Modular double-stranded DNA recognition defines specificity of the exonuclease TREX1 in cGAS-STING control

TREX1, but not other DEDDh exonucleases, restrains cyclic GMP-AMP synthase- stimulator of interferon genes (cGAS-STING) activation by cytosolic DNA. Here, we examined the mechanisms underlying this specificity. Biochemical comparison of TREX1 and TREX2 mapped TREX1's ability to degrade dsDNA and suppress cGAS signaling to a single amino acid, R128. Metazoan TREX proteins containing an R128-equivalent functioned as double-stranded DNA (dsDNA) nucleases, often alongside cGAS-like receptors, defining an evolutionarily conserved family. Structurally, TREX1-R128 contacted the non-substrate strand to enable efficient dsDNA binding and cleavage. Examination of conserved structural features of the R128-containing TREX family further identified a substrate-sensing loop that inserted into the active-site cleft and stabilized the substrate strand. In human TREX1, a distinct auxiliary DNA-binding surface (B-site) strengthened DNA engagement and cGAS restraint independent of DNase activity, suggesting a competitive function. Autoimmune-associated TREX1 mutations mapped to TREX1-dsDNA interfaces, and mutations disrupting these interactions increased cGAS-STING signaling and antitumor immunity in murine models. Thus, a modular dsDNA-recognition architecture defines TREX1 specificity and regulation of the cGAS-STING pathway.