I would like to extract proteins from E. coli cells under native conditions to eventually incubate the protein solution together with an in vitro purified protein. However, I need to avoid any DNA contaminations.
The only protocol I could find that allows bulk extraction of proteins is the ammonium sulfate precipitation but I could not find anything anywhere if this methods does or does not also precipitate DNA. Does anyone know? It concerns me a bit that there are protocols describing DNA precipitation using ammonium sulfate (but I have to admit I didn't look at the protocols).
Also, suggestions about other methods are very welcome! Particularly if someone knows a protocol that allows native protein extraction and also DNA precipitation from the same sample.
DNA should precipitate at 20% saturation, while most proteins precipitate between 20 and 80% saturation.
If you need to remove DNA, what about addition of DNase?
If you proceed to further purification, at least partly, you'll remove the DNA anyway.
Polyethylene imine (PEI) precipitates DNA and not proteins. People typically use this after breakage of the bacteria, to selectively precipitate the DNA. I forget the percentage. I can look it up if you want. Ammonium sulfate will precipitate DNA and protein.
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