Surface differentiation of ferritin and apoferritin with atomic force microscopic techniques

In the study reported herein, we differentiated the structure of ferritin from that of its demetalated counterpart, apoferritin, using field-effect-based atomic force microscopic (AFM) techniques. When ferritin was subjected to conductive-mode AFM analysis, the protein resembled a pancake with a diameter of 10 nm adsorbed on the indium-doped tin-oxide substrate with its fourfold channel perpendicular to the substrate, whereas a flat, empty cavity was revealed for apoferritin. We also attempted to verify the conformational difference with magnetic-mode AFM. However, the resulting phase images failed to differentiate the proteins due to interference from the fringe effect. Despite this, the ferritin analysis revealed a sound correlation between the surface conductivity profiles and the phase profiles. In contrast, apoferritin showed a chaotic relationship in this respect. These results not only suggest that the magnetic domain of ferritin is limited to the iron aggregate in the core, but also demonstrate that AFM is a useful tool for protein conformation analysis.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Field-effect-based atomic force microscopy (AFM) shows application potential to supplement existing protein characterization techniques for protein conformation analysis. ► According to conductive-mode AFM analysis, ferritin rests on ITO with its fourfold channel perpendicular to the substrate, analogous to the quaternary architecture documented in the literature, whereas its demetalated counterpart, apoferritin, has an empty cavity around its molecular center. ► The AFM characterizations suggest that the magnetic domain of ferritin is limited to the iron aggregate in the core.