A tripartite fusion system for the selection of protein variants with increased stability in vivo.

We describe here a genetic selection system that directly links protein stability to antibiotic resistance, allowing one to directly select for mutations that stabilize proteins in vivo. Our technique is based on a tripartite fusion in which the protein to be stabilized is inserted into the middle of the reporter protein β-lactamase via a flexible linker. The gene encoding the inserted protein is then mutagenized using error-prone PCR and the resulting plasmid library plated on media supplemented with increasing concentrations of β-lactam antibiotic. Mutations that stabilize the protein of interest can easily be identified on the basis of their increased antibiotic resistance compared to cells expressing the unmutated tripartite fusion.