| 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 Rat position 232 (H232R). Based on the allele frequency of 58% of the population, H232R is considered to be a 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 IC50 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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