Molecular Evolution of Amelogenin in Mammals

Abstract

Abstract. An evolutionary analysis of mammalian amelogenin, the major protein of forming enamel, was conducted by comparison of 26 sequences (including 14 new ones) representative of the main mammalian lineages. Amelogenin shows highly conserved residues in the hydrophilic N-and C-terminal regions. The central hydrophobic region (most of exon 6) is more variable, but it has conserved a high amount of proline and glutamine located in triplets, PXQ, indicating that these residues play an important role. This region evolves more rapidly, and is less constrained, than the other well-conserved regions, which are subjected to strong constraints. The comparison of the substitution rates in relation to the CpG richness confirmed that the highly conserved regions are subjected to strong selective pressures. The amino acids located at important sites and the residues known to lead to amelogenesis imperfecta when substituted were present in all sequences examined. Evolutionary analysis of the variable region of exon 6 points to a particular zone, rich in either amino acid insertion or deletion. We consider this region a hot spot of mutation for the mammalian amelogenin. In this region, numerous triplet repeats (PXQ) have been inserted recently and independently in five lineages, while most of the hydrophobic exon 6 region probably had its origin in several rounds of triplet insertions, early in vertebrate evolution. The putative ancestral DNA sequence of the mammalian amelogenin was calculated using a maximum likelihood approach. The putative ancestral protein was composed of 177 residues. It already contained all important amino acid positions known to date, its hydrophobic variable region was rich in proline and glutamine, and it contained triplet repeats PXQ as in the modern sequences.

Intrinsically Disordered Proteins in Biomineralization

“…Amelogenin is involved in biomineralization as well as cell signaling events (Gruenbaum-Cohen et al, 2009;Lyngstadaas et al, 2009;Veis, 2003). The primary sequence of amelogenin is highly conserved, especially the N-terminal Tyr-rich and C-terminal charged regions (Delgado et al, 2005). Determination of amelogenin's secondary structure has been hampered by the self-association of the protein (Li et al, 2006).…”

The origins of polypeptide domains

“…Recently, species-specific differences in tooth structure and physiological differences between dog breeds have been correlated to alterations in the lengths of homo-amino acid repeats in specific proteins. (44,45) This suggests that, besides being either neutral or deleterious, some modifications of small repeat elements can result in positive (Darwinian) selection. (46,47) Analysis of one pathological incident of a Q-repeat expansion in a human family showed that homo-amino acid repeat expansion can occur by single-step duplications of complex largerthan-trinucleotide sequences.…”

“…Excessive expansion of Q-repeats in various otherwise unrelated human proteins results in similar neuropathologies;(34) however, no functional role for Q-repeats in normal proteins has been genetically established and we are aware of no proposal in which Q-repeat expansion would result in a fitness benefit or positive selection. Nevertheless, recent reports indicate that intragenic expansions or contractions of protein-coding repeats are correlated to vertebrate evolution of amelogenin, a protein that participates in tooth development, (44) and to the diverse and rapidly achieved phenotypic differences that have been selected between breeds of domestic dogs. (45) These studies show that small sequence duplications and deletions are frequent events that might commonly modify the activities of existing proteins.…”

“…In the case of amelogenin, these subtle modifications likely alter the higher-order assembly of protein/ hydroxyapatite complexes that determine the physical characteristics of tooth enamel. (44) In dogs, several polymorphisms have been mapped to regulatory transcription factors-the implication being that these mutations alter downstream gene regulation. (45,46) Additional evidence that single-generation sequence insertions and deletions might substantially alter protein function comes from analyses on chordate tbp genes.…”

Amelogenin Peptide Extract Increases Differentiation and Angiogenic and Local Factor Production and Inhibits Apoptosis in Human Osteoblasts

“…EMD is mainly composed of amelogenins (∼90%) [10], a highly hydrophobic protein family that shares high homology across species [13]. The remaining protein portion of EMD is composed of extracellular proteins and enzymes such as enamelin, ameloblastin, and proteases [14][15][16].…”