I am purifying a GST-tagged protein, and cleaving the tag with TEV protease. My buffer contains 20 mM HEPES, pH 7.5, 300 mM NaCl, 5 mM DTT. There is 75 mM reduced glutathione around from elution, and I add 0.5 mM EDTA for digestion. I incubated overnight (ratio 1 mg enzyme to 10 mg protein) at 4 degrees.
In the morning, the solution was cloudy. So I spun out the precipitate, and took samples to analyze by SDS-PAGE. Some of my protein precipitated, but most (90%) stayed in solution. The 27 kDa TEV protease precipitated, and this was evident because most of my fusion protein was not cleaved. Another lab member used the same prep of protease (different protein), and had no problems.
Anyone have any ideas as to what is going on here? Has anyone ever heard of TEV protease precipitating?