Protein pull-down assay is an in vitro affinity purification method that uses a bait protein immobilized on a solid support to enrich interacting proteins from a complex protein mixture. It serves as a foundational technique for confirming protein-protein interactions (PPIs) initially discovered by yeast two-hybrid screening or other discovery platforms, and for primary screening to identify novel interaction partners of a target protein under controlled biochemical conditions.
The principle is straightforward yet powerful: a recombinant bait protein fused to an affinity tag (GST, His6, or biotin) is expressed and purified, then immobilized onto a matching affinity resin. When a prey protein sample—derived from cell lysate, recombinant expression, or in vitro transcription/translation—is incubated with the bait-loaded resin, specific binding partners are captured. Subsequent washing removes non-specific contaminants, and elution followed by Western blot or mass spectrometry analysis confirms predicted interactions or reveals previously unknown ones. Profacgen provides comprehensive pull-down assay services optimized for sensitivity, specificity, and throughput.
The protein pull-down assay is a well-established, affinity-based technique for investigating protein–protein interactions (PPIs) in vitro. It is widely employed to validate known binding partners, identify novel interacting proteins, and map the structural domains critical for binding. The assay leverages the specificity of affinity purification: a recombinant "bait" protein, fused to an affinity tag (e.g., GST, His-tag, FLAG, or biotin), is immobilized onto a solid support such as glutathione-Sepharose beads, nickel-chelating resin, or streptavidin-coated magnetic beads. This immobilized bait is then incubated with a complex protein mixture—typically cell lysates or purified protein fractions—containing potential "prey" proteins. Under optimized conditions, prey proteins that specifically interact with the bait are co-captured on the beads. Following extensive washing to remove nonspecific binders, the retained protein complexes are eluted and analyzed.
Among the available tag systems, GST is often the preferred choice due to its dual benefits: it enhances the solubility of the bait protein and allows for gentle, competitive elution using reduced glutathione, preserving the native conformation of interacting complexes. Downstream detection and identification of captured prey proteins are commonly performed using SDS-PAGE followed by Coomassie staining or Western blotting for hypothesis-driven studies, or by liquid chromatography–tandem mass spectrometry (LC-MS/MS) for unbiased, discovery-based proteomic screening. The pull-down assay is highly versatile, accommodating diverse sample types—from bacterial lysates to mammalian tissue extracts—and can be configured in either a single-protein (binary) interaction format or a multi-protein complex co-purification format, providing a robust platform for dissecting cellular signaling networks and protein interaction landscapes.
Figure 1. GST pull-down assay.
| Tag | Affinity Ligand | Binding Affinity | Elution Method | Best Suited For |
|---|---|---|---|---|
| GST | Glutathione | KD ~10−6 M | Reduced glutathione or PreScission protease | Native folding; large bait proteins; elution under non-denaturing conditions |
| His6 | Ni2+-NTA / Co2+-TALON | KD ~10−13 M (multivalent) | Imidazole or EDTA | Small baits; denaturing conditions; high-throughput formats |
| Biotin | Streptavidin / NeutrAvidin | KD ~10−14 M | Biotin displacement or denaturation | Stringent washing; very weak interactions; membrane-associated proteins |
| FLAG | Anti-FLAG M2 antibody | KD ~10−8 M | FLAG peptide (DYKDDDDK) or low pH | Antibody-based capture; native elution; co-IP-like workflow |
| MBP | Amylose | KD ~10−7 M | Maltose | Improving solubility of aggregation-prone baits |
Western Blot Detection
Confirm specific interactions using antibody-based detection.
Mass Spectrometry (MS) Identification
Discover novel interacting proteins without prior knowledge.
To complement your protein interaction analysis, explore our comprehensive portfolio of related screening and profiling services.
Background:
A drug discovery team had identified by Y2H screening that the transcription factor FOXO3 interacted with the histone acetyltransferase p300, but they needed to define the minimal interaction domain to design a disrupting peptide for therapeutic development.
Our Solution:
Profacgen expressed a series of FOXO3 deletion mutants as GST fusions (full-length, N-terminal, forkhead domain, C-terminal transactivation domain, and a series of 50-residue truncations). Each GST-bait was purified and used in pull-down assays against recombinant p300 fragments. Interactions were detected by Western blot using anti-p300 antibody.
Final Results:
The pull-down series pinpointed a 36-residue segment (FOXO3 residues 412–447) as the minimal p300-binding region. Alanine scanning of this segment identified 3 critical hydrophobic residues (L415, L419, F440) essential for binding. A cell-penetrating peptide mimicking this region disrupted the FOXO3-p300 interaction in cells (IC50 = 8 μM), reduced target gene HIF-1α expression by 45%, and inhibited tumor growth in a xenograft model by 52%. The pull-down data directly enabled the peptide therapeutic program.
Background:
A cancer genomics group discovered a novel fusion protein (BCR-ABL2) in chronic myeloid leukemia but could not determine its interacting partners using standard co-IP due to the lack of a specific antibody.
Our Solution:
Profacgen expressed the BCR-ABL2 fusion with an N-terminal biotin acceptor peptide (BAP) tag in HEK293 cells. The biotinylated fusion protein was captured on streptavidin-coated magnetic beads and incubated with K562 leukemia cell lysate. Captured proteins were eluted and identified by LC-MS/MS, with biotin-BAP alone as a negative control. Triplicate experiments ensured reproducibility.
Final Results:
MS identified 127 proteins specifically pulled down by BCR-ABL2 (enrichment >5-fold over biotin-BAP control). Of these, 34 were known BCR-ABL interactors, validating the approach. Three novel binding partners—RHOA, PARD3, and TIAM1—were implicated in cytoskeletal regulation and cell polarity. Functional validation confirmed that BCR-ABL2 expression disrupted normal cell polarity, a previously unrecognized oncogenic mechanism. The biotin pull-down approach overcame the antibody limitation and revealed actionable therapeutic targets.
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