Abstract
Author SummaryThe microstructure of vertebrate bones and teeth is controlled by polyproline-rich protein matrices (such as amelogenin) that serve as a scaffold to control the assembly of biological apatites. In tooth enamel, amphibians have large amelogenin subunits and thin enamel while mammals have smaller amelogenin subunits in tandem with elongated crystals and complex prismatic organization. Using specific peptides and frog amelogenin overexpressed in mice, we confirmed the effect of the length of the elongated polyproline repeat on reduced matrix subunit dimensions and enhanced apatite crystal length. Three-dimensional structures solved by NMR (nuclear magnetic resonance) and surface modeling algorithms indicate that elongated polyproline repeat stretches in amelogenins affect the dimensions of the supramolecular matrix through an increase in polyproline II helices, resulting in a compaction of supramolecular subunit dimensions. We propose that the availability of readily shaped apatites and innovative mechanisms based on amelogenin-repeat motifsthat compartmentalize and shape biological minerals was essential for the rise of early vertebrates, enabling the manufacture of strong teeth and backbones that might have given vertebrates a decisive survival advantage in the competition for food and in the sophistication of locomotion.
Highlights
Proline-rich regions occur in a wide variety of functionally significant proteins, including mucins, snow flea antifreeze proteins, prolamine storage proteins, pancreatic polypeptide hormones, neuropeptides, Alzheimer amyloid, prion proteins, and tooth enamel proteins [1,2]
Proline-rich, tripeptide tandem repeat proteins participate on all levels of biological mineralization and include members as diverse as the Haliotis rufescens protein Lustrin A involved in the extracellular deposition of shell and pearl, the Strongylocentrotus purpuratus protein SM50 contributing to the mineralization of sea urchin teeth and spicules from magnesium calcite and protodolomite, as well as vertebrate collagen I and the tetrapod tooth enamel protein amelogenin [6,7]
Using specific peptides and frog amelogenin overexpressed in mice, we confirmed the effect of the length of the elongated polyproline repeat on reduced matrix subunit dimensions and enhanced apatite crystal length
Summary
Proline-rich regions occur in a wide variety of functionally significant proteins, including mucins, snow flea antifreeze proteins, prolamine storage proteins, pancreatic polypeptide hormones, neuropeptides, Alzheimer amyloid, prion proteins, and tooth enamel proteins [1,2]. Many proline-rich proteins contain repetitive motifs and adopt left-handed polyproline II helical conformations (PPII) [3,4]. These PPII helices are more mobile than other periodic structures, e.g. a-helices or b-sheets [5], but exhibit well-defined molecular backbone conformation due to the rigidity of the proline ring. Proline-rich, tripeptide tandem repeat proteins participate on all levels of biological mineralization and include members as diverse as the Haliotis rufescens protein Lustrin A involved in the extracellular deposition of shell and pearl, the Strongylocentrotus purpuratus protein SM50 contributing to the mineralization of sea urchin teeth and spicules from magnesium calcite and protodolomite, as well as vertebrate collagen I and the tetrapod tooth enamel protein amelogenin [6,7]
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