Abstract
The conserved target of rapamycin complex 1 (TORC1) integrates nutrient signals to orchestrate cell growth and proliferation. Leucine availability is conveyed to control TORC1 activity via the leu-tRNA synthetase/EGOC-GTPase module in yeast and mammals, but the mechanisms sensing leucine remain only partially understood. We show here that both leucine and its α-ketoacid metabolite, α-ketoisocaproate, effectively activate the yeast TORC1 kinase via both EGOC GTPase-dependent and -independent mechanisms. Leucine and α-ketoisocaproate are interconverted by ubiquitous branched-chain aminotransferases (BCAT), which in yeast are represented by the mitochondrial and cytosolic enzymes Bat1 and Bat2, respectively. BCAT yeast mutants exhibit severely compromised TORC1 activity, which is partially restored by expression of Bat1 active site mutants, implicating both catalytic and structural roles of BCATs in TORC1 control. We find that Bat1 interacts with branched-chain amino acid metabolic enzymes and, in a leucine-dependent fashion, with the tricarboxylic acid (TCA)-cycle enzyme aconitase. BCAT mutation perturbed TCA-cycle intermediate levels, consistent with a TCA-cycle block, and resulted in low ATP levels, activation of AMPK, and TORC1 inhibition. We propose the biosynthetic capacity of BCAT and its role in forming multicomplex metabolons connecting branched-chain amino acids and TCA-cycle metabolism governs TCA-cycle flux to activate TORC1 signaling. Because mammalian mitochondrial BCAT is known to form a supramolecular branched-chain α-keto acid dehydrogenase enzyme complex that links leucine metabolism to the TCA-cycle, these findings establish a precedent for understanding TORC1 signaling in mammals.
Highlights
The Target of Rapamycin Complex 1 (TORC1) is functionally and structurally conserved throughout eukaryotes and senses and responds to nutrients to promote cell growth and inhibit catabolic processes such as autophagy
In all organisms from yeasts to mammals the target of rapamycin TORC1 pathway controls growth in response to nutrients such as leucine, but the leucine sensing mechanisms are only partially characterized. We show that both leucine and its α-ketoacid metabolite, α-ketoisocaproate, are capable of activating TORC1 kinase via EGOC GTPase
We find branched-chain aminotransferases (BCAT) interacts with several branched-chain amino acid metabolic enzymes, and in a leucine-dependent fashion with the tricarboxylic acid (TCA)-cycle enzyme aconitase
Summary
The Target of Rapamycin Complex 1 (TORC1) is functionally and structurally conserved throughout eukaryotes and senses and responds to nutrients to promote cell growth and inhibit catabolic processes such as autophagy. Under leucine-starvation conditions, the yeast SEA (Seh1-associated) complex and its mammalian ortholog GATOR activate the GTPase activity of, and thereby inhibit, the EGO and Rag GTPase complexes [4,5,6]. Activation of TORC1 controls yeast growth via phosphorylation of three major effector branches: activation of ribosome biogenesis via the protein kinase Sch, and repression of autophagy, nitrogen, and stress responses via Atg13/Atg and Tap42-PP2A [15,16,17,18,19]. Ammonium starvation, heat, oxidative, and osmotic stresses, and low levels of carbon, phosphate, and energy, control yeast TORC1 activity by additional mechanisms involving Rho, the AMP-regulated, MAPK, PAS, and Hog kinases, and stress granule sequestration [20,21,22,23]
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