EVIDENCE FOR AMELOGENIN NANOSPHERES AS FUNCTIONAL COMPONENTS OF SECRETORY-STAGE ENAMEL MATRIX

Amelogenins are the principal proteins of the extracellular matrix of developing dental enamel and are postulated to function in the process es of biomineralization of the developing tooth although the molecular mechanisms concerned are poorly understood. Recent imaging studies, e mploying dynamic light scattering, atomic force, and transmission elec tron microscopy (TEM) have shown that a recombinant amelogenin (M(r) s imilar to 20 000 Da) spontaneously forms supramolecular quasi-spherica l aggregates (''nanospheres'') of 15-20 nm in diameter. By comparison with in vitro experiments employing the recombinant amelogenin we show that the nanospheres appear as electron-lucent structures when treate d with conventional electron microscopy contrast reagents (phosphotung state or uranyl acetate) and we speculate that this property derives f rom the hydrophobic nature of the amelogenin protein. Employing TEM pr eparations, developing enamel from mouse, bovine, and hamster we demon strate that the amelogenin nanospheres occur as beaded rows of electro n-lucent structures aligned with, and separating, the enamel mineral c rystallites. We postulate that the amelogenin monomers self-assemble t o form nanospheres which function to space the initial crystallites, c ontrol crystal habit, inhibit intercrystalline fusions, and, through t he apposition of their surfaces, create anionic channels which facilit ate ion transport within the mineralizing matrix. (C) 1995 Academic Pr ess, Inc.