Yeast Synthetic Minimal Biosensors for Evaluating Protein Production.

The unfolded protein response (UPR) is a highly conserved cellular response in eukaryotic cells to counteract endoplasmic reticulum (ER) stress, typically triggered by unfolded protein accumulation. In addition to its relevance to human diseases like cancer, the induction of the UPR has a significant impact on the recombinant protein production in eukaryotic cell factories, including the industrial workhorseSaccharomyces cerevisiae. Being able to accurately detect and measure this ER stress response in single cells, enables the rapid optimization of protein production conditions and high-throughput strain selection strategies. Current methodologies to monitor the UPR in S. cerevisiae are often temporally and spatially removed from the cultivation stage or lack updated systematic evaluation. To this end, we constructed and systematically evaluated a series of high-throughput UPR sensors by different designs, incorporating either yeast native UPR promoters or novel synthetic minimal UPR promoters. The native promoters of DER1 and ERO1 were identified to have suitable UPR biosensor properties and served as an expression level guide for orthogonal sensor benchmarking. Our best synthetic minimal sensor is only 98 bp in length, has minimal homology to other native yeast sequences and displayed superior sensor characteristics. The synthetic minimal UPR sensor was able to accurately distinguish between cells expressing different heterologous proteins and between the different secretion levels of the same protein. This work demonstrated the potential of synthetic UPR biosensors as high-throughput tools to predict the protein production capacity of strains, interrogate protein properties hampering their secretion, and guide rational engineering strategies for optimal heterologous protein production.