Multiple effects of protein phosphatase 2A on nutrient-induced signalling in the yeast Saccharomyces cerevisiae

The trehalose-degrading enzyme trehalase is activated upon addition of gluc ose to derepressed cells or in response to nitrogen source addition to nitr ogen-starved glucose-repressed yeast (Saccharomyces cerevisiae) cells. Treh alase activation is mediated by phosphorylation. Inactivation involves deph osphorylation, as trehalase protein levels do not change upon multiple acti vation/inactivation cycles. Purified trehalase can be inactivated by incuba tion with protein phosphatase 2A (PP2A) in vitro. To test whether PP2A was involved in trehalase inactivation in vivo, we overexpressed the yeast PP2A isoform Pph22, Unexpectedly, the moderate (approximately threefold) overex pression of Pph22 that we obtained increased basal trehalase activity and r endered this activity unresponsive to the addition of glucose or a nitrogen source. Concomitant with higher basal trehalase activity, cells overexpres sing Pph22 did not store trehalose efficiently and were heat sensitive. Aft er the addition of glucose or of a nitrogen source to starved cells, Pph22- overexpressing cells showed a delayed exit from stationary phase, a delayed induction of ribosomal gene expression and constitutive repression of stre ss-regulated element-controlled genes, Deletion of the SCH9 gene encoding a protein kinase involved in nutrient-induced signal transduction restored g lucose-induced trehalase activation in Pph22-overexpressing cells. Taken to gether, our results indicate that yeast PP2A overexpression leads to the ac tivation of nutrient-induced signal transduction pathways in the absence of nutrients.