GST fusion protein - two dominant bands - (May/10/2016 )
Hello. I'm new to GST purification. I expressed a fusion protein: GST-'Protein X'. I can see expression in the soluble lysate upon over-expression. When purified with Glutathione Sepharose, the A280 reading of the eluent would indicate a ~0.5-0.8 mg/mL concentration yet when loading the sample, I'm not getting anywhere near that amount of protein. I blanked with elution buffer.
Additionally, the SDS-PAGE shows a faint band corresponding to the fusion protein and a predominant band around the size of GST alone (which is the same size as 'Protein X'). Does anyone know how I could get GST alone? There is a thrombin cleavage site but I didn't perform the cleavage! Any ideas would be much appreciated!
Do you have triton or anything in your buffer that would absorb at 280nm instead of protein? Or doe the spectrum look more like DNA (max at 260nm)?
Sounds like you've got some proteolytic cleavage happening from endogenous ecoli proteases. This is not as unusual as you would hope.
The two simplest solutions are (i) use protease inhibitors when you lyse your cells or (ii) use shorter inductions times (not sure how long your expressing for but sometimes shortening this time can help).
If those don't work, consider a different fusion protein (SUMO, GFP).
Also consider Prescission Protease site instead of thrombin. This is advantageous because you once you cleave the GST from your protein, you can re-run the mix over the GST column and remove all cleaved GST from your protein as well as the prescission protease. You can't do this with thrombin since it does not come with affinity tags.
Thank you labtastic. After I posted, I was wondering about premature translation termination. The target protein does contain some repeat sequence at the N-term. Thoughts?
I have heard lots of people talk about pre-mature translation termination but I have yet to run across this as a serious issue in my experience.
Nevertheless, if this is the problem, you can solve it by putting in a c-terminal his tag. This way you only purify full-length protein and not GST alone (whether the free GST is caused by proteolysis or pre-mature truncation). Then bind your protein to a GST column if you need and continue as usual.
Do you think that GST located in C-terminal is as effective as in N-terminal in order to improve the solubility of the heterologous protein?
paramyosin on Tue Jul 18 09:57:39 2017 said:
Do you think that GST located in C-terminal is as effective as in N-terminal in order to improve the solubility of the heterologous protein?
Depends entirely on your protein. I have seen several situations in which a C-terminal solubility tag was as effective as N-terminal. I also know of many proteins in which C-terminal anything (affinity or solubility tag) destroys expression and/or activity.
Only one way for you to find out.
Thank you labtastic, in my experience, there is always some (important) production of GST alone, which is contaminating your GST-target protein. It seems to be degradation but I think it is just due to synthesis of the GST alone. This problem would disappear placing the GST in C-terminal.
As the production of the proteins always begins at the N-terminus, I thought that GST located in N-terminus would favour the production of the downstream target protein in a folded and soluble way as a nascent process, but maybe it can confer stabilization to the target protein also in C-terminal as a whole protein.
I will try :)
paramyosin on Wed Jul 19 08:14:22 2017 said:
Thank you labtastic, in my experience, there is always some (important) production of GST alone, which is contaminating your GST-target protein. It seems to be degradation but I think it is just due to synthesis of the GST alone. This problem would disappear placing the GST in C-terminal.
As the production of the proteins always begins at the N-terminus, I thought that GST located in N-terminus would favour the production of the downstream target protein in a folded and soluble way as a nascent process, but maybe it can confer stabilization to the target protein also in C-terminal as a whole protein.
I will try
I agree intuitively it would seem that solubility tags would only be effective at the N-terminus. While this may often be the case, from experience I know this is not always true. C-terminal solubility tags have their place.
All this means that there is way more to transcription/translation/folding than we understand. Which is why it's always best to try the experiment than to talk yourself out of it. It's always probably why we still have jobs. ;)