Nucleotidyltransferase that catalyzes the formation of cyclic GMP-AMP (cGAMP) from ATP
and GTP and plays a key role in innate immunity. Catalysis involves both the formation
of a 2′,5′ phosphodiester linkage at the GpA step and the formation of a 3′,5′
phosphodiester linkage at the ApG step, producing c[G(2′,5′)pA(3′,5′)p] . Acts as a key
cytosolic DNA sensor, the presence of double-stranded DNA (dsDNA) in the cytoplasm
being a danger signal that triggers the immune responses . Binds cytosolic DNA directly,
leading to activation and synthesis of cGAMP, a second messenger that binds to and
activates TMEM173/STING, thereby triggering type-I interferon production.