Endocytosis of the plasma membrane carboxylate transporter Jen1 in Saccharomyces cerevisiae under alkali stress: a mechanism for preventing loss of essential metabolites

In batch cultures of Saccharomyces cerevisiae grown on lactate, medium alkalinization coincides with endocytosis of the Jen1 lactate/pyruvate transporter. To assess how culture pH impacts Jen1 endocytosis, S. cerevisiae was grown in carbon-limited continuous cultures with a mixed ethanol-lactate feed. When applying a linearly increasing pH (6.75 to 7.25), extracellular lactate and pyruvate concentrations increased progressively. Up to pH 7.0, these concentrations aligned with the thermodynamic equilibrium of Jen1-mediated electroneutral carboxylate-proton symport, while Jen1 internalization became more pronounced above pH 7.0. Transcriptional upregulation of genes involved in oxidative phosphorylation, together with higher residual lactate levels, indicated increased energy demands at supra-optimal pH values. By analyzing transport, endocytosis, and transcriptional responses in actively growing continuous cultures, this study uncovers pH-dependent physiological challenges associated with electroneutral carboxylate/proton symport. The data support the hypothesis that Jen1 internalization evolved to prevent intracellular metabolite loss under unfavorable pH conditions. These findings provide novel insights into the physiological consequences of transporter energy coupling and the ecophysiological relevance of anion transporters when extracellular pH approaches or exceeds cytosolic pH.