The severe acute respiratory syndrome coronavirus (SARS-CoV) nsp1 protein has unique biological functions that have not been described in the viral proteins of any RNA viruses; expressed SARS-CoV nsp1 protein has been found to suppress host gene expression by promoting host mRNA degradation and inhibiting translation. We generated an nsp1 mutant (

Severe acute respiratory syndrome coronavirus nsp1 protein suppresses host gene expression, including type I interferon production, by promoting host mRNA degradation and inhibiting host translation, in infected cells. We present evidence that nsp1 uses a novel, two-pronged strategy to inhibit host translation/gene expression. Nsp1 bound to the 40S ribosomal subunit and inactivated the translational activity of the 40S subunits. Furthermore, the nsp1-40S ribosome complex induced the modification of the 5'-region of capped mRNA template and rendered the template RNA translationally incompetent. Nsp1 also induced RNA cleavage in templates carrying the internal ribosome entry site (IRES) from encephalomyocarditis virus, but not in those carrying IRESs from hepatitis C and cricket paralysis viruses, demonstrating that the nsp1-induced RNA modification was template-dependent. We speculate that the mRNAs that underwent the nsp1-mediated modification are marked for rapid turnover by the host RNA degradation machinery.

SARS coronavirus (SCoV) nonstructural protein (nsp) 1, a potent inhibitor of host gene expression, possesses a unique mode of action: it binds to 40S ribosomes to inactivate their translation functions and induces host mRNA degradation. Our previous study demonstrated that nsp1 induces RNA modification near the 5′-end of a reporter mRNA having a short 5′ untranslated region and RNA cleavage in the encephalomyocarditis virus internal ribosome entry site (IRES) region of a dicistronic RNA template, but not in those IRES elements from hepatitis C or cricket paralysis viruses. By using primarily cell-free, in vitro translation systems, the present study revealed that the nsp1 induced endonucleolytic RNA cleavage mainly near the 5′ untranslated region of capped mRNA templates. Experiments using dicistronic mRNAs carrying different IRESes showed that nsp1 induced endonucleolytic RNA cleavage within the ribosome loading region of type I and type II picornavirus IRES elements, but not that of classical swine fever virus IRES, which is characterized as a hepatitis C virus-like IRES. The nsp1-induced RNA cleavage of template mRNAs exhibited no apparent preference for a specific nucleotide sequence at the RNA cleavage sites. Remarkably, SCoV mRNAs, which have a 5′ cap structure and 3′ poly A tail like those of typical host mRNAs, were not susceptible to nsp1-mediated RNA cleavage and importantly, the presence of the 5′-end leader sequence protected the SCoV mRNAs from nsp1-induced endonucleolytic RNA cleavage. The escape of viral mRNAs from nsp1-induced RNA cleavage may be an important strategy by which the virus circumvents the action of nsp1 leading to the efficient accumulation of viral mRNAs and viral proteins during infection.

There is an urgent need to identify antivirals to curtail the COVID-19 pandemic. Herein, we report the sensitivity of SARS-CoV-2 to recombinant human interferons α and β (IFNα/β). Treatment with IFN-α or IFN-β at a concentration of 50 international units (IU) per milliliter reduces viral titers by 3.4 log or over 4 log, respectively, in Vero cells. The EC 50 of IFN-α and IFN-β treatment is 1.35 IU/ml and 0.76 IU/ml, respectively, in Vero cells. These results suggest that SARS-CoV-2 is more sensitive than many other human pathogenic viruses, including SARS-CoV. Overall, our results demonstrate the potential efficacy of human Type I IFN in suppressing SARS-CoV-2 infection, a finding which could inform future treatment options for COVID-19.

Severe acute respiratory syndrome (SARS) coronavirus nonstructural protein 1 (nsp1) binds to the 40S ribosomal subunit and inhibits translation, and it also induces a template-dependent endonucleolytic cleavage of host mRNAs. nsp1 inhibits the translation of cap-dependent and internal ribosome entry site (IRES)-driven mRNAs, including SARS coronavirus mRNAs, hepatitis C virus (HCV), and cricket paralysis virus (CrPV) IRES-driven mRNAs that are resistant to nsp1-induced RNA cleavage. We used an nsp1 mutant, nsp1-CD, lacking the RNA cleavage function, to delineate the mechanism of nsp1-mediated translation inhibition and identify the translation step(s) targeted by nsp1. nsp1 and nsp1-CD had identical inhibitory effects on mRNA templates that are resistant to nsp1-induced RNA cleavage, implying the validity of using nsp1-CD to dissect the translation inhibition function of nsp1. We provide evidence for a novel mode of action of nsp1. nsp1 inhibited the translation initiation step by targeting at least two separate stages: 48S initiation complex formation and the steps involved in the formation of the 80S initiation complex from the 48S complex. nsp1 had a differential, mRNA template-dependent, inhibitory effect on 48S and 80S initiation complex formation. nsp1 inhibited different steps of translation initiation on CrPV and HCV IRES, both of which initiate translation via an IRES-40S binary complex intermediate; nsp1 inhibited binary complex formation on CrPV IRES and 48S complex formation on HCV IRES. Collectively, the data revealed that nsp1 inhibited translation by exerting its effect on multiple stages of translation initiation, depending on the mechanism of initiation operating on the mRNA template. Severe acute respiratory syndrome (SARS) coronavirus (SCoV) is the causative agent of the respiratory disease SARS, which emerged in southern China in 2002 and spread to different countries of the world during a 2002-2003 epidemic (6, 18-20, 26, 30). SCoV carries a large, single-stranded, positive-sense RNA genome. The 5=-most two-thirds of the genome contains two large overlapping open reading frames that encode two polyproteins, which are cleaved by virus-encoded proteinases into 16 nonstructural proteins (nsp1 to nsp16), most of which are involved in viral RNA synthesis.Coronavirus (CoV) nsp1 proteins share a biological function to inhibit host gene expression; nsp1 of different CoVs employ different strategies to inhibit host gene expression (11,15,16,36). Several lines of evidence indicate a strong possibility that CoV nsp1 is a major CoV virulence factor (22,36,37,39). SCoV nsp1 suppresses the host innate immune functions by inhibiting type I interferon expression (22) and host antiviral signaling pathways (37) in infected cells. Mouse hepatitis virus nsp1 inhibits the type I interferon system, and a mutant virus lacking the nsp1 gene is severely attenuated in infected mice (39). Hence, studies using CoV nsp1 proteins have begun to tease out the common as well as the divergent mechanisms involved in the CoV-i...

The emergence of the severe acute respiratory syndrome coronavirus (SARS-CoV) has led to a renewed interest in studying the role of accessory proteins in regulating coronavirus infections in the natural host. A significant body of evidence has accumulated in the area of SARS-CoV and host interactions that indicate that the accessory proteins might play an important role in modulating the host response to virus infection and thereby, contribute to pathogenesis. In this review, we have compiled the current knowledge about SARS-CoV accessory proteins, obtained from studies in cell culture systems, reverse genetics and animal models, to shed some light into the possible role of these proteins in the propagation and virulence of SARS-CoV in its natural host. We conclude by providing some questions for future studies that will greatly advance our knowledge about the biological significance and contributions of the accessory proteins in the development of SARS in humans.

Junín virus (JUNV), an arenavirus, is the causative agent of Argentine hemorrhagic fever, an infectious human disease with 15–30% case fatality. The pathogenesis of AHF is still not well understood. Elevated levels of interferon and cytokines are reported in AHF patients, which might be correlated to the severity of the disease. However the innate immune response to JUNV infection has not been well evaluated. Previous studies have suggested that the virulent strain of JUNV does not induce IFN in human macrophages and monocytes, whereas the attenuated strain of JUNV was found to induce IFN response in murine macrophages via the TLR-2 signaling pathway. In this study, we investigated the interaction between JUNV and IFN pathway in human epithelial cells highly permissive to JUNV infection. We have determined the expression pattern of interferon-stimulated genes (ISGs) and IFN-β at both mRNA and protein levels during JUNV infection. Our results clearly indicate that JUNV infection activates the type I IFN response. STAT1 phosphorylation, a downstream marker of activation of IFN signaling pathway, was readily detected in JUNV infected IFN-competent cells. Our studies also demonstrated for the first time that RIG-I was required for IFN production during JUNV infection. IFN activation was detected during infection by either the virulent or attenuated vaccine strain of JUNV. Curiously, both virus strains were relatively insensitive to human IFN treatment. Our studies collectively indicated that JUNV infection could induce host type I IFN response and provided new insights into the interaction between JUNV and host innate immune system, which might be important in future studies on vaccine development and antiviral treatment.

The nsp1 protein of transmissible gastroenteritis virus (TGEV), an alphacoronavirus, efficiently suppressed protein synthesis in mammalian cells. Unlike the nsp1 protein of severe acute respiratory syndrome coronavirus, a betacoronavirus, the TGEV nsp1 protein was unable to bind 40S ribosomal subunits or promote host mRNA degradation. TGEV nsp1 also suppressed protein translation in cell-free HeLa cell extract; however, it did not affect translation in rabbit reticulocyte lysate (RRL). Our data suggested that HeLa cell extracts and cultured host cells, but not RRL, contain a host factor(s) that is essential for TGEV nsp1-induced translational suppression.