Transporters reflected in specific properties of nonconventional yeasts
Some yeast species survive extreme changes in the environmental pH, temperature or osmotic pressure. We aim to identify and characterize specific transporters whose activity contributes to the ability to survive adverse environmental conditions. Acquired knowledge will help to improve the properties of yeast species used in industrial processes.
Yeasts occupy various habitats in the nature where they are exposed to changes of their environment. The most frequent are changes in the concentration of osmolytes (salts, sugars etc.). To prevent damages and survive stressful conditions, yeast cells respond with a series of physiological processes that aim to adapt to new conditions. One of these mechanisms is the increased synthesis or efflux of osmoprotective compounds to compensate the changes in the surrounding osmotic pressure. The small molecule of glycerol plays this role in most of the yeast species. The environment with high concentration of salts or sugars is called hyperosmotic and upon hypoosmotic conditions, the concentration of solutes is much higher inside the cells than outside, e.g. on pure water. Yeast species, which are able to survive very well the sudden changes in osmotic conditions are called osmotolerant. One of the aims of our department is to elucidate the mechanisms responsible for the high osmotolerance of some non-conventional yeast species (Zygosaccharomyces rouxii, Debaryomyces hansenii, Pichia farinosa etc.), and the improvement of osmotolerance of species used in the industry, e.g. Saccharomyces bayanus, S. kudravzevii, S. paradoxus or Dekkera bruxellensis.
Changes in the intracellular glycerol content upon the hypo- and hyperosmotic stresses
Exposure of cells to hypoosmotic stress results in a rapid influx of water (in the direction of water concentration gradient) and cells swell. To avoid rupture and to obtain normal cell volume, cells rapidly release the intracellular glycerol (via the Fps1 channel). On the other hand, hyperosmotic stress causes enormous loss of intracellular water and cells shrink. In response to these changes cells start to synthetize glycerol, close Fps1 channels to prevent efflux of glycerol and activate the uptake of glycerol via the Stl1 transporter from the extracellular environment. When the normal cell volume is restored and the protection mechanisms stabilized, cells start to grow and divide.
Changes in cell volume and glycerol transport upon hypo- and hyperosmotic stress.
Comparison of yeast growth upon hyperosmotic conditions.
Osmotolerance is the ability to survive in environment with high concentrations of salts or sugars. Our results show that osmotolerant yeasts are not only able to survive these conditions, but many of them require small concentrations of salt for their optimal growth. For example yeast species as D. hansenii, P. farinosa and Z. rouxii grow very well in the presence of high concentration of salts (2 M NaCl = 6 %; for comparison salty food contains maximally 2 % NaCl), (Bubnova et al., 2014). The exceptionality of these species is based in their ability to effectively transport glycerol from the environment into the cells.
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The model yeast S. cerevisiae has two transport systems for glycerol, encoded by FPS1 and STL1 genes. Fps1 channel is used for rapid efflux of glycerol upon hypoosmotic conditions. Contrary, the Stl1 transporter is empoyed for an active transport of glycerol into the cells. The expression and activity of Stl1 is not high enough in S. cerevisiae, thus this yeast is relatively osmosensitive. The non-conventional osmotolerant yeast species are assumed to possess highly active transporters for glycerol uptake. The aim of our work is to characterize these efficient transporters in non-conventional yeast species and elucidate their contribution to the high osmotolerance.
Increased production of glycerol in cells of Z. rouxii without Stl1 transporter
Cells of the osmotolerant yeast Z. rouxii with functional Stl1 glycerol transporter (WT) produce less glycerol than calls lacking this transporter (stl1∆). This result confirms the role of Stl1 in osmoregulation in this yeast species (Duskova et al., 2015).
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