The expanded amelogenin polyproline region preferentially binds to apatite versus carbonate and promotes apatite crystal elongation

Abstract

The transition from invertebrate calcium carbonate-based calcite and aragonite exo- and endoskeletons to the calcium phosphate-based vertebrate backbones and jaws composed of microscopic hydroxyapatite crystals is one of the great revolutions in the evolution of terrestrial organisms. To identify potential factors that might have played a role in such a transition, three key domains of the vertebrate tooth enamel protein amelogenin were probed for calcium mineral/protein interactions and their ability to promote calcium phosphate and calcium carbonate crystal growth. Under calcium phosphate crystal growth conditions, only the carboxy-terminus augmented polyproline repeat peptide, but not the N-terminal peptide nor the polyproline repeat peptide alone, promoted the formation of thin and parallel crystallites resembling those of bone and initial enamel. In contrast, under calcium carbonate crystal growth conditions, all three amelogenin-derived polypeptides caused calcium carbonate to form fused crystalline conglomerates. When examined for long-term crystal growth, polyproline repeat peptides of increasing length promoted the growth of shorter calcium carbonate crystals with broader basis, contrary to the positive correlation between polyproline repeat element length and apatite mineralization published earlier. To determine whether the positive correlation between polyproline repeat element length and apatite crystal growth versus the inverse correlation between polyproline repeat length and calcium carbonate crystal growth were related to the binding affinity of the polyproline domain to either apatite or carbonate, a parallel series of calcium carbonate and calcium phosphate/apatite protein binding studies was conducted. These studies demonstrated a remarkable binding affinity between the augmented amelogenin polyproline repeat region and calcium phosphates, and almost no binding to calcium carbonates. In contrast, the amelogenin N-terminus bound to both carbonate and apatite, but preferentially to calcium carbonate. Together, these studies highlight the specific binding affinity of the augmented amelogenin polyproline repeat region to calcium phosphates versus calcium carbonate, and its unique role in the growth of thin apatite crystals as they occur in vertebrate biominerals. Our data suggest that the rise of apatite-based biominerals in vertebrates might have been facilitated by a rapid evolution of specialized polyproline repeat proteins flanked by a charged domain, resulting in apatite crystals with reduced width, increased length, and tailored biomechanical properties.

SM50 repeat-polypeptides self-assemble into discrete matrix subunits and promote appositional calcium carbonate crystal growth during sea urchin tooth biomineralization

“…For the design of the SM50 repeat peptide, a 26 amino acid stretch between amino acids 227 and 252 containing four successive polyproline repeat motifs was selected (Gopinathan et al, 2014). SM50 227–252 was chosen as a representative polyproline type II repeat peptide because each of the four repeat motifs contained an N-terminal proline and a C-terminal glutamine, and repeat motif length alternated between six and seven amino acids.…”

“…While in vertebrates, cells secrete an extracellular matrix to shape the size and properties of the future tooth, mineral nucleation in echinoderms occurs within vacuoles inside of the odontoblastic syncytium, and mineral is deposited once syncytial vacuoles have come into contact with the growing tooth surface (Märkel, 1969; Kniprath, 1974, Märkel et al 1977). A common element among vertebrate and invertebrate teeth is the use of unique biomineralization proteins that facilitate the assembly of biological mineral platelets into complex odontogenic minerals (Gopinathan et al 2014). In the present study we have focused on the biomineralization process in echinoderm teeth and especially on role of polyproline repeat proteins such as SM50 in sea urchin tooth mineralization.…”

“…Moreover, it has been demonstrated that the evolution of structural biomineralization proteins has occurred independently and multiple times among metazoans (Jackson et al, 2009). While biomineralization genes might have evolved independently to respond to different and rapidly changing dietary habits throughout the course of evolution, many deuterostome biomineralization proteins share polyproline tandem repeat elements as a common sequence motif (Gopinathan et al, 2014). These polyproline repeat elements commonly occur as polyproline type II helices and have been associated with their intrinsically disordered molecular structure and their suitability as biomineralization modulators (Delak et al 2009).…”

“…These polyproline repeat elements commonly occur as polyproline type II helices and have been associated with their intrinsically disordered molecular structure and their suitability as biomineralization modulators (Delak et al 2009). The most prominent sea urchin biomineralization protein, SM50, has a molecular weight of 50kDa (Sucov et al, 1987; Richardson et al, 1989; Killian and Wilt, 1996) and features a total of 15 PXX tripeptide repeats, 12 PXXXXXQ-PXXXXQ combined septa-hexa-peptide repeats, and 5 other polyproline tripeptide tandem repeats (Gopinathan et al 2014). Previous studies using the vertebrate tooth enamel biomineralization protein amelogenin as a model have demonstrated that the length of these polyproline repeat motif stretches is a powerful regulator of crystal growth and habit through its effect on the compaction of matrix subunits (Jin et al 2009).…”

Function and repair of dental enamel – Potential role of epithelial transport processes of ameloblasts

“…In a recent in silico work we found that the major biomineralization proteins can be characterized by a high percentage of disordered residues, including dentin sialophosphoprotein (96%), dentin matrix protein 1 (96%), bone sialoprotein (85%), osteopontin (92%), amelogenin (67%), ameloblastin (82%), and enamelin (92%) [14]. The tooth enamel protein amelogenin is a typical intrinsically disordered biomineralization protein as it controls the growth of apatite crystals for tooth enamel formation [15,16].…”