The Iron Metabolism Receptors, TfR1 and TfR2, Bind and Stabilize Transferrin Through Non‐Conserved Interactions

Abstract

Hereditary hemochromatosis, a disease marked by chronic iron overload from insufficient expression of the hormone hepcidin, is one of the most common genetic diseases in people of Northern European ancestry. One form of HH (Type III) results from mutations in the transferrin receptor‐2 (TfR2). How TfR2 functions to promote hepcidin expression remains unclear, though it appears to be an important part of a mechanism to modulate hepcidin expression based on systemic iron levels. This responsiveness may be achieved through TfR2's competition with its homolog, TfR1, for their ligand, the iron transport protein, transferrin (Tf). TfR1 is expected to be the preferred receptor for diferric, or holo‐, Tf and that TfR2 will bind holo‐Tf when Tf‐iron saturation levels are elevated. To better understand the nature of these protein‐protein interactions, a surface plasmon resonance binding study with the full‐length receptors was conducted for the first time. In agreement with previous studies, holo‐Tf binds to TfR1 significantly stronger than TfR2. However, the binding constant for Tf‐TfR2 is still far below that of physiological holo‐Tf levels, inconsistent with the hypothetical model and suggests that other factors mediate the interaction. Apo‐Tf only weakly binds TfR2 at serum pH, thus is not be able to effectively compete with holo‐Tf under these conditions. Like TfR1, TfR2 is able to bind apo‐Tf with higher affinity at the lower pH of endosomes, suggesting that it could function to take up iron, where stabilization of the apo‐Tf‐TfR complex is key. Tf‐binding Tf‐binding to a TfR2 chimera containing the TfR1‐helical domain indicates that the differences in the helical domain account for differences in on‐rate of Tf, but non‐conserved inter‐receptor interactions are necessary stabilize the Tf‐TfR1/TfR2 chimera complex. One possible site of stabilization, the apical arm junction, is not important for TfR1‐Tf complex stabilization. The junction is critical for the TfR2‐Tf interaction, however; highlighting one non‐conserved stabilization mechanism. Our results highlight the differences in Tf interactions with the two TfRs.Support or Funding InformationNIH T‐32 Training Grant GMMBI0231C; NIH R01DK072166This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.