Structural basis of STING binding with and phosphorylation by TBK1

Area of Science:

• Immunology
• Molecular Biology
• Structural Biology

Background:

• Cytoplasmic DNA invasion signals danger, activating host immunity via the cGAS-STING pathway.
• Cyclic GMP-AMP synthase (cGAS) senses DNA, producing cyclic GMP-AMP (cGAMP) to activate stimulator of interferon genes (STING).
• Activated STING recruits and activates TBK1 kinase, leading to IRF3 phosphorylation and type-I interferon induction, but the activation mechanism is unclear.

Purpose of the Study:

• To elucidate the structural mechanism by which cGAMP-bound STING activates TBK1 kinase.
• To understand how STING is phosphorylated by TBK1 to initiate downstream immune signaling.

Main Methods:

• Cryo-electron microscopy (cryo-EM) to determine the structure of human TBK1 complexed with cGAMP-bound STING.
• Mutational analyses to validate the proposed interaction model.

Main Results:

• The cryo-EM structure reveals STING's C-terminal tail inserting into a groove within the TBK1 dimer.
• This binding mode prevents direct phosphorylation of STING by TBK1's active site, suggesting a requirement for oligomerization.
• Mutational data support a model where cGAMP induces STING and TBK1 oligomerization, facilitating STING phosphorylation.

Conclusions:

• STING phosphorylation by TBK1 is dependent on the oligomerization of both proteins, induced by cGAMP binding.
• This structural insight clarifies a key step in the cGAS-STING innate immune signaling pathway.
• The findings provide a mechanistic basis for understanding DNA-triggered immune activation.