Probing drug-target engagement of soluble Gαi1 protein using the SolThermoBRET thermal shift assay

IntroductionSensitive protein stability assays used during hit confirmation conventionally require high quantities of purified protein. Here, we describe a novel high-throughput 384-well BRET-based thermostability assay allowing for the ultrasensitive determination of Gi1 protein stability. Using this method we make use of the environmentally sensitive dye, SYPRO Red, which interacts with hydrophobic regions in the protein that become exposed upon denaturation. Assays are functional in crude cell lysates, without any requirement for protein purification. MethodsThe SolThermoBRET method measures resonance energy transfer between a thermally stabilised nano-luciferase (tsNLuc) genetically engineered to the N-terminus of Gi1 protein, and SYPRO Red, a fluorescent dye that binds to lipophilic residues exposed upon protein unfolding in response to thermal denaturation. Samples in 384-well PCR plate were subjected to a temperature gradient (30 - 60 {degrees}C) on a PCR block and incubated for 5 minutes. Isothermal SolThermoBRET at a fixed temperature 45{degrees}C allowed the measurement of IC50 curves to various ligands. Furimazine (10 M) was added to the samples and BRET read on the BMG Labtech PHERAstar FSX at room temperature. Melt curves were fit to a Boltzmann sigmoidal equation to obtain Tm values. ResultsLigand-induced stabilisation of Gi1 protein was demonstrated in non-purified samples. A range of Tm values were measured with increasing stabilisation observed for nucleotides/sides in the following order GDP<GTP<GppNHp<GTP{gamma}S. Correlation was observed between SolThermoBRET derived Tms determined in the presence of high ligand concentrations and IC50 determinations using Isothermal SolThermoBRET. ConclusionsSolThermoBRET represents a sensitive nanoscale method for the detection of changes in soluble protein thermostability. This method is attractive for hit conformation and the identification of novel ligands targeting soluble proteins.