REGULATION OF SALT TOLERANCE IN FISSION YEAST BY A PROTEIN-PHOSPHATASE-Z-LIKE SER THR PROTEIN PHOSPHATASE/

In the yeast Saccharomyces cerevisiae, Na+ efflux is mediated by the E na1 ATPase, and the expression of the ENA1 gene is regulated by the Pp z1 and Ppz2 Ser/Thr protein phosphatases. On the contrary, in the fiss ion yeast Schizosaccharomyces pombe, effective output of Na+ is attrib uted to the H+/Na+ antiporter encoded by the sod2 gene. We have isolat ed a S. pombe gene (pzh1) that encodes a 515-amino-acid protein that i s 78% identical, from residue 193 to the COOH terminus, to the PPZ1 an d PPZ2 gene products. Bacterially expressed Pzh1p shows enzymatic char acteristics virtually identical to those of recombinant Ppz1p. When ex pressed in high-copy number from the PPZ1 promoter, the pzh1 ORF rescu es the caffeine-induced lytic defect and slightly decreases the high s alt tolerance of S. cerevisiae ppz1 Delta mutants. Disruption of pzh1 yields viable S. pombe cells and has virtually no effect on tolerance to caffeine or osmotic stress, but it renders the cells highly toleran t to Na+ and Li+, and hypersensitive to K+. Although lack of pzh1 resu lts in a 2-3-fold increase in sod2 mRNA, the pzh1 mutation significant ly increases salt tolerance in the absence of the sod2 gene, suggestin g that the phosphatase also regulates a Sod2-independent mechanism. Th erefore, the finding of a PPZ-like protein phosphatase involved in the regulation of salt tolerance in fission yeast reveals unexpected aspe cts of cation homeostasis in this organism.