Diverse innate immune factors protect yeast from lethal viral pathogenesis

In recent years, newly characterized anti-viral systems have proven to be remarkably conserved from bacteria to mammals, demonstrating that unique insights into these systems can be gained by studying microbial organisms. Despite the enthusiasm generated by these findings, the key microbial model organism Saccharomyces cerevisiae (budding yeast) has been minimally exploited for studies of viral defense, primarily because it is not infected with exogenously transmitted viruses. However, most yeast strains are infected with an endogenous double stranded RNA (dsRNA) virus called L-A, and previous studies identified conserved antiviral systems that attenuate L-A replication. Although these systems do not completely eradicate L-A, we show here that they do prevent proteostatic stress and lethality caused by L-A over-proliferation. Exploiting this new finding, we demonstrate that the genetic screening methods available in yeast can be used to identify additional conserved antiviral systems. Using these approaches, we discovered antiviral functions for the yeast homologs of polyA-binding protein (PABPC1) and the La-domain containing protein Larp1, which are both involved in viral innate immunity in humans. We also identified new antiviral functions for the RNA exonucleases REX2 and MYG1, both of which have distinct but poorly characterized human and bacterial homologs. These findings highlight the potential of yeast as a powerful model system for the discovery and characterization of conserved antiviral systems. Significance StatementThe budding yeast Saccharomyces cerevisiae has been minimally exploited for investigation of host-virus interactions despite its chronic infection with a double-stranded RNA virus called L-A. Controverting its presumed harmless nature, we show here that L-A causes pathogenesis in cells lacking parallel-acting viral attenuation pathways. Taking advantage of the genetic tools available in budding yeast, we identify several highly conserved proteins to play a role in antiviral defense. Some of these have been recently identified in humans to be involved in viral innate immunity, thus highlighting the potential of budding yeast as a model organism to identify and investigate new antiviral systems.