Screening Libraries for Improved Solubility: Using E. coli Dihydrofolate Reductase as a Reporter

Low protein solubility is a problem in many areas of protein science. Although chemical methods have been developed to solubilize proteins these are not always effective and add to the cost of producing the protein. One way of overcoming these difficulties is to evolve the protein to be more soluble. A major hurdle in this process is the ability to select mutant proteins with enhanced solubility from a large library of randomly mutated proteins. In this article, we describe such a method. The method relies on the fact that increasing the expression of dihydrofolate reductase (DHFR) makes Escherichia coli resistant to Trimethoprim (TMP). Proteins fused to DHFR will produce chimeras with altered levels of resistance to TMP. This variation in TMP resistance can be used to identify mutant proteins with enhanced solubility.