Using yeast to study transport and structure-function relationship in aquaglyceroporins

Paper in proceeding, 2005

Aquaporins are small membrane proteins that transport water while the closely related aquaglyceroporins also can be permeable to polyols, urea and even arsenic. These substances can pass the pore in both directions by facilitated diffusion. Aquaporins are represented in organisms ranging from archaea to human, and their discovery was awarded the Nobel Prize in chemistry in 2003. Eleven different aquaporins (0–10) have been identified in mammals. Of these, AQP3, 7, 9 and 10 are aquaglyceroporins. They are expressed in a tissue-specific manner and play key roles Abstracts 196 in the regulation of water balance. Examples for relevant applications are transpiration, water retention in the kidneys and glycerol transport following fat metabolism. Aquaglyceroporins are probably also an entry point for arsenic in the liver. To study the function of aquaglyceroporins, we have developed a test system in Saccharomyces cerevisiae. When exposed to a hyperosmotic stress, yeast cells uses glycerol as a compatible solute to regain the turgor pressure decreased by water loss. When aquaglyceroporins are expressed in such cells, they cause sensitivity to hyperosmotic stress, due to glycerol loss through the aquaglyceroporins. When expressed in a strain deficient in glycerol production, the sensitivity of that strain to high levels of certain polyols is suppressed because the polyol can be taken up by the cell through the aquaglyceroporin. We have employed this system of conditional osmotic shock to design a genetic screen which has made it possible to identify residues responsible for the regulation of the yeast aquaglyceroporin Fps1. The genetic screen is being further developed to unravel key residues in channel specificity as well as the mode of action of potential inhibitors. We are also using this system to study mammalian aquaglyceroporins.