Hichin, a chitin binding protein is essential for the self-assembly of organic frameworks and calcium carbonate during shell formation

Abstract

Shell biomineralization is a process where inorganic minerals accumulate upon a chitinous scaffold under the control of multifunctional matrix proteins. In this study, we cloned a novel matrix protein gene from the mantle of Hyriopsis cumingii. The predicted protein, hichin, contains a chitin-binding domain and exhibited the highest expressional level in mantle tissue, with positive signals mainly detected in dorsal epithelial cells of the pallial mantle according to in situ hybridization, indicating its possible involvement in shell nacreous layer biomineralization. RNA interference showed that hichin suppression induced disordered self-assembly of the insoluble framework in the nacreous layer, and that the newly formed calcium carbonate crystals could not bind to organic frameworks. Furthermore, hichin was primarily responsible for building the framework during initial nacre deposition in pearl formation. Moreover, the chitin-binding domain of hichin also provided crystal morphology regulation in vitro crystallization assay. These results indicated that hichin is involved in the self-assembly of organic frameworks and morphological regulation in shell nacreous layer.