E. coli fusion-tag soluble protein purification

• E. coli expression strains (e.g., BL21(DE3))
• Plasmid vectors with affinity tags (His6
• GST, MBP, Fh8, CSQ-tag, Trx)
• IPTG for induction
• Lysis buffer (e.g., PBS with protease inhibitors)
• Ni-NTA resin
• Amylose resin
• Chitin resin
• Hydrophobic interaction chromatography (HIC) matrix
• Imidazole (for competitive elution)
• Column buffers (binding, wash, elution)
• Proteases (TEV, enterokinase)
• Dialysis buffer
• Glycerol
• Triton X-100
• SDS-PAGE reagents
• Western blot components
• MALDI-TOF MS equipment
• Fluorescence microscopy
• Spectrophotometer
• Centrifuge
• Sonicator or microfluidizer
• FPLC system.

• 1.Clone the gene of interest into an expression vector with a chosen affinity tag (e.g., His6, MBP, GST) using restriction digestion or Golden Gate cloning.

• 2.Transform the plasmid into competent E. coli BL21(DE3) cells.

• 3.Inoculate culture in LB medium and grow at 37°C until OD600 reaches ~0.6-0.8.

• 4.Induce protein expression with IPTG (0.1-1 mM) and incubate at 16-25°C for 4-16 hours.

• 5.Harvest cells by centrifugation (4,000 × g, 15 min, 4°C).

• 6.Resuspend pellet in lysis buffer and lyse cells via sonication or microfluidization.

• 7.Clarify lysate by centrifugation (12,000 × g, 20 min, 4°C).

• 8.Load supernatant onto affinity resin pre-equilibrated with binding buffer.

• 9.Wash column with wash buffer to remove non-specifically bound proteins.

• 10.Elute target protein using competitive elution (e.g., imidazole for His-tag) or specific elution conditions (e.g., low pH for Fh8-HIC).

• 11.Dialyze or desalt the eluted protein into storage buffer.

• 12.If required, cleave the tag using site-specific protease (e.g., TEV or enterokinase) and perform second purification step to remove cleaved tag and protease.

• 13.Analyze purity and identity by SDS-PAGE, Western blot, and mass spectrometry.14.Confirm bioactivity via functional assays (e.g., cell proliferation, enzyme activity).

If protein is insoluble:

Solution: test alternative fusion tags (e.g., MBP, GST, Trx, Fh8, CSQ-tag) or use co-expression of solubility enhancers.

If low yield:

Solution: optimize induction temperature (lower temp improves solubility), reduce IPTG concentration, or extend induction time.

For contamination (e.g., NADH oxidation activity in MBP-fusion proteins):

Solution: add 0.1% Triton X-100 and 2% glycerol to buffer or switch to His-tag + Ni-NTA purification.

If poor elution:

Solution: increase imidazole concentration or adjust pH.

For incomplete tag removal:

Solution: ensure sufficient protease concentration and incubation time; validate cleavage efficiency via Western blot.

If protein aggregates during purification:

Solution: include reducing agents (e.g., DTT) or chaperones in lysis buffer.