Rift Valley Fever Virus NSs Protein Promotes Post-Transcriptional Downregulation of Protein Kinase PKR and Inhibits eIF2α Phosphorylation

Tetsuro Ikegami,Krishna Narayanan,Wataru Kamitani,Shinji Makino,Sungyong Won,C J Peters,Michael Gale

doi:10.1371/journal.ppat.1000287

Tetsuro Ikegami, Krishna Narayanan + Show 5 more

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https://doi.org/10.1371/journal.ppat.1000287

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Abstract

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.

Highlights

Rift Valley fever virus (RVFV) is a mosquito-borne zoonotic pathogen, which is distributed in sub-Saharan Africa [1] and has caused large outbreaks in Madagascar [2], Egypt [3,4,5], Saudi Arabia [6], and Yemen [6]
We found that NSs decreased PKR abundance and prevented eIF2a phosphorylation in infected cells, allowing efficient viral translation and replication
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

Summary

Introduction

Rift Valley fever virus (RVFV) is a mosquito-borne zoonotic pathogen, which is distributed in sub-Saharan Africa [1] and has caused large outbreaks in Madagascar [2], Egypt [3,4,5], Saudi Arabia [6], and Yemen [6]. RVFV infection in adult ruminants causes febrile illness and a high rate of abortions, while some newborn animals less than 1–2 weeks of age develop an acute infection which results in higher mortality rates than those in adults [8]. Humans infected with RVFV usually develop an acute febrile myalgic syndrome; a small percentage of patients have a lethal illness that results in hepatic damage, hemorrhagic fever-like illness, encephalitis and/or retinal vasculitis [8]. The S segment encodes N and NSs genes and uses an ambi-sense strategy to express the N and NSs proteins in infected cells; N mRNA encoding N protein is transcribed from the viral-sense (negative-sense) S segment, while NSs mRNA encoding NSs protein is transcribed from the antiviral-sense (positive-sense) S segment.

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