SPECTROSCOPY, IMAGING, OTHER TECHNIQUES |
Exploring Transferrin - Receptor Interactions at the Single Molecule Level
Alexandre Yersin 1*, Toshiya Osada 1 and Atsushi Ikai 1
1 Tokyo Institute of Technology
* To whom correspondence should be addressed. E-mail: ayersin{at}bio.titech.ac.jp.
Submitted on June 7, 2007
Revised on July 7, 2007
Accepted on 29 August 2007
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Abstract |
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Interaction between the iron transporter protein transferrin (Tf) and its receptor at the cell surface is fundamental for most living organisms. Tf receptor (TfR) binds iron-loaded Tf (holo-Tf) and transports it to endosomes, where acidic pH favors iron release. Iron-free Tf (apo-Tf) is then brought back to the cell surface and dissociates from TfR. Here we investigated Tf-TfR interaction at the single molecule level under different conditions encountered during Tf cycle. An atomic force microscope tip functionalized with holo-Tf or apo-Tf was used to probe TfR. We tested both purified TfR anchored to a mica substrate and in situ TfR at the surface of living cells. Dynamic force measurements showed similar results for TfR on mica or at the cell surface but revealed striking differences between holo-Tf-TfR and apo-Tf-TfR interactions. First, the forces necessary to unbind holo-Tf and TfR are always stronger compared to apo-Tf-TfR interaction. Second, dissociation of holo-Tf-TfR complex involves overcoming two energy barriers while apo-Tf-TfR unbinding pathway comprises only one energy barrier. These results agree with the model proposing differences in the contact points between holo-Tf-TfR and apo-Tf-TfR interactions (Giannetti et al, 2003. PLoS biology. 1:E51).
Key Words:
AFM, Atomic force microscopy, HeLa cells, dynamic force spectroscopy, iron
