Endocytosis of the Saccharomyces cerevisiae Jen1 monocarboxylate-proton symporter under mildly alkaline conditions: a mechanism to prevent metabolite loss?

The Saccharomyces cerevisiae Jen1 transporter mediates electroneutral proton symport of lactate and pyruvate. In lactate-grown batch cultures, growth-coupled medium alkalinization was previously shown to coincide with endocytosis of Jen1. To investigate the physiological relevance of pH-dependent Jen1 endocytosis, S. cerevisiae was grown in carbon-limited continuous cultures on a mixed ethanol-lactate feed. When applying a linearly increasing pH (6.75–7.25) to these cultures, lactate and pyruvate concentrations in the external medium progressively increased. Up to a culture pH of 7.0, these extracellular concentrations aligned with a predicted thermodynamic equilibrium of reversible, Jen1-mediated electroneutral carboxylate-proton symport. Consistent with earlier reports, pronounced Jen1 internalization occurred above pH 7.0. At these mildly alkaline pH values, a more pronounced increase of residual lactate concentrations and transcriptional upregulation of genes involved in oxidative phosphorylation were consistent with increased cellular energy demands. This study reveals how pH-dependent regulation of carboxylate transporters shapes cellular adaptation to changing environmental conditions. Insights into these regulatory mechanisms can inform strategies to optimize microbial cell factories operating under variable pH regimes in industrial settings. The integrated analysis of transport, Jen1 localization, and transcriptional responses in growing continuous cultures uncovered physiological challenges associated with electroneutral carboxylate/proton symport under mildly alkaline conditions. The data support the hypothesis that Jen1 internalization evolved to prevent intracellular metabolite loss under unfavorable pH conditions.