Evolution and Regulation of Antiviral Nucleotide Signaling

Abstract

Cells utilize diverse methods for detecting and responding to viral infection. In mammals, the enzyme cyclic GMP–AMP synthase (cGAS) binds directly to viral DNA in the cytosol and becomes activated to synthesize the nucleotide second messenger 2′–5′/3′–5′ cyclic GMP–AMP (2′3′-cGAMP). 2′3′-cGAMP diffuses throughout the cell and is detected by Stimulator of Interferon Genes (STING), which activates downstream antiviral signaling. While pathogen strategies for inhibiting cGAS or STING have been identified, no viruses are known to target 2′3′-cGAMP during infection. Here, using a biochemical screen of 24 mammalian viruses, we identify poxins (poxvirus immune nucleases) as a family of 2′3′-cGAMP-specific degrading enzymes, and define the mechanism of cleavage. The closest homologs of vaccinia virus poxin outside mammalian poxviruses are less than 20% identical, and exist in the genomes of insect viruses, and insects in the family Lepidoptera. Using a family-wide biochemical and structural analysis, we reconstruct the evolutionary history of poxin enzymes, finding that these proteins likely evolved in the context of RNA virus polyproteins, and were horizontally transferred to insects and other viruses. Identification of functional poxin homologs in genomes of insects and non-mammalian viruses supports the emerging model that cGAS-like enzymes control antiviral responses in diverse organisms. In order to understand the function of divergent bacterial cGAS-like enzymes, we used biochemical and structural analysis to find that these proteins synthesize alternative second messengers allowing diversification of antiviral immune responses in different organisms. Together, my work highlights the opposing evolutionary forces within virus-host conflicts for control of antiviral nucleotide signaling pathways and shows how strategies for regulating signaling are shared across species and domain boundaries through horizontal transfer.