Pull Down Assay
A pull down assay is an in vitro method used to capture and identify protein protein interactions by using a tagged bait protein immobilized on a solid support to purify binding partners from a complex lysate. This guide is for molecular biologists, biochemists, and cell biologists who need a practical, source bounded framework to design, execute, and interpret pull down experiments with rigor. NCBI Bookshelf offers foundational background on affinity purification methods. EMBL EBI Training provides complementary resources on experimental design, making it a useful starting point for beginners and experienced researchers alike.
At a Glance
| Aspect | Key Information |
|---|---|
| Purpose | Identify protein protein interactions or validate a suspected interaction |
| Principle | An immobilized bait protein captures prey proteins from a sample |
| Bait types | Recombinant protein, tagged protein, or purified native protein |
| Common tags | GST, His, biotin, FLAG, HA, MBP |
| Detection methods | Western blot, mass spectrometry, silver stain, Coomassie blue |
| Key controls | Beads only, bait only (no lysate), lysate only (no bait), non specific competitor |
| Typical duration | 2 to 6 hours for binding, plus analysis time |
Core Concepts and Decision Points
A pull down assay depends on the specific affinity between a tagged bait and its cognate binding resin. The bait is expressed and purified, then immobilized on beads. A lysate containing potential prey proteins is incubated with the bait beads. After washing, bound proteins are eluted and analyzed. Galaxy Training Network includes workflows for analyzing mass spectrometry data from pull down experiments, highlighting the importance of computational steps in the pipeline.
Decision Point 1: Bait Selection and Tag
Choose a tag that does not disrupt the bait's native conformation or binding site. GST tags are large and may require cleavage. His tags are small but can produce background from histidine rich proteins. Biotin tags offer high affinity but require careful control for endogenous biotinylated proteins. For plant interactome mapping, researchers often use tandem affinity purification tags to reduce false positives, as discussed in Mapping the dynamic plant interactome: from in vitro assays to in vivo quantitative approaches. Consider the compatibility of the tag with your downstream detection method.
Decision Point 2: Lysate Preparation
The lysate must preserve the native state of prey proteins. Use a gentle lysis buffer with protease and phosphatase inhibitors. The salt concentration and detergent type (e.g., NP 40, Triton X 100) affect non specific binding. Test multiple buffer conditions in pilot experiments. Bioconductor offers packages for analyzing pull down mass spectrometry data, but raw data quality depends on clean lysates.
Decision Point 3: Controls for Specificity
Include a negative control with beads only (no bait) to measure non specific bead binding. Use a non specific bait (e.g., GST alone) to control for tag mediated interactions. A lysate only control (no bait) assesses background noise. For quantitative comparisons, use a bait with a known mutation that disrupts binding as a specificity control. In a study of duck plague virus, researchers used pull down to show UL31 interaction with ICP4, ICP22, and VP16, and they validated specificity with reciprocal co immunoprecipitation The UL31 protein of duck plague virus promotes viral replication and interacts with ICP4, ICP22, and VP16 proteins.
Pull Down Workflow and Implementation Steps
Follow these steps for a typical pull down assay. Adapt volumes based on your bait concentration and bead capacity.
Step 1: Bait Immobilization
Incubate your purified tagged bait with the appropriate affinity resin (e.g., glutathione agarose for GST, Ni NTA for His). Use a ratio that saturates the beads without excess bait. Typically 50 to 100 micrograms of bait per 20 microliters of bead slurry. Incubate at 4 degrees Celsius for 30 to 60 minutes with gentle rotation.
Step 2: Bead Washing
Wash the beads three to five times with binding buffer to remove unbound bait. Aspirate carefully to avoid losing beads. Keep an aliquot of the wash to check bait retention by SDS PAGE.
Step 3: Lysate Incubation
Pre clear the lysate by incubating with blank beads for 30 minutes at 4 degrees Celsius. Then add the pre cleared lysate to the bait beads. Use a protein amount that is in excess relative to the bait (e.g., 500 micrograms to 2 milligrams total protein). Incubate for 2 to 4 hours at 4 degrees Celsius with rotation. Longer incubation can increase capture but also background.
Step 4: Washing
Wash the beads five to seven times with wash buffer (similar to binding buffer but may include higher salt or detergent). Collect the final wash for analysis.
Step 5: Elution
Elute bound proteins using a method compatible with your tag. For GST, use reduced glutathione. For His, use imidazole. Alternatively, boil beads in SDS sample buffer. For mass spectrometry, elute with a volatile buffer or directly digest on beads.
Step 6: Analysis
Separate eluates by SDS PAGE and visualize by silver stain or Coomassie blue. For specific prey detection, perform Western blot. For discovery, submit to mass spectrometry. NCBI Sequence Read Archive stores proteomics data from large scale studies, though pull down data are more commonly deposited in PRIDE or MassIVE.
Quality Checks and Validation
Validate every pull down experiment with at least these checks:
- Bait presence: Confirm that your bait is properly expressed and immobilized. Run a sample of the beads before and after incubation on a gel.
- Binding specificity: Compare the band pattern of the elution from bait beads versus beads only. Distinct bands specific to the bait indicate potential interactors.
- Reproducibility: Perform at least two biological replicates. Technical replicates (same lysate, separate reactions) confirm consistency.
- Reciprocal pull down: If possible, tag the prey protein and repeat the pull down to confirm the interaction in the opposite orientation. This was done in a study of LRG1 and its effect on neutrophil mitochondrial homeostasis in bladder cancer LRG1 Drives Pathological Angiogenesis by Disrupting Neutrophil Mitochondrial Homeostasis in Bladder Cancer.
- Quantitative controls: Use a known interactor as a positive control. If no positive control exists, include a spike in of a purified known interactor to validate capture efficiency.
Common Mistakes and Troubleshooting
| Mistake | Consequence | Solution |
|---|---|---|
| Insufficient washing | High background, many non specific bands | Increase wash number or stringency (salt, detergent) |
| Overloading bait | Aggregation, reduced binding capacity | Use saturating but not excess bait |
| Poor lysate quality | Low prey recovery, proteolysis | Add fresh inhibitors, work quickly at 4 degrees Celsius |
| Wrong tag choice | Bait inactive or insoluble | Test alternative tags or cleave tag post purification |
| No negative control | Impossible to assess specificity | Always include beads only and tag only controls |
| Elution interfering with downstream analysis | Imidazole or glutathione may inhibit MS | Dialyze or use on bead digestion |
In a study on DHX15 and its interaction with HNRNPL and RBM33 in acute myeloid leukaemia, the authors used pull down to demonstrate complex formation and validated the findings with RNA immunoprecipitation DHX15 affects AML1-ETO9a splicing together with HNRNPL, RBM33 in AML1-ETO-positive acute myeloid leukaemia. This underscores that pull down is often one tool in a validation pipeline.
Limits of Interpretation
Pull down assays are performed outside the cellular environment. An interaction detected in a pull down may not occur in living cells due to compartmentalization, post translational modifications, or transient binding. Conversely, a weak in vitro interaction might be physiologically relevant if it occurs at high local concentrations. The assay does not distinguish between direct and indirect interactions. A prey protein may be captured because it binds to another prey that directly binds the bait. To test direct binding, use purified prey protein or crosslinking.
The presence of many bands can indicate abundant contaminants or aggregation prone proteins. Always validate key interactions using orthogonal methods such as co immunoprecipitation, bimolecular fluorescence complementation, or surface plasmon resonance. For plant systems, in vivo approaches like split luciferase complementation can confirm interactions, as reviewed in the plant interactome mapping paper Mapping the dynamic plant interactome: from in vitro assays to in vivo quantitative approaches. Also note that some proteins are intrinsically sticky and appear as false positives across many pull downs. Resources like the CRAPome database help filter such contaminants.
In genetic studies, pull down can be used to test effects of mutations. For example, a 24 bp insertion in the BCO2 gene region in chickens was associated with yellow skin, and pull down could potentially identify altered binding partners of BCO2 A regulatory 24-bp insertion at the BCO2 gene region is associated with yellow skin in chickens. However, the assay's limits require careful interpretation of such findings.
Frequently Asked Questions
1. What is the difference between a pull down assay and co immunoprecipitation? In co immunoprecipitation, an antibody captures the endogenous bait from a lysate. In pull down, the bait is typically a recombinant tagged protein immobilized on a resin. Pull down allows you to control bait concentration and tag, but co IP uses native bait and may better represent cellular conditions. NCBI Bookshelf compares these techniques.
2. How do I reduce non specific binding in my pull down? Increase washes with higher salt (500 mM NaCl) or mild detergent (0.5% NP 40). Pre clear the lysate with blank beads. Add BSA or non fat milk to the binding buffer as a blocking agent. Also, reduce incubation time and temperature.
3. Can pull down detect weak or transient interactions? Yes, with crosslinking. Use a reversible crosslinker (e.g., DSP) before lysis or during incubation to stabilize weak interactions. After elution, reverse the crosslink. Without crosslinking, transient interactions are often lost during washing. EMBL EBI Training covers crosslinking strategies.
4. What is the minimum amount of prey protein needed for detection? For Western blot detection, you need low nanogram amounts. For mass spectrometry, the sensitivity depends on the instrument. Typically, 1 to 10 micrograms of prey protein per sample is sufficient for identification. You can increase input lysate or concentrate the eluate to improve detection.
References and Further Reading
- NCBI Bookshelf for fundamentals of protein interaction methods.
- EMBL EBI Training for protocols and experimental design.
- Galaxy Training Network for computational analysis of pull down mass spectrometry data.
- Bioconductor for R packages to analyze and visualize interaction data.
- NCBI Sequence Read Archive for storing and retrieving sequencing and proteomics data.
- LRG1 Drives Pathological Angiogenesis by Disrupting Neutrophil Mitochondrial Homeostasis in Bladder Cancer example of pull down validation in disease context.
- Branch architecture and flowering time are separately regulated by GhFL and GhSP2 in cotton pull down used to study plant development interactions.
- DHX15 affects AML1-ETO9a splicing together with HNRNPL, RBM33 in AML1-ETO-positive acute myeloid leukaemia illustrates pull down with splicing factors.
- Mapping the dynamic plant interactome: from in vitro assays to in vivo quantitative approaches comprehensive review of pull down techniques in plants.
- A regulatory 24-bp insertion at the BCO2 gene region is associated with yellow skin in chickens genetic variant studied by pull down.