N-terminal Sequence Analysis of Bovine Dentin Phosphophoryn after Conversion of Phosphoseryl to S-propylcysteinyl Residues

Bovine dentin phosphophoryn (BDP), a protein rich in aspartyl (Asp) and O-phosphoseryl (Ser[P]) residues, is synthesized by odontoblasts and is believed to be involved in matrix-mediated biomineralization of dentin. We have purified BDP, using selective precipitation and ion exchange chromatography, from an EDTA soluble dentin extract and converted the Ser(P) residues to S-propylcysteinyl residues that are stable to Edman degradation, facilitating the determination of the amino acid sequence of the N-terminal 38 residues. After the initial Asp-Ser(P)-Pro-Asn-Ser(P)-Ser(P)-Asp-Glu-Ser(P)-Asn-Gly-, the sequence contained the repeated motifs Asp-Ser(P) and Asp-Ser(P)-Ser(P). Purified BDP migrated as a single band on gradient SDS-PAGE with an apparent molecular weight of 156 kDa. This value was consistent with the molecular weight of the dephosphorylated protein of 105 kDa determined by means of MALDI mass spectrometry.