Sphingolipid regulation by yeast Mdm1 supports adaptive remodeling of the methionine transporter Mup1

Membrane lipid composition influences endocytic remodeling of nutrient transporters, yet how lipid metabolism is spatially coordinated to support sustained adaptation to nutrient limitations remains unclear. Here, we investigated whether the endoplasmic reticulum (ER)–vacuole tether Mdm1 links sphingolipid (SL) homeostasis to regulation of the high-affinity methionine permease Mup1 in budding yeast. To test this, we examined Mup1 trafficking, amino acid homeostasis, and SL composition in mdm1 Δ cells during starvation. We found that loss of Mdm1 causes persistent retention of Mup1 at the plasma membrane (PM), accompanied by reduced intracellular methionine and broad amino acid depletion. Lipidomic analyses revealed decreased sphingoid bases and altered ceramide composition in mdm1 Δ cells. Importantly, supplementation with the SL precursor phytosphingosine restored SL pools, rescued Mup1 endocytosis, and improved amino acid homeostasis. Consistent with a chronic amino acid restriction-like state, mdm1 Δ cells exhibited extended chronological lifespan. Together, these findings support a model in which Mdm1 functions as a spatial organizer of SL metabolism, contributing to adaptive endocytic remodeling of Mup1, thereby linking ER–vacuole contact site function to PM proteostasis and metabolic adaptation.