Multiple Targets on the Gln3 Transcription Activator Are Cumulatively Required for Control of Its Cytoplasmic Sequestration

A remarkable characteristic of nutritional homeostatic mechanisms is the breadth of metabolite concentrations to which they respond, and the resolution of those responses; adequate but rarely excessive. Two general ways of achieving such exquisite control are known: stoichiometric mechanisms where increasing metabolite concentrations elicit proportionally increasing responses, and the actions of multiple independent metabolic signals that cumulatively generate appropriately measured responses. Intracellular localization of the nitrogen-responsive transcription activator, Gln3, responds to four distinct nitrogen environments: nitrogen limitation or short-term starvation, i.e., nitrogen catabolite repression (NCR), long-term starvation, glutamine starvation, and rapamycin inhibition of mTorC1. We have previously identified unique sites in Gln3 required for rapamycin-responsiveness, and Gln3-mTor1 interaction. Alteration of the latter results in loss of about 50% of cytoplasmic Gln3 sequestration. However, except for the Ure2-binding domain, no evidence exists for a Gln3 site responsible for the remaining cytoplasmic Gln3-Myc(13) sequestration in nitrogen excess. Here, we identify a serine/threonine-rich ( Gln3(477-493)) region required for effective cytoplasmic Gln3-Myc(13) sequestration in excess nitrogen. Substitutions of alanine but not aspartate for serines in this peptide partially abolish cytoplasmic Gln3 sequestration. Importantly, these alterations have no effect on the responses of Gln3-Myc(13) to rapamycin, methionine sulfoximine, or limiting nitrogen. However, cytoplasmic Gln3-Myc(13) sequestration is additively, and almost completely, abolished when mutations in the Gln3-Tor1 interaction site are combined with those in Gln3(477-493) cytoplasmic sequestration site. These findings clearly demonstrate that multiple individual regulatory pathways cumulatively control cytoplasmic Gln3 sequestration.