ROLE OF PROTEIN PHOSPHATASE 2A IN THE CONTROL OF GLYCOGEN-METABOLISM IN YEAST

The yeast homologues of mammalian protein phosphatase 2A (PP2A) are en coded by two genes, PPH21 and PPH22. To evaluate the role of these pho sphatases in the control of glycogen metabolism, wild-type cells and m utants carrying deletions of PPH21 or PPH22 were studied. Our results indicate that the lack of a single gene product does not result in sig nificant changes in glycogen content, glycogen synthase, and glycogen phosphorylase activities. Since the double disruption is very detrimen tal to the cell, the effect of lack of PP2A was evaluated by using str ain H336, which carries a deletion of the PPH22 gene and has the PPH22 gene placed under the control of the GAL1 promoter, under conditions that allowed either progressive depletion or overexpression of PPH22. When grown on galactose, H336 cells contain 2-3-fold more PP2A activit y than control cells. After 14 h in glucose, however, PP2A activity in strain H336 is markedly reduced. The decrease in PP2A activity correl ates with a reduced accumulation of glycogen and a more pronounced ina ctivation of glycogen synthase while glycogen phosphorylase becomes mo re resistant to inactivation. These observations suggest a role for PP 2A in controlling the activation states of both enzymes. The total amo unt of phosphorylase was also higher in the PP2A-depleted cells, as de termined by both enzymic and immunochemical techniques. However, North ern-blot analysis revealed that this is not due to an increase in the phosphorylase mRNA, which is in fact reduced in these cells. In contra st, overexpression of PP2A causes an increased expression of glycogen phosphorylase and a resulting failure to accumulate glycogen. We concl ude that PP2A is involved in regulating both the amounts and the activ ation states of glycogen synthase and glycogen phosphorylase.