Regulation of the branched-chain amino acid pathway and its crosstalk to carbon metabolism in yeast

In Saccharomyces cerevisiae, the TORC1 pathway regulates the transition from rapid fermentative growth to respiration during the diauxic shift, by tightly coordinating energy- and biomass-producing pathways. Although leucine and other components of the branched-chain amino acid (BCAA) pathway are known TORC1 regulators, how the BCAA pathway is controlled across this transition and influences the crosstalk between central carbon and amino acid metabolism remains unclear. By integrating high-throughput flow cytometry and untargeted LC-MS metabolomics into a thermodynamically curated genome-scale model, we profiled protein levels, metabolite dynamics, and cellular context during the diauxic shift of a GFP-tagged strain library of the BCAA pathway in wild-type and mutants lacking non-essential genes for BCAA biosynthesis. This analysis revealed that the BCAA pathway operates in two branches, with only the leucine-committed branch exhibiting a fermentative signature aligned with TORC1 activity. We further identified key regulatory elements and showed that dysregulation of the BCAA pathway disrupts distant metabolic circuits, including central carbon metabolism. Our findings highlight the dynamic role of the BCAA pathway in metabolic network integration during the diauxic shift.