Three functionally distinct classes of cGAS proteins in nature revealed by self-DNA-induced interferon responses

Innate immune pattern recognition receptors (PRRs) emerged early in evolution. It is generally assumed that structurally homologous proteins in distinct species will operate via similar mechanisms. We tested this prediction through the study of interferon responses to self-DNA by the enzymatic PRR cyclic GMP-AMP synthase (cGAS). Contrary to expectations, we identified three functional classes of this PRR in mammals. Class 1 proteins (including human) contained a catalytic domain that was intrinsically self-DNA reactive and stimulated interferon responses in diverse cell types. This reactivity was prevented by an upstream N-terminal domain. Class 2 and 3 proteins were either not self-DNA reactive (including chimpanzee) or included proteins whose N-terminal domain promoted self-DNA reactivity (mouse). While self-DNA reactivity of Class 1 cGAS was linked to an ability to access intra-mitochondrial DNA, mitochondrial localization was not associated with other classes. These studies reveal unexpected diversity in the mechanisms of self-DNA reactivity of a PRR. One Sentence Summary The regulation of self-DNA reactivity of cGAS is evolutionarily diverse in mammals.