Alterations in the Ure2 αcap domain elicit different gata factor responses to rapamycin treatment and nitrogen limitation

Abstract

Background: TorC1, excess nitrogen, and Ure2 negatively regulate Gln3 and Gat1. Results: Nitrogen catabolite repression-sensitive control of Gln3/Gat1 is normal in ure2αcap mutants that no longer respond to TorC1 inhibitor, rapamycin. Conclusion: Different regions of Ure2 are associated with the Gln3/Gat1 response to TorC1 inhibition and nitrogen availability. Significance: Gln3, Gat1, and Ure2 respond to TorC1 and nitrogen availability via distinct regulatory pathways. Ure2 is a phosphoprotein and central negative regulator of nitrogen-responsive Gln3/Gat1 localization and their ability to activate transcription. This negative regulation is achieved by the formation of Ure2-Gln3 and-Gat1 complexes that are thought to sequester these GATA factors in the cytoplasm of cells cultured in excess nitrogen. Ure2 itself is a dimer the monomer of which consists of two core domains and a flexible protrudingαcap. Here, we show that alterations in thiscap abolish rapamycin-elicited nuclear Gln3 and, to a more limited extent, Gat1 localization. In contrast, these alterations have little demonstrable effect on the Gln3 and Gat1 responses to nitrogen limitation. Using two-dimensional PAGE we resolved eight rather than the two previously reported Ure2 isoforms and demonstrated Ure2 dephosphorylation to be stimulus-specific, occurring after rapamycin treatment but only minimally if at all in nitrogen-limited cells. Alteration of theαcap significantly diminished the response of Ure2 dephosphorylation to the TorC1 inhibitor, rapamycin. Furthermore, in contrast to Gln3, rapamycin-elicited Ure2 dephosphorylation occurred independently of Sit4 and Pph21/22 (PP2A) as well as Siw14, Ptc1, and Ppz1. Together, our data suggest that distinct regions of Ure2 are associated with the receipt and/or implementation of signals calling for cessation of GATA factor sequestration in the cytoplasm. This in turn is more consistent with the existence of distinct pathways for TorC1-and nitrogen limitation-dependent control than it is with these stimuli representing sequential steps in a single regulatory pathway. © 2013 by The American Society for Biochemistry and Molecular Biology, Inc.