Self-Assembly of Filamentous
Amelogenin Requires Calcium
and Phosphate: From Dimers via Nanoribbons to Fibrils
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Abstract
Enamel matrix self-assembly has long been suggested as
the driving
force behind aligned nanofibrous hydroxyapatite 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 4.0β6.0. 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
5β7 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