Purification of nonamelogenin proteins from bovine secretory enamel

The developing enamel matrix is composed of two groups of proteins that can be generally classified as amelogenins and nonamelogenins. The hydrophobic amelogenins represent the majority of the developing enamel matrix proteins, whereas nonamelogenins include the more hydrophilic enamelins, proteinases, and other minor protein components, which represent a small proportion of the matrix. This report describes the purification and partial amino acid sequences of two previously unknown proteins isolated from developing bovine enamel. These proteins were prepared by extracting bovine secretory stage enamel with low ionic strength buffer, followed by ammonium sulfate fractionation. The proteins were purified by ion-exchange, affinity, and reversed-phase chromatography. We propose to designate the proteins BEgp (a glycoprotein) and BEpa. A partial sequence was also obtained from a third protein (BEpb) which was nearly identical to BEpa. Antibodies were prepared to a synthetic peptide based on the N-terminal sequence of BEpa and subsequent immunoblots of various bovine tissues showed a major component of approximately 25 kDa specifically in enamel and ameloblasts. Little or no cross-reactivity of the antibody was found to bovine proteins extracted from heart, lung, kidney, liver, dental pulp, or bone. Similar analyses of both rat secretory stage and maturation stage enamel showed two bands of 28 kDa and 29 kDa. Immunohistochemical localization in rat incisors, showed specific staining of the enamel, secretory granules, and Golgi apparatus in ameloblasts. No sequence homology with known proteins could be demonstrated for BEgp or BEpa, suggesting that these components of developing enamel are novel tooth-specific proteins.