A genetic analysis of the role of calcineurin and calmodulin in Ca++-dependent improvement of NaCl tolerance of Saccharomyces cerevisiae

Abstract

Mutants of Saccharomyces cerevisiae lacking activity of the Ca²⁺/calmodulin-dependent protein phosphatase calcineurin, show sensitivity to high concentrations of sodium that is partly reversed by the external supply of Ca²⁺. On long-time exposure to NaCl stress the mutants display an increased intracellular Na⁺/K⁺ ratio which is partially corrected by the addition of Ca²⁺, improving the sodium efflux of not only calcineurin-defective cells but also wild-type cells. We also demonstrate that the NaCl sensitivity of cmd mutants, expressing modified forms of calmodulin that do not bind Ca²⁺, is strongly reversed by the addition of Ca²⁺. This effect is highly dependent on calcineurin, since the NaCl tolerance of a cmd1-3 strain, carrying an additional mutation in calcineurin, is only slightly assisted by Ca²⁺. A striking characteristic of the loss of function of calcineurin is a several-fold increased content of intracellular Ca²⁺, localized mainly in subcellular compartment(s). If the compartmentalized Ca²⁺ pool is brought back to normal levels by an additional inactivating mutation of the vacuolar Ca²⁺-transporting ATPase, such double mutants do not significantly improve their tolerance to NaCl.