The biotech industry relies on cell factories for production of pharmaceutical proteins, of which several are among the top-selling medicines. There is, therefore, considerable interest in improving the efficiency of protein production by cell factories. Protein secretion involves numerous intracellular processes with many underlying mechanisms still remaining unclear. Here, we use RNA-seq to study the genome-wide transcriptional response to protein secretion in mutant yeast strains. We find that many cellular processes have to be attuned to support efficient protein secretion. In particular, altered energy metabolism resulting in reduced respiration and increased fermentation, as well as balancing of amino-acid biosynthesis and reduced thiamine biosynthesis seem to be particularly important. We confirm our findings by inverse engineering and physiological characterization and show that by tuning metabolism cells are able to efficiently secrete recombinant proteins. Our findings provide increased understanding of which cellular regulations and pathways are associated with efficient protein secretion.
Beta-elemene, a sesquiterpene and the major component of the medicinal herb Curcuma wenyujin, has antitumor activity against various types of cancer and could potentially serve as a potent antineoplastic drug. However, its current mode of production through extraction from plants has been inefficient and suffers from limited natural resources. Here, we engineered a yeast cell factory for the sustainable production of germacrene A, which can be transformed to beta-elemene by a one-step chemical reaction in vitro. Two heterologous germacrene A synthases (GASs) converting farnesyl pyrophosphate (FPP) to germacrene A were evaluated in yeast for their ability to produce germacrene A. Thereafter, several metabolic engineering strategies were used to improve the production level. Overexpression of truncated 3-hydroxyl-3-methylglutaryl-CoA reductase and fusion of FPP synthase with GAS, led to a sixfold increase in germacrene A production in shake-flask culture. Finally, 190.7 mg/l of germacrene A was achieved. The results reported in this study represent the highest titer of germacrene A reported to date. These results provide a basis for creating an efficient route for further industrial application re-placing the traditional extraction of beta-elemene from plant sources.
The yeast Saccharomyces cerevisiae is widely used to produce biopharmaceutical proteins. However, the limited capacity of the secretory pathway may reduce its productivity. Here, we increased the secretion of a heterologous ␣-amylase, a model protein used for studying the protein secretory pathway in yeast, by moderately overexpressing SEC16, which is involved in protein translocation from the endoplasmic reticulum to the Golgi apparatus. The moderate overexpression of SEC16 increased ␣-amylase secretion by generating more endoplasmic reticulum exit sites. The production of reactive oxygen species resulting from the heterologous ␣-amylase production was reduced. A genome-wide expression analysis indicated decreased endoplasmic reticulum stress in the strain that moderately overexpressed SEC16, which was consistent with a decreased volume of the endoplasmic reticulum. Additionally, fewer mitochondria were observed. Finally, the moderate overexpression of SEC16 was shown to improve the secretion of two other recombinant proteins, Trichoderma reesei endoglucanase I and Rhizopus oryzae glucan-1,4-␣-glucosidase, indicating that this mechanism is of general relevance.IMPORTANCE There is an increasing demand for recombinant proteins to be used as enzymes and pharmaceuticals. The yeast Saccharomyces cerevisiae is a cell factory that is widely used to produce recombinant proteins. Our study revealed that moderate overexpression of SEC16 increased recombinant protein secretion in S. cerevisiae. This new strategy can be combined with other targets to engineer cell factories to efficiently produce protein in the future. KEYWORDS protein secretion, mitochondria, ERES, ROS, SEC16T he yeast Saccharomyces cerevisiae is a widely used cell factory for the production of recombinant proteins (1). The advantages of S. cerevisiae are fast growth, easy cultivation, and ability to perform posttranslational modifications and secrete proteins to the extracellular medium, which facilitates the purification of the protein (2, 3). However, S. cerevisiae naturally secretes only a few proteins, such as aspartyl protease, invertase, and ␣-factor pheromone (4, 5); therefore, it has evolved a secretory pathway with a relatively low capacity. Many different strategies have been used to engineer this pathway with the objective of improving heterologous protein production (6-8). When expressing a protein that has a secretory signal peptide, the protein is cotranslationally translocated into the endoplasmic reticulum (ER) lumen, where disulfide bond formation occurs with the initial glycosylation (9). The secretion of recombinant proteins is affected by their signal peptide sequences (10). A synthetic leader sequence increases human insulin precursor production, whereas the use of the ␣-factor leader sequence increased ␣-amylase production (7). Excessive mannose glycosylation of recombinant proteins can reduce the efficiency of protein secretion, and it was recently found that
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