Improving expression and assembly of difficult-to-express heterologous proteins in Saccharomyces cerevisiae by culturing at a sub-physiological temperature.

Background: Escherichia coli heat labile toxin B subunit (LTB) is one of the most popular oral vaccine adjuvants and intestine adsorption enhancers. It is often expressed as a fusion partner with target antigens to enhance their immunogenicity as well as gut absorbability. However, high expression levels of a fusion protein are critical to the outcome of immunization experiments and the success of subsequent vaccine development efforts. In order to improve the expression and functional assembly of LTB-fusion proteins using Saccharomyces cerevisiae, we compared their expression under culture conditions at a sub-physiological temperature 20 °C with their expression under a standard 30 °C.

Results: The assembled expression of LTB-EDIII (LTB fused to the envelope domain III (EDIII) of Dengue virus serotype 2), which was expressed at the level of 20 µg/L in our previous study, was higher when the expression temperature was 20 °C as opposed to 30 °C. We also tested whether the expression and functional assembly of a difficult-to-express LTB fusion protein could be increased. The assembled expression of the difficult-to-express LTB-VP1 fusion protein (LTB fused to VP1 antigen of Foot-and-Mouth Disease Virus) dramatically increased, although the total amount of expressed protein was still lower than that of LTB-EDIII. Slight but significant increase in the expression of well-known reporter protein eGFP, which has previously been shown to be increased by cultivation at 20 °C, was also observed in our expression system. As no significant changes in corresponding transcripts levels and cell growth were observed between 20 °C and 30 °C, we infer that translation and post-translational assembly are responsible for these enhancements.

Conclusions: The effects of lowering the expression temperature from 30 °C to 20 °C on protein expression and folding levels in S. cerevisiae, using several proteins as models, are reported. When heterologous proteins are expressed at 20 °C, a greater amount of (specially, more assembled) functional proteins accumulated than at 30 °C. Although further studies are required to understand the molecular mechanisms, our results suggest that lowering the expression temperature is a convenient strategy for improving the expression of relatively complexly structured and difficult-to-express proteins in S. cerevisiae.