Extended Data Fig. 1: Structural analysis of bacterial STING–cyclic dinucleotide complex formation.
From: STING cyclic dinucleotide sensing originated in bacteria
a, Phylogenetic tree of all CBASS-associated bacterial STING homologues based on structure-guided sequence alignment and previous bioinformatics analysis10,35. STING homologues investigated in this study are highlighted in orange, and a star denotes determined STING crystal structures. All TM–STING fusions cluster together. b, Crystal structure of a STING receptor from the bacterium C. granulosa (CgSTING) in the apo state reveals an open configuration with a solvent exposed cyclic-dinucleotide-binding pocket at the dimeric interface (monomers in gold and grey for clarity). The CgSTING structure confirms that both divergent TM–STING and TIR–STING fusions are members of the same structurally conserved family of STING receptors. c, Comparison of the CgSTING, FsSTING–3′,3′-cGAMP and human STING–2′,3′-cGAMP structures demonstrates conservation of an open-to-closed β-strand lid movement upon ligand binding. d, Overlay of the β-strand lid of CgSTING (grey) and FsSTING (orange) shows both inward translation and slight rotation resulting in a displacement of about 5 Å. R153 of FsSTING stacks between the bases of 3′,3′-cGAMP and R151 is splayed away from ligand. e, Comparison of the human STING and FsSTING lid region shows conserved contacts from β-strand arginine residues. Unlike in bacterial STING, human STING R232 makes an additional contact with the cyclic dinucleotide phosphodiester backbone that is critical for recognition of the 2′–5′ linkage in 2′,3′-cGAMP. A detailed comparison is in Extended Data Fig. 10b. f, Modelling of 2′,3′-cGAMP into the FsSTING–3′,3′-cGAMP structure demonstrates an additional feature of bacterial STING preventing recognition of 2′–5′-linked cyclic dinucleotides. Although the overall cyclic dinucleotide conformation is shared between human and bacterial STING, the α-helix ending at P264 in human STING is a half-turn longer in FsSTING (also ending in a proline) which places a conserved T173 residue in a position that occludes where the free 3′-OH of 2′,3′-cGAMP would be positioned. g, Structure-guided alignment of FsSTING and human STING cyclic-dinucleotide-binding domains. FsSTING and human STING exhibit no detectable sequence homology but share a conserved structural fold. Key residues involved in cyclic dinucleotide binding that are shared between bacterial and human STING are boxed in orange and human STING specific cyclic dinucleotide contacts are boxed in red. In FsSTING, D169 directly reads out the guanine base of c-di-GMP.
