Rift Valley fever virus virulence factor NSs targets ATF4- and XBP1-driven unfolded protein response

ABSTRACT Rift Valley fever virus (RVFV) is a mosquito-borne arbovirus that infects humans and domestic ruminants, significantly impacting public health and the economy in African and Middle Eastern countries. RVFV non-structural protein NSs is a major viral virulence factor and possesses multiple functions to counteract host antiviral responses. One of its functions is to block host transcription by promoting degradation of subunits of the host transcription factor IIH (TFIIH). However, the biological significance of this function in the RVFV life cycle remains unclear. We used RVFV strain MP-12 and its mutant lacking the TFIIH degradation function (NSs-mut virus) to identify host genes targeted by NSs-mediated transcription suppression. We found that NSs-mut virus, but not MP-12, upregulated unfolded protein response (UPR) target genes compared to uninfected cells. Both viruses activated all three ER stress sensors, including PERK and IRE1. However, only MP-12 inhibited the PERK and IRE1 pathways by inhibiting the expression of their respective downstream mediators ATF4 and XBP1. ATF4 expression was inhibited at the transcriptional level, whereas XBP1 expression was inhibited at the post-transcriptional level. ATF4 depletion negatively impacted virus titer and the production of non-plaque-forming particles, resulting in alteration of the particle-to-PFU ratio of the viruses. We also showed that activation of the PERK pathway plays a pro-survival role in NSs-mut virus-infected cells, suggesting the involvement of the PERK pathway in RVFV-induced cytotoxicity. Taken together, our study revealed that RVFV NSs inhibits two out of the three UPR pathways and promotes RVFV-induced cytotoxicity by inhibiting the PERK pathway. IMPORTANCE The Rift Valley fever virus (RVFV) NSs protein is a major virulence factor with multiple functions, including inhibition of general host transcription. RVFV carrying a mutated NSs that lacks the transcription suppression function is less cytotoxic in vitro and less virulent in mice compared to its parental virus, suggesting that NSs-mediated host transcription suppression is important for RVFV virulence. Experiments using this mutant virus and its parental RVFV revealed that NSs inhibits the expression of unfolded protein response (UPR) mediators ATF4 and XBP1, which act downstream of the PERK and IRE1 pathways, respectively. Our data also showed that activation of PERK promotes cell survival in RVFV infection. These data suggest that the NSs-mediated inhibition of UPR pathways contributes to RVFV pathogenicity.