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Shell matrix proteins (SMPs) which participate in shell formation are important components in shells of Pinctada fucata. The 72 unique SMPs of P. fucata were identified before, among which, PNU9, which is a part of an EDTA-insoluble matrix (EISM) of prism and nacre, got our attention. The expression of PNU9 mRNA concentrated on the outer fold (OF) and mantle pallial (MP) of the mantle, which is an important tissue for biomineralization. The result of a shell notching assay suggested that PNU9 participated in shell damage repairing processes. The injection of dsRNA of PNU9 could lead to the overgrowth of nacreous lamellas but had no significant influence on the prismatic layer. According to the results of the crystallization experiment in vitro, rPNU9 (mbp-PNU9) was able to inhibit crystal growth during aragonite crystallization and influence the morphology of calcite. What's more, rPNU9 could inhibit precipitation of calcium carbonate and the amorphous calcium carbonate (ACC) transition in the aragonite system. The above results may offer new thoughts on the role of shell matrix proteins in the biomineralization process of the nacreous layer. [ABSTRACT FROM AUTHOR]

The shells of pearl oysters, Pinctada fucata, are composed of calcite and aragonite and possess remarkable mechanical properties. These shells are formed under the regulation of macromolecules, especially shell matrix proteins (SMPs). Identification of diverse SMPs will lay a foundation for understanding biomineralization process. Here, we identified 72 unique SMPs using liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis of proteins extracted from the shells of P. fucata combined with a draft genome. Of 72 SMPs, 17 SMPs are related to both the prismatic and nacreous layers. Moreover, according to the diverse domains found in the SMPs, we hypothesize that in addition to controlling CaCO3 crystallization and crystal organization, these proteins may potentially regulate the extracellular microenvironment and communicate between cells and the extracellular matrix (ECM). Immunohistological localization techniques identify the SMPs in the mantle, shells and synthetic calcite. Together, these proteomic data increase the repertoires of the shell matrix proteins in P. fucata and suggest that shell formation in P. fucata may involve tight regulation of cellular activities and the extracellular microenvironment. [ABSTRACT FROM AUTHOR]