Co-Immunoprecipitation (Co-IP) is the most widely used method for detecting and identifying protein–protein interactions under native conditions. It uses a specific antibody to capture a bait protein and any proteins bound to it from a cell lysate.

Principle

An antibody against the target (bait) protein is immobilized on a solid support — typically Protein A/G agarose beads or magnetic beads. When the antibody-conjugated beads are incubated with a cell or tissue lysate, the bait protein is captured along with any proteins that are physically associated with it. After washing away unbound material, the bound proteins are eluted and analyzed by Western blot or mass spectrometry.

Protocol Overview

1. Prepare the lysate using a mild, non-denaturing lysis buffer that preserves protein interactions (e.g., 50 mM Tris pH 7.5, 150 mM NaCl, 1% NP-40, protease inhibitors). Keep samples on ice.
2. Pre-clear the lysate by incubating with blank beads (no antibody) to remove proteins that bind non-specifically to the bead surface.
3. Incubate the pre-cleared lysate with the antibody-conjugated beads at 4 °C for 1–4 hours (or overnight) with gentle rotation.
4. Wash beads 3–5 times with cold lysis buffer. Stringency can be increased by raising the salt concentration (250–500 mM NaCl) or adding a mild detergent (0.1% Tween-20).
5. Elute bound proteins with SDS-PAGE sample buffer and heat (5 minutes at 95 °C).
6. Analyze by Western blot (for known interactors) or mass spectrometry (for discovery).

Controls

Rigorous controls are essential:

• Isotype control: use a non-specific antibody of the same isotype to distinguish specific binding from background.
• Bead-only control: beads without antibody to identify proteins binding to the bead matrix.
• Knockdown/knockout control: lysate from cells lacking the bait protein demonstrates antibody specificity.
• Reverse Co-IP: perform the reciprocal IP using an antibody against the suspected interactor.

Crosslinking

Antibodies themselves co-elute with the target and can interfere with mass spectrometry. Crosslinking the antibody to the beads (using DSS or BS³) before incubation prevents antibody elution. A control with crosslinked beads must confirm that the antibody is still functional after crosslinking.

Variants

• Native Co-IP: uses non-denaturing lysis buffer. Preserves native complexes but may co-precipitate indirectly associated proteins.
• Crosslinking Co-IP: a reversible crosslinker (e.g., formaldehyde, DSP) stabilizes weak or transient interactions before lysis.
• Nuclear Co-IP: for transcription factor complexes, use a nuclear extraction protocol and include DNase I to release chromatin-bound proteins.