FoodWrite talked about affinity tags not so long ago well here we talk about fusion tags which include the family of affinity tags.
What Are Fusion Tags?
Fusion tags are genetically encoded peptide or protein sequences that are fused to a target protein (usually at the N- or C-terminus) to enhance expression, solubility, purification, detection, or localization.
They serve various purposes and can include:
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Affinity tags (for purification)
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Solubility tags (to prevent aggregation)
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Reporter tags (for visualization or tracking)
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Cleavage tags (for post-purification removal)
So: affinity tags are a subset of fusion tags.
Types of Fusion Tags
1. Affinity Tags (subset of fusion tags)
Used primarily for protein purification using affinity chromatography.
| Tag | Binding Partner | Notes |
|---|---|---|
| His-tag (6xHis) | Ni²⁺ or Co²⁺ (IMAC) | Small, widely used |
| FLAG-tag | Anti-FLAG antibody | Highly specific |
| Strep-tag | Strep-Tactin or streptavidin | Mild elution |
| GST-tag | Glutathione resin | Large tag (~26 kDa), helps solubility |
| MBP-tag | Amylose resin | Improves solubility, large (~42 kDa) |
2. Solubility Tags
Improve folding and solubility of difficult-to-express proteins in E. coli or other systems.
| Tag | Notes |
|---|---|
| MBP (Maltose-Binding Protein) | Also serves as affinity tag |
| SUMO | Small Ubiquitin-like Modifier; also allows precise cleavage |
| Trx (Thioredoxin) | Enhances solubility and oxidative folding |
3. Reporter Tags
Used to visualize, quantify, or track protein expression.
| Tag | Notes |
|---|---|
| GFP (Green Fluorescent Protein) | Fluorescent, non-invasive tracking |
| Luciferase | Bioluminescent; used in cell assays |
| β-galactosidase (LacZ) | Colorimetric assays |
4. Cleavage Tags
Allow removal of the tag after purification using specific proteases.
| Protease | Recognizes | Example Tags |
|---|---|---|
| TEV | ENLYFQ↓G | His-TEV-target |
| Thrombin | LVPR↓GS | GST-thrombin-target |
| Factor Xa | IEGR↓ | FLAG-FXa-target |
These are often included between the tag and the target protein.
Summary: Are Fusion Tags and Affinity Tags the Same?
| Feature | Fusion Tag | Affinity Tag |
|---|---|---|
| Purpose | General (expression, solubility, purification, visualization) | Specific (purification) |
| Examples | GFP, MBP, His-tag, GST | His-tag, FLAG-tag, Strep-tag |
| Subset? | Broad category | Subset of fusion tags |
So, all affinity tags are fusion tags, but not all fusion tags are affinity tags.
Use in Bioprocessing
In industrial and research bioprocessing:
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Fusion tags are added during recombinant protein expression (e.g., in E. coli, CHO, HEK293).
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Affinity tags simplify purification, reducing cost and time.
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Cleavable tags are often removed before therapeutic or structural applications.
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