Comparative mass spectrometric analyses of enamel matrix proteins from five species suggest a common pathway of post-secretory proteolytic processing

This study was undertaken to examine probable initial pathway(s) of amelogenin proteolysis, making comparisons between species and thus searching for a common theme. Specimens of developing dental enamel matrix mere obtained from (i) mouse, 6 days post natal, (ii) male pig, (iii) female bovine, (iv) rat, and (v) female human. In collaboration with the Mass Spectrometry Facility of the School of Pharmacy, University of California, San Francisco, samples of the lyophilized proteins were analyzed by liquid chromatography-mass spectrometry. The results were complex, a large number (15-30 components) being identified in each case. Mass values obtained for each sample were compared with computed values derived from segments of the known amino acid sequences for the principal amelogenins of the five species. Putative identity with an experimental figure was accepted when the mass numbers agreed to within +/-2.0 daltons. In each case it was found that some components could be identified with sequences of the parent amelogenin. In the case of the mouse and rat strong evidence was obtained for sequential proteolytic processing from the carboxy-terminus of both the 180 and 156 residue amelogenins. A comparison between the five species showed, a fragment (cow, man, pig and mouse) uniquely identified as being derived by the processing of the parent amelogenin to the first proline residue from the carboxy-terminus, leading to the cleavage of 11 residues of the anionic carboxy-terminal sequence. In addition, it was found in each case, that mass identity of experimental data with the known sequences was only obtained assuming the presence of a single phosphorylated residue.