Diversified Biomineralization Roles of Pteria penguin Pearl Shell Lectins as Matrix Proteins.

Previously, we isolated jacalin-related lectins termed PPL2, PPL3 (PPL3A, 3B and 3C) and PPL4 from the mantle secretory fluid of Pteria penguin (Mabe) pearl shell. They showed the sequence homology with the plant lectin family, jacalin-related ß-prism fold lectins (JRLs). While PPL3s and PPL4 shared only 35%-50% homology to PPL2A, respectively, they exhibited unique carbohydrate binding properties based on the multiple glycan-binding profiling data sets from frontal affinity chromatography analysis. In this paper, we investigated biomineralization properties of these lectins and compared their biomineral functions. It was found that these lectins showed different effects on CaCO3 crystalization, respectively, although PPL3 and PPL2A showed similar carbohydrate binding specificities. PPL3 suppressed the crystal growth of CaCO3 calcite, while PPL2A increased the number of contact polycrystalline calcite composed of more than one crystal with various orientations. Furthermore, PPL4 alone showed no effect on CaCO3 crystalization; however, PPL4 regulated the size of crystals collaborated with N-acetyl-D-glucosamine and chitin oligomer, which are specific in recognizing carbohydrates for PPL4. These observations highlight the unique functions and molecular evolution of this lectin family involved in the mollusk shell formation.

Novel matrix proteins of Pteria penguin pearl oyster shell nacre homologous to the jacalin-related ß-prism fold lectins.

Nacreous layers of pearl oyster are one of the major functional biominerals. By participating in organic compound-crystal interactions, they assemble into consecutive mineral lamellae-like photonic crystals. Their biomineralization mechanisms are controlled by macromolecules; however, they are largely unknown. Here, we report two novel lectins termed PPL2A and PPL2B, which were isolated from the mantle and the secreted fluid of Pteria penguin oyster. PPL2A is a hetero-dimer composed of α and γ subunits, and PPL2B is a homo-dimer of ß subunit, all of which surprisingly shared sequence homology with the jacalin-related plant lectin. On the basis of knockdown experiments at the larval stage, the identification of PPLs in the shell matrix, and in vitro CaCO3 crystallization analysis, we conclude that two novel jacalin-related lectins participate in the biomineralization of P. penguin nacre as matrix proteins. Furthermore, it was found that trehalose, which is specific recognizing carbohydrates for PPL2A and is abundant in the secreted fluid of P. penguin mantle, functions as a regulatory factor for biomineralization via PPL2A. These observations highlight the unique functions, diversity and molecular evolution of this lectin family involved in the mollusk shell formation.