Lic4, a nuclear phosphoprotein that cooperates with calcineurin to regulate cation homeostasis in Saccharomyces cerevisiae

The target of the immunosuppressants cyclosporin A(CsA) and FK506 is calcineurin, a highly conserved protein phosphatase that is required for T-cell activation and the regulation of ion homeostasis in yeast. Here we identify two genes, PMR2B and LIC4 which, when overexpressed, suppress the cation-sensitive phenotype of yeast cells lacking calcineurin. PMR2B encodes a Na⁺/Li⁺-specific plasma membrane pump and is similar to PMR2A, whose expression is known to be regulated by calcineurin. LIC4 (lithium comvertas) encodes a novel 33-kDa protein with no identity to known proteins. LIC4 overexpression suppresses the Li⁺-sensitive phenotype of calcineurin mutants but not the defect in recovery from pheromone arrest or viability of calcineurin dependent mutants, indicating a specific role in cation homeostasis. Similarly, lic4 mutations increase the Li⁺ sensitivity of both wild-type and calcineurin mutant strains, and reduce expression of pmr2A in calcineurin mutant strains, indicating that calcineurin and Lic4 may regulate parallel cation homeostatic pathways. lic4 mutations also exacerbate the Li⁺-sensitive phenotype of hal3 mutant strains, and overexpression of either Lic4 or Hal3 suppresses the salt sensitivity of mutant strains lacking calcineurin, Hal3, or Lic4, either singly or in combination. Taken together, these observations suggest that calcineurin, Hal3, and Lic4 cooperatively regulate the response of yeast cells to?cation stress. Lic4 is phosphoprotein in vivo and a calcineurin substrate in vitro. By indirect and direct immunofluorescence detection of HA- and GFP-tagged proteins, Lic4 is localized in the nucleus in wild-type cells but predominantly cytoplasmic in cells lacking calcineurin. Taken together, our findings support a model in which calcineurin and Lic4 are components of signalling cascades that regulate cation stress responses in yeast.