Key takeaways: His-tag is the default choice for most applications—small, inexpensive, and compatible with denaturing conditions. GST-tag offers higher specificity and can improve solubility, but its large size (26 kDa) often requires removal before functional assays. Choose based on your downstream application, not just purification efficiency.
Overview: The Two Most Popular Affinity Tags
Polyhistidine (His-tag) and glutathione S-transferase (GST-tag) together account for over 80% of recombinant protein purifications worldwide. Each has distinct advantages that make it optimal for specific scenarios. Understanding these tradeoffs will save you time and improve your results.
Head-to-Head Comparison
| Feature | His-Tag (6xHis) | GST-Tag |
|---|---|---|
| Size | ~1 kDa (6 amino acids) | ~26 kDa (211 amino acids) |
| Affinity matrix | Ni-NTA or Co-NTA agarose | Glutathione agarose |
| Binding capacity | 5-50 mg/mL resin | 5-10 mg/mL resin |
| Elution | Imidazole (150-500 mM) | Reduced glutathione (10-50 mM) |
| Purity (single step) | 80-95% | 90-98% |
| Works under denaturing conditions? | Yes | No (GST must be folded) |
| Solubility enhancement | Minimal | Strong—GST is highly soluble |
| Tag removal needed? | Usually optional | Often required (26 kDa interferes) |
| Cost per purification | $ (low) | $$ (moderate) |
When to Choose His-Tag
His-tag is the best choice when:
- You are expressing proteins for antibody production or immunization—the small tag rarely elicits an interfering immune response
- Your protein is expressed in inclusion bodies and requires denaturing purification (Ni-NTA works in 6-8 M urea)
- You need to minimize cost for large-scale production—Ni-NTA resin is the most economical affinity matrix
- Your downstream application is structural biology (X-ray crystallography, cryo-EM)—the minimal tag rarely interferes with crystal packing
- You want to keep the tag on for detection (anti-His antibodies are widely available and inexpensive)
Browse our His-tagged recombinant proteins for ELISA, Western Blot, and SPR applications.
When to Choose GST-Tag
GST-tag is the better choice when:
- Your target protein is poorly soluble—GST is one of the most soluble proteins known and often pulls its fusion partner into solution
- You need very high purity in a single step—GST purification routinely achieves >95% purity without optimization
- You are performing GST pull-down assays to study protein-protein interactions (the tag itself is the assay tool)
- Your protein is toxic to E. coli—GST fusion often reduces toxicity by sequestering the target protein
Important caveat: The 26 kDa GST tag must be removed for most functional assays (enzymatic studies, cell-based assays, SPR/BLI). Factor Xa, thrombin, or TEV protease cleavage sites are typically engineered between GST and the target protein.
Tag Removal: What to Expect
His-tag removal is optional for most applications. The 6xHis tag adds only ~1 kDa and rarely interferes with protein function. However, for therapeutic proteins or crystallography of small proteins (<20 kDa), tag removal is recommended using TEV or thrombin protease.
GST-tag removal is almost always required for functional studies. After cleavage, a second round of glutathione affinity chromatography captures the free GST and uncleaved fusion protein, leaving purified tag-free target in the flow-through.
Cost and Scalability
For screening 10-50 constructs (typical in early-stage research), the cost difference between His and GST purification is negligible. For production-scale work (>10 mg final product), His-tag purification is approximately 3-5× less expensive due to lower resin costs and higher binding capacity.
Recommendation by Application
| Application | Recommended Tag |
|---|---|
| ELISA coating antigen | His-tag (small, minimal interference) |
| Western Blot standard | His-tag (easy detection with anti-His) |
| SPR/BLI kinetics | His-tag (can immobilize via anti-His capture) |
| Immunization/antibody production | His-tag or tag-free |
| Pull-down / protein interaction | GST-tag (tag serves as assay handle) |
| Crystallography | His-tag (minimal size, removable) |
| Difficult-to-express proteins | GST-tag (solubility enhancement) |
Need help choosing the right tagged protein? Our recombinant protein catalog includes both His-tagged and GST-tagged options with full specifications. Contact us for custom protein production with your preferred tag configuration.
Frequently Asked Questions
Can I use both His-tag and GST-tag on the same protein?
Yes. Many expression vectors include both tags in tandem (typically GST at the N-terminus and His at the C-terminus, separated by a protease site). This enables two-step purification: GST affinity first, followed by cleavage and Ni-NTA to remove free GST and uncleaved protein. The result is exceptionally pure, tag-free protein.
Does His-tag affect protein structure?
For most proteins, the 6xHis tag is unstructured and does not affect folding or activity. However, if the tag is placed at the N-terminus of a protein where the N-terminal region is critical for function (e.g., signal peptides, certain enzymes), C-terminal His-tag placement is recommended.
What imidazole concentration should I use for His-tag elution?
Start with 150 mM imidazole. If purity is low (many contaminant bands on SDS-PAGE), include 20-50 mM imidazole in the wash buffer and elute at 250 mM. Most His-tagged proteins elute between 150-300 mM imidazole.