Mass Spectrometry of Native Rat Amelogenins: Primary Transcripts, Secretory Isoforms, and C-terminal Degradation

Cloning technologies have established unambiguously that amelogenins always seem larger in molecular weight (Mr) by gel electrophoresis (SDS-PAGE) than by mass spectrometry (MS). This has caused many problems relating cloned versions of amelogenin to proteins actually secreted by ameloblasts in vivo. In this study, discrete protein fractions at 31-20 kDa (Mr SDS) were prepared from freeze-dried rat incisor enamel by techniques optimized for preserving protein integrity. N-terminal sequence and amino acid compositional analyses indicated that the major protein forming these fractions was amelogenin. As expected, the molecular weights estimated by matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ESI) MS were significantly less than their apparent molecular weights estimated by SDS-PAGE. Plots of MrSDS vs. M rMS for all fractions showed high linear correlation (r = 0.992). Analysis of MS data further indicated that the major protein in the 27-kDa fraction corresponded to the R...