title: 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.
authors.affiliation: Oral Biology, University of Illinois at Chicago Brodie Laboratory for Craniofacial Genetics, University of Illinois at Chicago College of Dentistry Chicago, IL, USA.
authors.forename: Gokul
authors.initials: G
authors.lastname: Gopinathan
authors.affiliation: Biocytogen, One Innovation Drive Worcester, MA, USA.
authors.forename: Tianquan
authors.initials: T
authors.lastname: Jin
authors.affiliation: Department of Periodontology, Stomatological Hospital, Jilin University Changchun, China.
authors.forename: Min
authors.initials: M
authors.lastname: Liu
authors.affiliation: Oral Biology, University of Illinois at Chicago Brodie Laboratory for Craniofacial Genetics, University of Illinois at Chicago College of Dentistry Chicago, IL, USA.
authors.forename: Steve
authors.initials: S
authors.lastname: Li
authors.affiliation: University of Illinois at Chicago Department of Orthodontics, University of Illinois at Chicago College of Dentistry Chicago, IL, USA.
authors.forename: Phimon
authors.initials: P
authors.lastname: Atsawasuwan
authors.affiliation: University of Illinois at Chicago Department of Orthodontics, University of Illinois at Chicago College of Dentistry Chicago, IL, USA.
authors.forename: Maria-Therese
authors.initials: MT
authors.lastname: Galang
authors.affiliation: Department of Medicine, University of Chicago Chicago, IL, USA.
authors.forename: Michael
authors.initials: M
authors.lastname: Allen
authors.affiliation: Oral Biology, University of Illinois at Chicago Brodie Laboratory for Craniofacial Genetics, University of Illinois at Chicago College of Dentistry Chicago, IL, USA.
authors.forename: Xianghong
authors.initials: X
authors.lastname: Luan
authors.affiliation: Oral Biology, University of Illinois at Chicago Brodie Laboratory for Craniofacial Genetics, University of Illinois at Chicago College of Dentistry Chicago, IL, USA.
authors.forename: Thomas G H
authors.initials: TG
authors.lastname: Diekwisch
keywords: amelogenin; calcium carbonate; hydroxyapatite; polyproline repeat proteins; vertebrate evolution