Organic Molecules in the Molluscan Shells Regulate the Fine Microstructures of Biominerals

Molluskan shells are typical biominerals consisting of calcium carbonates and few organic molecules. To protect and support the soft tissues of mollusks, the shells have fine microstructures with strong toughness and stiffness. <i>Pinctada fucata</i> that is used for the pearl aqua culture in Japan has multiple layers in the shell. The inner part of the shell is the nacreous layer. The nacreous layer shows the luster and interference color, because the platelet calcium carbonates sandwiched by thin organic frameworks build the lamellar microstructure (thickness of calcium carbonate is about 300-400 nm, thickness of organic framework is about a few nm). The outer part of the shell is the prismatic layer consisting of prisms (diameter is about 30-60 mm) of calcium carbonates surrounded by organic frameworks (the thickness of organic framework is about a few mm). The prismatic layer grows fast and function as a barrier from the environmental pressure and erosion. The hinge of bivalve is composed of the ligament that contains the nanofiber (diameter is about 50 nm) of calcium carbonates surrounded by dense organic frameworks (the thickness of organic framework is about 2~5 nm). When the bivalve shell opens and closes, nanofiber of the ligament becomes the flexible structure and resists against the pressure. To make these various microstructures in one shell, <i>P. fucata</i> has the fine mechanism regulating the crystal nucleation, growth, orientation, polymorph and morphology through non-classical pathway using organic molecules. Although the organic molecules are mainly chitin and protein, the structures, and functions of proteins in the shell are unclear.<br/>In the nacreous layer, we identified a novel matrix protein of Pif consisting of two proteins, Pif 97 and Pif 80¹. Pif 80 is high acidic protein and has calcium carbonate binding activity. Pif 97 has chitin-binding domain and VWA domain that has protein-protein interaction. <i>In vitro</i> and <i>in vivo</i> experimental analyses revealed that Pif has key roles to make the organic framework and control the shape of platelet calcium carbonates. In the prismatic layer, we identified a novel matrix protein of prismalin-14 that has acidic region in the N- and C-terminal and Gly/Tyr-rich region. Prismalin-14 is the mediator between chitin scaffolds and calcium carbonates^2,3. The calcite crystal of prisms contains small angle grain boundaries caused by a dense intracrystalline organic matrix network to improve mechanical strength. We identified chitin and chitinolytic enzymes as components of this intracrystalline organic matrix. In the ligament, we identified a novel acidic peptide of LICP that consists of 10 amino acids with N-terminal pyroglutamate amino acid. LICP inhibits the <i>c</i>-axis of calcium carbonate crystal and keeps the small crystal. The small tablets of crystals are aligned and connected each other to make the long and thin nanofiber in the ligament⁴. We also noted that the formation of organic frameworks is regulated by tissue inhibitor of metalloproteinase (TIMP) and matrix metalloproteinase (MMP) that degrade the LMP in the scaffold of organic frameworks.<br/>These new insights of matrix proteins in molluskan shells will develop the new insights and technologies for material and environmental sciences.