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I wholly agree with the below. I am not sure how well E.coli can correctly fold snaky misfolded/unfolded protein that are chaperoned by folded tags! Not to rule out that tags do it sometimes... "Folded by association" for insoluble proteins has often not worked well for me. Sometimes, when it 'works' for me and my colleagues, removal of the tag leads to insoluble protein/aggregation etc. I am dealing with SUMO tagged proteins that have enhanced solubility but severe degradation issues. Screening for pH, buffers and all the good-old stuff folks have suggested here is a good approach. Sometimes autoinduction protocols, which keep from 'overexpression' of protein in the cell might be an approach to explore after the above tests. Good luck! Raji >First of all, using a carrying protein (like GST, MBP) can be disconcerting. >These proteins are very soluble and can solubilize an insoluble protein in >testing condition. So you have something soluble but your protein of >interest can be misfolded or can precipitate when the carrying protein was >cleaved. So keep in mind that a soluble carried protein is not always a good >protein.