The Tor1,2 protein kinases globally influence many cellular processes including nitrogen-responsive gene expression that correlates with intracellular localization of GATA transcription activators Gln3 and Gat1/Nil1. Gln3-Myc 13 and Gat1-Myc 13 are restricted to the cytoplasm of cells provided with good nitrogen sources, e.g. glutamine. Following the addition of the Tor1,2 inhibitor, rapamycin, both transcription factors relocate to the nucleus. Gln3-Myc 13 localization is highly dependent upon Ure2 and type 2A-related phosphatase, Sit4. Ure2 is required for Gln3 to be restricted to the cytoplasm of cells provided with good nitrogen sources, and Sit4 is required for its location to the nucleus following rapamycin treatment. The paucity of analogous information concerning Gat1 regulation prompted us to investigate the effects of deleting SIT4 and URE2 on Gat1-Myc 13 localization, DNA binding, and NCR-sensitive transcription. Our data demonstrate that Tor pathway control of NCR-responsive transcription bifurcates at the regulation of Gln3 and Gat1. Gat1-Myc 13 localization is not strongly influenced by deleting URE2, nor is its nuclear targeting following rapamycin treatment strongly dependent on Sit4. ChIP experiments demonstrated that Gat1-Myc 13 can bind to the DAL5 promoter in the absence of Gln3. Gln3-Myc 13 , on the other hand, cannot bind to DAL5 in the absence of Gat1. We conclude that: (i) Tor pathway regulation of Gat1 differs markedly from that of Gln3, (ii) nuclear targeting of Gln3-Myc 13 is alone insufficient for its recruitment to the DAL5 promoter, and (iii) the Tor pathway continues to play an important regulatory role in NCR-sensitive transcription even after Gln3-Myc 13 is localized to the nucleus.

Gln3, the major activator of nitrogen catabolite repression (NCR)-sensitive transcription, is often used as an assay of Tor pathway regulation in Saccharomyces cerevisiae. Gln3 is cytoplasmic in cells cultured with repressive nitrogen sources (Gln) and nuclear with derepressive ones (Pro) or after treating Glngrown cells with the Tor inhibitor, rapamycin (Rap). In Raptreated or Pro-grown cells, Sit4 is posited to dephosphorylate Gln3, which then dissociates from a Gln3-Ure2 complex and enters the nucleus. However, in contrast with this view, Sit4-dependent Gln3 dephosphorylation is greater in Gln than Pro. Investigating this paradox, we show that PP2A (another Tor pathway phosphatase)-dependent Gln3 dephosphorylation is regulated oppositely to that of Sit4, being greatest in Pro-and least in Gln-grown cells. It thus parallels nuclear Gln3 localization and NCR-sensitive transcription. However, because PP2A is not required for nuclear Gln3 localization in Pro, PP2A-dependent Gln3 dephosphorylation and nuclear localization are likely parallel responses to derepressive nitrogen sources. In contrast, Rap-induced nuclear Gln3 localization absolutely requires all four PP2A components (Pph21/22, Tpd3, Cdc55, and Rts1). In pph21⌬22⌬, tpd3⌬, or cdc55⌬ cells, however, Gln3 is dephosphorylated to the same level as in Rap-treated wildtype cells, indicating Rap-induced Gln3 dephosphorylation is insufficient to achieve nuclear localization. Finally, PP2A-dependent Gln3 dephosphorylation parallels conditions where Gln3 is mostly nuclear, while Sit4-dependent and Rap-induced dephosphorylation parallels those where Gln3 is mostly cytoplasmic, suggesting the effects of these phosphatases on Gln3 may occur in different cellular compartments. (Tor) 2 is widely acknowledged as a global regulator whose actions control and integrate an array of cellular processes ranging from transcription to cell division (1-4). One of the principal roles of Tor is participation in the adjustment of cell metabolism and cell biology to intra-and extracellular nutrient fluctuations. Investigation of the Saccharomyces cerevisiae Tor proteins (Tor1 and Tor2) has added much to our understanding of their mammalian homologue (mTor) (1). Central to many of these investigations has been the GATA-binding transcriptional activator, Gln3, well known for its control of nitrogen catabolite repression (NCR)-sensitive transcription (5-9). Target of RapamycinNitrogen Catabolite Repression in S. cerevisiae-NCR is the regulatory mechanism through which S. cerevisiae responds to environmental nitrogen availability and selectively utilizes good nitrogen sources in preference to poorer ones (5-9). It permits cells to rapidly and successfully cope with nutritional changes ranging from starvation after a rain storm to luxurious nitrogen excess when situated in decaying fruit. NCR-sensitive regulation is achieved by controlling the action of Gln3. When cells are growing in the presence of sufficient quantities of a good nitrogen source (e.g. glutamine), Gln3 is restricted to ...

Tor1,2 control of type 2A-related phosphatase activities in Saccharomyces cerevisiae has been reported to be responsible for the regulation of Gln3 phosphorylation and intracellular localization in response to the nature of the nitrogen source available. According to the model, excess nitrogen stimulates Tor1,2 to phosphorylate Tip41 and/or Tap42. Tap42 then complexes with and inactivates Sit4 phosphatase, thereby preventing it from dephosphorylating Gln3. Phosphorylated Gln3 complexes with Ure2 and is sequestered in the cytoplasm. When Tor1,2 kinase activities are inhibited by limiting nitrogen, or rapamycin-treatment, Tap42 can no longer complex with Sit4. Active Sit4 dephosphorylates Gln3, which can then localize to the nucleus and activate transcription. The paucity of experimental data directly correlating active Sit4 and Pph3 with Gln3 regulation prompted us to assay Gln3-Myc 13 phosphorylation and intracellular localization in isogenic wild type, sit4, pph3, and sit4pph3 deletion strains. We found that Sit4 actively brought about Gln3-Myc 13 dephosphorylation in both good (glutamine or ammonia) and poor (proline) nitrogen sources. This Sit4 activity masked nitrogen source-dependent changes in Gln3-Myc 13 phosphorylation which were clearly visible when SIT4 was deleted. The extent of Sit4 requirement for Gln3 nuclear localization was both nitrogen source-and strain-dependent. In some strains, Sit4 was not even required for Gln3 nuclear localization in untreated or rapamycin-treated, prolinegrown cells or Msx-treated, ammonia-grown cells.

The ability of the cell to sense environmental conditions and alter gene expression in response to them is critical to its survival. In Saccharomyces cerevisiae, the Tor1/2 serine/threonine kinases are global regulators situated at the top of a signal cascade reported to receive and transmit nutritional signals associated with the nitrogen supply of the cell. At the other end of that cascade is Gln3, one of two transcriptional activators responsible for most nitrogen catabolic gene expression. When nitrogen is in excess, Tor1/2 are active, and Gln3 is phosphorylated and localizes to the cytoplasm. If Tor1/2 are inhibited by rapamycin or mutation, Gln3 becomes dephosphorylated, accumulates in the nucleus, and mediates nitrogen catabolite repression (NCR)-sensitive transcription. The observations that Gln3 also accumulates in the nuclei of cells provided with poor nitrogen sources or during nitrogen starvation has led to the conclusion that Tor1/2 control intracellular Gln3 localization and NCR-sensitive transcription by regulating Gln3 phosphorylation/dephosphorylation. To test this model, we compared Gln3 phosphorylation states and intracellular localizations under a variety of physiological conditions known to elicit different levels of NCRsensitive transcription. Our data indicate that: (i) observable Gln3 phosphorylation levels do not correlate in a consistent way with the quality or quantity of the nitrogen source provided, the intracellular localization of Gln3, or the capacity to support NCR-sensitive transcription. (ii) Gln3-Myc 13 is hyperphosphorylated during nitrogen and carbon starvation, but this uniform response does not correlate with Gln3 intracellular localization. (iii) Gln3-Myc 13 dephosphorylation and nuclear localization correlate with one another at early but not late times after rapamycin treatment. These data suggest that rapamycin treatment and growth with poor nitrogen sources bring about nuclear accumulation of Gln3 but likely do so by different mechanisms or by a common mechanism involving molecules other than Gln3 and/or other than the levels of Gln3-Myc 13 phosphorylation thus far detected by others and ourselves.

In yeast, rapamycin (Rap)-inhibited TorC1, and the phosphatases it regulates (Sit4 and PP2A) are components of a conserved pathway regulating the response of eukaryotic cells to nutrient availability. TorC1 and intracellular nitrogen levels regulate the localization of Gln3 and Gat1, the activators of nitrogen catabolite repression (NCR)-sensitive genes whose products are required to utilize poor nitrogen sources. In nitrogen excess, Gln3 and Gat1 are cytoplasmic, and NCR-sensitive transcription is repressed. During nitrogen limitation or Rap treatment, Gln3 and Gat1 are nuclear, and transcription is derepressed. We previously demonstrated that the Sit4 and Pph21/22-Tpd3-Cdc55/Rts1 requirements for nuclear Gln3 localization differ. We now show that Sit4 and Pph21/22-Tpd3-Cdc55/Rts1 requirements for NCR-sensitive and Rap-induced nuclear Gat1 localization markedly differ from those of Gln3. Our data suggest that Gln3 and Gat1 localizations are controlled by two different regulatory pathways. Gln3 localization predominantly responds to intracellular nitrogen levels, as reflected by its stronger NCR-sensitivity, weaker response to Rap treatment, and strong response to methionine sulfoximine (Msx, a glutamine synthetase inhibitor). In contrast, Gat1 localization predominantly responds to TorC1 regulation as reflected by its weaker NCR sensitivity, stronger response to Rap, and immunity to the effects of Msx. Nuclear Gln3 localization in prolinegrown (nitrogen limited) cells exhibits no requirement for Pph21/22-Tpd3/Cdc55, whereas nuclear Gat1 localization under these conditions is absolutely dependent on Pph21/22-Tpd3/Cdc55. Furthermore, the extent to which Pph21/22-Tpd3-Cdc55 is required for the TorC1 pathway (Rap) to induce nuclear Gat1 localization is regulated in parallel with Pph21/22-Tpd3-Cdc55-dependent Gln3 dephosphorylation and NCRsensitive transcription, being highest in limiting nitrogen and lowest when nitrogen is in excess.One consequence of motif, gene, or genome duplication is the subsequent existence of multiple proteins with the same or overlapping functions. In some cases the duplicated genes evolve, acquiring easily distinguishable new functions or regulatory profiles (1-4). In others, the differences are more subtle, and a more careful study is required before they become apparent. This is the case with the Saccharomyces cerevisiae GATA family transcription factors, Gln3 and Gat1/Nil1. Both Gln3 and Gat1 are responsible for nitrogen catabolite repression (NCR) 2 -sensitive expression of genes encoding the transport and enzyme proteins needed to scavenge poor nitrogen sources (e.g. proline) when more readily usable ones are limiting or unavailable (5-8). Gln3 and Gat1 binding sites in NCR-sensitive promoters contain the core sequence GATAA that binds the well characterized zinc finger motif that defines this family of proteins (9 -11).The most apparent differences in the regulation of the GATA factors occur at the level of transcription. GAT1 expression is NCR-sensitive, Gln3-dependent, Gat1-auto-a...

Regulated intracellular localization of Gln3, the transcriptional activator responsible for nitrogen catabolite repression (NCR)-sensitive transcription, permits Saccharomyces cerevisiae to utilize good nitrogen sources (e.g. glutamine and ammonia) in preference to poor ones (e.g. proline). During nitrogen starvation or growth in medium containing a poor nitrogen source, Gln3 is nuclear and NCR-sensitive transcription is high. However, when cells are grown in excess nitrogen, Gln3 is localized to the cytoplasm with a concomitant decrease in gene expression. Treating cells with the Tor protein inhibitor, rapamycin, mimics nitrogen starvation. Recently, carbon starvation has been reported to cause nuclear localization of Gln3 and increased NCR-sensitive transcription. Here we show that nuclear localization of Gln3 during carbon starvation derives from its indirect effects on nitrogen metabolism, i.e. Gln3 does not move into the nucleus of carbon-starved cells if glutamine rather than ammonia is provided as the nitrogen source. In addition, these studies have clearly shown Gln3 is not uniformly distributed in the cytoplasm, but rather localizes to punctate or tubular structures. Analysis of these images by deconvolution microscopy suggests that Gln3 is concentrated in or associated with a highly structured system in the cytosol, one that is possibly vesicular in nature. This finding may impact significantly on how we view (i) the mechanism by which Tor regulates the intracellular localization of Gln3 and (ii) how proteins move into and out of the nucleus.

The Tor1/2p signal transduction pathway regulates nitrogen catabolite repression (NCR)-sensitive (GAP1, GAT1, DAL5) and retrograde (CIT2, DLD3, IDH1/2) gene expression by controlling intracellular localization of the transcription activators, Gln3p and Gat1p, and Rtg1p and Rtg3p, respectively. The accepted pathway for this regulation is NH 3 or excess nitrogen ٜ Mks1p ٜ Ure2p ٜ Gln3p 3 DAL5, and rapamycin or limiting nitrogen ٜ Torp 3 Tap42 ٜ Mks1p 3 Rtg1/3p 3 CIT2, respectively. In current models, Mks1p positively regulates both Gln3p (and DAL5 expression) and Rtg1/3p (and CIT2 expression). Here, in contrast, we show the following. (i) Mks1p is a strong negative regulator of CIT2 expression and does not effect NCR-sensitive expression of DAL5 or GAP1. (ii) Retrograde carbon and NCR-sensitive nitrogen metabolism are not linked by the quality of the nitrogen source, i.e. its ability to elicit NCR, but by the product of its catabolism, i.e. glutamate or ammonia. (iii) In some instances, we can dissociate rapamycininduced CIT2 expression from Mks1p function, i.e. rapamycin does not suppress Mks1p-mediated down-regulation of CIT2 expression. These findings suggest that currently accepted models of Tor1/2p signal transduction pathway regulation require revision.

Ure2, the protein that negatively regulates GATA factor (Gln3, Gat1)-mediated transcription in Saccharomyces cerevisiae, possesses prion-like characteristics. Identification of metabolic and environmental factors that influence prion formation as well as any activities that prions or prion precursors may possess are important to understanding them and developing treatment strategies for the diseases in which they participate. Ure2 exhibits primary sequence and three-dimensional homologies to known glutathione S-transferases. However, multiple attempts over nearly 2 decades to demonstrate Ure2-mediated S-transferase activity have been unsuccessful, leading to the possibility that Ure2 may well not participate in glutathionation reactions. Here we show that Ure2 is required for detoxification of glutathione S-transferase substrates and cellular oxidants. ure2⌬ mutants are hypersensitive to cadmium and nickel ions and hydrogen peroxide. They are only slightly hypersensitive to diamide, which is nitrogen source-dependent, and minimally if at all hypersensitive to 1-chloro-2,4-dinitrobenzene, the most commonly used substrate for glutathione S-transferase enzyme assays. Therefore, Ure2 shares not only structural homology with various glutathione S-transferases, but ure2 mutations possess the same phenotypes as mutations in known S. cerevisiae and Schizosaccharomyces pombe glutathione S-transferase genes. These findings are consistent with Ure2 serving as a glutathione S-transferase in S. cerevisiae.Several neurodegenerative conditions derive from the same pathogenetic mechanism, i.e. a change in protein conformation, polymerization, and plaque formation. These conditions have been called conformational diseases such as Alzheimer's and the prionoses. Recent studies have demonstrated that a protein associated with such disease, amyloid ␤-protein, protects neurons from metal-induced oxidative damage (1). Neither the molecular basis for this activity nor how it is affected by environment and neuronal metabolism is yet known. The use of eukaryotic model systems, such as the yeast Saccharomyces cerevisiae, has greatly facilitated our acquisition of information about mammalian proteins, their functions and interactions, and how their synthesis and activities are regulated and integrated. In particular, the genetic study of prions has been