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.

The mosquito-borne flaviviruses include important human pathogens such as dengue, Zika, West Nile, and yellow fever viruses, which pose a serious threat for global health. Recent genetic screens identified endoplasmic reticulum (ER)-membrane multiprotein complexes, including the oligosaccharyltransferase (OST) complex, as critical flavivirus host factors. Here, we show that a chemical modulator of the OST complex termed NGI-1 has promising antiviral activity against flavivirus infections. We demonstrate that NGI-1 blocks viral RNA replication and that antiviral activity does not depend on inhibition of the N-glycosylation function of the OST. Viral mutants adapted to replicate in cells deficient of the OST complex showed resistance to NGI-1 treatment, reinforcing the on-target activity of NGI-1. Lastly, we show that NGI-1 also has strong antiviral activity in primary and disease-relevant cell types. This study provides an example for advancing from the identification of genetic determinants of infection to a host-directed antiviral compound with broad activity against flaviviruses.

Cohesin is a multi-subunit nuclear protein complex that coordinates sister chromatid separation during cell division. Highly frequent somatic mutations in genes encoding core cohesin subunits have been reported in multiple cancer types. Here, using a genome-wide CRISPR-Cas9 screening approach to identify host dependency factors and novel innate immune regulators of rotavirus (RV) infection, we demonstrate that the loss of STAG2, an important component of the cohesin complex, confers resistance to RV replication in cell culture and human intestinal enteroids. Mechanistically, STAG2 deficiency results in spontaneous genomic DNA damage and robust interferon (IFN) expression via the cGAS-STING cytosolic DNA-sensing pathway. The resultant activation of JAK-STAT signaling and IFN-stimulated gene (ISG) expression broadly protects against virus infections, including RVs. Our work highlights a previously undocumented role of the cohesin complex in regulating IFN homeostasis and identifies new therapeutic avenues for manipulating the innate immunity.