THE PROTEIN PHOSPHATASE CALCINEURIN IS ESSENTIAL FOR NACL TOLERANCE OF SACCHAROMYCES-CEREVISIAE

NaCl-sensitive yeast mutants were isolated to identify genes essential for NaCl tolerance. Complementation of a mutant highly sensitive to N a+ and Li+ led to the isolation of the CNB1 gene. This gene encodes th e regulatory subunit (CNB) of the Ca2+/calmodulin-dependent protein ph osphatase calcineurin. Cells deficient in CNB accumulated Li+ due to r educed expression of ENA1, a gene encoding a P-type ATPase involved in Na+ and Li+ efflux. In addition, the K+ transport system of cnb1 Delt a cells was not converted to the high affinity state that facilitates better discrimination of K+ over Na+. Thus the cnb1 Delta strain resem bled a trk1 mutant. These results indicate that adaptation to NaCl str ess in Saccharomyces cerevisiae requires a signal transduction pathway involving Ca2+ and protein phosphorylation-dephosphorylation. In this pathway, calcineurin would coordinate gene expression and activity of ion transporters to facilitate ion homeostasis.