BOTH ISOFORMS OF PROTEIN PHOSPHATASE-Z ARE ESSENTIAL FOR THE MAINTENANCE OF CELL-SIZE AND INTEGRITY IN SACCHAROMYCES-CEREVISIAE IN RESPONSETO OSMOTIC-STRESS

The sequences of two genes encoding the protein-serine/threonine-phosp hatases PPZ1 and PPZ2 from Saccharomyces cerevisiae have been determin ed. The molecular masses of PPZ1 and PPZ2 are 77.5 and 78.5 kDa, respe ctively, and each protein consists of two distinct domains. The C-term inal half of each molecule is 93% identical in PPZ1 and PPZ2, and comp rises the protein-phosphatase catalytic domain, while the N-terminal h alves, which are rich in serine and asparagine (PPZ1) or serine and ar ginine (PPZ2), are only 43% identical. Both N-termini start with the a mino acids Met-Gly-Asn, suggesting that after removal of the initiatin g methionine, the N-terminal glycine of the mature protein is myristoy lated. Disruption of the gene encoding either PPZ1 or PPZ2 leads to an increase in cell size and cell lysis, the latter being more pronounce d in cells disrupted in PPZ1. Haploid cells carrying a double disrupti on of PPZ1 and PPZ2 genes also show a marked increase in cell size and cell lysis, which can be significantly reduced by the addition of 1 M sorbitol to the growth medium. These results suggest that PPZ1 and PP Z2 play a role in regulating osmotic stability.