cGAS-like receptors sense RNA and control 3'2'-cGAMP signaling in Drosophila.

Cyclic GMP-AMP synthase (cGAS) is a cytosolic DNA sensor that produces the second messenger 2'3'-cGAMP and controls activation of innate immunity in mammalian cells1-5 . Animal genomes typically encode multiple proteins with predicted homology to cGAS6-10 , but the function of these uncharacterized enzymes is unknown. Here we show that cGAS-like receptors (cGLRs) are innate immune sensors capable of recognizing divergent molecular patterns and catalyzing synthesis of distinct nucleotide second messenger signals. Crystal structures of human and insect cGLRs reveal a nucleotidyltransferase signaling core shared with cGAS and a diversified primary ligand-binding surface modified with significant insertions and deletions. We demonstrate that cGLR surface remodeling enables altered ligand specificity and use a forward biochemical screen to identify cGLR1 as a double-stranded RNA sensor in the model organism Drosophila melanogaster. Surprisingly, RNA recognition activates Drosophila cGLR1 to synthesize the novel product cG[3'-5']pA[2'-5']p (3'2'-cGAMP). A crystal structure of Drosophila Stimulator of Interferon Genes (STING) in complex with 3'2'-cGAMP explains selective isomer recognition and we demonstrate that 3'2'-cGAMP induces an enhanced antiviral state in vivo that protects from viral infection. Similar to radiation of Toll-like receptors in pathogen immunity, our results establish cGLRs as a diverse family of metazoan pattern recognition receptors.