title: Characterization of the interaction between diferric transferrin and transferrin receptor 2 by functional assays and atomic force microscopy.
abstract: Transferrin receptor 2 (TfR2), a homologue of the classical transferrin receptor 1 (TfR1), is found in two isoforms, alpha and beta. Like TfR1, TfR2alpha is a type II membrane protein, but the beta form lacks transmembrane portions and therefore is likely to be an intracellular protein. To investigate the functional properties of TfR2alpha, we expressed the protein with FLAG tagging in transferrin-receptor-deficient Chinese hamster ovary cells. The association constant for the binding of diferric transferrin (Tf) to TfR2alpha is 5.6x10(6) M(-)(1), which is about 50 times lower than that for the binding of Tf to TfR1, with correspondingly reduced rates of iron uptake. Evidence for Tf internalization and recycling via TfR2alpha without degradation, as in the TfR1 pathway, was also found. The interaction of TfR2alpha with Tf was further investigated using atomic force microscopy, a powerful tool used for investigating the interaction between a ligand and its receptor at the single-molecule level on the living cell surface. Dynamic force microscopy reveals a difference in the interactions of Tf with TfR2alpha and TfR1, with Tf-TfR1 unbinding characterized by two energy barriers, while only one is present for Tf-TfR2. We speculate that this difference may reflect Tf binding to TfR2alpha by a single lobe, whereas two lobes of Tf participate in binding to TfR1. The difference in the binding properties of Tf to TfR1 and TfR2alpha may help account for the different physiological roles of the two receptors.
authors.affiliation: Division of Gastroenterology and Hematology/Oncology, Department of Medicine, Asahikawa Medical College, 2-1-1-1 Midorigaoka-Higashi, Asahikawa, Hokkaido 078-8510, Japan. ikuta@asahikawa-med.ac.jp <ikuta@asahikawa-med.ac.jp>
authors.forename: Katsuya
authors.initials: K
authors.lastname: Ikuta
authors.forename: Alexandre
authors.initials: A
authors.lastname: Yersin
authors.forename: Atsushi
authors.initials: A
authors.lastname: Ikai
authors.forename: Philip
authors.initials: P
authors.lastname: Aisen
authors.forename: Yutaka
authors.initials: Y
authors.lastname: Kohgo
mesh_terms: D000818:Animals; D015703:Antigens, CD / Q000378:metabolism; D016466:CHO Cells; D006224:Cricetinae; D003412:Cricetulus; D006801:Humans; D007501:Iron / Q000378:metabolism; D007700:Kinetics; D018625:Microscopy, Atomic Force; D011485:Protein Binding; D011990:Receptors, Transferrin / Q000378:metabolism*; D014168:Transferrin / Q000378:metabolism*
chemical_list: D015703:Antigens, CD; C068126:CD71 antigen; D011990:Receptors, Transferrin; C503640:TFR2 protein, human; D014168:Transferrin; C028283:diferric transferrin; D007501:Iron