title: Self-assembly of filamentous amelogenin requires calcium and phosphate: from dimers via nanoribbons to fibrils.
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 4.0-6.0. 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.
authors.affiliation: Department of Preventative and Restorative Dental Sciences, University of California, 707 Parnassus Avenue, San Francisco, CA 94143, USA.
authors.forename: Olga
authors.initials: O
authors.lastname: Martinez-Avila
authors.forename: Shenping
authors.initials: S
authors.lastname: Wu
authors.forename: Seung Joong
authors.initials: SJ
authors.lastname: Kim
authors.forename: Yifan
authors.initials: Y
authors.lastname: Cheng
authors.forename: Feroz
authors.initials: F
authors.lastname: Khan
authors.forename: Ram
authors.initials: R
authors.lastname: Samudrala
authors.forename: Andrej
authors.initials: A
authors.lastname: Sali
authors.forename: Jeremy A
authors.initials: JA
authors.lastname: Horst
authors.forename: Stefan
authors.initials: S
authors.lastname: Habelitz
mesh_terms: D053523:Amelogenin / Q000737:chemistry*; D002118:Calcium / Q000737:chemistry; D003746:Dental Enamel Proteins / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D018625:Microscopy, Atomic Force; D046529:Microscopy, Electron, Transmission; D037742:Nanotubes, Carbon; D010710:Phosphates / Q000737:chemistry; D055503:Protein Multimerization
chemical_list: D053523:Amelogenin; D003746:Dental Enamel Proteins; D037742:Nanotubes, Carbon; D010710:Phosphates; C109069:enamel matrix proteins; D002118:Calcium