Investigation of Pathways that Regulate the Expression of Recombinant Proteins in Yeast

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Developments in biotechnology over the last half century have revolutionized our lives by enabling a broad range of novel medical and industrial applications. One key component has been our ability to manipulate cells to produce "recombinant" proteins that are not normally expressed by the host cells. These host cells, used like factories, can mass-produce a large array of recombinant proteins that can be used as drugs or catalysts. For instance, human insulin and erythropoietin are produced in cell factories to treat diabetes and anemia, respectively. Similarly, host cells can produce industrial enzymes like proteases (enzymes that break proteins), which are added to laundry detergent. A major issue is that only a small regiment of host cells is available, and many potentially novel recombinant proteins cannot be properly expressed using currently available host cells. This hurdle averts the discovery of new biotechnology applications. One reason for this is that recombinant proteins cannot always reach their 3D conformation (each protein adopts a specific shape for its function) due to a lack of appropriate support in the host cellular environment. These recombinant proteins then misfold (i.e. adopt a wrong conformation), which triggers their degradation in host cells. We noticed that minor genetic variations in yeast cells can have dramatic effects in the stability of model proteins that we have studied. Therefore, we propose to take advantage of yeast biodiversity and large collections of yeast mutant strains to screen for potential novel host cells that display enhanced capacity to express a given recombinant protein. We will investigate which yeast host cells can produce higher levels of a family of enzymes that are notably used for bioremediation of waste water, in food processing and biosensors. Many of these enzymes that originate from fungi or plant can be expressed and secreted from yeast host cells. We have screened a large library of diverse natural or industrial yeast cells and we have identified several strains from different origins that secrete higher levels of the assessed enzyme. We will further characterize these strains to determine why that is the case. In parallel, we will screen a large library of mutant cells to determine which mutations cause higher enzyme expression. Using this information, we will bioengineer further optimized host cells for recombinant expression. Finally, we will establish a new method to assess potentially new host cells to see how broadly they can be used for recombinant protein expression. Notably, we will express together a mixed pool of related but different recombinant enzymes and determine which ones are secreted or not in a given assessed strain. One of our goals is to establish new screening methods to help design new host cells. In parallel, we also want to uncover potentially novel biological pathways that regulate the cellular environment and protein homeostasis.