A novel basic matrix protein of Pinctada fucata, PNU9, functions as inhibitor during crystallization of aragonite

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.