Vertebrate mineralized matrix proteins: Structure and function

The mineralized matrices of enamel, cementum, dentin, calcified cartilage and bone are similar in their ability to form a microenvironment that facilitates deposition of hydroxyapatite. However, they are not identical, as witnessed by the nature of apatite crystals that are formed. Enamel is devoid of collagen, and is composed of enamelins, amelogenins, tuftelin and ameloblastin, first described at this meeting. Cementum, dentin and bone matrices are composed primarily of type I collagen, however, each matrix may also contain unique moieties. The exact composition of cementum is not fully known, but in dentin there are unique matrix proteins, phosphophoryn (dentin phosphoprotein, DPP), a distinctive dentin matrix protein (DMP-1), and dentin sialoprotein (DSP). In bone, dentin and cementum, the matrix proteins include proteoglycans (versican, decorin, biglycan) and hyaluronan, glycoproteins which are often phosphorylated and sulfated (osteonectin, RGD-containing proteins) and gla-containing proteins (matrix gla protein, protein S, osteocalcin). The exact nature of all the non-collagenous proteins of calcified cartilage is not yet fully known. While there are no definitive functions for any of the mineralized matrix proteins to date, they most likely participate in regulation of cell metabolism, matrix deposition and mineralization, and bone turnover.