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 proline-grown (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 NCR-sensitive transcription, being highest in limiting nitrogen and lowest when nitrogen is in excess.
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