Description
A fast and easy way to identify new binders to human AQ STING.
Stimulator of interferon genes (STING), also known as transmembrane protein 173 (TMEM173), is a protein playing a major role in innate immunity.
The AQ and HAQ variants are found in respectively 5.2% and 20.4% of the world population. Both variants display the same binding pocket, since the R71H mutation is part of the transmembrane domain, and not expressed in the recombinant protein.
Upon intracellular cytosolic DNA release from pathogens such as viruses and bacteria, 2’-3’cGAMP binds to STING protein and triggers the secretion of type 1 interferon. The AQ variant shows a strong affinity to natural 2'-3'cGAMP and other known compounds.
Identifying new STING ligands is therefore a way to control immunity and fight autoinflammatory diseases.
Assay principle
The HTRF AQ STING binding assay is a competitive assay format which uses d2-labeled AQ STING ligand, a 6His tagged human AQ STING protein, and an anti 6His Cryptate-labeled antibody. Your compound competes with the AQ STING ligand-d2 and thereby prevents FRET from occurring.
Assay protocol
The Human AQ STING binding assay can be run in a 96- or 384-well low volume white plate (20 µL final). As described here, samples or standards are dispensed directly into the assay plate. The human His-tagged AQ STING protein is then added, followed by the dispensing of the HTRF reagents: anti 6His antibody labeled with Terbium cryptate and AQ STING ligand labeled with d2. The reagents labeled with HTRF fluorophores may be pre-mixed and added in a single dispensing step. No washing steps are needed. The protocol can be further miniaturized or upscaled by simply resizing each addition volume proportionally.
Compound screening
Various compounds known to be STING ligands were added to the assay.
2'3'cGAMP displays a high affinity for the AQ variant in the nM range. Similarly, the other bacterial compounds, such as 3'3' cGAMP and the reference compound ADU-S100, show Ki constants in the nM, whereas cyclic di-AMP and cyclic di-GMP are in the hundred nM range.
DMXAA, a non-specific compound, also a well-known mouse STING binder, has no effect on the assay, confirming the human specificity of the kit.
STING simplified pathway
STING, for STimulator of INterferon Genes, is a cytoplasmic homodimeric protein localized in the endoplasmic reticulum, and which plays an essential role in innate immunity. Upon pathogen infection or mitochrondrial shrinking during apoptosis, floating dsDNAs bind and activate a DNA sensor, cyclic GMP-AMP synthase (cGAS). Activated cGAS leads to the production of 2’-3’cGAMP, a cyclic dinucleotide, which then binds to STING proteins. In turn, phosphorylated STING interacts with TANK-binding-kinase-I (TBK1), leading to the recruitment and activation of the active interferon regulatory factor (IRF3) dimer. Nuclear translocation of the IRF3 dimer leads to the transcription of genes encoding IFN-α/β. In addition, the STING pathway controls NF-κB dependent inflammatory cytokine expression. As a negative feedback loop, the DNA-stimulated cGAS-STING-TBK1 pathway also triggers STING protein degradation through p62 SQSTM1 associated autophagy, to switch off IFNb production.
