential mechanisms of impairment on host antiviral immunity and therapeutic approaches

The coronavirus disease 2019 (COVID-19) is a global infectious disease aroused by RNA virus severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Patients may suffer from severe respiratory failure or even die, posing a huge challenge to global public health. Retinoic acid-inducible gene I (RIG-I) is one of the major pattern recognition receptors, function to recognize RNA viruses and mediate the innate immune response. RIG-1 and melanoma differentiation-associated gene 5 contain an N-terminal caspase recruitment domain that is activated upon detection of viral RNA in the cytoplasm of virus-infected cells. Activated RIG-I and mitochondrial antiviral signaling (MAVS) protein trigger a series of corresponding immune responses such as the production of type I interferon against viral infection. In this review, we are summarizing the role of the structural, nonstructural, and accessory proteins from SARS-CoV-2 on the RIG-I-MAVS pathway, and exploring the potential mechanism how SARS-CoV-2 could evade the host antiviral response. We then proposed that modulation of the RIG-I-MAVS signaling pathway might be a novel and effective therapeutic strategy to against COVID-19 as well as the constantly mutating coronavirus.

Synergistic interactions of repurposed drugs that inhibit Nsp1, a major virulence factor for COVID-19

Nsp1 is one of the first proteins expressed from the SARS-CoV-2 genome and is a major virulence factor for COVID-19. A rapid multiplexed assay for detecting the action of Nsp1 was developed in cultured lung cells. The assay is based on the acute cytopathic effects induced by Nsp1. Virtual screening was used to stratify compounds that interact with two functional Nsp1 sites: the RNA-binding groove and C-terminal helix-loop-helix region. Experimental screening focused on compounds that could be readily repurposed to treat COVID-19. Multiple synergistic combinations of compounds that significantly inhibited Nsp1 action were identified. Among the most promising combinations are Ponatinib, Rilpivirine, and Montelukast, which together, reversed the toxic effects of Nsp1 to the same extent as null mutations in the Nsp1 gene.

The key features of SARS-CoV-2 leader and NSP1 required for viral escape of NSP1-mediated repression

SARS-CoV-2, responsible for the ongoing global pandemic, must overcome a conundrum faced by all viruses. To achieve its own replication and spread, it simultaneously depends on and subverts cellular mechanisms. At the early stage of infection, SARS-CoV-2 expresses the viral nonstructural protein 1 (NSP1), which inhibits host translation by blocking the mRNA entry tunnel on the ribosome; this interferes with the binding of cellular mRNAs to the ribosome. Viral mRNAs, on the other hand, overcome this blockade. We show that NSP1 enhances expression of mRNAs containing the SARS-CoV-2 leader. The first stem-loop (SL1) in the viral leader is both necessary and sufficient for this enhancement mechanism. Our analysis pinpoints specific residues within SL1 (three cytosine residues at the positions 15, 19, and 20) and another within NSP1 (R124), which are required for viral evasion, and thus might present promising drug targets. We target SL1 with the antisense oligo (ASO) to efficiently and specifically down-regulate SARS-CoV-2 mRNA. Additionally, we carried out analysis of a functional interactome of NSP1 using BioID and identified components of antiviral defense pathways. Our analysis therefore suggests a mechanism by which NSP1 inhibits the expression of host genes while enhancing that of viral RNA. This analysis helps reconcile conflicting reports in the literature regarding the mechanisms by which the virus avoids NSP1 silencing.

Pharmaceutical Prospects of Curcuminoids for the Remedy of COVID-19: Truth or Myth

Coronavirus disease 2019 (COVID-19) is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which is a positive-strand RNA virus, and has rapidly spread worldwide as a pandemic. The vaccines, repurposed drugs, and specific treatments have led to a surge of novel therapies and guidelines nowadays; however, the epidemic of COVID-19 is not yet fully combated and is still in a vital crisis. In repositioning drugs, natural products are gaining attention because of the large therapeutic window and potent antiviral, immunomodulatory, anti-inflammatory, and antioxidant properties. Of note, the predominant curcumoid extracted from turmeric (Curcuma longa L.) including phenolic curcumin influences multiple signaling pathways and has demonstrated to possess anti-inflammatory, antioxidant, antimicrobial, hypoglycemic, wound healing, chemopreventive, chemosensitizing, and radiosensitizing spectrums. In this review, all pieces of current information related to curcumin-used for the treatment and prevention of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection through in vitro, in vivo, and in silico studies, clinical trials, and new formulation designs are retrieved to re-evaluate the applications based on the pharmaceutical efficacy of clinical therapy and to provide deep insights into knowledge and strategy about the curcumin's role as an immune booster, inflammatory modulator, and therapeutic agent against COVID-19. Moreover, this study will also afford a favorable application or approach with evidence based on the drug discovery and development, pharmacology, functional foods, and nutraceuticals for effectively fighting the COVID-19 pandemic.

Translational Control of COVID-19 and Its Therapeutic Implication

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative agent of COVID-19, which has broken out worldwide for more than two years. However, due to limited treatment, new cases of infection are still rising. Therefore, there is an urgent need to understand the basic molecular biology of SARS-CoV-2 to control this virus. SARS-CoV-2 replication and spread depend on the recruitment of host ribosomes to translate viral messenger RNA (mRNA). To ensure the translation of their own mRNAs, the SARS-CoV-2 has developed multiple strategies to globally inhibit the translation of host mRNAs and block the cellular innate immune response. This review provides a comprehensive picture of recent advancements in our understanding of the molecular basis and complexity of SARS-CoV-2 protein translation. Specifically, we summarize how this viral infection inhibits host mRNA translation to better utilize translation elements for translation of its own mRNA. Finally, we discuss the potential of translational components as targets for therapeutic interventions.

Parsing the role of NSP1 in SARS-CoV-2 infection

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the cause of the ongoing coronavirus disease 19 (COVID-19) pandemic. Despite its urgency, we still do not fully understand the molecular basis of SARS-CoV-2 pathogenesis and its ability to antagonize innate immune responses. SARS-CoV-2 leads to shutoff of cellular protein synthesis and over-expression of nsp1, a central shutoff factor in coronaviruses, inhibits cellular gene translation. However, the diverse molecular mechanisms nsp1 employs as well as its functional importance in infection are still unresolved. By overexpressing various nsp1 mutants and generating a SARS-CoV-2 mutant in which nsp1 does not bind ribosomes, we untangle the effects of nsp1. We uncover that nsp1, through inhibition of translation and induction of mRNA degradation, is the main driver of host shutoff during SARS-CoV-2 infection. Furthermore, we find the propagation of nsp1 mutant virus is inhibited specifically in cells with intact interferon (IFN) response as well as in-vivo , in infected hamsters, and this attenuation is associated with stronger induction of type I IFN response. This illustrates that nsp1 shutoff activity has an essential role mainly in counteracting the IFN response. Overall, our results reveal the multifaceted approach nsp1 uses to shut off cellular protein synthesis and uncover the central role it plays in SARS-CoV-2 pathogenesis, explicitly through blockage of the IFN response.

Minding the message: tactics controlling RNA decay, modification, and translation in virus-infected cells

With their categorical requirement for host ribosomes to translate mRNA, viruses provide a wealth of genetically tractable models to investigate how gene expression is remodeled post-transcriptionally by infection-triggered biological stress. By co-opting and subverting cellular pathways that control mRNA decay, modification, and translation, the global landscape of post-transcriptional processes is swiftly reshaped by virus-encoded factors. Concurrent host cell-intrinsic countermeasures likewise conscript post-transcriptional strategies to mobilize critical innate immune defenses. Here we review strategies and mechanisms that control mRNA decay, modification, and translation in animal virus-infected cells. Besides settling infection outcomes, post-transcriptional gene regulation in virus-infected cells epitomizes fundamental physiological stress responses in health and disease.

Stable Cell Clones Harboring Self-Replicating SARS-CoV-2 RNAs for Drug Screen

The development of antivirals against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has been hampered by the lack of efficient cell-based replication systems that are amenable to high-throughput screens in biosafety level 2 laboratories. Here we report that stable cell clones harboring autonomously replicating SARS-CoV-2 RNAs without spike (S), membrane (M), and envelope (E) genes can be efficiently derived from the baby hamster kidney (BHK-21) cell line when a pair of mutations were introduced into the non-structural protein 1 (Nsp1) of SARS-CoV-2 to ameliorate cellular toxicity associated with virus replication. In a proof-of-concept experiment we screened a 273-compound library using replicon cells and identified three compounds as novel inhibitors of SARS-CoV-2 replication. Altogether, this work establishes a robust, cell-based system for genetic and functional analyses of SARS-CoV-2 replication and for the development of antiviral drugs.

IMPORTANCE SARS-CoV-2 replicon systems that have been reported up to date were unsuccessful in deriving stable cell lines harboring non-cytopathic replicons. The transient expression of viral sgmRNA or a reporter gene makes it impractical for industry-scale screening of large compound libraries using these systems. Here, for the first time, we derived stable cell clones harboring the SARS-CoV-2 replicon. These clones may now be conveniently cultured in a standard BSL-2 laboratory for high throughput screen of compound libraries. Additionally, our stable replicon cells represent a new model system to study SARS-CoV-2 replication.

Inflammasome activation at the crux of severe COVID-19

The COVID-19 pandemic, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), results in life-threatening disease in a minority of patients, especially elderly people and those with co-morbidities such as obesity and diabetes. Severe disease is characterized by dysregulated cytokine release, pneumonia and acute lung injury, which can rapidly progress to acute respiratory distress syndrome, disseminated intravascular coagulation, multisystem failure and death. However, a mechanistic understanding of COVID-19 progression remains unclear. Here we review evidence that SARS-CoV-2 directly or indirectly activates inflammasomes, which are large multiprotein assemblies that are broadly responsive to pathogen-associated and stress-associated cellular insults, leading to secretion of the pleiotropic IL-1 family cytokines (IL-1β and IL-18), and pyroptosis, an inflammatory form of cell death. We further discuss potential mechanisms of inflammasome activation and clinical efforts currently under way to suppress inflammation to prevent or ameliorate severe COVID-19.

Construction of Non-infectious SARS-CoV-2 Replicons and Their Application in Drug Evaluation

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has caused a devastating pandemic worldwide. Vaccines and antiviral drugs are the most promising candidates for combating this global epidemic, and scientists all over the world have made great efforts to this end. However, manipulation of the SARS-CoV-2 should be performed in the biosafety level 3 laboratory. This makes experiments complicated and time-consuming. Therefore, a safer system for working with this virus is urgently needed. Here, we report the construction of plasmid-based, non-infectious SARS-CoV-2 replicons with turbo-green fluorescent protein and/or firefly luciferase reporters by reverse genetics using transformation-associated recombination cloning in Saccharomyces cerevisiae. Replication of these replicons was achieved simply by direct transfection of cells with the replicon plasmids as evident by the expression of reporter genes. Using SARS-CoV-2 replicons, the inhibitory effects of E64-D and remdesivir on SARS-CoV-2 replication were confirmed, and the half-maximal effective concentration (EC50) value of remdesivir and E64-D was estimated by different quantification methods respectively, indicating that these SARS-CoV-2 replicons are useful tools for antiviral drug evaluation.

Structural Basis and Function of the N Terminus of SARS-CoV-2 Nonstructural Protein 1

Nonstructural protein 1 (Nsp1) of severe acute respiratory syndrome coronaviruses (SARS-CoVs) is an important pathogenic factor that inhibits host protein translation by means of its C terminus. However, its N-terminal function remains elusive. Here, we determined the crystal structure of the N terminus (amino acids [aa] 11 to 125) of SARS-CoV-2 Nsp1 at a 1.25-Å resolution. Further functional assays showed that the N terminus of SARS-CoVs Nsp1 alone loses the ability to colocalize with ribosomes and inhibit protein translation. The C terminus of Nsp1 can colocalize with ribosomes, but its protein translation inhibition ability is significantly weakened. Interestingly, fusing the C terminus of Nsp1 with enhanced green fluorescent protein (EGFP) or other proteins in place of its N terminus restored the protein translation inhibitory ability to a level equivalent to that of full-length Nsp1. Thus, our results suggest that the N terminus of Nsp1 is able to stabilize the binding of the Nsp1 C terminus to ribosomes and act as a nonspecific barrier to block the mRNA channel, thus abrogating host mRNA translation.

Genes with 5' terminal oligopyrimidine tracts preferentially escape global suppression of translation by the SARS-CoV-2 Nsp1 protein

Viruses rely on the host translation machinery to synthesize their own proteins. Consequently, they have evolved varied mechanisms to co-opt host translation for their survival. SARS-CoV-2 relies on a nonstructural protein, Nsp1, for shutting down host translation. However, it is currently unknown how viral proteins and host factors critical for viral replication can escape a global shutdown of host translation. Here, using a novel FACS-based assay called MeTAFlow, we report a dose-dependent reduction in both nascent protein synthesis and mRNA abundance in cells expressing Nsp1. We perform RNA-seq and matched ribosome profiling experiments to identify gene-specific changes both at the mRNA expression and translation levels. We discover that a functionally coherent subset of human genes is preferentially translated in the context of Nsp1 expression. These genes include the translation machinery components, RNA binding proteins, and others important for viral pathogenicity. Importantly, we uncovered a remarkable enrichment of 5' terminal oligo-pyrimidine (TOP) tracts among preferentially translated genes. Using reporter assays, we validated that 5' UTRs from TOP transcripts can drive preferential expression in the presence of Nsp1. Finally, we found that LARP1, a key effector protein in the mTOR pathway, may contribute to preferential translation of TOP transcripts in response to Nsp1 expression. Collectively, our study suggests fine-tuning of host gene expression and translation by Nsp1 despite its global repressive effect on host protein synthesis.

Non-Canonical Translation Initiation Mechanisms Employed by Eukaryotic Viral mRNAs

Viruses exploit the translation machinery of an infected cell to synthesize their proteins. Therefore, viral mRNAs have to compete for ribosomes and translation factors with cellular mRNAs. To succeed, eukaryotic viruses adopt multiple strategies. One is to circumvent the need for m7G-cap through alternative instruments for ribosome recruitment. These include internal ribosome entry sites (IRESs), which make translation independent of the free 5' end, or cap-independent translational enhancers (CITEs), which promote initiation at the uncapped 5' end, even if located in 3' untranslated regions (3' UTRs). Even if a virus uses the canonical cap-dependent ribosome recruitment, it can still perturb conventional ribosomal scanning and start codon selection. The pressure for genome compression often gives rise to internal and overlapping open reading frames. Their translation is initiated through specific mechanisms, such as leaky scanning, 43S sliding, shunting, or coupled termination-reinitiation. Deviations from the canonical initiation reduce the dependence of viral mRNAs on translation initiation factors, thereby providing resistance to antiviral mechanisms and cellular stress responses. Moreover, viruses can gain advantage in a competition for the translational machinery by inactivating individual translational factors and/or replacing them with viral counterparts. Certain viruses even create specialized intracellular "translation factories", which spatially isolate the sites of their protein synthesis from cellular antiviral systems, and increase availability of translational components. However, these virus-specific mechanisms may become the Achilles' heel of a viral life cycle. Thus, better understanding of the unconventional mechanisms of viral mRNA translation initiation provides valuable insight for developing new approaches to antiviral therapy.

I(nsp1)ecting SARS-CoV-2-ribosome interactions

While SARS-CoV-2 is causing modern human history’s most serious health crisis and upending our way of life, clinical and basic research on the virus is advancing rapidly, leading to fascinating discoveries. Two studies have revealed how the viral virulence factor, nonstructural protein 1 (Nsp1), binds human ribosomes to inhibit host cell translation. Here, we examine the main conclusions on the molecular activity of Nsp1 and its role in suppressing innate immune responses. We discuss different scenarios potentially explaining how the viral RNA can bypass its own translation blockage and speculate on the suitability of Nsp1 as a therapeutic target.

Simeoni et al discuss how recent structural work has improved our understanding of SARS-CoV-2 Nsp1-mediated translation inhibition and how Nsp1 inhibition could impact host immune responses and suppress viral replication.

The viral protein NSP1 acts as a ribosome gatekeeper for shutting down host translation and fostering SARS-CoV-2 translation

SARS-CoV-2 coronavirus is responsible for Covid-19 pandemic. In the early phase of infection, the single-strand positive RNA genome is translated into non-structural proteins (NSP). One of the first proteins produced during viral infection, NSP1, binds to the host ribosome and blocks the mRNA entry channel. This triggers translation inhibition of cellular translation. In spite of the presence of NSP1 on the ribosome, viral translation proceeds however. The molecular mechanism of the so-called viral evasion to NSP1 inhibition remains elusive. Here, we confirm that viral translation is maintained in the presence of NSP1. The evasion to NSP1-inhibition is mediated by the cis-acting RNA hairpin SL1 in the 5'UTR of SARS-CoV-2. NSP1-evasion can be transferred on a reporter transcript by SL1 transplantation. The apical part of SL1 is only required for viral translation. We show that NSP1 remains bound on the ribosome during viral translation. We suggest that the interaction between NSP1 and SL1 frees the mRNA accommodation channel while maintaining NSP1 bound to the ribosome. Thus, NSP1 acts as a ribosome gatekeeper, shutting down host translation or fostering SARS-CoV-2 translation depending on the presence of the SL1 5'UTR hairpin. SL1 is also present and necessary for translation of sub-genomic RNAs in the late phase of the infectious program. Consequently, therapeutic strategies targeting SL1 should affect viral translation at early and late stages of infection. Therefore, SL1 might be seen as a genuine 'Achille heel' of the virus.