Structural allostery and binding of the transferrin*receptor complex.
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.
Xu
Guozhong
G
Case Center for Proteomics and Mass Spectrometry, Case Western Reserve University, Cleveland, Ohio 44106, USA.
Liu
Rutao
R
Zak
Olga
O
Aisen
Philip
P
Chance
Mark R
MR
D000494:Allosteric Regulation; D000595:Amino Acid Sequence; D001665:Binding Sites; D020285:Cryoelectron Microscopy; D006801:Humans; D007501:Iron / Q000378:metabolism; D013058:Mass Spectrometry; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011487:Protein Conformation; D011990:Receptors, Transferrin / Q000737:chemistry* / Q000378:metabolism* / Q000648:ultrastructure
D011990:Receptors, Transferrin; D007501:Iron