Direct stenting with sirolimus-eluting stents

Tor pathway control of the nitrogen-responsive DAL5 gene bifurcates at the level of Gln3 and Gat1 regulation in Saccharomyces cerevisiae

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.

Stress-responsive Gln3 localization in Saccharomyces cerevisiae is separable from and can overwhelm nitrogen source regulation

Intracellular localization of Saccharomyces cerevisiae GATA family transcription activator, Gln3, is used as a downstream readout of rapamycin-inhibited Tor1,2 control of Tap42 and Sit4 activities. Gln3 is cytoplasmic in cells provided with repressive nitrogen sources such as glutamine and is nuclear in cells growing with a derepressive nitrogen source such as proline or those treated with rapamycin or methionine sulfoximine (Msx). Although gross Gln3-Myc13 phosphorylation levels in wild type cells do not correlate with nitrogen source-determined intracellular Gln3-Myc13 localization, the phosphorylation levels are markedly influenced by several environmental perturbations. Msx treatment increases Snf1-independent Gln3-Myc13 phosphorylation, whereas carbon starvation increases both Snf1-dependent and -independent Gln3-Myc13 phosphorylation. Here we demonstrate that a broad spectrum of environmental stresses (temperature, osmotic, and oxidative) increase Gln3-Myc13 phosphorylation. In parallel, these stresses elicit rapid (<5 min for NaCl) Gln3-Myc13 relocalization from the nucleus to the cytoplasm. The response of Gln3-Myc13 localization to stressful conditions can completely overwhelm its response to nitrogen source quality or inhibitor-generated disruption of the Tor1,2 signal transduction pathway. Adding NaCl to cells cultured under conditions in which Gln3-Myc13 is normally nuclear, i.e. proline-grown, nitrogen-starved, Msx-, caffeine-, and rapamycin-treated wild type cells, or ure2Delta cells, results in its prompt relocalization to the cytoplasm. Together these data identify a major new level of regulation to which Gln3 responds, and adds a new dimension to mechanistic studies of the regulation of this transcription factor.

Saccharomyces cerevisiae Sit4 phosphatase is active irrespective of the nitrogen source provided, and Gln3 phosphorylation levels become nitrogen source-responsive in a sit4-deleted strain

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, proline-grown cells or Msx-treated, ammonia-grown cells.

Ammonia-specific regulation of Gln3 localization in Saccharomyces cerevisiae by protein kinase Npr1

Events directly regulating Gln3 intracellular localization and nitrogen catabolite repression (NCR)-sensitive transcription in Saccharomyces cerevisiae are interconnected with many cellular processes that influence the utilization of environmental metabolites. Among them are intracellular trafficking of the permeases that transport nitrogenous compounds and their control by the Tor1,2 signal transduction pathway. Npr1 is a kinase that phosphorylates and thereby stabilizes NCR-sensitive permeases, e.g. Gap1 and Mep2. It is also a phosphoprotein for which phosphorylation and kinase activity are regulated by Tor1,2 via Tap42 and Sit4. Npr1 has been reported to negatively regulate nuclear localization of Gln3 in SD (ammonia)-grown cells. Thus we sought to distinguish whether Npr1: (i) functions directly as a component of NCR control; or (ii) influences Gln3 localization indirectly, possibly as a consequence of participating in protein trafficking. If Npr1 functions directly, then the ability of all good nitrogen sources to restrict Gln3 to the cytoplasm should be lost in an npr1Delta just as occurs when URE2 (encoding this well studied negative Gln3 regulator) is deleted. We show that nuclear localization of Gln3-Myc(13) in an npr1Delta occurred only with ammonia as the nitrogen source. Other good nitrogen sources, e.g. glutamine, serine, or asparagine, restricted Gln3-Myc(13) to the cytoplasm of both wild type and npr1Delta cells. In other words, the npr1Delta did not possess the uniform phenotype for all repressive nitrogen sources characteristic of ure2Delta. This suggests that the connection between Gln3 localization and Npr1 is indirect, arising from the influence of Npr1 on the ability of cells to utilize ammonia as a repressive nitrogen source.