HFE and transferrin directly compete for transferrin receptor in solution and at the cell surface

Transferrin receptor (TfR) is a dimeric cell surface protein that binds both the serum iron transport protein transferrin (Fe-Tf) and HFE, the protein mutated in pa-tients with the iron overload disorder hereditary hemo-chromatosis. HFE and Fe-Tf can bind simultaneously to TfR to form a ternary complex, but HFE binding to TfR lowers the apparent affinity of the Fe-Tf/TfR interac-tion. This apparent affinity reduction could result from direct competition between HFE and Fe-Tf for their overlapping binding sites on each TfR polypeptide chain, from negative cooperativity, or from a combina-tion of both. To explore the mechanism of the affinity reduction, we constructed a heterodimeric TfR that con-tains mutations such that one TfR chain binds only HFE and the other binds only Fe-Tf. Binding studies using