GST-protein purification: degradation product - (Apr/14/2007 )

My vote goes with the pwnererr's suggestion of two plasmids in your system. Grow up some cells and do a plasmid prep. You'll see straight away if you have a mixture.
BTW, what proportion of target protein and contaminant are you getting? Does the contaminant bind to the GSH beads? If not, wash really well. If it does stick, let it. When you digest your fusion to release the target protein, the contaminant will stay on the column. Even if it does come off, you can remove it by a sizing column.

---

You may not need to try many type of induction and growing at the beginning. Just grow and induce 1ml of your bacteria as you normally do and go through all the expected steps. Run gel and commassie blue to see what happened at which step. Then you will consider the possible problem and solution. Keep us update? ^___^

---

Dear everybody,

Thanks for the idea that my protein starts degrading in the cells. It seems to be the case, I see several bands below mine after GST-immunostaining samples of bug, lysate, flow-through. So now I'm trying to minimize degradation during growing. Any ideas are welcome.

What I have done before:
inoculate media 1:10 with started culture
grow it till OD600: 0.6-1.2 shaking at 37C
induce with IPTG 1mM
grow RT overnight

With MagicMedia there is no need for induction, so after inoculation I let it grow RT shaking 24 hrs.

Do you think glucose or shorter growing period could help me?


To some comments:
The degradation product is GST positive and in some cases it is just as strong as the target protein.
Unfortunately it isn't likely that I can get rid of it by cut off, since the little difference between sizes (I've already tried Centricon).
The contaminant happily binds the beads and comes off only during elution. Unfortunately I need the GST-tag fused to my protein for the experiments I need these proteins.

Thanks again

---

Shorten the time my help. I am not sure about temp. Sometimes do just 2h at 37 may have less degradation, sometimes it won't help. i am afraid that you will have to try. What is the GST protein for, anyway, do it have to be very pure? If can't do anything about degradation (I am assuming that you did have pure clone), and you want very pure protein, then you may have to do something like FPLC.

---

Thanks for the ideas, that is what I have started to test. I did a 6-hr RT growing, the yield was bad, but the degradation decreased, too. But I'll definitely try 2-3hrs at 37C (it would be convenient, too). The other thing I'm doing is to play with the IPTG concentration and try adding glucose. Any comment on that is appreciated.

I need a pure GST-tagged protein because I need it for a FRET-based binding assay. So,at least the proportion of the target protein has to be much higher than the degradation.
What is FPLC, by the way? Is it a kind of liquid chromatography? I've been thinking about HPLC or any type of chromatography, but I have no clue, which one is applicable to separate 26 and 37 kDa proteins? I am not familiar with these techniques.

Anyway, I'll try different growing periods and induction and glucose and come back with the protocol, if I find the right one. Thanks for you help.

---

http://en.wikipedia.org/wiki/Fast_protein_..._chromatography

The exact information you have to look for from GE Healthcare website. According to them, the proteins can be separated based on size. Have to ask the expert, I think.

As for IPTG, we normally use from 0.1-1mM IPTG. You should not use higher than 1mM. I think lower the amount of IPTG may help. I am not sure about your express induction. For us, we just pick one colony, dump in 400 ml of media, o/n then purify protein. So one day of growth in total (since pick colony to harvest cells).

I never use glucose before, can't help you with that. Sorry.

Good luck

---

Use about 1mM for IPTG (common concentration). IF you want to play around with the conc, perhaps 0.5mM to 5mM. Glucose is just another supplement. Not necessary needed. Adding another parameter will make things more complicated. Try to grow your bacteria at lower temp, better folding.

Well, GST tag, you need resin for purification. But for really good quality, go for HPLC or FPLC. I think FPLC will be a better choice. Yes, it is a kind of liquid chromatography. But you really must have able to express a very high amount of desired protein before heading for HPLC or FPLC. All the best!

---

Of course they can be separated by size. With a long enough column you can separate anything by size. FPLC is nice but it still doesn't take anyway from the fact that size exclusion sucks and should be avoided at all costs. These days there are soo much better ways to purify protein that by size. There are numerous reasons why size exclusion sucks, they include but are not limited that it is a low resolution and low compacity method and of course it takes forever. Stick to some sort of ion exchanger.

Don't bother adding glucose or playing with the IPTG concentration. These things will not help you and are a complete waste of time. The expression is pretty much all or nothing from these plasmids. That is, 0.1mM IPTG will probably give nearly the same expression level as 1mM. It is not very titratable.

---

Yes, ion exchanger is good, but the problem here is possibly due to degradation. To purify a protein from its own degradation products, ion exchanger may not be as helpful as size separation. Of course, since the size different here is not so good either (26 and 37kD), it will be long and slow, I am afraid.

---

Exactly, you can easily manipulate the pH to make either the 26 or 36kD protein stick to the exchanger. It is at least worth a short before using a size exclusion column.

---