Stabilizing proteins through saturation suppressor mutagenesis

Abstract

While there have been recent, transformative advances in the area of protein structure prediction, prediction of point mutations that improve protein stability remains challenging. It is possible to construct and screen large mutant libraries for improved activity or ligand binding, however reliable screens for mutants that improve protein stability do not exist, especially for proteins that are well folded and relatively stable. We demonstrate that incorporation of a single, specific destabilizing, (parent inactive) mutation into each member of a deep mutational scanning library followed by screening for suppressors, allows for robust and accurate identification of stabilizing mutations. When coupled to FACS sorting of a yeast surface display library of the bacterial toxin CcdB, followed by deep sequencing of sorted populations, multiple stabilizing mutations could be identified after a single round of sorting. Multiple libraries with different parent inactive mutations could be pooled and simultaneously screened to further enhance the accuracy of identification of stabilizing mutations. Individual stabilizing mutations could be combined to result in a multimutant with increase in thermal melting temperature of about 20 °C and enhanced tolerance to high temperature exposure. The method employs small library sizes and can be readily extended to other display and screening formats to rapidly isolate stabilized protein mutants.