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.
Highlights
Many living things have the ability to convert inorganic ions into solid minerals through a process termed biomineralization (1, 2)
During our research on extrapallial fluid (EPF), we found that EPF can retard further crystallization of the synthesized amorphous calcium carbonate (ACC), which is the most unstable form of CaCO3 (22, 23)
As for the mollusk, to support the growth of the shell, ions involved in shell formation (Ca2ϩ, HCO3Ϫ, and CO32Ϫ) are actively exchanged between the EPF, the hemolymph, and the environment (10)
Summary
Animals—The oyster, P. fucata, was purchased from Guofa Pearl Farm, Beihai, Guangxi Province, China. For the crystal growth experiment performed with the mixture of ACCBP and ␣-Bgt (molar ratio: 1:2), the proteins were incubated 24 h in advance. The labeled proteins were diluted with CaCO3-saturated Tris-HCl buffer (10 mM pH 7.5, prepared by mixing 1 g of reagent grade calcite with 100 ml of Tris-HCl buffer and stirring for 24 h) to a final concentration of 0.1 M and incubated with the glass slides for 1 h at room temperature. RA-␣-Bgt-labeled ACCBP was incubated with freshly synthesized calcite that was identified with Raman spectra, in a solution of saturated CaCO3. ACCBP does not affect the growth of normal nacre lamellae This result explains why the content of ACCBP in the shell matrix is so low that it cannot be detected on Western blots of total matrix proteins (Fig. 3A).
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