Mechanism for multiple ligand recognition by the human transferrin receptor

Area of Science:

• Biochemistry
• Molecular Biology
• Cell Biology

Background:

• Transferrin receptor 1 (TfR) is essential for cellular iron uptake.
• Iron import involves TfR binding to iron-loaded transferrin (Fe-Tf) and subsequent iron release in endosomes.
• HFE protein, implicated in hereditary hemochromatosis, competes with Fe-Tf for TfR binding.

Purpose of the Study:

• To quantitatively determine binding affinities of TfR mutants to HFE, Fe-Tf, and apo-Tf.
• To elucidate the specific binding sites and interactions between TfR and different transferrin forms.
• To propose a structure-based model for TfR-mediated iron release.

Main Methods:

• Quantitative surface plasmon resonance (SPR) assays were employed.
• Site-directed mutagenesis of TfR was performed.
• Binding affinities were measured at different pH conditions (7.4 and 6.3).

Main Results:

• Fe-Tf and HFE bind to an overlapping site on the TfR helical domain.
• Mutations in the TfR protease-like domain affect Fe-Tf binding, while helical domain mutations affect apo-Tf binding.
• Binding data revealed distinct footprints for Fe-Tf and apo-Tf on TfR, suggesting pH-dependent conformational changes.

Conclusions:

• The study provides detailed binding footprints of Fe-Tf and apo-Tf on TfR.
• A model is proposed where Tf C-lobe contacts the TfR helical domain and Tf N-lobe contacts the protease-like domain.
• Differential binding suggests pH-induced conformational changes in Tf or TfR influence iron release mechanism.