Activation of the RAS/Cyclic AMP Pathway Suppresses a TOR Deficiency in Yeast

Abstract

The TOR (target of rapamycin) and RAS/cyclic AMP (cAMP) signaling pathways are the two major pathways controlling cell growth in response to nutrients in yeast. In this study we examine the functional interaction between TOR and the RAS/cAMP pathway. First, activation of the RAS/cAMP signaling pathway confers pronounced resistance to rapamycin. Second, constitutive activation of the RAS/cAMP pathway prevents several rapamycin-induced responses, such as the nuclear translocation of the transcription factor MSN2 and induction of stress genes, the accumulation of glycogen, the induction of autophagy, the down-regulation of ribosome biogenesis (ribosomal protein gene transcription and RNA polymerase I and III activity), and the down-regulation of the glucose transporter HXT1. Third, many of these TOR-mediated responses are independent of the previously described TOR effectors TAP42 and the type 2A-related protein phosphatase SIT4. Conversely, TOR-controlled TAP42/SIT4-dependent events are not affected by the RAS/cAMP pathway. Finally, and importantly, TOR controls the subcellular localization of both the protein kinase A catalytic subunit TPK1 and the RAS/cAMP signaling-related kinase YAK1. Our findings suggest that TOR signals through the RAS/cAMP pathway, independently of TAP42/SIT4. Therefore, the RAS/cAMP pathway may be a novel TOR effector branch.The Saccharomyces cerevisiae targets of rapamycin, TOR1 and TOR2, are functionally and structurally conserved protein kinases that control a large and diverse set of growth-related readouts in response to nutrient availability (53). In yeast, rapamycin treatment or TOR depletion results in several physiological changes characteristic of starved cells, including inhibition of translation initiation, inhibition of ribosome biogenesis, specific changes in transcription, sorting and turnover of nutrient permeases, accumulation of storage carbohydrates (such as glycogen), and induction of autophagy (reviewed in references 12 and 52). Nitrogen (in particular glutamine) and possibly carbon are important nutrients in the context of yeast TOR signaling (13,56). Mammalian TOR (mTOR) controls translation and other growth-related processes in response to amino acids and a growth factor signal (e.g., insulin). The growth factor input into the mTOR pathway is via phosphatidylinositol 3-kinase (PI3K), PDK1, and protein kinase B (PKB; also known as Akt). The precise nature of the link between mTOR and PI3K signaling remains to be determined but may involve the PKB-mediated phosphorylation and inhibition of the tuberous sclerosis protein complex (TSC1-TSC2) upstream of mTOR. Control of translation by mTOR is via activation of S6 kinase (S6K) and inhibition of the eIF4E inhibitor 4E-BP1 (reviewed in reference 30).The yeast TORs control several readouts via a phosphatase switch composed of the type 2A-related phosphatase SIT4, the PP2A/SIT4-associated protein TAP42, and the TAP42-interacting protein TIP41 (16,29,31). Under good nutrient conditions, TOR promotes the binding of SIT4 to TAP...

Functional Analysis of the ATG8 Homologue Ao atg8 and Role of Autophagy in Differentiation and Germination in Aspergillus oryzae

“…In A. nidulans and Aspergillus fumigatus, the Ras signaling pathway, which plays an important role in the control of cell growth and response to nutrients, is involved in germination and autophagy (2,4,24). Furthermore, in S. cerevisiae, a relationship has been reported between the Ras signaling pathway and the Tor kinase (central negative regulator of autophagy) (29), which supports our data. Thus, the involvement of autophagy in conidial germination has been demonstrated for the first time in filamentous fungi.…”

Arsenic Toxicity to Saccharomyces cerevisiae Is a Consequence of Inhibition of the TORC1 Kinase Combined with a Chronic Stress Response

“…Arsenic seems to drive this response system into a dead-end situation where chronic activation of stress genes leads to severe growth inhibition. Our data indicate that although PKA may have an important influence on RP gene transcription under some conditions, TORC1 activity plays a key role even when PKA levels are high (Marion et al, 2004;Zurita-Martinez and Cardenas, 2005;Chen and Powers, 2006), arguing against models in which PKA acts downstream of TOR to regulate ribosome biogenesis (Schmelzle et al, 2004). One key question that remains is the mechanism by which arsenic inhibits TORC1.…”

“…Hyperactive PKA prevents cytoplasmic accumulation of Sfp1 and also activates Fhl1/Ihf1, thus promoting RP gene transcription Schawalder et al, 2004;Schmelzle et al, 2004). Because in strains with zero PKA activity the shift of Sfp1 localization is not disturbed (Warner, 1999;Marion et al, 2004), PKA activity below a threshold is most probably a precondition for inhibition of Sfp1.…”

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

“…Furthermore, feedback regulation may involve other carbon source-dependent pathways as well, for instance the main glucose repression pathway, since no glucose-induced cAMP increase was observed in a snf1D mutant (Arguelles et al 1990). Finally, it is important to note that PKA activity is further finetuned by modulation of its subcellular localization, phosphorylation state and abundance (Werner-Washburne et al 1991;Griffioen et al 2000Griffioen et al , 2001Schmelzle et al 2004; http://doc.rero.ch Portela and Moreno 2006). However, compared to cAMPmediated control, this regulation of PKA appears to be less important for short-term signalling events.…”

“…Finally, PKA is a known inhibitor of autophagy, a degradative process that recycles non-essential proteins and organelles during periods of nutrient starvation (Budovskaya et al 2004;Schmelzle et al 2004;Yorimitsu and Klionsky 2005). The key players involved in autophagy are the Atg proteins and three of these, i.e.…”