| Assay Method Information | |
| | STING SPA Binding Assay |
| Description: | The human STING SPA binding assay measures displacement of tritium labeled 2′, 3′cGAMP (cyclic (guanosine-(2′→5′)-monophosphate-adenosine-(3′→5′)-monophosphate) to biotinylated STING protein. A soluble version of recombinant STING was expressed in E.coli that lacks the four transmembrane domains and contains residues 139-379 of Q86WV6 with an R at position 232 (H232R). Based on the allele frequency of 58% of the population, H232R is considered to be wild type (Yi, et. al., Single Nucleotide Polymorphisms of Human STING can affect innate immune response to cyclic dinucleotides PLOS ONE. 2013, 8(10), e77846). The STING construct has an N-terminal HIS tag, followed by a TEV protease cleavage site and an AVI tag to allow directed biotinylation by BirA biotin ligase (Beckett et al., A minimal peptide substrate in biotin holoenzyme synthetase-catalyzed biotinylation. (1999) Protein Science 8, 921-929). The HIS tag is cleaved after purification and prior to biotinylation.The assay was run in 1536-well plates in a total volume of 8 μL per well by adding 8 nM [3H]-2′3′-cGAMP and 40 nM biotin-STING protein in assay buffer [25 mM HEPES (Corning 25-060-C1) pH 7.5, 150 mM NaCl (Sigma S5150), 0.5 mg/mL BSA (Gibco 15260-037), 0.001% Tween-20 (Sigma P7949), molecular grade water (Corning 46-000-CM)]. Test compounds (80 nL) were added with an acoustic dispenser (EDC Biosystems) in 100% DMSO for a final assay concentration of 1% DMSO. Plates were centrifuged for 1 min and incubated for 60 min at room temperature. Finally, (2 μL) polystyrene streptavidin SPA beads (PerkinElmer RPNQ0306) were added and plates were sealed and centrifuged for 1 min at room temperature. Plates were dark adapted for 2 h and read on a ViewLux (Perkin Elmer) for 12 min per plate. A saturation binding curve for [3H]-2′3′-cGAMP showed a KD of 3.6±0.3 nM for binding to STING, comparable to reported values for the natural ligand (Zhang et al., Cyclic GMP-AMP containing mixed phosphodiester linkages is an endogenous high-affinity ligand for STING.Other natural ligands including cyclic-di-GMP also returned values in this assay within the expected range. Reference compound is cGAMP and results are reported as percent inhibition and ICso values. Binding to mouse STING used a construct similar to the one described above containing residues 138-378 of Q3TBT3. |
| Affinity data for this assay | |
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