Fig. 2

Molecular insight into STING activation. a Upper panel, the schematic domain representation of human STING (molecular weight, 42 kDa); bottom panel, conformational changes of STING activation. In steady-state, the transmembrane helix of two protomers forms a domain-swapped architecture. The STING LBD dimer presents a V shape, with a deep cleft between the two protomers to accommodate the CDNs, as the first pocket. (Graphic modified from Fig. 4 of ref. ³³ and Extended Data Fig. 7 of ref. ⁴⁰). b Cartoon representation of the structure in two orthogonal side views of activated STING tetramer with 2’3’-cGAMP and C53. (STING tetramer bound to both cGAMP and C53, PDB ID: 7SII). c Insight into the 2’-3-cGAMP-binding pocket. Residues offer key interaction with 2’3’-cGAMP and C53. The guanidinium groups of R238 on the lid sheet hang down into the core of LBD and forms direct interaction with the bottom backbone phosphates. The phosphate of 2’-5’ phosphodiester peripherally contacts with the R232 on one side. The free 3’-OH of guanosine forms a direct or water-mediated hydrogen bond to two Ser162 residues from the lower part of the pocket, whereas 2’-OH of adenosine is free of interaction. The guanine base directly interacts with the side groups of Glu260 and Thr263, while the adenosine forms only interaction with the main-chain carbonyl oxygen of Val239. (cGAMP bound human STING CTD structure, PDB ID: 4KSY). d Two-dimensional diagram of the interactions between STING ligand binding domain and 2’3’-cGAMP in the cytosolic side. e Two four-helix bundles are connected at the ER or Golgi luminal side by the N-terminal residues to maintain the TMD–TMD interaction between STING dimers. (STING tetramer bound to both cGAMP and C53, PDB ID: 7SII). f Two-dimensional diagram of the interactions between STING transmembrane domain and C53. g The C-terminal tail (CTT) of STING contains two conserved motifs for TBK1 and IRF3 binding, PLPLRT/SD motif and pLxIS motif (p, hydrophilic; x, nonaromatic) (up). The IRF3 CTT harbors a similar conserved cLxIS (c, charged residue) consensus motif. h Model of TBK1 activation and STING and IRF3 phosphorylation upon STING oligomerization. TBK1 sits above and binds to the CTT of STING dimer, but phosphorylates the CTT of an adjacent STING dimer. IRF-3 contains the binding surface for both the pLxIS motif of STING to mediate its recruitment and phosphorylation by TBK1 and also a pLxIS motif of its own to mediate its dimerization. Close-up views of the STING–TBK1 interface and STING–IRF3 interface are zoomed in and presented in red and yellow dashed boxes, respectively. (TBK1–STING tail complex, PDB: 6NT9; Phosphated STING tail–IRF3 complex, PDB: 5JEJ; cGAMP-bound chicken STING tetramer, PDB: 6NT8). All structural figures were generated with PyMOL (https://www.pymol.org). cGAMP cyclic GMP–AMP, IFN interferon, IRF3 interferon regulatory factor 3, STING stimulator of interferon genes, TBK1 TANK-binding kinase 1