The Matrix Protein Cysrichin, a Galaxin-like Protein from Hyriopsis cumingii, Induces Vaterite Formation In Vitro

SIMPLE SUMMARY: Vaterite has been widely used in the pharmaceutical industry because of its large specific surface area, high solubility, high biocompatibility, low specific gravity, and other beneficial characteristics. It is an internal factor that affects the quality of freshwater pearls. The mollusk shell comprises more than 95% calcium carbonate and less than 5% organic matrix. The organic matrix of the shell includes shell matrix proteins, polysaccharides, and a small number of lipids, among which shell matrix proteins play a crucial role in the formation of shells and pearls, according to previous studies. In our present study, we cloned a novel matrix protein, cysrichin, and our in vitro experiments confirmed that the cysrichin peptide induced vaterite crystals. Cysrichin has repeat amino acid sequences and a modular structure that may play a key role in establishing a structural framework and inducing crystal nucleation and growth. We analyzed cysrichin at both molecular and protein levels and confirmed that it was an important matrix protein; this finding provided some new clues to the mineralization of mussels. ABSTRACT: In this study, we cloned a novel matrix protein, cysrichin, with 16.03% homology and a similar protein structure to the coral biomineralized protein galaxin. Tissue expression analysis showed that cysrichin was mainly expressed in mantle and gill tissues. In situ hybridization indicated that cysrichin mRNA was detected in the entire epithelium region of mantle tissue. RNAi analysis and shell notching experiment confirmed that cysrichin participates in the prismatic layer and nacreous layer formation of the shell. An in vitro crystallization experiment showed that the cysrichin protein induced lotus-shaped and round-shaped crystals, which were identified as vaterite crystals. These results may provide new clues for understanding the formation of vaterite in freshwater shellfish.