Identification of nacre matrix protein genes hic14 and hic19 and their roles in crystal growth and pearl formation in the mussel Hyriopsis cumingii.

Biological mineralization is a highly programmed process in which inorganic minerals reassociate under the strict control of the extracellular matrix to form minerals with special functions and patterns. Shells are biominerals, and their synthesis is finely regulated by organic matrix including matrix proteins, polysaccharides, lipids, pigments, free amino acids, and small peptides. In this study, two matrix protein genes, hic14 and hic19, were isolated from the mantle of the mussel Hyriopsis cumingii. Tissue expression analysis showed that both proteins were expressed mainly in the mantle, and in situ hybridization of mantle tissues showed that they were specifically expressed in the dorsal epithelial cells of mantle pallial. Therefore, hic14 and hic19 were both nacreous layer matrix proteins. In the pearl insertion experiment, hic14 and hic19 kept low expression during pearl sac formation and disordered calcium carbonate deposition, and increased significantly during pearl nacre accumulation, which showed that both proteins participated in the mineralization of pearl nacre. In the RNA interference experiment, shell nacre tablet growth was inhibited after crystal nucleation due to the decreased expression of hic14, and crystal morphology and arrangement of nacre were highly modified after expression of hic19 was inhibited. These results provided further evidence that both hic14 and hic19 participated in nacreous layer biomineralization.