Small Scale GST-Tag Purification Under Nature Conditions

互联网2013-09-06

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<center> Small Scale GST-Tag Purification Under Nature Conditions According to AMERSHAM-BIOSCIENCES-GST fusion protein Handbook (pdf) </center>

<big><big>Aliquot of Cell Pellet after Induction </big> </big>

The idea is to aliquot cells after induction, and keep at -80ºC enough cell pellet samples for optimization of small scale purification procedure and further scale-up. Once you set up the best purification conditions at low scale, you can scale-up the procedure.

Example:
1)   Grow 1L culture
2)   Induce (IPTG, salt induction, etc. etc.)
3)   Spin cell culture 10min 8000rpm 4ºC, discharge supernatant
4)   Resuspend cell pellet at 4ºC very gently with 100ml cold PBS buffer. Aliquot as following:
            a) 10 tubes (1.5ml plastic tubes) with 1ml suspension (it means 10ml original culture per tube);
            b) 4 tubes (15ml plastic tubes) with 10ml suspension (it means 100ml original culture per tube)
            c) 1 tube (50ml plastic tube) with 50ml suspension (it means 500ml original culture).
5)   Spin 10min 8000rpm 4ºC, discharge supernatant
6)   Keep cell pellet at -80º

Equilibration of Glutathione Agarose

Place 20ul beads (40ul suspension) of glutathione agarose in 1.5ml plastic tube.

Wash with 2 x 1.5ml H₂ O and 2x 1.5ml lysis buffer (washing: mix, spin 3min 3500rpm, discharge supernatant).

Protein Extraction

Resuspend pellet of 10ml cell culture in 1ml lysis buffer (or 100ml bacterial culture for very low expression level).

Suggested Lysis buffer : 140mM NaCl; 2.7mM KCL; 10mM Na₂ HPO₄ ; 1.8mM KH₂ PO_4; pH 7.3 (PBS)
optional 0.02% NaN₃ (azide)
optional protease inhibitors

Optional additives to the lysis buffer
a) 1mM PMSF or protease inhibitor cocktail 1:200 (cocktail for bacterial cells #P-8849 from Sigma)
b) Dnase 100U/ml or 25-50ug/ml (SIGMA DN-25). Incubate 10min 4°C in the presence of 10mMMgCl₂ .
c) Lysozime 0.2mg/ml. Incubate 10min 4°C.
d) ßME, DTT or DTE up to 10mM for proteins with many cysteines.
e) 0.1-2% Triton X-100, NP40; or any other detergent that do not affect the biological activity of your protein.

Sonicate in ice bucket 3 x 10sec or more if the cells are not completely disrupted (Lysis is complete when the cloudy cell suspension becomes translucent. Avoid protein denaturation by frothing).

Spin 5min 13000rpm 4°C. Separate soluble proteins (supernatant) from insoluble or inclusion bodies proteins (pellet). Use supernatant for next step. Keep sample of 40ul of supernatant for PAGE-SDS: soluble proteins

Resuspend pellet in another 1ml lysis buffer and keep sample of 40ul for PAGE-SDS: insoluble proteins, or unlysed cells .

Protein Purification

1) Mix supernatant of last step gently with the equilibrated resin 60 min at 4°C.

2) Spin 3min 3500rpm 4°C. Discharge supernatant and keep sample of 40ul for PAGE-SDS: unbound proteins (this material could be use again in case of overloading).

3) Wash beads with 2x1.5ml PBS (washing: mix, spin 3min 3500rpm, keep supernatant aside). Keep sample of 40ul for PAGE-SDS of each washing .

4) Elute recombinant protein with 3 x 50ul elution buffer: 50mM TrisHCl pH8.0 10mM reduce glutathione (mix gently, incubate at RT for 5min for each elution, spin 3min 3500rpm , keep supernatant aside). Keep sample of 40ul for PAGE-SDS of each elution .

5) Resuspend beads in 50ul H₂ O + 20ul 5x sample buffer. Mix and spin. Keep sample of 40ul for PAGE-SDS: protein not eluted (or SDS extracted beads) .

6) Run on PAGE-SDS: crude supernatant; resuspended pellet; unbound, washings, elutions, and SDS extracted beads. MW of GST alone: 29000

Analysis of results - Troubleshooting

Expect over-expressed protein to be found only in the crude supernatant and in the elution of the Glutathione agarose.

If most of the protein remains insoluble after extraction, try
a) To change lysis buffer by adding ßME, DTT, glycerol, detergents or more NaCl. If only part of it is insoluble,
b) Or re-extract pellet with more buffer,
c) Or use more lysis buffer during extraction,
d) Or perform a more intensive sonication,
e) Or incubate with lysozyme before sonication.
f) Or try the denaturating protocol extraction (4-6 M guanidine-HCl; 4-8 M urea; alkaline pH>9.0; 0.5-2% Triton X-100; 0.5-2% N-lauroylsarcosine or other detergents)

If protein does not bind to the Glutathione agarose, there are several options to choose:
a) Check the Glutathione agarose: binding of a cell sonicate containing only GST
b) If only partially bound, use more resin, or bind for longer time (The longer the duration of purification, the greater the risk of protein degradation).
c) Add 1-10mM DTT prior to cell lysis (sometimes can increase significantly the binding); or try additives as glycerol, detergents or more NaCl
d) Purify protein under denaturating conditions.
e) Move tag to the opposite end of the protein.

If fusion protein is poorly eluted, try:
a) Decrease flow rate, or try overnight elution (The longer the duration of purification, the greater the risk of protein degradation)
b) Increase concentration of glutathione. First 15mM and then 20-40mM.
c) Increase ionic strength to 0.1-0.2M NaCl.
d) Increase the volume of elution buffer
e) Add a non-ionic detergent (as 0.1% Triton X-100 or 2% N-octyl glucoside) to reduce non-specific hydrophobic interactions that may prevent solubilization or elution of the fusion protein.

If multiple proteins bands are seen in the elution try:
a) If you suspect protein degradation (you can check previously with western blot using antibodies against GST or the fusion protein) try to work all the time at 4°C and use protease inhibitors during lysis. (The longer the duration of purification, the greater the risk of protein degradation)
b) Decrease resin volume (allows higher competition between fusion protein and contaminants for the same sites on the resin)
c) Increase ionic strength to 0.1- 2M NaCl or use detergents that not destroy the protein activity during washing step
d) Increase the volume of the washing step.
e) Consider an additional purification step before or after purification.