The Sch9 protein kinase in the yeast Saccharomyces cerevisiae controls cAPK activity and is required for nitrogen activation of the fermentable-growth-medium-induced (FGM) pathway

Abstract

In cells of the yeast Sacchammyces cerevisiae, trehalase activation, repression of CTTl (catalase), SSA3 (Hsp7O) and other STRE-controlled genes, feedback inhibition of CAMP synthesis and to some extent induction of ribosomal protein genes is controlled by the Ras-adenylate cyclase pathway and by the fermentable-growth-medium-induced pathway (FGM pathway). When derepressed cells are shifted from a non-fermentable carbon source to glucose, the Ras-adenylate cyclase pathway is transiently activated while the FGM pathway triggers a more lasting activation of the same targets when the cells become glucose-repressed. Activation of the FGM pathway is not mediated by CAMP but requires catalytic activity of CAMP-dependent protein kinase (cAPK; Tpkl, 2 or 3). This study shows that elimination of Sch9, a protein kinase with homology to the catalytic subunits of cAPK, affects all target systems in derepressed cells in a way consistent with higher activity of cAPK in wiwo. In witm measurements with trehalase and kemptide as substrates confirmed that elimination of Sch9 enhances cAPK activity about two-to threefold, in both the absence and presence of CAMP. In wiwo it similarly affected the basal and final level but not the extent of the glucose-induced responses in derepressed cells. The reduction in growth rate caused by deletion of SCHS is unlikely to be responsible for the increase in cAPK activity since reduction of growth rate generally leads to lower cAPK activity in yeast. On the other hand, deletion of SCHS abolished the responses of the protein kinase A targets in glucoserepressed cells. Re-addition of nitrogen to cells starved for nitrogen in the presence of glucose failed to trigger activation of trehalase, caused strongly reduced and aberrant repression of m l and SSA3, and failed to induce the upshift in RPL25 expression. From these results three conclusions can be drawn: (1) Sch9 either directly or indirectly reduces the activity of protein kinase A; (2) Sch9 is not required for glucose-induced activation of the Rasadenylate cyclase pathway; and (3) Sch9 is required for nitrogen-induced activation of the FGM pathway. The latter indicates that Sch9 might be the target of the FGM pathway rather than cAPK itself.

Life in the midst of scarcity: adaptations to nutrient availability in Saccharomyces cerevisiae

“…In the nucleolus, Fhl1 and Ifh1 were proposed to have additional functions, such as repressing rRNA transcription, which would provide an interesting link between rRNA and RP synthesis. Different studies indicated that Sch9 should be involved in the regulation of RP and Ribi gene expression and it was proposed that this protein kinase could be the effector allowing for TORC1 control independent of Fhl1 and Sfp1 (Crauwels et al 1997a;Jorgensen et al 2004;Roosen et al 2005; see also below). Another protein involved in TORC1-mediated transcriptional control of ribosome biosynthesis is Hmo1.…”

“…Subsequently, Sch9 was reported to be essential for the proper nutritional regulation of PKA-dependent phenotypes in glucose-repressed cells, such as the activation of trehalase, the repression of stress response genes and the induction of ribosomal protein genes (Crauwels et al 1997a). These findings led to the concept of the socalled Fermentable Growth Medium (FGM) pathway that is believed to ensure the maintenance of PKA-dependent phenotypes of yeast cells growing on medium containing a rapidly fermented sugar and all essential nutrients, after the transient activation of the cAMP-PKA pathway that occurs during lag phase when cells prepare themselves for fermentative growth (Thevelein 1994;Crauwels et al 1997a). As the FGM pathway appeared to operate independent of cAMP, it was postulated that Sch9 could control the activity of PKA at the level of the free catalytic Tpk subunits (Thevelein 1994;Crauwels et al 1997a).…”

“…These findings led to the concept of the socalled Fermentable Growth Medium (FGM) pathway that is believed to ensure the maintenance of PKA-dependent phenotypes of yeast cells growing on medium containing a rapidly fermented sugar and all essential nutrients, after the transient activation of the cAMP-PKA pathway that occurs during lag phase when cells prepare themselves for fermentative growth (Thevelein 1994;Crauwels et al 1997a). As the FGM pathway appeared to operate independent of cAMP, it was postulated that Sch9 could control the activity of PKA at the level of the free catalytic Tpk subunits (Thevelein 1994;Crauwels et al 1997a). More recently, however, it was found that PKA and Sch9 act in parallel .…”

“…Best studied is its role in the regulation of the protein kinase Rim15, which in turn controls the expression of stress-responsive genes through the transcriptional activators Gis1 and Msn2/4 that, respectively, bind to the PDS and STRE elements (Crauwels et al 1997a;Pedruzzi et al 2003;Cameroni et al 2004;Roosen et al 2005). Sch9 phosphorylates Rim15 at Ser 1061 in vitro and this phosphorylation event is necessary for normal cytoplasmic retention of Rim15 (Pedruzzi et al 2003;Wanke et al 2008).…”

Genome-wide coexpression dynamics: Theory and application

“…SCH9 encodes a serine͞threonine protein kinase. It is required for the nitrogen activation of the fermentable growth median-induced pathway (18). Because the sch9-delete mutant has a longer nondividing life span than the wild type (17), this brings out a likely association between the efficient usage of the urea cycle and longevity physiology.…”