The pull-down assay is an in vitro method used to determine a physical interaction between two or more proteins. Pull-down assays are useful for both confirming the existence of a protein–protein interaction predicted by other research techniques (e.g., co-immunoprecipitation) and as an initial screening assay for identifying previously unknown protein–protein interactions.
The basic principle of pull down assay is to utilize a tag fused protein (such as GST-tag, His-tag and biotin-tag) immobilized to affinity resin as the bait protein. Proteins binding to the bait protein (prey protein) can be captured and “pulled down” when the target protein or cell lysate flows through.