His-Tag Protein Purification Protocol: Native Ni-NTA/IMAC Workflow

Immobilised metal affinity chromatography (IMAC) is one of the most widely used methods for purifying recombinant proteins carrying a polyhistidine tag, commonly 6×His. Histidine side chains coordinate immobilised nickel or cobalt ions on the resin. Under native conditions, this can provide rapid enrichment from clarified lysates while retaining protein activity and structure when the target remains soluble.

Before starting

• Confirm the tag position and whether it is expected to be accessible in the folded protein.
• Determine whether the recombinant protein is mainly soluble or present in inclusion bodies.
• Select nickel or cobalt resin according to desired capacity, selectivity and downstream use.
• Keep all buffers compatible with the IMAC chemistry; strong chelators such as EDTA can strip metal ions from many resins.

Typical buffer strategy

Exact buffer composition should be optimised for the target protein and resin. A common native IMAC strategy uses a near-neutral Tris- or phosphate-based buffer containing salt, low imidazole during binding/washing, and higher imidazole for elution. Low imidazole concentrations in binding and wash buffers can suppress weak non-specific interactions, while substantially higher imidazole concentrations competitively elute His-tagged protein. The optimum concentrations depend on the target protein, metal ion, resin chemistry and sample composition.

Native His-tag purification protocol

1. Express the His-tagged protein. Use the host system and induction/expression conditions appropriate to the construct.
2. Harvest cells. Keep the sample cold and include protease inhibitors where compatible with downstream purification.
3. Lyse under native conditions. Use mechanical, enzymatic or detergent-assisted lysis suited to the expression host. Avoid conditions that precipitate or denature the target.
4. Clarify the lysate. Centrifuge at high speed to remove insoluble material and collect the supernatant.
5. Save a lysate fraction. Retain lysate and insoluble pellet samples for SDS-PAGE so expression and solubility can be assessed.
6. Equilibrate resin. Wash/equilibrate Ni-NTA or cobalt resin with binding buffer. Do not allow the resin bed to dry.
7. Load clarified lysate. Apply at a flow rate that allows binding. For gravity columns, avoid loading faster than the resin manufacturer's recommendation.
8. Collect flow-through. Save a fraction for SDS-PAGE to check whether target protein failed to bind.
9. Wash. Use several column volumes of wash buffer. Low to moderate imidazole is commonly used to reduce binding of native host proteins.
10. Collect wash fractions. Analyse them if target loss is suspected.
11. Elute. Increase imidazole or use a resin-compatible alternative elution strategy. Collect multiple fractions rather than one large fraction.
12. Assess purity. Run lysate, flow-through, wash and elution fractions by SDS-PAGE.
13. Pool appropriate fractions. Pool only fractions containing the desired purity and concentration.
14. Buffer-exchange if required. Remove imidazole by dialysis, desalting or ultrafiltration and place the protein into its final storage/assay buffer.

Optimising purity versus yield

• High background proteins: increase low-level imidazole in binding/wash buffer, increase salt where compatible or use cobalt resin for greater selectivity.
• Target in flow-through: reduce imidazole in binding buffer, slow loading, check pH and confirm the tag is accessible.
• Target in pellet: optimise expression temperature, induction strength or solubility before changing chromatography.
• Protein precipitates after elution: reduce concentration, adjust salt/pH, add compatible stabilisers or perform rapid buffer exchange.

Native versus denaturing purification

The His-tag interaction with Ni-NTA does not require native tertiary structure, so denaturing purification with urea or guanidine is possible. This is useful for inclusion-body proteins but requires a separate refolding strategy if native activity is needed. Use native purification whenever the soluble, active form is the objective.

Troubleshooting

References and primary methods

1. Hochuli E, Bannwarth W, Döbeli H, Gentz R, Stüber D. Genetic approach to facilitate purification of recombinant proteins with a novel metal chelate adsorbent. Bio/Technology. 1988;6:1321–1325.
2. Bornhorst JA, Falke JJ. Purification of proteins using polyhistidine affinity tags. Methods Enzymol. 2000;326:245–254. doi:10.1016/S0076-6879(00)26058-8.
3. Kimple ME, Sondek J. Overview of affinity tags for protein purification. Curr Protoc Protein Sci. 2004;36:9.9.1–9.9.19. doi:10.1002/0471140864.ps0909s36.

Choosing nickel versus cobalt IMAC

Nickel resins generally provide high binding capacity and are widely used for routine 6×His purification. Cobalt resins often provide greater selectivity and lower background, though capacity can be lower. If purity is the priority for a difficult lysate, cobalt can be worth testing; if yield from a strongly expressed soluble target is the main priority, nickel is often a practical first choice.

Buffer compatibility

• pH: His-tag binding is favoured at neutral to mildly alkaline pH because histidines must remain available for metal coordination.
• Imidazole: low concentrations in binding/wash buffer suppress weak non-specific histidine-rich proteins; high concentrations competitively elute the target.
• EDTA/strong chelators: can strip nickel or cobalt and should generally be avoided unless the resin protocol explicitly allows them.
• Reducing agents: low concentrations of DTT or β-mercaptoethanol may be tolerated by some resins, but compatibility is resin-specific.
• Detergents: non-ionic detergents may improve solubility for membrane-associated targets but should be kept within resin and downstream-assay limits.

Reading the SDS-PAGE fractions

Always analyse at least four fractions: clarified lysate, flow-through, wash and elution. If the target is strong in flow-through, binding failed. If the target appears mainly in wash fractions, the wash is too stringent. If many contaminants remain in the elution while the target is retained well, increase selectivity during wash rather than simply increasing elution imidazole.

What to do after purification

• Measure protein concentration using an assay compatible with the final buffer.
• Assess purity by SDS-PAGE and, where relevant, western blot using anti-His or target-specific antibody.
• Buffer-exchange out imidazole if it interferes with downstream assays.
• Check activity before and after concentration if the protein is an enzyme or binding reagent.
• Aliquot and freeze using a protein-specific storage formulation to minimise freeze-thaw cycles.

Tag cleavage

If the construct contains a protease cleavage site, the His tag can be removed after purification. Cleavage conditions should be optimised for protease amount, time and temperature. A useful strategy is to perform a second IMAC step after cleavage: uncleaved His-tagged protein, free tag and His-tagged protease can bind the resin while the cleaved target flows through, depending on the exact construct and protease.

When native IMAC is not enough

IMAC is an affinity-enrichment step, not a guarantee of homogeneity. For structural biology or demanding biochemical assays, a second purification stage such as ion-exchange chromatography or size-exclusion chromatography can remove oligomers, aggregates and residual host proteins. The purification strategy should be driven by downstream quality requirements rather than a fixed number of steps.

Frequently asked questions

Why is my His-tagged protein in the flow-through?

The tag may be buried, the pH may be too low, imidazole may be too high, resin capacity may be exceeded, or the protein may not actually be soluble.

Why does my purified sample contain many bacterial proteins?

Native host proteins with exposed histidines can bind IMAC resin. Increase low-level imidazole and salt during washing, reduce resin overloading or test cobalt resin.

Should I purify under denaturing conditions?

Only when the target is insoluble or native folding is not required during capture. A denatured purification generally creates a separate refolding problem that must be solved before functional use.

Can I use the purified protein directly in an assay?

Sometimes, but remove imidazole or other incompatible components and verify concentration, purity and activity first.

Explore more with ABMIUM

ABMIUM supplies recombinant proteins and can help match purification strategy to tag, host and downstream assay. Explore recombinant proteins, use Find your MATCH, or see the pull-down assay protocol for affinity-based interaction studies.