The L-A dsRNA Virus and Its Satellites: Totiviruses and Killers in Saccharomyces cerevisiae
A secreted protein toxin encoded by a satellite dsRNA-enabled dissection of the genetic control of replication and expression of a single-segment dsRNA virus, the first Totivirus, L-A. Among the then-novel findings were i. “head-full replication”, ii. a supposedly forbidden “T = 2” capsid symmetry based on an asymmetric dimer, iii. a host N-acetyltransferase whose modification of the coat protein is necessary for packaging, iv. Kex1 and Kex2 pro-toxin peptidases leading to discovery of the pre-pro-insulin processing enzymes, and v. specific viral (+) strand sites/structures needed for RNA packaging and (-) strand synthesis. L-A viral (+) strands made in the particle are extruded to the cytoplasm. Those destined for translation are 5′ 7meGMP-capped by a coat protein activity that steals the cap from cellular mRNAs. (+) strands destined for encapsidation in new coats are not capped. Three host-encoded anti-viral systems were found, one based on blocking translation of the viral non-polyA mRNAs (Ski2,3,8 complex), another a 5′->3′ exoribonuclease specific for uncapped molecules (such as the viral (+) strands)(Ski1/Xrn1), and the third a mitochondrial nuclease released in cells undergoing meiosis/sporulation (Nuc1). All of these systems protect cells from virus-induced pathology and have clear animal homologs. The 3′ polyA of yeast mRNAs is dispensable for translation in ski2Δ slh1Δ cells, and such cells are healthy unless the L-A and M dsRNAs are present, suggesting that this polyA is primarily a device allowing cells to distinguish viral and cellular mRNAs. We suggest that the ribosome-associated Ski2,3,8 proteins block 60S subunit joining on polyA− mRNAs. Recent evidence of roles for other cellular components controlling viral expression and replication suggests that yeast viruses will continue to be a fertile area for study of viral pathogenesis and host anti-viral systems.