SELF-ASSEMBLY OF A RECOMBINANT AMELOGENIN PROTEIN GENERATES SUPRAMOLECULAR STRUCTURES

Amelogenin proteins are the principal constituents of the extracellula r organic matrix associated with the nucleation and growth of the carb onated calcium hydroxyapatite (HAP)-containing mineral phase of dental enamel. Amelogenins are believed to function in controlling the sizes and organization of the developing enamel crystals. Previous studies have shown that enamel proteins exhibit unusual reversible aggregation properties. The present studies were designed to test the hypothesis that self-assembly of recombinant amelogenin generates supramolecular structures that are indistinguishable from the electron-dense particle s associated with HAP crystal growth in vivo. A recombinant amelogenin analog of the murine 180-residue protein was analyzed by high-resolut ion size exclusion chromatography, atomic force (AFM), and transmissio n electron (TEM) microscopy. It was found that the amelogenin formed s upramolecular aggregates which were in a concentration-dependent equil ibrium with protein monomers. Imaging of the amelogenin by both AFM an d TEM techniques revealed spherical aggregate structures of about 18 n m diameter which were seen to be similar to electron-dense enamel stru ctures observed in vivo. We interpret these results to suggest that, i n vivo, the amelogenin protein self-assembles through functional motif s of the protein primary structure, generating specific supramolecular aggregates which we hypothesize function to control the ultrastructur al organization of the developing enamel crystallites. (C) 1994 Academ ic Press, Inc.