Binding of matrix proteins to developing enamel crystals: An atomic force microscopy study

The control of hydroxyapatite crystal initiation and growth during enamel development is thought to be mediated by the proteins of the extracellular matrix. However, the precise nature of these critical mineral-protein interactions remains obscure. In this study, fluid tapping mode atomic force microscopy was used to image, for the first time, the binding of extracellular proteins found in enamel matrix (amelogenin and albumin) to developing enamel crystals. Both albumin and amelogenin were found to be associated with the crystal surfaces under conditions close to physiological; however, the binding of the two proteins was distinctly different. Albumin appeared to bind as a monomer, whereas amelogenin was bound as aggregates resembling previously described "nanosphere" structures. Both proteins were arrayed on the crystal surface in a distinctive banding pattern, perpendicular to the c-axis. This pattern was coincident with recently identified positively charged domains on the crystal surface, suggesting that an electrostatic interaction with the net negatively charged proteins controls the proteins' spatial distribution on the mineral surface. Desorption using phosphate buffers of increasing ionic strength indicated that both amelogenin and albumin were tightly bound to the crystals, but amelogenin alone was observed to bind more strongly to certain crystal faces, suggesting a more specific role for this protein in the control of crystal morphology and growth.