title: The molecular organization of the beta-sheet region in Corneous beta-proteins (beta-keratins) of sauropsids explains its stability and polymerization into filaments.
abstract: The hard corneous material of avian and reptilian scales, claws, beak and feathers is mainly derived from the presence of proteins formerly known as beta-keratins but now termed Corneous beta-proteins of sauropsids to distinguish them from keratins, which are members of the intermediate filament protein family. The modeling of the conserved 34 amino acid residues long central beta-sheet region of Corneous beta-proteins using an ab initio protein folding and structure prediction algorithm indicates that this region is formed by four antiparallel beta-sheets. Molecular dynamic simulations and Molecular Mechanics/Poisson Boltzmann Surface Area (MM-PBSA) analysis showed that the disposition of polar and apolar amino acids within the beta-region gives rise to an amphipathic core whose stability is further increased, especially in an aqueous environment, by the association into a dimer due to apolar interactions and specific amino-acid interactions. The dimers in turn polymerize into a 3nm thick linear beta-filament due to van der Waals and hydrogen-bond interactions. It is suggested that once this nuclear core of anti-parallel sheets evolved in the genome of a reptilian ancestor of the extant reptiles and birds about 300 millions years ago, new properties emerged in the corneous material forming scales, claws, beaks and feathers in these amniotes based on the tendency of these unique corneous proteins to form stable filaments different from keratin intermediate filaments or sterical structures formed by other corneous proteins so far known.
authors.affiliation: Department of Chemistry "Giacomo Ciamician", University of Bologna, Italy. Electronic address: matteo.calvaresi3@unibo.it.
authors.forename: Matteo
authors.initials: M
authors.lastname: Calvaresi
authors.affiliation: Research Division of Biology and Pathobiology of the Skin, Department of Dermatology, Medical University of Vienna, Austria.
authors.forename: Leopold
authors.initials: L
authors.lastname: Eckhart
authors.affiliation: Comparative Histolab and Department of Bigea, University of Bologna, Italy.
authors.forename: Lorenzo
authors.initials: L
authors.lastname: Alibardi
mesh_terms: D000818:Animals; D030161:Avian Proteins / Q000737:chemistry*; D001717:Birds; D019143:Evolution, Molecular; D006860:Hydrogen Bonding; D057927:Hydrophobic and Hydrophilic Interactions; D008958:Models, Molecular; D056004:Molecular Dynamics Simulation; D058105:Polymerization; D017433:Protein Structure, Secondary; D012104:Reptiles; D030162:Reptilian Proteins / Q000737:chemistry*; D053800:beta-Keratins / Q000737:chemistry*
chemical_list: D030161:Avian Proteins; D030162:Reptilian Proteins; D053800:beta-Keratins
keywords: 3D-modeling; Birds; Corneous beta-proteins; Polymer formation; Reptiles