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 (nsp1-mt) that neither promoted host mRNA degradation nor suppressed host protein synthesis in expressing cells. Both a SARS-CoV mutant virus, encoding the nsp1-mt protein (SARS-CoV-mt), and a wild-type virus (SARS-CoV-WT) replicated efficiently and exhibited similar one-step growth kinetics in susceptible cells. Both viruses accumulated similar amounts of virus-specific mRNAs and nsp1 protein in infected cells, whereas the amounts of endogenous host mRNAs were clearly higher in SARS-CoV-mt-infected cells than in SARS-CoV-WT-infected cells, in both the presence and absence of actinomycin D. Further, SARS-CoV-WT replication strongly inhibited host protein synthesis, whereas host protein synthesis inhibition in SARS-CoV-mt-infected cells was not as efficient as in SARS-CoV-WT-infected cells. These data revealed that nsp1 indeed promoted host mRNA degradation and contributed to host protein translation inhibition in infected cells. Notably, SARS-CoV-mt infection, but not SARS-CoV-WT infection, induced high levels of beta interferon (IFN) mRNA accumulation and high titers of type I IFN production. These data demonstrated that SARS-CoV nsp1 suppressed host innate immune functions, including type I IFN expression, in infected cells and suggested that SARS-CoV nsp1 most probably plays a critical role in SARS-CoV virulence.

Severe acute respiratory syndrome (SARS) coronavirus (SCoV) causes a recently emerged human disease associated with pneumonia. The 5′ end two-thirds of the single-stranded positive-sense viral genomic RNA, gene 1, encodes 16 mature proteins. Expression of nsp1, the most N-terminal gene 1 protein, prevented Sendai virus-induced endogenous IFN-β mRNA accumulation without inhibiting dimerization of IFN regulatory factor 3, a protein that is essential for activation of the IFN-β promoter. Furthermore, nsp1 expression promoted degradation of expressed RNA transcripts and host endogenous mRNAs, leading to a strong host protein synthesis inhibition. SCoV replication also promoted degradation of expressed RNA transcripts and host mRNAs, suggesting that nsp1 exerted its mRNA destabilization function in infected cells. In contrast to nsp1-induced mRNA destablization, no degradation of the 28S and 18S rRNAs occurred in either nsp1-expressing cells or SCoV-infected cells. These data suggested that, in infected cells, nsp1 promotes host mRNA degradation and thereby suppresses host gene expression, including proteins involved in host innate immune functions. SCoV nsp1-mediated promotion of host mRNA degradation may play an important role in SCoV pathogenesis.

The SARS coronavirus protein nsp1 can suppress host gene expression at a post-transcriptional level, with previous work showing a reduction in mRNA abundance. Now a direct effect on protein synthesis is revealed, as nsp1 modifies transcripts and also inactivates the 40S ribosomal subunit. 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 and 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 IRES elements from hepatitis C or 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.

Rift Valley fever (RVF) is an emerging zoonotic disease distributed in sub-Saharan African countries and the Arabian Peninsula. The disease is caused by the Rift Valley fever virus (RVFV) of the family Bunyaviridae and the genus Phlebovirus. The virus is transmitted by mosquitoes, and virus replication in domestic ruminant results in high rates of mortality and abortion. RVFV infection in humans usually causes a self-limiting, acute and febrile illness; however, a small number of cases progress to neurological disorders, partial or complete blindness, hemorrhagic fever, or thrombosis. This review describes the pathology of RVF in human patients and several animal models, and summarizes the role of viral virulence factors and host factors that affect RVFV pathogenesis.

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. Severe acute respiratory syndrome (SARS) coronavirus (SCoV) is the causative agent of SARS. The nsp1 protein of SCoV blocks host protein synthesis, including type I interferon, a general inhibitor of virus replication, in infected cells. This finding suggests that SCoV nsp1 protein plays a key role in the severe symptoms that accompany SARS infection. Nsp1 binds to the 40S ribosome subunit, which is an essential component for protein synthesis, and inactivates the translation activity of the ribosome. Furthermore, nsp1 binding to the 40S ribosome induces the modification of host mRNAs, leading to the accelerated decay of these RNAs in SCoV-infected cells. We found that the nature of nsp1-induced RNA modification was RNA cleavage and that nsp1 did not recognize specific nucleotides in host mRNAs to induce this cleavage. Interestingly, nsp1 did not induce RNA cleavage in SCoV mRNAs. These data indicate that nsp1 induces RNA cleavage of host mRNAs to suppress the expression of host genes, including those having antiviral functions; yet viral mRNAs are spared from such cleavage events, which, most likely, facilitate efficient SCoV protein synthesis and virus replication in infected cells.

Rift Valley fever virus (RVFV) (genus Phlebovirus, family Bunyaviridae) is a negative-stranded RNA virus with a tripartite genome. RVFV is transmitted by mosquitoes and causes fever and severe hemorrhagic illness among humans, and fever and high rates of abortions in livestock. A nonstructural RVFV NSs protein inhibits the transcription of host mRNAs, including interferon-β mRNA, and is a major virulence factor. The present study explored a novel function of the RVFV NSs protein by testing the replication of RVFV lacking the NSs gene in the presence of actinomycin D (ActD) or α-amanitin, both of which served as a surrogate of the host mRNA synthesis suppression function of the NSs. In the presence of the host-transcriptional inhibitors, the replication of RVFV lacking the NSs protein, but not that carrying NSs, induced double-stranded RNA-dependent protein kinase (PKR)–mediated eukaryotic initiation factor (eIF)2α phosphorylation, leading to the suppression of host and viral protein translation. RVFV NSs promoted post-transcriptional downregulation of PKR early in the course of the infection and suppressed the phosphorylated eIF2α accumulation. These data suggested that a combination of RVFV replication and NSs-induced host transcriptional suppression induces PKR-mediated eIF2α phosphorylation, while the NSs facilitates efficient viral translation by downregulating PKR and inhibiting PKR-mediated eIF2α phosphorylation. Thus, the two distinct functions of the NSs, i.e., the suppression of host transcription, including that of type I interferon mRNAs, and the downregulation of PKR, work together to prevent host innate antiviral functions, allowing efficient replication and survival of RVFV in infected mammalian hosts. The mosquito-borne bunyavirus Rift Valley fever virus (RVFV) devastates both humans and domestic animals; it causes abortions in ruminants and complications such as hemorrhage, encephalitis, or retinal vasculitis in humans. A major RVFV virulence factor, NSs, disables host cell mRNA synthesis. Here we describe our new evidence that showed NSs working in a second way; in addition to inhibiting host cell transcription, NSs kept translation active in infected cells. It is well-established that activated protein kinase PKR phosphorylates a translation factor, eIF2α, and then this phosphorylated eIF2α suppresses translation. We found that NSs decreased PKR abundance and prevented eIF2α phosphorylation in infected cells, allowing efficient viral translation and replication. In contrast, when cells were infected with an RVFV mutant lacking NSs in the presence of transcriptional inhibitors that mimic the transcription inhibition function of NSs, the PKR reduction did not occur and phoshorylated eIF2α was accumulated, resulting in the inhibition of virus gene expression and replication. Thus, NSs functions in two ways to help RVFV replicate in mammalian hosts: its newly identified PKR downregulation function secures efficient viral translation, and its host transcription inhibition function suppresses the expression of host innate antiviral functions.