MASS-SPECTROGRAPHIC ANALYSIS OF A PORCINE AMELOGENIN IDENTIFIES A SINGLE PHOSPHORYLATED LOCUS

The amelogenins of the extracellular matrix of developing dental ename l, 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, mo use and rat have now been elucidated by the interpretation of cDNA seq uences or by direct amino acid sequence determinations [2-6] demonstra ting a high degree of sequence homology between species [1]. However, the nature of post-translational modification of these proteins is les s 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 th at the lower molecular weight (5-7 kDa) bovine and porcine amelogenin polypeptides (TRAP and LRAP) contained a single phospho-serine residue at position (16)Ser and, since these polypeptides are derived by prot eolytic processing from the higher molecular weight ''parent'' ameloge nins ( 18-25 kDa), we concluded that these precursor molecules must al so be phosphorylated, as has previously been suggested [10]. In contra st to these observations, an extensive amino acid sequencing study of porcine amelogenins has recently reported no evidence for such phospho rylation [11]. We now report that a new analysis of the major porcine( ''20K'') amelogenin provides positive evidence for porcine amelogenin phosphorylation.