Fluorescent Thermal Shift Assays for Identifying Small Molecule Ligands
Fluorescent thermal shift assay (FTSA; also known as ThermoFluor, differential scanning fluorimetry (DSF), thermal shift assay (TSA) and temperature-dependent fluorescence (TdF)) is a high-throughput, simple and inexpensive screening method to identify low-molecular weight ligands that bind to and stabilise purified protein. The temperature at which a protein unfolds is measured by an increase in the fluorescence of a specific dye that has affinity for the hydrophobic parts of the protein, which are exposed as the protein unfolds. A simple data fitting procedure allows a quick calculation of the difference in the temperature of the transition midpoint in the presence and absence of ligand, which is related to the binding affinity of the small molecule. Assays can be performed using state-of-the-art, real-time, quantitative PCR (qPCR) instruments, meaning hundreds of results can be obtained in a few hours. FTSA is now regularly being used as both a primary and secondary screening method in the drug discovery process. This chapter contains a detailed discussion on the optimal experimental set-up, data analysis, strengths and limitations of the assay and its use in drug discovery. Examples of successful applications of FTSA are highlighted as well as a summary of the new ‘in-cell’ TSA type methods, Cellular thermal shift assay (CETSA), Fast parallel proteolysis (FastPP) and thermal proteome profiling (TPP).