Immunoprecipitation (IP) Protocol
An immunoprecipitation is a technique used to separate an antigen or protein from a mixture of other proteins by binding to an antibody in solution or an immobilized antibody. A complex sample mixture containing the antigen of interest is flowed over a solid support containing the immobilized antibody. The antigen binds the antibody forming a complex while all other non-specific proteins are washed away. Alternatively, antibody and antigen are mixed together and then flowed over a solid support designed to bind the antibody (Protein A, for example) with the antigen in tow. The antigen-antibody complex is disrupted and the antigen is collected resulting in a more homogenous/concentrated protein mixture. See our antibody validation page for immunoprecipitation data.
- Wash adherent cells twice in the dish or flask with ice-cold PBS and drain off PBS. Wash non-adherent cells in PBS and centrifuge at 800 to 1000 rpm in a table-top centrifuge for 5 minutes to pellet the cells.
- Add ice-cold modified RIPA buffer to cells (1mL per 107 cells/100 mm dish/150 cm2 flask; 0.5mL per 5 x 106 cells/60 mm dish/ 75 cm2 flask).
- Scrape adherent cells off the dish or flask with either a rubber policeman or a plastic cell scraper that has been cooled in ice-cold distilled water. Transfer the cell suspension into a centrifuge tube. Gently rock the suspension on either a rocker or an orbital shaker at 4°C for 15 minutes to lyse cells.
- Centrifuge the lysate at 14,000xg in a precooled centrifuge for 15 minutes. Immediately transfer the supernatant to a fresh centrifuge tube and discard the pellet.
- To prepare Protein A or G agarose/sepharose, wash the beads twice with PBS and restore to a 50% slurry with PBS. It is recommended to cut the tip off of the pipette when manipulating agarose beads to avoid disruption of the beads.
- Pre-clear the cell lysate by adding 100μl of either Protein A or G agarose/sepharose bead slurry (50%) per 1mL of cell lysate and incubating at 4°C for 10 minutes on a rocker or orbital shaker. Pre-clearing the lysate will reduce non-specific binding of proteins to the agarose or sepharose when it is used later on in the assay.
- Remove the Protein A or G beads by centrifugation at 14,000x g at 4°C for 10 minutes. Transfer the supernatant to a fresh centrifuge tube.
- Determine the protein concentration of the cell lysate (e.g. be sure to use a cell lysis buffer that is compatible with your protein concentration assay).
- Dilute the cell lysate to approximately 1 μg/μl total cell protein with PBS to reduce the concentration of the detergents in the buffer. A more concentrated cell lysate (i.e., 10 μg/μl) may be necessary to immunoprecipitate a protein, which is found in low levels in a cell model.
- Add the recommended volume of the immunoprecipitating antibody (see antibody datasheet for detailed information) to 500μl (i.e., 500 μg) of cell lysate. The optimal amount of antibody that will quantitatively immunoprecipitate the protein of interest should be empirically determined for each cell model.
- Gently rock the cell lysate/antibody mixture for either 2 hours or overnight at 4°C on a rocker or an orbital shaker.
- Capture the immunocomplex by adding 100μl Protein A or G agarose/sepharose bead slurry (50 μl packed beads) and gently rocking on either a rocker or orbital shaker for either 1 hour or overnight at 4°C. In many instances, immunocomplex capture can be enhanced by adding 2 μg of a bridging antibody (e.g., rabbit- anti-mouse IgG). This is especially important with antibodies, which bind poorly to Protein A, such as mouse IgG1 or antibodies generated in chicken.
- Collect the agarose/sepharose beads by pulse centrifugation (i.e., 5 seconds in the microcentrifuge at 14,000 rpm). Discard the supernatant and wash the beads 3 times with 800μl ice-cold modified RIPA buffer. Occasionally, washing with modified RIPA buffer will strip the immunocomplex off of the agarose/sepharose beads. In these cases, washing with the milder PBS is recommended.
- Resuspend the agarose/sepharose beads in 60μl 2x sample buffer and mix gently. This will allow for sufficient volume to run three lanes.
The agarose/sepharose beads are boiled for 5 minutes to dissociate the immunocomplexes from the beads. The beads are collected by centrifugation and SDS-PAGE is performed with the supernatant. Alternatively, the supernatant can be transferred to a fresh microcentrifuge tube and stored frozen at –20°C for later use. Frozen supernatant should be reboiled for 5 minutes directly prior to loading on a gel.
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