3'3'-cGAMP

3′,3′-Cyclic GMP-AMP (3′3′-cGAMP, CAS: 849214-04-6) is a naturally occurring cyclic dinucleotide synthesized by bacteria. It couples one guanylate and one adenylate through two 3′5′ phosphodiester bonds in a head-to-tail cyclization. It is a geometric isomer of the 2′3′-cGAMP produced by cGAS - the only difference lies in the phosphodiester linkage: both bonds in 3′3′-cGAMP adopt the 3′5′ configuration, whereas 2′3′-cGAMP contains one 2′5′ and one 3′5′ bond.

Bacteria synthesize 3′3′-cGAMP through cyclonucleotide synthases such as DncV (the CBASS antiphage system) to counter phage invasion. Mammalian cells do not produce 3′3′-cGAMP themselves; once the STING ligand-binding domain binds 3′3′-cGAMP, STING dimerizes and translocates to the Golgi, where it recruits and activates TBK1, and TBK1 phosphorylates IRF3 and IKKβ to launch type I interferon and NF-κB pathways. This activation bypasses upstream cGAS, so the host reads bacterial 3′3′-cGAMP as an infection signal. 3′3′-cGAMP binds STING with lower affinity than 2′3′-cGAMP and activates it more weakly, reflecting STING's selectivity for the phosphodiester geometry.

Researchers use 3′3′-cGAMP as a STING-activating ligand to study how bacterial cyclic dinucleotides trigger host immunity, and as a control molecule of distinct phosphodiester geometry to map the binding preference of the STING ligand-binding domain.

References:

[1] Davies BW, Bogard RW, Young TS, Mekalanos JJ. Coordinated regulation of accessory genetic elements produces cyclic di-nucleotides for V. cholerae virulence. Cell. 2012;149(2):358-370.

[2] Cohen D, Melamed S, Millman A, et al. Cyclic GMP-AMP signalling protects bacteria against viral infection. Nature. 2019;574(7779):691-695.

[3] Severin GB, Ramliden MS, Hawver LA, et al. Direct activation of a phospholipase by cyclic GMP-AMP in El Tor Vibrio cholerae. PNAS. 2018;115(26):E6048-E6055.