Reverse overshot water wheel retroendocytosis of apotransferrin extrudes cellular iron
ABSTRACT Iron (Fe) a vital micronutrient for all organisms, must be managed judiciously because both deficiency or excess can trigger severe pathology. Although cellular Fe import is well understood, its export is thought to be limited to transmembrane extrusion through ferroportin (also known as Slc40a1) the only known mammalian Fe exporter. Utilizing primary cells and cell lines (including those with no discernible expression of ferroportin on their surface) we demonstrate that upon Fe loading, the multifunctional enzyme glyceraldehyde 3 phosphate dehydrogenase (GAPDH) which is recruited to the cell surface, 'treadmills' apotransferrin in and out of the cell. Kinetic analysis utilizing labeled ligand, GAPDH knockdown cells, 55Fe labeled cells and pharmacological inhibitors of endocytosis confirmed GAPDH dependent apotransferrin internalization as a prerequisite for cellular Fe export. These studies define an unusual rapid recycling process of retroendocytosis for cellular Fe extrusion, a process mirroring receptor mediated internalization that has never before been considered for maintenance of cellular cationic homeostasis. Modulation of this unusual pathway could provide insights for management of Fe overload disorders. Summary: We present evidence that retroendocytosis of apotransferrin mediated by multifunctional GAPDH extrudes excess cellular Fe in a mechanism reminiscent of pumping out of water by a reverse overshot water wheel.
Journal of Cell Science