Expression of an immunocomplex consisting of Fc fragment fused with a consensus dengue envelope domain III in Saccharomyces cerevisiae

Expression of virus-like particles (VLPs) of foot-and-mouth disease virus (FMDV) using Saccharomyces cerevisiae

We engineered Saccharomyces cerevisiae to express structural proteins of foot-and-mouth disease virus (FMDV) and produce virus-like particles (VLPs). The gene, which encodes four structural capsid proteins (VP0 (VP4 and VP2), VP3, and VP1), followed by a translational "ribosomal skipping" sequence consisting of 2A and protease 3C, was codon-optimized and chemically synthesized. The cloned gene was used to transform S. cerevisiae 2805 strain. Western blot analysis revealed that the polyprotein consisting of VP0, VP3, and VP1 was processed into the discrete capsid proteins. Western blot analysis of 3C confirmed the presence of discrete 3C protein, suggesting that the 2A sequence functioned as a "ribosomal skipping" signal in the yeast for an internal re-initiation of 3C translation from a monocistronic transcript, thereby indicating polyprotein processing by the discrete 3C protease. Moreover, a band corresponding to only VP2, which was known to be non-enzymatically processed from VP0 to both VP4 and VP2 during viral assembly, further validated the assembly of processed capsid proteins into VLPs. Electron microscopy showed the presence of the characteristic icosahedral VLPs. Our results clearly demonstrate that S. cerevisiae processes the viral structural polyprotein using a viral 3C protease and the resulting viral capsid subunits are assembled into virion particles. KEY POINTS: • Ribosomal skipping by self-cleaving FMDV peptide in S. cerevisiae. • Proteolytic processing of a structural polyprotein from a monocistronic transcript. • Assembly of the processed viral capsid proteins into a virus-like particle.

Metabolic engineering of Saccharomyces cerevisiae for the biosynthesis of a fungal pigment from the phytopathogenic fungus Cladosporium phlei

BACKGROUND

Cladosporium phlei is a phytopathogenic fungus that produces a pigment called phleichrome. This fungal perylenequinone plays an important role in the production of a photosensitizer that is a necessary component of photodynamic therapy. We applied synthetic biology to produce phleichrome using Saccharomyces cerevisiae.

RESULTS

The gene Cppks1, which encodes a non-reducing polyketide synthase (NR-PKS) responsible for the biosynthesis of phleichrome in C. phlei, was cloned into a yeast episomal vector and used to transform S. cerevisiae. In addition, a gene encoding a phosphopantetheinyl transferase (PPTase) of Aspergillus nidulans was cloned into a yeast integrative vector and also introduced into S. cerevisiae for the enzymatic activation of the protein product of Cppks1. Co-transformed yeasts were screened on a leucine/uracil-deficient selective medium and the presence of both integrative as well as episomal recombinant plasmids in the yeast were confirmed by colony PCR. The episomal vector for Cppks1 expression was so dramatically unstable during cultivation that most cells lost their episomal vector rapidly in nonselective media. This loss was also observed to a less degree in selective media. This data strongly suggests that the presence of the Cppks1 gene exerts a significant detrimental effect on the growth of transformed yeast cells and that selection pressure is required to maintain the Cppks1-expressing vector. The co-transformants on the selective medium showed the distinctive changes in pigmentation after a period of prolonged cultivation at 20 °C and 25 °C, but not at 30 °C. Furthermore, thin layer chromatography (TLC) revealed the presence of a spot corresponding with the purified phleichrome in the extract from the cells of the co-transformants. Liquid chromatography (LC/MS/MS) verified that the newly expressed pigment was indeed phleichrome.

CONCLUSION

Our results indicate that metabolic engineering by multiple gene expression is possible and capable of producing fungal pigment phleichrome in S. cerevisiae. This result adds to our understanding of the characteristics of fungal PKS genes, which exhibit complex structures and diverse biological activities.

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-EDIII₂ (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-EDIII₂. 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.