MUTATIONS IN THE SACCHAROMYCES-CEREVISIAE TYPE 2A PROTEIN PHOSPHATASECATALYTIC SUBUNIT REVEAL ROLES IN CELL-WALL INTEGRITY, ACTIN CYTOSKELETON ORGANIZATION AND MITOSIS

Temperature-sensitive mutations were generated in the Saccharomyces ce revisiae PPH22 gene that, together with its homologue PPH21, encode th e catalytic subunit of type 2A protein phosphatase (PP2A). At the rest rictive temperature (37 degrees), cells dependent solely on pph22(ts) alleles for PP2A function displayed a rapid arrest of proliferation. T s(-) pph22 mutant cells underwent lysis at 37 degrees, showing an acco mpanying viability loss that was suppressed by inclusion of 1 M sorbit ol in the growth medium. Ts(-) pph22 mutant cells also displayed defec ts in bud morphogenesis and polarization of the cortical actin cytoske leton at 37 degrees. PP2A is therefore required for maintenance of cel l integrity and polarized growth. On transfer from 24 degrees to 37 de grees, Ts(-) pph22 mutant cells accumulated a 2N DNA content indicatin g a cell cycle block before completion of mitosis. However, during pro longed incubation at 37 degrees, many Ts(-) pph22 mutant cells progres sed through an aberrant nuclear division and accumulated multiple nucl ei. Ts(-) pph22 mutant cells also accumulated aberrant microtubule str uctures at 37 degrees, while under semi-permissive conditions they wer e sensitive to the microtubule-destabilizing agent benomyl, suggesting that PP2A is required for normal microtubule function. Remarkably, th e multiple defects of Ts(-) pph22 mutant cells were suppressed by a vi able allele (SSD1-v1) of the polymorphic SSD1 gene.