Sit4 and PP2A Dephosphorylate Transcription Activator Gln3 When TorC1 Is Up‐As Well As Down‐Regulated
Authors: Tate Jennifer J. , Cooper Terrance G. 
Published: June 2021

The yeast Saccharomyces cerevisiae lives in a boom and bust nutritional environment. Sophisticated regulatory systems have evolved to rapidly cope with these changes while preserving intracellular homeostasis. Target of Rapamycin Complex 1 (TorC1), a Ser/Thr kinase complex that is up‐regulated by excess nitrogen and down‐regulated by limiting nitrogen or rapamycin treatment, plays a central role in global nutrient‐responsive regulation. Two of TorC1's downstream targets are Gln3 and Gat1, the GATA‐family activators, whose localization and function are responsible for Nitrogen Catabolite Repression‐ (NCR‐) sensitive transcription. In nitrogen‐rich environments, where TorC1 is up‐regulated, Gln3 is phosphorylated, cytoplasmic (in a Gln3‐Ure2 complex) and NCR‐sensitive transcription is repressed. In contrast, when TorC1 is down‐regulated by rapamycin treatment, Gln3 is dephosphorylated, dissociates from Ure2, relocates to the nucleus and NCR‐sensitive transcription is derepressed. It is generally accepted that in excess nitrogen, TorC1 binds to and inhibits the Tap42‐Sit4 and Tap42‐PP2A phosphatase complexes that dephosphorylate Gln3, whereas when TorC1 is down‐regulated by limiting nitrogen or is inhibited by rapamycin, the phosphatase complexes are released from TorC1 and become active. They dephosphorylate Gln3, causing it to dissociate from Ure2, enter the nucleus and activate NCR‐sensitive transcription.Though convincing, the above scenario has been complicated by a paradoxical observation, i.e., Sit4 dephosphorylates Gln3 more in nitrogen excess than in nitrogen limiting conditions. This paradox motivated us to revisit the roles of Sit4 and PP2A in Gln3 regulation. Our observations resolve this paradox and add new dimensions to our understanding of nitrogen‐responsive transcription factor regulation: (i) Contrary to expectation, we show (by four different methods) that Gln3 is more phosphorylated when situated in the nucleus than when it is sequestered in the cytoplasm. (ii) In nitrogen excess, where TorC1 is up‐regulated, Gln3 cycles out of the nucleus into the cytoplasm. As it arrives in the cytoplasm, Gln3 is dephosphorylated by Sit4 and PP2A. (iii) In both excess and limiting nitrogen conditions, Sit4, PP2A and Ure2 are all required to maintain cytoplasmic Gln3 in this dephosphorylated form.We suggest Sit4 and PP2A function not only when TorC1 is down‐regulated but also when it is up‐regulated. This is possible because there are two forms of Sit4 and PP2A, free and complexed with Tap42. Only 5% of Sit4 and PP2A form a complex with Tap42. Tap42‐Sit4 and Tap42‐PP2A complex activities are controlled by TorC1, whereas free Sit4 and PP2A are not. We deduce that it is the free Sit4 and PP2A that dephosphorylate Gln3 upon its arrival in the cytoplasm. These observations predict that Gln3 must contain multiple distinct phosphatase targets. In agreement, we show that a C‐terminal Gln3 domain is required for rapamycin‐elicited, Sit4‐dependent nuclear Gln3 localization. However, an N‐terminal Gln3 domain also targeted for rapamycin‐elicited, Sit4‐dependent Gln3 dephosphorylation, enters the nucleus in nitrogen excess (TorC1 is up‐regulated) and in the absence of Sit4.Support or Funding InformationGM35642‐27This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.