Nitrogen-depedent regulation of Rtg1 and Rtg3 in TOR pathway
Many key signaling molecules are conserved from yeast to man. mTOR is a protein kinase involved in nutrient and growth factor signaling in humans that has a yeast homolog with a similar role, TOR1. Like its human counterpart, yeast TOR1 is also inhibited by rapamycin. Signaling downstream of the TOR kinase pathways regulates the nuclear localization of several transcription factors in response to the carbon and nitrogen sources in the nutritional environment. Yeast cells activate the transcription factor GLN3 through the TOR pathway to control the use of nitrogen in response to nitrogen sensing. TOR phosphorylation of GLN3 when nitrogen is abundant causes cytoplasmic localization and inactivation of this transcription factor. When nitrogen is abundant, URE2 also interacts with GLN3 to keep it out of the nucleus and repressed. TOR also regulates GLN3 through regulation of the phosphatase SIT4 that dephosphorylates GLN3 inducing nuclear localization. The preferred nitrogen sources for yeast cells are glutamine and glutamate, which are derived from TCA cycle intermediates. The use of urea or ammonia as a nitrogen source is associated with induction of genes involved in the TCA cycle by the transcription factors RTG1 and RTG3. When glutamine is present, it causes repression of RTG1 and RTG3 by TOR1. The set of genes regulated downstream of TOR affect cell growth associated processes such as translation and nutrient use. The ability of TOR to selectively regulate different sets of genes in response to either carbon or nitrogen sources indicates that TOR can interact independently with these downstream effectors.
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