1. Response to nutritional environmental stress

   The nutrient sources in the environment that microorganisms need to survive are oscillating, exposing them to nutrient starvation and excess. Microorganisms adapt to these nutritional stresses by reorganizing their own components. We are studying the role of proteolysis (autophagy, endocytosis, etc.) in this dynamic restructuring of cells, using budding yeast as a model.

2. Production of recombinant proteins using koji mold

   The koji mold (Aspergillus oryzae) secretes and produces a large amount of hydrolytic enzymes that break down high-molecular weight compounds such as starch and protein contained in grains. Because of this characteristic, it has been used in the brewing and fermentation industry in Japan for a long time (and is now recognized as the “national microbes” of Japan). We are conducting research on the development of recombinant protein production using the high enzyme productivity of koji mold.

• Elucidation of the degradation mechanism of membrane transporters during the transition from respiration to fermentation in yeast

The yeast Saccharomyces cerevisiae has been used for basic research as a model organism for eukaryotes and has long been used for alcoholic fermentation around the world. S. cerevisiae is also an unusual creature in that it ferments alcohol in the presence of glucose and other fermentable carbon sources (in nature, fruits, tree sap and flower nectar) even in the presence of oxygen. Even if other carbon sources are available, the fermentable carbon source is preferentially consumed. When the fermentable carbon source in the environment is exhausted, the energy production shifts from glycolytic-fermentative to respiration-based energy production, and the gene expression changes drastically. Jen1 is a lactate/pyruvate transporter localized in the plasma membrane, and its expression is significantly activated by the conversion from fermentation to respiration (i.e., glucose depletion). However, when a fermentable carbon source is replenished, Jen1 on the plasma membrane is rapidly degraded. In our laboratory, we have clarified the mechanism by which Jen1 is selectively degraded in response to glucose. When respiring yeast cells are exposed to glucose, the C-terminal region of Jen1 is recognized and ubiquitinated by the Rod1-Rsp5 ubiquitin ligase complex, leading to its degradation in the vacuole via endocytosis. When this C-terminal region (20 amino acid residues) was fused to another differentially regulated transporter (methionine transporter), it was degraded in a glucose- and Rod1-dependent manner. In other words, this region is a Rod1-dependent glucose-responsive degron (degradation control region). These results are expected to contribute not only to the elucidation of the regulation of nutrient uptake in response to changes in nutrient sources in S. cerevisiae, but also to the elucidation of the regulation of sugar uptake in mammals.