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      Sit4 and PP2A Dephosphorylate Nitrogen Catabolite Repression-Sensitive Gln3 When TorC1 Is Up- as Well as Down-Regulated

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      Genetics
      Genetics Society of America

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          Abstract

          Saccharomyces cerevisiae lives in boom and bust nutritional environments. Sophisticated regulatory systems have evolved to rapidly cope with these changes while preserving intracellular homeostasis. Target of Rapamycin Complex 1 (TorC1), is a serine/threonine kinase complex and a principle nitrogen-responsive regulator. TorC1 is activated by excess nitrogen and downregulated by limiting nitrogen. Two of TorC1’s many downstream targets are Gln3 and Gat1 —GATA-family transcription activators—whose localization and function are Nitrogen Catabolite Repression- (NCR-) sensitive. In nitrogen replete environments, TorC1 is activated, thereby inhibiting the P Tap42-Sit4 and P Tap42-PP2A ( Pph21 / Pph22 - Tpd3 , Pph21 ,22- Rts1 / Cdc55 ) phosphatase complexes. Gln3 is phosphorylated, sequestered in the cytoplasm and NCR-sensitive transcription repressed. In nitrogen-limiting conditions, TorC1 is downregulated and P Tap42-Sit4 and P Tap42-PP2A are active. They dephosphorylate Gln3 , which dissociates from Ure2 , relocates to the nucleus, and activates transcription. A paradoxical observation, however, led us to suspect that Gln3 control was more complex than appreciated, i.e. , Sit4 dephosphorylates Gln3 more in excess than in limiting nitrogen conditions. This paradox motivated us to reinvestigate the roles of these phosphatases in Gln3 regulation. We discovered that: (i) Sit4 and PP2A actively function both in conditions where TorC1 is activated as well as down-regulated; (ii) nuclear Gln3 is more highly phosphorylated than when it is sequestered in the cytoplasm; (iii) in nitrogen-replete conditions, Gln3 relocates from the nucleus to the cytoplasm, where it is dephosphorylated by Sit4 and PP2A; and (iv) in nitrogen excess and limiting conditions, Sit4 , PP2A, and Ure2 are all required to maintain cytoplasmic Gln3 in its dephosphorylated form.

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          Author and article information

          Journal
          Genetics
          Genetics
          Genetics Society of America
          0016-6731
          1943-2631
          June 18 2019
          : genetics.302371.2019
          Article
          10.1534/genetics.119.302371
          6707456
          31213504
          808f7bc7-01ac-4089-a12d-13684e30ec26
          © 2019
          History

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