Protein Pull-Down Assay Protocol: Bait-Prey Interaction, Controls and Analysis

Pull-down assays are affinity-purification experiments used to test or discover protein–protein interactions. A bait protein or binding domain is immobilised on a compatible resin, incubated with a source of candidate prey proteins, washed to remove non-specific binders, and eluted for analysis by SDS-PAGE, western blotting or mass spectrometry.

Pull-down assay vs co-immunoprecipitation

When to use a pull-down assay

• Confirm a suspected bait–prey interaction using purified components
• Screen a lysate for proteins that bind a defined bait
• Map an interaction domain using bait truncations or mutations
• Compare binding under different buffer, ligand or modification conditions
• Enrich interactors for western blotting or mass spectrometry

Choose the affinity system

• GST: GST-tagged bait captured on glutathione resin.
• His tag: polyhistidine-tagged bait captured on nickel or cobalt chelate resin.
• Biotin: biotinylated bait captured on streptavidin/avidin resin.
• Other tags: HA, Myc, FLAG and related systems can support affinity capture where validated.

Thermo Fisher describes pull-down as a small-scale affinity-purification method in which a tagged bait replaces the antibody-based capture used in co-IP. The correct system depends on bait expression, tag accessibility, background binding and downstream analysis.

Equipment

• Microcentrifuge and cold centrifuge where required
• Tube rotator or end-over-end mixer
• Affinity resin, magnetic beads or spin columns appropriate to the tag
• Protein assay for bait/prey quantification
• SDS-PAGE and western blot apparatus or access to mass spectrometry
• Ice bucket and low-binding tubes where appropriate

Reagents

• Purified or expressed bait protein
• Compatible affinity resin
• Prey-containing cell/tissue lysate or purified prey protein
• Non-denaturing lysis/binding buffer
• Protease inhibitors and phosphatase inhibitors where relevant
• Wash buffer
• Elution buffer compatible with the affinity system and downstream analysis

Experimental design before starting

Decide whether this is discovery or confirmation

For confirmation, purified bait and purified prey can reduce confounding proteins. For discovery, a complex lysate may be more biologically informative but increases non-specific binding and usually requires stronger controls plus mass-spectrometry validation.

Preserve the interaction

Buffer pH, ionic strength, detergent, cofactors and temperature can determine whether a real interaction survives. Thermo Fisher explicitly notes that every protein pair can require unique binding conditions. Avoid assuming one “universal” buffer is optimal.

Detailed pull-down protocol

1. Immobilise the bait

1. Prepare the bait protein in a buffer compatible with the affinity resin.
2. Equilibrate the resin according to the manufacturer's instructions.
3. Bind bait to the resin at a loading amount within the resin capacity.
4. Wash away unbound bait.
5. Reserve an aliquot where useful to confirm bait loading by SDS-PAGE or western blotting.

2. Prepare the prey sample

1. Lyse cells or tissue under non-denaturing conditions with fresh protease inhibitors.
2. Keep the sample cold to limit degradation and complex rearrangement.
3. Clarify the lysate by centrifugation.
4. Measure protein concentration and normalise across comparison groups.
5. Reserve an input aliquot before exposure to the resin.

For one Thermo Fisher mammalian-lysate example, crude lysate is incubated on ice for approximately 30 minutes then clarified at 12,000 × g for 5 minutes. Treat this as a kit-specific starting point rather than a universal condition.

3. Optional pre-clearing

Incubate the prey lysate with control resin lacking bait when non-specific resin binding is expected to be high. Collect the supernatant for the actual pull-down.

4. Incubate bait with prey

1. Combine immobilised bait with the prey-containing sample.
2. Incubate with gentle mixing at 4°C or another condition validated for the interaction.
3. Use sufficient time for binding without unnecessarily prolonging exposure to proteases or allowing weak complexes to rearrange.

5. Wash

1. Collect the resin by centrifugation, spin column or magnet as appropriate.
2. Wash multiple times using a buffer that removes non-specific proteins without disrupting the target interaction.
3. For difficult backgrounds, optimise salt and detergent gradually rather than making a large jump in stringency.
4. Consider retaining wash fractions during development so you can see whether bait or prey is being lost.

6. Elute bound complexes

Choose an elution strategy compatible with both the tag chemistry and the downstream readout:

• Competitive elution: glutathione for GST, imidazole for His-tag systems, free biotin/desthiobiotin for selected streptavidin systems.
• Low-pH elution: useful for some affinity systems but requires rapid neutralisation if native protein is needed.
• Denaturing elution: SDS sample buffer for direct SDS-PAGE/western blot analysis.

7. Analyse bait and prey

• SDS-PAGE with total-protein staining for major interactors
• Western blotting for a suspected prey protein
• Mass spectrometry for discovery workflows

Essential controls

• Input: confirms that prey was present before the pull-down.
• Resin-only control: measures proteins binding the support itself.
• Tag-only control: detects binding to GST, His-associated resin context or other tag components.
• Irrelevant bait control: estimates non-specific binding to a similarly prepared protein.
• Positive interaction control: where a known binder exists.
• Bait loading control: confirms equivalent bait immobilisation across conditions.

Thermo Fisher recommends analysing lysates before and after exposure to the support and using bait-/prey-free controls to identify artifactual binding.

How to interpret a pull-down western blot

A convincing prey band should be present in the input, enriched in the bait pull-down, and absent or much weaker in resin-only/tag-only/irrelevant-bait controls. If the bait is also immunoblotted, confirm comparable bait loading before comparing prey recovery.

Does a pull-down prove direct binding?

Only when the experiment uses sufficiently purified components and excludes bridging molecules. A bait incubated with whole-cell lysate can recover an indirect complex, just as co-IP can. Direct interaction should be supported by purified proteins, orthogonal biophysical assays or appropriate domain/mutation experiments where the biological question requires it.

Optimisation priorities

Limitations

• Affinity tags can alter bait folding, localisation or binding.
• Recombinant bait may lack native post-translational modifications.
• Lysate pull-down can capture indirect complexes.
• High-abundance proteins may dominate discovery experiments.
• Binding conditions can favour interactions that are weak or absent in cells.

Troubleshooting

Frequently asked questions

What is the difference between pull-down and co-IP?

Co-IP uses an antibody to capture the bait from lysate. Pull-down uses an affinity-tagged or otherwise immobilised bait. See the Co-IP protocol.

Which tag is best?

There is no universal best tag. GST can aid solubility but is relatively large; His tags are compact but metal-chelate resins can have characteristic background; biotin–streptavidin is extremely strong and versatile. Choose around bait biology and downstream needs.

Can I use pull-down for mass spectrometry?

Yes. Use MS-compatible buffers, controls and elution conditions, and include replicate/quantitative criteria to distinguish true interactors from abundant background proteins.

Related ABMIUM methods

• Co-Immunoprecipitation (Co-IP) Protocol
• Immunoprecipitation Protocol
• Western Blot Protocol

Explore reagents for protein-interaction workflows

Need an IP-compatible antibody, recombinant bait/prey protein or help comparing validation evidence? Use Find your MATCH or contact ABMIUM. We can search across the ABMIUM catalogue by target, tag, species, application and validation evidence.

References and technical sources

1. Thermo Fisher Scientific — Overview of protein–protein interaction analysis.
2. Thermo Fisher Scientific — Pull-down assays.
3. Thermo Fisher Scientific — Pierce GST Protein Interaction Pull-Down Kit user guide.