Viral and metazoan poxins are cGAMP-specific nucleases that restrict cGAS-STING signaling

Cytosolic DNA triggers innate immune responses through activation of cyclic GMP–AMP synthase (cGAS) and production of the cyclic dinucleotide second messenger 2′3′ cGAMP(1–4). 2′3′ cGAMP is a potent inducer of immune signaling, but no intracellular nucleases are known to cleave 2′3′ cGAMP and prevent activation of the receptor stimulator of interferon genes (STING)(5–7). Through a biochemical screen analyzing 24 mammalian viruses, here we identify poxvirus immune nucleases (poxins) as a family of 2′3′ cGAMP-specific degrading enzymes. Poxins cleave 2′3′ cGAMP to restrict STING-dependent signaling, and deletion of the poxin gene (B2R) attenuates vaccinia virus replication in vivo. Crystal structures of vaccinia virus poxin in pre- and post-reactive states define the mechanism of selective 2′3′ cGAMP degradation through metal-independent cleavage of the 3′–5′ bond, converting 2′3′ cGAMP into linear Gp[2′–5′]Ap[3′]. Poxins are conserved in mammalian poxviruses, and remarkably, we further identify functional poxin homologues in the genomes of moths and butterflies and the baculoviruses which infect them. Baculovirus and insect host poxin homologues retain selective 2′3′ cGAMP degradation activity, suggesting an ancient role for poxins in cGAS-STING regulation. Our results define poxins as a family of 2′3′ cGAMP-specific nucleases and demonstrate a mechanism for how viruses evade innate immunity.