ATP and α-ketoglutarate link nitrogen availability to TORC1-Npr1 signaling in yeast

The TORC1 (Target of Rapamycin Complex 1) kinase complex coordinates eukaryotic cell growth and metabolism in response to nutrient availability. In yeast, one of the proteins under TORC1 control is the Npr1 kinase mediating nitrogen control of several plasma membrane transporters, yet the metabolic signals governing this pathway remain poorly understood. Here, we first investigated the previously described and unexpected activation of TORC1-Npr1 signaling upon acute glucose starvation. We show that this response occurs only under specific nitrogen conditions, notably in cells grown on proline or glutamine, and depends on increased ATP production driven by stimulated respiration. In proline-grown cells, this respiration is fueled by catabolism of α-ketoglutarate, while high α-ketoglutarate levels negatively affect TORC1-Npr1 activation. We next analyzed TORC1-Npr1 activation following NH₄⁺ addition to proline-grown cells. Our results confirm that glutamine accumulation is not required for this activation and instead show that NH₄⁺ assimilation is associated with a decrease in α-ketoglutarate levels and a sustained increase in ATP production, concomitant with TORC1-Npr1 activation. In cells lacking the NH₄⁺-assimilating glutamate dehydrogenases, both increased ATP production and TORC1-Npr1 activation are impaired. Together, these findings identify ATP and α-ketoglutarate as important metabolic determinants of TORC1-Npr1 signaling in response to nitrogen availability.