Immobilized metal affinity chromatography purifies polyhistidine-tagged recombinant proteins by binding to immobilized transition metal ions (Ni²⁺ or Co²⁺) via histidine side chain imidazole rings and eluting with imidazole or low pH.
Principle
A polyhistidine tag — typically 6 to 10 consecutive histidine residues — is fused to the N- or C-terminus of the recombinant protein. The imidazole side chain of histidine coordinates with chelated divalent metal ions (Ni²⁺, Co²⁺, Cu²⁺, Zn²⁺) immobilized on a chromatographic support via nitrilotriacetic acid or iminodiacetic acid chelators. Ni-NTA (nitrilotriacetic acid) provides four coordination sites to the Ni²⁺ ion, leaving two sites available for histidine binding. The interaction is reversible: imidazole (20–500 mM) competitively displaces the histidine tag, or low pH (4.5–5.5) protonates the histidine side chains, disrupting metal coordination.
Resin Types
Ni-NTA agarose (Qiagen, Thermo Fisher) is the most widely used IMAC resin. Binding capacity is typically 5–10 mg of 6×His-tagged protein per mL of settled resin. Ni-Sepharose (GE Healthcare) offers higher flow rates for FPLC applications. TALON (Clontech/TaKaRa) uses Co²⁺-carboxymethylaspartate resin, providing higher specificity with lower non-specific binding than Ni-NTA, at the cost of lower capacity. Magnetic beads (Dynabeads His-Tag, MagnaHis) enable batch purification from small volumes. High-capacity resins (up to 40 mg/mL) are available for preparative purification.
Buffers
Lysis/loading buffer: 20–50 mM Tris or phosphate pH 7.5–8.0, 300–500 mM NaCl (suppresses ionic interactions), 10–20 mM imidazole (reduces non-specific binding). Wash buffer: same composition with 20–40 mM imidazole. Elution buffer: 200–500 mM imidazole or pH gradient to 4.5. The NaCl concentration is critical — lower salt increases non-specific binding of acidic proteins. Additives such as 1 mM DTT or TCEP prevent oxidation of surface cysteines. For membrane proteins, 0.1–1% detergents compatible with IMAC (DDM, CHAPS, digitonin) can be included.
Protocol
Lyse cells in lysis buffer with protease inhibitors. Clarify by centrifugation (20,000 g, 30 min) and filtration (0.45 µm). Equilibrate resin with 5 volumes of lysis buffer. Incubate clarified lysate with resin (batch mode: 30 min rotating at 4 °C, or column mode: load at 0.5–1 mL/min). Wash with 10–20 column volumes of wash buffer until UV baseline stabilizes. Elute with 5–10 volumes of elution buffer, collecting fractions. Analyze fractions by SDS-PAGE and Western blot or Bradford assay.
Tag Placement and Cleavage
The 6×His tag can be placed at either terminus. N-terminal tags are more common and produce higher expression in E. coli, but C-terminal tags reduce the risk of affecting N-terminal signal sequences. The tag can be removed by incorporating a TEV, thrombin, or Factor Xa protease cleavage site between the tag and the protein. Cleavage is performed after elution, followed by a subtractive IMAC step to remove the cleaved tag and uncleaved protein.
Applications
His-tag/IMAC is the most widely used protein purification method in molecular biology. It works for soluble proteins expressed in E. coli, yeast, insect cells, and mammalian cells. It also purifies membrane proteins solubilized in detergents, inclusion body proteins purified under denaturing conditions (8 M urea or 6 M guanidine-HCl), and protein complexes via sequential tagging of interacting partners (co-IMAC). The small tag size rarely interferes with protein function, making it ideal for affinity capture, structural studies, and enzymatic assays.
