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J. Biol. Chem., Vol. 282, Issue 32, 23253-23263, August 10, 2007

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A Novel Extrapallial Fluid Protein Controls the Morphology of Nacre Lamellae in the Pearl Oyster, Pinctada fucata^*

Zhuojun Ma¹, Jing Huang¹, Juan Sun, Guannan Wang, Changzhong Li, Liping Xie², and Rongqing Zhang³

From the Institute of Marine Biotechnology, Department of Biological Sciences and Biotechnology, Tsinghua University, Beijing, 100084, China and the Protein Science Laboratory of the Ministry of Education, Tsinghua University, Beijing, 100084, China

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 CaCO₃ precipitated from the EPF. Crystal binding experiments further revealed that ACCBP could recognize different CaCO₃ 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 CaCO₃-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.

Received for publication, January 2, 2007 , and in revised form, June 8, 2007.

The nucleotide sequence(s) reported in this paper has been submitted to the GenBank^TM/EBI Data Bank with accession number(s) DQ473430 [GenBank] .

^* This work was supported in part by the National High Technology Research and Development Program of China (2006AA09Z441 and 2006AA09Z413) and the National Natural Science Foundation of China (no. 30530600 and no. 30221003). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ These authors contributed equally to this work.

² To whom correspondence may be addressed: Dept. of Biological Sciences and Biotechnology, Tsinghua University, Beijing, 100084, China. Tel.: 86-10-62772899; Fax: 86-10-62772899; E-mail: lpxie{at}mail.tsinghua.edu.cn. ³ To whom correspondence may be addressed: Dept. of Biological Sciences and Biotechnology, Tsinghua University, Beijing, 100084, China. Tel.: 86-10-62772899; Fax: 86-10-62772899; E-mail: rqzhang{at}mail.tsinghua.edu.cn.

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