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A quick procedure for purification of functional recombinant proteins over-expressed in E.coli.

Expression of cloned cDNAs in E. coli is a widespread means for obtaining recombinant proteins in large quantities. However, a major inconvenience of the system is that, even though over-expressed, the proteins of interest rarely exceed 20% of the total proteins. Painstaking chromatographic procedures and/or potentially detrimental renaturation steps are thus commonly utilized in order to get a native product sufficiently purified for functional analysis. We present here a high salt lysis and fractionation procedure which makes it possible to obtain purified proteins in soluble native form in about 2 hours. This procedure relies on the observation that most over-expressed proteins are more susceptible to precipitation by ammonium sulfate than are bacterial proteins, probably because of the facilitation of homoaggregate formation by the predominant protein species. It also takes advantage of low temperature growth conditions (which excludes the use of expression systems relying on thermal induction) that usually limit inclusion body formation and increase significantly the proportion of soluble recombinant proteins. The proteins partially purified according to this method have been used successfully in functional assays. However, if purification to near homogeneity is required, a single chromatographic step past this stage appears to be sufficient in most cases. Cultures are grown and induced 28°C, using resuspended