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

Division of Oral Biology, Faculty of Dentistry, McGill University, 3640 University Street, Montreal, QC, Canada H3A 2B2

A.W. Bell

Department of Anatomy and Cell Biology, Faculty of Medicine, McGill University

J.P. Simmer

Department of Pediatric Dentistry, School of Dentistry, University of Texas, Health Science Center at San Antonio, TX, USA 78284-7888

C.E. Smith

Division of Oral Biology, Faculty of Dentistry, McGill University, 3640 University Street, Montreal, QC, Canada H3A 2B2, Department of Anatomy and Cell Biology, Faculty of Medicine, McGill University

Cloning technologies have established unambiguously that amelogenins always seem larger in molecular weight (M_r) 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 (M_r ^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 M_r^SDS vs. M _r^MS 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 R180 secretory isoform of rat amelogenin, whereas some minor proteins in the 23-kDa fraction likely corresponded to a R156 secretory isoform. This was in contrast to major proteins forming the 25-, 24-, and 23-kDa fractions (M_r^SDS), which seemed to represent proteolytic fragments of R 180 progressively altered at the P₁₆₉-A₁₇₀, P₁₆₄-L₁₆₅, and F₁₅₁-S₁₅₂ C-terminal cleavage sites, respectively. Proteins in the 20-kDa fraction (MrSDS) most closely matched by ESI-MS fragments of the R156 secretory isoform that were C-terminally-modified at the equivalent P₁₆₄-L₁₆₅ site.

Key Words: enamel matrix proteins • amelogenin • mobility by SDS-PAGE • mass spectrometry.

Journal of Dental Research, Vol. 79, No. 3, 840-849 (2000)
DOI: 10.1177/00220345000790031001


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