The yeast A kinases differentially regulate iron uptake and respiratory function

Abstract

Tpk2, but not Tpk1 or Tpk3, is required for pseudohyphal growth. Genome-wide transcriptional profiling has revealed unique signatures for each of the three A kinases leading to the identification of additional functional diversity among these proteins. Tpk2 negatively regulates genes involved in iron uptake and positively regulates genes involved in trehalose degradation and water homeostasis. Tpk1 is required for the derepression of branched chain amino acid biosynthesis genes that seem to have a second role in the maintenance of iron levels and DNA stability within mitochondria. The fact that TPK2 mutants grow better than wild types on nonfermentable carbon sources and on media deficient in iron supports the unique role of Tpk2 in respiratory growth and carbon source use.I n yeast, cAMP acting through the A kinases (PKA) provides a key regulatory signal for growth on diverse carbon sources. Growth on fermentable carbon sources (e.g., glucose, fructose, and sucrose) requires a higher basal level of cAMP than does growth on nonfermentable carbon sources (e.g., ethanol, glycerol, and acetate). Therefore, the level of cAMP must decrease in order for cells to switch from growth on fermentable carbon sources to growth on nonfermentable carbon sources (the diauxic shift; ref. 1). Addition of glucose to yeast cells growing on a nonfermentable carbon source or starved for glucose results in a transient peak in intracellular cAMP levels. This transition to fermentation requires both the transient increase of cAMP and the cAMP-dependent PKA (2). Activated PKA shifts the metabolic flux away from gluconeogenesis and toward glycolysis by regulating key enzymes in these processes including fructose-1,6-bisphosphatase and phosphofructokinase-2 (3). Phosphorylation by PKA inactivates the transcription factor Adr1, a positive regulatory factor for the transcription of the respiratory enzyme Adh2 (4). In addition, PKA promotes the breakdown of glycogen and trehalose by inhibiting enzymes involved in synthesis (trehalose synthase and glycogen synthase) and activating enzymes involved in breakdown (trehalase and glycogen phosphorylase) of these storage carbohydrates.The transcriptional changes occurring in the transition from fermentative growth to respiratory growth have been monitored by genome expression arrays (5). As glucose is depleted, transcription of genes involved in respiration, the tricarboxylic acid cycle, the glyoxylate cycle, gluconeogenesis, and storage carbohydrate synthesis is induced, whereas transcription of genes involved in glycolysis and protein synthesis is repressed (5). Consistent with the shift to respiratory growth, cytoplasmic ribosomal protein genes are repressed, and mitochondrial ribosomal genes are induced. In view of the role of cAMP and the PKAs in the use of carbon sources, these results raise the question of whether the PKAs function in respiratory growth and whether they are redundant for this function.To elucidate more broadly the functional differences between the PKA catalytic subunits, we ...

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

“…This implies as well that there is probably a large overlap in the functions of Tpk1-3, which is further consistent with the high level of protein sequence similarity between the different catalytic subunits (Toda et al 1987b). Nevertheless, there are several examples of separate, specific functions for the different subunits, such as the induction of pseudohyphal growth, the expression control of genes involved in iron uptake or branched amino acid biosynthesis, or the regulation of mitochondrial enzymes (Robertson and Fink 1998;Pan and Heitman 1999;Robertson et al 2000;Zhu et al 2000;Singh et al 2004;Chevtzoff et al 2005;Ptacek et al 2005). Given their central and essential role, the activity of the catalytic subunits is tightly regulated.…”

The CaCTR1 gene is required for high-affinity iron uptake and is transcriptionally controlled by a copper-sensing transactivator encoded by CaMAC1

“…The regulation of genes that facilitate iron uptake in C. albicans has been reported to be under the control of the CaTup1p transcriptional repressor (Knight et al, 2002). Interestingly, cDNA micro-arrays have shown that transcriptional repression of ScFRE2, ScFET3, ScFTR1, ScCCC2 and the ScARN genes in response to high-iron conditions in S. cerevisiae is under the control of the protein kinase ScTpk2p (Robertson et al, 2000). The ScTPK2 gene and its functional homologue in C. albicans are also involved in the regulation of morphogenesis via the cAMP-activated pathway (reviewed by Gancedo, 2001;Sonneborn et al, 2000).…”

Sfp1 is a stress- and nutrient-sensitive regulator of ribosomal protein gene expression

“…Although the PKA catalytic subunits, encoded by TPK1, TPK2, and TPK3, are redundant in most cases, they sometimes show different substrate specificities and functions (40)(41)(42). To determine whether a specific PKA subunit controls Sfp1 nuclear localization, we generated strains containing a single PKA catalytic subunit and lacking the regulatory subunit Bcy1.…”