The molecular mechanism for receptor-stimulated iron release from the plasma iron transport protein transferrin.
Human transferrin receptor 1 (TfR) binds iron-loaded transferrin (Fe-Tf) and transports it to acidic endosomes where iron is released in a TfR-facilitated process. Consistent with our hypothesis that TfR binding stimulates iron release from Fe-Tf at acidic pH by stabilizing the apo-Tf conformation, a TfR mutant (W641A/F760A-TfR) that binds Fe-Tf, but not apo-Tf, cannot stimulate iron release from Fe-Tf, and less iron is released from Fe-Tf inside cells expressing W641A/F760A-TfR than cells expressing wild-type TfR (wtTfR). Electron paramagnetic resonance spectroscopy shows that binding at acidic pH to wtTfR, but not W641A/F760A-TfR, changes the Tf iron binding site > or =30 A from the TfR W641/F760 patch. Mutation of Tf histidine residues predicted to interact with the W641/F760 patch eliminates TfR-dependent acceleration of iron release. Identification of TfR and Tf residues critical for TfR-facilitated iron release, yet distant from a Tf iron binding site, demonstrates that TfR transmits long-range conformational changes and stabilizes the conformation of apo-Tf to accelerate iron release from Fe-Tf.
Giannetti
Anthony M
AM
Division of Biology 114-96, California Institute of Technology, Pasadena, California 91125, USA.
Halbrooks
Peter J
PJ
Mason
Anne B
AB
Vogt
Todd M
TM
Enns
Caroline A
CA
Björkman
Pamela J
PJ
D015703:Antigens, CD / Q000235:genetics / Q000378:metabolism*; D001665:Binding Sites; D004578:Electron Spin Resonance Spectroscopy; D006639:Histidine / Q000378:metabolism; D006801:Humans; D007501:Iron / Q000097:blood* / Q000378:metabolism; D009154:Mutation; D010649:Phenylalanine / Q000378:metabolism; D021381:Protein Transport; D011990:Receptors, Transferrin / Q000235:genetics / Q000378:metabolism*; D014168:Transferrin / Q000378:metabolism*; D014364:Tryptophan / Q000378:metabolism
D015703:Antigens, CD; C068126:CD71 antigen; D011990:Receptors, Transferrin; D014168:Transferrin; D010649:Phenylalanine; D006639:Histidine; D014364:Tryptophan; D007501:Iron