bioRxiv2021
The essential biometal manganese (Mn) serves as a cofactor for several enzymes that are crucial for the prevention of human diseases. Whether intracellular Mn levels may be sensed and modulate intracellular signaling events has so far remained largely unexplored. The highly conserved target of rapamycin complex 1 (TORC1, mTORC1 in mammals) protein kinase requires divalent metal cofactors such as magnesium (Mg2+) to phosphorylate effectors as part of a homeostatic process that coordinates cell growth and metabolism with nutrient and/or growth factor availability. Here, our genetic approaches reveal that TORC1 activity is stimulated in vivo by elevated cytoplasmic Mn levels, which can be induced by loss of the Golgi-resident Mn2+ transporter Pmr1 and which depends on the natural resistance-associated macrophage protein (NRAMP) metal ion transporters Smf1 and Smf2. Accordingly, genetic interventions that increase cytoplasmic Mn2+ levels antagonize the effects of rapamycin in triggering autophagy, mitophagy, and Rtg1-Rtg3-dependent mitochondrion-to-nucleus retrograde signaling. Surprisingly, our in vitro protein kinase assays uncovered that Mn2+ activates TORC1 substantially better than Mg2+, which is primarily due to its ability to lower the Km for ATP, thereby allowing more efficient ATP coordination in the catalytic cleft of TORC1. These findings, therefore, provide both a mechanism to explain our genetic observations in yeast and a rationale for how fluctuations in trace amounts of Mn can become physiologically relevant. Supporting this notion, TORC1 is also wired to feedback control mechanisms that impinge on Smf1 and Smf2. Finally, we also show that Mn2+-mediated control of TORC1 is evolutionarily conserved in mammals, which may prove relevant for our understanding of the role of Mn in human diseases. Significance StatementThe target of rapamycin complex 1 (TORC1, mTORC1 in mammals) is a central, highly conserved controller of cell growth and aging in eukaryotes. Our study shows that the essential biometal manganese (Mn) acts as a primordial activator of TORC1 and that NRAMP metal ion transporters control TORC1 activity by regulating cytoplasmic Mn2+ levels. Moreover, TORC1 activity regulates Mn2+ levels through feedback circuits impinging on NRAMP transporters. Altogether, our results indicate that Mn homeostasis is highly regulated and modulates key cellular processes such as autophagy, mitophagy, and Rtg1-3 complex-dependent retrograde response. These findings open new perspectives for the understanding of neurodegenerative disorders and aging-related processes