GST-tag pull-down uses glutathione S-transferase (GST) fusion proteins immobilized on glutathione agarose to purify recombinant proteins from expression systems or to capture and identify protein-protein interactions from cell lysates.
Principle
The 26 kDa GST protein from Schistosoma japonicum is fused to the target protein. GST binds glutathione (GSH, γ-glutamyl-cysteinyl-glycine) immobilized on agarose beads with high affinity (K_d ≈ 0.1 µM). The fusion protein is captured from crude lysates, washed extensively, and eluted with 10–20 mM reduced glutathione under non-denaturing conditions. For interaction studies, the immobilized GST fusion protein serves as bait to capture interacting proteins from a lysate, which are then identified by Western blot or mass spectrometry.
Glutathione Resins
Glutathione agarose is typically cross-linked agarose beads with glutathione coupled through the sulfur atom. Binding capacity is 5–15 mg of GST fusion protein per mL of resin. Reduced glutathione is used as the competitive eluent. Preswelling and equilibration in PBS or similar buffer (pH 7.3–8.0) is required before use. Glutathione magnetic beads enable rapid pull-downs in microcentrifuge tubes for small-scale interaction studies.
Expression and Lysis
GST fusion proteins are expressed in E. coli BL21(DE3) strains from pGEX vectors (GE Healthcare), which include a tac promoter inducible with IPTG. Expression at 18–25 °C overnight improves solubility of difficult targets. Lysis in PBS (140 mM NaCl, 2.7 mM KCl, 10 mM Na₂HPO₄, 1.8 mM KH₂PO₄, pH 7.3) with 1% Triton X-100 or 0.1% Tween-20 reduces non-specific binding. DTT (1–5 mM) maintains GST in its active reduced form. Protease inhibitors and DNase/RNase improve lysate quality.
Purification Protocol
Equilibrate glutathione agarose with PBS. Incubate clarified lysate with resin (batch: 30–60 min, 4 °C; column: gravity flow). Wash with 20 volumes of PBS plus 0.1% Triton X-100. Elute with 10 mM reduced glutathione in 50 mM Tris-HCl pH 8.0. Collect 0.5 mL fractions. Dialyze eluted protein to remove glutathione for downstream applications. Remove the GST tag by thrombin or PreScission protease cleavage at a designed recognition site, followed by a subtractive glutathione agarose step to capture free GST and uncleaved fusion.
Pull-Down Protocol for Interactions
Immobilize 50–200 µg of GST fusion bait on 25–50 µL of glutathione agarose. Incubate with cell lysate (0.5–2 mg total protein) in binding buffer (PBS + 0.1% NP-40) for 1–2 hours at 4 °C with rotation. Wash 3–5 times with binding buffer. Elute with 2× SDS sample buffer and analyze by SDS-PAGE followed by Coomassie staining or Western blot. Include GST-only beads as a negative control to identify proteins binding nonspecifically to GST or the resin. Competitor glutathione (10 mM) pre-incubation demonstrates specific competition.
Advantages and Limitations
GST tag often enhances expression and solubility of its fusion partner, making it valuable for difficult proteins. Glutathione elution is mild and preserves protein activity. The large size (26 kDa) can, however, interfere with some functional assays, mandating tag removal. Dimerization of GST can cause unwanted oligomerization of fusion proteins. The tag is ideal for affinity capture interaction studies but less suitable if the 26 kDa addition to molecular weight is problematic for structural work.
