title: Elongated polyproline motifs facilitate enamel evolution through matrix subunit compaction.
abstract: Vertebrate body designs rely on hydroxyapatite as the principal mineral component of relatively light-weight, articulated endoskeletons and sophisticated tooth-bearing jaws, facilitating rapid movement and efficient predation. Biological mineralization and skeletal growth are frequently accomplished through proteins containing polyproline repeat elements. Through their well-defined yet mobile and flexible structure polyproline-rich proteins control mineral shape and contribute many other biological functions including Alzheimer's amyloid aggregation and prolamine plant storage. In the present study we have hypothesized that polyproline repeat proteins exert their control over biological events such as mineral growth, plaque aggregation, or viscous adhesion by altering the length of their central repeat domain, resulting in dramatic changes in supramolecular assembly dimensions. In order to test our hypothesis, we have used the vertebrate mineralization protein amelogenin as an exemplar and determined the biological effect of the four-fold increased polyproline tandem repeat length in the amphibian/mammalian transition. To study the effect of polyproline repeat length on matrix assembly, protein structure, and apatite crystal growth, we have measured supramolecular assembly dimensions in various vertebrates using atomic force microscopy, tested the effect of protein assemblies on crystal growth by electron microscopy, generated a transgenic mouse model to examine the effect of an abbreviated polyproline sequence on crystal growth, and determined the structure of polyproline repeat elements using 3D NMR. Our study shows that an increase in PXX/PXQ tandem repeat motif length results (i) in a compaction of protein matrix subunit dimensions, (ii) reduced conformational variability, (iii) an increase in polyproline II helices, and (iv) promotion of apatite crystal length. Together, these findings establish a direct relationship between polyproline tandem repeat fragment assemblies and the evolution and the design of vertebrate mineralized tissue microstructures. Our findings reveal that in the greater context of chordate evolution, the biological control of apatite growth by polyproline-based matrix assemblies provides a molecular basis for the evolution of the vertebrate body plan.
authors.affiliation: Brodie Laboratory for Craniofacial Genetics, University of Illinois at Chicago College of Dentistry, Chicago, Illinois, United States of America.
authors.forename: Tianquan
authors.initials: T
authors.lastname: Jin
authors.forename: Yoshihiro
authors.initials: Y
authors.lastname: Ito
authors.forename: Xianghong
authors.initials: X
authors.lastname: Luan
authors.forename: Smit
authors.initials: S
authors.lastname: Dangaria
authors.forename: Cameron
authors.initials: C
authors.lastname: Walker
authors.forename: Michael
authors.initials: M
authors.lastname: Allen
authors.forename: Ashok
authors.initials: A
authors.lastname: Kulkarni
authors.forename: Carolyn
authors.initials: C
authors.lastname: Gibson
authors.forename: Richard
authors.initials: R
authors.lastname: Braatz
authors.forename: Xiubei
authors.initials: X
authors.lastname: Liao
authors.forename: Thomas G H
authors.initials: TG
authors.lastname: Diekwisch
mesh_terms: D000566:Amelogenesis; D053523:Amelogenin / Q000737:chemistry / Q000378:metabolism*; D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D000818:Animals; D001031:Apatites / Q000378:metabolism*; D005075:Biological Evolution; D040761:Biomimetic Materials; D002417:Cattle; D003460:Crystallization; D003743:Dental Enamel / Q000378:metabolism; D006801:Humans; D051379:Mice; D008822:Mice, Transgenic; D008969:Molecular Sequence Data; D054118:Nanospheres; D010455:Peptides / Q000378:metabolism*; D011894:Rana pipiens; D014714:Vertebrates / Q000378:metabolism*
chemical_list: D053523:Amelogenin; D001031:Apatites; D010455:Peptides; C011083:polyproline