共免疫沉淀与下拉实验

Co-immunoprecipitation uses an antibody to capture a target protein and its binding partners, while pull-down assays use an affinity-tagged bait protein instead of an antibody. Both methods isolate protein complexes from solution, but pull-down assays employ affinity chromatography with immobilized ligands specific to the engineered tag, such as histidine residues or biotin.

Co-immunoprecipitation captures intact protein complexes by allowing an antibody to bind the target protein while it remains bound to its interaction partner. The antibody-protein complex is then immobilized on protein A-sepharose beads. After washing away unbound proteins, the entire complex is released by boiling in reducing SDS-PAGE sample-loading buffer for analysis.

Affinity tags, such as histidine residues or biotin sequences, are engineered onto bait proteins to enable specific capture on immobilized affinity ligands. These tags allow the bait protein to bind to solid supports like streptavidin-sepharose beads, facilitating isolation of the bait and its protein interaction partners from complex cell lysates.

Co-immunoprecipitation and pull-down assays are generally used for analyzing stable or strong protein interactions because the washing and centrifugation steps remove weakly bound or transient proteins. Recent developments like HaloTag, a genetically-encoded protein fusion tag capable of chemically reacting with haloalkane solid supports, have simplified the study of transient protein interactions.

Pull-down assays can test whether proteins bind to signaling molecules like calmodulin in the presence or absence of second messengers such as calcium ions. By comparing binding under different conditions, researchers can determine how calcium ions regulate protein-protein interactions within signaling pathways and mediate biological responses.

The procedure begins by preparing protein A-sepharose beads in PBS buffer, then adding cell lysate and antibody, and rotating the mixture at 4°C for one hour. After centrifugation and washing to remove unbound proteins, the antibody-protein complex is released by boiling in reducing SDS-PAGE sample-loading buffer for analysis by immunoblotting.

Pull-down assays release captured protein complexes by washing with elution buffer containing a competitive analyte specific for the affinity tag, rather than boiling. This gentler elution method preserves protein complex integrity. The supernatant is then incubated at room temperature for 30 minutes and analyzed by immunoblotting to identify bound proteins.