A conserved vesicular circuit enables metabolic adaptation and maintains systemic heme homeostasis
Metabolic homeostasis depends on adaptive control of intracellular metabolite flux, yet how such control is reconfigured when canonical transport pathways fail is unknown. Here we define a conserved vesicular circuit that preserves systemic heme balance by rerouting intracellular heme flux. We show that loss of the intestinal heme exporter MRP-5 in Caenorhabditis elegans causes lethal heme sequestration within endolysosomal compartments. This defect is bypassed by disabling the vesicular adaptor AP-3, which stabilizes and reroutes the heme importer HRG-1, restoring heme export without increasing cytosolic heme. Unbiased genetics and transcriptomics identify two previously uncharacterized SLC49A family members, HRG-13 and HRG-14, as heme exporters with distinct affinities that engage in a vesicular importer-exporter handoff. Live imaging reveals heme-enhanced contacts between HRG-1 and SLC49A-containing vesicles, consistent with direct vesicular transfer. This circuitry extends to vertebrates as disruption of the SLC49A3 homolog impairs erythropoiesis in zebrafish and causes intracellular heme overload, premature hemoglobinization and apoptosis in differentiating human erythroid cells. Together, these findings establish SLC49A3 proteins as conserved heme exporters and uncover a general principle of metabolic adaptation in which reprogramming intracellular compartmentalization, rather than increasing nutrient supply, restores systemic homeostasis.