Biomineralization in the presence of calcium-binding phosphoprotein particles.

The innermost shell lamella, which coats the inner surface of the shells in the estuarine clam Rangia cuneata, is a dynamic structure with a variable composition. In some populations the lamella is a phosphoprotein-rich structure devoid of crystalline mineral, and in others it is a glucosamine-rich structure often containing barite (BaSO4) inclusions. Mineral depositions was artificially stimulated in Rangia containing glucosamine-rich lamellae by scratching the inner shell surface. After stimulation, the lamellae were transformed into phosphoprotein-rich structures in which aragonite (CaCO3) was deposited. The mineral grew in spherulitic and dumbbell-shaped clusters characteristic of aragonite precipitated from strictly inorganic solutions. This study demonstrates that phosphoprotein particles accumulate in the innermost shell lamella during stimulated biomineralization but neither inhibit mineral deposition nor influence the crystal habits. Since phosphoprotein particles are high capacity calcium-binding proteins, they may be the source and transport vehicle for the calcium ions utilized in shell mineralization.