The Effect Of Osmotic Stress And Hspi2 On Passive And Active Transport In The Yeast

ABSTRACT The yeast Saccharomyces cerevisiae is used extensively in many important industrial applications that often expose these organisms to a range of stresses. Consequently, a considerable amount research has focused on understanding the nature of stress response in yeast and the development of more stress resistant yeast. The stress protein Hsp12 has since garnered interest, as it is upregulated in response to a wide variety of environmental stresses. Using an Hsp12-GFP construct, we confirmed that Hsp12 was induced in response to the osmolytes mannitol and salt. In the first part of this thesis, we used the non-fluorescent probe fluorescein diacetate to measure membrane permeability after osmotic shock on cells in which the ABC transporters have been inactivated. Intracellular esterases rapidly convert FDA into fluorescein and accumulation of fluorescein inside the cell was used as a measure of FDA diffusion. We also used a fluorophore rhodamine 6G that is commonly used to measure passive diffusion into yeast cells. Fluorescein is a substrate for the ABC transporter Pdrl 2, which is induced in cells subjected to weak acid stress. Contrary to previous research we have shown that cells grown in normal conditions also show ATP-dependent efflux of fluorescein. RT-PCR confirmed that PDR12 was only expressed in cells subjected to weak acid stress, which suggests that our lab strains may constitutively express another ABC transporter. After osmotic shock, the hsp12 cells displayed reduced diffusion rates, which suggest that Hsp12 may maintain the integrity of the plasma membrane thereby reducing permeability. In the second part of the thesis, we investigated the effect of osmotic stress on the activity of ABC transporters. We found that after osmotic shock wild-type cells displayed enhanced rates of fluorescein efflux whereas the opposite was observed for hsp12 cells, which suggest that Hsp12 modulates the activity of ABC transporters during stress. We have also found that hsp12 cells may possess compensatory mechanisms that confer a phenotype similar to that of stressed wild-type cells and therefore mask the effect of the HSP12 deletional mutation.