The Cytosolic DNA Sensor cGAS Forms an Oligomeric Complex with DNA and Undergoes Switch-like Conformational Changes in the Activation Loop

•DNA binding switches an activation loop in cGAS•DNA binding induces dimerization of cGAS, forming a 2:2 complex•cGAS dimerization is essential for its activation•The two cGAS DNA binding surfaces are critical for inducing type I interferons

SummaryThe presence of DNA in the cytoplasm is a danger signal that triggers immune and inflammatory responses. Cytosolic DNA binds to and activates cyclic GMP-AMP (cGAMP) synthase (cGAS), which produces the second messenger cGAMP. cGAMP binds to the adaptor protein STING and activates a signaling cascade that leads to the production of type I interferons and other cytokines. Here, we report the crystal structures of human cGAS in its apo form, representing its autoinhibited conformation as well as in its cGAMP- and sulfate-bound forms. These structures reveal switch-like conformational changes of an activation loop that result in the rearrangement of the catalytic site. The structure of DNA-bound cGAS reveals a complex composed of dimeric cGAS bound to two molecules of DNA. Functional analyses of cGAS mutants demonstrate that both the protein-protein interface and the two DNA binding surfaces are critical for cGAS activation. These results provide insights into the mechanism of DNA sensing by cGAS.

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