Structural mechanism of cytosolic DNA sensing by cGAS

Cytosolic DNA arising from intracellular bacterial or viral infections is a powerful pathogen-associated molecular pattern (PAMP) that leads to innate immune host defence by the production of type I interferon and inflammatory cytokines. Recognition of cytosolic DNA by the recently discovered cyclic-GMP-AMP (cGAMP) synthase (cGAS) induces the production of cGAMP to activate the stimulator of interferon genes (STING). Here we report the crystal structure of cGAS alone and in complex with DNA, ATP and GTP along with functional studies. Our results explain the broad DNA sensing specificity of cGAS, show how cGAS catalyses dinucleotide formation and indicate activation by a DNA-induced structural switch. cGAS possesses a remarkable structural similarity to the antiviral cytosolic double-stranded RNA sensor 2′-5′oligoadenylate synthase (OAS1), but contains a unique zinc thumb that recognizes B-form double-stranded DNA. Our results mechanistically unify dsRNA and dsDNA innate immune sensing by OAS1 and cGAS nucleotidyl transferases. Cytosolic DNA arising from intracellular bacterial or viral infections induces type I interferon through activation of the DNA sensor cGAS, which catalyses the synthesis of cyclic dinucleotide which in turn activates STING; here the crystal structures of a carboxy-terminal fragment of cGAS alone and in complex with UTP and DNA–ATP–GTP complex are determined. The mechanism of sensing and signalling of cytosolic DNA by the innate immune system is a topic of intense research interest as it is the means by which invading bacteria and viruses are detected. Cytosolic DNA is known to induce type I interferon through activation of the DNA sensor cyclic-GMP-AMP synthetase (cGAS), which catalyses the synthesis of a cyclic dinucleotide which in turn activates a protein known as STING (stimulator of interferon genes). Karl-Peter Hopfner and co-workers present the crystal structures of a C-terminal fragment of cGAS alone, in complex with UTP, and as a DNA–ATP–GTP complex. In a complementary paper [in this issue], Veit Hornung and coworkers show that the product of cGAS is distinct from previously characterized cyclic dinucleotides. Rather it is an unorthodox cyclic dinucleotide with a 2′–5′ linkage between guanosine and adenosine. This two-step synthesis of cGAMP(2′–5′) could be a focus for the development of specific inhibitors for the treatment of autoimmune diseases that engage the cGAS–STING axis.