The iridescent sheen of nacre — the mother of pearl that coats the inner shell of molluscs — arises from the light-scattering properties of its finely layered structure. This comprises crystals of aragonite — a form of calcium carbonate — aligned within organic molecules. Hiromichi Nagasawa and his colleagues at the University of Tokyo have now discovered two proteins key to the formation of this material.
Studying the shell of the Japanese pearl oyster (Pinctada fucata), the researchers found that one protein, Pif 80, binds specifically to the aragonite crystals. When calcium carbonate was crystallized with organic material lacking Pif 80 and another protein, Pif 97, aragonite crystals grew randomly, with no iridescent effect.
The team thinks that the two proteins form a complex that helps to guide the formation of aragonite crystals, which are then assembled within sheets of organic material.