His-Tagged Protein Purification

I. Objective and Scope

Purify recombinant proteins carrying a histidine tag (His-tag) by specific affinity to chelating resins or magnetic beads loaded with transition-metal ions such as Ni²⁺. This method is applicable to soluble proteins with N-terminal, C-terminal, or dual-terminal His tags expressed in bacterial or eukaryotic systems, as well as inclusion-body proteins purified under denaturing conditions.

II. Principle of Purification

The imidazole group on histidine side chains can form coordination bonds with transition-metal ions such as Ni²⁺ and Co²⁺. When these metal ions are immobilized on chromatographic resins or magnetic beads via multidentate ligands (e.g., IDA, NTA, TED), His-tagged proteins bind selectively, whereas most other proteins do not form stable coordination complexes, enabling separation from complex background proteins. Coordination between Ni²⁺ and the His tag is mainly mediated by the N atoms in the imidazole ring. In addition to histidine, amino acids capable of coordinating with metal ions include cysteine and tryptophan. The approximate binding strength of transition-metal ions to His tags follows: Cu²⁺ > Ni²⁺ > Zn²⁺ > Co²⁺.

III. Types of His Tags

1.Tandem histidines: 4–10 His residues in tandem, most commonly 6×His.

2.Repeated HN modules: His-Asn-His-Asn-His-Asn-His-Asn-His-Asn-His-Asn (6×HN).

3.HAT tag:

Lys-Asp-His-Leu-Ile-His-Asn-Val-His-Lys-Glu-Glu-His-Ala-His-Ala-His-Asn-Lys.

IV. Selection of Ni-Chelating Resins/Beads and Ligands

1.Structures and characteristics of commonly used ligands

Typical ligands include IDA, NTA,and TED. Their coordination valency with Ni²⁺, binding strength to His tags, and loading capacity differ, generally summarized as follows:

（1）IDA: forms tridentate coordination with Ni²⁺, leaving more sites available for His tags; relatively strong binding and high capacity, but may increase nonspecific binding and co-binding of impurities.

（2）NTA: forms tetradentate coordination with Ni²⁺; moderate His-tag binding and moderate capacity, balancing binding strength and selectivity.

（3）TED: forms pentadentate coordination with Ni²⁺, leaving the fewest sites for His tags; relatively weaker binding and lower capacity, but offers advantages in selectivity and resistance to contamination.

2.Tolerance conditions and use of reducing agents

Ni resins or beads with these ligands generally tolerate 8 M urea and 6 M guanidine hydrochloride, enabling denaturing His-tag purification.

（1）Ni-IDA Affinity Resin: avoid reducing agents in samples and buffers, as they may reduce Ni²⁺, turn the resin brown, and abolish binding.

（2）Ni-NTA Affinity Resin: low concentrations of reducing agents are acceptable, such as 5 mM DTE, 5 mM DTT, 20 mM β-mercaptoethanol, 5 mM TCEP, or 10 mM reduced glutathione.

（3）Ni-TED Affinity Resin: tolerate higher concentrations of reducing agents and also withstand 1 M NaOH and 100 mM EDTA; suitable for purification of His-tagged proteins expressed in eukaryotic cells and for more stringent cleaning/regeneration conditions.

V. Column-Based Purification Workflow for His-Tagged Proteins

The following procedure applies to prepacked or self-packed Ni-chelating affinity columns. The recommended pH for all steps is 7.0–8.0.

1.Column equilibration

Equilibrate the resin with a buffer system as close as possible to the lysis supernatant. Common equilibration buffers are 50 mM Tris or 20 mM phosphate buffer (pH 7.5–8.0) containing 300 mM NaCl, optionally supplemented with 10–20 mM imidazole to suppress nonspecific binding. Before loading, equilibrate with at least 5–10 column volumes (CV) until the baseline is stable.

2.Sample loading

Adjust loading amount and flow rate according to protein content and resin volume. Typically, ensure the sample residence time in the resin is ≥1 min to allow sufficient binding of His-tagged proteins. Loading may be performed by gravity flow or a low-pressure pump, avoiding excessive pressure and foam formation.

3.Washing

Wash with equilibration buffer or a wash buffer with slightly elevated imidazole (e.g., 20–40 mM imidazole). The wash volume is usually ≥5 CV. Increasing wash volume and moderately raising imidazole concentration helps remove nonspecifically bound proteins and improves elution purity.

4.Elution

Choose elution strategies based on experimental goals:

Linear gradient elution uses a linear increase of imidazole from low to high concentrations (e.g., 20–500 mM), suitable for initial condition scouting.Stepwise gradient elution sets multiple fixed imidazole plateaus (e.g., 50, 100, 200, 300 mM) to elute proteins with different binding strengths, facilitating optimization and fractionated collection.Single-step elution applies one high imidazole concentration (e.g., 250–300 mM) directly, suitable when impurity levels are low.Collect elution fractions and analyze by grading assays (e.g., SDS-PAGE). Pool fractions containing the target protein and proceed to buffer exchange or polishing chromatography.

VI. Magnetic Bead-Based Purification Workflow for His-Tagged Proteins

The following procedure applies to Ni-chelating magnetic beads used in batch binding on a rotator or shaker. The recommended pH for all steps is 7.0–8.0.

1.Equilibration

Equilibrate beads with the same or compatible buffer as the lysis supernatant, typically 50 mM Tris or 20 mM phosphate buffer containing 300 mM NaCl (optionally 10–20 mM imidazole). Resuspend beads in equilibration buffer, mix gently, place on a magnetic rack to separate, discard the supernatant, and repeat 2–3 times.

2.Incubation/binding

Add the sample to the equilibrated bead suspension. Incubate with gentle mixing on a rotator or shaker for 0.5–2 h to ensure full binding of His-tagged proteins to bead-bound Ni²⁺. After incubation, place on a magnetic rack to collect beads and discard the supernatant.

3.Washing

Add wash buffer (same as or similar to the column wash buffer), mix gently, then separate on a magnetic rack and discard the supernatant. Repeat washing 3–5 times to remove nonspecifically bound proteins. Adjust imidazole concentration and wash number as needed for purity.

4.Elution

Add high-imidazole elution buffer to the washed beads and incubate on a rotator or shaker for 2–3 min to dissociate the target protein. Place on a magnetic rack, separate beads, and collect the supernatant as the eluted His-tagged protein. To improve recovery and concentration, elute 1–2 additional times and pool elution fractions.

Aladdin: https://www.aladdinsci.com/