Self-Assembly of Filamentous Amelogenin Requires Calcium and Phosphate: From Dimers via Nanoribbons to Fibrils

Enamel matrix self-assembly has long been,. suggested as the driving force behind aligned nanofibrous hydroxyaptite. formation. We tested if amelogenin, the main enamel matrix protein, can self assemble into ribbon-like structures in physiologic solutions. Ribbons 17 nm wide were,observed to grow several micrometers in length, requiring calcium, phosphate, and pH 4.07-6.0. The pH range suggests that the formation. of ion bridges through protonated histidine residues is essential to self assembly, supported by a statistical analysis of 212 phosphate-binding proteins predicting. 12 phosphate binding histidines. Thermophoretic analysis verified the importance of calcium and Phosphate in self assembly X-ray scattering characterized amelogenin dimers with dimensions fitting the cross-section of the amelogenin ribbon leading to the hypothesis that antiparallel dimers are the building blocks Of the ribbons Over 5-7 days, ribbons self-organized-into bundles composed Of aligned ribbons mimicking the structure of enamel crystallites in enamel rods. These observations confirm reports of filamentous Organic components in developing enamel and provide a new Model for matrix-templated enamel mineralization.