Role of protein phosphatase 2A in the control of glycogen metabolism in yeast.

The yeast homologues of mammalian protein phosphatase 2A (PP2A) are encoded by two genes, PPH21 and PPH22. To evaluate the role of these phosphatases in the control of glycogen metabolism, wild-type cells and mutants carrying deletions of PPH21 or PPH22 were studied. Our results indicate that the lack of a single gene product does not result in significant changes in glycogen content, glycogen synthase, and glycogen phosphorylase activities. Since the double disruption is very detrimental to the cell, the effect of lack of PP2A was evaluated by using strain H336, which carries a deletion of the PPH21 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 activity than control cells. After 14 h in glucose, however, PP2A activity in strain H336 is markedly reduced. The decrease in PP2A activity correlates with a reduced accumulation of glycogen and a more pronounced inactivation of glycogen synthase while glycogen phosphorylase becomes more resistant to inactivation. These observations suggest a role for PP2A in controlling the activation states of both enzymes. The total amount of phosphorylase was also higher in the PP2A-depleted cells, as determined by both enzymic and immunochemical techniques. However, Northern-blot analysis revealed that this is not due to an increase in the phosphorylase mRNA, which is in fact reduced in these cells. In contrast, overexpression of PP2A causes an increased expression of glycogen phosphorylase and a resulting failure to accumulate glycogen. We conclude that PP2A is involved in regulating both the amounts and the activation states of glycogen synthase and glycogen phosphorylase.