A CRISPR screening approach shows that endoplasmic reticulum (ER)-associated protein complexes, including the oligosaccharyltransferase (OST) protein complex, are important for infection by dengue virus and other related mosquito-borne flaviviruses, whereas hepatitis C virus is dependent on distinct entry factors, RNA binding proteins and FAD biosynthesis. Jan Carette and colleagues use a CRISPR screening approach to identify cellular genes with important roles in the lifecycle of two important human flaviviruses: dengue virus and hepatitis C virus. The authors show that endoplasmic-reticulum-associated protein complexes, including the oligosaccharyltransferase (OST) protein complex, are important for infection by dengue virus and other related mosquito-borne flaviviruses, whereas hepatitis C virus is dependent on distinct entry factors, RNA binding proteins and FAD biosynthesis. Also in this issue of Nature, Michael Diamond and colleagues report that the endoplasmic-reticulum-associated signal peptidase complex is required for infection by numerous flaviviruses, including West Nile, dengue and Zika viruses, but not for infection by other types of virus or for host protein synthesis. The Flaviviridae are a family of viruses that cause severe human diseases. For example, dengue virus (DENV) is a rapidly emerging pathogen causing an estimated 100 million symptomatic infections annually worldwide1. No approved antivirals are available to date, and clinical trials with a tetravalent dengue vaccine showed disappointingly low protection rates2. Hepatitis C virus (HCV) also remains a major medical problem, with 160 million chronically infected patients worldwide and only expensive treatments available3. Despite distinct differences in their pathogenesis and modes of transmission, the two viruses share common replication strategies4. A detailed understanding of the host functions that determine viral infection is lacking. Here we use a pooled CRISPR genetic screening strategy5,6 to comprehensively dissect host factors required for these two highly important Flaviviridae members. For DENV, we identified endoplasmic-reticulum (ER)-associated multi-protein complexes involved in signal sequence recognition, N-linked glycosylation and ER-associated degradation. DENV replication was nearly completely abrogated in cells deficient in the oligosaccharyltransferase (OST) complex. Mechanistic studies pinpointed viral RNA replication and not entry or translation as the crucial step requiring the OST complex. Moreover, we show that viral non-structural proteins bind to the OST complex. The identified ER-associated protein complexes were also important for infection by other mosquito-borne flaviviruses including Zika virus, an emerging pathogen causing severe birth defects7. By contrast, the most significant genes identified in the HCV screen were distinct and included viral receptors, RNA-binding proteins and enzymes involved in metabolism. We found an unexpected link between intracellular flavin adenine dinucleotide (FAD) levels and HCV replication. This study shows notable divergence in host-dependency factors between DENV and HCV, and illuminates new host targets for antiviral therapy.

More than 140 post-transcriptional modifications (PTMs) are known to decorate cellular RNAs, but their incidence, identity and significance in viral RNA are still largely unknown. We have developed an agnostic analytical approach to comprehensively survey PTMs on viral and cellular RNAs. Specifically, we used mass spectrometry to analyze PTMs on total RNA isolated from cells infected with Zika virus, Dengue virus, hepatitis C virus (HCV), poliovirus and human immunodeficiency virus type 1. All five RNA viruses significantly altered global PTM landscapes. Examination of PTM profiles of individual viral genomes isolated by affinity capture revealed a plethora of PTMs on viral RNAs, which far exceeds the handful of well-characterized modifications. Direct comparison of viral epitranscriptomes identified common and virus-specific PTMs. In particular, specific dimethylcytosine modifications were only present in total RNA from virus-infected cells, and in intracellular HCV RNA, and viral RNA from Zika and HCV virions. Moreover, dimethylcytosine abundance during viral infection was modulated by the cellular DEAD-box RNA helicase DDX6. By opening the Pandora’s box on viral PTMs, this report presents numerous questions and hypotheses on PTM function and strongly supports PTMs as a new tier of regulation by which RNA viruses subvert the host and evade cellular surveillance systems.

Idiopathic pulmonary arterial hypertension (PAH [IPAH]) is an insidious and potentially fatal disease linked to a mutation or reduced expression of bone morphogenetic protein receptor 2 (BMPR2). Because intravascular inflammatory cells are recruited in IPAH pathogenesis, we hypothesized that reduced BMPR2 enhances production of the potent chemokine granulocyte macrophage colony-stimulating factor (GM-CSF) in response to an inflammatory perturbation. When human pulmonary artery (PA) endothelial cells deficient in BMPR2 were stimulated with tumor necrosis factor (TNF), a twofold increase in GM-CSF was observed and related to enhanced messenger RNA (mRNA) translation. The mechanism was associated with disruption of stress granule formation. Specifically, loss of BMPR2 induced prolonged phospho-p38 mitogen-activated protein kinase (MAPK) in response to TNF, and this increased GADD34–PP1 phosphatase activity, dephosphorylating eukaryotic translation initiation factor (eIF2α), and derepressing GM-CSF mRNA translation. Lungs from IPAH patients versus unused donor controls revealed heightened PA expression of GM-CSF co-distributing with increased TNF and expanded populations of hematopoietic and endothelial GM-CSF receptor α (GM-CSFRα)–positive cells. Moreover, a 3-wk infusion of GM-CSF in mice increased hypoxia-induced PAH, in association with increased perivascular macrophages and muscularized distal arteries, whereas blockade of GM-CSF repressed these features. Thus, reduced BMPR2 can subvert a stress granule response, heighten GM-CSF mRNA translation, increase inflammatory cell recruitment, and exacerbate PAH.

The poxvirus vaccinia virus phosphorylates serine/threonine residues in the human small ribosomal subunit RACK1, converting it to a plant-like state to favour translation of poxvirus mRNAs It has been proposed that ribosomes have the capacity to control mRNA-specific translation by changes in their subunit composition or post-translational modifications. Derek Walsh and colleagues reveal a new way that viruses usurp the cellular machinery to facilitate their replication. They show that poxviruses customize host ribosomes to boost translation of viral mRNAs. The vaccinia virus phosphorylates serine/threonine residues in the ribosomal subunit RACK1, creating a RACK1 loop with a negative charge. This results in the preferential translation of poxvirus mRNAs that carry adenosine repeats in the 5′ UTR, which act as translational enhancers. Ribosomes have the capacity to selectively control translation through changes in their composition that enable recognition of specific RNA elements1. However, beyond differential subunit expression during development2,3, evidence for regulated ribosome specification within individual cells has remained elusive1. Here we report that a poxvirus kinase phosphorylates serine/threonine residues in the human small ribosomal subunit protein, receptor for activated C kinase (RACK1), that are not phosphorylated in uninfected cells or cells infected by other viruses. These modified residues cluster in an extended loop in RACK1, phosphorylation of which selects for translation of viral or reporter mRNAs with 5′ untranslated regions that contain adenosine repeats, so-called polyA-leaders. Structural and phylogenetic analyses revealed that although RACK1 is highly conserved, this loop is variable and contains negatively charged amino acids in plants, in which these leaders act as translational enhancers. Phosphomimetics and inter-species chimaeras have shown that negative charge in the RACK1 loop dictates ribosome selectivity towards viral RNAs. By converting human RACK1 to a charged, plant-like state, poxviruses remodel host ribosomes so that adenosine repeats erroneously generated by slippage of the viral RNA polymerase4 confer a translational advantage. Our findings provide insight into ribosome customization through trans-kingdom mimicry and the mechanics of species-specific leader activity that underlie poxvirus polyA-leaders4.