Abstract

Macromolecular interactions play a central role in many biological processes. Protein‐protein interactions have mostly been studied by co‐immunoprecipitation, which cannot provide quantitative information on all possible molecular connections present in the complex. We will review a new approach that allows cellular proteins and biomolecular complexes to be studied in real‐time at the single‐molecule level. This technique is called single‐molecule pull‐down (SiMPull), because it integrates principles of conventional immunoprecipitation with the powerful single‐molecule fluorescence microscopy. SiMPull is used to count how many of each protein is present in the physiological complexes found in cytosol and membranes. Concurrently, it serves as a single‐molecule biochemical tool to perform functional studies on the pulled‐down proteins. In this review, we will focus on the detailed methodology of SiMPull, its salient features and a wide range of biological applications in comparison with other biosensing tools.