EMSA Protocol: Gel-Shift Assay for Protein-DNA and Protein-RNA Binding

The electrophoretic mobility shift assay (EMSA), also called a gel-shift assay or band-shift assay, is a native electrophoresis method for detecting protein–DNA or protein–RNA complexes. A labelled nucleic-acid probe is incubated with a purified protein or nuclear/cell extract, then resolved on a non-denaturing gel. Protein-bound probe migrates more slowly than free probe, producing a shifted band.

EMSA is valuable because it directly visualises complex formation and can test sequence specificity using competitor probes. A supershift experiment adds an antibody to the complex; a further reduction in mobility can support the presence of a specific protein in the nucleic-acid complex.

Important: a shifted band demonstrates complex formation under the assay conditions, but sequence specificity and protein identity require competition and/or supershift controls. Buffer composition must be optimised for each protein–nucleic acid system.

When to use EMSA

• Test whether a transcription factor binds a candidate DNA motif.
• Compare wild-type and mutated binding sequences.
• Assess protein–RNA binding.
• Test the effect of cofactors, phosphorylation or small molecules on nucleic-acid binding.
• Identify a protein in a complex by antibody supershift.
• Estimate apparent binding affinity with a quantitative concentration series.

Equipment required

• Native polyacrylamide gel apparatus.
• Cold-room or chilled electrophoresis setup where needed for unstable complexes.
• Gel imager matched to radioactive, fluorescent, chemiluminescent or biotin detection.
• Low-binding tubes.
• Purified protein or nuclear-extract preparation equipment.

Reagents

• Double-stranded DNA or RNA probe containing the binding sequence.
• Detection label: fluorescent, biotin, radioactive or another validated label.
• Purified protein or nuclear/cell extract.
• Native binding buffer.
• Glycerol or another density component for native loading.
• Optional non-specific competitor such as poly(dI-dC), where appropriate.
• Unlabelled specific competitor probe.
• Unlabelled non-specific or mutated competitor probe.
• Target-specific antibody for supershift where required.

Probe design

• Use a probe long enough to contain the complete binding element plus flanking sequence, but avoid unnecessary length that creates additional binding sites.
• For double-stranded DNA, anneal complementary oligonucleotides carefully and verify stoichiometry.
• Prepare an unlabelled identical competitor.
• Prepare a mutated competitor or probe that disrupts the predicted binding motif without changing overall length/composition excessively.
• Purify labelled probe where unincorporated label could raise background.

Binding-buffer development

EMSA conditions balance complex stability with electrophoretic mobility. A reasonable starting screen may include a near-neutral pH buffer, moderate monovalent salt, glycerol, low reducing agent concentration if required, magnesium for proteins that need it, and non-specific competitor DNA when using complex extracts. Hellman and Fried emphasise that the optimal buffer depends on the interacting system.

• Salt: too low can increase non-specific binding; too high can disrupt specific complexes.
• Glycerol: can stabilise complexes and assist loading, but excessive viscosity affects migration.
• Divalent cations: include only if required for binding/folding.
• Detergent: small amounts can reduce aggregation for some proteins but may destabilise complexes.

Detailed EMSA protocol

1. Prepare labelled probe. Generate the DNA/RNA probe at known concentration and quality.
2. Prepare free-probe control. Set aside a reaction containing labelled probe but no protein.
3. Prepare protein/extract. Keep purified protein or nuclear extract cold and avoid repeated freeze–thaw cycles.
4. Assemble binding reaction. Combine binding buffer, any non-specific competitor, protein and labelled probe. Add probe consistently to all tubes.
5. Incubate to equilibrium or a fixed validated time. A short room-temperature or cold incubation is common, but the correct time depends on association kinetics and stability.
6. Prepare specific-competition reaction. Add a molar excess of unlabelled probe identical to the labelled probe before or together with labelled probe as defined by the assay.
7. Prepare non-specific/mutated competition reaction. Add equivalent excess of unrelated or binding-site-mutated DNA/RNA.
8. Optional supershift. Add a target-specific antibody to the binding reaction. Incubate long enough for antibody association without precipitating the complex.
9. Prepare native gel. Use a non-denaturing polyacrylamide percentage appropriate for probe and complex size. Pre-run if required to stabilise temperature and ionic conditions.
10. Add native loading component. Do not add SDS or boiling/reducing denaturation conditions.
11. Load samples promptly. Keep the timing consistent between reactions.
12. Electrophorese under native conditions. Run at a voltage and temperature that preserve the complex. Cold electrophoresis can help unstable interactions.
13. Detect labelled nucleic acid. Use the appropriate imaging method without saturating the shifted band.
14. Interpret controls together. Compare free probe, protein-bound shift, specific competition, non-specific competition and supershift.

Essential controls

• Free probe: defines unbound-probe mobility.
• Protein + probe: shows formation of one or more complexes.
• Specific unlabelled competitor: should reduce a sequence-specific shift.
• Mutated/non-specific competitor: should compete less effectively if the interaction is sequence-specific.
• Supershift antibody: can support the presence of a particular protein in the complex.
• Isotype antibody control: distinguishes specific supershift effects from generic immunoglobulin effects.

Interpreting EMSA patterns

• A single shifted band can represent one dominant stoichiometric complex.
• Multiple shifted bands can represent different oligomeric states, additional proteins, alternative binding sites or partially assembled complexes.
• Smearing can indicate unstable complexes, protein degradation, aggregation or overloading.
• Specific competitor should reduce the target shift more strongly than mutated/non-specific competitor.
• A supershift supports target-protein participation, but absence of supershift does not exclude binding because the antibody can fail to recognise the native complex or can disrupt it.

Quantitative EMSA and apparent affinity

To estimate an apparent dissociation constant, keep labelled probe concentration constant and titrate protein across a range spanning mostly free to mostly bound probe. Quantify bound and free probe under non-saturating imaging conditions. Plot fraction bound versus free protein concentration and fit an appropriate binding model. Interpret cautiously when protein concentration is not known accurately, complexes contain multiple components or binding is cooperative.

Using nuclear extracts

Nuclear extracts reproduce more of the cellular protein environment but create additional non-specific binding and complex heterogeneity. Include a non-specific nucleic-acid competitor and stronger sequence controls. If a specific complex is suspected, combine competition with a supershift antibody or compare extract from knockout/depleted cells.

Warnings and common analytical traps

• A shift alone does not prove sequence specificity. Use competitor controls.
• A supershift antibody can disrupt rather than supershift a complex. A negative supershift is not definitive.
• Do not heat or denature samples before a native gel.
• Do not overload probe or protein. Non-specific binding and aggregation can dominate.
• Do not derive K_d from one or two concentrations. Use a proper binding curve.

Limitations

• Mobility does not directly report complex molecular weight.
• In-gel stability can differ from solution equilibrium.
• Extract-based EMSA cannot always identify every component in a complex.
• Some protein–nucleic acid complexes dissociate during electrophoresis.
• Labels can alter probe behaviour if positioned poorly.

Troubleshooting

Frequently asked questions

Can EMSA be used for RNA?

Yes. The same native mobility principle applies to protein–RNA complexes, with RNase-free handling and RNA-compatible probe design.

What gel percentage should I use?

There is no universal percentage. Lower-percentage gels can help resolve larger complexes; higher percentages better separate small probes and compact complexes. Optimise empirically.

What does a supershift prove?

It supports the presence of the antibody-recognised protein in the shifted complex. It does not establish that the protein contacts the nucleic acid directly.

References and journal sources

1. Hellman LM, Fried MG. Electrophoretic mobility shift assay (EMSA) for detecting protein-nucleic acid interactions. Nat Protoc. 2007;2:1849–1861. doi:10.1038/nprot.2007.249.
2. Garner MM, Revzin A. A gel electrophoresis method for quantifying the binding of proteins to specific DNA regions. Nucleic Acids Res. 1981;9:3047–3060. doi:10.1093/nar/9.13.3047.

Relevant ABMIUM products

• RELA Protein – Transcription factor p65 — a catalogue example of a recombinant DNA-binding transcription factor. Before EMSA use, confirm that the supplied construct and preparation are functionally competent for the intended DNA-binding assay.
• Anti-NF-κB p65/RELA Antibody Picoband® — suitable as an orthogonal detection reagent based on listed applications, but not automatically a validated EMSA supershift antibody; native-complex compatibility must be demonstrated.

Explore more with ABMIUM

For another DNA/RNA-binding protein, explore recombinant proteins or use Find your MATCH with the target, sequence system and downstream assay. Related interaction workflows: Pull-Down and Co-IP.