Label-free quantification of membrane-ligand interactions using backscattering interferometry

Methods to measure affinities of membrane proteins and soluble ligands are cumbersome and often rely on truncations or other modifications of the membrane protein or ligand. Baksh et al . show that backscattering interferometry is a sensitive and accurate technology for the label-free quantification of ligand–membrane receptor interactions. Although membrane proteins are ubiquitous within all living organisms and represent the majority of drug targets, a general method for direct, label-free measurement of ligand binding to native membranes has not been reported. Here we show that backscattering interferometry (BSI) can accurately quantify ligand-receptor binding affinities in a variety of membrane environments. By detecting minute changes in the refractive index of a solution, BSI allows binding interactions of proteins with their ligands to be measured at picomolar concentrations. Equilibrium binding constants in the micromolar to picomolar range were obtained for small- and large-molecule interactions in both synthetic and cell-derived membranes without the use of labels or supporting substrates. The simple and low-cost hardware, high sensitivity and label-free nature of BSI should make it readily applicable to the study of many membrane-associated proteins of biochemical and pharmacological interest.