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Molecular & Cellular Proteomics 4:1959-1967, 2005.
© 2005 by The American Society for Biochemistry and Molecular Biology, Inc.

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Research

Structural Allostery and Binding of the Transferrin·Receptor Complex^*,S

Guozhong Xu^,, Rutao Liu^,¶, Olga Zak^¶, Philip Aisen^¶ and Mark R. Chance^,||

From the Case Center for Proteomics and Mass Spectrometry, Case Western Reserve University, Cleveland, Ohio 44106 and the ^¶ Department of Physiology and Biophysics, Albert Einstein College of Medicine, Bronx, New York 10461

The structural allostery and binding interface for the human serum transferrin (Tf)·transferrin receptor (TfR) complex were identified using radiolytic footprinting and mass spectrometry. We have determined previously that the transferrin C-lobe binds to the receptor helical domain. In this study we examined the binding interactions of full-length transferrin with receptor and compared these data with a model of the complex derived from cryoelectron microscopy (cryo-EM) reconstructions (Cheng, Y., Zak, O., Aisen, P., Harrison, S. C. & Walz, T. (2004) Structure of the human transferrin receptor·transferrin complex. Cell 116, 565–576). The footprinting results provide the following novel conclusions. First, we report characteristic oxidations of acidic residues in the C-lobe of native Tf and basic residues in the helical domain of TfR that were suppressed as a function of complex formation; this confirms ionic interactions between these protein segments as predicted by cryo-EM data and demonstrates a novel method for detecting ion pair interactions in the formation of macromolecular complexes. Second, the specific side-chain interactions between the C-lobe and N-lobe of transferrin and the corresponding interactions sites on the transferrin receptor predicted from cryo-EM were confirmed in solution. Last, the footprinting data revealed allosteric movements of the iron binding C- and N-lobes of Tf that sequester iron as a function of complex formation; these structural changes promote tighter binding of the metal ion and facilitate efficient ion transport during endocytosis.

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