Immunoprecipitation — Proteinase Antibodies

Immunoprecipitation (IP) with a Triple Point rabbit polyclonal antibody is a routine technique for pulling down a proteinase target from lysate, either to enrich for detection of low-abundance targets or to isolate protein complexes containing the target. Because proteinases are enzymatically active, IP requires some methodological adjustments over a "generic" IP protocol — specifically to preserve enzymatic activity where the downstream assay needs it, and to avoid autocatalytic degradation of the IP'd material.

This protocol describes IP of proteinases from cell or tissue lysate, using either magnetic Protein A/G beads or sepharose beads, with elution conditions matched to the downstream application. For Western blot detection of IP'd material, see the last section.

Bead selection — Protein A, Protein G, or Protein A/G

Rabbit IgG binds Protein A with high affinity and Protein G with moderate affinity. Both will work for a Triple Point rabbit polyclonal antibody. The practical choice comes down to bead format and downstream application:

Lysis buffer — preserving active enzyme vs preserving epitope

The choice of lysis buffer depends on whether you need the IP'd material to be enzymatically active or only immunodetectable:

For active-enzyme IP (activity assay downstream)

Use a mild non-denaturing lysis buffer with a class-appropriate protease inhibitor for other classes but not the target's own class. For a serine proteinase target, add PMSF or AEBSF only if you want to inactivate the target; omit them if you want active enzyme. Typical buffer: 50 mM Tris-HCl pH 7.4 (or 7.5 for neutral proteinases; pH 5.5-6.0 for aspartic and cysteine proteinases that require acidic activation state), 150 mM NaCl, 1% Triton X-100 or 0.5% NP-40, 1 mM EDTA (omit for metalloproteinases if activity is required — use 1 mM CaCl2 + 1 mM ZnCl2 instead), plus class-appropriate inhibitors for other proteinases.

For denaturing IP (Western blot detection only)

Use RIPA buffer or a stronger lysis buffer containing SDS. Denatures the target so it cannot autoactivate or cleave binding partners, but allows immunoprecipitation via linear epitopes. Typical buffer: 50 mM Tris-HCl pH 7.4, 150 mM NaCl, 1% Triton X-100, 0.5% sodium deoxycholate, 0.1% SDS, complete protease inhibitor cocktail. Note that some antibodies lose epitope binding under RIPA conditions — test empirically.

Class-specific inhibitor guidance

See the class-specific Western blot protocols for the appropriate inhibitor cocktail per proteinase class: serine proteinases, cysteine proteinases, aspartic proteinases, metalloproteinases.

The IP procedure — standard workflow

1. Prepare lysate. 500 μL of lysate at 1-2 mg/mL total protein is typical for a single IP. Clear by centrifugation at 14,000 × g for 10 minutes at 4°C. Save 25 μL as "input" for the Western blot.
2. Pre-clear (optional but recommended). Add 20 μL of the same bead type (no antibody) to the cleared lysate. Rotate 30 minutes at 4°C. Remove beads (magnetic separation or centrifugation). This step removes proteins that bind non-specifically to the bead matrix.
3. Antibody binding. Add 2-5 μg of Triple Point rabbit polyclonal antibody to the pre-cleared lysate. Rotate overnight at 4°C, or 2 hours at 4°C for high-abundance targets.
4. Bead capture. Add 20-30 μL of Protein A magnetic beads (equilibrated in lysis buffer). Rotate 1 hour at 4°C.
5. Wash. Separate beads (magnetic stand). Wash 3-5 times with 1 mL lysis buffer, 5 minutes rotation per wash. For stringent washing (to reduce background), wash the final 1-2 times with buffer containing 300 mM NaCl.
6. Elute. See elution section below — the correct elution depends on the downstream application.
7. Analyse. Western blot input (2-5% of starting material), unbound (2-5% of the post-IP supernatant), and IP eluate.

Elution — avoiding re-activation of latent enzymes

Standard IP elution uses either low-pH glycine (100 mM glycine, pH 2.5-3.0) or high-temperature SDS sample buffer. Both work for detection but have different implications for a proteinase target.

Blot detection of IP'd material — the antibody interference problem

When you elute in SDS sample buffer and run the IP eluate on a Western blot, the blot will show two bright bands from the co-eluted antibody: heavy chain at ~50 kDa and light chain at ~25 kDa. If your target of interest runs at either of these MWs (a common problem for MMPs at 50 kDa, cathepsins at 25-30 kDa, granzymes at 30 kDa), the antibody band will obscure the target band.

Three approaches to work around this:

1. Use a different-species primary for detection

IP with rabbit polyclonal, detect on the Western blot with a mouse monoclonal (if available) against the same target. The anti-rabbit secondary used for detection will not bind the IP'd rabbit IgG bands. Requires a validated second antibody against the same target.

2. Use IgG-light-chain-specific secondary

Some suppliers sell secondary antibodies that only bind the intact IgG or only the light chain, avoiding the heavy chain band. Reduces but does not eliminate the interference.

3. Crosslink the antibody to the bead before IP

Crosslink your Triple Point polyclonal to the Protein A beads with disuccinimidyl suberate (DSS) or dimethyl pimelimidate (DMP) before adding lysate. The antibody stays on the beads through the IP and elution, so only the target elutes. Requires a one-time crosslinking step and reduces the effective antibody available for capture, but gives the cleanest eluate.

IP-Western blot — the standard workflow

The most common use of proteinase IP is enrichment for Western blot detection when the target is at low endogenous abundance. In this workflow:

1. Lyse 5-10× the volume you would normally load for a Western blot — typically 500-1000 μL instead of 50 μL.
2. IP with 2-5 μg of Triple Point polyclonal.
3. Elute in 30 μL SDS sample buffer.
4. Load 10-15 μL of the eluate on the gel (representing 33-50% of the IP).
5. Detect with the same or a different antibody against the target.
6. Compare band intensity in the IP lane against input (5% loaded on the same gel) to quantify enrichment factor. A well-optimised IP typically gives 10-50× enrichment over the input.

Controls for IP experiments

Every IP needs at least three lanes on the detection blot:

• Input: 2-5% of the starting lysate. Confirms the target was present before IP.
• Unbound (flow-through): 2-5% of the post-IP supernatant. Should show partial depletion of the target if the IP is working — complete depletion suggests low-abundance target or high-efficiency IP; partial depletion is typical.
• IP eluate: The enriched material.

Additionally, run an IgG control IP in parallel: same lysate, same beads, but with an irrelevant rabbit IgG (e.g., pre-immune serum, or a rabbit polyclonal against an unrelated target not expressed in the sample) at the same concentration. The IgG control IP eluate should not contain the target at detectable levels. If it does, your IP has significant non-specific binding — typically caused by insufficient washing, sticky sample, or antibody cross-reactivity.

Troubleshooting

No target in IP eluate

Confirm the antibody binds the target in native (non-denatured) form — some antibodies raised against denatured peptide immunogen bind linear epitopes only and fail on native protein. Try RIPA lysis or add 0.1% SDS to the lysate before IP (denatures the target but keeps epitope accessible).

High non-specific binding to beads

Pre-clear the lysate longer, use a stricter wash (higher salt, or add 0.05% Tween-20 to the wash buffer), or reduce the antibody amount — 5 μg per IP is usually more than needed and drives non-specific binding at high concentrations.

Target degraded in eluate

The target is autoactivating and cleaving itself during the IP. Add class-specific inhibitors to the lysis buffer, keep everything at 4°C throughout, and elute in SDS sample buffer immediately after the final wash rather than storing the beads.

Antibody heavy chain obscures target

See the antibody interference section above — use a different-species detection antibody, crosslink the IP antibody to beads, or run the gel to resolve the target from the 50 kDa heavy chain (a 12% or 15% gel gives better resolution in the 25-50 kDa range than a 10% gel).

Related

• Western blot protocol for aspartic proteinases
• Western blot protocol for cysteine proteinases
• Western blot protocol for metalloproteinases
• Western blot protocol for serine proteinases
• Controls and validation
• Dilution optimization
• Quality and validation approach