RESUMEN
The trehalose-degrading enzyme trehalase is activated upon addition of glucose to derepressed cells or in response to nitrogen source addition to nitrogen-starved glucose-repressed yeast (Saccharomyces cerevisiae) cells. Trehalase activation is mediated by phosphorylation. Inactivation involves dephosphorylation, as trehalase protein levels do not change upon multiple activation/inactivation cycles. Purified trehalase can be inactivated by incubation 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) overexpression of Pph22 that we obtained increased basal trehalase activity and rendered this activity unresponsive to the addition of glucose or a nitrogen source. Concomitant with higher basal trehalase activity, cells overexpressing Pph22 did not store trehalose efficiently and were heat sensitive. After 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 stress-regulated element-controlled genes. Deletion of the SCH9 gene encoding a protein kinase involved in nutrient-induced signal transduction restored glucose-induced trehalase activation in Pph22-overexpressing cells. Taken together, our results indicate that yeast PP2A overexpression leads to the activation of nutrient-induced signal transduction pathways in the absence of nutrients.
Asunto(s)
Fosfoproteínas Fosfatasas/metabolismo , Saccharomyces cerevisiae/metabolismo , Transducción de Señal , Trehalasa/metabolismo , Secuencia de Bases , Dominio Catalítico , Activación Enzimática , Inhibidores Enzimáticos/farmacología , Glucosa/metabolismo , Glicerol/metabolismo , Datos de Secuencia Molecular , Nitrógeno/metabolismo , Ácido Ocadaico/farmacología , Fosfoproteínas Fosfatasas/genética , Regiones Promotoras Genéticas , Proteínas Quinasas/genética , Proteínas Quinasas/metabolismo , Proteína Fosfatasa 2 , Proteínas Ribosómicas/efectos de los fármacos , Proteínas Ribosómicas/genética , Proteínas Ribosómicas/metabolismo , Trehalasa/antagonistas & inhibidores , Trehalasa/aislamiento & purificaciónRESUMEN
A variety of results has been obtained consistent with activation of neutral trehalase in Saccharomyces cerevisiae through direct phosphorylation by cAMP-dependent protein kinase (PKA). A series of neutral trehalase mutant alleles, in which all evolutionarily conserved putative phosphorylation sites were changed into alanine, was tested for activation in vitro (by PKA) and in vivo (by glucose addition). None of the mutations alone affected the activation ratio, whereas all mutations combined resulted in an inactive enzyme. All mutant alleles were expressed to similar levels, as shown by Western blotting. Several of the point mutations significantly lowered the specific activity. Using this series of mutants with different activity levels we show an inverse relationship between trehalase activity and heat-shock survival during glucose-induced trehalose mobilization. This is consistent with a stress-protective function of trehalose. On the other hand, reduction of trehalase activity below a certain threshold level impaired recovery from a sublethal heat shock. This suggests that trehalose breakdown is required for efficient recovery from heat shock, and that the presence of trehalase protein alone is not sufficient for efficient heat-stress recovery.