Transferrin-mediated cellular iron delivery.

Essential to iron homeostasis is the transport of iron by the bilobal protein human serum transferrin (hTF). Each lobe (N- and C-lobe) of hTF forms a deep cleft which binds a single Fe(3+). Iron-bearing hTF in the blood binds tightly to the specific transferrin receptor (TFR), a homodimeric transmembrane protein. After undergoing endocytosis, acidification of the endosome initiates the release of Fe(3+) from hTF in a TFR-mediated process. Iron-free hTF remains tightly bound to the TFR at acidic pH; following recycling back to the cell surface, it is released to sequester more iron. Efficient delivery of iron is critically dependent on hTF/TFR interactions. Therefore, identification of the pH-specific contacts between hTF and the TFR is crucial. Recombinant protein production has enabled deconvolution of this complex system. The studies reviewed herein support a model in which pH-induced interrelated events control receptor-stimulated iron release from each lobe of hTF.