The Saccharomyces cerevisiae phosphotyrosyl phosphatase activator proteinsare required for a subset of the functions disrupted by protein phosphatase 2A mutations

In Saccharomyces cerevisiae, PTPA is encoded by two genes, YPA1 and YPA2. I n order to examine the biological role of PTPA as potential regulator of pr otein phosphatase 2A (PP2A), we compared the phenotypes of the ypa Delta mu tants with these of PP2A-deficient strains. While deletion of both YPA gene s is lethal, deletion of YPA1 alone results in a phenotype resembling that of PP2A-deficient strains in specific aspects such as aberrant bud morpholo gy, abnormal actin distribution, and similar growth defects under various g rowth conditions. These phenotypes were even more pronounced when YPA1 was deleted in a pph21 Delta genetic background. Moreover, ypa Delta mutants ar e hypersensitive to nocodazole and show inappropriate mitotic spindle forma tion as previously described for mutants in the catalytic subunit of PP2A, suggesting that Ypa, like PP2A, has a function in mitotic spindle formation . These results are consistent with an in vivo role of Ypa as a regulator o f PP2A However, unlike a PP2A-deficient strain, ypa Delta mutants do not sh ow a Ga arrest. Therefore, Ypa does not seem to play a role in the regulati on of PP2A at this stage of the cell cycle. These results imply that Ypa re gulates a specific subset of PP2A functions, possibly by controlling the su bunit composition of PP2A (C) 2001 Academic Press.