Mass-spectrographic analysis of a porcine amelogenin identifies a single phosphorylated locus

Abstract

The amelogenins of the extracellular matrix of developing dental enamel, comprise a family of tissue-specific proteins which are postulated to play a central role in the biomineralization of dental enamel [1]. The primary structures of amelogenins derived from cow, pig, human, mouse and rat have now been elucidated by the interpretation of cDNA sequences or by direct amino acid sequence determinations [2–6] demonstrating a high degree of sequence homology between species [1]. However, the nature of post-translational modification of these proteins is less clear. In particular, early reports of amelogenin phosphorylation [7–8] have proved to be difficult to confirm by direct chemical analyses [1].

Using mass spectrographic analysis, we recently [9], reported that the lower molecular weight (5–7 kDa) bovine and porcine amelogenin polypeptides (TRAP and LRAP) contained a single phospho-serine residue at position ¹⁶Ser and, since these polypeptides are derived by proteolytic processing from the higher molecular weight “parent” amelogenins (18–25 kDa), we concluded that these precursor molecules must also be phosphorylated, as has previously been suggested [10]. In contrast to these observations, an extensive amino acid sequencing study of porcine amelogenins has recently reported no evidence for such phosphorylation [11].

We now report that a new analysis of the major porcine(“20K”) amelogenin provides positive evidence for porcine amelogenin phosphorylation.