An SRR1 domain-containing protein is required for efficient Orsay virus replication in Caenorhabditis elegans

Viruses depend on their hosts for completing their life cycle, and a better understanding of virus replication can inform therapeutic strategies. Using the Orsay virus-Caenorhabditis elegans experimental platform, we identified by a forward genetic screen the host gene Y55F3BL.4 (renamed viro-9) as a novel host factor critical for Orsay virus replication. Three distinct mutations of viro-9 each resulted in a >1,000-fold reduction in Orsay viral load, demonstrating a pro-viral function of viro-9. viro-9 had no previously described function in C. elegans, but in the absence of viral infection, deletion of the viro-9 locus by CRISPR/Cas9 led to a reduction in brood size and a shortened lifespan. VIRO-9 contains a sensitivity to red light reduced (SRR1) protein domain. While SRR1 domains are present in diverse organisms, including plants, yeast, and mammals, little is known about their function. The Caenorhabditis briggsae ortholog of viro-9, CBG23913, can functionally complement the C. elegans viro-9 defect, demonstrating that the pro-viral function of the SRR1 domain is conserved over at least 80 million years of evolution. Furthermore, we identified three conserved amino acid residues within the SRR1 domain that are required for Orsay virus infection. This study provides the first insights into amino acids necessary for functionality of the SRR1 domain and demonstrates the essential role of viro-9 in virus infection.