Protein interactions during assembly of the enamel organic extracellular matrix

Enamel is the outermost covering of teeth and contains the largest hydroxyapatite crystallites formed in the vertebrate body. Enamel forms extracellularly through the ordered assembly of a protein scaffolding that regulates crystallite dimensions. The two most studied proteins of the enamel extracellular matrix (ECM) are amelogenin and tuftelin. The underlying mechanism for assembly of the proteins within the enamel extracellular matrix and the regulatory role of crystallite-protein interactions have proven elusive. We used the two-hybrid system to identify and define minimal protein domains responsible for supra molecular assembly of the enamel ECM. We show that amelogenin proteins self-assemble, and this self-assembly depends on the amino-terminal 42 residues interacting either directly or indirectly with a 17-residue domain in the carboxyl region. Amelogenin and tuftelin fail to interact with each other. Based upon this data, and advances in the field, a model for amelogenin assemblies that direct enamel biomineralization is presented.