Functional roles for viral RNA ribosylation in mammalian cells

In mammalian cells, the addition of ADP-ribose to proteins and DNA plays well established roles in regulating cell function. Recently, RNA ribosylation was also found in mammalian cells under conditions of cell stress, though the functional consequences remain unclear. Here we find that infection with chikungunya virus, a positive strand RNA virus that causes frequent widespread epidemics, increases overall levels of RNA ribosylation in human fibroblasts. During infection, viral RNA is ribosylated by the PARP12 ribosyltransferase, which is counteracted by the viral nsP3 ribosylhydrolase. Increased viral RNA ribosylation resulted in decreased translation in cell-free systems and infected fibroblasts, and more rapid viral RNA decay. Further, ribosylated RNA potently induced the expression of antiviral host response genes. Together these data show the first functional consequences of RNA ribosylation in mammalian cells by showing that RNA ribosylation inhibits translation, decreases RNA stability, and creates a novel pathogen-associated molecular pattern that activates the host innate immune response. As macrodomains are present in multiple unrelated viruses, our data suggest RNA ribosylation is a novel component of cellular antiviral sensing pathway. These results define a framework for understanding how RNA ribosylation regulates RNA metabolism during viral infection.