title: A novel extrapallial fluid protein controls the morphology of nacre lamellae in the pearl oyster, Pinctada fucata.
abstract: Mollusk shell nacre is known for its superior mechanical properties and precisely controlled biomineralization process. However, the question of how mollusks control the morphology of nacre lamellae remains unresolved. Here, a novel 38-kDa extrapallial fluid (EPF) protein, named amorphous calcium carbonate-binding protein (ACCBP), may partially answer this question. Although sequence analysis indicated ACCBP is a member of the acetylcholine-binding protein family, it is actively involved in the shell mineralization process. In vitro, ACCBP can inhibit the growth of calcite and induce the formation of amorphous calcium carbonate. When ACCBP functions were restrained in vivo, the nacre lamellae grew in a screw-dislocation pattern, and low crystallinity CaCO(3) precipitated from the EPF. Crystal binding experiments further revealed that ACCBP could recognize different CaCO(3) crystal phases and crystal faces. With this capacity, ACCBP could modify the morphology of nacre lamellae by inhibiting the growth of undesired aragonite crystal faces and meanwhile maintain the stability of CaCO(3)-supersaturated body fluid by ceasing the nucleation and growth of calcite. Furthermore, the crystal growth inhibition capacity of ACCBP was proved to be directly related to its acetylcholine-binding site. Our results suggest that a "safeguard mechanism" of undesired crystal growth is necessary for shell microstructure formation.
authors.affiliation: Institute of Marine Biotechnology, Department of Biological Sciences and Biotechnology, Tsinghua University, Beijing, 100084, China.
authors.forename: Zhuojun
authors.initials: Z
authors.lastname: Ma
authors.forename: Jing
authors.initials: J
authors.lastname: Huang
authors.forename: Juan
authors.initials: J
authors.lastname: Sun
authors.forename: Guannan
authors.initials: G
authors.lastname: Wang
authors.forename: Changzhong
authors.initials: C
authors.lastname: Li
authors.forename: Liping
authors.initials: L
authors.lastname: Xie
authors.forename: Rongqing
authors.initials: R
authors.lastname: Zhang
mesh_terms: D000109:Acetylcholine / Q000737:chemistry; D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D002038:Bungarotoxins / Q000737:chemistry; D002119:Calcium Carbonate / Q000737:chemistry; D002352:Carrier Proteins / Q000737:chemistry* / Q000494:pharmacology; D003460:Crystallization; D008954:Models, Biological; D008969:Molecular Sequence Data; D049911:Pinctada; D017434:Protein Structure, Tertiary; D017386:Sequence Homology, Amino Acid
chemical_list: D002038:Bungarotoxins; D002352:Carrier Proteins; D002119:Calcium Carbonate; D000109:Acetylcholine