Molluscan biomineralization: The proteinaceous shell constituents of Pinna nobilis L.

The shell of molluscs is a remarkable example of a natural composite biomaterial, synthesized at ambient temperature. Consequently, many consider it as a model for trying to develop at little cost new biomimetic materials of superior mechanical properties. The peculiar resistance of shells to fracture lies in an organic matrix, which is closely associated with the mineral phase. This matrix regulates the crystal growth, by allowing nucleation of the crystals only where appropriate, by favoring crystal elongation in privileged directions, and by inhibiting their growth. This matrix is a mixture of glycoproteins and polysaccharides, the primary structure and function of which are poorly known. Pinna nobilis, the Mediterranean fan mussel, is one of the few molluscs for which molecular data on shell proteins are available. In the present paper, we review what is known, at molecular level, on the macromolecular constituents of the shell of P. nobilis by describing three proteins, the characterization of which is still going on: the first one, mucoperlin, is specific of the nacreous layer. The two others were obtained from the calcitic prismatic layer and one of them was localized by immunogold and further tested to check its ability to modify the shapes of CaCO3 crystals. The data shown here, together with data obtained from the pearl oyster or the abalone, put into question some common ideas about molluscan mineralization.