Micelle structure of amelogenin in porcine secretory enamel.

ABSTRACT

Even during the secretory stage of amelogenesis, enamel crystals thicken as amelogenins (the major protein component) decrease. To explain this phenomenon, we propose a model for amelogenin structure and function based upon the hypothesis that amelogenin forms micelles. Solubility and hydrophobicity analyses suggest that all but the hydrophilic amelogenin C-terminal regions aggregate via hydrophobic bonds to form a micelle core. Amelogenin micelles may form super-assemblies via their C-termini (KTKREEVD), which contain complementary positive (KTKR) and negative (EEVD) elements. Disassembly of the micelles through controlled proteolysis provides space for crystal growth. Initial cleavage (by enamelysin) removes the surface-accessible amelogenin C-terminus, exposing the middle portion to cleavage (by EMSP1). As a result, the 13-kDa amelogenin, a rod-shaped domain based upon ultrafiltration and transmission electron microscopy studies, is released. This model explains how amelogenin is able to 'space' and support the ribbon-like crystals and continuously yield space as the crystals thicken, until they are sufficiently mature to support themselves.