S.E. Hart wrote:
> Hi,
>> I've been trying to express a protein in the cytosol of E. coli. I'm only
> getting very low yields. The protein is predicted to contain a disordered
> region of at least 40 amino acids. Would this be enough to explain the low
> yields? The disordered region is at the very N-terminus - I truncated the
> protein in a loop region after two transmembrane regions. Why do
> disordered regions have a negative effect on expression yields - is it
> just because they are likely to be targets for proteases or are the other
> factors? I've found that adding a tag to the protein greatly increases the
> yields. Is that just because the disordered region is no longer at the
> N-terminus? I appreciate that tags can have other positive effects e.g. on
> the solubility of the protein.
Have you done an SDS-PAGE with the entire lysed bacteria, and is there
no sign of your protein? Often expressed proteins aggregate inside the
bacteria and form "inclusion bodies". If you then use the cytosol as a
source of your protein, yields will, of course, be very poor.
In such a case you isolate the inclusion bodies by centrifugation,
solubilise them in caotropic agents like urea or guanidinium chloride
and then try and re-fold them by slow removal of the caotrope, either by
dialysis or after binding to a column.
Some tags, most notably maltose binding protein (MBP), can reduce the
tendency of expressed protein to form inclusion bodies and keep them in
solution. The molecular mechanism behind this is not well understood,
protein purification is still more art than science.
