Enhanced Green Fluorescent Protein (eGFP) is an engineered variant of the wild-type green fluorescent protein originally discovered in the jellyfish Aequorea victoria. It is an indispensable non-invasive biomarker characterized by bright, stable fluorescence with an excitation peak at 488 nm and an emission peak at 509 nm. Enhanced green fluorescent protein allows scientists to track gene expression, visualize cellular localization, and monitor protein-protein interactions in real time.
As with every other recombinant protein, the production of enhanced green fluorescent protein requires a recombinant expression system.
Escherichia coli is the most commonly used expression system due to its rapid doubling time, well-characterized genetics, inexpensive media requirements, and capacity for high-density, high-yield protein production.
Upstream processing begins with vector design. The eGFP gene is cloned into an expression plasmid, such as a pET vector, under the control of a strong T7 promoter.
A hexahistidine tag (6x-His) is incorporated at either the N- or C-terminus to simplify downstream purification. A specific protease cleavage site, such as TEV protease, is often placed between the tag and eGFP to allow optional tag removal after purification.
The recombinant plasmid is transformed into an expression strain, such as E. coli BL21(DE3). E. coli is grown in rich media such as Luria-Bertani (LB). The goal is to achieve mid-log phase. (OD600 ≈ 0.6 – 0.8). Isopropyl β-D-1-thiogalactopyranoside is added to induce recombinant protein expression.
Following induction, the culture temperature is lowered to 18–25 °C for overnight incubation. Reducing temperature slows down protein synthesis. This provides eGFP with the time required to fold its chromophore correctly. This controlled expression rate prevents the protein from aggregating into insoluble inclusion bodies.
Once overnight incubation is finished, downstream processing begins by harvesting the bacterial biomass. The E. coli culture is centrifuged at low speed (4,000–6,000 × g) to pellet the cells, after which the media supernatant is discarded.
Releasing intracellular eGFP into solution requires a combination of physical and enzymatic methods to break down the bacterial cell wall systematically.
The cell pellet is resuspended in a lysis buffer to facilitate disruption. The cell envelope is then physically ruptured using either ultrasonic waves (sonication) on ice or high-pressure homogenization (French Press).
Lysozyme is added to degrade the peptidoglycan cell wall. Non-ionic detergents assist in solubilizing membranes. DNase I is also introduced to digest genomic DNA released during disruption. This step significantly reduces lysate viscosity.
Once the disruption is complete, the crude lysate undergoes high-speed centrifugation (15,000–20,000 × g). Centrifugation settles insoluble debris and aggregates at the bottom of the tube. This leaves a supernatant that contains functional, soluble eGFP.
The clarified supernatant contains the 6x-His-tagged eGFP. The supernatant is loaded onto an IMAC column packed with nickel (Ni2+) or cobalt (Co2+) resin. The tags bind to the metal ions, and most host proteins pass through.
A buffer containing a low concentration of imidazole (10 – 20 mM) is used to wash the column. This removes non-specifically bound host contaminants. In the end, enhanced green fluorescent protein is eluted by applying a higher imidazole concentration gradient or step (250 – 500 mM), which competes with the His-tag for binding sites on the metal resin.
Applications such as structural biology require high-purity protein, which requires additional purification steps.
Also known as gel filtration, SEC separates eGFP (≈27 kDa) from higher-molecular-weight aggregates, degradation fragments, and residual imidazole. It also serves as a buffer exchange step.
eGFP has a stable structure and distinct hydrophobic patches, which are advantageous for HIC. The sample is loaded in high-salt conditions and eluted with a decreasing salt gradient. This isolates the properly folded target protein.
The purity of the eluted eGFP is evaluated using Sodium Dodecyl Sulfate–Polyacrylamide Gel Electrophoresis (SDS-PAGE). Following electrophoresis, the gel is stained with Coomassie Blue to visualize the protein sample. Pure eGFP appears as a distinct single band at approximately 27 kDa, indicating the absence of host protein contaminants.
Total protein concentration is determined using standard colorimetric assays such as the BCA (Bicinchoninic Acid) or Bradford assay to measure the final yield. Spectrophotometry can also be used to measure absorbance directly at 488 nm. The exact yield is calculated via eGFP's known molar extinction coefficient.
Proper folding is essential for signal intensity. A fluorometer or microplate reader confirms functional activity. Excitation and emission spectra are recorded to verify peak maxima near 488 nm and 509 nm. This ensures that the purified recombinant protein is fully functional.
If the protein aggregates, lower the temperature to 18 °C. The issue of inclusion bodies can also be resolved by implementing a denaturing-refolding protocol with urea or guanidine hydrochloride.
To prevent enzymatic degradation of eGFP, add a broad-spectrum protease inhibitor cocktail to the lysis buffer. Additionally, maintain all samples at 4 °C throughout cell disruption and downstream purification.
For sensitive downstream applications like X-ray crystallography, incubate the purified protein with recombinant TEV protease to cleave the 6x-His tag. Afterward, run a reverse-IMAC step to separate and collect the pure, untagged eGFP.
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