Co-Immunoprecipitation Protocol: Detecting Protein-Protein Interactions

LabProtocol.co Team·2026-03-22·8 min read
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Co-Immunoprecipitation Protocol: Detecting Protein-Protein Interactions

Co-immunoprecipitation (Co-IP) is the definitive biochemical method for confirming that two proteins physically interact in their native state. Unlike yeast two-hybrid or proximity ligation assays, Co-IP captures endogenous protein complexes under near-physiological conditions — making it the gold standard for validating protein-protein interactions.

This protocol walks you through the complete Co-IP workflow: cell lysis under non-denaturing conditions, antibody-bead coupling, immunoprecipitation, stringent washing, and detection by western blot.

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Materials and Reagents

Lysis and Wash Buffers

  • Non-denaturing lysis buffer: 50 mM Tris-HCl pH 7.4, 150 mM NaCl, 1 mM EDTA, 1% NP-40 (or 0.5% Triton X-100). Do NOT use SDS or deoxycholate (they disrupt protein complexes).
  • Protease inhibitor cocktail (Roche cOmplete Mini, EDTA-free)
  • Phosphatase inhibitors if studying phosphorylation-dependent interactions (1 mM Na3VO4, 10 mM NaF)
  • Wash buffer: Same as lysis buffer, or lysis buffer with reduced detergent (0.1-0.5% NP-40) for stringent washing
  • High-salt wash (optional): Lysis buffer with 300-500 mM NaCl (reduces non-specific binding)

Immunoprecipitation Components

  • Primary antibody against the bait protein (2-5 ug per IP)
  • Protein A/G magnetic beads (e.g., Thermo Fisher Dynabeads Protein A/G, or Pierce Protein A/G Plus Agarose)
  • Normal IgG from the same species as the primary antibody (negative control)
  • Magnetic rack (for magnetic beads) or microcentrifuge (for agarose beads)

Detection

  • 4x Laemmli sample buffer + beta-mercaptoethanol
  • SDS-PAGE gels, transfer membranes, antibodies, ECL (see western blot protocol)
  • Detection antibody against the prey protein (for western blot)
  • Optional: conformation-specific secondary antibody (e.g., Rockland TrueBlot) to avoid detecting denatured IgG heavy/light chain bands

Step-by-Step Co-IP Protocol

Step 1: Prepare Cell Lysate

  1. Start with a large amount of cells — Co-IP requires more protein than a standard western blot. Use 1-2 x 10^7 cells (approximately one confluent 10 cm dish) or 5-10 mg total protein.
  2. Wash cells twice with ice-cold PBS.
  3. Add 1 mL ice-cold non-denaturing lysis buffer (with protease inhibitors).
  4. Scrape cells and transfer to a 1.5 mL tube.
  5. Incubate on ice for 30 minutes, vortexing gently every 10 minutes.
  6. Centrifuge at 14,000 x g for 15 minutes at 4 degrees C.
  7. Transfer the clarified supernatant to a fresh tube. Save 50 uL as "input" (5-10% of total lysate) for western blot.

Step 2: Pre-Clear the Lysate (Optional but Recommended)

Pre-clearing removes proteins that bind non-specifically to the beads.

  1. Add 25 uL Protein A/G beads to the lysate.
  2. Rotate at 4 degrees C for 1 hour.
  3. Remove beads using a magnetic rack or brief centrifugation.
  4. Transfer pre-cleared lysate to a fresh tube.

Step 3: Add Antibody and Beads

Option A — Add antibody first, then beads:

  1. Add 2-5 ug primary antibody (against the bait protein) to the pre-cleared lysate.
  2. Rotate at 4 degrees C overnight (or 2-4 hours minimum).
  3. Add 25-50 uL Protein A/G magnetic beads.
  4. Rotate at 4 degrees C for 1-2 hours.

Option B — Pre-couple antibody to beads:

  1. Incubate 2-5 ug antibody with 25-50 uL beads in 200 uL PBS-T for 1-2 hours at room temperature.
  2. Wash beads 2x with lysis buffer.
  3. Add antibody-coupled beads to the lysate.
  4. Rotate at 4 degrees C for 2-4 hours or overnight.

Controls (run in parallel):

  • IgG control: Use the same amount of normal IgG (same species) instead of the specific antibody. This shows non-specific binding.
  • Beads-only control: Beads without any antibody. This shows bead background.

Step 4: Wash the Beads

This is the most critical step for reducing background.

  1. Place the tube on a magnetic rack (or centrifuge briefly for agarose beads). Remove the supernatant.
  2. Add 1 mL wash buffer.
  3. Rotate at 4 degrees C for 5 minutes.
  4. Remove wash buffer.
  5. Repeat for a total of 4-5 washes. For high background, include one wash with high-salt buffer (300 mM NaCl) or increase detergent to 0.5% NP-40.

Step 5: Elute and Prepare Samples

  1. After the final wash, remove all residual buffer carefully.
  2. Add 30-50 uL 2x Laemmli sample buffer (with beta-mercaptoethanol).
  3. Heat at 95 degrees C for 5 minutes to denature proteins and release them from the beads.
  4. Centrifuge briefly and collect the eluate (use the magnetic rack to separate beads).
  5. Load on SDS-PAGE gel alongside the input sample and IgG control.

Step 6: Western Blot Detection

  1. Run SDS-PAGE and transfer to PVDF or nitrocellulose membrane.
  2. Probe with antibody against the prey protein (the interaction partner you want to detect).
  3. If the prey co-precipitates with the bait, you should see a band in the IP lane but NOT in the IgG control lane.
  4. Strip and reprobe with antibody against the bait protein to confirm successful immunoprecipitation.

Tip: Use conformation-specific secondary antibodies (e.g., TrueBlot anti-rabbit IgG HRP) to avoid detecting denatured IgG heavy chain (~50 kDa) and light chain (~25 kDa) bands, which can obscure prey proteins of similar size.

Troubleshooting

No Co-IP Signal (Bait Pulled Down, but No Prey)

  • Interaction is disrupted by lysis conditions: Try gentler detergent (0.5% NP-40 or 0.1% digitonin instead of 1% NP-40). Some complexes are detergent-sensitive.
  • Interaction is transient or low-affinity: Cross-link with DSP (dithiobis[succinimidyl propionate]) at 1 mM for 30 minutes before lysis. DSP is cleavable with DTT/beta-mercaptoethanol during elution.
  • Wrong antibody: The antibody may bind an epitope that overlaps with the interaction interface. Try a different antibody targeting a different region of the bait.
  • Insufficient input: Co-IP requires more protein than western blot. Use at least 500 ug to 1 mg total protein per IP.

High Background in IgG Control

  • Non-specific binding to beads: Pre-clear more aggressively (longer time, more beads).
  • Insufficient washing: Increase to 5-6 washes, add high-salt wash.
  • Too much antibody: Reduce from 5 ug to 2 ug per IP.
  • Sticky protein: Some abundant proteins (cytoskeletal proteins, heat shock proteins) bind non-specifically to everything. These are common Co-IP contaminants.

IgG Heavy/Light Chain Bands Obscure the Prey

  • Use conformation-specific secondary antibodies (TrueBlot or VeriBlot).
  • Alternatively, cross-link the antibody to the beads using BS3 (bis[sulfosuccinimidyl] suberate) at 5 mM for 30 minutes, then wash. This prevents the antibody from eluting with the sample.

Common Mistakes to Avoid

  1. Using denaturing lysis buffers — SDS and sodium deoxycholate destroy protein complexes. Use NP-40 or Triton X-100.
  2. Insufficient starting material — Co-IP needs 500 ug to 2 mg total protein per IP. A single well of a 6-well plate is not enough.
  3. Skipping the IgG control — without it, you cannot distinguish specific from non-specific interactions.
  4. Over-washing or under-washing — too few washes gives high background; too many washes or too-stringent conditions can disrupt the interaction. Optimize empirically.
  5. Ignoring the heavy chain band — if your prey protein is near 50 kDa or 25 kDa, it may co-migrate with IgG. Use TrueBlot secondaries.

Pro Tips

  • Reciprocal Co-IP — pull down with antibody against protein A and blot for protein B, then repeat by pulling down protein B and blotting for protein A. Reciprocal confirmation greatly strengthens the evidence.
  • Crosslinking for weak interactions — DSP (1 mM, 30 min, room temperature) stabilizes transient complexes. Cleavable with reducing agents during sample preparation.
  • Native PAGE — for very sensitive complexes, skip SDS-PAGE and use native or Blue Native PAGE to preserve the complex during electrophoresis.
  • Mass spectrometry — if you want to discover unknown interaction partners, send your IP eluate for LC-MS/MS analysis instead of (or in addition to) western blot.

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