Saccharomyces cerevisiae uses different mechanisms to adapt to changes in environmental osmolarity. Upon hyperosmotic shock, cells first mobilize a rapid rescue system that prevents excessive loss of ions and water; then in the adaptation period they accumulate a compatible solute (glycerol). When subjected to hypoosmotic shock, they rapidly release intracellular stocks of glycerol to reduce intracellular osmolarity and prevent bursting. The plasma membrane Nha1 alkali metal cation/H+ antiporter is not important in helping the cells to survive a sudden drop in external osmolarity, but is involved in the cell response to hyperosmotic shock. For this role, its long hydrophilic C-terminus is indispensable. The capacity of the Nha1 antiporter to transport potassium is regulated by Hog1 kinase. Upon sorbitol-mediated stress, the Nha1p potassium export activity decreases in order to maintain a higher intracellular concentration of solutes. The C-terminal-less Nha1 version is not inactivated and its potassium efflux activity renders cells very sensitive to hyperosmotic shock. Taken together, our results suggest an important role of Nha1p and its C-terminus in the immediate response to hyperosmotic shock as part of the rapid rescue mechanism.

Department of Membrane Transport Institute of Physiology Academy of Sciences CR Videnska 1083 142 20 Prague 4 Czech Republic

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