Real-time analysis of molecular assembly by kinetic flow cytometry

Flow cytometry is uniquely capable of making sensitive and quantitative multiparameter fluorescence measurements with discrimination of free from particle-bound fluorophore. Recent advances in mixing and sample delivery have extended these capabilities into the sub-second time domain. Access to these time scales has enabled us to use flow cytometry to measure molecular interactions. Using the general approach of immobilizing one molecule on a microsphere and fluorescently labeling another, we have been able to make real-time measurements of ligand-receptor and enzyme-substrate interactions involving proteins, nucleic acids, carbohydrates, and lipids. We are developing schemes for immobilizing active biological molecules in defined and homogeneous orientations relative to the surface. We are also developing approaches for homogeneous fluorescent labeling of active biomolecules and calibration schemes for quantitative measurements by flow cytometry. We will present several examples of applications of this new technology, including DNA-and protein-protein interactions, nucleic acid hybridization, and interactions on artificial membrane surfaces. These approaches should have wide applications for mechanistic analysis, diagnostics, and drug development.