Development of a screening platform for directed evolution using the reef coral fluorescent protein ZsGreen as a solubility reporter.

Soluble proteins, with high expression levels, are preferred candidates for structural and functional studies. In cases of low expression, aggregation or inclusion body formation, time-consuming searches for optimal expression or refolding conditions are required. We have developed a high-throughput solubility engineering and screening platform for proteins that are expressed in an insoluble form in Escherichia coli with the aim of obtaining a broad spectrum of best hits with increased solubility in difficult to express target proteins. This process has been developed using error-prone PCR to introduce random base changes in genes of interest. Expression of mutated proteins in fusion with the reef coral fluorescent protein ZsGreen as a solubility marker has enabled the selection of more soluble variants. We have used a colony picker to achieve high-throughput selection of E.coli expressing more soluble target protein-ZsGreen fusions, with increased fluorescence. The whole process enables us to complete one round of mutation, screening and analysis of 20,000 potential soluble clones within approximately 8 weeks. We describe the development of the methods using different model proteins and show one example, the kinase domain from the human EphB2 receptor, as a successful application of the whole platform.