Transferrin receptor (TfR) is a carrier protein for transferrin. It is needed for the import of iron into the cell and is regulated in response to intracellular iron concentration. It imports iron by internalizing the transferrin-iron complex through receptor-mediated endocytosis of holo-transferrin(iron containing transferrin). Earlier two transferrin receptors in humans, transferrin receptor 1 and transferrin receptor 2 were described . Subsequent work has revealed additional molecules involved in the trafficking of transferrin and associated iron into cells.
Low iron concentrations promote increased levels of transferrin receptor, to increase iron intake into the cell. Thus, transferrin receptor maintains cellular iron homeostasis.
TfR production in the cell is regulated according to iron levels by iron-responsive element-binding proteins, IRP1 and IRP2. In the absence of iron, one of these proteins (generally IRP2) binds to the hairpin like structure (IRE) that is in the 3' UTR of the TfR receptor mRNA. Once binding occurs, the mRNA is stabilized and degradation is inhibited.
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