Phosphoproteins from teeth and bone

The anionic non-collagenous proteins of the extracellular matrices of bone and dentine have been proposed as important participants in the mineralization of these tissues. Phosphorylated protein components have been implicated as mediators of the specific nucleation of the growth of hydroxyapatite crystals on the matrix collagen fibrils. However, the phosphoproteins of bone and dentine are quite different and it is difficult to postulate a common mechanism for the nucleation. If there is a common mechanism some particular domain on each molecule is likely to be involved. Therefore, we have initiated studies of the domain structure of the most highly phosphorylated dentine protein, phosphophoryn. At least three sequence domains with different character have been found, and studies are under way to relate these domains to the antigenic, collagen-binding and calcium ion-binding properties of phosphophoryn. The collagen- and calcium ion-binding regions appear to be localized within the same domains.