Gis4, a new component of the ion homeostasis system in the yeast Saccharomyces cerevisiae.

Gis4 is a new component of the system required for acquisition of salt tolerance in Saccharomyces cerevisiae. The gis4 Delta mutant is sensitive to Na+ and Li+ ions but not to osmotic stress. Genetic evidence suggests that Gis4 mediates its function in salt tolerance, at least partly, together with the Snf1 protein kinase and in parallel with the calcineurin protein phosphatase. When exposed to salt stress, mutants lacking gis4 Delta display a defect in maintaining low intracellular levels of Na+ and Li+ ions and exporting those ions from the cell. This defect is due to diminished expression of the ENA1 gene, which encodes the Na+ and Li+ export pump. The protein sequence of Gis4 is poorly conserved and does not reveal any hints to its molecular function. Gis4 is enriched at the cell surface, probably due to C-terminal farnesylation. The CAAX box at the C terminus is required for cell surface localization but does not seem to be strictly essential for the function of Gis4 in salt tolerance. Gis4 and Snf1 seem to share functions in the control of ion homeostasis and ENA1 expression but not in glucose derepression, the best known role of Snf1. Together with additional evidence that links Gis4 genetically and physically to Snf1, it appears that Gis4 may function in a pathway in which Snf1 plays a specific role in controlling ion homeostasis. Hence, it appears that the conserved Snf1 kinase plays roles in different pathways controlling nutrient as well as stress response.