Analyses for binding of the transferrin family of proteins to the transferrin receptor 2

Transferrin receptor 2 alpha (TfR2 alpha), the major product of the TfR2 gene, is the second receptor for transferrin (Tf), which can mediate cellular iron uptake in vitro. Homozygous mutations of TfR2 cause haemochromatosis, suggesting that TfR2 alpha may not be a simple iron transporter, but a regulator of iron by identifying iron-Tf. In this study, we analysed the ligand specificity of TfR2 alpha using human transferrin receptor 1 (TfR1) and TfR2 alpha-stably transfected and expressing cells and flow-cytometric techniques. We showed that human TfR2 alpha interacted with both human and bovine Tf, whereas human TfR1 interacted only with human Tf. Neither human TfR1 nor TfR2 alpha interacted with either lactoferrin or melanotransferrin. In addition, by creating point mutations in human TfR2 alpha, the RGD sequence in the extracellular domain of TfR2 alpha was shown to be crucial for Tf-binding. Furthermore, we demonstrated that mutated TfR2 alpha (Y250X), which has been reported in patients with hereditary haemochromatosis, also lost its ability to interact with both human and bovine Tf. Although human TfR1 and TfR2 alpha share an essential structure (RGD) for ligand-binding, they have clearly different ligand specificities, which may be related to the differences in their roles in iron metabolism.