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-p hosphoseryl (Ser[P]) residues, is synthesized by odontoblasts and is believ ed to be involved in matrix-mediated biomineralization of dentin. We have p urified 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, facilitatin g the determination of the amino acid sequence of the N-terminal 38 residue s. After the initial Asp-Ser(P)-Pro-Asn-Ser(P)-Ser(P)-Asp-Glu-Ser(P)-Asn-Gl y-, the sequence contained the repeated motifs Asp-Ser(P) and Asp-Ser(P)-Se r(P). Purified BDP migrated as a single band on gradient SD-PAGE with an ap parent molecular weight of 156 kDa. This value ii-as consistent with the mo lecular weight of the dephosphorylated protein of 105 kDa determined by mea ns of MALDI mass spectrometry.