Tor Signaling and Nutrient-based Signals Converge on Mks1p Phosphorylation to Regulate Expression of Rtg1p·Rtg3p-dependent Target Genes

Ivanka Dilova,Sofia Aronova,Jenny C.-Y Chen,Ted Powers

doi:10.1074/jbc.m409012200

Ivanka Dilova, Sofia Aronova + Show 2 more

Open Access

https://doi.org/10.1074/jbc.m409012200

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Abstract

The heterodimeric bZip/HLH transcription factors Rtg1p and Rtg3p regulate the expression of a concise set of metabolic genes (termed RTG target genes) required for de novo biosynthesis of glutamate and glutamine. Several components have now been identified that control both the intracellular localization as well as activity of the Rtg1p.Rtg3p complex, yet the precise upstream regulatory signals involved remain unclear. For example, it has been proposed that Rtg1p.Rtg3p activity is repressed by glutamate, acting through the mitochondrial retrograde response pathway or, alternatively, by glutamine, acting through the Tor kinase pathway. Here we demonstrate that RTG target gene regulation is remarkably complex, with glutamate and glutamine as well as ammonia collaborating as potentially distinct signals to regulate RTG target gene expression. We show that both Tor and these nutrient-based signals converge on Mks1p, the immediate upstream inhibitor of Rtg1p.Rtg3p, and that a direct correlation exists between the degree of Mks1p phosphorylation and the extent of RTG target gene repression. Finally, we find that Tor- and glutamine-mediated RTG-target gene repression can be experimentally uncoupled, indicating that glutamine and Tor act, at least in part, independently to inhibit this pathway.

Highlights

Normal cell proliferation requires that cells adjust their protein biosynthetic and metabolic activity in response to nutrient availability and other environmental cues
We and others show that Tor negatively regulates a concise cluster of genes, termed RTG target genes, which encode a variety of enzymes involved in the anaplerotic production of ␣-ketoglutarate for use in de novo biosynthesis of glutamate and glutamine [13, 14]
We find that the role of proline can be attributed to its conversion to glutamate and that rapamycin-sensitive RTG target gene repression requires the availability of ammonia and/or glutamine

Summary

Introduction

Normal cell proliferation requires that cells adjust their protein biosynthetic and metabolic activity in response to nutrient availability and other environmental cues. We and others show that Tor negatively regulates a concise cluster of genes, termed RTG target genes, which encode a variety of enzymes involved in the anaplerotic production of ␣-ketoglutarate for use in de novo biosynthesis of glutamate and glutamine [13, 14] These genes were characterized originally by Butow and co-workers [15,16,17] as being under the control of a mitochondria-to-nucleus signaling pathway or retrograde response pathway that adjusts their transcription in response to the respiratory state of the cell. Our present results suggest that glutamine likely acts in concert with, rather than uniquely upstream of Tor. We find that all metabolic signals converge on Mks1p to influence its phosphorylation state and, regulate the intracellular localization and activity of the Rtg1p1⁄7Rtg3p complex

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