SMIM1 drives an alternative iron uptake pathway
Iron is essential for numerous biological processes, and its abundance is critical in human cells to prevent anemia or toxic overload. Although transferrin receptor-mediated iron uptake is the canonical route, human cells also possess an unstudied alternative pathway. To reveal this missing mechanism, we developed a large-scale, gain-of-function approach to assign genes to a catalogue of quantitative cellular traits. Applying this genetic strategy, we identified SMIM1, the antigen responsible for the rare Vel blood group and a transmembrane protein with unknown function, as a key regulator of iron homeostasis. We found that SMIM1 expression completely suppresses the iron deficiency caused by transferrin receptor loss. Mechanistically, SMIM1 mediates iron uptake through clathrin-dependent endocytosis of hemoglobin (Hb). SMIM1 directly binds Hb via its extracellular HKCK motif, leading to its transport to lysosomes and subsequent iron release. In the human population, genetic variants in SMIM1 influence Hb levels and erythrocyte traits. SMIM1-deficient mice display iron deficiency in early erythroid precursors in response to erythropoietic stress, underscoring the physiological relevance of this process.