GST-Tagged Protein Purification
GST-Tagged Protein Purification
I. Overview and Principle
GST (Glutathione S-transferase) tag is a commonly used fusion tag with a molecular weight of ~26 kDa and good water solubility. It is often used to improve soluble expression of hydrophobic proteins or proteins prone to inclusion body formation. Using chromatography resins immobilized with reduced glutathione (GSH), GST-tagged recombinant proteins can be purified by specific affinity binding. In most cases, a single-step GSH affinity purification yields higher purity for GST-tagged proteins than a single-step Ni²⁺ affinity purification for His-tagged proteins. Depending on vector design, the GST tag can be removed by site-specific proteases such as PreScission Protease, Thrombin, or Factor Xa.
GSH affinity resins are covalently coupled with reduced GSH on the surface. GST-tagged proteins bind specifically via their native glutathione-binding site to GSH on the resin, enabling selective capture from lysates or culture supernatants containing abundant impurities. Proteins are then eluted by high concentrations of GSH or by changing pH/ionic strength, achieving purification.
II. Materials and Reagents
2.1 Reagents
NP-40 (non-ionic detergent)
EDTA solution (e.g., 500 mM stock)
Magnesium sulfate (MgSO₄, e.g., 5 mM solution)
Acetate buffer–related reagents (e.g., sodium acetate/acetic acid)
20% ethanol solution
PBS-related salts
2.2 Instruments and Equipment
Ultrasonic disruptor or high-pressure homogenizer
Ice-bath setup (ice bucket or recirculating cooling system)
High-speed refrigerated centrifuge
Prepacked or self-packed chromatography columns
Peristaltic pump or protein chromatography system (e.g., standard protein FPLC system)
pH meter
UV spectrophotometer (for A280 or A260/280 measurement)
SDS-PAGE electrophoresis system and imaging system
III. Sample Preparation Workflow
1.Purification of soluble GST-tagged proteins expressed in the cytoplasm of E. coli
① Resuspend collected E. coli pellets in equilibration buffer (recommended ratio: 1 g wet pellet to 5–10 mL equilibration buffer). After thorough resuspension, if the suspension is viscous, add a small amount of nuclease, mix gently, keep on ice, and lyse cells by sonication.
Tip1: If the target protein is protease-sensitive, add protease inhibitors such as PMSF during lysis.
Tip2: Add a small amount of non-ionic detergent (e.g., NP-40, final concentration ≤0.1% of total volume) to improve purity.
② After sonication, centrifuge the lysate at 12,000×g, 4°C for 20 min, and collect the supernatant.
③ Load the collected supernatant onto the column for subsequent GST affinity purification.
2.Purification of secreted GST-tagged proteins
① Culture medium without GSH: after centrifugation to remove cells and debris, directly load the supernatant for column purification.
② Culture medium containing GSH: first add ammonium sulfate for protein precipitation. Collect the precipitate, fully dissolve in equilibration buffer, centrifuge to remove insoluble material, and load the supernatant onto the column under the same conditions.
3.Purification of GST-tagged proteins expressed in the periplasm of E. coli
Prepare the following solutions in advance:
1）Buffer S: 30 mM Tris-HCl, 20% sucrose, pH 8.0
2）500 mM EDTA
3）5 mM MgSO₄
① Based on wet weight (example: 1 g cells), add 80 mL Buffer S and 160 µL of 500 mM EDTA. Stir gently to mix and continue stirring for ~10 min.
② Centrifuge at 8,000×g, 4°C for 20 min and discard the supernatant. Resuspend the pellet in 80 mL 5 mM MgSO₄, stir gently on ice until dissolved, then centrifuge again at 8,000×g, 4°C for 20 min.
③ Load the resulting supernatant onto the column for purification.
4.Purification of GST-tagged proteins from inclusion bodies expressed in E. coli
① Resuspend collected E. coli pellets in equilibration buffer (ratio: 1 g wet pellet to 5–10 mL equilibration buffer). If viscous, add a small amount of nuclease, keep on ice, and lyse by sonication.
② Centrifuge at 5,000×g, 4°C for 20 min and discard the supernatant. Resuspend the pellet in an equal volume of equilibration buffer, sonicate again to disperse the pellet as much as possible, then centrifuge again under the same conditions.
③ Add the required denaturant to the pellet (or protein solution) and refold protein using an appropriate refolding method. After refolding, transfer the refolded solution into a dialysis bag and dialyze against equilibration buffer until fully equilibrated.
④ After dialysis, centrifuge to remove insoluble material and load the supernatant onto the column.
IV. Column-Based Purification Procedure
① Estimate total amount of GST-tagged protein in the sample and, together with resin binding capacity, calculate the required resin volume. Add the corresponding resin volume to the column.
② After the resin fully settles to the bottom, pre-equilibrate with ≥3× resin volume of purified water, then equilibrate further with 5× resin volume of equilibration buffer.
③ Load the sample to be purified, ensuring that the residence time from entering to exiting the resin is >1 min to allow sufficient binding.
④ Wash with 10× resin volume of wash buffer to remove nonspecifically bound impurities as much as possible.
⑤ Elute with elution buffer and collect eluates. It is recommended to collect fractions in separate tubes. Determine target peak fractions by downstream analyses (e.g., SDS-PAGE, absorbance) before pooling.
V. Resin Regeneration Procedure
For purifying the same protein, the same batch of resin can be reused up to 3 consecutive times before regeneration. For purifying different GST-tagged proteins, regenerate the resin as follows:
① Wash with 5× resin volume of Regeneration Buffer I (0.1 M Tris-HCl, 0.5 M NaCl, 0.1% SDS, pH 8.5).
② Then wash with 5× resin volume of Regeneration Buffer II (0.1 M acetate buffer, 0.5 M NaCl, 0.1% SDS, pH 4.5).
③ Wash thoroughly with 5× resin volume of purified water to remove residual salts and detergents.
④ Wash again with 5× resin volume of PBS to restore neutral buffer conditions close to storage conditions.
⑤ Finally, add sufficient 20% ethanol solution to cover the resin and store sealed at 4°C.
VI. FAQ
Q1: Why can’t GST-tagged recombinant proteins bind to the resin?
Common reasons include: incorrect expression of the GST tag (confirm by plasmid sequencing and anti-GST Western blotting); overly harsh sonication causing protein denaturation (reduce power, shorten time, or switch to enzymatic lysis); equilibration/wash buffer pH outside 6.5–8.0 preventing binding; high concentrations of reducing agents such as DTT in the sample (reduce or omit); and excessive loading flow rate leading to insufficient contact time (lower the flow rate).
Q2: Why does the purified product contain many contaminating proteins?
High impurity levels are often related to lysis conditions, resin amount, and protein stability: overly strong sonication partially denatures GST-tagged proteins, reducing specific binding and increasing nonspecific adsorption—use gentler lysis; excessive resin volume increases nonspecific adsorption—reduce resin according to binding capacity; target protein degradation leads to small fragments mixing with impurities—add protease inhibitors during lysis or use protease-deficient strains; if GST-tagged proteins form complexes with impurities and co-bind, increase salt concentration in wash buffer, add 1% Triton X-100, or add 5 mM DTT in equilibration buffer to weaken nonspecific interactions.
Q3: Why can’t GST-tagged proteins be eluted by the elution buffer?
First check whether GSH concentration in the elution buffer is too low (increase stepwise if needed, up to 60 mM) and whether pH is too low (adjust to ~pH 9.0 to improve elution). If salt concentration is insufficient and nonspecific interactions are strong, increase NaCl and try adding ~0.1% Triton X-100. Also note that GSH is easily oxidized—use freshly prepared elution buffer. If the target protein is significantly degraded, some fragments may bind tightly or adopt abnormal conformations that resist elution; in this case, add protease inhibitors during lysis and washing, or use protease-deficient strains to reduce degradation.
Aladdin: https://www.aladdinsci.com/