PROTEIN-TO-PROTEIN INTERACTIONS - CRITERIA DEFINING THE ASSEMBLY OF THE ENAMEL ORGANIC MATRIX

Enamel crystallites form in a protein matrix located proximal to the a meloblast cell layer. This unique organic extracellular matrix is cons tructed from structural protein components biosynthesized and secreted by ameloblasts. To date, three distinct classes of enamel matrix prot eins have been cloned. These are the amelogenins, tuftelin, and amelob lastin, with recent data implicating ameloblastin gene expression duri ng cementogenesis. The organic enamel extracellular matrix undergoes a ssembly to provide a three-dimensional array of protein domains that c arry out the physiologic function of guiding enamel hydroxyapatite cry stallite formation. Using the yeast two-hybrid system, we have surveye d these three known enamel gene products for their ability to direct s elf-assembly. We measured the capacity of the enamel gene products to direct protein-to-protein interactions, a characteristic of enamel pro teins predicated to be required for self-assembly. We provide addition al evidence for the self-assembly nature of amelogenin and tuftelin. A meloblastin self-assembly could not be demonstrated, nor were protein- to-protein interactions observed between ameloblastin and either amelo genin or tuftelin. Within the limits of the yeast two-hybrid assay, th ese findings constrain the emerging model of enamel matrix assembly by helping to define the limits of enamel matrix protein-protein interac tions that are believed to guide enamel mineral crystallite formation.