Part:BBa_K608408

GST tag

Results

The non-purified, sonificated supernatant of a 50ml E.coli culture pellet was resuspended in 7.5ml PBS. This sample delivered 319,4 µg/µl of protein detected with a Bradford assay. A second identical sample was GST-purified with a Glutathion-Sepharose pull down assay and resulted in 1ml of sample with a protein concentration of 1,6 µg/µl. However both concentrations were a little above the straight calibration line values. Therefore the results are not perfectly quantified, but the numbers are still significant enough to provide a good proof of the functionality of the assay.

Then the GFP absorbance of 100µl of supernatant of both samples was measured with a plate reader:

Non-purified: 180630

GST-purified: 243011

Thus the ratio of GFP absorbance / protein concentration is:

Non-purified: 565

GST-purified: 143540

If the values of the Bradford assay are assumed as correct, the purified sample has a 35.8X higher GFP concentration than the non-purified sample. Due to 3 washing steps it can be assumed that there is barely any other protein left except GFP. Furthermore it can be estimated that only 17% of the total GFP amount present in the 50ml cell culture, could be captured using this assay.

Methods

Plate reader:

The fluorescence intensity and protein concentration were measured with the FLUOstar Omega, which is a multi-mode microplate reader.

Samples were pipetted into the microplate and analyzed via the plate reader. In this experiment we focused on the protein concentration and the fluorescence intensity of GFP.

Bradford-assay: A method to determine the total protein concentration.

To analyze the protein concentration of the samples, Coomassie Brillant Blue was pippeted to each sample. With the binding of the dye to the proteins the color changes from dark red to blue. The more protein in the solution the more Coomassie dye can bind to proteins and the more the color changes into blue. The absorption of bound Coomassie dye is 595nm. The absorbance is proportional with the amount of bound dye. With a series of Bovine Serum Albumin (BSA) measurements the exact protein concentration of the samples can be determined. BSA acts like a “marker” because the concentration of BSA is known and with a linear calibration line the exact protein concentration can be detected.

Usage and Biology

Prepare construct

Cloning: Clone the GST Biobrick part BBa_K608408 beforte the N-terminus of the protein you wish to purify. You may want to put a Glycine linker inbetween the two parts, for better expression and folding proterties of the fusion protein. DO NOT USE THE RFC10 STANDARD, but another, which allows fusion proteins. We recommend the Gibson Assembly, it worked conviently for us.

Vector: You may want to use a vector especially designed for protein expression. The iGEM vectors are all high copy plasmids, which can lead to an overload of protein in your E. coli cells. This can result in toxic effects or inclusion bodies.

Transformation: you may want to chose a E. coli strain especially designed for protein expression. It is helpful for a successfull protein purification to use a protease deficient strain such as BL21.

Cell culture and Lysis

Grow cells overnight after transformation in 2ml medium.

Inoculate 250mL LB in a 1l flask

Optional: induce with IPTG (500 µL)

centrifuging cells in 50ml falcon tubes

resuspend pellets in 7,5µl ice cold PBS

sonificatie cells (4x 1 min with pause, maximal power, 1 pulse per second)

Add 1,5X Protease inhibitor and 25µl of 10mM Lysozyme and let rotate at room temperature for 30min

Centrifuge lysate for 10min at 500xg

Preparation of glutathione sepharose beads. You may add 1X protease inhibitor to all used PBS.

Gently shake the bottle of sepharose to resuspend the matrix.

Use a pipet to remove sufficient slurry for use and transfer to an 15 ml falcon tube. (Dispense 1.33 ml of original sepharose slurry per ml of final volume required.)

Sendiment the matrix by centrifugation at 500xg for 5 min. Carefully decant the supernatant.

Wash the sepharose by adding 10 ml of cold 1xPBS per 1.33 ml of the original slurry of glutathione sepharose dispensed. Invert to mix. (Sepharose must be thoroughly washed with PBS to remove the 20% ethanol storage solution. Residual ethanol may interfere with subsequent procedures).

Sediment the matrix by centrifugation at 500xg for 5 minutes. Decant the supernatant.

For each 1.33 ml of the original slurry, add 1 ml of 1xPBS. This produces a 50% slurry. Mix well prior to the subsequent pipetting steps. (Sepharose 4B equilibrated with PBS may be stored at 4°C for up to a month.

Purification of fusion proteins

Transfer supernatant from step 2)h) on top of slurry from step 4)f)

Rotate falcon for 30 min

Centrifuge at 500xg for 5min

Collect supernatant in a separate 50ml falcon ( in case the protein does not attach well to the beads, this supernatant can be used for a second purification)

Add 10ml of cold PBS with protease inhibitor to flask with the beads

Rotate falcon for 10 min

Repeat steps c) - f)

Centrifuge at 500xg for 5min

Collect supernatant in a separate 50ml falcon (in case the protein does not attach well to the beads, this supernatant can be used for a second purification)

Eluate beads with 1ml elution buffer (50mM Tris pH 8, 10mM Glutathione) – transfer slurry into a 2ml eppi

Centrifuge eppi at 13000rpm to make a stable pellet

Transfer supernatant to a new eppi. Take care not to take up beads from the pellet.

Optional: repeat j) – l) to wash the rest off the beads

Part:BBa_K608408

Results

Methods

Usage and Biology

Further reading